image/ycbcr: new package.

R=r, rsc, nigeltao_gnome
CC=golang-dev, raph
https://golang.org/cl/4374043

src/pkg/Makefile

@@ -105,6 +105,7 @@ DIRS=\
 	image\
 	image/jpeg\
 	image/png\
+	image/ycbcr\
 	index/suffixarray\
 	io\
 	io/ioutil\

src/pkg/image/ycbcr/Makefile

+# 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.
+
+include ../../../Make.inc
+
+TARG=image/ycbcr
+GOFILES=\
+	ycbcr.go\
+
+include ../../../Make.pkg

src/pkg/image/ycbcr/ycbcr.go

+// 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.
+
+// The ycbcr package provides images from the Y'CbCr color model.
+//
+// JPEG, VP8, the MPEG family and other codecs use this color model. Such
+// codecs often use the terms YUV and Y'CbCr interchangeably, but strictly
+// speaking, the term YUV applies only to analog video signals.
+//
+// Conversion between RGB and Y'CbCr is lossy and there are multiple, slightly
+// different formulae for converting between the two. This package follows
+// the JFIF specification at http://www.w3.org/Graphics/JPEG/jfif3.pdf.
+package ycbcr
+
+import (
+	"image"
+)
+
+// RGBToYCbCr converts an RGB triple to a YCbCr triple. All components lie
+// within the range [0, 255].
+func RGBToYCbCr(r, g, b uint8) (uint8, uint8, uint8) {
+	// The JFIF specification says:
+	//	Y' =  0.2990*R + 0.5870*G + 0.1140*B
+	//	Cb = -0.1687*R - 0.3313*G + 0.5000*B + 128
+	//	Cr =  0.5000*R - 0.4187*G - 0.0813*B + 128
+	// http://www.w3.org/Graphics/JPEG/jfif3.pdf says Y but means Y'.
+	r1 := int(r)
+	g1 := int(g)
+	b1 := int(b)
+	yy := (19595*r1 + 38470*g1 + 7471*b1 + 1<<15) >> 16
+	cb := (-11056*r1 - 21712*g1 + 32768*b1 + 257<<15) >> 16
+	cr := (32768*r1 - 27440*g1 - 5328*b1 + 257<<15) >> 16
+	if yy < 0 {
+		yy = 0
+	} else if yy > 255 {
+		yy = 255
+	}
+	if cb < 0 {
+		cb = 0
+	} else if cb > 255 {
+		cb = 255
+	}
+	if cr < 0 {
+		cr = 0
+	} else if cr > 255 {
+		cr = 255
+	}
+	return uint8(yy), uint8(cb), uint8(cr)
+}
+
+// YCbCrToRGB converts a YCbCr triple to an RGB triple. All components lie
+// within the range [0, 255].
+func YCbCrToRGB(y, cb, cr uint8) (uint8, uint8, uint8) {
+	// The JFIF specification says:
+	//	R = Y' + 1.40200*(Cr-128)
+	//	G = Y' - 0.34414*(Cb-128) - 0.71414*(Cr-128)
+	//	B = Y' + 1.77200*(Cb-128)
+	// http://www.w3.org/Graphics/JPEG/jfif3.pdf says Y but means Y'.
+	yy1 := int(y)<<16 + 1<<15
+	cb1 := int(cb) - 128
+	cr1 := int(cr) - 128
+	r := (yy1 + 91881*cr1) >> 16
+	g := (yy1 - 22554*cb1 - 46802*cr1) >> 16
+	b := (yy1 + 116130*cb1) >> 16
+	if r < 0 {
+		r = 0
+	} else if r > 255 {
+		r = 255
+	}
+	if g < 0 {
+		g = 0
+	} else if g > 255 {
+		g = 255
+	}
+	if b < 0 {
+		b = 0
+	} else if b > 255 {
+		b = 255
+	}
+	return uint8(r), uint8(g), uint8(b)
+}
+
