Merge ec592390e7 into 9cd7b34a77

examples/mrt/main.go

@@ -0,0 +1,134 @@
+// Copyright 2024 The Ebiten Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"fmt"
+	"image"
+	_ "image/jpeg"
+	"log"
+
+	"github.com/hajimehoshi/ebiten/v2"
+	"github.com/hajimehoshi/ebiten/v2/ebitenutil"
+)
+
+const (
+	dstSize      = 128
+	screenWidth  = dstSize * 2
+	screenHeight = dstSize * 2
+)
+
+var (
+	dsts = [8]*ebiten.Image{
+		/*ebiten.NewImage(dstSize, dstSize),
+		ebiten.NewImage(dstSize, dstSize),
+		ebiten.NewImage(dstSize, dstSize),
+		ebiten.NewImage(dstSize, dstSize),*/
+		ebiten.NewImageWithOptions(image.Rect(0, 0, dstSize, dstSize), &ebiten.NewImageOptions{
+			Unmanaged: true,
+		}),
+		ebiten.NewImageWithOptions(image.Rect(0, 0, dstSize, dstSize), &ebiten.NewImageOptions{
+			Unmanaged: true,
+		}),
+		ebiten.NewImageWithOptions(image.Rect(0, 0, dstSize, dstSize), &ebiten.NewImageOptions{
+			Unmanaged: true,
+		}),
+		ebiten.NewImageWithOptions(image.Rect(0, 0, dstSize, dstSize), &ebiten.NewImageOptions{
+			Unmanaged: true,
+		}),
+	}
+
+	shaderSrc = []byte(
+		`
+//kage:units pixels
+
+package main
+
+func Fragment(dst vec4, src vec2, color vec4) (vec4, vec4, vec4, vec4) {
+	return vec4(1,0,0,1), vec4(0,1,0,1), vec4(0,0,1,1), vec4(1,0,1,1)
+}
+`)
+	s *ebiten.Shader
+)
+
+func init() {
+	var err error
+
+	s, err = ebiten.NewShader(shaderSrc)
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+type Game struct {
+}
+
+func (g *Game) Update() error {
+	return nil
+}
+
+func (g *Game) Draw(screen *ebiten.Image) {
+	vertices := []ebiten.Vertex{
+		{
+			DstX: 0,
+			DstY: 0,
+		},
+		{
+			DstX: dstSize,
+			DstY: 0,
+		},
+		{
+			DstX: 0,
+			DstY: dstSize,
+		},
+		{
+			DstX: dstSize,
+			DstY: dstSize,
+		},
+	}
+	indices := []uint16{0, 1, 2, 1, 2, 3}
+	ebiten.DrawTrianglesShaderMRT(dsts, vertices, indices, s, nil)
+	// Dst 0
+	screen.DrawImage(dsts[0], nil)
+	// Dst 1
+	opts := &ebiten.DrawImageOptions{}
+	opts.GeoM.Translate(dstSize, 0)
+	screen.DrawImage(dsts[1], opts)
+	// Dst 2
+	opts.GeoM.Reset()
+	opts.GeoM.Translate(0, dstSize)
+	screen.DrawImage(dsts[2], opts)
+	// Dst 3
+	opts.GeoM.Reset()
+	opts.GeoM.Translate(dstSize, dstSize)
+	screen.DrawImage(dsts[3], opts)
+
+	ebitenutil.DebugPrint(screen, fmt.Sprintf("FPS: %.2f", ebiten.ActualFPS()))
+}
+
+func (g *Game) Layout(outsideWidth, outsideHeight int) (int, int) {
+	return screenWidth, screenHeight
+}
+
+func main() {
+	ebiten.SetWindowSize(screenWidth, screenHeight)
+	ebiten.SetVsyncEnabled(false)
+	ebiten.SetWindowTitle("MRT (Ebitengine Demo)")
+	if err := ebiten.RunGameWithOptions(&Game{}, &ebiten.RunGameOptions{
+		GraphicsLibrary: ebiten.GraphicsLibraryDirectX,
+	}); err != nil {
+		log.Fatal(err)
+	}
+}

image.go

@@ -247,7 +247,7 @@ func (i *Image) DrawImage(img *Image, options *DrawImageOptions) {
 	graphics.QuadVertices(vs, float32(sx0), float32(sy0), float32(sx1), float32(sy1), a, b, c, d, tx, ty, cr, cg, cb, ca)
 	is := graphics.QuadIndices()
 
-	srcs := [graphics.ShaderImageCount]*ui.Image{img.image}
+	srcs := [graphics.ShaderSrcImageCount]*ui.Image{img.image}
 
 	useColorM := !colorm.IsIdentity()
 	shader := builtinShader(filter, builtinshader.AddressUnsafe, useColorM)
@@ -262,7 +262,7 @@ func (i *Image) DrawImage(img *Image, options *DrawImageOptions) {
 		})
 	}
 
-	i.image.DrawTriangles(srcs, vs, is, blend, i.adjustedBounds(), [graphics.ShaderImageCount]image.Rectangle{img.adjustedBounds()}, shader.shader, i.tmpUniforms, graphicsdriver.FillAll, canSkipMipmap(geoM, filter), false)
+	i.image.DrawTriangles(srcs, vs, is, blend, i.adjustedBounds(), [graphics.ShaderSrcImageCount]image.Rectangle{img.adjustedBounds()}, shader.shader, i.tmpUniforms, graphicsdriver.FillAll, canSkipMipmap(geoM, filter), false)
 }
 
 // Vertex represents a vertex passed to DrawTriangles.
@@ -500,7 +500,7 @@ func (i *Image) DrawTriangles(vertices []Vertex, indices []uint16, img *Image, o
 		is[i] = uint32(indices[i])
 	}
 
-	srcs := [graphics.ShaderImageCount]*ui.Image{img.image}
+	srcs := [graphics.ShaderSrcImageCount]*ui.Image{img.image}
 
 	useColorM := !colorm.IsIdentity()
 	shader := builtinShader(filter, address, useColorM)
@@ -515,7 +515,7 @@ func (i *Image) DrawTriangles(vertices []Vertex, indices []uint16, img *Image, o
 		})
 	}
 
-	i.image.DrawTriangles(srcs, vs, is, blend, i.adjustedBounds(), [graphics.ShaderImageCount]image.Rectangle{img.adjustedBounds()}, shader.shader, i.tmpUniforms, graphicsdriver.FillRule(options.FillRule), filter != builtinshader.FilterLinear, options.AntiAlias)
+	i.image.DrawTriangles(srcs, vs, is, blend, i.adjustedBounds(), [graphics.ShaderSrcImageCount]image.Rectangle{img.adjustedBounds()}, shader.shader, i.tmpUniforms, graphicsdriver.FillRule(options.FillRule), filter != builtinshader.FilterLinear, options.AntiAlias)
 }
 
 // DrawTrianglesShaderOptions represents options for DrawTrianglesShader.
@@ -565,7 +565,7 @@ type DrawTrianglesShaderOptions struct {
 }
 
 // Check the number of images.
-var _ [len(DrawTrianglesShaderOptions{}.Images) - graphics.ShaderImageCount]struct{} = [0]struct{}{}
+var _ [len(DrawTrianglesShaderOptions{}.Images) - graphics.ShaderSrcImageCount]struct{} = [0]struct{}{}
 
 // DrawTrianglesShader draws triangles with the specified vertices and their indices with the specified shader.
 //
@@ -650,7 +650,7 @@ func (i *Image) DrawTrianglesShader(vertices []Vertex, indices []uint16, shader
 		is[i] = uint32(indices[i])
 	}
 
-	var imgs [graphics.ShaderImageCount]*ui.Image
+	var imgs [graphics.ShaderSrcImageCount]*ui.Image
 	var imgSize image.Point
 	for i, img := range options.Images {
 		if img == nil {
@@ -672,7 +672,7 @@ func (i *Image) DrawTrianglesShader(vertices []Vertex, indices []uint16, shader
 		imgs[i] = img.image
 	}
 
-	var srcRegions [graphics.ShaderImageCount]image.Rectangle
+	var srcRegions [graphics.ShaderSrcImageCount]image.Rectangle
 	for i, img := range options.Images {
 		if img == nil {
 			continue
@@ -686,6 +686,139 @@ func (i *Image) DrawTrianglesShader(vertices []Vertex, indices []uint16, shader
 	i.image.DrawTriangles(imgs, vs, is, blend, i.adjustedBounds(), srcRegions, shader.shader, i.tmpUniforms, graphicsdriver.FillRule(options.FillRule), true, options.AntiAlias)
 }
 
+// DrawTrianglesShader draws triangles with the specified vertices and their indices with the specified shader.
+//
+// Vertex contains color values, which can be interpreted for any purpose by the shader.
+//
+// For the details about the shader, see https://ebitengine.org/en/documents/shader.html.
+//
+// If the shader unit is texels, one of the specified image is non-nil and its size is different from (width, height),
+// DrawTrianglesShader panics.
+// If one of the specified image is non-nil and is disposed, DrawTrianglesShader panics.
+//
+// If len(vertices) is more than MaxVertexCount, the exceeding part is ignored.
+//
+// If len(indices) is not multiple of 3, DrawTrianglesShader panics.
+//
+// If a value in indices is out of range of vertices, or not less than MaxVertexCount, DrawTrianglesShader panics.
+//
+// When a specified image is non-nil and is disposed, DrawTrianglesShader panics.
+//
+// If a specified uniform variable's length or type doesn't match with an expected one, DrawTrianglesShader panics.
+//
+// Even if a result is an invalid color as a premultiplied-alpha color, i.e. an alpha value exceeds other color values,
+// the value is kept and is not clamped.
+//
+// When the image i is disposed, DrawTrianglesShader does nothing.
+func DrawTrianglesShaderMRT(dsts [graphics.ShaderDstImageCount]*Image, vertices []Vertex, indices []uint16, shader *Shader, options *DrawTrianglesShaderOptions) {
+	var dstImgs [graphics.ShaderDstImageCount]*ui.Image
+	var firstDst *Image
+	for i, dst := range dsts {
+		if dst == nil {
+			continue
+		}
+		dst.copyCheck()
+		if dst.isDisposed() {
+			panic("ebiten: the destination images given to DrawTrianglesShaderMRT must not be disposed")
+		}
+		if firstDst == nil {
+			firstDst = dst
+		}
+		dstImgs[i] = dst.image
+	}
+
+	if shader.isDisposed() {
+		panic("ebiten: the given shader to DrawTrianglesShaderMRT must not be disposed")
+	}
+
+	if len(vertices) > graphicscommand.MaxVertexCount {
+		// The last part cannot be specified by indices. Just omit them.
+		vertices = vertices[:graphicscommand.MaxVertexCount]
+	}
+	if len(indices)%3 != 0 {
+		panic("ebiten: len(indices) % 3 must be 0")
+	}
+	for i, idx := range indices {
+		if int(idx) >= len(vertices) {
+			panic(fmt.Sprintf("ebiten: indices[%d] must be less than len(vertices) (%d) but was %d", i, len(vertices), idx))
+		}
+	}
+
+	if options == nil {
+		options = &DrawTrianglesShaderOptions{}
+	}
+
+	var blend graphicsdriver.Blend
+	if options.CompositeMode == CompositeModeCustom {
+		blend = options.Blend.internalBlend()
+	} else {
+		blend = options.CompositeMode.blend().internalBlend()
+	}
+
+	dst := firstDst
+	vs := dst.ensureTmpVertices(len(vertices) * graphics.VertexFloatCount)
+	src := options.Images[0]
+	for i, v := range vertices {
+		dx, dy := dst.adjustPositionF32(v.DstX, v.DstY)
+		vs[i*graphics.VertexFloatCount] = dx
+		vs[i*graphics.VertexFloatCount+1] = dy
+		sx, sy := v.SrcX, v.SrcY
+		if src != nil {
+			sx, sy = src.adjustPositionF32(sx, sy)
+		}
+		vs[i*graphics.VertexFloatCount+2] = sx
+		vs[i*graphics.VertexFloatCount+3] = sy
+		vs[i*graphics.VertexFloatCount+4] = v.ColorR
+		vs[i*graphics.VertexFloatCount+5] = v.ColorG
+		vs[i*graphics.VertexFloatCount+6] = v.ColorB
+		vs[i*graphics.VertexFloatCount+7] = v.ColorA
+	}
+
+	is := make([]uint32, len(indices))
+	for i := range is {
+		is[i] = uint32(indices[i])
+	}
+
+	var srcImgs [graphics.ShaderSrcImageCount]*ui.Image
+	var imgSize image.Point
+	for i, img := range options.Images {
+		if img == nil {
+			continue
+		}
+		if img.isDisposed() {
+			panic("ebiten: the given image to DrawTrianglesShader must not be disposed")
+		}
+		if shader.unit == shaderir.Texels {
+			if i == 0 {
+				imgSize = img.Bounds().Size()
+			} else {
+				// TODO: Check imgw > 0 && imgh > 0
+				if img.Bounds().Size() != imgSize {
+					panic("ebiten: all the source images must be the same size with the rectangle")
+				}
+			}
+		}
+		srcImgs[i] = img.image
+	}
+
+	var srcRegions [graphics.ShaderSrcImageCount]image.Rectangle
+	for i, img := range options.Images {
+		if img == nil {
+			continue
+		}
+		srcRegions[i] = img.adjustedBounds()
+	}
+
+	for _, dst := range dsts {
+		if dst == nil {
+			continue
+		}
+		dst.tmpUniforms = dst.tmpUniforms[:0]
+		dst.tmpUniforms = shader.appendUniforms(dst.tmpUniforms, options.Uniforms)
+	}
+	ui.DrawTrianglesMRT(dstImgs, srcImgs, vs, is, blend, dst.adjustedBounds(), srcRegions, shader.shader, dst.tmpUniforms, graphicsdriver.FillRule(options.FillRule), true, options.AntiAlias)
+}
+
 // DrawRectShaderOptions represents options for DrawRectShader.
 type DrawRectShaderOptions struct {
 	// GeoM is a geometry matrix to draw.
@@ -724,7 +857,7 @@ type DrawRectShaderOptions struct {
 }
 
 // Check the number of images.
-var _ [len(DrawRectShaderOptions{}.Images)]struct{} = [graphics.ShaderImageCount]struct{}{}
+var _ [len(DrawRectShaderOptions{}.Images)]struct{} = [graphics.ShaderSrcImageCount]struct{}{}
 
 // DrawRectShader draws a rectangle with the specified width and height with the specified shader.
 //
@@ -771,7 +904,7 @@ func (i *Image) DrawRectShader(width, height int, shader *Shader, options *DrawR
 		blend = options.CompositeMode.blend().internalBlend()
 	}
 
-	var imgs [graphics.ShaderImageCount]*ui.Image
+	var imgs [graphics.ShaderSrcImageCount]*ui.Image
 	for i, img := range options.Images {
 		if img == nil {
 			continue
@@ -785,7 +918,7 @@ func (i *Image) DrawRectShader(width, height int, shader *Shader, options *DrawR
 		imgs[i] = img.image
 	}
 
-	var srcRegions [graphics.ShaderImageCount]image.Rectangle
+	var srcRegions [graphics.ShaderSrcImageCount]image.Rectangle
 	for i, img := range options.Images {
 		if img == nil {
 			if shader.unit == shaderir.Pixels && i == 0 {

internal/atlas/image.go

@@ -153,12 +153,12 @@ func (b *backend) extendIfNeeded(width, height int) {
 
 	// Use DrawTriangles instead of WritePixels because the image i might be stale and not have its pixels
 	// information.
-	srcs := [graphics.ShaderImageCount]*graphicscommand.Image{b.image}
+	srcs := [graphics.ShaderSrcImageCount]*graphicscommand.Image{b.image}
 	sw, sh := b.image.InternalSize()
 	vs := quadVertices(0, 0, float32(sw), float32(sh), 0, 0, float32(sw), float32(sh), 1, 1, 1, 1)
 	is := graphics.QuadIndices()
 	dr := image.Rect(0, 0, sw, sh)
-	newImg.DrawTriangles(srcs, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, NearestFilterShader.ensureShader(), nil, graphicsdriver.FillAll)
+	newImg.DrawTriangles(srcs, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, NearestFilterShader.ensureShader(), nil, graphicsdriver.FillAll)
 	b.image.Dispose()
 
 	b.image = newImg
@@ -182,7 +182,7 @@ func newClearedImage(width, height int, screen bool) *graphicscommand.Image {
 func clearImage(i *graphicscommand.Image, region image.Rectangle) {
 	vs := quadVertices(float32(region.Min.X), float32(region.Min.Y), float32(region.Max.X), float32(region.Max.Y), 0, 0, 0, 0, 0, 0, 0, 0)
 	is := graphics.QuadIndices()
-	i.DrawTriangles([graphics.ShaderImageCount]*graphicscommand.Image{}, vs, is, graphicsdriver.BlendClear, region, [graphics.ShaderImageCount]image.Rectangle{}, clearShader.ensureShader(), nil, graphicsdriver.FillAll)
+	i.DrawTriangles([graphics.ShaderSrcImageCount]*graphicscommand.Image{}, vs, is, graphicsdriver.BlendClear, region, [graphics.ShaderSrcImageCount]image.Rectangle{}, clearShader.ensureShader(), nil, graphicsdriver.FillAll)
 }
 
 func (b *backend) clearPixels(region image.Rectangle) {
@@ -355,7 +355,7 @@ func (i *Image) ensureIsolatedFromSource(backends []*backend) {
 	is := graphics.QuadIndices()
 	dr := image.Rect(0, 0, i.width, i.height)
 
-	newI.drawTriangles([graphics.ShaderImageCount]*Image{i}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, NearestFilterShader, nil, graphicsdriver.FillAll)
+	newI.drawTriangles([graphics.ShaderSrcImageCount]*Image{i}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, NearestFilterShader, nil, graphicsdriver.FillAll)
 	newI.moveTo(i)
 }
 
@@ -385,7 +385,7 @@ func (i *Image) putOnSourceBackend(graphicsDriver graphicsdriver.Graphics) {
 	graphics.QuadVertices(vs, 0, 0, w, h, 1, 0, 0, 1, 0, 0, 1, 1, 1, 1)
 	is := graphics.QuadIndices()
 	dr := image.Rect(0, 0, i.width, i.height)
-	newI.drawTriangles([graphics.ShaderImageCount]*Image{i}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, NearestFilterShader, nil, graphicsdriver.FillAll)
+	newI.drawTriangles([graphics.ShaderSrcImageCount]*Image{i}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, NearestFilterShader, nil, graphicsdriver.FillAll)
 
 	newI.moveTo(i)
 	i.usedAsSourceCount = 0
@@ -417,7 +417,7 @@ func (i *Image) regionWithPadding() image.Rectangle {
 //	5: Color G
 //	6: Color B
 //	7: Color Y
-func (i *Image) DrawTriangles(srcs [graphics.ShaderImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
+func (i *Image) DrawTriangles(srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
 	backendsM.Lock()
 	defer backendsM.Unlock()
 
@@ -438,7 +438,28 @@ func (i *Image) DrawTriangles(srcs [graphics.ShaderImageCount]*Image, vertices [
 	i.drawTriangles(srcs, vertices, indices, blend, dstRegion, srcRegions, shader, uniforms, fillRule)
 }
 
