Bug fix: Consider color.Color.RGBA returns alpha-premultiplied values

colormatrix.go

@@ -17,7 +17,6 @@ limitations under the License.
 package ebiten
 
 import (
-	"image/color"
 	"math"
 )
 
@@ -81,28 +80,26 @@ func Monochrome() ColorMatrix {
 	}
 }
 
-// ScaleColor returns a color matrix that scales a color matrix by clr.
-func ScaleColor(clr color.Color) ColorMatrix {
-	rf, gf, bf, af := rgba(clr)
+// ScaleColor returns a color matrix that scales a color matrix by the given color (r, g, b, a).
+func ScaleColor(r, g, b, a float64) ColorMatrix {
 	return ColorMatrix{
 		[ColorMatrixDim - 1][ColorMatrixDim]float64{
-			{rf, 0, 0, 0, 0},
-			{0, gf, 0, 0, 0},
-			{0, 0, bf, 0, 0},
-			{0, 0, 0, af, 0},
+			{r, 0, 0, 0, 0},
+			{0, g, 0, 0, 0},
+			{0, 0, b, 0, 0},
+			{0, 0, 0, a, 0},
 		},
 	}
 }
 
-// TranslateColor returns a color matrix that translates a color matrix by clr.
-func TranslateColor(clr color.Color) ColorMatrix {
-	rf, gf, bf, af := rgba(clr)
+// TranslateColor returns a color matrix that translates a color matrix by the given color (r, g, b, a).
+func TranslateColor(r, g, b, a float64) ColorMatrix {
 	return ColorMatrix{
 		[ColorMatrixDim - 1][ColorMatrixDim]float64{
-			{1, 0, 0, 0, rf},
-			{0, 1, 0, 0, gf},
-			{0, 0, 1, 0, bf},
-			{0, 0, 0, 1, af},
+			{1, 0, 0, 0, r},
+			{0, 1, 0, 0, g},
+			{0, 0, 1, 0, b},
+			{0, 0, 0, 1, a},
 		},
 	}
 }
@@ -123,11 +120,11 @@ func RotateHue(theta float64) ColorMatrix {
 	}
 }
 
-func rgba(clr color.Color) (float64, float64, float64, float64) {
-	r, g, b, a := clr.RGBA()
-	rf := float64(r) / float64(math.MaxUint16)
-	gf := float64(g) / float64(math.MaxUint16)
-	bf := float64(b) / float64(math.MaxUint16)
-	af := float64(a) / float64(math.MaxUint16)
+func rgba(r, g, b, a uint8) (float64, float64, float64, float64) {
+	const max = math.MaxUint8
+	rf := float64(r) / max
+	gf := float64(g) / max
+	bf := float64(b) / max
+	af := float64(a) / max
 	return rf, gf, bf, af
 }

ebitenutil/debugprint.go

@@ -18,6 +18,7 @@ package ebitenutil
 
 import (
 	"github.com/hajimehoshi/ebiten"
+	"github.com/hajimehoshi/ebiten/internal"
 	"github.com/hajimehoshi/ebiten/internal/assets"
 	"image/color"
 )
@@ -34,7 +35,7 @@ func DebugPrint(r *ebiten.RenderTarget, str string) {
 	defaultDebugPrintState.DebugPrint(r, str)
 }
 
-func (d *debugPrintState) drawText(r *ebiten.RenderTarget, str string, x, y int, clr color.Color) {
+func (d *debugPrintState) drawText(rt *ebiten.RenderTarget, str string, x, y int, c color.Color) {
 	parts := []ebiten.ImagePart{}
 	locationX, locationY := 0, 0
 	for _, c := range str {
@@ -53,11 +54,10 @@ func (d *debugPrintState) drawText(r *ebiten.RenderTarget, str string, x, y int,
 		})
 		locationX += assets.TextImageCharWidth
 	}
-	geom := ebiten.GeometryMatrixI()
-	geom.Concat(ebiten.TranslateGeometry(float64(x)+1, float64(y)))
-	clrm := ebiten.ColorMatrixI()
-	clrm.Concat(ebiten.ScaleColor(clr))
-	r.DrawImage(d.textTexture, parts, geom, clrm)
+	geo := ebiten.TranslateGeometry(float64(x)+1, float64(y))
+	r, g, b, a := internal.RGBA(c)
+	clr := ebiten.ScaleColor(r, g, b, a)
+	rt.DrawImage(d.textTexture, parts, geo, clr)
 }
 
