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	}