src/pkg/image/color/ycbcr.go

// 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}
}