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image/ycbcr: new package.
R=r, rsc, nigeltao_gnome CC=golang-dev, raph https://golang.org/cl/4374043
This commit is contained in:
Nigel Tao
parent
420a17e371
commit
ce89a233a2
@ -105,6 +105,7 @@ DIRS=\
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image\
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image/jpeg\
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image/png\
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image/ycbcr\
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index/suffixarray\
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io\
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io/ioutil\
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11
src/pkg/image/ycbcr/Makefile
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11
src/pkg/image/ycbcr/Makefile
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@ -0,0 +1,11 @@
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# Copyright 2011 The Go Authors. All rights reserved.
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# Use of this source code is governed by a BSD-style
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# license that can be found in the LICENSE file.
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include ../../../Make.inc
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TARG=image/ycbcr
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GOFILES=\
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ycbcr.go\
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include ../../../Make.pkg
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174
src/pkg/image/ycbcr/ycbcr.go
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174
src/pkg/image/ycbcr/ycbcr.go
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@ -0,0 +1,174 @@
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// Copyright 2011 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// The ycbcr package provides images from the Y'CbCr color model.
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//
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// JPEG, VP8, the MPEG family and other codecs use this color model. Such
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// codecs often use the terms YUV and Y'CbCr interchangeably, but strictly
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// speaking, the term YUV applies only to analog video signals.
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//
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// Conversion between RGB and Y'CbCr is lossy and there are multiple, slightly
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// different formulae for converting between the two. This package follows
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// the JFIF specification at http://www.w3.org/Graphics/JPEG/jfif3.pdf.
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package ycbcr
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import (
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"image"
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)
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// RGBToYCbCr converts an RGB triple to a YCbCr triple. All components lie
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// within the range [0, 255].
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func RGBToYCbCr(r, g, b uint8) (uint8, uint8, uint8) {
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// The JFIF specification says:
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// Y' = 0.2990*R + 0.5870*G + 0.1140*B
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// Cb = -0.1687*R - 0.3313*G + 0.5000*B + 128
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// Cr = 0.5000*R - 0.4187*G - 0.0813*B + 128
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// http://www.w3.org/Graphics/JPEG/jfif3.pdf says Y but means Y'.
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r1 := int(r)
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g1 := int(g)
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b1 := int(b)
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yy := (19595*r1 + 38470*g1 + 7471*b1 + 1<<15) >> 16
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cb := (-11056*r1 - 21712*g1 + 32768*b1 + 257<<15) >> 16
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cr := (32768*r1 - 27440*g1 - 5328*b1 + 257<<15) >> 16
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if yy < 0 {
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yy = 0
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} else if yy > 255 {
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yy = 255
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}
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if cb < 0 {
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cb = 0
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} else if cb > 255 {
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cb = 255
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}
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if cr < 0 {
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cr = 0
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} else if cr > 255 {
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cr = 255
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}
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return uint8(yy), uint8(cb), uint8(cr)
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}
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// YCbCrToRGB converts a YCbCr triple to an RGB triple. All components lie
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// within the range [0, 255].
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func YCbCrToRGB(y, cb, cr uint8) (uint8, uint8, uint8) {
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// The JFIF specification says:
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// R = Y' + 1.40200*(Cr-128)
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// G = Y' - 0.34414*(Cb-128) - 0.71414*(Cr-128)
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// B = Y' + 1.77200*(Cb-128)
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// http://www.w3.org/Graphics/JPEG/jfif3.pdf says Y but means Y'.
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yy1 := int(y)<<16 + 1<<15
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cb1 := int(cb) - 128
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cr1 := int(cr) - 128
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r := (yy1 + 91881*cr1) >> 16
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g := (yy1 - 22554*cb1 - 46802*cr1) >> 16
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b := (yy1 + 116130*cb1) >> 16
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if r < 0 {
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r = 0
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} else if r > 255 {
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r = 255
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}
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if g < 0 {
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g = 0
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} else if g > 255 {
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g = 255
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}
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if b < 0 {
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b = 0
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} else if b > 255 {
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b = 255
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}
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return uint8(r), uint8(g), uint8(b)
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}
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// YCbCrColor represents a fully opaque 24-bit Y'CbCr color, having 8 bits for
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// each of one luma and two chroma components.
