ebiten: guarantee invalid color values are not clamped

Closes #2798

image.go

@@ -587,6 +587,9 @@ var _ [len(DrawTrianglesShaderOptions{}.Images) - graphics.ShaderImageCount]stru
 //
 // 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 (i *Image) DrawTrianglesShader(vertices []Vertex, indices []uint16, shader *Shader, options *DrawTrianglesShaderOptions) {
 	i.copyCheck()
@@ -742,6 +745,9 @@ var _ [len(DrawRectShaderOptions{}.Images)]struct{} = [graphics.ShaderImageCount
 // If no source images are specified, imageSrc0Size returns a valid size only when the unit is pixels,
 // but always returns 0 when the unit is texels (default).
 //
+// 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, DrawRectShader does nothing.
 func (i *Image) DrawRectShader(width, height int, shader *Shader, options *DrawRectShaderOptions) {
 	i.copyCheck()
@@ -954,6 +960,9 @@ func (i *Image) at(x, y int) (r, g, b, a byte) {
 //
 // Set implements the standard draw.Image's Set.
 //
+// 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.
+//
 // If the image is disposed, Set does nothing.
 func (i *Image) Set(x, y int, clr color.Color) {
 	i.copyCheck()
@@ -1028,6 +1037,9 @@ func (i *Image) Deallocate() {
 //
 // WritePixels also works on a sub-image.
 //
+// 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 is disposed, WritePixels does nothing.
 func (i *Image) WritePixels(pixels []byte) {
 	i.copyCheck()

image_test.go

@@ -4582,3 +4582,44 @@ func TestImageDrawImageAfterDeallocation(t *testing.T) {
 		}
 	}
 }
+
+// Issue #2798
+func TestImageInvalidPremultipliedAlphaColor(t *testing.T) {
+	// This test checks the rendering result when Set and WritePixels use an invalid premultiplied alpha color.
+	// The result values are kept and not clamped.
+
+	const (
+		w = 16
+		h = 16
+	)
+
+	dst := ebiten.NewImage(w, h)
+	dst.Set(0, 0, color.RGBA{R: 0xff, G: 0xc0, B: 0x80, A: 0x40})
+	dst.Set(0, 1, color.RGBA{R: 0xff, G: 0xc0, B: 0x80, A: 0x00})
+	if got, want := dst.At(0, 0).(color.RGBA), (color.RGBA{R: 0xff, G: 0xc0, B: 0x80, A: 0x40}); got != want {
+		t.Errorf("got: %v, want: %v", got, want)
+	}
+	if got, want := dst.At(0, 1).(color.RGBA), (color.RGBA{R: 0xff, G: 0xc0, B: 0x80, A: 0x00}); got != want {
+		t.Errorf("got: %v, want: %v", got, want)
+	}
+
+	pix := make([]byte, 4*w*h)
+	for j := 0; j < h; j++ {
+		for i := 0; i < w; i++ {
+			pix[4*(j*16+i)] = byte(i)
+			pix[4*(j*16+i)+1] = byte(j)
+			pix[4*(j*16+i)+2] = 0x80
+			pix[4*(j*16+i)+3] = byte(i - j)
+		}
+	}
+	dst.WritePixels(pix)
+	for j := 0; j < h; j++ {
+		for i := 0; i < w; i++ {
+			got := dst.At(i, j)
+			want := color.RGBA{R: byte(i), G: byte(j), B: 0x80, A: byte(i - j)}
+			if got != want {
+				t.Errorf("At(%d, %d): got: %v, want: %v", i, j, got, want)
+			}
+		}
+	}
+}

shader_test.go

@@ -2429,3 +2429,61 @@ func Fragment(dstPos vec4, srcPos vec2, color vec4) vec4 {
 		}
 	}
 }
+
+// Issue #2798
+func TestShaderInvalidPremultipliedAlphaColor(t *testing.T) {
+	// This test checks the rendering result when the shader returns an invalid premultiplied alpha color.
+	// The result values are kept and not clamped.
+
+	const w, h = 16, 16
+
+	dst := ebiten.NewImage(w, h)
+	s, err := ebiten.NewShader([]byte(`//kage:unit pixels
+
+package main
+
+func Fragment(dstPos vec4, srcPos vec2, color vec4) vec4 {
+	return vec4(1, 0.75, 0.5, 0.25)
+}
+`))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	dst.DrawRectShader(w, h, s, nil)
+
+	for j := 0; j < h; j++ {
+		for i := 0; i < w; i++ {
+			got := dst.At(i, j).(color.RGBA)
+			want := color.RGBA{R: 0xff, G: 0xc0, B: 0x80, A: 0x40}
+			if !sameColors(got, want, 2) {
+				t.Errorf("dst.At(%d, %d): got: %v, want: %v", i, j, got, want)
+			}
+		}
+	}
+
+	dst.Clear()
+	s, err = ebiten.NewShader([]byte(`//kage:unit pixels
+
+package main
+
+func Fragment(dstPos vec4, srcPos vec2, color vec4) vec4 {
+	return vec4(1, 0.75, 0.5, 0)
+}
+`))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	dst.DrawRectShader(w, h, s, nil)
+
+	for j := 0; j < h; j++ {
+		for i := 0; i < w; i++ {
+			got := dst.At(i, j).(color.RGBA)
+			want := color.RGBA{R: 0xff, G: 0xc0, B: 0x80, A: 0}
+			if !sameColors(got, want, 2) {
+				t.Errorf("dst.At(%d, %d): got: %v, want: %v", i, j, got, want)
+			}
+		}
+	}
+}