libgo: Update to weekly.2012-01-20.

From-SVN: r183540

9 files changed, 355 insertions, 154 deletions

diff --git a/libgo/go/image/color/color.go b/libgo/go/image/color/color.go
index 2948db7f38c..29a7b8a4001 100644
--- a/libgo/go/image/color/color.go
+++ b/libgo/go/image/color/color.go
@@ -152,26 +152,35 @@ func (m *modelFunc) Convert(c Color) Color {
 	return m.f(c)
 }
 
-// RGBAModel is the Model for RGBA colors.
-var RGBAModel Model = ModelFunc(func(c Color) Color {
+// Models for the standard color types.
+var (
+	RGBAModel    Model = ModelFunc(rgbaModel)
+	RGBA64Model  Model = ModelFunc(rgba64Model)
+	NRGBAModel   Model = ModelFunc(nrgbaModel)
+	NRGBA64Model Model = ModelFunc(nrgba64Model)
+	AlphaModel   Model = ModelFunc(alphaModel)
+	Alpha16Model Model = ModelFunc(alpha16Model)
+	GrayModel    Model = ModelFunc(grayModel)
+	Gray16Model  Model = ModelFunc(gray16Model)
+)
+
+func rgbaModel(c Color) Color {
 	if _, ok := c.(RGBA); ok {
 		return c
 	}
 	r, g, b, a := c.RGBA()
 	return RGBA{uint8(r >> 8), uint8(g >> 8), uint8(b >> 8), uint8(a >> 8)}
-})
+}
 
-// RGBAModel is the Model for RGBA64 colors.
-var RGBA64Model Model = ModelFunc(func(c Color) Color {
+func rgba64Model(c Color) Color {
 	if _, ok := c.(RGBA64); ok {
 		return c
 	}
 	r, g, b, a := c.RGBA()
 	return RGBA64{uint16(r), uint16(g), uint16(b), uint16(a)}
-})
+}
 
-// NRGBAModel is the Model for NRGBA colors.
-var NRGBAModel Model = ModelFunc(func(c Color) Color {
+func nrgbaModel(c Color) Color {
 	if _, ok := c.(NRGBA); ok {
 		return c
 	}
@@ -187,10 +196,9 @@ var NRGBAModel Model = ModelFunc(func(c Color) Color {
 	g = (g * 0xffff) / a
 	b = (b * 0xffff) / a
 	return NRGBA{uint8(r >> 8), uint8(g >> 8), uint8(b >> 8), uint8(a >> 8)}
-})
+}
 
-// NRGBAModel is the Model for NRGBA64 colors.
-var NRGBA64Model Model = ModelFunc(func(c Color) Color {
+func nrgba64Model(c Color) Color {
 	if _, ok := c.(NRGBA64); ok {
 		return c
 	}
@@ -206,45 +214,41 @@ var NRGBA64Model Model = ModelFunc(func(c Color) Color {
 	g = (g * 0xffff) / a
 	b = (b * 0xffff) / a
 	return NRGBA64{uint16(r), uint16(g), uint16(b), uint16(a)}
-})
+}
 
-// AlphaModel is the Model for Alpha colors.
-var AlphaModel Model = ModelFunc(func(c Color) Color {
+func alphaModel(c Color) Color {
 	if _, ok := c.(Alpha); ok {
 		return c
 	}
 	_, _, _, a := c.RGBA()
 	return Alpha{uint8(a >> 8)}
-})
+}
 
-// Alpha16Model is the Model for Alpha16 colors.
-var Alpha16Model Model = ModelFunc(func(c Color) Color {
+func alpha16Model(c Color) Color {
 	if _, ok := c.(Alpha16); ok {
 		return c
 	}
 	_, _, _, a := c.RGBA()
 	return Alpha16{uint16(a)}
-})
+}
 
-// GrayModel is the Model for Gray colors.
-var GrayModel Model = ModelFunc(func(c Color) Color {
+func grayModel(c Color) Color {
 	if _, ok := c.(Gray); ok {
 		return c
 	}
 	r, g, b, _ := c.RGBA()
 	y := (299*r + 587*g + 114*b + 500) / 1000
 	return Gray{uint8(y >> 8)}
-})
+}
 
-// Gray16Model is the Model for Gray16 colors.
-var Gray16Model Model = ModelFunc(func(c Color) Color {
+func gray16Model(c Color) Color {
 	if _, ok := c.(Gray16); ok {
 		return c
 	}
 	r, g, b, _ := c.RGBA()
 	y := (299*r + 587*g + 114*b + 500) / 1000
 	return Gray16{uint16(y)}
-})
+}
 
 // Palette is a palette of colors.
 type Palette []Color
@@ -290,13 +294,10 @@ func (p Palette) Index(c Color) int {
 	return ret
 }
 
+// Standard colors.
 var (
-	// Black is an opaque black Color.
-	Black = Gray16{0}
-	// White is an opaque white Color.
-	White = Gray16{0xffff}
-	// Transparent is a fully transparent Color.
+	Black       = Gray16{0}
+	White       = Gray16{0xffff}
 	Transparent = Alpha16{0}
-	// Opaque is a fully opaque Color.
-	Opaque = Alpha16{0xffff}
+	Opaque      = Alpha16{0xffff}
 )
diff --git a/libgo/go/image/color/ycbcr.go b/libgo/go/image/color/ycbcr.go
index c79816dd45a..4c2f29ea021 100644
--- a/libgo/go/image/color/ycbcr.go
+++ b/libgo/go/image/color/ycbcr.go
@@ -4,8 +4,7 @@
 
 package color
 
-// RGBToYCbCr converts an RGB triple to a Y'CbCr triple. All components lie
-// within the range [0, 255].
+// RGBToYCbCr converts an RGB triple to a Y'CbCr triple.
 func RGBToYCbCr(r, g, b uint8) (uint8, uint8, uint8) {
 	// The JFIF specification says:
 	//	Y' =  0.2990*R + 0.5870*G + 0.1140*B
@@ -36,8 +35,7 @@ func RGBToYCbCr(r, g, b uint8) (uint8, uint8, uint8) {
 	return uint8(yy), uint8(cb), uint8(cr)
 }
 
