Add Src and Over draw operators.

R=r, rsc
CC=golang-dev
https://golang.org/cl/207096

3 files changed, 95 insertions, 63 deletions

diff --git a/src/pkg/exp/4s/xs.go b/src/pkg/exp/4s/xs.go
index e1d28c8850..c5493e719e 100644
--- a/src/pkg/exp/4s/xs.go
+++ b/src/pkg/exp/4s/xs.go
@@ -266,14 +266,14 @@ func setpiece(p *Piece) {
 	draw.Draw(bb2, r2, draw.White, draw.ZP)
 	draw.Draw(bb2, r.Add(delta), bb, bbr.Min)
 	draw.Draw(bb2mask, r2, draw.Transparent, draw.ZP)
-	draw.DrawMask(bb2mask, r, draw.Opaque, bbr.Min, bbmask, draw.ZP, draw.SoverD)
-	draw.DrawMask(bb2mask, r.Add(delta), draw.Opaque, bbr.Min, bbmask, draw.ZP, draw.SoverD)
+	draw.DrawMask(bb2mask, r, draw.Opaque, bbr.Min, bbmask, draw.ZP, draw.Over)
+	draw.DrawMask(bb2mask, r.Add(delta), draw.Opaque, bbr.Min, bbmask, draw.ZP, draw.Over)
 }
 
 func drawpiece() {
-	draw.DrawMask(screen, br.Add(pos), bb, bbr.Min, bbmask, draw.ZP, draw.SoverD)
+	draw.DrawMask(screen, br.Add(pos), bb, bbr.Min, bbmask, draw.ZP, draw.Over)
 	if suspended {
-		draw.DrawMask(screen, br.Add(pos), draw.White, draw.ZP, whitemask, draw.ZP, draw.SoverD)
+		draw.DrawMask(screen, br.Add(pos), draw.White, draw.ZP, whitemask, draw.ZP, draw.Over)
 	}
 }
 
@@ -282,7 +282,7 @@ func undrawpiece() {
 	if collider(pos, br.Max) {
 		mask = bbmask
 	}
-	draw.DrawMask(screen, br.Add(pos), draw.White, bbr.Min, mask, bbr.Min, draw.SoverD)
+	draw.DrawMask(screen, br.Add(pos), draw.White, bbr.Min, mask, bbr.Min, draw.Over)
 }
 
 func rest() {
@@ -349,7 +349,7 @@ func drawboard() {
 	}
 	score(0)
 	if suspended {
-		draw.DrawMask(screen, screenr, draw.White, draw.ZP, whitemask, draw.ZP, draw.SoverD)
+		draw.DrawMask(screen, screenr, draw.White, draw.ZP, whitemask, draw.ZP, draw.Over)
 	}
 }
 
@@ -375,7 +375,7 @@ func movepiece() bool {
 	if collider(pos, br2.Max) {
 		mask = bb2mask
 	}
-	draw.DrawMask(screen, br2.Add(pos), bb2, bb2r.Min, mask, bb2r.Min, draw.SoverD)
+	draw.DrawMask(screen, br2.Add(pos), bb2, bb2r.Min, mask, bb2r.Min, draw.Over)
 	pos.Y += DY
 	display.FlushImage()
 	return true
@@ -444,7 +444,7 @@ func horiz() bool {
 	for j := 0; j < h; j++ {
 		r.Min.Y = rboard.Min.Y + lev[j]*pcsz
 		r.Max.Y = r.Min.Y + pcsz
-		draw.DrawMask(screen, r, draw.White, draw.ZP, whitemask, draw.ZP, draw.SoverD)
+		draw.DrawMask(screen, r, draw.White, draw.ZP, whitemask, draw.ZP, draw.Over)
 		display.FlushImage()
 	}
 	PlaySound(whoosh)
@@ -457,7 +457,7 @@ func horiz() bool {
 		for j := 0; j < h; j++ {
 			r.Min.Y = rboard.Min.Y + lev[j]*pcsz
 			r.Max.Y = r.Min.Y + pcsz
-			draw.DrawMask(screen, r, draw.White, draw.ZP, whitemask, draw.ZP, draw.SoverD)
+			draw.DrawMask(screen, r, draw.White, draw.ZP, whitemask, draw.ZP, draw.Over)
 		}
 		display.FlushImage()
 	}
diff --git a/src/pkg/exp/draw/draw.go b/src/pkg/exp/draw/draw.go
index 1888e69a58..bf5a08479e 100644
--- a/src/pkg/exp/draw/draw.go
+++ b/src/pkg/exp/draw/draw.go
@@ -15,7 +15,14 @@ import "image"
 // A Porter-Duff compositing operator.
 type Op int
 
