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 // 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 color // RGBToYCbCr converts an RGB triple to a Y'CbCr triple. 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 Y'CbCr triple to an RGB triple. 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) } // YCbCr represents a fully opaque 24-bit Y'CbCr color, having 8 bits each for // one luma and two chroma components. // // 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, and Y' (luma) // is Y (luminance) after applying gamma correction. // // 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. type YCbCr struct { Y, Cb, Cr uint8 } func (c YCbCr) 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 } // YCbCrModel is the Model for Y'CbCr colors. var YCbCrModel Model = ModelFunc(yCbCrModel) func yCbCrModel(c Color) Color { if _, ok := c.(YCbCr); ok { return c } r, g, b, _ := c.RGBA() y, u, v := RGBToYCbCr(uint8(r>>8), uint8(g>>8), uint8(b>>8)) return YCbCr{y, u, v} }