github.com/hbdrawn/golang@v0.0.0-20141214014649-6b835209aba2/src/image/color/ycbcr.go (about)

     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  }