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# Source file src/image/color/ycbcr.go

```     1	// Copyright 2011 The Go Authors. All rights reserved.
2	// Use of this source code is governed by a BSD-style
4
5	package color
6
7	// RGBToYCbCr converts an RGB triple to a Y'CbCr triple.
8	func RGBToYCbCr(r, g, b uint8) (uint8, uint8, uint8) {
9		// The JFIF specification says:
10		//	Y' =  0.2990*R + 0.5870*G + 0.1140*B
11		//	Cb = -0.1687*R - 0.3313*G + 0.5000*B + 128
12		//	Cr =  0.5000*R - 0.4187*G - 0.0813*B + 128
13		// http://www.w3.org/Graphics/JPEG/jfif3.pdf says Y but means Y'.
14
15		r1 := int32(r)
16		g1 := int32(g)
17		b1 := int32(b)
18		yy := (19595*r1 + 38470*g1 + 7471*b1 + 1<<15) >> 16
19		cb := (-11056*r1 - 21712*g1 + 32768*b1 + 257<<15) >> 16
20		cr := (32768*r1 - 27440*g1 - 5328*b1 + 257<<15) >> 16
21		if yy < 0 {
22			yy = 0
23		} else if yy > 0xff {
24			yy = 0xff
25		}
26		if cb < 0 {
27			cb = 0
28		} else if cb > 0xff {
29			cb = 0xff
30		}
31		if cr < 0 {
32			cr = 0
33		} else if cr > 0xff {
34			cr = 0xff
35		}
36		return uint8(yy), uint8(cb), uint8(cr)
37	}
38
39	// YCbCrToRGB converts a Y'CbCr triple to an RGB triple.
40	func YCbCrToRGB(y, cb, cr uint8) (uint8, uint8, uint8) {
41		// The JFIF specification says:
42		//	R = Y' + 1.40200*(Cr-128)
43		//	G = Y' - 0.34414*(Cb-128) - 0.71414*(Cr-128)
44		//	B = Y' + 1.77200*(Cb-128)
45		// http://www.w3.org/Graphics/JPEG/jfif3.pdf says Y but means Y'.
46
47		yy1 := int32(y) * 0x10100 // Convert 0x12 to 0x121200.
48		cb1 := int32(cb) - 128
49		cr1 := int32(cr) - 128
50		r := (yy1 + 91881*cr1) >> 16
51		g := (yy1 - 22554*cb1 - 46802*cr1) >> 16
52		b := (yy1 + 116130*cb1) >> 16
53		if r < 0 {
54			r = 0
55		} else if r > 0xff {
56			r = 0xff
57		}
58		if g < 0 {
59			g = 0
60		} else if g > 0xff {
61			g = 0xff
62		}
63		if b < 0 {
64			b = 0
65		} else if b > 0xff {
66			b = 0xff
67		}
68		return uint8(r), uint8(g), uint8(b)
69	}
70
71	// YCbCr represents a fully opaque 24-bit Y'CbCr color, having 8 bits each for
72	// one luma and two chroma components.
73	//
74	// JPEG, VP8, the MPEG family and other codecs use this color model. Such
75	// codecs often use the terms YUV and Y'CbCr interchangeably, but strictly
76	// speaking, the term YUV applies only to analog video signals, and Y' (luma)
77	// is Y (luminance) after applying gamma correction.
78	//
79	// Conversion between RGB and Y'CbCr is lossy and there are multiple, slightly
80	// different formulae for converting between the two. This package follows
81	// the JFIF specification at http://www.w3.org/Graphics/JPEG/jfif3.pdf.
82	type YCbCr struct {
83		Y, Cb, Cr uint8
84	}
85
86	func (c YCbCr) RGBA() (uint32, uint32, uint32, uint32) {
87		// This code is a copy of the YCbCrToRGB function above, except that it
88		// returns values in the range [0, 0xffff] instead of [0, 0xff]. There is a
89		// subtle difference between doing this and having YCbCr satisfy the Color
90		// interface by first converting to an RGBA. The latter loses some
91		// information by going to and from 8 bits per channel.