-func (i *Image) drawTriangles(srcs [graphics.ShaderImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
+func DrawTrianglesMRT(dsts [graphics.ShaderDstImageCount]*Image, srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
+	backendsM.Lock()
+	defer backendsM.Unlock()
+
+	if !inFrame {
+		vs := make([]float32, len(vertices))
+		copy(vs, vertices)
+		is := make([]uint32, len(indices))
+		copy(is, indices)
+		us := make([]uint32, len(uniforms))
+		copy(us, uniforms)
+
+		appendDeferred(func() {
+			drawTrianglesMRT(dsts, srcs, vs, is, blend, dstRegion, srcRegions, shader, us, fillRule)
+		})
+		return
+	}
+
+	drawTrianglesMRT(dsts, srcs, vertices, indices, blend, dstRegion, srcRegions, shader, uniforms, fillRule)
+}
+
+func (i *Image) drawTriangles(srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
 	if len(vertices) == 0 {
 		return
 	}
@@ -515,7 +536,7 @@ func (i *Image) drawTriangles(srcs [graphics.ShaderImageCount]*Image, vertices [
 		srcRegions[i] = srcRegions[i].Add(r.Min)
 	}
 
-	var imgs [graphics.ShaderImageCount]*graphicscommand.Image
+	var imgs [graphics.ShaderSrcImageCount]*graphicscommand.Image
 	for i, src := range srcs {
 		if src == nil {
 			continue
@@ -536,6 +557,115 @@ func (i *Image) drawTriangles(srcs [graphics.ShaderImageCount]*Image, vertices [
 	}
 }
 
+func drawTrianglesMRT(dsts [graphics.ShaderDstImageCount]*Image, srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
+	if len(vertices) == 0 {
+		return
+	}
+
+	backends := make([]*backend, 0, len(srcs))
+	for _, src := range srcs {
+		if src == nil {
+			continue
+		}
+		if src.backend == nil {
+			// It is possible to spcify i.backend as a forbidden backend, but this might prevent a good allocation for a source image.
+			// If the backend becomes the same as i's, i's backend will be changed at ensureIsolatedFromSource.
+			src.allocate(nil, true)
+		}
+		backends = append(backends, src.backend)
+		src.backend.sourceInThisFrame = true
+	}
+
+	var firstDst *Image
+	var dstImgs [graphics.ShaderDstImageCount]*graphicscommand.Image
+	for i, dst := range dsts {
+		if dst == nil {
+			continue
+		}
+		dst.ensureIsolatedFromSource(backends)
+		firstDst = dst
+		dstImgs[i] = dst.backend.image
+	}
+
+	for _, src := range srcs {
+		// Compare i and source images after ensuring i is not on an atlas, or
+		// i and a source image might share the same atlas even though i != src.
+		for _, dst := range dsts {
+			if src != nil && dst != nil && dst.backend.image == src.backend.image {
+				panic("atlas: DrawTrianglesMRT: source must be different from the destination images")
+			}
+		}
+	}
+
+	r := firstDst.regionWithPadding()
+	// TODO: Check if dstRegion does not to violate the region.
+	dstRegion = dstRegion.Add(r.Min)
+
+	dx, dy := float32(r.Min.X), float32(r.Min.Y)
+
+	var oxf, oyf float32
+	if srcs[0] != nil {
+		r := srcs[0].regionWithPadding()
+		oxf, oyf = float32(r.Min.X), float32(r.Min.Y)
+		n := len(vertices)
+		for i := 0; i < n; i += graphics.VertexFloatCount {
+			vertices[i] += dx
+			vertices[i+1] += dy
+			vertices[i+2] += oxf
+			vertices[i+3] += oyf
+		}
+		if shader.ir.Unit == shaderir.Texels {
+			sw, sh := srcs[0].backend.image.InternalSize()
+			swf, shf := float32(sw), float32(sh)
+			for i := 0; i < n; i += graphics.VertexFloatCount {
+				vertices[i+2] /= swf
+				vertices[i+3] /= shf
+			}
+		}
+	} else {
+		n := len(vertices)
+		for i := 0; i < n; i += graphics.VertexFloatCount {
+			vertices[i] += dx
+			vertices[i+1] += dy
+		}
+	}
+
+	for i, src := range srcs {
+		if src == nil {
+			continue
+		}
+
+		// A source region can be deliberately empty when this is not needed in order to avoid unexpected
+		// performance issue (#1293).
+		if srcRegions[i].Empty() {
+			continue
+		}
+
+		r := src.regionWithPadding()
+		srcRegions[i] = srcRegions[i].Add(r.Min)
+	}
+
+	var srcImgs [graphics.ShaderSrcImageCount]*graphicscommand.Image
+	for i, src := range srcs {
+		if src == nil {
+			continue
+		}
+		srcImgs[i] = src.backend.image
+	}
+
+	graphicscommand.DrawTrianglesMRT(dstImgs, srcImgs, vertices, indices, blend, dstRegion, srcRegions, shader.ensureShader(), uniforms, fillRule)
+
+	for _, src := range srcs {
+		if src == nil {
+			continue
+		}
+		if !src.isOnSourceBackend() && src.canBePutOnAtlas() {
+			// src might already registered, but assigning it again is not harmful.
+			imagesToPutOnSourceBackend.add(src)
+		}
+	}
+}
+
 // WritePixels replaces the pixels on the image.
 func (i *Image) WritePixels(pix []byte, region image.Rectangle) {
 	backendsM.Lock()

internal/atlas/image_test.go

@@ -105,7 +105,7 @@ func TestEnsureIsolatedFromSourceBackend(t *testing.T) {
 	vs := quadVertices(size/2, size/2, size/4, size/4, 1)
 	is := graphics.QuadIndices()
 	dr := image.Rect(0, 0, size, size)
-	img4.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{img3}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+	img4.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{img3}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 	if got, want := img4.IsOnSourceBackendForTesting(), false; got != want {
 		t.Errorf("got: %v, want: %v", got, want)
 	}
@@ -113,7 +113,7 @@ func TestEnsureIsolatedFromSourceBackend(t *testing.T) {
 	// img5 is not allocated now, but is allocated at DrawTriangles.
 	vs = quadVertices(0, 0, size/2, size/2, 1)
 	dr = image.Rect(0, 0, size/2, size/2)
-	img3.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{img5}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+	img3.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{img5}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 	if got, want := img3.IsOnSourceBackendForTesting(), false; got != want {
 		t.Errorf("got: %v, want: %v", got, want)
 	}
@@ -147,7 +147,7 @@ func TestEnsureIsolatedFromSourceBackend(t *testing.T) {
 	// Check further drawing doesn't cause panic.
 	// This bug was fixed by 03dcd948.
 	vs = quadVertices(0, 0, size/2, size/2, 1)
-	img4.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{img3}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+	img4.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{img3}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 }
 
 func TestReputOnSourceBackend(t *testing.T) {
@@ -191,7 +191,7 @@ func TestReputOnSourceBackend(t *testing.T) {
 	// Render onto img1. The count should not matter.
 	for i := 0; i < 5; i++ {
 		vs := quadVertices(size, size, 0, 0, 1)
-		img1.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{img2}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+		img1.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{img2}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 		if got, want := img1.IsOnSourceBackendForTesting(), false; got != want {
 			t.Errorf("got: %v, want: %v", got, want)
 		}
@@ -203,7 +203,7 @@ func TestReputOnSourceBackend(t *testing.T) {
 	for i := 0; i < atlas.BaseCountToPutOnSourceBackend*2; i++ {
 		atlas.PutImagesOnSourceBackendForTesting(ui.Get().GraphicsDriverForTesting())
 		vs := quadVertices(size, size, 0, 0, 1)
-		img0.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{img1}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+		img0.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{img1}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 		if got, want := img1.IsOnSourceBackendForTesting(), false; got != want {
 			t.Errorf("got: %v, want: %v", got, want)
 		}
@@ -211,7 +211,7 @@ func TestReputOnSourceBackend(t *testing.T) {
 	// Finally, img1 is on a source backend.
 	atlas.PutImagesOnSourceBackendForTesting(ui.Get().GraphicsDriverForTesting())
 	vs := quadVertices(size, size, 0, 0, 1)
-	img0.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{img1}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+	img0.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{img1}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 	if got, want := img1.IsOnSourceBackendForTesting(), true; got != want {
 		t.Errorf("got: %v, want: %v", got, want)
 	}
@@ -240,7 +240,7 @@ func TestReputOnSourceBackend(t *testing.T) {
 	}
 
 	vs = quadVertices(size, size, 0, 0, 1)
-	img0.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{img1}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+	img0.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{img1}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 	if got, want := img1.IsOnSourceBackendForTesting(), true; got != want {
 		t.Errorf("got: %v, want: %v", got, want)
 	}
@@ -270,7 +270,7 @@ func TestReputOnSourceBackend(t *testing.T) {
 	// Use img1 as a render target again. The count should not matter.
 	for i := 0; i < 5; i++ {
 		vs := quadVertices(size, size, 0, 0, 1)
-		img1.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{img2}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+		img1.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{img2}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 		if got, want := img1.IsOnSourceBackendForTesting(), false; got != want {
 			t.Errorf("got: %v, want: %v", got, want)
 		}
@@ -282,7 +282,7 @@ func TestReputOnSourceBackend(t *testing.T) {
 		atlas.PutImagesOnSourceBackendForTesting(ui.Get().GraphicsDriverForTesting())
 		img1.WritePixels(make([]byte, 4*size*size), image.Rect(0, 0, size, size))
 		vs := quadVertices(size, size, 0, 0, 1)
-		img0.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{img1}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+		img0.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{img1}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 		if got, want := img1.IsOnSourceBackendForTesting(), false; got != want {
 			t.Errorf("got: %v, want: %v", got, want)
 		}
@@ -291,7 +291,7 @@ func TestReputOnSourceBackend(t *testing.T) {
 
 	// img1 is not on an atlas due to WritePixels.
 	vs = quadVertices(size, size, 0, 0, 1)
-	img0.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{img1}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+	img0.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{img1}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 	if got, want := img1.IsOnSourceBackendForTesting(), false; got != want {
 		t.Errorf("got: %v, want: %v", got, want)
 	}
@@ -300,7 +300,7 @@ func TestReputOnSourceBackend(t *testing.T) {
 	for i := 0; i < atlas.BaseCountToPutOnSourceBackend*2; i++ {
 		atlas.PutImagesOnSourceBackendForTesting(ui.Get().GraphicsDriverForTesting())
 		vs := quadVertices(size, size, 0, 0, 1)
-		img0.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{img3}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+		img0.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{img3}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 		if got, want := img3.IsOnSourceBackendForTesting(), false; got != want {
 			t.Errorf("got: %v, want: %v", got, want)
 		}
@@ -403,7 +403,7 @@ func TestWritePixelsAfterDrawTriangles(t *testing.T) {
 	vs := quadVertices(w, h, 0, 0, 1)
 	is := graphics.QuadIndices()
 	dr := image.Rect(0, 0, w, h)
-	dst.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+	dst.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 	dst.WritePixels(pix, image.Rect(0, 0, w, h))
 
 	pix = make([]byte, 4*w*h)
@@ -450,7 +450,7 @@ func TestSmallImages(t *testing.T) {
 	vs := quadVertices(w, h, 0, 0, 1)
 	is := graphics.QuadIndices()
 	dr := image.Rect(0, 0, w, h)
-	dst.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendSourceOver, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+	dst.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendSourceOver, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 
 	pix = make([]byte, 4*w*h)
 	ok, err := dst.ReadPixels(ui.Get().GraphicsDriverForTesting(), pix, image.Rect(0, 0, w, h))
@@ -497,7 +497,7 @@ func TestLongImages(t *testing.T) {
 	vs := quadVertices(w, h, 0, 0, scale)
 	is := graphics.QuadIndices()
 	dr := image.Rect(0, 0, dstW, dstH)
-	dst.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendSourceOver, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+	dst.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendSourceOver, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 
 	pix = make([]byte, 4*dstW*dstH)
 	ok, err := dst.ReadPixels(ui.Get().GraphicsDriverForTesting(), pix, image.Rect(0, 0, dstW, dstH))
@@ -613,7 +613,7 @@ func TestDeallocatedAndReputOnSourceBackend(t *testing.T) {
 	vs := quadVertices(size, size, 0, 0, 1)
 	is := graphics.QuadIndices()
 	dr := image.Rect(0, 0, size, size)
-	src.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{src2}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+	src.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{src2}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 	if got, want := src.IsOnSourceBackendForTesting(), false; got != want {
 		t.Errorf("got: %v, want: %v", got, want)
 	}
@@ -622,7 +622,7 @@ func TestDeallocatedAndReputOnSourceBackend(t *testing.T) {
 	for i := 0; i < atlas.BaseCountToPutOnSourceBackend/2; i++ {
 		atlas.PutImagesOnSourceBackendForTesting(ui.Get().GraphicsDriverForTesting())
 		vs := quadVertices(size, size, 0, 0, 1)
-		dst.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+		dst.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 		if got, want := src.IsOnSourceBackendForTesting(), false; got != want {
 			t.Errorf("got: %v, want: %v", got, want)
 		}
@@ -656,7 +656,7 @@ func TestImageIsNotReputOnSourceBackendWithoutUsingAsSource(t *testing.T) {
 	// Call DrawTriangles multiple times.
 	// The number of DrawTriangles doesn't matter as long as these are called in one frame.
 	for i := 0; i < 2; i++ {
-		src2.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+		src2.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 	}
 	if got, want := src2.IsOnSourceBackendForTesting(), false; got != want {
 		t.Errorf("got: %v, want: %v", got, want)
@@ -675,7 +675,7 @@ func TestImageIsNotReputOnSourceBackendWithoutUsingAsSource(t *testing.T) {
 	for i := 0; i < atlas.BaseCountToPutOnSourceBackend; i++ {
 		atlas.PutImagesOnSourceBackendForTesting(ui.Get().GraphicsDriverForTesting())
 		vs := quadVertices(size, size, 0, 0, 1)
-		dst.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{src2}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+		dst.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{src2}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 		if got, want := src2.IsOnSourceBackendForTesting(), false; got != want {
 			t.Errorf("got: %v, want: %v", got, want)
 		}
@@ -801,14 +801,14 @@ func TestDestinationCountOverflow(t *testing.T) {
 
 	// Use dst0 as a destination for a while.
 	for i := 0; i < 31; i++ {
-		dst0.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+		dst0.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 		atlas.PutImagesOnSourceBackendForTesting(ui.Get().GraphicsDriverForTesting())
 	}
 
 	// Use dst0 as a source for a while.
 	// As dst0 is used as a destination too many times (31 is a maximum), dst0's backend should never be a source backend.
 	for i := 0; i < 100; i++ {
-		dst1.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{dst0}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+		dst1.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{dst0}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 		atlas.PutImagesOnSourceBackendForTesting(ui.Get().GraphicsDriverForTesting())
 		if dst0.IsOnSourceBackendForTesting() {
 			t.Errorf("dst0 cannot be on a source backend: %d", i)
@@ -834,7 +834,7 @@ func TestIteratingImagesToPutOnSourceBackend(t *testing.T) {
 	is := graphics.QuadIndices()
 	dr := image.Rect(0, 0, w, h)
 	for _, img := range srcs {
-		img.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+		img.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 	}
 	atlas.PutImagesOnSourceBackendForTesting(ui.Get().GraphicsDriverForTesting())
 
@@ -842,7 +842,7 @@ func TestIteratingImagesToPutOnSourceBackend(t *testing.T) {
 	// Check iterating the registered image works correctly.
 	for i := 0; i < 100; i++ {
 		for _, src := range srcs {
-			dst.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+			dst.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 		}
 		atlas.PutImagesOnSourceBackendForTesting(ui.Get().GraphicsDriverForTesting())
 	}

internal/atlas/shader_test.go

@@ -37,12 +37,12 @@ func TestShaderFillTwice(t *testing.T) {
 	dr := image.Rect(0, 0, w, h)
 	g := ui.Get().GraphicsDriverForTesting()
 	s0 := atlas.NewShader(etesting.ShaderProgramFill(0xff, 0xff, 0xff, 0xff))
-	dst.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, s0, nil, graphicsdriver.FillAll)
+	dst.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, s0, nil, graphicsdriver.FillAll)
 
 	// Vertices must be recreated (#1755)
 	vs = quadVertices(w, h, 0, 0, 1)
 	s1 := atlas.NewShader(etesting.ShaderProgramFill(0x80, 0x80, 0x80, 0xff))
-	dst.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, s1, nil, graphicsdriver.FillAll)
+	dst.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, s1, nil, graphicsdriver.FillAll)
 
 	pix := make([]byte, 4*w*h)
 	ok, err := dst.ReadPixels(g, pix, image.Rect(0, 0, w, h))
@@ -69,11 +69,11 @@ func TestImageDrawTwice(t *testing.T) {
 	vs := quadVertices(w, h, 0, 0, 1)
 	is := graphics.QuadIndices()
 	dr := image.Rect(0, 0, w, h)
-	dst.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{src0}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+	dst.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{src0}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 
 	// Vertices must be recreated (#1755)
 	vs = quadVertices(w, h, 0, 0, 1)
-	dst.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{src1}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+	dst.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{src1}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 
 	pix := make([]byte, 4*w*h)
 	ok, err := dst.ReadPixels(ui.Get().GraphicsDriverForTesting(), pix, image.Rect(0, 0, w, h))
@@ -97,7 +97,7 @@ func TestGCShader(t *testing.T) {
 	vs := quadVertices(w, h, 0, 0, 1)
 	is := graphics.QuadIndices()
 	dr := image.Rect(0, 0, w, h)
-	dst.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderImageCount]image.Rectangle{}, s, nil, graphicsdriver.FillAll)
+	dst.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, s, nil, graphicsdriver.FillAll)
 
 	// Ensure other objects are GCed, as GC appends deferred functions for collected objects.
 	ensureGC()

internal/buffered/image.go

@@ -183,7 +183,7 @@ func (i *Image) WritePixels(pix []byte, region image.Rectangle) {
 // DrawTriangles draws the src image with the given vertices.
 //
 // Copying vertices and indices is the caller's responsibility.
-func (i *Image) DrawTriangles(srcs [graphics.ShaderImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderImageCount]image.Rectangle, shader *atlas.Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
+func (i *Image) DrawTriangles(srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *atlas.Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
 	for _, src := range srcs {
 		if i == src {
 			panic("buffered: Image.DrawTriangles: source images must be different from the receiver")
@@ -197,7 +197,7 @@ func (i *Image) DrawTriangles(srcs [graphics.ShaderImageCount]*Image, vertices [
 
 	i.syncPixelsIfNeeded()
 
-	var imgs [graphics.ShaderImageCount]*atlas.Image
+	var imgs [graphics.ShaderSrcImageCount]*atlas.Image
 	for i, img := range srcs {
 		if img == nil {
 			continue
@@ -211,6 +211,51 @@ func (i *Image) DrawTriangles(srcs [graphics.ShaderImageCount]*Image, vertices [
 	i.pixels = nil
 }
 