-// YCbCrToRGB converts a Y'CbCr triple to an RGB triple. All components lie
-// within the range [0, 255].
+// YCbCrToRGB converts a Y'CbCr triple to an RGB triple.
 func YCbCrToRGB(y, cb, cr uint8) (uint8, uint8, uint8) {
 	// The JFIF specification says:
 	//	R = Y' + 1.40200*(Cr-128)
@@ -89,11 +87,13 @@ func (c YCbCr) RGBA() (uint32, uint32, uint32, uint32) {
 }
 
 // YCbCrModel is the Model for Y'CbCr colors.
-var YCbCrModel Model = ModelFunc(func(c Color) Color {
+var YCbCrModel Model = ModelFunc(yCbCrModel)
+
+func yCbCrModel(c Color) Color {
 	if _, ok := c.(YCbCr); ok {
 		return c
 	}
 	r, g, b, _ := c.RGBA()
 	y, u, v := RGBToYCbCr(uint8(r>>8), uint8(g>>8), uint8(b>>8))
 	return YCbCr{y, u, v}
-})
+}
diff --git a/libgo/go/image/draw/draw.go b/libgo/go/image/draw/draw.go
index 228ed6e719c..3b6679f7c7e 100644
--- a/libgo/go/image/draw/draw.go
+++ b/libgo/go/image/draw/draw.go
@@ -171,7 +171,7 @@ func drawFillOver(dst *image.RGBA, r image.Rectangle, src *image.Uniform) {
 	sr, sg, sb, sa := src.RGBA()
 	// The 0x101 is here for the same reason as in drawRGBA.
 	a := (m - sa) * 0x101
-	i0 := (r.Min.Y-dst.Rect.Min.Y)*dst.Stride + (r.Min.X-dst.Rect.Min.X)*4
+	i0 := dst.PixOffset(r.Min.X, r.Min.Y)
 	i1 := i0 + r.Dx()*4
 	for y := r.Min.Y; y != r.Max.Y; y++ {
 		for i := i0; i < i1; i += 4 {
@@ -195,7 +195,7 @@ func drawFillSrc(dst *image.RGBA, r image.Rectangle, src *image.Uniform) {
 	// The built-in copy function is faster than a straightforward for loop to fill the destination with
 	// the color, but copy requires a slice source. We therefore use a for loop to fill the first row, and
 	// then use the first row as the slice source for the remaining rows.
-	i0 := (r.Min.Y-dst.Rect.Min.Y)*dst.Stride + (r.Min.X-dst.Rect.Min.X)*4
+	i0 := dst.PixOffset(r.Min.X, r.Min.Y)
 	i1 := i0 + r.Dx()*4
 	for i := i0; i < i1; i += 4 {
 		dst.Pix[i+0] = uint8(sr >> 8)
@@ -213,8 +213,8 @@ func drawFillSrc(dst *image.RGBA, r image.Rectangle, src *image.Uniform) {
 
 func drawCopyOver(dst *image.RGBA, r image.Rectangle, src *image.RGBA, sp image.Point) {
 	dx, dy := r.Dx(), r.Dy()
-	d0 := (r.Min.Y-dst.Rect.Min.Y)*dst.Stride + (r.Min.X-dst.Rect.Min.X)*4
-	s0 := (sp.Y-src.Rect.Min.Y)*src.Stride + (sp.X-src.Rect.Min.X)*4
+	d0 := dst.PixOffset(r.Min.X, r.Min.Y)
+	s0 := src.PixOffset(sp.X, sp.Y)
 	var (
 		ddelta, sdelta int
 		i0, i1, idelta int
@@ -261,8 +261,8 @@ func drawCopyOver(dst *image.RGBA, r image.Rectangle, src *image.RGBA, sp image.
 
