exp/draw: fast path for drawing overlapping image.RGBAs.

R=r, nigeltao
CC=golang-dev
https://golang.org/cl/2145045

src/pkg/exp/draw/draw.go

@@ -74,12 +74,8 @@ func DrawMask(dst Image, r image.Rectangle, src image.Image, sp image.Point, mas
 					return
 				}
 				if src0, ok := src.(*image.RGBA); ok {
-					if dst0 == src0 && r.Overlaps(r.Add(sp.Sub(r.Min))) {
-						// TODO(nigeltao): Implement a fast path for the overlapping case.
-					} else {
-						drawCopyOver(dst0, r, src0, sp)
-						return
-					}
+					drawCopyOver(dst0, r, src0, sp)
+					return
 				}
 			} else if mask0, ok := mask.(*image.Alpha); ok {
 				if src0, ok := src.(image.ColorImage); ok {
@@ -94,12 +90,8 @@ func DrawMask(dst Image, r image.Rectangle, src image.Image, sp image.Point, mas
 					return
 				}
 				if src0, ok := src.(*image.RGBA); ok {
-					if dst0 == src0 && r.Overlaps(r.Add(sp.Sub(r.Min))) {
-						// TODO(nigeltao): Implement a fast path for the overlapping case.
-					} else {
-						drawCopySrc(dst0, r, src0, sp)
-						return
-					}
+					drawCopySrc(dst0, r, src0, sp)
+					return
 				}
 			}
 		}
@@ -181,18 +173,42 @@ func drawFillOver(dst *image.RGBA, r image.Rectangle, src image.ColorImage) {
 }
 
 func drawCopyOver(dst *image.RGBA, r image.Rectangle, src *image.RGBA, sp image.Point) {
-	x0, x1 := r.Min.X, r.Max.X
-	y0, y1 := r.Min.Y, r.Max.Y
-	for y, sy := y0, sp.Y; y != y1; y, sy = y+1, sy+1 {
-		dbase := y * dst.Stride
-		dpix := dst.Pix[dbase+x0 : dbase+x1]
-		sbase := sy * src.Stride
-		spix := src.Pix[sbase+sp.X:]
-		for i, rgba := range dpix {
+	dx0, dx1 := r.Min.X, r.Max.X
+	dy0, dy1 := r.Min.Y, r.Max.Y
+	nrows := dy1 - dy0
+	sx0, sx1 := sp.X, sp.X+dx1-dx0
+	d0 := dy0*dst.Stride + dx0
+	d1 := dy0*dst.Stride + dx1
+	s0 := sp.Y*src.Stride + sx0
+	s1 := sp.Y*src.Stride + sx1
+	var (
+		ddelta, sdelta int
+		i0, i1, idelta int
+	)
+	if r.Min.Y < sp.Y || r.Min.Y == sp.Y && r.Min.X <= sp.X {
+		ddelta = dst.Stride
+		sdelta = src.Stride
+		i0, i1, idelta = 0, d1-d0, +1
+	} else {
+		// If the source start point is higher than the destination start point, or equal height but to the left,
+		// then we compose the rows in right-to-left, bottom-up order instead of left-to-right, top-down.
+		d0 += (nrows - 1) * dst.Stride
+		d1 += (nrows - 1) * dst.Stride
+		s0 += (nrows - 1) * src.Stride
+		s1 += (nrows - 1) * src.Stride
+		ddelta = -dst.Stride
+		sdelta = -src.Stride
+		i0, i1, idelta = d1-d0-1, -1, -1
+	}
+	for ; nrows > 0; nrows-- {
+		dpix := dst.Pix[d0:d1]
+		spix := src.Pix[s0:s1]
+		for i := i0; i != i1; i += idelta {
 			// For unknown reasons, even though both dpix[i] and spix[i] are
 			// image.RGBAColors, on an x86 CPU it seems fastest to call RGBA
 			// for the source but to do it manually for the destination.
 			sr, sg, sb, sa := spix[i].RGBA()
+			rgba := dpix[i]
 			dr := uint32(rgba.R)
 			dg := uint32(rgba.G)
 			db := uint32(rgba.B)
@@ -205,6 +221,10 @@ func drawCopyOver(dst *image.RGBA, r image.Rectangle, src *image.RGBA, sp image.
 			da = (da*a)/m + sa
 			dpix[i] = image.RGBAColor{uint8(dr >> 8), uint8(dg >> 8), uint8(db >> 8), uint8(da >> 8)}
 		}
+		d0 += ddelta
+		d1 += ddelta
+		s0 += sdelta
+		s1 += sdelta
 	}
 }
 