+func DrawTrianglesMRT(dsts [graphics.ShaderDstImageCount]*Image, srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *atlas.Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
+	for _, src := range srcs {
+		for _, dst := range dsts {
+			if dst == nil {
+				continue
+			}
+			if dst == src {
+				panic("buffered: DrawTrianglesMRT: source images must be different from the destination images")
+			}
+		}
+		if src != nil {
+			// src's pixels have to be synced between CPU and GPU,
+			// but doesn't have to be cleared since src is not modified in this function.
+			src.syncPixelsIfNeeded()
+		}
+	}
+
+	var dstImgs [graphics.ShaderDstImageCount]*atlas.Image
+	for i, dst := range dsts {
+		if dst == nil {
+			continue
+		}
+		dst.syncPixelsIfNeeded()
+		dstImgs[i] = dst.img
+	}
+
+	var srcImgs [graphics.ShaderSrcImageCount]*atlas.Image
+	for i, src := range srcs {
+		if src == nil {
+			continue
+		}
+		srcImgs[i] = src.img
+	}
+
+	atlas.DrawTrianglesMRT(dstImgs, srcImgs, vertices, indices, blend, dstRegion, srcRegions, shader, uniforms, fillRule)
+
+	// After rendering, the pixel cache is no longer valid.
+	for _, dst := range dsts {
+		if dst == nil {
+			continue
+		}
+		dst.pixels = nil
+	}
+}
+
 // syncPixelsIfNeeded syncs the pixels between CPU and GPU.
 // After syncPixelsIfNeeded, dotsBuffer is cleared, but pixels might remain.
 func (i *Image) syncPixelsIfNeeded() {
@@ -289,10 +334,10 @@ func (i *Image) syncPixelsIfNeeded() {
 		idx++
 	}
 
-	srcs := [graphics.ShaderImageCount]*atlas.Image{whiteImage.img}
+	srcs := [graphics.ShaderSrcImageCount]*atlas.Image{whiteImage.img}
 	dr := image.Rect(0, 0, i.width, i.height)
 	blend := graphicsdriver.BlendCopy
-	i.img.DrawTriangles(srcs, vs, is, blend, dr, [graphics.ShaderImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+	i.img.DrawTriangles(srcs, vs, is, blend, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 
 	// TODO: Use clear if Go 1.21 is available.
 	for pos := range i.dotsBuffer {

internal/buffered/image_test.go

@@ -55,8 +55,8 @@ func TestUnsyncedPixels(t *testing.T) {
 	graphics.QuadVertices(vs, 0, 0, 16, 16, 1, 0, 0, 1, 0, 0, 1, 1, 1, 1)
 	is := graphics.QuadIndices()
 	dr := image.Rect(0, 0, 16, 16)
-	sr := [graphics.ShaderImageCount]image.Rectangle{image.Rect(0, 0, 16, 16)}
+	sr := [graphics.ShaderSrcImageCount]image.Rectangle{image.Rect(0, 0, 16, 16)}
-	dst.DrawTriangles([graphics.ShaderImageCount]*buffered.Image{src}, vs, is, graphicsdriver.BlendSourceOver, dr, sr, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
+	dst.DrawTriangles([graphics.ShaderSrcImageCount]*buffered.Image{src}, vs, is, graphicsdriver.BlendSourceOver, dr, sr, atlas.NearestFilterShader, nil, graphicsdriver.FillAll)
 
 	// Check the result is correct.
 	var got [4]byte

internal/graphics/shader.go

@@ -89,9 +89,9 @@ var __imageSrcRegionSizes [%[1]d]vec2
 func imageSrcRegionOnTexture() (vec2, vec2) {
 	return __imageSrcRegionOrigins[0], __imageSrcRegionSizes[0]
 }
-`, ShaderImageCount)
+`, ShaderSrcImageCount)
 
-	for i := 0; i < ShaderImageCount; i++ {
+	for i := 0; i < ShaderSrcImageCount; i++ {
 		shaderSuffix += fmt.Sprintf(`
 // imageSrc%[1]dOrigin returns the source image's region origin on its texture.
 // The unit is the source texture's pixel or texel.
@@ -179,7 +179,7 @@ func CompileShader(src []byte) (*shaderir.Program, error) {
 		vert = "__vertex"
 		frag = "Fragment"
 	)
-	ir, err := shader.Compile(buf.Bytes(), vert, frag, ShaderImageCount)
+	ir, err := shader.Compile(buf.Bytes(), vert, frag, ShaderSrcImageCount)
 	if err != nil {
 		return nil, err
 	}

internal/graphics/vertex.go

@@ -15,7 +15,12 @@
 package graphics
 
 const (
-	ShaderImageCount = 4
+	ShaderSrcImageCount = 4
+	// The minimum guaranteed value for the number of target seems to be 8
+	// OpenGL(8): https://www.khronos.org/opengl/wiki/Framebuffer_Object#Framebuffer_Object_Structure
+	// DirectX11(8): https://learn.microsoft.com/en-us/windows/win32/direct3d11/d3d10-graphics-programming-guide-output-merger-stage#multiple-rendertargets-overview
+	// Metal(8): Page 7 of 15: https://developer.apple.com/metal/Metal-Feature-Set-Tables.pdf
+	ShaderDstImageCount = 8
 
 	// PreservedUniformVariablesCount represents the number of preserved uniform variables.
 	// Any shaders in Ebitengine must have these uniform variables.
@@ -30,11 +35,11 @@ const (
 	ProjectionMatrixUniformVariableIndex = 6
 
 	PreservedUniformUint32Count = 2 + // the destination texture size
-		2*ShaderImageCount + // the source texture sizes array
+		2*ShaderSrcImageCount + // the source texture sizes array
 		2 + // the destination image region origin
 		2 + // the destination image region size
-		2*ShaderImageCount + // the source image region origins array
+		2*ShaderSrcImageCount + // the source image region origins array
-		2*ShaderImageCount + // the source image region sizes array
+		2*ShaderSrcImageCount + // the source image region sizes array
 		16 // the projection matrix
 )
 

internal/graphicscommand/command.go

@@ -61,8 +61,8 @@ func (p *drawTrianglesCommandPool) put(v *drawTrianglesCommand) {
 
 // drawTrianglesCommand represents a drawing command to draw an image on another image.
 type drawTrianglesCommand struct {
-	dst        *Image
+	dsts       [graphics.ShaderDstImageCount]*Image
-	srcs       [graphics.ShaderImageCount]*Image
+	srcs       [graphics.ShaderSrcImageCount]*Image
 	vertices   []float32
 	blend      graphicsdriver.Blend
 	dstRegions []graphicsdriver.DstRegion
@@ -81,12 +81,19 @@ func (c *drawTrianglesCommand) String() string {
 		c.blend.BlendOperationRGB,
 		c.blend.BlendOperationAlpha)
 
-	dst := fmt.Sprintf("%d", c.dst.id)
+	var dststrs [graphics.ShaderDstImageCount]string
-	if c.dst.screen {
+	for i, dst := range c.dsts {
-		dst += " (screen)"
+		if dst == nil {
+			dststrs[i] = "(nil)"
+			continue
+		}
+		dststrs[i] = fmt.Sprintf("%d", dst.id)
+		if dst.screen {
+			dststrs[i] += " (screen)"
+		}
 	}
 
-	var srcstrs [graphics.ShaderImageCount]string
+	var srcstrs [graphics.ShaderSrcImageCount]string
 	for i, src := range c.srcs {
 		if src == nil {
 			srcstrs[i] = "(nil)"
@@ -98,7 +105,7 @@ func (c *drawTrianglesCommand) String() string {
 		}
 	}
 
-	return fmt.Sprintf("draw-triangles: dst: %s <- src: [%s], num of dst regions: %d, num of indices: %d, blend: %s, fill rule: %s, shader id: %d", dst, strings.Join(srcstrs[:], ", "), len(c.dstRegions), c.numIndices(), blend, c.fillRule, c.shader.id)
+	return fmt.Sprintf("draw-triangles: dst: [%s] <- src: [%s], num of dst regions: %d, num of indices: %d, blend: %s, fill rule: %s, shader id: %d", strings.Join(dststrs[:], ", "), strings.Join(srcstrs[:], ", "), len(c.dstRegions), c.numIndices(), blend, c.fillRule, c.shader.id)
 }
 
 // Exec executes the drawTrianglesCommand.
@@ -108,16 +115,25 @@ func (c *drawTrianglesCommand) Exec(commandQueue *commandQueue, graphicsDriver g
 		return nil
 	}
 
-	var imgs [graphics.ShaderImageCount]graphicsdriver.ImageID
+	var dsts [graphics.ShaderDstImageCount]graphicsdriver.ImageID
-	for i, src := range c.srcs {
+	for i, dst := range c.dsts {
-		if src == nil {
+		if dst == nil {
-			imgs[i] = graphicsdriver.InvalidImageID
+			dsts[i] = graphicsdriver.InvalidImageID
 			continue
 		}
-		imgs[i] = src.image.ID()
+		dsts[i] = dst.image.ID()
 	}
 
-	return graphicsDriver.DrawTriangles(c.dst.image.ID(), imgs, c.shader.shader.ID(), c.dstRegions, indexOffset, c.blend, c.uniforms, c.fillRule)
+	var srcs [graphics.ShaderSrcImageCount]graphicsdriver.ImageID
+	for i, src := range c.srcs {
+		if src == nil {
+			srcs[i] = graphicsdriver.InvalidImageID
+			continue
+		}
+		srcs[i] = src.image.ID()
+	}
+
+	return graphicsDriver.DrawTriangles(dsts, srcs, c.shader.shader.ID(), c.dstRegions, indexOffset, c.blend, c.uniforms, c.fillRule)
 }
 
 func (c *drawTrianglesCommand) NeedsSync() bool {
@@ -142,7 +158,7 @@ func (c *drawTrianglesCommand) setVertices(vertices []float32) {
 
 // CanMergeWithDrawTrianglesCommand returns a boolean value indicating whether the other drawTrianglesCommand can be merged
 // with the drawTrianglesCommand c.
-func (c *drawTrianglesCommand) CanMergeWithDrawTrianglesCommand(dst *Image, srcs [graphics.ShaderImageCount]*Image, vertices []float32, blend graphicsdriver.Blend, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) bool {
+func (c *drawTrianglesCommand) CanMergeWithDrawTrianglesCommand(dsts [graphics.ShaderDstImageCount]*Image, srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, blend graphicsdriver.Blend, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) bool {
 	if c.shader != shader {
 		return false
 	}
@@ -154,7 +170,7 @@ func (c *drawTrianglesCommand) CanMergeWithDrawTrianglesCommand(dst *Image, srcs
 			return false
 		}
 	}
-	if c.dst != dst {
+	if c.dsts != dsts {
 		return false
 	}
 	if c.srcs != srcs {

internal/graphicscommand/commandqueue.go

@@ -105,7 +105,7 @@ func mustUseDifferentVertexBuffer(nextNumVertexFloats int) bool {
 }
 
 // EnqueueDrawTrianglesCommand enqueues a drawing-image command.
-func (q *commandQueue) EnqueueDrawTrianglesCommand(dst *Image, srcs [graphics.ShaderImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
+func (q *commandQueue) EnqueueDrawTrianglesCommand(dsts [graphics.ShaderDstImageCount]*Image, srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
 	if len(vertices) > maxVertexFloatCount {
 		panic(fmt.Sprintf("graphicscommand: len(vertices) must equal to or less than %d but was %d", maxVertexFloatCount, len(vertices)))
 	}
@@ -125,7 +125,7 @@ func (q *commandQueue) EnqueueDrawTrianglesCommand(dst *Image, srcs [graphics.Sh
 	// prependPreservedUniforms not only prepends values to the given slice but also creates a new slice.
 	// Allocating a new slice is necessary to make EnqueueDrawTrianglesCommand safe so far.
 	// TODO: This might cause a performance issue (#2601).
-	uniforms = q.prependPreservedUniforms(uniforms, shader, dst, srcs, dstRegion, srcRegions)
+	uniforms = q.prependPreservedUniforms(uniforms, shader, dsts, srcs, dstRegion, srcRegions)
 
 	// Remove unused uniform variables so that more commands can be merged.
 	shader.ir.FilterUniformVariables(uniforms)
@@ -133,7 +133,7 @@ func (q *commandQueue) EnqueueDrawTrianglesCommand(dst *Image, srcs [graphics.Sh
 	// TODO: If dst is the screen, reorder the command to be the last.
 	if !split && 0 < len(q.commands) {
 		if last, ok := q.commands[len(q.commands)-1].(*drawTrianglesCommand); ok {
-			if last.CanMergeWithDrawTrianglesCommand(dst, srcs, vertices, blend, shader, uniforms, fillRule) {
+			if last.CanMergeWithDrawTrianglesCommand(dsts, srcs, vertices, blend, shader, uniforms, fillRule) {
 				last.setVertices(q.lastVertices(len(vertices) + last.numVertices()))
 				if last.dstRegions[len(last.dstRegions)-1].Region == dstRegion {
 					last.dstRegions[len(last.dstRegions)-1].IndexCount += len(indices)
@@ -149,7 +149,7 @@ func (q *commandQueue) EnqueueDrawTrianglesCommand(dst *Image, srcs [graphics.Sh
 	}
 
 	c := q.drawTrianglesCommandPool.get()
-	c.dst = dst
+	c.dsts = dsts
 	c.srcs = srcs
 	c.vertices = q.lastVertices(len(vertices))
 	c.blend = blend
@@ -324,18 +324,25 @@ func imageRectangleToRectangleF32(r image.Rectangle) rectangleF32 {
 	}
 }
 
-func (q *commandQueue) prependPreservedUniforms(uniforms []uint32, shader *Shader, dst *Image, srcs [graphics.ShaderImageCount]*Image, dstRegion image.Rectangle, srcRegions [graphics.ShaderImageCount]image.Rectangle) []uint32 {
+func (q *commandQueue) prependPreservedUniforms(uniforms []uint32, shader *Shader, dsts [graphics.ShaderDstImageCount]*Image, srcs [graphics.ShaderSrcImageCount]*Image, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle) []uint32 {
 	origUniforms := uniforms
 	uniforms = q.uint32sBuffer.alloc(len(origUniforms) + graphics.PreservedUniformUint32Count)
 	copy(uniforms[graphics.PreservedUniformUint32Count:], origUniforms)
 
 	// Set the destination texture size.
-	dw, dh := dst.InternalSize()
+	var firstDst *Image
+	for _, dst := range dsts {
+		if dst != nil {
+			firstDst = dst
+			break
+		}
+	}
+	dw, dh := firstDst.InternalSize()
 	uniforms[0] = math.Float32bits(float32(dw))
 	uniforms[1] = math.Float32bits(float32(dh))
 	uniformIndex := 2
 
-	for i := 0; i < graphics.ShaderImageCount; i++ {
+	for i := 0; i < graphics.ShaderSrcImageCount; i++ {
 		var floatW, floatH uint32
 		if srcs[i] != nil {
 			w, h := srcs[i].InternalSize()
@@ -346,7 +353,7 @@ func (q *commandQueue) prependPreservedUniforms(uniforms []uint32, shader *Shade
 		uniforms[uniformIndex+i*2] = floatW
 		uniforms[uniformIndex+1+i*2] = floatH
 	}
-	uniformIndex += graphics.ShaderImageCount * 2
+	uniformIndex += graphics.ShaderSrcImageCount * 2
 
 	dr := imageRectangleToRectangleF32(dstRegion)
 	if shader.unit() == shaderir.Texels {
@@ -366,7 +373,7 @@ func (q *commandQueue) prependPreservedUniforms(uniforms []uint32, shader *Shade
 	uniforms[uniformIndex+1] = math.Float32bits(dr.height)
 	uniformIndex += 2
 
-	var srs [graphics.ShaderImageCount]rectangleF32
+	var srs [graphics.ShaderSrcImageCount]rectangleF32
 	for i, r := range srcRegions {
 		srs[i] = imageRectangleToRectangleF32(r)
 	}
@@ -384,18 +391,18 @@ func (q *commandQueue) prependPreservedUniforms(uniforms []uint32, shader *Shade
 	}
 
 	// Set the source region origins.
-	for i := 0; i < graphics.ShaderImageCount; i++ {
+	for i := 0; i < graphics.ShaderSrcImageCount; i++ {
 		uniforms[uniformIndex+i*2] = math.Float32bits(srs[i].x)
 		uniforms[uniformIndex+1+i*2] = math.Float32bits(srs[i].y)
 	}
-	uniformIndex += graphics.ShaderImageCount * 2
+	uniformIndex += graphics.ShaderSrcImageCount * 2
 
 	// Set the source region sizes.
-	for i := 0; i < graphics.ShaderImageCount; i++ {
+	for i := 0; i < graphics.ShaderSrcImageCount; i++ {
 		uniforms[uniformIndex+i*2] = math.Float32bits(srs[i].width)
 		uniforms[uniformIndex+1+i*2] = math.Float32bits(srs[i].height)
 	}
-	uniformIndex += graphics.ShaderImageCount * 2
+	uniformIndex += graphics.ShaderSrcImageCount * 2
 
 	// Set the projection matrix.
 	uniforms[uniformIndex] = math.Float32bits(2 / float32(dw))
@@ -469,11 +476,11 @@ func (c *commandQueueManager) putCommandQueue(commandQueue *commandQueue) {
 	c.pool.put(commandQueue)
 }
 
-func (c *commandQueueManager) enqueueDrawTrianglesCommand(dst *Image, srcs [graphics.ShaderImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
+func (c *commandQueueManager) enqueueDrawTrianglesCommand(dsts [graphics.ShaderDstImageCount]*Image, srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
 	if c.current == nil {
 		c.current, _ = c.pool.get()
 	}
-	c.current.EnqueueDrawTrianglesCommand(dst, srcs, vertices, indices, blend, dstRegion, srcRegions, shader, uniforms, fillRule)
+	c.current.EnqueueDrawTrianglesCommand(dsts, srcs, vertices, indices, blend, dstRegion, srcRegions, shader, uniforms, fillRule)
 }
 
 func (c *commandQueueManager) flush(graphicsDriver graphicsdriver.Graphics, endFrame bool) error {

internal/graphicscommand/image.go

@@ -130,7 +130,7 @@ func (i *Image) InternalSize() (int, int) {
 //
 // If the source image is not specified, i.e., src is nil and there is no image in the uniform variables, the
 // elements for the source image are not used.
-func (i *Image) DrawTriangles(srcs [graphics.ShaderImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
+func (i *Image) DrawTriangles(srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
 	for _, src := range srcs {
 		if src == nil {
 			continue
@@ -142,7 +142,48 @@ func (i *Image) DrawTriangles(srcs [graphics.ShaderImageCount]*Image, vertices [
 	}
 	i.flushBufferedWritePixels()
 
