Source file src/pkg/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 3 // license that can be found in the LICENSE file. 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 r1 := int(r) 15 g1 := int(g) 16 b1 := int(b) 17 yy := (19595*r1 + 38470*g1 + 7471*b1 + 1<<15) >> 16 18 cb := (-11056*r1 - 21712*g1 + 32768*b1 + 257<<15) >> 16 19 cr := (32768*r1 - 27440*g1 - 5328*b1 + 257<<15) >> 16 20 if yy < 0 { 21 yy = 0 22 } else if yy > 255 { 23 yy = 255 24 } 25 if cb < 0 { 26 cb = 0 27 } else if cb > 255 { 28 cb = 255 29 } 30 if cr < 0 { 31 cr = 0 32 } else if cr > 255 { 33 cr = 255 34 } 35 return uint8(yy), uint8(cb), uint8(cr) 36 } 37 38 // YCbCrToRGB converts a Y'CbCr triple to an RGB triple. 39 func YCbCrToRGB(y, cb, cr uint8) (uint8, uint8, uint8) { 40 // The JFIF specification says: 41 // R = Y' + 1.40200*(Cr-128) 42 // G = Y' - 0.34414*(Cb-128) - 0.71414*(Cr-128) 43 // B = Y' + 1.77200*(Cb-128) 44 // http://www.w3.org/Graphics/JPEG/jfif3.pdf says Y but means Y'. 45 yy1 := int(y)<<16 + 1<<15 46 cb1 := int(cb) - 128 47 cr1 := int(cr) - 128 48 r := (yy1 + 91881*cr1) >> 16 49 g := (yy1 - 22554*cb1 - 46802*cr1) >> 16 50 b := (yy1 + 116130*cb1) >> 16 51 if r < 0 { 52 r = 0 53 } else if r > 255 { 54 r = 255 55 } 56 if g < 0 { 57 g = 0 58 } else if g > 255 { 59 g = 255 60 } 61 if b < 0 { 62 b = 0 63 } else if b > 255 { 64 b = 255 65 } 66 return uint8(r), uint8(g), uint8(b) 67 } 68 69 // YCbCr represents a fully opaque 24-bit Y'CbCr color, having 8 bits each for 70 // one luma and two chroma components. 71 // 72 // JPEG, VP8, the MPEG family and other codecs use this color model. Such 73 // codecs often use the terms YUV and Y'CbCr interchangeably, but strictly 74 // speaking, the term YUV applies only to analog video signals, and Y' (luma) 75 // is Y (luminance) after applying gamma correction. 76 // 77 // Conversion between RGB and Y'CbCr is lossy and there are multiple, slightly 78 // different formulae for converting between the two. This package follows 79 // the JFIF specification at http://www.w3.org/Graphics/JPEG/jfif3.pdf. 80 type YCbCr struct { 81 Y, Cb, Cr uint8 82 } 83 84 func (c YCbCr) RGBA() (uint32, uint32, uint32, uint32) { 85 r, g, b := YCbCrToRGB(c.Y, c.Cb, c.Cr) 86 return uint32(r) * 0x101, uint32(g) * 0x101, uint32(b) * 0x101, 0xffff 87 } 88 89 // YCbCrModel is the Model for Y'CbCr colors. 90 var YCbCrModel Model = ModelFunc(yCbCrModel) 91 92 func yCbCrModel(c Color) Color { 93 if _, ok := c.(YCbCr); ok { 94 return c 95 } 96 r, g, b, _ := c.RGBA() 97 y, u, v := RGBToYCbCr(uint8(r>>8), uint8(g>>8), uint8(b>>8)) 98 return YCbCr{y, u, v} 99 }