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type YCbCrColor struct {
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Y, Cb, Cr uint8
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}
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func (c YCbCrColor) RGBA() (uint32, uint32, uint32, uint32) {
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r, g, b := YCbCrToRGB(c.Y, c.Cb, c.Cr)
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return uint32(r) * 0x101, uint32(g) * 0x101, uint32(b) * 0x101, 0xffff
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}
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func toYCbCrColor(c image.Color) image.Color {
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if _, ok := c.(YCbCrColor); ok {
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return c
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}
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r, g, b, _ := c.RGBA()
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y, u, v := RGBToYCbCr(uint8(r>>8), uint8(g>>8), uint8(b>>8))
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return YCbCrColor{y, u, v}
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}
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// YCbCrColorModel is the color model for YCbCrColor.
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var YCbCrColorModel image.ColorModel = image.ColorModelFunc(toYCbCrColor)
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// SubsampleRatio is the chroma subsample ratio used in a YCbCr image.
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type SubsampleRatio int
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const (
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SubsampleRatio444 SubsampleRatio = iota
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SubsampleRatio422
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SubsampleRatio420
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)
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// YCbCr is an in-memory image of YCbCr colors. There is one Y sample per pixel,
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// but each Cb and Cr sample can span one or more pixels.
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// YStride is the Y slice index delta between vertically adjacent pixels.
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// CStride is the Cb and Cr slice index delta between vertically adjacent pixels
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// that map to separate chroma samples.
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// It is not an absolute requirement, but YStride and len(Y) are typically
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// multiples of 8, and:
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// For 4:4:4, CStride == YStride/1 && len(Cb) == len(Cr) == len(Y)/1.
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// For 4:2:2, CStride == YStride/2 && len(Cb) == len(Cr) == len(Y)/2.
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// For 4:2:0, CStride == YStride/2 && len(Cb) == len(Cr) == len(Y)/4.
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type YCbCr struct {
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Y []uint8
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Cb []uint8
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Cr []uint8
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YStride int
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CStride int
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SubsampleRatio SubsampleRatio
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Rect image.Rectangle
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}
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func (p *YCbCr) ColorModel() image.ColorModel {
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return YCbCrColorModel
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}
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func (p *YCbCr) Bounds() image.Rectangle {
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return p.Rect
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}
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func (p *YCbCr) At(x, y int) image.Color {
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if !p.Rect.Contains(image.Point{x, y}) {
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return YCbCrColor{}
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}
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switch p.SubsampleRatio {
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case SubsampleRatio422:
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i := x / 2
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return YCbCrColor{
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p.Y[y*p.YStride+x],
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p.Cb[y*p.CStride+i],
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p.Cr[y*p.CStride+i],
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}
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case SubsampleRatio420:
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i, j := x/2, y/2
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return YCbCrColor{
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p.Y[y*p.YStride+x],
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p.Cb[j*p.CStride+i],
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p.Cr[j*p.CStride+i],
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}
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}
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// Default to 4:4:4 subsampling.
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return YCbCrColor{
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p.Y[y*p.YStride+x],
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p.Cb[y*p.CStride+x],
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p.Cr[y*p.CStride+x],
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}
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}
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func (p *YCbCr) Opaque() bool {
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return true
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}
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33
src/pkg/image/ycbcr/ycbcr_test.go
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33
src/pkg/image/ycbcr/ycbcr_test.go
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@ -0,0 +1,33 @@
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// Copyright 2011 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package ycbcr
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import (
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"testing"
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)
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func delta(x, y uint8) uint8 {
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if x >= y {
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return x - y
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}
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return y - x
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}
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// Test that a subset of RGB space can be converted to YCbCr and back to within
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// 1/256 tolerance.
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func TestRoundtrip(t *testing.T) {
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for r := 0; r < 255; r += 7 {
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for g := 0; g < 255; g += 5 {
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for b := 0; b < 255; b += 3 {
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r0, g0, b0 := uint8(r), uint8(g), uint8(b)
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y, cb, cr := RGBToYCbCr(r0, g0, b0)
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r1, g1, b1 := YCbCrToRGB(y, cb, cr)
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if delta(r0, r1) > 1 || delta(g0, g1) > 1 || delta(b0, b1) > 1 {
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t.Fatalf("r0, g0, b0 = %d, %d, %d r1, g1, b1 = %d, %d, %d", r0, g0, b0, r1, g1, b1)
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}
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}
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}
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}
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}
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