Source File
ycbcr.go
Belonging Package
image/color
// 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 (, , 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
// https://www.w3.org/Graphics/JPEG/jfif3.pdf says Y but means Y'.
:= int32()
:= int32()
:= int32()
// yy is in range [0,0xff].
//
// Note that 19595 + 38470 + 7471 equals 65536.
:= (19595* + 38470* + 7471* + 1<<15) >> 16
// The bit twiddling below is equivalent to
//
// cb := (-11056*r1 - 21712*g1 + 32768*b1 + 257<<15) >> 16
// if cb < 0 {
// cb = 0
// } else if cb > 0xff {
// cb = ^int32(0)
// }
//
// but uses fewer branches and is faster.
// Note that the uint8 type conversion in the return
// statement will convert ^int32(0) to 0xff.
// The code below to compute cr uses a similar pattern.
//
// Note that -11056 - 21712 + 32768 equals 0.
:= -11056* - 21712* + 32768* + 257<<15
if uint32()&0xff000000 == 0 {
>>= 16
} else {
= ^( >> 31)
}
// Note that 32768 - 27440 - 5328 equals 0.
:= 32768* - 27440* - 5328* + 257<<15
if uint32()&0xff000000 == 0 {
>>= 16
} else {
= ^( >> 31)
}
return uint8(), uint8(), uint8()
}
// YCbCrToRGB converts a Y'CbCr triple to an RGB triple.
func (, , 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)
// https://www.w3.org/Graphics/JPEG/jfif3.pdf says Y but means Y'.
//
// Those formulae use non-integer multiplication factors. When computing,
// integer math is generally faster than floating point math. We multiply
// all of those factors by 1<<16 and round to the nearest integer:
// 91881 = roundToNearestInteger(1.40200 * 65536).
// 22554 = roundToNearestInteger(0.34414 * 65536).
// 46802 = roundToNearestInteger(0.71414 * 65536).
// 116130 = roundToNearestInteger(1.77200 * 65536).
//
// Adding a rounding adjustment in the range [0, 1<<16-1] and then shifting
// right by 16 gives us an integer math version of the original formulae.
// R = (65536*Y' + 91881 *(Cr-128) + adjustment) >> 16
// G = (65536*Y' - 22554 *(Cb-128) - 46802*(Cr-128) + adjustment) >> 16
// B = (65536*Y' + 116130 *(Cb-128) + adjustment) >> 16
// A constant rounding adjustment of 1<<15, one half of 1<<16, would mean
// round-to-nearest when dividing by 65536 (shifting right by 16).
// Similarly, a constant rounding adjustment of 0 would mean round-down.
//
// Defining YY1 = 65536*Y' + adjustment simplifies the formulae and
// requires fewer CPU operations:
// R = (YY1 + 91881 *(Cr-128) ) >> 16
// G = (YY1 - 22554 *(Cb-128) - 46802*(Cr-128)) >> 16
// B = (YY1 + 116130 *(Cb-128) ) >> 16
//
// The inputs (y, cb, cr) are 8 bit color, ranging in [0x00, 0xff]. In this
// function, the output is also 8 bit color, but in the related YCbCr.RGBA
// method, below, the output is 16 bit color, ranging in [0x0000, 0xffff].
// Outputting 16 bit color simply requires changing the 16 to 8 in the "R =
// etc >> 16" equation, and likewise for G and B.
//
// As mentioned above, a constant rounding adjustment of 1<<15 is a natural
// choice, but there is an additional constraint: if c0 := YCbCr{Y: y, Cb:
// 0x80, Cr: 0x80} and c1 := Gray{Y: y} then c0.RGBA() should equal
// c1.RGBA(). Specifically, if y == 0 then "R = etc >> 8" should yield
// 0x0000 and if y == 0xff then "R = etc >> 8" should yield 0xffff. If we
// used a constant rounding adjustment of 1<<15, then it would yield 0x0080
// and 0xff80 respectively.
//
// Note that when cb == 0x80 and cr == 0x80 then the formulae collapse to:
// R = YY1 >> n
// G = YY1 >> n
// B = YY1 >> n
// where n is 16 for this function (8 bit color output) and 8 for the
// YCbCr.RGBA method (16 bit color output).
