Fix error in YCC PixOffset and SubImage.

ycc.go

@@ -38,7 +38,7 @@ type ycc struct {
 // PixOffset returns the index of the first element of Pix that corresponds to
 // the pixel at (x, y).
 func (p *ycc) PixOffset(x, y int) int {
-	return (y * p.Stride) + (x * 3)
+	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*3
 }
 
 func (p *ycc) Bounds() image.Rectangle {

ycc_test.go

@@ -18,6 +18,7 @@ package resize
 
 import (
 	"image"
+	"image/color"
 	"testing"
 )
 
@@ -119,3 +120,95 @@ func TestConvertYCbCr(t *testing.T) {
 		}
 	}
 }
+
+func TestYCbCr(t *testing.T) {
+	rects := []image.Rectangle{
+		image.Rect(0, 0, 16, 16),
+		image.Rect(1, 0, 16, 16),
+		image.Rect(0, 1, 16, 16),
+		image.Rect(1, 1, 16, 16),
+		image.Rect(1, 1, 15, 16),
+		image.Rect(1, 1, 16, 15),
+		image.Rect(1, 1, 15, 15),
+		image.Rect(2, 3, 14, 15),
+		image.Rect(7, 0, 7, 16),
+		image.Rect(0, 8, 16, 8),
+		image.Rect(0, 0, 10, 11),
+		image.Rect(5, 6, 16, 16),
+		image.Rect(7, 7, 8, 8),
+		image.Rect(7, 8, 8, 9),
+		image.Rect(8, 7, 9, 8),
+		image.Rect(8, 8, 9, 9),
+		image.Rect(7, 7, 17, 17),
+		image.Rect(8, 8, 17, 17),
+		image.Rect(9, 9, 17, 17),
+		image.Rect(10, 10, 17, 17),
+	}
+	subsampleRatios := []image.YCbCrSubsampleRatio{
+		image.YCbCrSubsampleRatio444,
+		image.YCbCrSubsampleRatio422,
+		image.YCbCrSubsampleRatio420,
+		image.YCbCrSubsampleRatio440,
+	}
+	deltas := []image.Point{
+		image.Pt(0, 0),
+		image.Pt(1000, 1001),
+		image.Pt(5001, -400),
+		image.Pt(-701, -801),
+	}
+	for _, r := range rects {
+		for _, subsampleRatio := range subsampleRatios {
+			for _, delta := range deltas {
+				testYCbCr(t, r, subsampleRatio, delta)
+			}
+		}
+		if testing.Short() {
+			break
+		}
+	}
+}
+
+func testYCbCr(t *testing.T, r image.Rectangle, subsampleRatio image.YCbCrSubsampleRatio, delta image.Point) {
+	// Create a YCbCr image m, whose bounds are r translated by (delta.X, delta.Y).
+	r1 := r.Add(delta)
+	img := image.NewYCbCr(r1, subsampleRatio)
+
+	// Initialize img's pixels. For 422 and 420 subsampling, some of the Cb and Cr elements
+	// will be set multiple times. That's OK. We just want to avoid a uniform image.
+	for y := r1.Min.Y; y < r1.Max.Y; y++ {
+		for x := r1.Min.X; x < r1.Max.X; x++ {
+			yi := img.YOffset(x, y)
+			ci := img.COffset(x, y)
+			img.Y[yi] = uint8(16*y + x)
+			img.Cb[ci] = uint8(y + 16*x)
+			img.Cr[ci] = uint8(y + 16*x)
+		}
+	}
+
+	m := imageYCbCrToYCC(img)
+
+	// Make various sub-images of m.
+	for y0 := delta.Y + 3; y0 < delta.Y+7; y0++ {
+		for y1 := delta.Y + 8; y1 < delta.Y+13; y1++ {
+			for x0 := delta.X + 3; x0 < delta.X+7; x0++ {
+				for x1 := delta.X + 8; x1 < delta.X+13; x1++ {
+					subRect := image.Rect(x0, y0, x1, y1)
+					sub := m.SubImage(subRect).(*ycc)
+
+					// For each point in the sub-image's bounds, check that m.At(x, y) equals sub.At(x, y).
+					for y := sub.Rect.Min.Y; y < sub.Rect.Max.Y; y++ {
+						for x := sub.Rect.Min.X; x < sub.Rect.Max.X; x++ {
+							color0 := m.At(x, y).(color.YCbCr)
+							color1 := sub.At(x, y).(color.YCbCr)
+							if color0 != color1 {
+								t.Errorf("r=%v, subsampleRatio=%v, delta=%v, x=%d, y=%d, color0=%v, color1=%v",
+									r, subsampleRatio, delta, x, y, color0, color1)
+								return
+							}
+						}
+					}
+				}
+			}
+		}
+	}
+}