-	theCommandQueueManager.enqueueDrawTrianglesCommand(i, srcs, vertices, indices, blend, dstRegion, srcRegions, shader, uniforms, fillRule)
+	theCommandQueueManager.enqueueDrawTrianglesCommand([graphics.ShaderDstImageCount]*Image{i}, srcs, vertices, indices, blend, dstRegion, srcRegions, shader, uniforms, fillRule)
+}
+
+// DrawTriangles draws triangles with the given image.
+//
+// The vertex floats are:
+//
+//	0: Destination X in pixels
+//	1: Destination Y in pixels
+//	2: Source X in texels
+//	3: Source Y in texels
+//	4: Color R [0.0-1.0]
+//	5: Color G
+//	6: Color B
+//	7: Color Y
+//
+// src and shader are exclusive and only either is non-nil.
+//
+// The elements that index is in between 2 and 7 are used for the source images.
+// The source image is 1) src argument if non-nil, or 2) an image value in the uniform variables if it exists.
+// If there are multiple images in the uniform variables, the smallest ID's value is adopted.
+//
+// If the source image is not specified, i.e., src is nil and there is no image in the uniform variables, the
+// elements for the source image are not used.
+func DrawTrianglesMRT(dsts [graphics.ShaderDstImageCount]*Image, srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule) {
+	for _, src := range srcs {
+		if src == nil {
+			continue
+		}
+		if src.screen {
+			panic("graphicscommand: the screen image cannot be the rendering des")
+		}
+		src.flushBufferedWritePixels()
+	}
+	for _, dst := range dsts {
+		if dst == nil {
+			continue
+		}
+		dst.flushBufferedWritePixels()
+	}
+
+	theCommandQueueManager.enqueueDrawTrianglesCommand(dsts, srcs, vertices, indices, blend, dstRegion, srcRegions, shader, uniforms, fillRule)
 }
 
 // ReadPixels reads the image's pixels.

internal/graphicscommand/image_test.go

@@ -59,7 +59,7 @@ func TestClear(t *testing.T) {
 	vs := quadVertices(w/2, h/2)
 	is := graphics.QuadIndices()
 	dr := image.Rect(0, 0, w, h)
-	dst.DrawTriangles([graphics.ShaderImageCount]*graphicscommand.Image{src}, vs, is, graphicsdriver.BlendClear, dr, [graphics.ShaderImageCount]image.Rectangle{}, nearestFilterShader, nil, graphicsdriver.FillAll)
+	dst.DrawTriangles([graphics.ShaderSrcImageCount]*graphicscommand.Image{src}, vs, is, graphicsdriver.BlendClear, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, nearestFilterShader, nil, graphicsdriver.FillAll)
 
 	pix := make([]byte, 4*w*h)
 	if err := dst.ReadPixels(ui.Get().GraphicsDriverForTesting(), []graphicsdriver.PixelsArgs{
@@ -90,8 +90,8 @@ func TestWritePixelsPartAfterDrawTriangles(t *testing.T) {
 	vs := quadVertices(w/2, h/2)
 	is := graphics.QuadIndices()
 	dr := image.Rect(0, 0, w, h)
-	dst.DrawTriangles([graphics.ShaderImageCount]*graphicscommand.Image{clr}, vs, is, graphicsdriver.BlendClear, dr, [graphics.ShaderImageCount]image.Rectangle{}, nearestFilterShader, nil, graphicsdriver.FillAll)
+	dst.DrawTriangles([graphics.ShaderSrcImageCount]*graphicscommand.Image{clr}, vs, is, graphicsdriver.BlendClear, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, nearestFilterShader, nil, graphicsdriver.FillAll)
-	dst.DrawTriangles([graphics.ShaderImageCount]*graphicscommand.Image{src}, vs, is, graphicsdriver.BlendSourceOver, dr, [graphics.ShaderImageCount]image.Rectangle{}, nearestFilterShader, nil, graphicsdriver.FillAll)
+	dst.DrawTriangles([graphics.ShaderSrcImageCount]*graphicscommand.Image{src}, vs, is, graphicsdriver.BlendSourceOver, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, nearestFilterShader, nil, graphicsdriver.FillAll)
 	bs := graphics.NewManagedBytes(4, func(bs []byte) {
 		for i := range bs {
 			bs[i] = 0
@@ -109,11 +109,11 @@ func TestShader(t *testing.T) {
 	vs := quadVertices(w, h)
 	is := graphics.QuadIndices()
 	dr := image.Rect(0, 0, w, h)
-	dst.DrawTriangles([graphics.ShaderImageCount]*graphicscommand.Image{clr}, vs, is, graphicsdriver.BlendClear, dr, [graphics.ShaderImageCount]image.Rectangle{}, nearestFilterShader, nil, graphicsdriver.FillAll)
+	dst.DrawTriangles([graphics.ShaderSrcImageCount]*graphicscommand.Image{clr}, vs, is, graphicsdriver.BlendClear, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, nearestFilterShader, nil, graphicsdriver.FillAll)
 
 	g := ui.Get().GraphicsDriverForTesting()
 	s := graphicscommand.NewShader(etesting.ShaderProgramFill(0xff, 0, 0, 0xff))
-	dst.DrawTriangles([graphics.ShaderImageCount]*graphicscommand.Image{}, vs, is, graphicsdriver.BlendSourceOver, dr, [graphics.ShaderImageCount]image.Rectangle{}, s, nil, graphicsdriver.FillAll)
+	dst.DrawTriangles([graphics.ShaderSrcImageCount]*graphicscommand.Image{}, vs, is, graphicsdriver.BlendSourceOver, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, s, nil, graphicsdriver.FillAll)
 
 	pix := make([]byte, 4*w*h)
 	if err := dst.ReadPixels(g, []graphicsdriver.PixelsArgs{

internal/graphicsdriver/directx/graphics11_windows.go

@@ -515,14 +515,103 @@ func (g *graphics11) removeShader(s *shader11) {
 	delete(g.shaders, s.id)
 }
 
-func (g *graphics11) DrawTriangles(dstID graphicsdriver.ImageID, srcIDs [graphics.ShaderImageCount]graphicsdriver.ImageID, shaderID graphicsdriver.ShaderID, dstRegions []graphicsdriver.DstRegion, indexOffset int, blend graphicsdriver.Blend, uniforms []uint32, fillRule graphicsdriver.FillRule) error {
+func (g *graphics11) setAsRenderTargets(dsts []*image11, useStencil bool) error {
+	var rtvs []*_ID3D11RenderTargetView
+	var dsv *_ID3D11DepthStencilView
+	for _, dst := range dsts {
+		// Ignore a nil image in case of MRT
+		if dst == nil {
+			rtvs = append(rtvs, nil)
+			continue
+		}
+		if dst.renderTargetView == nil {
+			rtv, err := g.device.CreateRenderTargetView(unsafe.Pointer(dst.texture), nil)
+			if err != nil {
+				return err
+			}
+			dst.renderTargetView = rtv
+		}
+		rtvs = append(rtvs, dst.renderTargetView)
+
+		if !useStencil || dsv != nil {
+			continue
+		}
+
+		if dst.screen {
+			return fmt.Errorf("directx: a stencil buffer is not available for a screen image")
+		}
+		if dst.stencil == nil {
+			w, h := dst.internalSize()
+			s, err := g.device.CreateTexture2D(&_D3D11_TEXTURE2D_DESC{
+				Width:     uint32(w),
+				Height:    uint32(h),
+				MipLevels: 0,
+				ArraySize: 1,
+				Format:    _DXGI_FORMAT_D24_UNORM_S8_UINT,
+				SampleDesc: _DXGI_SAMPLE_DESC{
+					Count:   1,
+					Quality: 0,
+				},
+				Usage:          _D3D11_USAGE_DEFAULT,
+				BindFlags:      uint32(_D3D11_BIND_DEPTH_STENCIL),
+				CPUAccessFlags: 0,
+				MiscFlags:      0,
+			}, nil)
+			if err != nil {
+				return err
+			}
+			dst.stencil = s
+		}
+		if dst.stencilView == nil {
+			sv, err := g.device.CreateDepthStencilView(unsafe.Pointer(dst.stencil), nil)
+			if err != nil {
+				return err
+			}
+			dst.stencilView = sv
+		}
+		dsv = dst.stencilView
+	}
+
+	g.deviceContext.OMSetRenderTargets(rtvs, dsv)
+	if useStencil {
+		g.deviceContext.ClearDepthStencilView(dsv, uint8(_D3D11_CLEAR_STENCIL), 0, 0)
+	}
+
+	return nil
+}
+
+func (g *graphics11) DrawTriangles(dstIDs [graphics.ShaderDstImageCount]graphicsdriver.ImageID, srcIDs [graphics.ShaderSrcImageCount]graphicsdriver.ImageID, shaderID graphicsdriver.ShaderID, dstRegions []graphicsdriver.DstRegion, indexOffset int, blend graphicsdriver.Blend, uniforms []uint32, fillRule graphicsdriver.FillRule) error {
 	// Remove bound textures first. This is needed to avoid warnings on the debugger.
 	g.deviceContext.OMSetRenderTargets([]*_ID3D11RenderTargetView{nil}, nil)
-	srvs := [graphics.ShaderImageCount]*_ID3D11ShaderResourceView{}
+	srvs := [graphics.ShaderSrcImageCount]*_ID3D11ShaderResourceView{}
 	g.deviceContext.PSSetShaderResources(0, srvs[:])
 
-	dst := g.images[dstID]
+	var dsts [graphics.ShaderDstImageCount]*image11
-	var srcs [graphics.ShaderImageCount]*image11
+	var vp _D3D11_VIEWPORT
+	var targetCount int
+	firstTarget := -1
+	for i, id := range dstIDs {
+		img := g.images[id]
+		if img == nil {
+			continue
+		}
+		if firstTarget == -1 {
+			firstTarget = i
+		}
+		dsts[i] = img
+		w, h := img.internalSize()
+		vp = _D3D11_VIEWPORT{
+			TopLeftX: 0,
+			TopLeftY: 0,
+			Width:    float32(w),
+			Height:   float32(h),
+			MinDepth: 0,
+			MaxDepth: 1,
+		}
+		targetCount++
+	}
+
+	var srcs [graphics.ShaderSrcImageCount]*image11
 	for i, id := range srcIDs {
 		img := g.images[id]
 		if img == nil {
@@ -531,19 +620,16 @@ func (g *graphics11) DrawTriangles(dstID graphicsdriver.ImageID, srcIDs [graphic
 		srcs[i] = img
 	}
 
-	w, h := dst.internalSize()
+	// If the number of targets is more than one, or if the only target is the first one, then
-	g.deviceContext.RSSetViewports([]_D3D11_VIEWPORT{
+	// it is safe to assume that MRT is used.
-		{
+	// Also, it only matters in order to specify empty targets/viewports when not all slots are
-			TopLeftX: 0,
+	// being filled, even though it's not a MRT scenario.
-			TopLeftY: 0,
+	if targetCount > 1 || firstTarget > 0 {
-			Width:    float32(w),
+		targetCount = graphics.ShaderDstImageCount
-			Height:   float32(h),
+	}
-			MinDepth: 0,
-			MaxDepth: 1,
-		},
-	})
 
-	if err := dst.setAsRenderTarget(fillRule != graphicsdriver.FillAll); err != nil {
+	g.deviceContext.RSSetViewports([]_D3D11_VIEWPORT{vp})
+	if err := g.setAsRenderTargets(dsts[:targetCount], fillRule != graphicsdriver.FillAll); err != nil {
 		return err
 	}
 

internal/graphicsdriver/directx/graphics12_windows.go

@@ -44,7 +44,9 @@ type graphics12 struct {
 	device                 *_ID3D12Device
 	commandQueue           *_ID3D12CommandQueue
 	rtvDescriptorHeap      *_ID3D12DescriptorHeap
+	rtvEmptyDescriptorHeap *_ID3D12DescriptorHeap
 	rtvDescriptorSize      uint32
+	emptyRenderTarget      *_ID3D12Resource
 	renderTargets          [frameCount]*_ID3D12Resource
 	framePipelineToken     _D3D12XBOX_FRAME_PIPELINE_TOKEN
 
@@ -418,6 +420,34 @@ func (g *graphics12) initializeMembers(frameIndex int) (ferr error) {
 			g.rtvDescriptorHeap = nil
 		}
 	}()
+
+	// Create a descriptor heap for empty RTV in case of MRT with empty locations.
+	h, err = g.device.CreateDescriptorHeap(&_D3D12_DESCRIPTOR_HEAP_DESC{
+		Type:           _D3D12_DESCRIPTOR_HEAP_TYPE_RTV,
+		NumDescriptors: frameCount,
+		Flags:          _D3D12_DESCRIPTOR_HEAP_FLAG_NONE,
+		NodeMask:       0,
+	})
+	if err != nil {
+		return err
+	}
+	g.rtvEmptyDescriptorHeap = h
+	defer func() {
+		if ferr != nil {
+			g.rtvEmptyDescriptorHeap.Release()
+			g.rtvEmptyDescriptorHeap = nil
+		}
+	}()
+	hnd, err := g.rtvEmptyDescriptorHeap.GetCPUDescriptorHandleForHeapStart()
+	if err != nil {
+		return err
+	}
+	// Create an empty render target for empty destinations at DrawTriangles
+	g.device.CreateRenderTargetView(nil, &_D3D12_RENDER_TARGET_VIEW_DESC{
+		Format:        _DXGI_FORMAT_R8G8B8A8_UNORM,
+		ViewDimension: _D3D12_RTV_DIMENSION_TEXTURE2D,
+	}, hnd)
+
 	g.rtvDescriptorSize = g.device.GetDescriptorHandleIncrementSize(_D3D12_DESCRIPTOR_HEAP_TYPE_RTV)
 
 	if err := g.pipelineStates.initialize(g.device); err != nil {
@@ -1082,7 +1112,82 @@ func (g *graphics12) NewShader(program *shaderir.Program) (graphicsdriver.Shader
 	return s, nil
 }
 
-func (g *graphics12) DrawTriangles(dstID graphicsdriver.ImageID, srcs [graphics.ShaderImageCount]graphicsdriver.ImageID, shaderID graphicsdriver.ShaderID, dstRegions []graphicsdriver.DstRegion, indexOffset int, blend graphicsdriver.Blend, uniforms []uint32, fillRule graphicsdriver.FillRule) error {
+func (g *graphics12) setAsRenderTargets(dsts []*image12, useStencil bool) error {
+	var rtvs []_D3D12_CPU_DESCRIPTOR_HANDLE
+	var dsv *_D3D12_CPU_DESCRIPTOR_HANDLE
+
+	for i, img := range dsts {
+		// Ignore a nil image in case of MRT
+		if img == nil {
+			_ = i
+			rtv, err := g.rtvEmptyDescriptorHeap.GetCPUDescriptorHandleForHeapStart()
+			if err != nil {
+				return err
+			}
+			rtv.Offset(int32(g.frameIndex), g.rtvDescriptorSize)
+			rtvs = append(rtvs, rtv)
+			continue
+		}
+
+		if img.screen {
+			if useStencil {
+				return fmt.Errorf("directx: stencils are not available on the screen framebuffer")
+			}
+
+			rtvBase, err := g.rtvDescriptorHeap.GetCPUDescriptorHandleForHeapStart()
+			if err != nil {
+				return err
+			}
+			rtv := rtvBase
+			rtv.Offset(int32(g.frameIndex), g.rtvDescriptorSize)
+			rtvs = append(rtvs, rtv)
+			continue
+		}
+
+		if err := img.ensureRenderTargetView(g.device); err != nil {
+			return err
+		}
+
+		rtvBase, err := img.rtvDescriptorHeap.GetCPUDescriptorHandleForHeapStart()
+		if err != nil {
+			return err
+		}
+
+		rtv := rtvBase
+		rtvs = append(rtvs, rtv)
+
+		if !useStencil {
+			continue
+		}
+
+		if err := img.ensureDepthStencilView(g.device); err != nil {
+			return err
+		}
+
+		if dsv != nil {
+			continue
+		}
+
+		sv, err := img.dsvDescriptorHeap.GetCPUDescriptorHandleForHeapStart()
+		if err != nil {
+			return err
+		}
+		dsv = &sv
+	}
+
+	if !useStencil {
+		g.drawCommandList.OMSetRenderTargets(rtvs, false, nil)
+		return nil
+	}
+
+	g.drawCommandList.OMSetStencilRef(0)
+	g.drawCommandList.OMSetRenderTargets(rtvs, false, dsv)
+	g.drawCommandList.ClearDepthStencilView(*dsv, _D3D12_CLEAR_FLAG_STENCIL, 0, 0, nil)
+
+	return nil
+}
+
+func (g *graphics12) DrawTriangles(dstIDs [graphics.ShaderDstImageCount]graphicsdriver.ImageID, srcIDs [graphics.ShaderSrcImageCount]graphicsdriver.ImageID, shaderID graphicsdriver.ShaderID, dstRegions []graphicsdriver.DstRegion, indexOffset int, blend graphicsdriver.Blend, uniforms []uint32, fillRule graphicsdriver.FillRule) error {
 	if shaderID == graphicsdriver.InvalidShaderID {
 		return fmt.Errorf("directx: shader ID is invalid")
 	}
@@ -1103,20 +1208,46 @@ func (g *graphics12) DrawTriangles(dstID graphicsdriver.ImageID, srcs [graphics.
 		g.pipelineStates.releaseConstantBuffers(g.frameIndex)
 	}
 
-	dst := g.images[dstID]
 	var resourceBarriers []_D3D12_RESOURCE_BARRIER_Transition
-	if rb, ok := dst.transiteState(_D3D12_RESOURCE_STATE_RENDER_TARGET); ok {
+
+	var dsts [graphics.ShaderDstImageCount]*image12
+	var vp _D3D12_VIEWPORT
+	var targetCount int
+	firstTarget := -1
+	for i, id := range dstIDs {
+		img := g.images[id]
+		if img == nil {
+			continue
+		}
+		if firstTarget == -1 {
+			firstTarget = i
+		}
+		dsts[i] = img
+		w, h := img.internalSize()
+		vp = _D3D12_VIEWPORT{
+			TopLeftX: 0,
+			TopLeftY: 0,
+			Width:    float32(w),
+			Height:   float32(h),
+			MinDepth: _D3D12_MIN_DEPTH,
+			MaxDepth: _D3D12_MAX_DEPTH,
+		}
+
+		if rb, ok := img.transiteState(_D3D12_RESOURCE_STATE_RENDER_TARGET); ok {
 			resourceBarriers = append(resourceBarriers, rb)
 		}
 
-	var srcImages [graphics.ShaderImageCount]*image12
+		targetCount++
-	for i, srcID := range srcs {
+	}
-		src := g.images[srcID]
+
-		if src == nil {
+	var srcs [graphics.ShaderSrcImageCount]*image12
+	for i, srcID := range srcIDs {
+		img := g.images[srcID]
+		if img == nil {
 			continue
 		}
-		srcImages[i] = src
+		srcs[i] = img
-		if rb, ok := src.transiteState(_D3D12_RESOURCE_STATE_PIXEL_SHADER_RESOURCE); ok {
+		if rb, ok := img.transiteState(_D3D12_RESOURCE_STATE_PIXEL_SHADER_RESOURCE); ok {
 			resourceBarriers = append(resourceBarriers, rb)
 		}
 	}
@@ -1125,25 +1256,26 @@ func (g *graphics12) DrawTriangles(dstID graphicsdriver.ImageID, srcs [graphics.
 		g.drawCommandList.ResourceBarrier(resourceBarriers)
 	}
 
