image/draw: improve performances if mask is *image.Alpha

The existing DrawMask method is generic and is therefore calling the At().RGBA() method for every pixel of the mask and the source.
Do a specific implementation when the mask is *image.Alpha (which is
common) and use use the PixOffset method to increase performances.

name                old time/op  new time/op  delta
RGBA2-12            1.60ms ± 0%  1.13ms ± 1%  -29.16%  (p=0.008 n=5+5)
GenericMaskOver-12   915µs ± 4%   926µs ± 1%     ~     (p=0.190 n=5+4)
RGBA64Over-12       1.53ms ± 3%  1.21ms ± 2%  -20.74%  (p=0.008 n=5+5)
GrayOver-12         1.36ms ± 2%  1.01ms ± 7%  -26.27%  (p=0.008 n=5+5)

Fixes: #46395
Change-Id: Iaeaa8cfcc6a3fe93eb19b361f3bf076e41cac5b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/323749
Reviewed-by: Nigel Tao <nigeltao@golang.org>
Trust: Nigel Tao <nigeltao@golang.org>
Trust: Andrew Gerrand <adg@golang.org>
Run-TryBot: Nigel Tao <nigeltao@golang.org>

3 files changed, 171 insertions, 0 deletions

diff --git a/src/image/draw/bench_test.go b/src/image/draw/bench_test.go
index 524ead2b8e..2b7c6d6ac4 100644
--- a/src/image/draw/bench_test.go
+++ b/src/image/draw/bench_test.go
@@ -266,3 +266,11 @@ func BenchmarkGenericSrc(b *testing.B) {
 func BenchmarkGenericMaskSrc(b *testing.B) {
 	bench(b, color.RGBA64Model, color.RGBA64Model, color.AlphaModel, Src)
 }
+
+func BenchmarkRGBA64Over(b *testing.B) {
+	bench(b, color.RGBAModel, color.RGBA64Model, color.AlphaModel, Over)
+}
+
+func BenchmarkGrayOver(b *testing.B) {
+	bench(b, color.RGBAModel, color.GrayModel, color.AlphaModel, Over)
+}
diff --git a/src/image/draw/draw.go b/src/image/draw/draw.go
index 5e81ddc395..d3c5b29097 100644
--- a/src/image/draw/draw.go
+++ b/src/image/draw/draw.go
@@ -160,6 +160,15 @@ func DrawMask(dst Image, r image.Rectangle, src image.Image, sp image.Point, mas
 				case *image.Uniform:
 					drawGlyphOver(dst0, r, src0, mask0, mp)
 					return
+				case *image.RGBA:
+					drawRGBAMaskOver(dst0, r, src0, sp, mask0, mp)
+					return
+				case *image.Gray:
+					drawGrayMaskOver(dst0, r, src0, sp, mask0, mp)
+					return
+				case image.RGBA64Image:
+					drawRGBA64ImageMaskOver(dst0, r, src0, sp, mask0, mp)
+					return
 				}
 			}
 		} else {
@@ -602,6 +611,156 @@ func drawGlyphOver(dst *image.RGBA, r image.Rectangle, src *image.Uniform, mask
 	}
 }
 