 func drawCopySrc(dst *image.RGBA, r image.Rectangle, src *image.RGBA, sp image.Point) {
 	n, dy := 4*r.Dx(), r.Dy()
-	d0 := (r.Min.Y-dst.Rect.Min.Y)*dst.Stride + (r.Min.X-dst.Rect.Min.X)*4
-	s0 := (sp.Y-src.Rect.Min.Y)*src.Stride + (sp.X-src.Rect.Min.X)*4
+	d0 := dst.PixOffset(r.Min.X, r.Min.Y)
+	s0 := src.PixOffset(sp.X, sp.Y)
 	var ddelta, sdelta int
 	if r.Min.Y <= sp.Y {
 		ddelta = dst.Stride
@@ -348,9 +348,6 @@ func drawNRGBASrc(dst *image.RGBA, r image.Rectangle, src *image.NRGBA, sp image
 func drawYCbCr(dst *image.RGBA, r image.Rectangle, src *image.YCbCr, sp image.Point) {
 	// An image.YCbCr is always fully opaque, and so if the mask is implicitly nil
 	// (i.e. fully opaque) then the op is effectively always Src.
-	var (
-		yy, cb, cr uint8
-	)
 	x0 := (r.Min.X - dst.Rect.Min.X) * 4
 	x1 := (r.Max.X - dst.Rect.Min.X) * 4
 	y0 := r.Min.Y - dst.Rect.Min.Y
@@ -359,12 +356,11 @@ func drawYCbCr(dst *image.RGBA, r image.Rectangle, src *image.YCbCr, sp image.Po
 	case image.YCbCrSubsampleRatio422:
 		for y, sy := y0, sp.Y; y != y1; y, sy = y+1, sy+1 {
 			dpix := dst.Pix[y*dst.Stride:]
-			for x, sx := x0, sp.X; x != x1; x, sx = x+4, sx+1 {
-				i := sx / 2
-				yy = src.Y[sy*src.YStride+sx]
-				cb = src.Cb[sy*src.CStride+i]
-				cr = src.Cr[sy*src.CStride+i]
-				rr, gg, bb := color.YCbCrToRGB(yy, cb, cr)
+			yi := (sy-src.Rect.Min.Y)*src.YStride + (sp.X - src.Rect.Min.X)
+			ciBase := (sy-src.Rect.Min.Y)*src.CStride - src.Rect.Min.X/2
+			for x, sx := x0, sp.X; x != x1; x, sx, yi = x+4, sx+1, yi+1 {
+				ci := ciBase + sx/2
+				rr, gg, bb := color.YCbCrToRGB(src.Y[yi], src.Cb[ci], src.Cr[ci])
 				dpix[x+0] = rr
 				dpix[x+1] = gg
 				dpix[x+2] = bb
@@ -374,12 +370,11 @@ func drawYCbCr(dst *image.RGBA, r image.Rectangle, src *image.YCbCr, sp image.Po
 	case image.YCbCrSubsampleRatio420:
 		for y, sy := y0, sp.Y; y != y1; y, sy = y+1, sy+1 {
 			dpix := dst.Pix[y*dst.Stride:]
-			for x, sx := x0, sp.X; x != x1; x, sx = x+4, sx+1 {
-				i, j := sx/2, sy/2
-				yy = src.Y[sy*src.YStride+sx]
-				cb = src.Cb[j*src.CStride+i]
-				cr = src.Cr[j*src.CStride+i]
-				rr, gg, bb := color.YCbCrToRGB(yy, cb, cr)
+			yi := (sy-src.Rect.Min.Y)*src.YStride + (sp.X - src.Rect.Min.X)
+			ciBase := (sy/2-src.Rect.Min.Y/2)*src.CStride - src.Rect.Min.X/2
+			for x, sx := x0, sp.X; x != x1; x, sx, yi = x+4, sx+1, yi+1 {
+				ci := ciBase + sx/2
+				rr, gg, bb := color.YCbCrToRGB(src.Y[yi], src.Cb[ci], src.Cr[ci])
 				dpix[x+0] = rr
 				dpix[x+1] = gg
 				dpix[x+2] = bb
@@ -390,11 +385,10 @@ func drawYCbCr(dst *image.RGBA, r image.Rectangle, src *image.YCbCr, sp image.Po
 		// Default to 4:4:4 subsampling.
 		for y, sy := y0, sp.Y; y != y1; y, sy = y+1, sy+1 {
 			dpix := dst.Pix[y*dst.Stride:]
-			for x, sx := x0, sp.X; x != x1; x, sx = x+4, sx+1 {
-				yy = src.Y[sy*src.YStride+sx]
-				cb = src.Cb[sy*src.CStride+sx]
-				cr = src.Cr[sy*src.CStride+sx]
-				rr, gg, bb := color.YCbCrToRGB(yy, cb, cr)
+			yi := (sy-src.Rect.Min.Y)*src.YStride + (sp.X - src.Rect.Min.X)
+			ci := (sy-src.Rect.Min.Y)*src.CStride + (sp.X - src.Rect.Min.X)
+			for x := x0; x != x1; x, yi, ci = x+4, yi+1, ci+1 {
+				rr, gg, bb := color.YCbCrToRGB(src.Y[yi], src.Cb[ci], src.Cr[ci])
 				dpix[x+0] = rr
 				dpix[x+1] = gg
 				dpix[x+2] = bb
@@ -405,9 +399,9 @@ func drawYCbCr(dst *image.RGBA, r image.Rectangle, src *image.YCbCr, sp image.Po
 }
 
 func drawGlyphOver(dst *image.RGBA, r image.Rectangle, src *image.Uniform, mask *image.Alpha, mp image.Point) {
-	i0 := (r.Min.Y-dst.Rect.Min.Y)*dst.Stride + (r.Min.X-dst.Rect.Min.X)*4
+	i0 := dst.PixOffset(r.Min.X, r.Min.Y)
 	i1 := i0 + r.Dx()*4
-	mi0 := (mp.Y-mask.Rect.Min.Y)*mask.Stride + mp.X - mask.Rect.Min.X
+	mi0 := mask.PixOffset(mp.X, mp.Y)
 	sr, sg, sb, sa := src.RGBA()
 	for y, my := r.Min.Y, mp.Y; y != r.Max.Y; y, my = y+1, my+1 {
 		for i, mi := i0, mi0; i < i1; i, mi = i+4, mi+1 {
@@ -451,7 +445,7 @@ func drawRGBA(dst *image.RGBA, r image.Rectangle, src image.Image, sp image.Poin
 	sx0 := sp.X + x0 - r.Min.X
 	mx0 := mp.X + x0 - r.Min.X
 	sx1 := sx0 + (x1 - x0)
-	i0 := (y0-dst.Rect.Min.Y)*dst.Stride + (x0-dst.Rect.Min.X)*4
+	i0 := dst.PixOffset(x0, y0)
 	di := dx * 4
 	for y := y0; y != y1; y, sy, my = y+dy, sy+dy, my+dy {
 		for i, sx, mx := i0, sx0, mx0; sx != sx1; i, sx, mx = i+di, sx+dx, mx+dx {
diff --git a/libgo/go/image/geom.go b/libgo/go/image/geom.go
index 667aee6259e..e123483314c 100644
--- a/libgo/go/image/geom.go
+++ b/libgo/go/image/geom.go
@@ -112,7 +112,7 @@ func (r Rectangle) Add(p Point) Rectangle {
 	}
 }
 
-// Add returns the rectangle r translated by -p.
+// Sub returns the rectangle r translated by -p.
 func (r Rectangle) Sub(p Point) Rectangle {
 	return Rectangle{
 		Point{r.Min.X - p.X, r.Min.Y - p.Y},
diff --git a/libgo/go/image/image.go b/libgo/go/image/image.go
index a0dd930c7c0..9dd676acedd 100644
--- a/libgo/go/image/image.go
+++ b/libgo/go/image/image.go
@@ -61,15 +61,21 @@ func (p *RGBA) At(x, y int) color.Color {
 	if !(Point{x, y}.In(p.Rect)) {
 		return color.RGBA{}
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*4
+	i := p.PixOffset(x, y)
 	return color.RGBA{p.Pix[i+0], p.Pix[i+1], p.Pix[i+2], p.Pix[i+3]}
 }
 