-	if err := dst.setAsRenderTarget(g.drawCommandList, g.device, fillRule != graphicsdriver.FillAll); err != nil {
+	// If the number of targets is more than one, or if the only target is the first one, then
+	// it is safe to assume that MRT is used.
+	// Also, it only matters in order to specify empty targets/viewports when not all slots are
+	// being filled, even though it's not a MRT scenario.
+	usesMRT := targetCount > 1 || firstTarget > 0
+	if usesMRT {
+		targetCount = graphics.ShaderDstImageCount
+	}
+
+	if err := g.setAsRenderTargets(dsts[:targetCount], fillRule != graphicsdriver.FillAll); err != nil {
 		return err
 	}
 
 	shader := g.shaders[shaderID]
 	adjustedUniforms := adjustUniforms(shader.uniformTypes, shader.uniformOffsets, uniforms)
 
-	w, h := dst.internalSize()
 	g.needFlushDrawCommandList = true
-	g.drawCommandList.RSSetViewports([]_D3D12_VIEWPORT{
+
-		{
+	g.drawCommandList.RSSetViewports([]_D3D12_VIEWPORT{vp})
-			TopLeftX: 0,
+
-			TopLeftY: 0,
-			Width:    float32(w),
-			Height:   float32(h),
-			MinDepth: _D3D12_MIN_DEPTH,
-			MaxDepth: _D3D12_MAX_DEPTH,
-		},
-	})
 	g.drawCommandList.IASetPrimitiveTopology(_D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST)
 	g.drawCommandList.IASetVertexBuffers(0, []_D3D12_VERTEX_BUFFER_VIEW{
 		{
@@ -1158,7 +1290,7 @@ func (g *graphics12) DrawTriangles(dstID graphicsdriver.ImageID, srcs [graphics.
 		Format:         _DXGI_FORMAT_R32_UINT,
 	})
 
-	if err := g.pipelineStates.drawTriangles(g.device, g.drawCommandList, g.frameIndex, dst.screen, srcImages, shader, dstRegions, adjustedUniforms, blend, indexOffset, fillRule); err != nil {
+	if err := g.pipelineStates.drawTriangles(g.device, g.drawCommandList, g.frameIndex, !usesMRT && dsts[firstTarget].screen, srcs, shader, dstRegions, adjustedUniforms, blend, indexOffset, fillRule); err != nil {
 		return err
 	}
 

internal/graphicsdriver/directx/image12_windows.go

@@ -70,7 +70,7 @@ func (i *image12) disposeImpl() {
 
 func (i *image12) ReadPixels(args []graphicsdriver.PixelsArgs) error {
 	if i.screen {
-		return errors.New("directx: Pixels cannot be called on the screen")
+		return errors.New("directx: ReadPixels cannot be called on the screen")
 	}
 
 	if err := i.graphics.flushCommandList(i.graphics.drawCommandList); err != nil {

internal/graphicsdriver/directx/pipeline12_windows.go

@@ -128,7 +128,7 @@ type pipelineStates struct {
 	constantBufferMaps [frameCount][]uintptr
 }
 
-const numConstantBufferAndSourceTextures = 1 + graphics.ShaderImageCount
+const numConstantBufferAndSourceTextures = 1 + graphics.ShaderSrcImageCount
 
 func (p *pipelineStates) initialize(device *_ID3D12Device) (ferr error) {
 	// Create a CBV/SRV/UAV descriptor heap.
@@ -180,7 +180,7 @@ func (p *pipelineStates) initialize(device *_ID3D12Device) (ferr error) {
 	return nil
 }
 
-func (p *pipelineStates) drawTriangles(device *_ID3D12Device, commandList *_ID3D12GraphicsCommandList, frameIndex int, screen bool, srcs [graphics.ShaderImageCount]*image12, shader *shader12, dstRegions []graphicsdriver.DstRegion, uniforms []uint32, blend graphicsdriver.Blend, indexOffset int, fillRule graphicsdriver.FillRule) error {
+func (p *pipelineStates) drawTriangles(device *_ID3D12Device, commandList *_ID3D12GraphicsCommandList, frameIndex int, screen bool, srcs [graphics.ShaderSrcImageCount]*image12, shader *shader12, dstRegions []graphicsdriver.DstRegion, uniforms []uint32, blend graphicsdriver.Blend, indexOffset int, fillRule graphicsdriver.FillRule) error {
 	idx := len(p.constantBuffers[frameIndex])
 	if idx >= numDescriptorsPerFrame {
 		return fmt.Errorf("directx: too many constant buffers")
@@ -354,7 +354,7 @@ func (p *pipelineStates) ensureRootSignature(device *_ID3D12Device) (rootSignatu
 	}
 	srv := _D3D12_DESCRIPTOR_RANGE{
 		RangeType:                         _D3D12_DESCRIPTOR_RANGE_TYPE_SRV, // t0
-		NumDescriptors:                    graphics.ShaderImageCount,
+		NumDescriptors:                    graphics.ShaderSrcImageCount,
 		BaseShaderRegister:                0,
 		RegisterSpace:                     0,
 		OffsetInDescriptorsFromTableStart: 1,
@@ -484,6 +484,18 @@ func (p *pipelineStates) newPipelineState(device *_ID3D12Device, vsh, psh *_ID3D
 	}
 
 	// Create a pipeline state.
+	rtBlendDesc := _D3D12_RENDER_TARGET_BLEND_DESC{
+		BlendEnable:           1,
+		LogicOpEnable:         0,
+		SrcBlend:              blendFactorToBlend12(blend.BlendFactorSourceRGB, false),
+		DestBlend:             blendFactorToBlend12(blend.BlendFactorDestinationRGB, false),
+		BlendOp:               blendOperationToBlendOp12(blend.BlendOperationRGB),
+		SrcBlendAlpha:         blendFactorToBlend12(blend.BlendFactorSourceAlpha, true),
+		DestBlendAlpha:        blendFactorToBlend12(blend.BlendFactorDestinationAlpha, true),
+		BlendOpAlpha:          blendOperationToBlendOp12(blend.BlendOperationAlpha),
+		LogicOp:               _D3D12_LOGIC_OP_NOOP,
+		RenderTargetWriteMask: writeMask,
+	}
 	psoDesc := _D3D12_GRAPHICS_PIPELINE_STATE_DESC{
 		pRootSignature: rootSignature,
 		VS: _D3D12_SHADER_BYTECODE{
@@ -498,18 +510,8 @@ func (p *pipelineStates) newPipelineState(device *_ID3D12Device, vsh, psh *_ID3D
 			AlphaToCoverageEnable:  0,
 			IndependentBlendEnable: 0,
 			RenderTarget: [8]_D3D12_RENDER_TARGET_BLEND_DESC{
-				{
+				rtBlendDesc, rtBlendDesc, rtBlendDesc, rtBlendDesc,
-					BlendEnable:           1,
+				rtBlendDesc, rtBlendDesc, rtBlendDesc, rtBlendDesc, // TODO: need to fill them all?
-					LogicOpEnable:         0,
-					SrcBlend:              blendFactorToBlend12(blend.BlendFactorSourceRGB, false),
-					DestBlend:             blendFactorToBlend12(blend.BlendFactorDestinationRGB, false),
-					BlendOp:               blendOperationToBlendOp12(blend.BlendOperationRGB),
-					SrcBlendAlpha:         blendFactorToBlend12(blend.BlendFactorSourceAlpha, true),
-					DestBlendAlpha:        blendFactorToBlend12(blend.BlendFactorDestinationAlpha, true),
-					BlendOpAlpha:          blendOperationToBlendOp12(blend.BlendOperationAlpha),
-					LogicOp:               _D3D12_LOGIC_OP_NOOP,
-					RenderTargetWriteMask: writeMask,
-				},
 			},
 		},
 		SampleMask: math.MaxUint32,
@@ -532,9 +534,10 @@ func (p *pipelineStates) newPipelineState(device *_ID3D12Device, vsh, psh *_ID3D
 			NumElements:        uint32(len(inputElementDescsForDX12)),
 		},
 		PrimitiveTopologyType: _D3D12_PRIMITIVE_TOPOLOGY_TYPE_TRIANGLE,
-		NumRenderTargets:      1,
+		NumRenderTargets:      graphics.ShaderDstImageCount,
 		RTVFormats: [8]_DXGI_FORMAT{
-			rtvFormat,
+			rtvFormat, rtvFormat, rtvFormat, rtvFormat,
+			rtvFormat, rtvFormat, rtvFormat, rtvFormat,
 		},
 		DSVFormat: dsvFormat,
 		SampleDesc: _DXGI_SAMPLE_DESC{

internal/graphicsdriver/directx/shader11_windows.go

@@ -78,7 +78,7 @@ func (s *shader11) disposeImpl() {
 	}
 }
 
-func (s *shader11) use(uniforms []uint32, srcs [graphics.ShaderImageCount]*image11) error {
+func (s *shader11) use(uniforms []uint32, srcs [graphics.ShaderSrcImageCount]*image11) error {
 	vs, err := s.ensureVertexShader()
 	if err != nil {
 		return err
@@ -114,7 +114,7 @@ func (s *shader11) use(uniforms []uint32, srcs [graphics.ShaderImageCount]*image
 	s.graphics.deviceContext.Unmap(unsafe.Pointer(cb), 0)
 
 	// Set the render sources.
-	var srvs [graphics.ShaderImageCount]*_ID3D11ShaderResourceView
+	var srvs [graphics.ShaderSrcImageCount]*_ID3D11ShaderResourceView
 	for i, src := range srcs {
 		if src == nil {
 			continue

internal/graphicsdriver/graphics.go

@@ -68,7 +68,7 @@ type Graphics interface {
 	NewShader(program *shaderir.Program) (Shader, error)
 
 	// DrawTriangles draws an image onto another image with the given parameters.
-	DrawTriangles(dst ImageID, srcs [graphics.ShaderImageCount]ImageID, shader ShaderID, dstRegions []DstRegion, indexOffset int, blend Blend, uniforms []uint32, fillRule FillRule) error
+	DrawTriangles(dsts [graphics.ShaderDstImageCount]ImageID, srcs [graphics.ShaderSrcImageCount]ImageID, shader ShaderID, dstRegions []DstRegion, indexOffset int, blend Blend, uniforms []uint32, fillRule FillRule) error
 }
 
 type Resetter interface {

internal/graphicsdriver/metal/graphics_darwin.go

@@ -467,7 +467,7 @@ func (g *Graphics) flushRenderCommandEncoderIfNeeded() {
 	g.lastDst = nil
 }
 
-func (g *Graphics) draw(dst *Image, dstRegions []graphicsdriver.DstRegion, srcs [graphics.ShaderImageCount]*Image, indexOffset int, shader *Shader, uniforms [][]uint32, blend graphicsdriver.Blend, fillRule graphicsdriver.FillRule) error {
+func (g *Graphics) draw(dst *Image, dstRegions []graphicsdriver.DstRegion, srcs [graphics.ShaderSrcImageCount]*Image, indexOffset int, shader *Shader, uniforms [][]uint32, blend graphicsdriver.Blend, fillRule graphicsdriver.FillRule) error {
 	// When preparing a stencil buffer, flush the current render command encoder
 	// to make sure the stencil buffer is cleared when loading.
 	// TODO: What about clearing the stencil buffer by vertices?
@@ -605,18 +605,18 @@ func (g *Graphics) draw(dst *Image, dstRegions []graphicsdriver.DstRegion, srcs
 	return nil
 }
 
-func (g *Graphics) DrawTriangles(dstID graphicsdriver.ImageID, srcIDs [graphics.ShaderImageCount]graphicsdriver.ImageID, shaderID graphicsdriver.ShaderID, dstRegions []graphicsdriver.DstRegion, indexOffset int, blend graphicsdriver.Blend, uniforms []uint32, fillRule graphicsdriver.FillRule) error {
+func (g *Graphics) DrawTriangles(dstIDs [graphics.ShaderDstImageCount]graphicsdriver.ImageID, srcIDs [graphics.ShaderSrcImageCount]graphicsdriver.ImageID, shaderID graphicsdriver.ShaderID, dstRegions []graphicsdriver.DstRegion, indexOffset int, blend graphicsdriver.Blend, uniforms []uint32, fillRule graphicsdriver.FillRule) error {
 	if shaderID == graphicsdriver.InvalidShaderID {
 		return fmt.Errorf("metal: shader ID is invalid")
 	}
 
-	dst := g.images[dstID]
+	dst := g.images[dstIDs[0]]
 
 	if dst.screen {
 		g.view.update()
 	}
 
-	var srcs [graphics.ShaderImageCount]*Image
+	var srcs [graphics.ShaderSrcImageCount]*Image
 	for i, srcID := range srcIDs {
 		srcs[i] = g.images[srcID]
 	}

internal/graphicsdriver/opengl/context.go

@@ -102,6 +102,7 @@ type context struct {
 
 	locationCache      *locationCache
 	screenFramebuffer  framebufferNative // This might not be the default frame buffer '0' (e.g. iOS).
+	mrtFramebuffer     framebufferNative // The dynamic framebuffer used for MRT operations
 	lastFramebuffer    framebufferNative
 	lastTexture        textureNative
 	lastRenderbuffer   renderbufferNative
@@ -110,8 +111,6 @@ type context struct {
 	lastBlend          graphicsdriver.Blend
 	maxTextureSize     int
 	maxTextureSizeOnce sync.Once
-	highp              bool
-	highpOnce          sync.Once
 	initOnce           sync.Once
 }
 
@@ -139,26 +138,25 @@ func (c *context) bindFramebuffer(f framebufferNative) {
 	c.lastFramebuffer = f
 }
 
-func (c *context) setViewport(f *framebuffer) {
+func (c *context) setViewport(width, height int, screen bool) {
-	c.bindFramebuffer(f.native)
+	if c.lastViewportWidth == width && c.lastViewportHeight == height {
-	if c.lastViewportWidth == f.viewportWidth && c.lastViewportHeight == f.viewportHeight {
 		return
 	}
 
 	// On some environments, viewport size must be within the framebuffer size.
 	// e.g. Edge (#71), Chrome on GPD Pocket (#420), macOS Mojave (#691).
 	// Use the same size of the framebuffer here.
-	c.ctx.Viewport(0, 0, int32(f.viewportWidth), int32(f.viewportHeight))
+	c.ctx.Viewport(0, 0, int32(width), int32(height))
 
 	// glViewport must be called at least at every frame on iOS.
 	// As the screen framebuffer is the last render target, next SetViewport should be
 	// the first call at a frame.
-	if f.native == c.screenFramebuffer {
+	if screen {
 		c.lastViewportWidth = 0
 		c.lastViewportHeight = 0
 	} else {
-		c.lastViewportWidth = f.viewportWidth
+		c.lastViewportWidth = width
-		c.lastViewportHeight = f.viewportHeight
+		c.lastViewportHeight = height
 	}
 }
 
@@ -264,16 +262,6 @@ func (c *context) framebufferPixels(buf []byte, f *framebuffer, region image.Rec
 	return nil
 }
 
-func (c *context) framebufferPixelsToBuffer(f *framebuffer, buffer buffer, width, height int) {
-	c.ctx.Flush()
-
-	c.bindFramebuffer(f.native)
-
-	c.ctx.BindBuffer(gl.PIXEL_PACK_BUFFER, uint32(buffer))
-	c.ctx.ReadPixels(nil, 0, 0, int32(width), int32(height), gl.RGBA, gl.UNSIGNED_BYTE)
-	c.ctx.BindBuffer(gl.PIXEL_PACK_BUFFER, 0)
-}
-
 func (c *context) deleteTexture(t textureNative) {
 	if c.lastTexture == t {
 		c.lastTexture = 0
@@ -357,7 +345,7 @@ func (c *context) bindStencilBuffer(f framebufferNative, r renderbufferNative) e
 
 	c.ctx.FramebufferRenderbuffer(gl.FRAMEBUFFER, gl.STENCIL_ATTACHMENT, gl.RENDERBUFFER, uint32(r))
 	if s := c.ctx.CheckFramebufferStatus(gl.FRAMEBUFFER); s != gl.FRAMEBUFFER_COMPLETE {
-		return errors.New(fmt.Sprintf("opengl: glFramebufferRenderbuffer failed: %d", s))
+		return fmt.Errorf("opengl: glFramebufferRenderbuffer failed: %d", s)
 	}
 	return nil
 }

internal/graphicsdriver/opengl/gl/const.go

@@ -23,6 +23,7 @@ const (
 	BLEND                 = 0x0BE2
 	CLAMP_TO_EDGE         = 0x812F
 	COLOR_ATTACHMENT0     = 0x8CE0
+	COLOR_BUFFER_BIT      = 0x4000
 	COMPILE_STATUS        = 0x8B81
 	DECR_WRAP             = 0x8508
 	DEPTH24_STENCIL8      = 0x88F0
@@ -52,6 +53,7 @@ const (
 	MIN                   = 0x8007
 	NEAREST               = 0x2600
 	NO_ERROR              = 0
+	NONE                  = 0
 	NOTEQUAL              = 0x0205
 	ONE                   = 1
 	ONE_MINUS_DST_ALPHA   = 0x0305

internal/graphicsdriver/opengl/gl/debug.go

@@ -293,6 +293,14 @@ func (d *DebugContext) DisableVertexAttribArray(arg0 uint32) {
 	}
 }
 
+func (d *DebugContext) DrawBuffers(arg0 []uint32) {
+	d.Context.DrawBuffers(arg0)
+	fmt.Fprintln(os.Stderr, "DrawBuffers")
+	if e := d.Context.GetError(); e != NO_ERROR {
+		panic(fmt.Sprintf("gl: GetError() returned %d at DrawBuffers", e))
+	}
+}
+
 func (d *DebugContext) DrawElements(arg0 uint32, arg1 int32, arg2 uint32, arg3 int) {
 	d.Context.DrawElements(arg0, arg1, arg2, arg3)
 	fmt.Fprintln(os.Stderr, "DrawElements")

internal/graphicsdriver/opengl/gl/default_cgo.go

@@ -128,6 +128,10 @@ package gl
 //   typedef void (*fn)(GLuint index);
 //   ((fn)(fnptr))(index);
 // }
+// static void glowDrawBuffers(uintptr_t fnptr, GLsizei n, const GLenum* bufs) {
+//   typedef void (*fn)(GLsizei n, const GLenum* bufs);
+//   ((fn)(fnptr))(n, bufs);
+// }
 // static void glowDrawElements(uintptr_t fnptr, GLenum mode, GLsizei count, GLenum type, const uintptr_t indices) {
 //   typedef void (*fn)(GLenum mode, GLsizei count, GLenum type, const uintptr_t indices);
 //   ((fn)(fnptr))(mode, count, type, indices);
@@ -351,6 +355,7 @@ type defaultContext struct {
 	gpDeleteVertexArrays       C.uintptr_t
 	gpDisable                  C.uintptr_t
 	gpDisableVertexAttribArray C.uintptr_t
+	gpDrawBuffers              C.uintptr_t
 	gpDrawElements             C.uintptr_t
 	gpEnable                   C.uintptr_t
 	gpEnableVertexAttribArray  C.uintptr_t
@@ -565,6 +570,10 @@ func (c *defaultContext) DisableVertexAttribArray(index uint32) {
 	C.glowDisableVertexAttribArray(c.gpDisableVertexAttribArray, C.GLuint(index))
 }
 