+func drawGrayMaskOver(dst *image.RGBA, r image.Rectangle, src *image.Gray, sp image.Point, mask *image.Alpha, mp image.Point) {
+	x0, x1, dx := r.Min.X, r.Max.X, 1
+	y0, y1, dy := r.Min.Y, r.Max.Y, 1
+	if r.Overlaps(r.Add(sp.Sub(r.Min))) {
+		if sp.Y < r.Min.Y || sp.Y == r.Min.Y && sp.X < r.Min.X {
+			x0, x1, dx = x1-1, x0-1, -1
+			y0, y1, dy = y1-1, y0-1, -1
+		}
+	}
+
+	sy := sp.Y + y0 - r.Min.Y
+	my := mp.Y + y0 - r.Min.Y
+	sx0 := sp.X + x0 - r.Min.X
+	mx0 := mp.X + x0 - r.Min.X
+	sx1 := sx0 + (x1 - x0)
+	i0 := dst.PixOffset(x0, y0)
+	di := dx * 4
+	for y := y0; y != y1; y, sy, my = y+dy, sy+dy, my+dy {
+		for i, sx, mx := i0, sx0, mx0; sx != sx1; i, sx, mx = i+di, sx+dx, mx+dx {
+			mi := mask.PixOffset(mx, my)
+			ma := uint32(mask.Pix[mi])
+			ma |= ma << 8
+			si := src.PixOffset(sx, sy)
+			sy := uint32(src.Pix[si])
+			sy |= sy << 8
+			sa := uint32(0xffff)
+
+			d := dst.Pix[i : i+4 : i+4] // Small cap improves performance, see https://golang.org/issue/27857
+			dr := uint32(d[0])
+			dg := uint32(d[1])
+			db := uint32(d[2])
+			da := uint32(d[3])
+
+			// dr, dg, db and da are all 8-bit color at the moment, ranging in [0,255].
+			// We work in 16-bit color, and so would normally do:
+			// dr |= dr << 8
+			// and similarly for dg, db and da, but instead we multiply a
+			// (which is a 16-bit color, ranging in [0,65535]) by 0x101.
+			// This yields the same result, but is fewer arithmetic operations.
+			a := (m - (sa * ma / m)) * 0x101
+
+			d[0] = uint8((dr*a + sy*ma) / m >> 8)
+			d[1] = uint8((dg*a + sy*ma) / m >> 8)
+			d[2] = uint8((db*a + sy*ma) / m >> 8)
+			d[3] = uint8((da*a + sa*ma) / m >> 8)
+		}
+		i0 += dy * dst.Stride
+	}
+}
+
+func drawRGBAMaskOver(dst *image.RGBA, r image.Rectangle, src *image.RGBA, sp image.Point, mask *image.Alpha, mp image.Point) {
+	x0, x1, dx := r.Min.X, r.Max.X, 1
+	y0, y1, dy := r.Min.Y, r.Max.Y, 1
+	if dst == src && r.Overlaps(r.Add(sp.Sub(r.Min))) {
+		if sp.Y < r.Min.Y || sp.Y == r.Min.Y && sp.X < r.Min.X {
+			x0, x1, dx = x1-1, x0-1, -1
+			y0, y1, dy = y1-1, y0-1, -1
+		}
+	}
+
+	sy := sp.Y + y0 - r.Min.Y
+	my := mp.Y + y0 - r.Min.Y
+	sx0 := sp.X + x0 - r.Min.X
+	mx0 := mp.X + x0 - r.Min.X
+	sx1 := sx0 + (x1 - x0)
+	i0 := dst.PixOffset(x0, y0)
+	di := dx * 4
+	for y := y0; y != y1; y, sy, my = y+dy, sy+dy, my+dy {
+		for i, sx, mx := i0, sx0, mx0; sx != sx1; i, sx, mx = i+di, sx+dx, mx+dx {
+			mi := mask.PixOffset(mx, my)
+			ma := uint32(mask.Pix[mi])
+			ma |= ma << 8
+			si := src.PixOffset(sx, sy)
+			sr := uint32(src.Pix[si+0])
+			sg := uint32(src.Pix[si+1])
+			sb := uint32(src.Pix[si+2])
+			sa := uint32(src.Pix[si+3])
+			sr |= sr << 8
+			sg |= sg << 8
+			sb |= sb << 8
+			sa |= sa << 8
+			d := dst.Pix[i : i+4 : i+4] // Small cap improves performance, see https://golang.org/issue/27857
+			dr := uint32(d[0])
+			dg := uint32(d[1])
+			db := uint32(d[2])
+			da := uint32(d[3])
+
+			// dr, dg, db and da are all 8-bit color at the moment, ranging in [0,255].
+			// We work in 16-bit color, and so would normally do:
+			// dr |= dr << 8
+			// and similarly for dg, db and da, but instead we multiply a
