exp/draw: fix drawRGBA when x0 > x1 and we have to draw right-to-left.

R=r
CC=golang-dev
https://golang.org/cl/1940042

diff --git a/src/pkg/exp/draw/draw.go b/src/pkg/exp/draw/draw.go
index 2c985a2d3eff0c3aad81fee59a08d3b102bbd9e3..b6b9eff4f8b62edd12636b1fb359a008b9f38760 100644 (file)
--- a/src/pkg/exp/draw/draw.go
+++ b/src/pkg/exp/draw/draw.go
@@ -293,9 +293,8 @@ func drawRGBA(dst *image.RGBA, r image.Rectangle, src image.Image, sp image.Poin
        for y := y0; y != y1; y, sy, my = y+dy, sy+dy, my+dy {
                sx := sp.X + x0 - r.Min.X
                mx := mp.X + x0 - r.Min.X
-               dbase := y * dst.Stride
-               dpix := dst.Pix[dbase+x0 : dbase+x1]
-               for i, rgba := range dpix {
+               dpix := dst.Pix[y*dst.Stride : (y+1)*dst.Stride]
+               for x := x0; x != x1; x, sx, mx = x+dx, sx+dx, mx+dx {
                        ma := uint32(m)
                        if mask != nil {
                                _, _, _, ma = mask.At(mx, my).RGBA()
@@ -303,6 +302,7 @@ func drawRGBA(dst *image.RGBA, r image.Rectangle, src image.Image, sp image.Poin
                        sr, sg, sb, sa := src.At(sx, sy).RGBA()
                        var dr, dg, db, da uint32
                        if op == Over {
+                               rgba := dpix[x]
                                dr = uint32(rgba.R)
                                dg = uint32(rgba.G)
                                db = uint32(rgba.B)
@@ -324,8 +324,7 @@ func drawRGBA(dst *image.RGBA, r image.Rectangle, src image.Image, sp image.Poin
                                db = sb * ma / m
                                da = sa * ma / m
                        }
-                       dpix[i] = image.RGBAColor{uint8(dr >> 8), uint8(dg >> 8), uint8(db >> 8), uint8(da >> 8)}
-                       sx, mx = sx+dx, mx+dx
+                       dpix[x] = image.RGBAColor{uint8(dr >> 8), uint8(dg >> 8), uint8(db >> 8), uint8(da >> 8)}
                }
        }
 }