100 lines
2.7 KiB
Go
100 lines
2.7 KiB
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}
|
|
}
|