//
// The solution is to make the rounding adjustment non-constant, and equal
// to 257*Y', which ranges over [0, 1<<16-1] as Y' ranges over [0, 255].
// YY1 is then defined as:
// YY1 = 65536*Y' + 257*Y'
// or equivalently:
// YY1 = Y' * 0x10101
:= int32() * 0x10101
:= int32() - 128
:= int32() - 128
// The bit twiddling below is equivalent to
//
// r := (yy1 + 91881*cr1) >> 16
// if r < 0 {
// r = 0
// } else if r > 0xff {
// r = ^int32(0)
// }
//
// but uses fewer branches and is faster.
// Note that the uint8 type conversion in the return
// statement will convert ^int32(0) to 0xff.
// The code below to compute g and b uses a similar pattern.
:= + 91881*
if uint32()&0xff000000 == 0 {
>>= 16
} else {
= ^( >> 31)
}
:= - 22554* - 46802*
if uint32()&0xff000000 == 0 {
>>= 16
} else {
= ^( >> 31)
}
:= + 116130*
if uint32()&0xff000000 == 0 {
>>= 16
} else {
= ^( >> 31)
}
return uint8(), uint8(), uint8()
}
// 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 https://www.w3.org/Graphics/JPEG/jfif3.pdf.
type YCbCr struct {
Y, Cb, Cr uint8
}
func ( YCbCr) () (uint32, uint32, uint32, uint32) {
// This code is a copy of the YCbCrToRGB function above, except that it
// returns values in the range [0, 0xffff] instead of [0, 0xff]. There is a
// subtle difference between doing this and having YCbCr satisfy the Color
// interface by first converting to an RGBA. The latter loses some
// information by going to and from 8 bits per channel.
//
// For example, this code:
// const y, cb, cr = 0x7f, 0x7f, 0x7f
// r, g, b := color.YCbCrToRGB(y, cb, cr)
// r0, g0, b0, _ := color.YCbCr{y, cb, cr}.RGBA()
// r1, g1, b1, _ := color.RGBA{r, g, b, 0xff}.RGBA()
// fmt.Printf("0x%04x 0x%04x 0x%04x\n", r0, g0, b0)
// fmt.Printf("0x%04x 0x%04x 0x%04x\n", r1, g1, b1)
// prints:
// 0x7e18 0x808d 0x7db9
// 0x7e7e 0x8080 0x7d7d
:= int32(.Y) * 0x10101
:= int32(.Cb) - 128
:= int32(.Cr) - 128
// The bit twiddling below is equivalent to
//
// r := (yy1 + 91881*cr1) >> 8
// if r < 0 {
// r = 0
// } else if r > 0xff {
// r = 0xffff
// }
//
// but uses fewer branches and is faster.
// The code below to compute g and b uses a similar pattern.
:= + 91881*
if uint32()&0xff000000 == 0 {
>>= 8
} else {
= ^( >> 31) & 0xffff
}
:= - 22554* - 46802*
if uint32()&0xff000000 == 0 {
>>= 8
} else {
= ^( >> 31) & 0xffff
}
:= + 116130*
if uint32()&0xff000000 == 0 {
>>= 8
} else {
= ^( >> 31) & 0xffff
}
return uint32(), uint32(), uint32(), 0xffff
}
// YCbCrModel is the [Model] for Y'CbCr colors.
var YCbCrModel Model = ModelFunc(yCbCrModel)
func yCbCrModel( Color) Color {
if , := .(YCbCr); {
return
}
, , , := .RGBA()
, , := RGBToYCbCr(uint8(>>8), uint8(>>8), uint8(>>8))
return YCbCr{, , }
}
// NYCbCrA represents a non-alpha-premultiplied Y'CbCr-with-alpha color, having
// 8 bits each for one luma, two chroma and one alpha component.
type NYCbCrA struct {
YCbCr
A uint8
}
func ( NYCbCrA) () (uint32, uint32, uint32, uint32) {
// The first part of this method is the same as YCbCr.RGBA.