@@ -265,17 +285,33 @@ func drawFillSrc(dst *image.RGBA, r image.Rectangle, src image.ColorImage) {
 func drawCopySrc(dst *image.RGBA, r image.Rectangle, src *image.RGBA, sp image.Point) {
 	dx0, dx1 := r.Min.X, r.Max.X
 	dy0, dy1 := r.Min.Y, r.Max.Y
+	nrows := dy1 - dy0
 	sx0, sx1 := sp.X, sp.X+dx1-dx0
 	d0 := dy0*dst.Stride + dx0
 	d1 := dy0*dst.Stride + dx1
-	s0 := sp.Y*dst.Stride + sx0
-	s1 := sp.Y*dst.Stride + sx1
-	for y := dy0; y < dy1; y++ {
+	s0 := sp.Y*src.Stride + sx0
+	s1 := sp.Y*src.Stride + sx1
+	var ddelta, sdelta int
+	if r.Min.Y <= sp.Y {
+		ddelta = dst.Stride
+		sdelta = src.Stride
+	} else {
+		// If the source start point is higher than the destination start point, then we compose the rows
+		// in bottom-up order instead of top-down. Unlike the drawCopyOver function, we don't have to
+		// check the x co-ordinates because the built-in copy function can handle overlapping slices.
+		d0 += (nrows - 1) * dst.Stride
+		d1 += (nrows - 1) * dst.Stride
+		s0 += (nrows - 1) * src.Stride
+		s1 += (nrows - 1) * src.Stride
+		ddelta = -dst.Stride
+		sdelta = -src.Stride
+	}
+	for ; nrows > 0; nrows-- {
 		copy(dst.Pix[d0:d1], src.Pix[s0:s1])
-		d0 += dst.Stride
-		d1 += dst.Stride
-		s0 += src.Stride
-		s1 += src.Stride
+		d0 += ddelta
+		d1 += ddelta
+		s0 += sdelta
+		s1 += sdelta
 	}
 }
 

src/pkg/exp/draw/draw_test.go

@@ -53,6 +53,16 @@ func hgradRed(alpha int) Image {
 	return m
 }
 
+func gradYellow(alpha int) Image {
+	m := image.NewRGBA(16, 16)
+	for y := 0; y < 16; y++ {
+		for x := 0; x < 16; x++ {
+			m.Set(x, y, image.RGBAColor{uint8(x * alpha / 15), uint8(y * alpha / 15), 0, uint8(alpha)})
+		}
+	}
+	return m
+}
+
 type drawTest struct {
 	desc     string
 	src      image.Image
@@ -94,24 +104,31 @@ var drawTests = []drawTest{
 	drawTest{"genericSrc", fillBlue(255), vgradAlpha(192), Src, image.RGBAColor{0, 0, 102, 102}},
 }
 
