image/draw: add draw tests where the destination image doesn't start

at (0, 0).

Also refactor the test to use the SubImage method rather than monkeying
with an image's Pix and Rect fields.

R=r
CC=golang-dev
https://golang.org/cl/4678045

src/pkg/image/draw/draw_test.go

@@ -154,22 +154,32 @@ var drawTests = []drawTest{
 	{"genericSrc", fillBlue(255), vgradAlpha(192), Src, image.RGBAColor{0, 0, 102, 102}},
 }
 
-func makeGolden(dst, src, mask image.Image, op Op) image.Image {
+func makeGolden(dst image.Image, r image.Rectangle, src image.Image, sp image.Point, mask image.Image, mp image.Point, 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()
-	sx0 := src.Bounds().Min.X - b.Min.X
-	sy0 := src.Bounds().Min.Y - b.Min.Y
-	var mx0, my0 int
+	sb := src.Bounds()
+	mb := image.Rect(-1e9, -1e9, 1e9, 1e9)
 	if mask != nil {
-		mx0 = mask.Bounds().Min.X - b.Min.X
-		my0 = mask.Bounds().Min.Y - b.Min.Y
+		mb = mask.Bounds()
 	}
 	golden := image.NewRGBA(b.Max.X, b.Max.Y)
-	for y := b.Min.Y; y < b.Max.Y; y++ {
-		my, sy := my0+y, sy0+y
-		for x := b.Min.X; x < b.Max.X; x++ {
-			mx, sx := mx0+x, sx0+x
+	for y := r.Min.Y; y < r.Max.Y; y++ {
+		sy := y + sp.Y - r.Min.Y
+		my := y + mp.Y - r.Min.Y
+		for x := r.Min.X; x < r.Max.X; x++ {
+			if !(image.Point{x, y}.In(b)) {
+				continue
+			}
+			sx := x + sp.X - r.Min.X
+			if !(image.Point{sx, sy}.In(sb)) {
+				continue
+			}
+			mx := x + mp.X - r.Min.X
+			if !(image.Point{mx, my}.In(mb)) {
+				continue
+			}
+
 			const M = 1<<16 - 1
 			var dr, dg, db, da uint32
 			if op == Over {
@@ -189,39 +199,53 @@ func makeGolden(dst, src, mask image.Image, op Op) image.Image {
 			})
 		}
 	}
-	golden.Rect = b
-	return golden
+	return golden.SubImage(b)
 }
 
 func TestDraw(t *testing.T) {
-loop:
+	rr := []image.Rectangle{
+		image.Rect(0, 0, 0, 0),
+		image.Rect(0, 0, 16, 16),
+		image.Rect(3, 5, 12, 10),
+		image.Rect(0, 0, 9, 9),
+		image.Rect(8, 8, 16, 16),
+		image.Rect(8, 0, 9, 16),
+		image.Rect(0, 8, 16, 9),
+		image.Rect(8, 8, 9, 9),
+		image.Rect(8, 8, 8, 8),
+	}
+	for _, r := range rr {
+	loop:
 		for _, test := range drawTests {
-		dst := hgradRed(255)
+			dst := hgradRed(255).(*image.RGBA).SubImage(r).(Image)
 			// 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)
+			golden := makeGolden(dst, image.Rect(0, 0, 16, 16), test.src, image.ZP, test.mask, image.ZP, test.op)
 			b := dst.Bounds()
 			if !b.Eq(golden.Bounds()) {
-			t.Errorf("draw %s: bounds %v versus %v", test.desc, dst.Bounds(), golden.Bounds())
+				t.Errorf("draw %v %s: bounds %v versus %v", r, test.desc, dst.Bounds(), golden.Bounds())
 				continue
 			}
 			// Draw the same combination onto the actual dst using the optimized DrawMask implementation.
-		DrawMask(dst, b, test.src, image.ZP, test.mask, image.ZP, test.op)
+			DrawMask(dst, image.Rect(0, 0, 16, 16), test.src, image.ZP, test.mask, image.ZP, test.op)
+			if image.Pt(8, 8).In(r) {
 				// 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) {
-			t.Errorf("draw %s: at (8, 8) %v versus %v", test.desc, dst.At(8, 8), test.expected)
+					t.Errorf("draw %v %s: at (8, 8) %v versus %v", r, test.desc, dst.At(8, 8), test.expected)
 					continue
 				}
+			}
 			// 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("draw %s: at (%d, %d), %v versus golden %v", test.desc, x, y, dst.At(x, y), golden.At(x, y))
+						t.Errorf("draw %v %s: at (%d, %d), %v versus golden %v", r, test.desc, x, y, dst.At(x, y), golden.At(x, y))
 						continue loop
 					}
 				}
 			}
 		}
+	}
 }
 
 func TestDrawOverlap(t *testing.T) {
@@ -230,19 +254,11 @@ func TestDrawOverlap(t *testing.T) {
 		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)
+				dst := m.SubImage(image.Rect(5, 5, 10, 10)).(*image.RGBA)
+				src := m.SubImage(image.Rect(5+xoff, 5+yoff, 10+xoff, 10+yoff)).(*image.RGBA)
 				b := dst.Bounds()
+				// Draw the (src, mask, op) onto a copy of dst using a slow but obviously correct implementation.
+				golden := makeGolden(dst, b, src, src.Bounds().Min, nil, image.ZP, op)
 				if !b.Eq(golden.Bounds()) {
 					t.Errorf("drawOverlap xoff=%d,yoff=%d: bounds %v versus %v", xoff, yoff, dst.Bounds(), golden.Bounds())
 					continue