+// PixOffset returns the index of the first element of Pix that corresponds to
+// the pixel at (x, y).
+func (p *RGBA) PixOffset(x, y int) int {
+	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*4
+}
+
 func (p *RGBA) Set(x, y int, c color.Color) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*4
+	i := p.PixOffset(x, y)
 	c1 := color.RGBAModel.Convert(c).(color.RGBA)
 	p.Pix[i+0] = c1.R
 	p.Pix[i+1] = c1.G
@@ -81,7 +87,7 @@ func (p *RGBA) SetRGBA(x, y int, c color.RGBA) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*4
+	i := p.PixOffset(x, y)
 	p.Pix[i+0] = c.R
 	p.Pix[i+1] = c.G
 	p.Pix[i+2] = c.B
@@ -98,7 +104,7 @@ func (p *RGBA) SubImage(r Rectangle) Image {
 	if r.Empty() {
 		return &RGBA{}
 	}
-	i := (r.Min.Y-p.Rect.Min.Y)*p.Stride + (r.Min.X-p.Rect.Min.X)*4
+	i := p.PixOffset(r.Min.X, r.Min.Y)
 	return &RGBA{
 		Pix:    p.Pix[i:],
 		Stride: p.Stride,
@@ -124,7 +130,7 @@ func (p *RGBA) Opaque() bool {
 	return true
 }
 
-// NewRGBA returns a new RGBA with the given width and height.
+// NewRGBA returns a new RGBA with the given bounds.
 func NewRGBA(r Rectangle) *RGBA {
 	w, h := r.Dx(), r.Dy()
 	buf := make([]uint8, 4*w*h)
@@ -150,7 +156,7 @@ func (p *RGBA64) At(x, y int) color.Color {
 	if !(Point{x, y}.In(p.Rect)) {
 		return color.RGBA64{}
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*8
+	i := p.PixOffset(x, y)
 	return color.RGBA64{
 		uint16(p.Pix[i+0])<<8 | uint16(p.Pix[i+1]),
 		uint16(p.Pix[i+2])<<8 | uint16(p.Pix[i+3]),
@@ -159,11 +165,17 @@ func (p *RGBA64) At(x, y int) color.Color {
 	}
 }
 
+// PixOffset returns the index of the first element of Pix that corresponds to
+// the pixel at (x, y).
+func (p *RGBA64) PixOffset(x, y int) int {
+	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*8
+}
+
 func (p *RGBA64) Set(x, y int, c color.Color) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*8
+	i := p.PixOffset(x, y)
 	c1 := color.RGBA64Model.Convert(c).(color.RGBA64)
 	p.Pix[i+0] = uint8(c1.R >> 8)
 	p.Pix[i+1] = uint8(c1.R)
@@ -179,7 +191,7 @@ func (p *RGBA64) SetRGBA64(x, y int, c color.RGBA64) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*8
+	i := p.PixOffset(x, y)
 	p.Pix[i+0] = uint8(c.R >> 8)
 	p.Pix[i+1] = uint8(c.R)
 	p.Pix[i+2] = uint8(c.G >> 8)
@@ -200,7 +212,7 @@ func (p *RGBA64) SubImage(r Rectangle) Image {
 	if r.Empty() {
 		return &RGBA64{}
 	}
-	i := (r.Min.Y-p.Rect.Min.Y)*p.Stride + (r.Min.X-p.Rect.Min.X)*8
+	i := p.PixOffset(r.Min.X, r.Min.Y)
 	return &RGBA64{
 		Pix:    p.Pix[i:],
 		Stride: p.Stride,
@@ -226,7 +238,7 @@ func (p *RGBA64) Opaque() bool {
 	return true
 }
 
-// NewRGBA64 returns a new RGBA64 with the given width and height.
+// NewRGBA64 returns a new RGBA64 with the given bounds.
 func NewRGBA64(r Rectangle) *RGBA64 {
 	w, h := r.Dx(), r.Dy()
 	pix := make([]uint8, 8*w*h)
@@ -252,15 +264,21 @@ func (p *NRGBA) At(x, y int) color.Color {
 	if !(Point{x, y}.In(p.Rect)) {
 		return color.NRGBA{}
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*4
+	i := p.PixOffset(x, y)
 	return color.NRGBA{p.Pix[i+0], p.Pix[i+1], p.Pix[i+2], p.Pix[i+3]}
 }
 
+// PixOffset returns the index of the first element of Pix that corresponds to
+// the pixel at (x, y).
+func (p *NRGBA) PixOffset(x, y int) int {
+	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*4
+}
+
 func (p *NRGBA) Set(x, y int, c color.Color) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*4
+	i := p.PixOffset(x, y)
 	c1 := color.NRGBAModel.Convert(c).(color.NRGBA)
 	p.Pix[i+0] = c1.R
 	p.Pix[i+1] = c1.G
@@ -272,7 +290,7 @@ func (p *NRGBA) SetNRGBA(x, y int, c color.NRGBA) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*4
+	i := p.PixOffset(x, y)
 	p.Pix[i+0] = c.R
 	p.Pix[i+1] = c.G
 	p.Pix[i+2] = c.B
@@ -289,7 +307,7 @@ func (p *NRGBA) SubImage(r Rectangle) Image {
 	if r.Empty() {
 		return &NRGBA{}
 	}
-	i := (r.Min.Y-p.Rect.Min.Y)*p.Stride + (r.Min.X-p.Rect.Min.X)*4
+	i := p.PixOffset(r.Min.X, r.Min.Y)
 	return &NRGBA{
 		Pix:    p.Pix[i:],
 		Stride: p.Stride,
@@ -315,7 +333,7 @@ func (p *NRGBA) Opaque() bool {
 	return true
 }
 
-// NewNRGBA returns a new NRGBA with the given width and height.
+// NewNRGBA returns a new NRGBA with the given bounds.
 func NewNRGBA(r Rectangle) *NRGBA {
 	w, h := r.Dx(), r.Dy()
 	pix := make([]uint8, 4*w*h)
@@ -341,7 +359,7 @@ func (p *NRGBA64) At(x, y int) color.Color {
 	if !(Point{x, y}.In(p.Rect)) {
 		return color.NRGBA64{}
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*8
+	i := p.PixOffset(x, y)
 	return color.NRGBA64{
 		uint16(p.Pix[i+0])<<8 | uint16(p.Pix[i+1]),
 		uint16(p.Pix[i+2])<<8 | uint16(p.Pix[i+3]),
@@ -350,11 +368,17 @@ func (p *NRGBA64) At(x, y int) color.Color {
 	}
 }
 