:= int32(.Y) * 0x10101
:= int32(.Cb) - 128
:= int32(.Cr) - 128
// The bit twiddling below is equivalent to
//
// r := (yy1 + 91881*cr1) >> 8
// if r < 0 {
// r = 0
// } else if r > 0xff {
// r = 0xffff
// }
//
// but uses fewer branches and is faster.
// The code below to compute g and b uses a similar pattern.
:= + 91881*
if uint32()&0xff000000 == 0 {
>>= 8
} else {
= ^( >> 31) & 0xffff
}
:= - 22554* - 46802*
if uint32()&0xff000000 == 0 {
>>= 8
} else {
= ^( >> 31) & 0xffff
}
:= + 116130*
if uint32()&0xff000000 == 0 {
>>= 8
} else {
= ^( >> 31) & 0xffff
}
// The second part of this method applies the alpha.
:= uint32(.A) * 0x101
return uint32() * / 0xffff, uint32() * / 0xffff, uint32() * / 0xffff,
}
// NYCbCrAModel is the [Model] for non-alpha-premultiplied Y'CbCr-with-alpha
// colors.
var NYCbCrAModel Model = ModelFunc(nYCbCrAModel)
func nYCbCrAModel( Color) Color {
switch c := .(type) {
case NYCbCrA:
return
case YCbCr:
return NYCbCrA{, 0xff}
}
, , , := .RGBA()
// Convert from alpha-premultiplied to non-alpha-premultiplied.
if != 0 {
= ( * 0xffff) /
= ( * 0xffff) /
= ( * 0xffff) /
}
, , := RGBToYCbCr(uint8(>>8), uint8(>>8), uint8(>>8))
return NYCbCrA{YCbCr{Y: , Cb: , Cr: }, uint8( >> 8)}
}
// RGBToCMYK converts an RGB triple to a CMYK quadruple.
func (, , uint8) (uint8, uint8, uint8, uint8) {
:= uint32()
:= uint32()
:= uint32()
:=
if < {
=
}
if < {
=
}
if == 0 {
return 0, 0, 0, 0xff
}
:= ( - ) * 0xff /
:= ( - ) * 0xff /
:= ( - ) * 0xff /
return uint8(), uint8(), uint8(), uint8(0xff - )
}
// CMYKToRGB converts a [CMYK] quadruple to an RGB triple.
func (, , , uint8) (uint8, uint8, uint8) {
:= 0xffff - uint32()*0x101
:= (0xffff - uint32()*0x101) * / 0xffff
:= (0xffff - uint32()*0x101) * / 0xffff
:= (0xffff - uint32()*0x101) * / 0xffff
return uint8( >> 8), uint8( >> 8), uint8( >> 8)
}
// CMYK represents a fully opaque CMYK color, having 8 bits for each of cyan,
// magenta, yellow and black.
//
// It is not associated with any particular color profile.
type CMYK struct {
C, M, Y, K uint8
}
func ( CMYK) () (uint32, uint32, uint32, uint32) {
// This code is a copy of the CMYKToRGB function above, except that it
// returns values in the range [0, 0xffff] instead of [0, 0xff].
:= 0xffff - uint32(.K)*0x101
:= (0xffff - uint32(.C)*0x101) * / 0xffff
:= (0xffff - uint32(.M)*0x101) * / 0xffff
:= (0xffff - uint32(.Y)*0x101) * / 0xffff
return , , , 0xffff
}
// CMYKModel is the [Model] for CMYK colors.
var CMYKModel Model = ModelFunc(cmykModel)
func cmykModel( Color) Color {
if , := .(CMYK); {
return
}
, , , := .RGBA()
, , , := RGBToCMYK(uint8(>>8), uint8(>>8), uint8(>>8))
return CMYK{, , , }
}
The pages are generated with Golds v0.6.9-preview. (GOOS=linux GOARCH=amd64) Golds is a Go 101 project developed by Tapir Liu. PR and bug reports are welcome and can be submitted to the issue list. Please follow @Go100and1 (reachable from the left QR code) to get the latest news of Golds. |