 func (d *debugPrintState) DebugPrint(r *ebiten.RenderTarget, str string) {
@@ -79,6 +79,6 @@ func (d *debugPrintState) DebugPrint(r *ebiten.RenderTarget, str string) {
 			panic(err)
 		}
 	}
-	d.drawText(r, str, 1, d.y+1, &color.RGBA{0x00, 0x00, 0x00, 0x80})
-	d.drawText(r, str, 0, d.y, &color.RGBA{0xff, 0xff, 0xff, 0xff})
+	d.drawText(r, str, 1, d.y+1, color.NRGBA{0x00, 0x00, 0x00, 0x80})
+	d.drawText(r, str, 0, d.y, color.NRGBA{0xff, 0xff, 0xff, 0xff})
 }

example/alphablending/main.go

@@ -52,13 +52,13 @@ func (g *Game) Update(r *ebiten.RenderTarget) error {
 	}
 	for i := 0; i < 10; i++ {
 		geo := ebiten.TranslateGeometry(15+float64(i)*(diff), 20)
-		clr := ebiten.ScaleColor(color.RGBA{0xff, 0xff, 0xff, 0x80})
+		clr := ebiten.ScaleColor(1.0, 1.0, 1.0, 0.5)
 		if err := ebiten.DrawWholeImage(g.tmpRenderTarget, g.ebitenImage, geo, clr); err != nil {
 			return err
 		}
 	}
 
-	r.Fill(color.RGBA{0x00, 0x00, 0x80, 0xff})
+	r.Fill(color.NRGBA{0x00, 0x00, 0x80, 0xff})
 	for i := 0; i < 10; i++ {
 		geo := ebiten.TranslateGeometry(0, float64(i)*(diff))
 		clr := ebiten.ColorMatrixI()

example/blocks/blocks/font.go

@@ -19,6 +19,7 @@ package blocks
 import (
 	"github.com/hajimehoshi/ebiten"
 	"image/color"
+	"math"
 )
 
 func init() {
@@ -32,7 +33,7 @@ func textWidth(str string) int {
 	return charWidth * len(str)
 }
 
-func drawText(r *ebiten.RenderTarget, images *Images, str string, ox, oy, scale int, clr color.Color) {
+func drawText(rt *ebiten.RenderTarget, images *Images, str string, ox, oy, scale int, c color.Color) {
 	fontImageId := images.GetImage("font")
 	parts := []ebiten.ImagePart{}
 
@@ -53,13 +54,19 @@ func drawText(r *ebiten.RenderTarget, images *Images, str string, ox, oy, scale
 		locationX += charWidth
 	}
 
-	geoMat := ebiten.ScaleGeometry(float64(scale), float64(scale))
-	geoMat.Concat(ebiten.TranslateGeometry(float64(ox), float64(oy)))
-	clrMat := ebiten.ScaleColor(clr)
-	r.DrawImage(fontImageId, parts, geoMat, clrMat)
+	geo := ebiten.ScaleGeometry(float64(scale), float64(scale))
+	geo.Concat(ebiten.TranslateGeometry(float64(ox), float64(oy)))
+	c2 := color.NRGBA64Model.Convert(c).(color.NRGBA64)
+	const max = math.MaxUint16
+	r := float64(c2.R) / max
+	g := float64(c2.G) / max
+	b := float64(c2.B) / max
+	a := float64(c2.A) / max
+	clr := ebiten.ScaleColor(r, g, b, a)
+	rt.DrawImage(fontImageId, parts, geo, clr)
 }
 
-func drawTextWithShadow(r *ebiten.RenderTarget, images *Images, str string, x, y, scale int, clr color.Color) {
-	drawText(r, images, str, x+1, y+1, scale, color.RGBA{0, 0, 0, 0x80})
-	drawText(r, images, str, x, y, scale, clr)
+func drawTextWithShadow(rt *ebiten.RenderTarget, images *Images, str string, x, y, scale int, clr color.Color) {
+	drawText(rt, images, str, x+1, y+1, scale, color.NRGBA{0, 0, 0, 0x80})
+	drawText(rt, images, str, x, y, scale, clr)
 }

example/blocks/blocks/gamescene.go

@@ -116,7 +116,7 @@ func (s *GameScene) Draw(r *ebiten.RenderTarget, images *Images) {
 	w, h := field.Size()
 	geoMat := ebiten.ScaleGeometry(float64(fieldWidth)/float64(w), float64(fieldHeight)/float64(h))
 	geoMat.Concat(ebiten.TranslateGeometry(20, 20)) // TODO: magic number?
-	colorMat := ebiten.ScaleColor(color.RGBA{0, 0, 0, 0x80})
+	colorMat := ebiten.ScaleColor(0.0, 0.0, 0.0, 0.5)
 	ebiten.DrawWholeImage(r, field, geoMat, colorMat)
 
 	geoMat = ebiten.GeometryMatrixI()

