ebiten: Add texture[ID]Size functions for shaders

Fixes #1239

internal/graphics/vertex.go

@@ -26,9 +26,17 @@ const (
 	//
 	// All the preversed uniform variables are vec2 so far.
 	PreservedUniformVariablesNum = 1 + // the destination texture size
+		ShaderImageNum + // the texture sizes
 		(ShaderImageNum - 1) // the offsets of the second and the following images
 )
 
+func TextureOffsetUniformVariableIndex(i int) int {
+	if i == 0 {
+		panic("graphics: the texture 0 doesn't have its offset")
+	}
+	return 1 + ShaderImageNum + i - 1
+}
+
 const (
 	IndicesNum     = (1 << 16) / 3 * 3 // Adjust num for triangles.
 	VertexFloatNum = 8

internal/graphicsdriver/opengl/graphics.go

@@ -304,8 +304,19 @@ func (g *Graphics) DrawShader(dst driver.ImageID, srcs [graphics.ShaderImageNum]
 	us[0].name = "U0"
 	us[0].value = []float32{float32(vw), float32(vh)}
 
-	for i, o := range offsets {
+	for i, src := range srcs {
+		img := g.images[src]
+		var w, h int
+		if img != nil {
+			w, h = img.framebufferSize()
+		}
 		const offset = 1
+		us[i+offset].name = fmt.Sprintf("U%d", i+offset)
+		us[i+offset].value = []float32{float32(w), float32(h)}
+	}
+
+	for i, o := range offsets {
+		const offset = 1 + graphics.ShaderImageNum
 		o := o
 		us[i+offset].name = fmt.Sprintf("U%d", i+offset)
 		us[i+offset].value = o[:]

internal/graphicsdriver/opengl/image.go

@@ -71,20 +71,26 @@ func (i *Image) Pixels() ([]byte, error) {
 	return p, nil
 }
 
+func (i *Image) framebufferSize() (int, int) {
+	if i.screen {
+		// The (default) framebuffer size can't be converted to a power of 2.
+		// On browsers, i.width and i.height are used as viewport size and
+		// Edge can't treat a bigger viewport than the drawing area (#71).
+		return i.width, i.height
+	}
+	return graphics.InternalImageSize(i.width), graphics.InternalImageSize(i.height)
+}
+
 func (i *Image) ensureFramebuffer() error {
 	if i.framebuffer != nil {
 		return nil
 	}
 
+	w, h := i.framebufferSize()
 	if i.screen {
-		// The (default) framebuffer size can't be converted to a power of 2.
-		// On browsers, c.width and c.height are used as viewport size and
-		// Edge can't treat a bigger viewport than the drawing area (#71).
-		i.framebuffer = newScreenFramebuffer(&i.graphics.context, i.width, i.height)
+		i.framebuffer = newScreenFramebuffer(&i.graphics.context, w, h)
 		return nil
 	}
-
-	w, h := graphics.InternalImageSize(i.width), graphics.InternalImageSize(i.height)
 	f, err := newFramebufferFromTexture(&i.graphics.context, i.textureNative, w, h)
 	if err != nil {
 		return err

internal/testing/shader.go

@@ -351,7 +351,7 @@ func ShaderProgramImages(imageNum int) shaderir.Program {
 					texPos,
 					{
 						Type:  shaderir.UniformVariable,
-						Index: 1 + i - 1,
+						Index: graphics.TextureOffsetUniformVariableIndex(i),
 					},
 				},
 			}

shader.go

@@ -36,6 +36,16 @@ func textureDstSize() vec2 {
 }
 `
 
+	for i := 0; i < graphics.ShaderImageNum; i++ {
+		shaderSuffix += fmt.Sprintf(`
+var __textureSize%[1]d vec2
+
+func texture%[1]dSize() vec2 {
+	return __textureSize%[1]d
+}
+`, i)
+	}
+
 	for i := 0; i < graphics.ShaderImageNum; i++ {
 		var offset string
 		if i >= 1 {