// 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,
	}

		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)
			}
		}
		if testing.Short() {
			break
		}
	}
}

func testYCbCr(t *testing.T, r Rectangle, subsampleRatio YCbCrSubsampleRatio, delta Point) {

	vRatio := v0 / d.comp[1].v
	var subsampleRatio image.YCbCrSubsampleRatio
	switch hRatio<<4 | vRatio {
	case 0x11:
		subsampleRatio = image.YCbCrSubsampleRatio444
	case 0x12:
		subsampleRatio = image.YCbCrSubsampleRatio440
	case 0x21:
		subsampleRatio = image.YCbCrSubsampleRatio422
	case 0x22:
		subsampleRatio = image.YCbCrSubsampleRatio420
	case 0x41:
		subsampleRatio = image.YCbCrSubsampleRatio411
	case 0x42:

	// that also allowed a CopyFromRGBA method if needed.

	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
	y1 := r.Max.Y - dst.Rect.Min.Y
	switch src.SubsampleRatio {
`

const post = `
	default:
		return false
	}
	return true
}
`

const sratioCase = `
	case image.YCbCrSubsampleRatio%s:
		for y, sy := y0, sp.Y; y != y1; y, sy = y+1, sy+1 {

			cr[i] = uint8(7 * i % 0x100)
		}
		src = &image.YCbCr{
			Y:              yy,
			Cb:             cb,
			Cr:             cr,
			YStride:        srcw,
			CStride:        srcw,
			SubsampleRatio: image.YCbCrSubsampleRatio444,
			Rect:           image.Rect(0, 0, srcw, srch),
		}
	default:
		b.Fatal("unknown source color model", scm)
	}

	var mask image.Image
	switch mcm {
	case nil:

	// that also allowed a CopyFromRGBA method if needed.

	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
	y1 := r.Max.Y - dst.Rect.Min.Y
	switch src.SubsampleRatio {

	case image.YCbCrSubsampleRatio444:
		for y, sy := y0, sp.Y; y != y1; y, sy = y+1, sy+1 {
			dpix := dst.Pix[y*dst.Stride:]
			yi := (sy-src.Rect.Min.Y)*src.YStride + (sp.X - src.Rect.Min.X)

func vgradCr() image.Image {
	m := &image.YCbCr{
		Y:              make([]byte, 16*16),
		Cb:             make([]byte, 16*16),
		Cr:             make([]byte, 16*16),
		YStride:        16,
		CStride:        16,
		SubsampleRatio: image.YCbCrSubsampleRatio444,
		Rect:           image.Rect(0, 0, 16, 16),
	}
	for y := 0; y < 16; y++ {
		for x := 0; x < 16; x++ {
			m.Cr[y*m.CStride+x] = uint8(y * 0x11)
		}
	}
	return m
}

		{"Uniform.C", Field, 0},
		{"White", Var, 0},
		{"YCbCr", Type, 0},
		{"YCbCr.CStride", Field, 0},
		{"YCbCr.Cb", Field, 0},
		{"YCbCr.Cr", Field, 0},
		{"YCbCr.Rect", Field, 0},
		{"YCbCr.SubsampleRatio", Field, 0},
		{"YCbCr.Y", Field, 0},
		{"YCbCr.YStride", Field, 0},
		{"YCbCrSubsampleRatio", Type, 0},
		{"YCbCrSubsampleRatio410", Const, 5},
		{"YCbCrSubsampleRatio411", Const, 5},

pkg image, type YCbCr struct, CStride int
pkg image, type YCbCr struct, Cb []uint8
pkg image, type YCbCr struct, Cr []uint8
pkg image, type YCbCr struct, Rect Rectangle
pkg image, type YCbCr struct, SubsampleRatio YCbCrSubsampleRatio
pkg image, type YCbCr struct, Y []uint8
pkg image, type YCbCr struct, YStride int
pkg image, type YCbCrSubsampleRatio int
pkg image, var Black *Uniform
pkg image, var ErrFormat error