Fixes #836.

R=gri
CC=golang-dev
https://golang.org/cl/1548042

src/pkg/exp/draw/draw.go

@@ -182,7 +182,7 @@ func drawCopyOver(dst *image.RGBA, r Rectangle, src *image.RGBA, sp Point) {
 	y0, y1 := r.Min.Y, r.Max.Y
 	for y, sy := y0, sp.Y; y != y1; y, sy = y+1, sy+1 {
 		dpix := dst.Pixel[y]
-		spix := src.Pixel[y]
+		spix := src.Pixel[sy]
 		for x, sx := x0, sp.X; x != x1; x, sx = x+1, sx+1 {
 			// For unknown reasons, even though both dpix[x] and spix[sx] are
 			// image.RGBAColors, on an x86 CPU it seems fastest to call RGBA

src/pkg/exp/draw/draw_test.go

@@ -149,3 +149,17 @@ loop:
 		}
 	}
 }
+
+// TestIssue836 verifies http://code.google.com/p/go/issues/detail?id=836.
+func TestIssue836(t *testing.T) {
+	a := image.NewRGBA(1, 1)
+	b := image.NewRGBA(2, 2)
+	b.Set(0, 0, image.RGBAColor{0, 0, 0, 5})
+	b.Set(1, 0, image.RGBAColor{0, 0, 5, 5})
+	b.Set(0, 1, image.RGBAColor{0, 5, 0, 5})
+	b.Set(1, 1, image.RGBAColor{5, 0, 0, 5})
+	Draw(a, Rect(0, 0, 1, 1), b, Pt(1, 1))
+	if !eq(image.RGBAColor{5, 0, 0, 5}, a.At(0, 0)) {
+		t.Errorf("Issue 836: want %v got %v", image.RGBAColor{5, 0, 0, 5}, a.At(0, 0))
+	}
+}