-const SoverD Op = 0
+const (
+	// Over specifies ``(src in mask) over dst''.
+	Over Op = iota
+	// Src specifies ``src in mask''.
+	Src
+)
+
+var zeroColor image.Color = image.AlphaColor{0}
 
 // A draw.Image is an image.Image with a Set method to change a single pixel.
 type Image interface {
@@ -23,14 +30,13 @@ type Image interface {
 	Set(x, y int, c image.Color)
 }
 
-// Draw calls DrawMask with a nil mask and an SoverD op.
+// Draw calls DrawMask with a nil mask and an Over op.
 func Draw(dst Image, r Rectangle, src image.Image, sp Point) {
-	DrawMask(dst, r, src, sp, nil, ZP, SoverD)
+	DrawMask(dst, r, src, sp, nil, ZP, Over)
 }
 
 // DrawMask aligns r.Min in dst with sp in src and mp in mask and then replaces the rectangle r
-// in dst with the result of a Porter-Duff composition. For the SoverD operator, the result
-// is ``(src in mask) over dst''. If mask is nil, this simplifies to ``src over dst''.
+// in dst with the result of a Porter-Duff composition. A nil mask is treated as opaque.
 // The implementation is simple and slow.
 // TODO(nigeltao): Optimize this.
 func DrawMask(dst Image, r Rectangle, src image.Image, sp Point, mask image.Image, mp Point, op Op) {
@@ -54,22 +60,25 @@ func DrawMask(dst Image, r Rectangle, src image.Image, sp Point, mask image.Imag
 	// TODO(nigeltao): Ensure that r is well formed, i.e. r.Max.X >= r.Min.X and likewise for Y.
 
 	// Fast paths for special cases. If none of them apply, then we fall back to a general but slow implementation.
-	if dst0, ok := dst.(*image.RGBA); ok && op == SoverD {
-		if mask == nil {
-			if src0, ok := src.(image.ColorImage); ok {
-				drawFill(dst0, r, src0)
-				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 {
-					drawCopy(dst0, r, src0, sp)
+	if dst0, ok := dst.(*image.RGBA); ok {
+		if op == Over {
+			// TODO(nigeltao): Implement a fast path for font glyphs (i.e. when mask is an image.Alpha).
+		} else {
+			if mask == nil {
+				if src0, ok := src.(image.ColorImage); ok {
+					drawFill(dst0, r, src0)
 					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 {
+						drawCopy(dst0, r, src0, sp)
+						return
+					}
+				}
 			}
 		}
-		// TODO(nigeltao): Implement a fast path for font glyphs (i.e. when mask is an image.Alpha).
 	}
 
