1 files changed, 144 insertions, 0 deletions
diff --git a/src/pkg/image/ycbcr.go b/src/pkg/image/ycbcr.go
new file mode 100644
index 000000000..c1a0b666f
--- /dev/null
+++ b/src/pkg/image/ycbcr.go
@@ -0,0 +1,144 @@
+// Copyright 2011 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
+
+import (
+	"image/color"
+)
+
+// YCbCrSubsampleRatio is the chroma subsample ratio used in a YCbCr image.
+type YCbCrSubsampleRatio int
+
+const (
+	YCbCrSubsampleRatio444 YCbCrSubsampleRatio = iota
+	YCbCrSubsampleRatio422
+	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.
+// CStride is the Cb and Cr slice index delta between vertically adjacent pixels
+// that map to separate chroma samples.
+// It is not an absolute requirement, but YStride and len(Y) are typically
+// multiples of 8, and:
+//	For 4:4:4, CStride == YStride/1 && len(Cb) == len(Cr) == len(Y)/1.
+//	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, Cb, Cr      []uint8
+	YStride        int
+	CStride        int
+	SubsampleRatio YCbCrSubsampleRatio
+	Rect           Rectangle
+}
+
+func (p *YCbCr) ColorModel() color.Model {
+	return color.YCbCrModel
+}
+
+func (p *YCbCr) Bounds() Rectangle {
+	return p.Rect
+}
+
+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:
+		return (y-p.Rect.Min.Y)*p.CStride + (x/2 - p.Rect.Min.X/2)
+	case YCbCrSubsampleRatio420:
+		return (y/2-p.Rect.Min.Y/2)*p.CStride + (x/2 - p.Rect.Min.X/2)
+	}
+	// Default to 4:4:4 subsampling.
+	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 {
+	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,
+	}
+}