92		//
93		// For example, this code:
94		//	const y, cb, cr = 0x7f, 0x7f, 0x7f
95		//	r, g, b := color.YCbCrToRGB(y, cb, cr)
96		//	r0, g0, b0, _ := color.YCbCr{y, cb, cr}.RGBA()
97		//	r1, g1, b1, _ := color.RGBA{r, g, b, 0xff}.RGBA()
98		//	fmt.Printf("0x%04x 0x%04x 0x%04x\n", r0, g0, b0)
99		//	fmt.Printf("0x%04x 0x%04x 0x%04x\n", r1, g1, b1)
100		// prints:
101		//	0x7e18 0x808e 0x7db9
102		//	0x7e7e 0x8080 0x7d7d
103
104		yy1 := int32(c.Y) * 0x10100 // Convert 0x12 to 0x121200.
105		cb1 := int32(c.Cb) - 128
106		cr1 := int32(c.Cr) - 128
107		r := (yy1 + 91881*cr1) >> 8
108		g := (yy1 - 22554*cb1 - 46802*cr1) >> 8
109		b := (yy1 + 116130*cb1) >> 8
110		if r < 0 {
111			r = 0
112		} else if r > 0xffff {
113			r = 0xffff
114		}
115		if g < 0 {
116			g = 0
117		} else if g > 0xffff {
118			g = 0xffff
119		}
120		if b < 0 {
121			b = 0
122		} else if b > 0xffff {
123			b = 0xffff
124		}
125		return uint32(r), uint32(g), uint32(b), 0xffff
126	}
127
128	// YCbCrModel is the Model for Y'CbCr colors.
129	var YCbCrModel Model = ModelFunc(yCbCrModel)
130
131	func yCbCrModel(c Color) Color {
132		if _, ok := c.(YCbCr); ok {
133			return c
134		}
135		r, g, b, _ := c.RGBA()
136		y, u, v := RGBToYCbCr(uint8(r>>8), uint8(g>>8), uint8(b>>8))
137		return YCbCr{y, u, v}
138	}
139
140	// RGBToCMYK converts an RGB triple to a CMYK quadruple.
141	func RGBToCMYK(r, g, b uint8) (uint8, uint8, uint8, uint8) {
142		rr := uint32(r)
143		gg := uint32(g)
144		bb := uint32(b)
145		w := rr
146		if w < gg {
147			w = gg
148		}
149		if w < bb {
150			w = bb
151		}
152		if w == 0 {
153			return 0, 0, 0, 0xff
154		}
155		c := (w - rr) * 0xff / w
156		m := (w - gg) * 0xff / w
157		y := (w - bb) * 0xff / w
158		return uint8(c), uint8(m), uint8(y), uint8(0xff - w)
159	}
160
161	// CMYKToRGB converts a CMYK quadruple to an RGB triple.
162	func CMYKToRGB(c, m, y, k uint8) (uint8, uint8, uint8) {
163		w := uint32(0xffff - uint32(k)*0x101)
164		r := uint32(0xffff-uint32(c)*0x101) * w / 0xffff
165		g := uint32(0xffff-uint32(m)*0x101) * w / 0xffff
166		b := uint32(0xffff-uint32(y)*0x101) * w / 0xffff
167		return uint8(r >> 8), uint8(g >> 8), uint8(b >> 8)
168	}
169
170	// CMYK represents a fully opaque CMYK color, having 8 bits for each of cyan,
171	// magenta, yellow and black.
172	//
173	// It is not associated with any particular color profile.
174	type CMYK struct {
175		C, M, Y, K uint8
176	}
177
178	func (c CMYK) RGBA() (uint32, uint32, uint32, uint32) {
179		// This code is a copy of the CMYKToRGB function above, except that it
180		// returns values in the range [0, 0xffff] instead of [0, 0xff].
181
182		w := uint32(0xffff - uint32(c.K)*0x101)
183		r := uint32(0xffff-uint32(c.C)*0x101) * w / 0xffff
184		g := uint32(0xffff-uint32(c.M)*0x101) * w / 0xffff
185		b := uint32(0xffff-uint32(c.Y)*0x101) * w / 0xffff
186		return uint32(r), uint32(g), uint32(b), 0xffff
187	}
188
189	// CMYKModel is the Model for CMYK colors.
190	var CMYKModel Model = ModelFunc(cmykModel)
191
192	func cmykModel(c Color) Color {
193		if _, ok := c.(CMYK); ok {
194			return c
195		}
196		r, g, b, _ := c.RGBA()
197		cc, mm, yy, kk := RGBToCMYK(uint8(r>>8), uint8(g>>8), uint8(b>>8))
198		return CMYK{cc, mm, yy, kk}
199	}
200
```

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