373 lines
11 KiB
Go
373 lines
11 KiB
Go
// Copyright 2011 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package color
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// RGBToYCbCr converts an RGB triple to a Y'CbCr triple.
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func RGBToYCbCr(r, g, b uint8) (uint8, uint8, uint8) {
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// The JFIF specification says:
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// Y' = 0.2990*R + 0.5870*G + 0.1140*B
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// Cb = -0.1687*R - 0.3313*G + 0.5000*B + 128
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// Cr = 0.5000*R - 0.4187*G - 0.0813*B + 128
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// https://www.w3.org/Graphics/JPEG/jfif3.pdf says Y but means Y'.
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r1 := int32(r)
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g1 := int32(g)
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b1 := int32(b)
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// yy is in range [0,0xff].
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//
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// Note that 19595 + 38470 + 7471 equals 65536.
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yy := (19595*r1 + 38470*g1 + 7471*b1 + 1<<15) >> 16
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// The bit twiddling below is equivalent to
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//
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// cb := (-11056*r1 - 21712*g1 + 32768*b1 + 257<<15) >> 16
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// if cb < 0 {
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// cb = 0
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// } else if cb > 0xff {
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// cb = ^int32(0)
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// }
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//
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// but uses fewer branches and is faster.
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// Note that the uint8 type conversion in the return
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// statement will convert ^int32(0) to 0xff.
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// The code below to compute cr uses a similar pattern.
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//
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// Note that -11056 - 21712 + 32768 equals 0.
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cb := -11056*r1 - 21712*g1 + 32768*b1 + 257<<15
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if uint32(cb)&0xff000000 == 0 {
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cb >>= 16
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} else {
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cb = ^(cb >> 31)
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}
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// Note that 32768 - 27440 - 5328 equals 0.
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cr := 32768*r1 - 27440*g1 - 5328*b1 + 257<<15
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if uint32(cr)&0xff000000 == 0 {
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cr >>= 16
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} else {
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cr = ^(cr >> 31)
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}
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return uint8(yy), uint8(cb), uint8(cr)
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}
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// YCbCrToRGB converts a Y'CbCr triple to an RGB triple.
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func YCbCrToRGB(y, cb, cr uint8) (uint8, uint8, uint8) {
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// The JFIF specification says:
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// R = Y' + 1.40200*(Cr-128)
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// G = Y' - 0.34414*(Cb-128) - 0.71414*(Cr-128)
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// B = Y' + 1.77200*(Cb-128)
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// https://www.w3.org/Graphics/JPEG/jfif3.pdf says Y but means Y'.
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//
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// Those formulae use non-integer multiplication factors. When computing,
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// integer math is generally faster than floating point math. We multiply
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// all of those factors by 1<<16 and round to the nearest integer:
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// 91881 = roundToNearestInteger(1.40200 * 65536).
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// 22554 = roundToNearestInteger(0.34414 * 65536).
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// 46802 = roundToNearestInteger(0.71414 * 65536).
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// 116130 = roundToNearestInteger(1.77200 * 65536).
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//
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// Adding a rounding adjustment in the range [0, 1<<16-1] and then shifting
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// right by 16 gives us an integer math version of the original formulae.
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// R = (65536*Y' + 91881 *(Cr-128) + adjustment) >> 16
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// G = (65536*Y' - 22554 *(Cb-128) - 46802*(Cr-128) + adjustment) >> 16
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// B = (65536*Y' + 116130 *(Cb-128) + adjustment) >> 16
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// A constant rounding adjustment of 1<<15, one half of 1<<16, would mean
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// round-to-nearest when dividing by 65536 (shifting right by 16).
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// Similarly, a constant rounding adjustment of 0 would mean round-down.
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//
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// Defining YY1 = 65536*Y' + adjustment simplifies the formulae and
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// requires fewer CPU operations:
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// R = (YY1 + 91881 *(Cr-128) ) >> 16
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// G = (YY1 - 22554 *(Cb-128) - 46802*(Cr-128)) >> 16
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// B = (YY1 + 116130 *(Cb-128) ) >> 16
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//
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// The inputs (y, cb, cr) are 8 bit color, ranging in [0x00, 0xff]. In this
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// function, the output is also 8 bit color, but in the related YCbCr.RGBA
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// method, below, the output is 16 bit color, ranging in [0x0000, 0xffff].
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// Outputting 16 bit color simply requires changing the 16 to 8 in the "R =
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// etc >> 16" equation, and likewise for G and B.
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//
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// As mentioned above, a constant rounding adjustment of 1<<15 is a natural
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// choice, but there is an additional constraint: if c0 := YCbCr{Y: y, Cb:
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// 0x80, Cr: 0x80} and c1 := Gray{Y: y} then c0.RGBA() should equal
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// c1.RGBA(). Specifically, if y == 0 then "R = etc >> 8" should yield
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// 0x0000 and if y == 0xff then "R = etc >> 8" should yield 0xffff. If we
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// used a constant rounding adjustment of 1<<15, then it would yield 0x0080
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// and 0xff80 respectively.