-func makeGolden(dst image.Image, t drawTest) image.Image {
+func makeGolden(dst, src, mask image.Image, op Op) image.Image {
 	// Since golden is a newly allocated image, we don't have to check if the
 	// input source and mask images and the output golden image overlap.
 	b := dst.Bounds()
-	golden := image.NewRGBA(b.Dx(), b.Dy())
+	sx0 := src.Bounds().Min.X - b.Min.X
+	sy0 := src.Bounds().Min.Y - b.Min.Y
+	var mx0, my0 int
+	if mask != nil {
+		mx0 = mask.Bounds().Min.X - b.Min.X
+		my0 = mask.Bounds().Min.Y - b.Min.Y
+	}
+	golden := image.NewRGBA(b.Max.X, b.Max.Y)
 	for y := b.Min.Y; y < b.Max.Y; y++ {
-		my, sy := y, y
+		my, sy := my0+y, sy0+y
 		for x := b.Min.X; x < b.Max.X; x++ {
-			mx, sx := x, x
+			mx, sx := mx0+x, sx0+x
 			const M = 1<<16 - 1
 			var dr, dg, db, da uint32
-			if t.op == Over {
+			if op == Over {
 				dr, dg, db, da = dst.At(x, y).RGBA()
 			}
-			sr, sg, sb, sa := t.src.At(sx, sy).RGBA()
+			sr, sg, sb, sa := src.At(sx, sy).RGBA()
 			ma := uint32(M)
-			if t.mask != nil {
-				_, _, _, ma = t.mask.At(mx, my).RGBA()
+			if mask != nil {
+				_, _, _, ma = mask.At(mx, my).RGBA()
 			}
 			a := M - (sa * ma / M)
 			golden.Set(x, y, image.RGBA64Color{
@@ -122,6 +139,7 @@ func makeGolden(dst image.Image, t drawTest) image.Image {
 			})
 		}
 	}
+	golden.Rect = b
 	return golden
 }
 
@@ -130,14 +148,14 @@ loop:
 	for _, test := range drawTests {
 		dst := hgradRed(255)
 		// Draw the (src, mask, op) onto a copy of dst using a slow but obviously correct implementation.
+		golden := makeGolden(dst, test.src, test.mask, test.op)
 		b := dst.Bounds()
-		golden := makeGolden(dst, test)
 		if !b.Eq(golden.Bounds()) {
 			t.Errorf("draw %s: bounds %v versus %v", test.desc, dst.Bounds(), golden.Bounds())
 			continue
 		}
 		// Draw the same combination onto the actual dst using the optimized DrawMask implementation.
-		DrawMask(dst, image.Rect(b.Min.X, b.Min.Y, b.Max.X, b.Max.Y), test.src, image.ZP, test.mask, image.ZP, test.op)
+		DrawMask(dst, b, test.src, image.ZP, test.mask, image.ZP, test.op)
 		// Check that the resultant pixel at (8, 8) matches what we expect
 		// (the expected value can be verified by hand).
 		if !eq(dst.At(8, 8), test.expected) {
@@ -156,6 +174,45 @@ loop:
 	}
 }
 
+func TestDrawOverlap(t *testing.T) {
+	for _, op := range []Op{Over, Src} {
+		for yoff := -2; yoff <= 2; yoff++ {
+		loop:
+			for xoff := -2; xoff <= 2; xoff++ {
+				m := gradYellow(127).(*image.RGBA)
+				dst := &image.RGBA{
+					Pix:    m.Pix,
+					Stride: m.Stride,
+					Rect:   image.Rect(5, 5, 10, 10),
+				}
+				src := &image.RGBA{
+					Pix:    m.Pix,
+					Stride: m.Stride,
+					Rect:   image.Rect(5+xoff, 5+yoff, 10+xoff, 10+yoff),
+				}
+				// Draw the (src, mask, op) onto a copy of dst using a slow but obviously correct implementation.
+				golden := makeGolden(dst, src, nil, op)
+				b := dst.Bounds()
+				if !b.Eq(golden.Bounds()) {
+					t.Errorf("drawOverlap xoff=%d,yoff=%d: bounds %v versus %v", xoff, yoff, dst.Bounds(), golden.Bounds())
+					continue
+				}
+				// Draw the same combination onto the actual dst using the optimized DrawMask implementation.
+				DrawMask(dst, b, src, src.Bounds().Min, nil, image.ZP, op)
+				// Check that the resultant dst image matches the golden output.
+				for y := b.Min.Y; y < b.Max.Y; y++ {
+					for x := b.Min.X; x < b.Max.X; x++ {
+						if !eq(dst.At(x, y), golden.At(x, y)) {
+							t.Errorf("drawOverlap xoff=%d,yoff=%d: at (%d, %d), %v versus golden %v", xoff, yoff, x, y, dst.At(x, y), golden.At(x, y))
+							continue loop
+						}
+					}
+				}
+			}
+		}
+	}
+}
+
 // TestIssue836 verifies http://code.google.com/p/go/issues/detail?id=836.
 func TestIssue836(t *testing.T) {
 	a := image.NewRGBA(1, 1)