Commit ce89a233 authored by Nigel Tao's avatar Nigel Tao

image/ycbcr: new package.

R=r, rsc, nigeltao_gnome
CC=golang-dev, raph
https://golang.org/cl/4374043
parent 420a17e3
......@@ -105,6 +105,7 @@ DIRS=\
image\
image/jpeg\
image/png\
image/ycbcr\
index/suffixarray\
io\
io/ioutil\
......
# 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
// 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
}
// 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)
}
}
}
}
}
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