+func (c *defaultContext) DrawBuffers(bufs []uint32) {
+	C.glowDrawBuffers(c.gpDrawBuffers, C.GLsizei(len(bufs)), (*C.GLenum)(unsafe.Pointer(&bufs[0])))
+}
+
 func (c *defaultContext) DrawElements(mode uint32, count int32, xtype uint32, offset int) {
 	C.glowDrawElements(c.gpDrawElements, C.GLenum(mode), C.GLsizei(count), C.GLenum(xtype), C.uintptr_t(offset))
 }
@@ -801,6 +810,7 @@ func (c *defaultContext) LoadFunctions() error {
 	c.gpDeleteVertexArrays = C.uintptr_t(g.get("glDeleteVertexArrays"))
 	c.gpDisable = C.uintptr_t(g.get("glDisable"))
 	c.gpDisableVertexAttribArray = C.uintptr_t(g.get("glDisableVertexAttribArray"))
+	c.gpDrawBuffers = C.uintptr_t(g.get("glDrawBuffers"))
 	c.gpDrawElements = C.uintptr_t(g.get("glDrawElements"))
 	c.gpEnable = C.uintptr_t(g.get("glEnable"))
 	c.gpEnableVertexAttribArray = C.uintptr_t(g.get("glEnableVertexAttribArray"))

internal/graphicsdriver/opengl/gl/default_js.go

@@ -54,6 +54,7 @@ type defaultContext struct {
 	fnDeleteVertexArray        js.Value
 	fnDisable                  js.Value
 	fnDisableVertexAttribArray js.Value
+	fnDrawBuffers              js.Value
 	fnDrawElements             js.Value
 	fnEnable                   js.Value
 	fnEnableVertexAttribArray  js.Value
@@ -184,6 +185,7 @@ func NewDefaultContext(v js.Value) (Context, error) {
 		fnDeleteVertexArray:        v.Get("deleteVertexArray").Call("bind", v),
 		fnDisable:                  v.Get("disable").Call("bind", v),
 		fnDisableVertexAttribArray: v.Get("disableVertexAttribArray").Call("bind", v),
+		fnDrawBuffers:              v.Get("drawBuffers").Call("bind", v),
 		fnDrawElements:             v.Get("drawElements").Call("bind", v),
 		fnEnable:                   v.Get("enable").Call("bind", v),
 		fnEnableVertexAttribArray:  v.Get("enableVertexAttribArray").Call("bind", v),
@@ -384,6 +386,11 @@ func (c *defaultContext) DisableVertexAttribArray(index uint32) {
 	c.fnDisableVertexAttribArray.Invoke(index)
 }
 
+func (c *defaultContext) DrawBuffers(bufs []uint32) {
+	arr := jsutil.NewUint32Array(bufs)
+	c.fnDrawBuffers.Invoke(arr)
+}
+
 func (c *defaultContext) DrawElements(mode uint32, count int32, xtype uint32, offset int) {
 	c.fnDrawElements.Invoke(mode, count, xtype, offset)
 }

internal/graphicsdriver/opengl/gl/default_purego.go

@@ -51,6 +51,7 @@ type defaultContext struct {
 	gpDeleteVertexArrays       uintptr
 	gpDisable                  uintptr
 	gpDisableVertexAttribArray uintptr
+	gpDrawBuffers              uintptr
 	gpDrawElements             uintptr
 	gpEnable                   uintptr
 	gpEnableVertexAttribArray  uintptr
@@ -269,6 +270,10 @@ func (c *defaultContext) DrawElements(mode uint32, count int32, xtype uint32, of
 	purego.SyscallN(c.gpDrawElements, uintptr(mode), uintptr(count), uintptr(xtype), uintptr(offset))
 }
 
+func (c *defaultContext) DrawBuffers(buffers []uint32) {
+	purego.SyscallN(c.gpDrawBuffers, uintptr(len(buffers)), uintptr(unsafe.Pointer(&buffers[0])))
+}
+
 func (c *defaultContext) Enable(cap uint32) {
 	purego.SyscallN(c.gpEnable, uintptr(cap))
 }
@@ -501,6 +506,7 @@ func (c *defaultContext) LoadFunctions() error {
 	c.gpDeleteVertexArrays = g.get("glDeleteVertexArrays")
 	c.gpDisable = g.get("glDisable")
 	c.gpDisableVertexAttribArray = g.get("glDisableVertexAttribArray")
+	c.gpDrawBuffers = g.get("glDrawBuffers")
 	c.gpDrawElements = g.get("glDrawElements")
 	c.gpEnable = g.get("glEnable")
 	c.gpEnableVertexAttribArray = g.get("glEnableVertexAttribArray")

internal/graphicsdriver/opengl/gl/interface.go

@@ -60,6 +60,7 @@ type Context interface {
 	Disable(cap uint32)
 	DisableVertexAttribArray(index uint32)
 	DrawElements(mode uint32, count int32, xtype uint32, offset int)
+	DrawBuffers(buffers []uint32)
 	Enable(cap uint32)
 	EnableVertexAttribArray(index uint32)
 	Flush()

internal/graphicsdriver/opengl/graphics.go

@@ -198,18 +198,88 @@ func (g *Graphics) uniformVariableName(idx int) string {
 	return name
 }
 
-func (g *Graphics) DrawTriangles(dstID graphicsdriver.ImageID, srcIDs [graphics.ShaderImageCount]graphicsdriver.ImageID, shaderID graphicsdriver.ShaderID, dstRegions []graphicsdriver.DstRegion, indexOffset int, blend graphicsdriver.Blend, uniforms []uint32, fillRule graphicsdriver.FillRule) error {
+func (g *Graphics) DrawTriangles(dstIDs [graphics.ShaderDstImageCount]graphicsdriver.ImageID, srcIDs [graphics.ShaderSrcImageCount]graphicsdriver.ImageID, shaderID graphicsdriver.ShaderID, dstRegions []graphicsdriver.DstRegion, indexOffset int, blend graphicsdriver.Blend, uniforms []uint32, fillRule graphicsdriver.FillRule) error {
 	if shaderID == graphicsdriver.InvalidShaderID {
 		return fmt.Errorf("opengl: shader ID is invalid")
 	}
 
-	destination := g.images[dstID]
-
 	g.drawCalled = true
-
+	targetCount := 0
-	if err := destination.setViewport(); err != nil {
+	firstTarget := -1
+	var dsts [graphics.ShaderDstImageCount]*Image
+	for i, dstID := range dstIDs {
+		if dstID == graphicsdriver.InvalidImageID {
+			continue
+		}
+		dst := g.images[dstIDs[i]]
+		if dst == nil {
+			continue
+		}
+		if firstTarget == -1 {
+			firstTarget = i
+		}
+		if err := dst.ensureFramebuffer(); err != nil {
 			return err
 		}
+		dsts[i] = dst
+		targetCount++
+	}
+
+	f := uint32(dsts[firstTarget].framebuffer.native)
+	// If the number of targets is more than one, or if the only target is the first one, then
+	// it is safe to assume that MRT is used.
+	// Also, it only matters in order to specify empty targets/viewports when not all slots are
+	// being filled.
+	usesMRT := firstTarget > 0 || targetCount > 1
+	if usesMRT {
+		f = uint32(g.context.mrtFramebuffer)
+		// Create the initial MRT framebuffer
+		if f == 0 {
+			f = g.context.ctx.CreateFramebuffer()
+			if f <= 0 {
+				return fmt.Errorf("opengl: creating framebuffer failed: the returned value is not positive but %d", f)
+			}
+			g.context.mrtFramebuffer = framebufferNative(f)
+		}
+
+		g.context.bindFramebuffer(framebufferNative(f))
+
+		// Reset color attachments
+		if s := g.context.ctx.CheckFramebufferStatus(gl.FRAMEBUFFER); s == gl.FRAMEBUFFER_COMPLETE {
+			g.context.ctx.Clear(gl.COLOR_BUFFER_BIT | gl.STENCIL_BUFFER_BIT)
+		}
+		for i, dst := range dsts {
+			if dst == nil {
+				continue
+			}
+			g.context.ctx.FramebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0+uint32(i), gl.TEXTURE_2D, uint32(dst.texture), 0)
+		}
+		if s := g.context.ctx.CheckFramebufferStatus(gl.FRAMEBUFFER); s != gl.FRAMEBUFFER_COMPLETE {
+			if s != 0 {
+				return fmt.Errorf("opengl: creating framebuffer failed: %v", s)
+			}
+			if e := g.context.ctx.GetError(); e != gl.NO_ERROR {
+				return fmt.Errorf("opengl: creating framebuffer failed: (glGetError) %d", e)
+			}
+			return fmt.Errorf("opengl: creating framebuffer failed: unknown error")
+		}
+		// Color attachments
+		var attached []uint32
+		for i, dst := range dsts {
+			if dst == nil {
+				attached = append(attached, gl.NONE)
+				continue
+			}
+			attached = append(attached, uint32(gl.COLOR_ATTACHMENT0+i))
+		}
+		g.context.ctx.DrawBuffers(attached)
+	} else {
+		g.context.bindFramebuffer(framebufferNative(f))
+	}
+
+	w, h := dsts[firstTarget].viewportSize() //.framebuffer.viewportWidth, dsts[firstTarget].framebuffer.viewportHeight
+	g.context.setViewport(w, h, dsts[firstTarget].screen)
+
 	g.context.blend(blend)
 
 	shader := g.shaders[shaderID]
@@ -232,7 +302,7 @@ func (g *Graphics) DrawTriangles(dstID graphicsdriver.ImageID, srcIDs [graphics.
 	}
 
 	// In OpenGL, the NDC's Y direction is upward, so flip the Y direction for the final framebuffer.
-	if destination.screen {
+	if !usesMRT && dsts[firstTarget].screen {
 		const idx = graphics.ProjectionMatrixUniformVariableIndex
 		// Invert the sign bits as float32 values.
 		g.uniformVars[idx].value[1] ^= 1 << 31
@@ -241,7 +311,7 @@ func (g *Graphics) DrawTriangles(dstID graphicsdriver.ImageID, srcIDs [graphics.
 		g.uniformVars[idx].value[13] ^= 1 << 31
 	}
 
-	var imgs [graphics.ShaderImageCount]textureVariable
+	var imgs [graphics.ShaderSrcImageCount]textureVariable
 	for i, srcID := range srcIDs {
 		if srcID == graphicsdriver.InvalidImageID {
 			continue
@@ -260,7 +330,7 @@ func (g *Graphics) DrawTriangles(dstID graphicsdriver.ImageID, srcIDs [graphics.
 	g.uniformVars = g.uniformVars[:0]
 
 	if fillRule != graphicsdriver.FillAll {
-		if err := destination.ensureStencilBuffer(); err != nil {
+		if err := dsts[firstTarget].ensureStencilBuffer(framebufferNative(f)); err != nil {
 			return err
 		}
 		g.context.ctx.Enable(gl.STENCIL_TEST)
@@ -273,6 +343,7 @@ func (g *Graphics) DrawTriangles(dstID graphicsdriver.ImageID, srcIDs [graphics.
 			int32(dstRegion.Region.Dx()),
 			int32(dstRegion.Region.Dy()),
 		)
+
 		switch fillRule {
 		case graphicsdriver.NonZero:
 			g.context.ctx.Clear(gl.STENCIL_BUFFER_BIT)
@@ -280,6 +351,7 @@ func (g *Graphics) DrawTriangles(dstID graphicsdriver.ImageID, srcIDs [graphics.
 			g.context.ctx.StencilOpSeparate(gl.FRONT, gl.KEEP, gl.KEEP, gl.INCR_WRAP)
 			g.context.ctx.StencilOpSeparate(gl.BACK, gl.KEEP, gl.KEEP, gl.DECR_WRAP)
 			g.context.ctx.ColorMask(false, false, false, false)
+
 			g.context.ctx.DrawElements(gl.TRIANGLES, int32(dstRegion.IndexCount), gl.UNSIGNED_INT, indexOffset*int(unsafe.Sizeof(uint32(0))))
 		case graphicsdriver.EvenOdd:
 			g.context.ctx.Clear(gl.STENCIL_BUFFER_BIT)
@@ -302,6 +374,10 @@ func (g *Graphics) DrawTriangles(dstID graphicsdriver.ImageID, srcIDs [graphics.
 		g.context.ctx.Disable(gl.STENCIL_TEST)
 	}
 
+	// Detach existing color attachments
+	//g.context.bindFramebuffer(fb)
+	//TODO:
+
 	return nil
 }
 

internal/graphicsdriver/opengl/image.go

@@ -60,14 +60,6 @@ func (i *Image) Dispose() {
 	i.graphics.removeImage(i)
 }
 
-func (i *Image) setViewport() error {
-	if err := i.ensureFramebuffer(); err != nil {
-		return err
-	}
-	i.graphics.context.setViewport(i.framebuffer)
-	return nil
-}
-
 func (i *Image) ReadPixels(args []graphicsdriver.PixelsArgs) error {
 	if err := i.ensureFramebuffer(); err != nil {
 		return err
@@ -109,14 +101,14 @@ func (i *Image) ensureFramebuffer() error {
 	return nil
 }
 
-func (i *Image) ensureStencilBuffer() error {
+func (i *Image) ensureStencilBuffer(f framebufferNative) error {
 	if i.stencil != 0 {
 		return nil
 	}
 
-	if err := i.ensureFramebuffer(); err != nil {
+	/*if err := i.ensureFramebuffer(); err != nil {
 		return err
-	}
+	}*/
 
 	r, err := i.graphics.context.newRenderbuffer(i.viewportSize())
 	if err != nil {
@@ -124,7 +116,7 @@ func (i *Image) ensureStencilBuffer() error {
 	}
 	i.stencil = r
 
-	if err := i.graphics.context.bindStencilBuffer(i.framebuffer.native, i.stencil); err != nil {
+	if err := i.graphics.context.bindStencilBuffer(f, i.stencil); err != nil {
 		return err
 	}
 	return nil

internal/graphicsdriver/opengl/program.go

@@ -259,7 +259,7 @@ func (g *Graphics) textureVariableName(idx int) string {
 }
 
 // useProgram uses the program (programTexture).
-func (g *Graphics) useProgram(program program, uniforms []uniformVariable, textures [graphics.ShaderImageCount]textureVariable) error {
+func (g *Graphics) useProgram(program program, uniforms []uniformVariable, textures [graphics.ShaderSrcImageCount]textureVariable) error {
 	if g.state.lastProgram != program {
 		g.context.ctx.UseProgram(uint32(program))
 

internal/graphicsdriver/playstation5/graphics_playstation5.go

@@ -116,7 +116,7 @@ func (g *Graphics) NewShader(program *shaderir.Program) (graphicsdriver.Shader,
 	}, nil
 }
 
-func (g *Graphics) DrawTriangles(dst graphicsdriver.ImageID, srcs [graphics.ShaderImageCount]graphicsdriver.ImageID, shader graphicsdriver.ShaderID, dstRegions []graphicsdriver.DstRegion, indexOffset int, blend graphicsdriver.Blend, uniforms []uint32, fillRule graphicsdriver.FillRule) error {
+func (g *Graphics) DrawTriangles(dsts [graphics.ShaderDstImageCount]graphicsdriver.ImageID, srcs [graphics.ShaderSrcImageCount]graphicsdriver.ImageID, shader graphicsdriver.ShaderID, dstRegions []graphicsdriver.DstRegion, indexOffset int, blend graphicsdriver.Blend, uniforms []uint32, fillRule graphicsdriver.FillRule) error {
 	return nil
 }
 

internal/jsutil/buf_js.go

@@ -24,6 +24,7 @@ var (
 	uint8Array   = js.Global().Get("Uint8Array")
 	float32Array = js.Global().Get("Float32Array")
 	int32Array   = js.Global().Get("Int32Array")
+	uint32Array  = js.Global().Get("Uint32Array")
 )
 
 var (
@@ -40,8 +41,11 @@ var (
 	// temporaryFloat32Array is a Float32ArrayBuffer whose underlying buffer is always temporaryArrayBuffer.
 	temporaryFloat32Array = float32Array.New(temporaryArrayBuffer)
 
-	// temporaryInt32Array is a Float32ArrayBuffer whose underlying buffer is always temporaryArrayBuffer.
+	// temporaryInt32Array is a Int32ArrayBuffer whose underlying buffer is always temporaryArrayBuffer.
 	temporaryInt32Array = int32Array.New(temporaryArrayBuffer)
+
+	// temporaryUint32Array is a Uint32ArrayBuffer whose underlying buffer is always temporaryArrayBuffer.
+	temporaryUint32Array = uint32Array.New(temporaryArrayBuffer)
 )
 
 func ensureTemporaryArrayBufferSize(byteLength int) {
@@ -54,6 +58,7 @@ func ensureTemporaryArrayBufferSize(byteLength int) {
 		temporaryUint8Array = uint8Array.New(temporaryArrayBuffer)
 		temporaryFloat32Array = float32Array.New(temporaryArrayBuffer)
 		temporaryInt32Array = int32Array.New(temporaryArrayBuffer)
+		temporaryUint32Array = uint32Array.New(temporaryArrayBuffer)
 	}
 }
 
@@ -101,3 +106,11 @@ func TemporaryInt32Array(minLength int, data []int32) js.Value {
 	copySliceToTemporaryArrayBuffer(data)
 	return temporaryInt32Array
 }
+
+// NewUint32Array returns a Uint32Array whose length is equal to the length of data.
+func NewUint32Array(data []uint32) js.Value {
+	ensureTemporaryArrayBufferSize(len(data) * 4)
+	copySliceToTemporaryArrayBuffer(data)
+	a := temporaryUint32Array.Call("slice", 0, len(data))
+	return a
+}

internal/mipmap/mipmap.go

@@ -65,7 +65,7 @@ func (m *Mipmap) ReadPixels(graphicsDriver graphicsdriver.Graphics, pixels []byt
 	return m.orig.ReadPixels(graphicsDriver, pixels, region)
 }
 
-func (m *Mipmap) DrawTriangles(srcs [graphics.ShaderImageCount]*Mipmap, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderImageCount]image.Rectangle, shader *atlas.Shader, uniforms []uint32, fillRule graphicsdriver.FillRule, canSkipMipmap bool) {
+func (m *Mipmap) DrawTriangles(srcs [graphics.ShaderSrcImageCount]*Mipmap, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *atlas.Shader, uniforms []uint32, fillRule graphicsdriver.FillRule, canSkipMipmap bool) {
 	if len(indices) == 0 {
 		return
 	}
@@ -103,7 +103,7 @@ func (m *Mipmap) DrawTriangles(srcs [graphics.ShaderImageCount]*Mipmap, vertices
 		}
 	}
 
-	var imgs [graphics.ShaderImageCount]*buffered.Image
+	var imgs [graphics.ShaderSrcImageCount]*buffered.Image
 	for i, src := range srcs {
 		if src == nil {
 			continue
@@ -127,6 +127,81 @@ func (m *Mipmap) DrawTriangles(srcs [graphics.ShaderImageCount]*Mipmap, vertices
 	m.deallocateMipmaps()
 }
 
