ebiten: add premultipliedAlpha flag to imageToBytes

This is a preparation for ColorEncoding option at NewImageFromImage. Updates #3314
2025-12-24 13:08:06 +08:00 · 2025-11-24 18:36:13 +09:00
parent 43f1312136
commit 869376395c
3 changed files with 137 additions and 52 deletions
--- a/image.go
+++ b/image.go
@@ -1511,7 +1511,7 @@ func NewImageFromImageWithOptions(source image.Image, options *NewImageFromImage
 		return i
 	}

-	i.WritePixels(imageToBytes(source))
+	i.WritePixels(imageToBytes(source, true))
 	return i
 }

--- a/imagetobytes.go
+++ b/imagetobytes.go
@@ -21,12 +21,13 @@ import (
 )

 // imageToBytes gets RGBA bytes from img.
+// premultipliedAlpha specifies whether the returned bytes are in premultiplied alpha format or not.
 //
 // Basically imageToBytes just calls draw.Draw.
 // If img is a paletted image, an optimized copying method is used.
 //
-// If img is *image.RGBA and its length is same as 4*width*height, imageToBytes returns its Pix.
-func imageToBytes(img image.Image) []byte {
+// imageToBytes might return img.Pix directly without copying when possible.
+func imageToBytes(img image.Image, premultipliedAlpha bool) []byte {
 	size := img.Bounds().Size()
 	w, h := size.X, size.Y

@@ -41,48 +42,70 @@ func imageToBytes(img image.Image) []byte {
 		y1 := b.Max.Y

 		palette := make([]uint8, len(img.Palette)*4)
-		for i, c := range img.Palette {
-			// Create a temporary slice to reduce boundary checks.
-			pl := palette[4*i : 4*i+4]
-			rgba := color.RGBAModel.Convert(c).(color.RGBA)
-			pl[0] = rgba.R
-			pl[1] = rgba.G
-			pl[2] = rgba.B
-			pl[3] = rgba.A
+		if premultipliedAlpha {
+			for i, c := range img.Palette {
+				// Create a temporary slice to reduce boundary checks.
+				pl := palette[4*i : 4*i+4]
+				rgba := color.RGBAModel.Convert(c).(color.RGBA)
+				pl[0] = rgba.R
+				pl[1] = rgba.G
+				pl[2] = rgba.B
+				pl[3] = rgba.A
+			}
+		} else {
+			for i, c := range img.Palette {
+				// Create a temporary slice to reduce boundary checks.
+				pl := palette[4*i : 4*i+4]
+				nrgba := color.NRGBAModel.Convert(c).(color.NRGBA)
+				pl[0] = nrgba.R
+				pl[1] = nrgba.G
+				pl[2] = nrgba.B
+				pl[3] = nrgba.A
+			}
 		}
 		// Even img is a subimage of another image, Pix starts with 0-th index.
-		idx0 := 0
-		idx1 := 0
+		var srcIdx, dstIdx int
 		d := img.Stride - (x1 - x0)
-		for j := 0; j < y1-y0; j++ {
-			for i := 0; i < x1-x0; i++ {
-				p := int(img.Pix[idx0])
-				copy(bs[idx1:idx1+4], palette[4*p:4*p+4])
-				idx0++
-				idx1 += 4
+		for range y1 - y0 {
+			for range x1 - x0 {
+				p := int(img.Pix[srcIdx])
+				copy(bs[dstIdx:dstIdx+4], palette[4*p:4*p+4])
+				srcIdx++
+				dstIdx += 4
 			}
-			idx0 += d
+			srcIdx += d
 		}
 		return bs
 	case *image.RGBA:
-		if len(img.Pix) == 4*w*h {
+		if premultipliedAlpha && len(img.Pix) == 4*w*h {
+			return img.Pix
+		}
+	case *image.NRGBA:
+		if !premultipliedAlpha && len(img.Pix) == 4*w*h {
 			return img.Pix
 		}
-		return imageToBytesSlow(img)
-	default:
-		return imageToBytesSlow(img)
 	}
+	return imageToBytesSlow(img, premultipliedAlpha)
 }

-func imageToBytesSlow(img image.Image) []byte {
+func imageToBytesSlow(img image.Image, premultipliedAlpha bool) []byte {
 	size := img.Bounds().Size()
 	w, h := size.X, size.Y
 	bs := make([]byte, 4*w*h)