 	x0, x1, dx := r.Min.X, r.Max.X, 1
@@ -89,42 +98,49 @@ func DrawMask(dst Image, r Rectangle, src image.Image, sp Point, mask image.Imag
 		sx := sp.X + x0 - r.Min.X
 		mx := mp.X + x0 - r.Min.X
 		for x := x0; x != x1; x, sx, mx = x+dx, sx+dx, mx+dx {
-			// TODO(nigeltao): Check that op == SoverD.
-			if mask == nil {
-				dst.Set(x, y, src.At(sx, sy))
-				continue
+			// A nil mask is equivalent to a fully opaque, infinitely large mask.
+			// We work in 16-bit color, so that multiplying two values does not overflow a uint32.
+			const M = 1<<16 - 1
+			ma := uint32(M)
+			if mask != nil {
+				_, _, _, ma = mask.At(mx, my).RGBA()
+				ma >>= 16
 			}
-			_, _, _, ma := mask.At(mx, my).RGBA()
-			switch ma {
-			case 0:
-				continue
-			case 0xFFFFFFFF:
+			switch {
+			case ma == 0:
+				if op == Over {
+					// No-op.
+				} else {
+					dst.Set(x, y, zeroColor)
+				}
+			case ma == M && op == Src:
 				dst.Set(x, y, src.At(sx, sy))
 			default:
-				dr, dg, db, da := dst.At(x, y).RGBA()
-				dr >>= 16
-				dg >>= 16
-				db >>= 16
-				da >>= 16
 				sr, sg, sb, sa := src.At(sx, sy).RGBA()
 				sr >>= 16
 				sg >>= 16
 				sb >>= 16
 				sa >>= 16
-				ma >>= 16
-				const M = 1<<16 - 1
-				a := sa * ma / M
-				dr = (dr*(M-a) + sr*ma) / M
-				dg = (dg*(M-a) + sg*ma) / M
-				db = (db*(M-a) + sb*ma) / M
-				da = (da*(M-a) + sa*ma) / M
 				if out == nil {
 					out = new(image.RGBA64Color)
 				}
-				out.R = uint16(dr)
-				out.G = uint16(dg)
-				out.B = uint16(db)
-				out.A = uint16(da)
+				if op == Over {
+					dr, dg, db, da := dst.At(x, y).RGBA()
+					dr >>= 16
+					dg >>= 16
+					db >>= 16
+					da >>= 16
+					a := M - (sa * ma / M)
+					out.R = uint16((dr*a + sr*ma) / M)
+					out.G = uint16((dg*a + sg*ma) / M)
+					out.B = uint16((db*a + sb*ma) / M)
+					out.A = uint16((da*a + sa*ma) / M)
+				} else {
+					out.R = uint16(sr * ma / M)
+					out.G = uint16(sg * ma / M)
+					out.B = uint16(sb * ma / M)
+					out.A = uint16(sa * ma / M)
+				}
 				dst.Set(x, y, out)
 			}
 		}
diff --git a/src/pkg/exp/draw/draw_test.go b/src/pkg/exp/draw/draw_test.go
index 61794dab88..64cf71cf61 100644
--- a/src/pkg/exp/draw/draw_test.go
+++ b/src/pkg/exp/draw/draw_test.go
@@ -57,31 +57,47 @@ type drawTest struct {
 	desc     string
 	src      image.Image
 	mask     image.Image
+	op       Op
 	expected image.Color
 }
 