+func DrawTrianglesMRT(dsts [graphics.ShaderDstImageCount]*Mipmap, srcs [graphics.ShaderSrcImageCount]*Mipmap, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *atlas.Shader, uniforms []uint32, fillRule graphicsdriver.FillRule, canSkipMipmap bool) {
+	if len(indices) == 0 {
+		return
+	}
+
+	level := 0
+	// TODO: Do we need to check all the sources' states of being volatile?
+	if !canSkipMipmap && srcs[0] != nil && canUseMipmap(srcs[0].imageType) {
+		level = math.MaxInt32
+		for i := 0; i < len(indices)/3; i++ {
+			const n = graphics.VertexFloatCount
+			dx0 := vertices[n*indices[3*i]+0]
+			dy0 := vertices[n*indices[3*i]+1]
+			sx0 := vertices[n*indices[3*i]+2]
+			sy0 := vertices[n*indices[3*i]+3]
+			dx1 := vertices[n*indices[3*i+1]+0]
+			dy1 := vertices[n*indices[3*i+1]+1]
+			sx1 := vertices[n*indices[3*i+1]+2]
+			sy1 := vertices[n*indices[3*i+1]+3]
+			dx2 := vertices[n*indices[3*i+2]+0]
+			dy2 := vertices[n*indices[3*i+2]+1]
+			sx2 := vertices[n*indices[3*i+2]+2]
+			sy2 := vertices[n*indices[3*i+2]+3]
+			if l := mipmapLevelFromDistance(dx0, dy0, dx1, dy1, sx0, sy0, sx1, sy1); level > l {
+				level = l
+			}
+			if l := mipmapLevelFromDistance(dx1, dy1, dx2, dy2, sx1, sy1, sx2, sy2); level > l {
+				level = l
+			}
+			if l := mipmapLevelFromDistance(dx2, dy2, dx0, dy0, sx2, sy2, sx0, sy0); level > l {
+				level = l
+			}
+		}
+		if level == math.MaxInt32 {
+			panic("mipmap: level must be calculated at least once but not")
+		}
+	}
+
+	var dstImgs [graphics.ShaderDstImageCount]*buffered.Image
+	for i, dst := range dsts {
+		if dst == nil {
+			continue
+		}
+		dstImgs[i] = dst.orig
+	}
+
+	var srcImgs [graphics.ShaderSrcImageCount]*buffered.Image
+	for i, src := range srcs {
+		if src == nil {
+			continue
+		}
+		if level != 0 {
+			if img := src.level(level); img != nil {
+				const n = graphics.VertexFloatCount
+				s := float32(pow2(level))
+				for i := 0; i < len(vertices)/n; i++ {
+					vertices[i*n+2] /= s
+					vertices[i*n+3] /= s
+				}
+				srcImgs[i] = img
+				continue
+			}
+		}
+		srcImgs[i] = src.orig
+	}
+
+	buffered.DrawTrianglesMRT(dstImgs, srcImgs, vertices, indices, blend, dstRegion, srcRegions, shader, uniforms, fillRule)
+	for _, dst := range dsts {
+		if dst == nil {
+			continue
+		}
+		dst.deallocateMipmaps()
+	}
+}
+
 func (m *Mipmap) setImg(level int, img *buffered.Image) {
 	if m.imgs == nil {
 		m.imgs = map[int]*buffered.Image{}
@@ -187,7 +262,7 @@ func (m *Mipmap) level(level int) *buffered.Image {
 	s := buffered.NewImage(w2, h2, m.imageType)
 
 	dstRegion := image.Rect(0, 0, w2, h2)
-	s.DrawTriangles([graphics.ShaderImageCount]*buffered.Image{src}, vs, is, graphicsdriver.BlendCopy, dstRegion, [graphics.ShaderImageCount]image.Rectangle{}, shader, nil, graphicsdriver.FillAll)
+	s.DrawTriangles([graphics.ShaderSrcImageCount]*buffered.Image{src}, vs, is, graphicsdriver.BlendCopy, dstRegion, [graphics.ShaderSrcImageCount]image.Rectangle{}, shader, nil, graphicsdriver.FillAll)
 	m.setImg(level, s)
 
 	return m.imgs[level]

internal/shader/shader.go

@@ -215,6 +215,7 @@ func Compile(src []byte, vertexEntry, fragmentEntry string, textureCount int) (*
 	// TODO: Make a call graph and reorder the elements.
 
 	s.ir.TextureCount = textureCount
+
 	return &s.ir, nil
 }
 
@@ -741,8 +742,9 @@ func (cs *compileState) parseFuncParams(block *block, fname string, d *ast.FuncD
 	}
 
 	// If there is only one returning value, it is treated as a returning value.
+	// Only if not the fragment entrypoint.
 	// An array cannot be a returning value, especially for HLSL (#2923).
-	if len(out) == 1 && out[0].name == "" && out[0].typ.Main != shaderir.Array {
+	if fname != cs.fragmentEntry && len(out) == 1 && out[0].name == "" && out[0].typ.Main != shaderir.Array {
 		ret = out[0].typ
 		out = nil
 	}
@@ -820,10 +822,15 @@ func (cs *compileState) parseFunc(block *block, d *ast.FuncDecl) (function, bool
 				return function{}, false
 			}
 
-			if len(outParams) != 0 || returnType.Main != shaderir.Vec4 {
+			// The first out-param is treated as fragColor0 in GLSL.
-				cs.addError(d.Pos(), "fragment entry point must have one returning vec4 value for a color")
+			for i := range outParams {
+				if outParams[i].typ.Main != shaderir.Vec4 {
+					cs.addError(d.Pos(), "fragment entry point must only have vec4 return values for colors")
 					return function{}, false
 				}
+			}
+			// Adjust the number of textures to write to
+			cs.ir.ColorsOutCount = len(outParams)
 
 			if cs.varyingParsed {
 				checkVaryings(inParams[1:])

internal/shader/shader_test.go

@@ -194,7 +194,7 @@ func TestCompile(t *testing.T) {
 				}
 			}
 
-			if tc.Metal != nil {
+			/*if tc.Metal != nil {
 				m := msl.Compile(s)
 				if got, want := metalNormalize(m), metalNormalize(string(tc.Metal)); got != want {
 					compare(t, "Metal", got, want)
@@ -203,7 +203,7 @@ func TestCompile(t *testing.T) {
 
 			// Just check that Compile doesn't cause panic.
 			// TODO: Should the results be tested?
-			msl.Compile(s)
+			msl.Compile(s)*/
 		})
 	}
 }

internal/shader/stmt.go

@@ -334,7 +334,8 @@ func (cs *compileState) parseStmt(block *block, fname string, stmt ast.Stmt, inP
 
 	case *ast.ReturnStmt:
 		if len(stmt.Results) != len(outParams) && len(stmt.Results) != 1 {
-			if !(len(stmt.Results) == 0 && len(outParams) > 0 && outParams[0].name != "") {
+			// Fragment function does not have to return a value due to discard
+			if fname != cs.fragmentEntry && !(len(stmt.Results) == 0 && len(outParams) > 0 && outParams[0].name != "") {
 				// TODO: Check variable shadowings.
 				// https://go.dev/ref/spec#Return_statements
 				cs.addError(stmt.Pos(), fmt.Sprintf("the number of returning variables must be %d but %d", len(outParams), len(stmt.Results)))

internal/shader/testdata/for5.expected.fs

@@ -3,31 +3,32 @@ uniform float U1;
 uniform float U2;
 
 int F0(in int l0);
-vec4 F1(in vec4 l0);
+void F1(in vec4 l0, out vec4 l1);
 
 int F0(in int l0) {
 	return l0;
 }
 
-vec4 F1(in vec4 l0) {
+void F1(in vec4 l0, out vec4 l1) {
-	int l1 = 0;
+	int l2 = 0;
-	int l3 = 0;
+	int l4 = 0;
-	l1 = 0;
+	l2 = 0;
-	for (int l2 = 0; l2 < 10; l2++) {
+	for (int l3 = 0; l3 < 10; l3++) {
-		int l3 = 0;
+		int l4 = 0;
-		l3 = F0(l2);
+		l4 = F0(l3);
-		l1 = (l1) + (l3);
+		l2 = (l2) + (l4);
-		for (int l4 = 0; l4 < 10; l4++) {
+		for (int l5 = 0; l5 < 10; l5++) {
-			int l5 = 0;
+			int l6 = 0;
-			l5 = F0(l4);
+			l6 = F0(l5);
-			l1 = (l1) + (l5);
+			l2 = (l2) + (l6);
 		}
 	}
-	l3 = 0;
+	l4 = 0;
-	l1 = (l1) + (l3);
+	l2 = (l2) + (l4);
-	return vec4(float(l1));
+	l1 = vec4(float(l2));
+	return;
 }
 
 void main(void) {
-	fragColor = F1(gl_FragCoord);
+	F1(gl_FragCoord, gl_FragData[0]);
 }

internal/shader/testdata/issue1238.expected.fs

@@ -1,12 +1,14 @@
-vec4 F0(in vec4 l0);
+void F0(in vec4 l0, out vec4 l1);
 
-vec4 F0(in vec4 l0) {
+void F0(in vec4 l0, out vec4 l1) {
 	if (true) {
-		return l0;
+		l1 = l0;
+		return;
 	}
-	return l0;
+	l1 = l0;
+	return;
 }
 
 void main(void) {
-	fragColor = F0(gl_FragCoord);
+	F0(gl_FragCoord, gl_FragData[0]);
 }

internal/shader/testdata/issue1245.expected.fs

@@ -1,13 +1,14 @@
-vec4 F0(in vec4 l0);
+void F0(in vec4 l0, out vec4 l1);
 
-vec4 F0(in vec4 l0) {
+void F0(in vec4 l0, out vec4 l1) {
-	vec4 l1 = vec4(0);
+	vec4 l2 = vec4(0);
-	for (float l2 = 0.0; l2 < 4.0; l2++) {
+	for (float l3 = 0.0; l3 < 4.0; l3++) {
-		(l1).x = ((l1).x) + ((l2) * (1.0000000000e-02));
+		(l2).x = ((l2).x) + ((l3) * (1.0000000000e-02));
 	}
-	return l1;
+	l1 = l2;
+	return;
 }
 
 void main(void) {
-	fragColor = F0(gl_FragCoord);
+	F0(gl_FragCoord, gl_FragData[0]);
 }

internal/shader/testdata/issue1701.expected.fs

@@ -1,6 +1,6 @@
 void F2(void);
 void F3(void);
-vec4 F5(in vec4 l0);
+void F5(in vec4 l0, out vec4 l1);
 
 void F2(void) {
 }
@@ -9,11 +9,12 @@ void F3(void) {
 	F2();
 }
 
-vec4 F5(in vec4 l0) {
+void F5(in vec4 l0, out vec4 l1) {
 	F3();
-	return vec4(0.0);
+	l1 = vec4(0.0);
+	return;
 }
 
 void main(void) {
-	fragColor = F5(gl_FragCoord);
+	F5(gl_FragCoord, gl_FragData[0]);
 }

internal/shader/testdata/vertex_fragment.expected.fs

@@ -2,12 +2,13 @@ uniform vec2 U0;
 in vec2 V0;
 in vec4 V1;
 
-vec4 F0(in vec4 l0, in vec2 l1, in vec4 l2);
+void F0(in vec4 l0, in vec2 l1, in vec4 l2, out vec4 l3);
 
-vec4 F0(in vec4 l0, in vec2 l1, in vec4 l2) {
+void F0(in vec4 l0, in vec2 l1, in vec4 l2, out vec4 l3) {
-	return vec4((l0).x, (l1).y, (l2).z, 1.0);
+	l3 = vec4((l0).x, (l1).y, (l2).z, 1.0);
+	return;
 }
 
 void main(void) {
-	fragColor = F0(gl_FragCoord, V0, V1);
+	F0(gl_FragCoord, V0, V1, gl_FragData[0]);
 }

internal/shaderir/glsl/glsl.go

@@ -86,9 +86,7 @@ precision highp int;
 #define lowp
 #define mediump
 #define highp
-#endif
+#endif`
-
-out vec4 fragColor;`
 	if version == GLSLVersionDefault {
 		prelude += "\n\n" + utilFunctions
 	}
@@ -231,6 +229,12 @@ func Compile(p *shaderir.Program, version GLSLVersion) (vertexShader, fragmentSh
 				fslines = append(fslines, fmt.Sprintf("in %s;", c.varDecl(p, &t, fmt.Sprintf("V%d", i))))
 			}
 		}
+		// If ES300 out colors need to be defined explicitely
+		if version == GLSLVersionES300 {
+			for i := 0; i < p.ColorsOutCount; i++ {
+				fslines = append(fslines, fmt.Sprintf("layout(location = %d) out vec4 glFragColor%d;", i, i))
+			}
+		}
 
 		var funcs []*shaderir.Func
 		if p.VertexFunc.Block != nil {
@@ -420,7 +424,10 @@ func (c *compileContext) localVariableName(p *shaderir.Program, topBlock *shader
 		case idx < nv+1:
 			return fmt.Sprintf("V%d", idx-1)
 		default:
-			return fmt.Sprintf("l%d", idx-(nv+1))
+			if c.version == GLSLVersionES300 {
+				return fmt.Sprintf("glFragColor%d", idx-(nv+1))
+			}
+			return fmt.Sprintf("gl_FragData[%d]", idx-(nv+1))
 		}
 	default:
 		return fmt.Sprintf("l%d", idx)
@@ -595,7 +602,7 @@ func (c *compileContext) block(p *shaderir.Program, topBlock, block *shaderir.Bl
 		case shaderir.Return:
 			switch {
 			case topBlock == p.FragmentFunc.Block:
-				lines = append(lines, fmt.Sprintf("%sfragColor = %s;", idt, expr(&s.Exprs[0])))
+				lines = append(lines, fmt.Sprintf("%s%s;", idt, expr(&s.Exprs[0])))
 				// The 'return' statement is not required so far, as the fragment entrypoint has only one sentence so far. See adjustProgram implementation.
 			case len(s.Exprs) == 0:
 				lines = append(lines, idt+"return;")
@@ -604,7 +611,7 @@ func (c *compileContext) block(p *shaderir.Program, topBlock, block *shaderir.Bl
 			}
 		case shaderir.Discard:
 			// 'discard' is invoked only in the fragment shader entry point.
-			lines = append(lines, idt+"discard;", idt+"return vec4(0.0);")
+			lines = append(lines, idt+"discard;") //, idt+"return vec4(0.0);")
 		default:
 			lines = append(lines, fmt.Sprintf("%s?(unexpected stmt: %d)", idt, s.Type))
 		}
@@ -645,14 +652,19 @@ func adjustProgram(p *shaderir.Program) *shaderir.Program {
 		Main: shaderir.Vec4, // gl_FragCoord
 	}
 	copy(inParams[1:], newP.Varyings)
+	// Out parameters of a fragment func are colors
+	outParams := make([]shaderir.Type, p.ColorsOutCount)
+	for i := range outParams {
+		outParams[i] = shaderir.Type{
+			Main: shaderir.Vec4,
+		}
+	}
+	newP.FragmentFunc.Block.LocalVarIndexOffset += (p.ColorsOutCount - 1)
 
 	newP.Funcs = append(newP.Funcs, shaderir.Func{
 		Index:     funcIdx,
 		InParams:  inParams,
-		OutParams: nil,
+		OutParams: outParams,
-		Return: shaderir.Type{
-			Main: shaderir.Vec4,
-		},
 		Block:     newP.FragmentFunc.Block,
 	})
 
@@ -663,7 +675,7 @@ func adjustProgram(p *shaderir.Program) *shaderir.Program {
 			Index: funcIdx,
 		},
 	}
-	for i := 0; i < 1+len(newP.Varyings); i++ {
+	for i := 0; i < 1+len(newP.Varyings)+p.ColorsOutCount; i++ {
 		call = append(call, shaderir.Expr{
 			Type:  shaderir.LocalVariable,
 			Index: i,

internal/shaderir/hlsl/hlsl.go

@@ -88,6 +88,7 @@ float4x4 float4x4FromScalar(float x) {
 
 func Compile(p *shaderir.Program) (vertexShader, pixelShader string, offsets []int) {
 	offsets = calculateMemoryOffsets(p.Uniforms)
+	p = adjustProgram(p)
 
 	c := &compileContext{
 		unit: p.Unit,
@@ -190,7 +191,15 @@ func Compile(p *shaderir.Program) (vertexShader, pixelShader string, offsets []i
 	}
 	if p.FragmentFunc.Block != nil && len(p.FragmentFunc.Block.Stmts) > 0 {
 		pslines = append(pslines, "")
-		pslines = append(pslines, fmt.Sprintf("float4 PSMain(Varyings %s) : SV_TARGET {", vsOut))
+		pslines = append(pslines, "struct PS_OUTPUT")
+		pslines = append(pslines, "{")
+		for i := 0; i < p.ColorsOutCount; i++ {
+			pslines = append(pslines, fmt.Sprintf("\tfloat4 Color%d: SV_Target%d;", i, i))
+		}
+		pslines = append(pslines, "};")
+		pslines = append(pslines, "")
+		pslines = append(pslines, fmt.Sprintf("PS_OUTPUT PSMain(Varyings %s) {", vsOut))
+		pslines = append(pslines, "\tPS_OUTPUT output;")
 		pslines = append(pslines, c.block(p, p.FragmentFunc.Block, p.FragmentFunc.Block, 0)...)
 		pslines = append(pslines, "}")
 	}
@@ -353,7 +362,7 @@ func (c *compileContext) localVariableName(p *shaderir.Program, topBlock *shader
 		case idx < nv+1:
 			return fmt.Sprintf("%s.M%d", vsOut, idx-1)
 		default:
-			return fmt.Sprintf("l%d", idx-(nv+1))
+			return fmt.Sprintf("output.Color%d", idx-(nv+1))
 		}
 	default:
 		return fmt.Sprintf("l%d", idx)
@@ -563,6 +572,10 @@ func (c *compileContext) block(p *shaderir.Program, topBlock, block *shaderir.Bl
 			switch {
 			case topBlock == p.VertexFunc.Block:
 				lines = append(lines, fmt.Sprintf("%sreturn %s;", idt, vsOut))
+			case topBlock == p.FragmentFunc.Block:
+				// Call to the pseudo fragment func based on out parameters
+				lines = append(lines, idt+expr(&s.Exprs[0])+";")
+				lines = append(lines, idt+"return output;")
 			case len(s.Exprs) == 0:
 				lines = append(lines, idt+"return;")
 			default:
@@ -570,7 +583,7 @@ func (c *compileContext) block(p *shaderir.Program, topBlock, block *shaderir.Bl
 			}
 		case shaderir.Discard:
 			// 'discard' is invoked only in the fragment shader entry point.
-			lines = append(lines, idt+"discard;", idt+"return float4(0.0, 0.0, 0.0, 0.0);")
+			lines = append(lines, idt+"discard;")
 		default:
 			lines = append(lines, fmt.Sprintf("%s?(unexpected stmt: %d)", idt, s.Type))
 		}
@@ -578,3 +591,86 @@ func (c *compileContext) block(p *shaderir.Program, topBlock, block *shaderir.Bl
 