-	dstImg := &image.RGBA{
-		Pix:    bs,
-		Stride: 4 * w,
-		Rect:   image.Rect(0, 0, w, h),
+	var dstImg draw.Image
+	if premultipliedAlpha {
+		dstImg = &image.RGBA{
+			Pix:    bs,
+			Stride: 4 * w,
+			Rect:   image.Rect(0, 0, w, h),
+		}
+	} else {
+		dstImg = &image.NRGBA{
+			Pix:    bs,
+			Stride: 4 * w,
+			Rect:   image.Rect(0, 0, w, h),
+		}
 	}
 	draw.Draw(dstImg, image.Rect(0, 0, w, h), img, img.Bounds().Min, draw.Src)
 	return bs
--- a/imagetobytes_test.go
+++ b/imagetobytes_test.go
@@ -39,61 +39,123 @@ func TestImageToBytes(t *testing.T) {
 	}
 	bigPalette := color.Palette(p)
 	cases := []struct {
-		In  image.Image
-		Out []uint8
+		Image  image.Image
+		Premul bool
+		Out    []uint8
 	}{
 		{
-			In: &image.Paletted{
-				Pix:    []uint8{0, 1, 1, 0},
+			Image: &image.Paletted{
+				Pix:    []uint8{0, 1, 1, 2},
 				Stride: 2,
 				Rect:   image.Rect(0, 0, 2, 2),
 				Palette: color.Palette([]color.Color{
-					color.Transparent, color.White,
+					color.Transparent, color.White, color.RGBA{0x80, 0x80, 0x80, 0x80},
 				}),
 			},
-			Out: []uint8{0, 0, 0, 0, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0, 0, 0, 0},
+			Premul: true,
+			Out:    []uint8{0, 0, 0, 0, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x80, 0x80, 0x80, 0x80},
 		},
 		{
-			In:  image.NewPaletted(image.Rect(0, 0, 240, 160), pal).SubImage(image.Rect(238, 158, 240, 160)),
-			Out: []uint8{0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff},
+			Image: &image.Paletted{
+				Pix:    []uint8{0, 1, 1, 2},
+				Stride: 2,
+				Rect:   image.Rect(0, 0, 2, 2),
+				Palette: color.Palette([]color.Color{
+					color.Transparent, color.White, color.RGBA{0x80, 0x80, 0x80, 0x80},
+				}),
+			},
+			Premul: false,
+			Out:    []uint8{0, 0, 0, 0, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x80},
 		},
 		{
-			In: &image.RGBA{
+			Image:  image.NewPaletted(image.Rect(0, 0, 240, 160), pal).SubImage(image.Rect(238, 158, 240, 160)),
+			Premul: true,
+			Out:    []uint8{0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff},
+		},
+		{
+			Image:  image.NewPaletted(image.Rect(0, 0, 240, 160), pal).SubImage(image.Rect(238, 158, 240, 160)),
+			Premul: false,
+			Out:    []uint8{0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff},
+		},
+		{
+			Image: &image.RGBA{
 				Pix:    []uint8{0, 0, 0, 0, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0, 0, 0, 0},
 				Stride: 8,
 				Rect:   image.Rect(0, 0, 2, 2),
 			},
-			Out: []uint8{0, 0, 0, 0, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0, 0, 0, 0},
+			Premul: true,
+			Out:    []uint8{0, 0, 0, 0, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0, 0, 0, 0},
 		},
 		{
-			In: &image.NRGBA{
+			Image: &image.RGBA{
+				Pix:    []uint8{0, 0, 0, 0, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0, 0, 0, 0},
+				Stride: 8,
+				Rect:   image.Rect(0, 0, 2, 2),
+			},
+			Premul: false,
+			Out:    []uint8{0, 0, 0, 0, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0, 0, 0, 0},