+// YCbCrColor represents a fully opaque 24-bit Y'CbCr color, having 8 bits for
+// each of one luma and two chroma components.
+type YCbCrColor struct {
+	Y, Cb, Cr uint8
+}
+
+func (c YCbCrColor) RGBA() (uint32, uint32, uint32, uint32) {
+	r, g, b := YCbCrToRGB(c.Y, c.Cb, c.Cr)
+	return uint32(r) * 0x101, uint32(g) * 0x101, uint32(b) * 0x101, 0xffff
+}
+
+func toYCbCrColor(c image.Color) image.Color {
+	if _, ok := c.(YCbCrColor); ok {
+		return c
+	}
+	r, g, b, _ := c.RGBA()
+	y, u, v := RGBToYCbCr(uint8(r>>8), uint8(g>>8), uint8(b>>8))
+	return YCbCrColor{y, u, v}
+}
+
+// YCbCrColorModel is the color model for YCbCrColor.
+var YCbCrColorModel image.ColorModel = image.ColorModelFunc(toYCbCrColor)
+
+// SubsampleRatio is the chroma subsample ratio used in a YCbCr image.
+type SubsampleRatio int
+
+const (
+	SubsampleRatio444 SubsampleRatio = iota
+	SubsampleRatio422
+	SubsampleRatio420
+)
+
+// YCbCr is an in-memory image of YCbCr 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              []uint8
+	Cb             []uint8
+	Cr             []uint8
+	YStride        int
+	CStride        int
+	SubsampleRatio SubsampleRatio
+	Rect           image.Rectangle
+}
+
+func (p *YCbCr) ColorModel() image.ColorModel {
+	return YCbCrColorModel
+}
+
+func (p *YCbCr) Bounds() image.Rectangle {
+	return p.Rect
+}
+
+func (p *YCbCr) At(x, y int) image.Color {
+	if !p.Rect.Contains(image.Point{x, y}) {
+		return YCbCrColor{}
+	}
+	switch p.SubsampleRatio {
+	case SubsampleRatio422:
+		i := x / 2
+		return YCbCrColor{
+			p.Y[y*p.YStride+x],
+			p.Cb[y*p.CStride+i],
+			p.Cr[y*p.CStride+i],
+		}
+	case SubsampleRatio420:
+		i, j := x/2, y/2
+		return YCbCrColor{
+			p.Y[y*p.YStride+x],
+			p.Cb[j*p.CStride+i],
+			p.Cr[j*p.CStride+i],
+		}
+	}
+	// Default to 4:4:4 subsampling.
+	return YCbCrColor{
+		p.Y[y*p.YStride+x],
+		p.Cb[y*p.CStride+x],
+		p.Cr[y*p.CStride+x],
+	}
+}
+
+func (p *YCbCr) Opaque() bool {
+	return true
+}

src/pkg/image/ycbcr/ycbcr_test.go

+// 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 ycbcr
+
+import (
+	"testing"
+)
+
+func delta(x, y uint8) uint8 {
+	if x >= y {
+		return x - y
+	}
+	return y - x
+}
+
+// Test that a subset of RGB space can be converted to YCbCr and back to within
+// 1/256 tolerance.
+func TestRoundtrip(t *testing.T) {
+	for r := 0; r < 255; r += 7 {
+		for g := 0; g < 255; g += 5 {
+			for b := 0; b < 255; b += 3 {
+				r0, g0, b0 := uint8(r), uint8(g), uint8(b)
+				y, cb, cr := RGBToYCbCr(r0, g0, b0)
+				r1, g1, b1 := YCbCrToRGB(y, cb, cr)
+				if delta(r0, r1) > 1 || delta(g0, g1) > 1 || delta(b0, b1) > 1 {
+					t.Fatalf("r0, g0, b0 = %d, %d, %d   r1, g1, b1 = %d, %d, %d", r0, g0, b0, r1, g1, b1)
+				}
+			}
+		}
+	}
+}