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//
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// Note that when cb == 0x80 and cr == 0x80 then the formulae collapse to:
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// R = YY1 >> n
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// G = YY1 >> n
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// B = YY1 >> n
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// where n is 16 for this function (8 bit color output) and 8 for the
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// YCbCr.RGBA method (16 bit color output).
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//
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// The solution is to make the rounding adjustment non-constant, and equal
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// to 257*Y', which ranges over [0, 1<<16-1] as Y' ranges over [0, 255].
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// YY1 is then defined as:
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// YY1 = 65536*Y' + 257*Y'
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// or equivalently:
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// YY1 = Y' * 0x10101
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yy1 := int32(y) * 0x10101
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cb1 := int32(cb) - 128
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cr1 := int32(cr) - 128
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// The bit twiddling below is equivalent to
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//
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// r := (yy1 + 91881*cr1) >> 16
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// if r < 0 {
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// r = 0
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// } else if r > 0xff {
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// r = ^int32(0)
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// }
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//
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// but uses fewer branches and is faster.
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// Note that the uint8 type conversion in the return
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// statement will convert ^int32(0) to 0xff.
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// The code below to compute g and b uses a similar pattern.
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r := yy1 + 91881*cr1
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if uint32(r)&0xff000000 == 0 {
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r >>= 16
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} else {
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r = ^(r >> 31)
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}
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g := yy1 - 22554*cb1 - 46802*cr1
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if uint32(g)&0xff000000 == 0 {
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g >>= 16
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} else {
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g = ^(g >> 31)
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}
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b := yy1 + 116130*cb1
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if uint32(b)&0xff000000 == 0 {
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b >>= 16
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} else {
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b = ^(b >> 31)
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}
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return uint8(r), uint8(g), uint8(b)
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}
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// YCbCr represents a fully opaque 24-bit Y'CbCr color, having 8 bits each for
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// one luma and two chroma components.
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//
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// JPEG, VP8, the MPEG family and other codecs use this color model. Such
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// codecs often use the terms YUV and Y'CbCr interchangeably, but strictly
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// speaking, the term YUV applies only to analog video signals, and Y' (luma)
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// is Y (luminance) after applying gamma correction.
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//
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// Conversion between RGB and Y'CbCr is lossy and there are multiple, slightly
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// different formulae for converting between the two. This package follows
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// the JFIF specification at https://www.w3.org/Graphics/JPEG/jfif3.pdf.
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type YCbCr struct {
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Y, Cb, Cr uint8
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}
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func (c YCbCr) RGBA() (uint32, uint32, uint32, uint32) {
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// This code is a copy of the YCbCrToRGB function above, except that it
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// returns values in the range [0, 0xffff] instead of [0, 0xff]. There is a
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// subtle difference between doing this and having YCbCr satisfy the Color
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// interface by first converting to an RGBA. The latter loses some
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// information by going to and from 8 bits per channel.
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//
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// For example, this code:
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// const y, cb, cr = 0x7f, 0x7f, 0x7f
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// r, g, b := color.YCbCrToRGB(y, cb, cr)
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// r0, g0, b0, _ := color.YCbCr{y, cb, cr}.RGBA()
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// r1, g1, b1, _ := color.RGBA{r, g, b, 0xff}.RGBA()
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// fmt.Printf("0x%04x 0x%04x 0x%04x\n", r0, g0, b0)
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// fmt.Printf("0x%04x 0x%04x 0x%04x\n", r1, g1, b1)
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// prints:
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// 0x7e18 0x808d 0x7db9
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// 0x7e7e 0x8080 0x7d7d
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yy1 := int32(c.Y) * 0x10101
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cb1 := int32(c.Cb) - 128
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cr1 := int32(c.Cr) - 128
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// The bit twiddling below is equivalent to
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//
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// r := (yy1 + 91881*cr1) >> 8
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// if r < 0 {
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// r = 0
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// } else if r > 0xff {
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// r = 0xffff
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// }
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//
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// but uses fewer branches and is faster.
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// The code below to compute g and b uses a similar pattern.
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r := yy1 + 91881*cr1
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if uint32(r)&0xff000000 == 0 {
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r >>= 8
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} else {
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r = ^(r >> 31) & 0xffff
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}
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g := yy1 - 22554*cb1 - 46802*cr1
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if uint32(g)&0xff000000 == 0 {
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g >>= 8
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} else {
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g = ^(g >> 31) & 0xffff
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}
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b := yy1 + 116130*cb1
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if uint32(b)&0xff000000 == 0 {
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b >>= 8
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} else {
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b = ^(b >> 31) & 0xffff
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}
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return uint32(r), uint32(g), uint32(b), 0xffff
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}
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// YCbCrModel is the Model for Y'CbCr colors.