+// PixOffset returns the index of the first element of Pix that corresponds to
+// the pixel at (x, y).
+func (p *NRGBA64) PixOffset(x, y int) int {
+	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*8
+}
+
 func (p *NRGBA64) Set(x, y int, c color.Color) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*8
+	i := p.PixOffset(x, y)
 	c1 := color.NRGBA64Model.Convert(c).(color.NRGBA64)
 	p.Pix[i+0] = uint8(c1.R >> 8)
 	p.Pix[i+1] = uint8(c1.R)
@@ -370,7 +394,7 @@ func (p *NRGBA64) SetNRGBA64(x, y int, c color.NRGBA64) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*8
+	i := p.PixOffset(x, y)
 	p.Pix[i+0] = uint8(c.R >> 8)
 	p.Pix[i+1] = uint8(c.R)
 	p.Pix[i+2] = uint8(c.G >> 8)
@@ -391,7 +415,7 @@ func (p *NRGBA64) SubImage(r Rectangle) Image {
 	if r.Empty() {
 		return &NRGBA64{}
 	}
-	i := (r.Min.Y-p.Rect.Min.Y)*p.Stride + (r.Min.X-p.Rect.Min.X)*8
+	i := p.PixOffset(r.Min.X, r.Min.Y)
 	return &NRGBA64{
 		Pix:    p.Pix[i:],
 		Stride: p.Stride,
@@ -417,7 +441,7 @@ func (p *NRGBA64) Opaque() bool {
 	return true
 }
 
-// NewNRGBA64 returns a new NRGBA64 with the given width and height.
+// NewNRGBA64 returns a new NRGBA64 with the given bounds.
 func NewNRGBA64(r Rectangle) *NRGBA64 {
 	w, h := r.Dx(), r.Dy()
 	pix := make([]uint8, 8*w*h)
@@ -443,15 +467,21 @@ func (p *Alpha) At(x, y int) color.Color {
 	if !(Point{x, y}.In(p.Rect)) {
 		return color.Alpha{}
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x - p.Rect.Min.X)
+	i := p.PixOffset(x, y)
 	return color.Alpha{p.Pix[i]}
 }
 
+// PixOffset returns the index of the first element of Pix that corresponds to
+// the pixel at (x, y).
+func (p *Alpha) PixOffset(x, y int) int {
+	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*1
+}
+
 func (p *Alpha) Set(x, y int, c color.Color) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x - p.Rect.Min.X)
+	i := p.PixOffset(x, y)
 	p.Pix[i] = color.AlphaModel.Convert(c).(color.Alpha).A
 }
 
@@ -459,7 +489,7 @@ func (p *Alpha) SetAlpha(x, y int, c color.Alpha) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x - p.Rect.Min.X)
+	i := p.PixOffset(x, y)
 	p.Pix[i] = c.A
 }
 
@@ -473,7 +503,7 @@ func (p *Alpha) SubImage(r Rectangle) Image {
 	if r.Empty() {
 		return &Alpha{}
 	}
-	i := (r.Min.Y-p.Rect.Min.Y)*p.Stride + (r.Min.X-p.Rect.Min.X)*1
+	i := p.PixOffset(r.Min.X, r.Min.Y)
 	return &Alpha{
 		Pix:    p.Pix[i:],
 		Stride: p.Stride,
@@ -499,7 +529,7 @@ func (p *Alpha) Opaque() bool {
 	return true
 }
 
-// NewAlpha returns a new Alpha with the given width and height.
+// NewAlpha returns a new Alpha with the given bounds.
 func NewAlpha(r Rectangle) *Alpha {
 	w, h := r.Dx(), r.Dy()
 	pix := make([]uint8, 1*w*h)
@@ -525,15 +555,21 @@ func (p *Alpha16) At(x, y int) color.Color {
 	if !(Point{x, y}.In(p.Rect)) {
 		return color.Alpha16{}
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*2
+	i := p.PixOffset(x, y)
 	return color.Alpha16{uint16(p.Pix[i+0])<<8 | uint16(p.Pix[i+1])}
 }
 
+// PixOffset returns the index of the first element of Pix that corresponds to
+// the pixel at (x, y).
+func (p *Alpha16) PixOffset(x, y int) int {
+	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*2
+}
+
 func (p *Alpha16) Set(x, y int, c color.Color) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*2
+	i := p.PixOffset(x, y)
 	c1 := color.Alpha16Model.Convert(c).(color.Alpha16)
 	p.Pix[i+0] = uint8(c1.A >> 8)
 	p.Pix[i+1] = uint8(c1.A)
@@ -543,7 +579,7 @@ func (p *Alpha16) SetAlpha16(x, y int, c color.Alpha16) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*2
+	i := p.PixOffset(x, y)
 	p.Pix[i+0] = uint8(c.A >> 8)
 	p.Pix[i+1] = uint8(c.A)
 }
@@ -558,7 +594,7 @@ func (p *Alpha16) SubImage(r Rectangle) Image {
 	if r.Empty() {
 		return &Alpha16{}
 	}
-	i := (r.Min.Y-p.Rect.Min.Y)*p.Stride + (r.Min.X-p.Rect.Min.X)*2
+	i := p.PixOffset(r.Min.X, r.Min.Y)
 	return &Alpha16{
 		Pix:    p.Pix[i:],
 		Stride: p.Stride,
@@ -584,7 +620,7 @@ func (p *Alpha16) Opaque() bool {
 	return true
 }
 
-// NewAlpha16 returns a new Alpha16 with the given width and height.
+// NewAlpha16 returns a new Alpha16 with the given bounds.
 func NewAlpha16(r Rectangle) *Alpha16 {
 	w, h := r.Dx(), r.Dy()
 	pix := make([]uint8, 2*w*h)
@@ -610,15 +646,21 @@ func (p *Gray) At(x, y int) color.Color {
 	if !(Point{x, y}.In(p.Rect)) {
 		return color.Gray{}
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x - p.Rect.Min.X)
+	i := p.PixOffset(x, y)
 	return color.Gray{p.Pix[i]}
 }
 