+			// (which is a 16-bit color, ranging in [0,65535]) by 0x101.
+			// This yields the same result, but is fewer arithmetic operations.
+			a := (m - (sa * ma / m)) * 0x101
+
+			d[0] = uint8((dr*a + sr*ma) / m >> 8)
+			d[1] = uint8((dg*a + sg*ma) / m >> 8)
+			d[2] = uint8((db*a + sb*ma) / m >> 8)
+			d[3] = uint8((da*a + sa*ma) / m >> 8)
+		}
+		i0 += dy * dst.Stride
+	}
+}
+
+func drawRGBA64ImageMaskOver(dst *image.RGBA, r image.Rectangle, src image.RGBA64Image, sp image.Point, mask *image.Alpha, mp image.Point) {
+	x0, x1, dx := r.Min.X, r.Max.X, 1
+	y0, y1, dy := r.Min.Y, r.Max.Y, 1
+	if image.Image(dst) == src && r.Overlaps(r.Add(sp.Sub(r.Min))) {
+		if sp.Y < r.Min.Y || sp.Y == r.Min.Y && sp.X < r.Min.X {
+			x0, x1, dx = x1-1, x0-1, -1
+			y0, y1, dy = y1-1, y0-1, -1
+		}
+	}
+
+	sy := sp.Y + y0 - r.Min.Y
+	my := mp.Y + y0 - r.Min.Y
+	sx0 := sp.X + x0 - r.Min.X
+	mx0 := mp.X + x0 - r.Min.X
+	sx1 := sx0 + (x1 - x0)
+	i0 := dst.PixOffset(x0, y0)
+	di := dx * 4
+	for y := y0; y != y1; y, sy, my = y+dy, sy+dy, my+dy {
+		for i, sx, mx := i0, sx0, mx0; sx != sx1; i, sx, mx = i+di, sx+dx, mx+dx {
+			mi := mask.PixOffset(mx, my)
+			ma := uint32(mask.Pix[mi])
+			ma |= ma << 8
+			srgba := src.RGBA64At(sx, sy)
+			d := dst.Pix[i : i+4 : i+4] // Small cap improves performance, see https://golang.org/issue/27857
+			dr := uint32(d[0])
+			dg := uint32(d[1])
+			db := uint32(d[2])
+			da := uint32(d[3])
+
+			// dr, dg, db and da are all 8-bit color at the moment, ranging in [0,255].
+			// We work in 16-bit color, and so would normally do:
+			// dr |= dr << 8
+			// and similarly for dg, db and da, but instead we multiply a
+			// (which is a 16-bit color, ranging in [0,65535]) by 0x101.
+			// This yields the same result, but is fewer arithmetic operations.
+			a := (m - (uint32(srgba.A) * ma / m)) * 0x101
+
+			d[0] = uint8((dr*a + uint32(srgba.R)*ma) / m >> 8)
+			d[1] = uint8((dg*a + uint32(srgba.G)*ma) / m >> 8)
+			d[2] = uint8((db*a + uint32(srgba.B)*ma) / m >> 8)
+			d[3] = uint8((da*a + uint32(srgba.A)*ma) / m >> 8)
+		}
+		i0 += dy * dst.Stride
+	}
+}
+
 func drawRGBA(dst *image.RGBA, r image.Rectangle, src image.Image, sp image.Point, mask image.Image, mp image.Point, op Op) {
 	x0, x1, dx := r.Min.X, r.Max.X, 1
 	y0, y1, dy := r.Min.Y, r.Max.Y, 1
diff --git a/src/image/draw/draw_test.go b/src/image/draw/draw_test.go
index 8a51409526..75a2896631 100644
--- a/src/image/draw/draw_test.go
+++ b/src/image/draw/draw_test.go
@@ -397,6 +397,10 @@ var drawTests = []drawTest{
 		Over, color.RGBA{81, 0, 102, 255}},
 	{"genericSrcSlowest", fillBlue(255), convertToSlowestRGBA(vgradAlpha(192)),
 		Src, color.RGBA{0, 0, 102, 102}},
+	// The source pixel is {0, 48, 0, 90}.
+	{"rgbaVariableMaskOver", vgradGreen(255), vgradAlpha(192), Over, color.RGBA{81, 54, 0, 255}},
+	// The source pixel is {136} in Gray-space, which is {136, 136, 136, 255} in RGBA-space.
+	{"grayVariableMaskOver", vgradGray(), vgradAlpha(192), Over, color.RGBA{136, 54, 54, 255}},
 }
 
 func makeGolden(dst image.Image, r image.Rectangle, src image.Image, sp image.Point, mask image.Image, mp image.Point, op Op) image.Image {