example/blocks/blocks/titlescene.go

@@ -47,7 +47,7 @@ func (s *TitleScene) Draw(r *ebiten.RenderTarget, images *Images) {
 	message := "PRESS SPACE TO START"
 	x := (ScreenWidth - textWidth(message)) / 2
 	y := ScreenHeight - 48
-	drawTextWithShadow(r, images, message, x, y, 1, color.RGBA{0x80, 0, 0, 0xff})
+	drawTextWithShadow(r, images, message, x, y, 1, color.NRGBA{0x80, 0, 0, 0xff})
 }
 
 func drawTitleBackground(r *ebiten.RenderTarget, images *Images, c int) {
@@ -78,5 +78,5 @@ func drawLogo(r *ebiten.RenderTarget, images *Images, str string) {
 	textWidth := textWidth(str) * scale
 	x := (ScreenWidth - textWidth) / 2
 	y := 32
-	drawTextWithShadow(r, images, str, x, y, scale, color.RGBA{0x00, 0x00, 0x80, 0xff})
+	drawTextWithShadow(r, images, str, x, y, scale, color.NRGBA{0x00, 0x00, 0x80, 0xff})
 }

example/paint/main.go

@@ -51,7 +51,7 @@ func (g *Game) Update(r *ebiten.RenderTarget) error {
 
 	if ebiten.IsMouseButtonPressed(ebiten.MouseButtonLeft) {
 		geo := ebiten.TranslateGeometry(float64(mx), float64(my))
-		clr := ebiten.ScaleColor(color.RGBA{0xff, 0x40, 0x40, 0xff})
+		clr := ebiten.ScaleColor(1.0, 0.25, 0.25, 1.0)
 		theta := 2.0 * math.Pi * float64(g.count%60) / 60.0
 		clr.Concat(ebiten.RotateHue(theta))
 		ebiten.DrawWholeImage(g.canvasRenderTarget, g.brushRenderTarget.Image(), geo, clr)

graphics.go

@@ -53,6 +53,7 @@ const (
 )
 
 // An Image represents an image to be rendered.
+// An image's pixels are stored as non alpha-premultiplied.
 type Image struct {
 	glTexture *opengl.Texture
 }

internal/math.go

@@ -16,6 +16,11 @@ limitations under the License.
 
 package internal
 
+import (
+	"image/color"
+	"math"
+)
+
 func NextPowerOf2(x uint64) uint64 {
 	x -= 1
 	x |= (x >> 1)
@@ -30,3 +35,13 @@ func NextPowerOf2(x uint64) uint64 {
 func NextPowerOf2Int(size int) int {
 	return int(NextPowerOf2(uint64(size)))
 }
+
+func RGBA(clr color.Color) (r, g, b, a float64) {
+	clr2 := color.NRGBA64Model.Convert(clr).(color.NRGBA64)
+	const max = math.MaxUint16
+	r = float64(clr2.R) / max
+	g = float64(clr2.G) / max
+	b = float64(clr2.B) / max
+	a = float64(clr2.A) / max
+	return
+}

internal/opengl/internal/shader/shader.go

@@ -87,7 +87,7 @@ void main(void) {
   color0 = (color_matrix * color0) + color_matrix_translation;
 
   // Photoshop-like RGBA blending
-  gl_FragColor.a = color0.a + color1.a - color0.a * color1.a;
+  gl_FragColor.a = color0.a + (1.0 - color0.a) * color1.a;
   gl_FragColor.rgb = (color0.a * color0.rgb + (1.0 - color0.a) * color1.a * color1.rgb) / gl_FragColor.a;
 }
 `,

rendertarget.go

@@ -22,7 +22,6 @@ import (
 	"github.com/hajimehoshi/ebiten/internal/opengl"
 	"github.com/hajimehoshi/ebiten/internal/opengl/internal/shader"
 	"image/color"
-	"math"
 )
 
 type innerRenderTarget struct {
@@ -50,20 +49,15 @@ func (r *innerRenderTarget) size() (width, height int) {
 }
 
 func (r *innerRenderTarget) Clear() error {
-	return r.Fill(color.RGBA{0, 0, 0, 0})
+	return r.Fill(color.Transparent)
 }
 
 func (r *innerRenderTarget) Fill(clr color.Color) error {
 	if err := r.glRenderTarget.SetAsViewport(); err != nil {
 		return err
 	}
-	const max = math.MaxUint16
-	cr, cg, cb, ca := clr.RGBA()
-	rf := gl.GLclampf(float64(cr) / max)
-	gf := gl.GLclampf(float64(cg) / max)
-	bf := gl.GLclampf(float64(cb) / max)
-	af := gl.GLclampf(float64(ca) / max)
-	gl.ClearColor(rf, gf, bf, af)
+	rf, gf, bf, af := internal.RGBA(clr)
+	gl.ClearColor(gl.GLclampf(rf), gl.GLclampf(gf), gl.GLclampf(bf), gl.GLclampf(af))
 	gl.Clear(gl.COLOR_BUFFER_BIT)
 	return nil
 }