+// PixOffset returns the index of the first element of Pix that corresponds to
+// the pixel at (x, y).
+func (p *Gray) PixOffset(x, y int) int {
+	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*1
+}
+
 func (p *Gray) Set(x, y int, c color.Color) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x - p.Rect.Min.X)
+	i := p.PixOffset(x, y)
 	p.Pix[i] = color.GrayModel.Convert(c).(color.Gray).Y
 }
 
@@ -626,7 +668,7 @@ func (p *Gray) SetGray(x, y int, c color.Gray) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x - p.Rect.Min.X)
+	i := p.PixOffset(x, y)
 	p.Pix[i] = c.Y
 }
 
@@ -640,7 +682,7 @@ func (p *Gray) SubImage(r Rectangle) Image {
 	if r.Empty() {
 		return &Gray{}
 	}
-	i := (r.Min.Y-p.Rect.Min.Y)*p.Stride + (r.Min.X-p.Rect.Min.X)*1
+	i := p.PixOffset(r.Min.X, r.Min.Y)
 	return &Gray{
 		Pix:    p.Pix[i:],
 		Stride: p.Stride,
@@ -653,7 +695,7 @@ func (p *Gray) Opaque() bool {
 	return true
 }
 
-// NewGray returns a new Gray with the given width and height.
+// NewGray returns a new Gray with the given bounds.
 func NewGray(r Rectangle) *Gray {
 	w, h := r.Dx(), r.Dy()
 	pix := make([]uint8, 1*w*h)
@@ -679,15 +721,21 @@ func (p *Gray16) At(x, y int) color.Color {
 	if !(Point{x, y}.In(p.Rect)) {
 		return color.Gray16{}
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*2
+	i := p.PixOffset(x, y)
 	return color.Gray16{uint16(p.Pix[i+0])<<8 | uint16(p.Pix[i+1])}
 }
 
+// PixOffset returns the index of the first element of Pix that corresponds to
+// the pixel at (x, y).
+func (p *Gray16) PixOffset(x, y int) int {
+	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*2
+}
+
 func (p *Gray16) Set(x, y int, c color.Color) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*2
+	i := p.PixOffset(x, y)
 	c1 := color.Gray16Model.Convert(c).(color.Gray16)
 	p.Pix[i+0] = uint8(c1.Y >> 8)
 	p.Pix[i+1] = uint8(c1.Y)
@@ -697,7 +745,7 @@ func (p *Gray16) SetGray16(x, y int, c color.Gray16) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*2
+	i := p.PixOffset(x, y)
 	p.Pix[i+0] = uint8(c.Y >> 8)
 	p.Pix[i+1] = uint8(c.Y)
 }
@@ -712,7 +760,7 @@ func (p *Gray16) SubImage(r Rectangle) Image {
 	if r.Empty() {
 		return &Gray16{}
 	}
-	i := (r.Min.Y-p.Rect.Min.Y)*p.Stride + (r.Min.X-p.Rect.Min.X)*2
+	i := p.PixOffset(r.Min.X, r.Min.Y)
 	return &Gray16{
 		Pix:    p.Pix[i:],
 		Stride: p.Stride,
@@ -725,7 +773,7 @@ func (p *Gray16) Opaque() bool {
 	return true
 }
 
-// NewGray16 returns a new Gray16 with the given width and height.
+// NewGray16 returns a new Gray16 with the given bounds.
 func NewGray16(r Rectangle) *Gray16 {
 	w, h := r.Dx(), r.Dy()
 	pix := make([]uint8, 2*w*h)
@@ -756,15 +804,21 @@ func (p *Paletted) At(x, y int) color.Color {
 	if !(Point{x, y}.In(p.Rect)) {
 		return p.Palette[0]
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x - p.Rect.Min.X)
+	i := p.PixOffset(x, y)
 	return p.Palette[p.Pix[i]]
 }
 
+// PixOffset returns the index of the first element of Pix that corresponds to
+// the pixel at (x, y).
+func (p *Paletted) PixOffset(x, y int) int {
+	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*1
+}
+
 func (p *Paletted) Set(x, y int, c color.Color) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x - p.Rect.Min.X)
+	i := p.PixOffset(x, y)
 	p.Pix[i] = uint8(p.Palette.Index(c))
 }
 
@@ -772,7 +826,7 @@ func (p *Paletted) ColorIndexAt(x, y int) uint8 {
 	if !(Point{x, y}.In(p.Rect)) {
 		return 0
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x - p.Rect.Min.X)
+	i := p.PixOffset(x, y)
 	return p.Pix[i]
 }
 
@@ -780,7 +834,7 @@ func (p *Paletted) SetColorIndex(x, y int, index uint8) {
 	if !(Point{x, y}.In(p.Rect)) {
 		return
 	}
-	i := (y-p.Rect.Min.Y)*p.Stride + (x - p.Rect.Min.X)
+	i := p.PixOffset(x, y)
 	p.Pix[i] = index
 }
 