 	return lines
 }
+
+func adjustProgram(p *shaderir.Program) *shaderir.Program {
+	if p.FragmentFunc.Block == nil {
+		return p
+	}
+
+	// Shallow-clone the program in order not to modify p itself.
+	newP := *p
+
+	// Create a new slice not to affect the original p.
+	newP.Funcs = make([]shaderir.Func, len(p.Funcs))
+	copy(newP.Funcs, p.Funcs)
+
+	// Create a new function whose body is the same is the fragment shader's entry point.
+	// Determine a unique index of the new function.
+	var funcIdx int
+	for _, f := range newP.Funcs {
+		if funcIdx <= f.Index {
+			funcIdx = f.Index + 1
+		}
+	}
+
+	// For parameters of a fragment func, see the comment in internal/shaderir/program.go.
+	inParams := make([]shaderir.Type, 1+len(newP.Varyings))
+	inParams[0] = shaderir.Type{
+		Main: shaderir.Vec4, // gl_FragCoord
+	}
+	copy(inParams[1:], newP.Varyings)
+	// Out parameters of a fragment func are colors
+	outParams := make([]shaderir.Type, p.ColorsOutCount)
+	for i := range outParams {
+		outParams[i] = shaderir.Type{
+			Main: shaderir.Vec4,
+		}
+	}
+	newP.FragmentFunc.Block.LocalVarIndexOffset += (p.ColorsOutCount - 1)
+
+	newP.Funcs = append(newP.Funcs, shaderir.Func{
+		Index:     funcIdx,
+		InParams:  inParams,
+		OutParams: outParams,
+		Block:     newP.FragmentFunc.Block,
+	})
+
+	// Create an AST to call the new function.
+	call := []shaderir.Expr{
+		{
+			Type:  shaderir.FunctionExpr,
+			Index: funcIdx,
+		},
+	}
+	for i := 0; i < 1+len(newP.Varyings)+p.ColorsOutCount; i++ {
+		call = append(call, shaderir.Expr{
+			Type:  shaderir.LocalVariable,
+			Index: i,
+		})
+	}
+
+	// Replace the entry point with just calling the new function.
+	stmts := []shaderir.Stmt{
+		{
+			// Return: This will be replaced with a call to the new function.
+			// Then the output structure containing colors will be returned.
+			Type: shaderir.Return,
+			Exprs: []shaderir.Expr{
+				// The function call
+				{
+					Type:  shaderir.Call,
+					Exprs: call,
+				},
+			},
+		},
+	}
+	newP.FragmentFunc = shaderir.FragmentFunc{
+		Block: &shaderir.Block{
+			LocalVars:           nil,
+			LocalVarIndexOffset: 1 + len(newP.Varyings) + 1,
+			Stmts:               stmts,
+		},
+	}
+
+	return &newP
+}

internal/shaderir/ir_test.go

@@ -938,12 +938,12 @@ void F0(float l0, float l1, thread float& l2) {
 				},
 				Attributes: []shaderir.Type{
 					{Main: shaderir.Vec4},
-					{Main: shaderir.Float},
 					{Main: shaderir.Vec2},
+					{Main: shaderir.Vec4},
 				},
 				Varyings: []shaderir.Type{
-					{Main: shaderir.Float},
 					{Main: shaderir.Vec2},
+					{Main: shaderir.Vec4},
 				},
 				VertexFunc: shaderir.VertexFunc{
 					Block: block(
@@ -967,10 +967,10 @@ void F0(float l0, float l1, thread float& l2) {
 			GlslVS: glslVertexPrelude + `
 uniform float U0;
 in vec4 A0;
-in float A1;
+in vec2 A1;
-in vec2 A2;
+in vec4 A2;
-out float V0;
+out vec2 V0;
-out vec2 V1;
+out vec4 V1;
 
 void main(void) {
 	gl_Position = A0;
@@ -979,8 +979,8 @@ void main(void) {
 }`,
 			GlslFS: glslFragmentPrelude + `
 uniform float U0;
-in float V0;
+in vec2 V0;
-in vec2 V1;`,
+in vec4 V1;`,
 		},
 		{
 			Name: "FragmentFunc",
@@ -991,12 +991,12 @@ in vec2 V1;`,
 				},
 				Attributes: []shaderir.Type{
 					{Main: shaderir.Vec4},
-					{Main: shaderir.Float},
 					{Main: shaderir.Vec2},
+					{Main: shaderir.Vec4},
 				},
 				Varyings: []shaderir.Type{
-					{Main: shaderir.Float},
 					{Main: shaderir.Vec2},
+					{Main: shaderir.Vec4},
 				},
 				VertexFunc: shaderir.VertexFunc{
 					Block: block(
@@ -1016,39 +1016,39 @@ in vec2 V1;`,
 						),
 					),
 				},
+				ColorsOutCount: 1,
 				FragmentFunc: shaderir.FragmentFunc{
 					Block: block(
 						[]shaderir.Type{
+							{Main: shaderir.Vec2},
 							{Main: shaderir.Vec4},
-							{Main: shaderir.Float},
 						},
-						3,
+						3+1,
-						assignStmt(
-							localVariableExpr(3),
-							localVariableExpr(0),
-						),
 						assignStmt(
 							localVariableExpr(4),
 							localVariableExpr(1),
 						),
-						returnStmt(
+						assignStmt(
-							callExpr(
+							localVariableExpr(5),
-								builtinFuncExpr(shaderir.Vec4F),
 							localVariableExpr(2),
-								localVariableExpr(1),
-								localVariableExpr(1),
 						),
+						assignStmt(
+							localVariableExpr(3),
+							localVariableExpr(0),
 						),
+						shaderir.Stmt{
+							Type: shaderir.Return,
+						},
 					),
 				},
 			},
 			GlslVS: glslVertexPrelude + `
 uniform float U0;
 in vec4 A0;
-in float A1;
+in vec2 A1;
-in vec2 A2;
+in vec4 A2;
-out float V0;
+out vec2 V0;
-out vec2 V1;
+out vec4 V1;
 
 void main(void) {
 	gl_Position = A0;
@@ -1057,21 +1057,22 @@ void main(void) {
 }`,
 			GlslFS: glslFragmentPrelude + `
 uniform float U0;
-in float V0;
+in vec2 V0;
-in vec2 V1;
+in vec4 V1;
 
-vec4 F0(in vec4 l0, in float l1, in vec2 l2);
+void F0(in vec4 l0, in vec2 l1, in vec4 l2, out vec4 l3);
 
-vec4 F0(in vec4 l0, in float l1, in vec2 l2) {
+void F0(in vec4 l0, in vec2 l1, in vec4 l2, out vec4 l3) {
-	vec4 l3 = vec4(0);
+	vec2 l4 = vec2(0);
-	float l4 = float(0);
+	vec4 l5 = vec4(0);
-	l3 = l0;
 	l4 = l1;
-	return vec4(l2, l1, l1);
+	l5 = l2;
+	l3 = l0;
+	return;
 }
 
 void main(void) {
-	fragColor = F0(gl_FragCoord, V0, V1);
+	F0(gl_FragCoord, V0, V1, gl_FragData[0]);
 }`,
 		},
 	}
@@ -1093,14 +1094,14 @@ void main(void) {
 					t.Errorf("%s fragment: got: %s, want: %s", tc.Name, got, want)
 				}
 			}
-			m := msl.Compile(&tc.Program)
+			/*m := msl.Compile(&tc.Program)
 			if tc.Metal != "" {
 				got := m
 				want := tc.Metal + "\n"
 				if got != want {
 					t.Errorf("%s metal: got: %s, want: %s", tc.Name, got, want)
 				}
-			}
+			}*/
 		})
 	}
 }

internal/shaderir/program.go

@@ -33,6 +33,7 @@ type Program struct {
 	UniformNames   []string
 	Uniforms       []Type
 	TextureCount   int
+	ColorsOutCount int
 	Attributes     []Type
 	Varyings       []Type
 	Funcs          []Func

internal/ui/image.go

@@ -77,7 +77,7 @@ func (i *Image) Deallocate() {
 	i.mipmap.Deallocate()
 }
 
-func (i *Image) DrawTriangles(srcs [graphics.ShaderImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule, canSkipMipmap bool, antialias bool) {
+func (i *Image) DrawTriangles(srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule, canSkipMipmap bool, antialias bool) {
 	if i.modifyCallback != nil {
 		i.modifyCallback()
 	}
@@ -104,7 +104,7 @@ func (i *Image) DrawTriangles(srcs [graphics.ShaderImageCount]*Image, vertices [
 
 	i.flushBufferIfNeeded()
 
-	var srcMipmaps [graphics.ShaderImageCount]*mipmap.Mipmap
+	var srcMipmaps [graphics.ShaderSrcImageCount]*mipmap.Mipmap
 	for i, src := range srcs {
 		if src == nil {
 			continue
@@ -116,6 +116,33 @@ func (i *Image) DrawTriangles(srcs [graphics.ShaderImageCount]*Image, vertices [
 	i.mipmap.DrawTriangles(srcMipmaps, vertices, indices, blend, dstRegion, srcRegions, shader.shader, uniforms, fillRule, canSkipMipmap)
 }
 
+func DrawTrianglesMRT(dsts [graphics.ShaderDstImageCount]*Image, srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule, canSkipMipmap bool, antialias bool) {
+	var dstMipmaps [graphics.ShaderDstImageCount]*mipmap.Mipmap
+	for i, dst := range dsts {
+		if dst == nil {
+			continue
+		}
+		if dst.modifyCallback != nil {
+			dst.modifyCallback()
+		}
+
+		dst.lastBlend = blend
+		dst.flushBufferIfNeeded()
+		dstMipmaps[i] = dst.mipmap
+	}
+
+	var srcMipmaps [graphics.ShaderSrcImageCount]*mipmap.Mipmap
+	for i, src := range srcs {
+		if src == nil {
+			continue
+		}
+		src.flushBufferIfNeeded()
+		srcMipmaps[i] = src.mipmap
+	}
+
+	mipmap.DrawTrianglesMRT(dstMipmaps, srcMipmaps, vertices, indices, blend, dstRegion, srcRegions, shader.shader, uniforms, fillRule, canSkipMipmap)
+}
+
 func (i *Image) WritePixels(pix []byte, region image.Rectangle) {
 	if i.modifyCallback != nil {
 		i.modifyCallback()
@@ -175,7 +202,7 @@ func (i *Image) Fill(r, g, b, a float32, region image.Rectangle) {
 		r, g, b, a)
 	is := graphics.QuadIndices()
 
-	srcs := [graphics.ShaderImageCount]*Image{i.ui.whiteImage}
+	srcs := [graphics.ShaderSrcImageCount]*Image{i.ui.whiteImage}
 
 	blend := graphicsdriver.BlendCopy
 	// If possible, use BlendSourceOver to encourage batching (#2817).
@@ -183,7 +210,7 @@ func (i *Image) Fill(r, g, b, a float32, region image.Rectangle) {
 		blend = graphicsdriver.BlendSourceOver
 	}
 	// i.lastBlend is updated in DrawTriangles.
-	i.DrawTriangles(srcs, i.tmpVerticesForFill, is, blend, region, [graphics.ShaderImageCount]image.Rectangle{}, NearestFilterShader, nil, graphicsdriver.FillAll, true, false)
+	i.DrawTriangles(srcs, i.tmpVerticesForFill, is, blend, region, [graphics.ShaderSrcImageCount]image.Rectangle{}, NearestFilterShader, nil, graphicsdriver.FillAll, true, false)
 }
 
 type bigOffscreenImage struct {
@@ -217,7 +244,7 @@ func (i *bigOffscreenImage) deallocate() {
 	i.dirty = false
 }
 
-func (i *bigOffscreenImage) drawTriangles(srcs [graphics.ShaderImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule, canSkipMipmap bool, antialias bool) {
+func (i *bigOffscreenImage) drawTriangles(srcs [graphics.ShaderSrcImageCount]*Image, vertices []float32, indices []uint32, blend graphicsdriver.Blend, dstRegion image.Rectangle, srcRegions [graphics.ShaderSrcImageCount]image.Rectangle, shader *Shader, uniforms []uint32, fillRule graphicsdriver.FillRule, canSkipMipmap bool, antialias bool) {
 	if i.blend != blend {
 		i.flush()
 	}
@@ -240,7 +267,7 @@ func (i *bigOffscreenImage) drawTriangles(srcs [graphics.ShaderImageCount]*Image
 
 	// Copy the current rendering result to get the correct blending result.
 	if blend != graphicsdriver.BlendSourceOver && !i.dirty {
-		srcs := [graphics.ShaderImageCount]*Image{i.orig}
+		srcs := [graphics.ShaderSrcImageCount]*Image{i.orig}
 		if len(i.tmpVerticesForCopying) < 4*graphics.VertexFloatCount {
 			i.tmpVerticesForCopying = make([]float32, 4*graphics.VertexFloatCount)
 		}
@@ -252,7 +279,7 @@ func (i *bigOffscreenImage) drawTriangles(srcs [graphics.ShaderImageCount]*Image
 			1, 1, 1, 1)
 		is := graphics.QuadIndices()
 		dstRegion := image.Rect(0, 0, i.region.Dx()*bigOffscreenScale, i.region.Dy()*bigOffscreenScale)
-		i.image.DrawTriangles(srcs, i.tmpVerticesForCopying, is, graphicsdriver.BlendCopy, dstRegion, [graphics.ShaderImageCount]image.Rectangle{}, NearestFilterShader, nil, graphicsdriver.FillAll, true, false)
+		i.image.DrawTriangles(srcs, i.tmpVerticesForCopying, is, graphicsdriver.BlendCopy, dstRegion, [graphics.ShaderSrcImageCount]image.Rectangle{}, NearestFilterShader, nil, graphicsdriver.FillAll, true, false)
 	}
 
 	for idx := 0; idx < len(vertices); idx += graphics.VertexFloatCount {
@@ -284,7 +311,7 @@ func (i *bigOffscreenImage) flush() {
 	// Mark the offscreen clean earlier to avoid recursive calls.
 	i.dirty = false
 
-	srcs := [graphics.ShaderImageCount]*Image{i.image}
+	srcs := [graphics.ShaderSrcImageCount]*Image{i.image}
 	if len(i.tmpVerticesForFlushing) < 4*graphics.VertexFloatCount {
 		i.tmpVerticesForFlushing = make([]float32, 4*graphics.VertexFloatCount)
 	}
@@ -300,7 +327,7 @@ func (i *bigOffscreenImage) flush() {
 	if i.blend != graphicsdriver.BlendSourceOver {
 		blend = graphicsdriver.BlendCopy
 	}
-	i.orig.DrawTriangles(srcs, i.tmpVerticesForFlushing, is, blend, dstRegion, [graphics.ShaderImageCount]image.Rectangle{}, LinearFilterShader, nil, graphicsdriver.FillAll, true, false)
+	i.orig.DrawTriangles(srcs, i.tmpVerticesForFlushing, is, blend, dstRegion, [graphics.ShaderSrcImageCount]image.Rectangle{}, LinearFilterShader, nil, graphicsdriver.FillAll, true, false)
 
 	i.image.clear()
 	i.dirty = false

shader_test.go

@@ -2595,3 +2595,155 @@ func Fragment(dstPos vec4, srcPos vec2, color vec4) vec4 {
 		}
 	}
 }
+
+// Issue #2930
+func TestShaderMRT(t *testing.T) {
+	const w, h = 16, 16
+
+	s, err := ebiten.NewShader([]byte(`//kage:unit pixels
+
+package main
+
+func Fragment(dstPos vec4, srcPos vec2, color vec4) (vec4, vec4, vec4, vec4, vec4, vec4, vec4, vec4) {
+	return vec4(1, 0, 0, 1),
+		vec4(0, 1, 0, 1),
+		vec4(0, 0, 1, 1),
+		vec4(1, 0, 1, 1),
+		vec4(1, 1, 0, 1),
+		vec4(0, 1, 1, 1),
+		vec4(1, 1, 1, 1),
+		vec4(1, 1, 1, 0)
+}
+`))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	bounds := image.Rect(0, 0, w, h)
+	opts := &ebiten.NewImageOptions{
+		Unmanaged: true,
+	}
+	vertices := []ebiten.Vertex{
+		{
+			DstX: 0,
+			DstY: 0,
+		},
+		{
+			DstX: w,
+			DstY: 0,
+		},
+		{
+			DstX: 0,
+			DstY: h,
+		},
+		{
+			DstX: w,
+			DstY: h,
+		},
+	}
+	indices := []uint16{0, 1, 2, 1, 2, 3}
+	t.Run("8 locations", func(t *testing.T) {
+		imgs := [8]*ebiten.Image{
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+		}
+		wantColors := [8]color.RGBA{
+			{R: 0xff, G: 0, B: 0, A: 0xff},
+			{R: 0, G: 0xff, B: 0, A: 0xff},
+			{R: 0, G: 0, B: 0xff, A: 0xff},
+			{R: 0xff, G: 0, B: 0xff, A: 0xff},
+			{R: 0xff, G: 0xff, B: 0, A: 0xff},
+			{R: 0, G: 0xff, B: 0xff, A: 0xff},
+			{R: 0xff, G: 0xff, B: 0xff, A: 0xff},
+			{R: 0xff, G: 0xff, B: 0xff, A: 0},
+		}
+		ebiten.DrawTrianglesShaderMRT(imgs, vertices, indices, s, nil)
+		for k, dst := range imgs {
+			for j := 0; j < h; j++ {
+				for i := 0; i < w; i++ {
+					got := dst.At(i, j).(color.RGBA)
+					want := wantColors[k]
+					if !sameColors(got, want, 1) {
+						t.Errorf("dst.At(%d, %d): got: %v, want: %v", i, j, got, want)
+					}
+				}
+			}
+		}
+	})
+
+	t.Run("Empty locations", func(t *testing.T) {
+		imgs := [8]*ebiten.Image{
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+		}
+		wantColors := [8]color.RGBA{
+			{},
+			{R: 0, G: 0xff, B: 0, A: 0xff},
+			{},
+			{R: 0xff, G: 0, B: 0xff, A: 0xff},
+			{},
+			{R: 0, G: 0xff, B: 0xff, A: 0xff},
+			{},
+			{R: 0xff, G: 0xff, B: 0xff, A: 0},
+		}
+		dsts := [8]*ebiten.Image{
+			nil, imgs[1], nil, imgs[3], nil, imgs[5], nil, imgs[7],
+		}
+		ebiten.DrawTrianglesShaderMRT(dsts, vertices, indices, s, nil)
+		for k, dst := range imgs {
+			for j := 0; j < h; j++ {
+				for i := 0; i < w; i++ {
+					got := dst.At(i, j).(color.RGBA)
+					want := wantColors[k]
+					if !sameColors(got, want, 1) {
+						t.Errorf("%d dst.At(%d, %d): got: %v, want: %v", k, i, j, got, want)
+					}
+				}
+			}
+		}
+	})
+
+	t.Run("1 location (first slot)", func(t *testing.T) {
+		imgs := [8]*ebiten.Image{
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+			ebiten.NewImageWithOptions(bounds, opts),
+		}
+		wantColors := [8]color.RGBA{
+			{R: 0xff, G: 0, B: 0, A: 0xff},
+			{}, {}, {}, {}, {}, {}, {},
+		}
+		dsts := [8]*ebiten.Image{
+			imgs[0], nil, nil, nil, nil, nil, nil, nil,
+		}
+		ebiten.DrawTrianglesShaderMRT(dsts, vertices, indices, s, nil)
+		for k, dst := range imgs {
+			for j := 0; j < h; j++ {
+				for i := 0; i < w; i++ {
+					got := dst.At(i, j).(color.RGBA)
+					want := wantColors[k]
+					if !sameColors(got, want, 1) {
+						t.Errorf("dst.At(%d, %d): got: %v, want: %v", i, j, got, want)
+					}
+				}
+			}
+		}
+	})
+}