+		},
+		{
+			Image: &image.NRGBA{
 				Pix:    []uint8{0, 0, 0, 0, 0xff, 0xff, 0xff, 0x80, 0x80, 0x80, 0x80, 0x80, 0, 0, 0, 0},
 				Stride: 8,
 				Rect:   image.Rect(0, 0, 2, 2),
 			},
-			Out: []uint8{0, 0, 0, 0, 0x80, 0x80, 0x80, 0x80, 0x40, 0x40, 0x40, 0x80, 0, 0, 0, 0},
+			Premul: true,
+			Out:    []uint8{0, 0, 0, 0, 0x80, 0x80, 0x80, 0x80, 0x40, 0x40, 0x40, 0x80, 0, 0, 0, 0},
 		},
 		{
-			In: &image.Paletted{
+			Image: &image.NRGBA{
+				Pix:    []uint8{0, 0, 0, 0, 0xff, 0xff, 0xff, 0x80, 0x80, 0x80, 0x80, 0x80, 0, 0, 0, 0},
+				Stride: 8,
+				Rect:   image.Rect(0, 0, 2, 2),
+			},
+			Premul: false,
+			Out:    []uint8{0, 0, 0, 0, 0xff, 0xff, 0xff, 0x80, 0x80, 0x80, 0x80, 0x80, 0, 0, 0, 0},
+		},
+		{
+			Image: &image.Paletted{
 				Pix:     []uint8{0, 64, 0, 0},
 				Stride:  2,
 				Rect:    image.Rect(0, 0, 2, 2),
 				Palette: bigPalette,
 			},
-			Out: []uint8{0, 0, 0, 0, 0xff, 0xff, 0xff, 0xff, 0, 0, 0, 0, 0, 0, 0, 0},
+			Premul: true,
+			Out:    []uint8{0, 0, 0, 0, 0xff, 0xff, 0xff, 0xff, 0, 0, 0, 0, 0, 0, 0, 0},
 		},
 		{
-			In: (&image.Paletted{
+			Image: &image.Paletted{
+				Pix:     []uint8{0, 64, 0, 0},
+				Stride:  2,
+				Rect:    image.Rect(0, 0, 2, 2),
+				Palette: bigPalette,
+			},
+			Premul: false,
+			Out:    []uint8{0, 0, 0, 0, 0xff, 0xff, 0xff, 0xff, 0, 0, 0, 0, 0, 0, 0, 0},
+		},
+		{
+			Image: (&image.Paletted{
 				Pix:     []uint8{0, 64, 0, 0},
 				Stride:  2,
 				Rect:    image.Rect(0, 0, 2, 2),
 				Palette: bigPalette,
 			}).SubImage(image.Rect(1, 0, 2, 1)),
-			Out: []uint8{0xff, 0xff, 0xff, 0xff},
+			Premul: true,
+			Out:    []uint8{0xff, 0xff, 0xff, 0xff},
+		},
+		{
+			Image: (&image.Paletted{
+				Pix:     []uint8{0, 64, 0, 0},
+				Stride:  2,
+				Rect:    image.Rect(0, 0, 2, 2),
+				Palette: bigPalette,
+			}).SubImage(image.Rect(1, 0, 2, 1)),
+			Premul: false,
+			Out:    []uint8{0xff, 0xff, 0xff, 0xff},
 		},
 	}
 	for i, c := range cases {
-		got := ebiten.ImageToBytes(c.In)
+		got := ebiten.ImageToBytes(c.Image, c.Premul)
 		want := c.Out
 		if !bytes.Equal(got, want) {
 			t.Errorf("Test %d: got: %v, want: %v", i, got, want)
@@ -104,23 +166,23 @@ func TestImageToBytes(t *testing.T) {
 func BenchmarkImageToBytesRGBA(b *testing.B) {
 	img := image.NewRGBA(image.Rect(0, 0, 4096, 4096))
 	b.ResetTimer()
-	for i := 0; i < b.N; i++ {
-		ebiten.ImageToBytes(img)
+	for range b.N {
+		ebiten.ImageToBytes(img, true)
 	}
 }

 func BenchmarkImageToBytesNRGBA(b *testing.B) {
 	img := image.NewNRGBA(image.Rect(0, 0, 4096, 4096))
 	b.ResetTimer()
-	for i := 0; i < b.N; i++ {
-		ebiten.ImageToBytes(img)
+	for range b.N {
+		ebiten.ImageToBytes(img, true)
 	}
 }

 func BenchmarkImageToBytesPaletted(b *testing.B) {
 	img := image.NewPaletted(image.Rect(0, 0, 4096, 4096), palette.Plan9)
 	b.ResetTimer()
-	for i := 0; i < b.N; i++ {
-		ebiten.ImageToBytes(img)
+	for range b.N {
+		ebiten.ImageToBytes(img, true)
 	}
 }