1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
|
// 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
}
|