 var drawTests = []drawTest{
-	// Uniform mask (0% opaque) mask.
-	drawTest{"nop", vgradGreen(255), fillAlpha(0), image.RGBAColor{136, 0, 0, 255}},
-	// Uniform mask (100%, 75%, nil) and vertical-gradient source.
-	drawTest{"copy", vgradGreen(90), fillAlpha(255), image.RGBAColor{0, 48, 0, 90}},
-	drawTest{"copyAlpha", vgradGreen(90), fillAlpha(192), image.RGBAColor{100, 36, 0, 255}},
-	drawTest{"copyNil", vgradGreen(90), nil, image.RGBAColor{0, 48, 0, 90}},
+	// Uniform mask (0% opaque).
+	drawTest{"nop", vgradGreen(255), fillAlpha(0), Over, image.RGBAColor{136, 0, 0, 255}},
+	drawTest{"clear", vgradGreen(255), fillAlpha(0), Src, image.RGBAColor{0, 0, 0, 0}},
 	// Uniform mask (100%, 75%, nil) and uniform source.
-	drawTest{"fill", fillBlue(90), fillAlpha(255), image.RGBAColor{0, 0, 90, 90}},
-	drawTest{"fillAlpha", fillBlue(90), fillAlpha(192), image.RGBAColor{100, 0, 68, 255}},
-	drawTest{"fillNil", fillBlue(90), nil, image.RGBAColor{0, 0, 90, 90}},
-	// Variable mask. In detail, at (x, y) == (8, 8):
+	// At (x, y) == (8, 8):
+	// The destination pixel is {136, 0, 0, 255}.
+	// The source pixel is {0, 0, 90, 90}.
+	drawTest{"fill", fillBlue(90), fillAlpha(255), Over, image.RGBAColor{88, 0, 90, 255}},
+	drawTest{"fillSrc", fillBlue(90), fillAlpha(255), Src, image.RGBAColor{0, 0, 90, 90}},
+	drawTest{"fillAlpha", fillBlue(90), fillAlpha(192), Over, image.RGBAColor{100, 0, 68, 255}},
+	drawTest{"fillAlphaSrc", fillBlue(90), fillAlpha(192), Src, image.RGBAColor{0, 0, 68, 68}},
+	drawTest{"fillNil", fillBlue(90), nil, Over, image.RGBAColor{88, 0, 90, 255}},
+	drawTest{"fillNilSrc", fillBlue(90), nil, Src, image.RGBAColor{0, 0, 90, 90}},
+	// Uniform mask (100%, 75%, nil) and variable source.
+	// At (x, y) == (8, 8):
+	// The destination pixel is {136, 0, 0, 255}.
+	// The source pixel is {0, 48, 0, 90}.
+	drawTest{"copy", vgradGreen(90), fillAlpha(255), Over, image.RGBAColor{88, 48, 0, 255}},
+	drawTest{"copySrc", vgradGreen(90), fillAlpha(255), Src, image.RGBAColor{0, 48, 0, 90}},
+	drawTest{"copyAlpha", vgradGreen(90), fillAlpha(192), Over, image.RGBAColor{100, 36, 0, 255}},
+	drawTest{"copyAlphaSrc", vgradGreen(90), fillAlpha(192), Src, image.RGBAColor{0, 36, 0, 68}},
+	drawTest{"copyNil", vgradGreen(90), nil, Over, image.RGBAColor{88, 48, 0, 255}},
+	drawTest{"copyNilSrc", vgradGreen(90), nil, Src, image.RGBAColor{0, 48, 0, 90}},
+	// Variable mask and variable source.
+	// At (x, y) == (8, 8):
 	// The destination pixel is {136, 0, 0, 255}.
 	// The source pixel is {0, 0, 255, 255}.
 	// The mask pixel's alpha is 102, or 40%.
-	drawTest{"generic", fillBlue(255), vgradAlpha(192), image.RGBAColor{81, 0, 102, 255}},
+	drawTest{"generic", fillBlue(255), vgradAlpha(192), Over, image.RGBAColor{81, 0, 102, 255}},
+	drawTest{"genericSrc", fillBlue(255), vgradAlpha(192), Src, image.RGBAColor{0, 0, 102, 102}},
 }
 
 func TestDraw(t *testing.T) {
 	for _, test := range drawTests {
 		dst := hgradRed(255)
-		DrawMask(dst, Rect(0, 0, 16, 16), test.src, ZP, test.mask, ZP, SoverD)
+		DrawMask(dst, Rect(0, 0, 16, 16), test.src, ZP, test.mask, ZP, test.op)
 		if !eq(dst.At(8, 8), test.expected) {
 			t.Errorf("draw %s: %v versus %v", test.desc, dst.At(8, 8), test.expected)
 		}