@@ -796,7 +850,7 @@ func (p *Paletted) SubImage(r Rectangle) Image {
 			Palette: p.Palette,
 		}
 	}
-	i := (r.Min.Y-p.Rect.Min.Y)*p.Stride + (r.Min.X-p.Rect.Min.X)*1
+	i := p.PixOffset(r.Min.X, r.Min.Y)
 	return &Paletted{
 		Pix:     p.Pix[i:],
 		Stride:  p.Stride,
diff --git a/libgo/go/image/jpeg/reader.go b/libgo/go/image/jpeg/reader.go
index ed1a962586d..2e412ad1716 100644
--- a/libgo/go/image/jpeg/reader.go
+++ b/libgo/go/image/jpeg/reader.go
@@ -203,8 +203,7 @@ func (d *decoder) makeImg(h0, v0, mxx, myy int) {
 		return
 	}
 	var subsampleRatio image.YCbCrSubsampleRatio
-	n := h0 * v0
-	switch n {
+	switch h0 * v0 {
 	case 1:
 		subsampleRatio = image.YCbCrSubsampleRatio444
 	case 2:
@@ -214,16 +213,8 @@ func (d *decoder) makeImg(h0, v0, mxx, myy int) {
 	default:
 		panic("unreachable")
 	}
-	b := make([]byte, mxx*myy*(1*8*8*n+2*8*8))
-	d.img3 = &image.YCbCr{
-		Y:              b[mxx*myy*(0*8*8*n+0*8*8) : mxx*myy*(1*8*8*n+0*8*8)],
-		Cb:             b[mxx*myy*(1*8*8*n+0*8*8) : mxx*myy*(1*8*8*n+1*8*8)],
-		Cr:             b[mxx*myy*(1*8*8*n+1*8*8) : mxx*myy*(1*8*8*n+2*8*8)],
-		SubsampleRatio: subsampleRatio,
-		YStride:        mxx * 8 * h0,
-		CStride:        mxx * 8,
-		Rect:           image.Rect(0, 0, d.width, d.height),
-	}
+	m := image.NewYCbCr(image.Rect(0, 0, 8*h0*mxx, 8*v0*myy), subsampleRatio)
+	d.img3 = m.SubImage(image.Rect(0, 0, d.width, d.height)).(*image.YCbCr)
 }
 
 // Specified in section B.2.3.
diff --git a/libgo/go/image/tiff/reader.go b/libgo/go/image/tiff/reader.go
index 00a51db9f4c..dc5a87a9fb9 100644
--- a/libgo/go/image/tiff/reader.go
+++ b/libgo/go/image/tiff/reader.go
@@ -223,8 +223,8 @@ func (d *decoder) decode(dst image.Image, ymin, ymax int) error {
 		}
 	case mRGB:
 		img := dst.(*image.RGBA)
-		min := (ymin-img.Rect.Min.Y)*img.Stride - img.Rect.Min.X*4
-		max := (ymax-img.Rect.Min.Y)*img.Stride - img.Rect.Min.X*4
+		min := img.PixOffset(0, ymin)
+		max := img.PixOffset(0, ymax)
 		var off int
 		for i := min; i < max; i += 4 {
 			img.Pix[i+0] = d.buf[off+0]
@@ -235,16 +235,16 @@ func (d *decoder) decode(dst image.Image, ymin, ymax int) error {
 		}
 	case mNRGBA:
 		img := dst.(*image.NRGBA)
-		min := (ymin-img.Rect.Min.Y)*img.Stride - img.Rect.Min.X*4
-		max := (ymax-img.Rect.Min.Y)*img.Stride - img.Rect.Min.X*4
+		min := img.PixOffset(0, ymin)
+		max := img.PixOffset(0, ymax)
 		if len(d.buf) != max-min {
 			return FormatError("short data strip")
 		}
 		copy(img.Pix[min:max], d.buf)
 	case mRGBA:
 		img := dst.(*image.RGBA)
-		min := (ymin-img.Rect.Min.Y)*img.Stride - img.Rect.Min.X*4
-		max := (ymax-img.Rect.Min.Y)*img.Stride - img.Rect.Min.X*4
+		min := img.PixOffset(0, ymin)
+		max := img.PixOffset(0, ymax)
 		if len(d.buf) != max-min {
 			return FormatError("short data strip")
 		}
diff --git a/libgo/go/image/ycbcr.go b/libgo/go/image/ycbcr.go
index 81f3c9fe03d..c1a0b666f83 100644
--- a/libgo/go/image/ycbcr.go
+++ b/libgo/go/image/ycbcr.go
@@ -17,6 +17,18 @@ const (
 	YCbCrSubsampleRatio420
 )
 
+func (s YCbCrSubsampleRatio) String() string {
+	switch s {
+	case YCbCrSubsampleRatio444:
+		return "YCbCrSubsampleRatio444"
+	case YCbCrSubsampleRatio422:
+		return "YCbCrSubsampleRatio422"
+	case YCbCrSubsampleRatio420:
+		return "YCbCrSubsampleRatio420"
+	}
+	return "YCbCrSubsampleRatioUnknown"
+}
+
 // YCbCr is an in-memory image of Y'CbCr colors. There is one Y sample per
 // pixel, but each Cb and Cr sample can span one or more pixels.
 // YStride is the Y slice index delta between vertically adjacent pixels.
@@ -28,9 +40,7 @@ const (
 //	For 4:2:2, CStride == YStride/2 && len(Cb) == len(Cr) == len(Y)/2.
 //	For 4:2:0, CStride == YStride/2 && len(Cb) == len(Cr) == len(Y)/4.
 type YCbCr struct {
-	Y              []uint8
-	Cb             []uint8
-	Cr             []uint8
+	Y, Cb, Cr      []uint8
 	YStride        int
 	CStride        int
 	SubsampleRatio YCbCrSubsampleRatio
@@ -49,39 +59,86 @@ func (p *YCbCr) At(x, y int) color.Color {
 	if !(Point{x, y}.In(p.Rect)) {
 		return color.YCbCr{}
 	}
+	yi := p.YOffset(x, y)
+	ci := p.COffset(x, y)
+	return color.YCbCr{
+		p.Y[yi],
+		p.Cb[ci],
+		p.Cr[ci],
+	}
+}
+
+// YOffset returns the index of the first element of Y that corresponds to
+// the pixel at (x, y).
+func (p *YCbCr) YOffset(x, y int) int {
+	return (y-p.Rect.Min.Y)*p.YStride + (x - p.Rect.Min.X)
+}
+
+// COffset returns the index of the first element of Cb or Cr that corresponds
+// to the pixel at (x, y).
+func (p *YCbCr) COffset(x, y int) int {
 	switch p.SubsampleRatio {
 	case YCbCrSubsampleRatio422:
-		i := x / 2
-		return color.YCbCr{
-			p.Y[y*p.YStride+x],
-			p.Cb[y*p.CStride+i],
-			p.Cr[y*p.CStride+i],
-		}
+		return (y-p.Rect.Min.Y)*p.CStride + (x/2 - p.Rect.Min.X/2)
 	case YCbCrSubsampleRatio420:
-		i, j := x/2, y/2
-		return color.YCbCr{
-			p.Y[y*p.YStride+x],
-			p.Cb[j*p.CStride+i],
-			p.Cr[j*p.CStride+i],
-		}
+		return (y/2-p.Rect.Min.Y/2)*p.CStride + (x/2 - p.Rect.Min.X/2)
 	}
 	// Default to 4:4:4 subsampling.
-	return color.YCbCr{
-		p.Y[y*p.YStride+x],
-		p.Cb[y*p.CStride+x],
-		p.Cr[y*p.CStride+x],
-	}
+	return (y-p.Rect.Min.Y)*p.CStride + (x - p.Rect.Min.X)
 }
 
 // SubImage returns an image representing the portion of the image p visible
 // through r. The returned value shares pixels with the original image.
 func (p *YCbCr) SubImage(r Rectangle) Image {
-	q := new(YCbCr)
-	*q = *p
-	q.Rect = q.Rect.Intersect(r)
-	return q
+	r = r.Intersect(p.Rect)
+	// If r1 and r2 are Rectangles, r1.Intersect(r2) is not guaranteed to be inside
+	// either r1 or r2 if the intersection is empty. Without explicitly checking for
+	// this, the Pix[i:] expression below can panic.
+	if r.Empty() {
+		return &YCbCr{
+			SubsampleRatio: p.SubsampleRatio,
+		}
+	}
+	yi := p.YOffset(r.Min.X, r.Min.Y)
+	ci := p.COffset(r.Min.X, r.Min.Y)
+	return &YCbCr{
+		Y:              p.Y[yi:],
+		Cb:             p.Cb[ci:],
+		Cr:             p.Cr[ci:],
+		SubsampleRatio: p.SubsampleRatio,
+		YStride:        p.YStride,
+		CStride:        p.CStride,
+		Rect:           r,
+	}
 }
 
 func (p *YCbCr) Opaque() bool {
 	return true
 }
+
+// NewYCbCr returns a new YCbCr with the given bounds and subsample ratio.
+func NewYCbCr(r Rectangle, subsampleRatio YCbCrSubsampleRatio) *YCbCr {
+	w, h, cw, ch := r.Dx(), r.Dy(), 0, 0
+	switch subsampleRatio {
+	case YCbCrSubsampleRatio422:
+		cw = (r.Max.X+1)/2 - r.Min.X/2
+		ch = h
+	case YCbCrSubsampleRatio420:
+		cw = (r.Max.X+1)/2 - r.Min.X/2
+		ch = (r.Max.Y+1)/2 - r.Min.Y/2
+	default:
+		// Default to 4:4:4 subsampling.
+		cw = w
+		ch = h
+	}
+	b := make([]byte, w*h+2*cw*ch)
+	return &YCbCr{
+		Y:              b[:w*h],
+		Cb:             b[w*h+0*cw*ch : w*h+1*cw*ch],
+		Cr:             b[w*h+1*cw*ch : w*h+2*cw*ch],
+		SubsampleRatio: subsampleRatio,
+		YStride:        w,
+		CStride:        cw,
+		Rect:           r,
+	}
+}
diff --git a/libgo/go/image/ycbcr_test.go b/libgo/go/image/ycbcr_test.go
new file mode 100644
index 00000000000..eb8b1950bfe
--- /dev/null
+++ b/libgo/go/image/ycbcr_test.go
@@ -0,0 +1,104 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package image_test
+
+import (
+	. "image"
+	"image/color"
+	"testing"
+)
+
+func TestYCbCr(t *testing.T) {
+	rects := []Rectangle{
+		Rect(0, 0, 16, 16),
+		Rect(1, 0, 16, 16),
+		Rect(0, 1, 16, 16),
+		Rect(1, 1, 16, 16),
+		Rect(1, 1, 15, 16),
+		Rect(1, 1, 16, 15),
+		Rect(1, 1, 15, 15),
+		Rect(2, 3, 14, 15),
+		Rect(7, 0, 7, 16),
+		Rect(0, 8, 16, 8),
+		Rect(0, 0, 10, 11),
+		Rect(5, 6, 16, 16),
+		Rect(7, 7, 8, 8),
+		Rect(7, 8, 8, 9),
+		Rect(8, 7, 9, 8),
+		Rect(8, 8, 9, 9),
+		Rect(7, 7, 17, 17),
+		Rect(8, 8, 17, 17),
+		Rect(9, 9, 17, 17),
+		Rect(10, 10, 17, 17),
+	}
+	subsampleRatios := []YCbCrSubsampleRatio{
+		YCbCrSubsampleRatio444,
+		YCbCrSubsampleRatio422,
+		YCbCrSubsampleRatio420,
+	}
+	deltas := []Point{
+		Pt(0, 0),
+		Pt(1000, 1001),
+		Pt(5001, -400),
+		Pt(-701, -801),
+	}
+	for _, r := range rects {
+		for _, subsampleRatio := range subsampleRatios {
+			for _, delta := range deltas {
+				testYCbCr(t, r, subsampleRatio, delta)
+			}
+		}
+	}
+}
+
+func testYCbCr(t *testing.T, r Rectangle, subsampleRatio YCbCrSubsampleRatio, delta Point) {
+	// Create a YCbCr image m, whose bounds are r translated by (delta.X, delta.Y).
+	r1 := r.Add(delta)
+	m := NewYCbCr(r1, subsampleRatio)
+
+	// Test that the image buffer is reasonably small even if (delta.X, delta.Y) is far from the origin.
+	if len(m.Y) > 100*100 {
+		t.Errorf("r=%v, subsampleRatio=%v, delta=%v: image buffer is too large",
+			r, subsampleRatio, delta)
+		return
+	}
+
+	// Initialize m'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 := m.YOffset(x, y)
+			ci := m.COffset(x, y)
+			m.Y[yi] = uint8(16*y + x)
+			m.Cb[ci] = uint8(y + 16*x)
+			m.Cr[ci] = uint8(y + 16*x)
+		}
+	}
+
+	// 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 := Rect(x0, y0, x1, y1)
+					sub := m.SubImage(subRect).(*YCbCr)
+
+					// 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
+							}
+						}
+					}
+				}
+			}
+		}
+	}
+}