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var YCbCrModel Model = ModelFunc(yCbCrModel)
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func yCbCrModel(c Color) Color {
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if _, ok := c.(YCbCr); ok {
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return c
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}
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r, g, b, _ := c.RGBA()
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y, u, v := RGBToYCbCr(uint8(r>>8), uint8(g>>8), uint8(b>>8))
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return YCbCr{y, u, v}
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}
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// NYCbCrA represents a non-alpha-premultiplied Y'CbCr-with-alpha color, having
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// 8 bits each for one luma, two chroma and one alpha component.
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type NYCbCrA struct {
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YCbCr
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A uint8
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}
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func (c NYCbCrA) RGBA() (uint32, uint32, uint32, uint32) {
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// The first part of this method is the same as YCbCr.RGBA.
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yy1 := int32(c.Y) * 0x10101
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cb1 := int32(c.Cb) - 128
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cr1 := int32(c.Cr) - 128
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// The bit twiddling below is equivalent to
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//
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// r := (yy1 + 91881*cr1) >> 8
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// if r < 0 {
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// r = 0
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// } else if r > 0xff {
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// r = 0xffff
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// }
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//
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// but uses fewer branches and is faster.
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// The code below to compute g and b uses a similar pattern.
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r := yy1 + 91881*cr1
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if uint32(r)&0xff000000 == 0 {
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r >>= 8
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} else {
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r = ^(r >> 31) & 0xffff
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}
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g := yy1 - 22554*cb1 - 46802*cr1
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if uint32(g)&0xff000000 == 0 {
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g >>= 8
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} else {
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g = ^(g >> 31) & 0xffff
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}
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b := yy1 + 116130*cb1
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if uint32(b)&0xff000000 == 0 {
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b >>= 8
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} else {
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b = ^(b >> 31) & 0xffff
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}
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// The second part of this method applies the alpha.
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a := uint32(c.A) * 0x101
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return uint32(r) * a / 0xffff, uint32(g) * a / 0xffff, uint32(b) * a / 0xffff, a
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}
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// NYCbCrAModel is the Model for non-alpha-premultiplied Y'CbCr-with-alpha
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// colors.
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var NYCbCrAModel Model = ModelFunc(nYCbCrAModel)
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func nYCbCrAModel(c Color) Color {
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switch c := c.(type) {
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case NYCbCrA:
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return c
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case YCbCr:
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return NYCbCrA{c, 0xff}
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}
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r, g, b, a := c.RGBA()
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// Convert from alpha-premultiplied to non-alpha-premultiplied.
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if a != 0 {
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r = (r * 0xffff) / a
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g = (g * 0xffff) / a
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b = (b * 0xffff) / a
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}
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y, u, v := RGBToYCbCr(uint8(r>>8), uint8(g>>8), uint8(b>>8))
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return NYCbCrA{YCbCr{Y: y, Cb: u, Cr: v}, uint8(a >> 8)}
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}
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// RGBToCMYK converts an RGB triple to a CMYK quadruple.
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func RGBToCMYK(r, g, b uint8) (uint8, uint8, uint8, uint8) {
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rr := uint32(r)
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gg := uint32(g)
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bb := uint32(b)
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w := rr
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if w < gg {
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w = gg
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}
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if w < bb {
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w = bb
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}
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if w == 0 {
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return 0, 0, 0, 0xff
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}
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c := (w - rr) * 0xff / w
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m := (w - gg) * 0xff / w
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y := (w - bb) * 0xff / w
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return uint8(c), uint8(m), uint8(y), uint8(0xff - w)
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}
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// CMYKToRGB converts a CMYK quadruple to an RGB triple.
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func CMYKToRGB(c, m, y, k uint8) (uint8, uint8, uint8) {
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w := 0xffff - uint32(k)*0x101
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r := (0xffff - uint32(c)*0x101) * w / 0xffff
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g := (0xffff - uint32(m)*0x101) * w / 0xffff
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b := (0xffff - uint32(y)*0x101) * w / 0xffff
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return uint8(r >> 8), uint8(g >> 8), uint8(b >> 8)
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}
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// CMYK represents a fully opaque CMYK color, having 8 bits for each of cyan,
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// magenta, yellow and black.
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//
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// It is not associated with any particular color profile.
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type CMYK struct {
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C, M, Y, K uint8
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}
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func (c CMYK) RGBA() (uint32, uint32, uint32, uint32) {
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// This code is a copy of the CMYKToRGB function above, except that it
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// returns values in the range [0, 0xffff] instead of [0, 0xff].
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w := 0xffff - uint32(c.K)*0x101
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r := (0xffff - uint32(c.C)*0x101) * w / 0xffff
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g := (0xffff - uint32(c.M)*0x101) * w / 0xffff
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b := (0xffff - uint32(c.Y)*0x101) * w / 0xffff
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return r, g, b, 0xffff
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}
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// CMYKModel is the Model for CMYK colors.
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var CMYKModel Model = ModelFunc(cmykModel)
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func cmykModel(c Color) Color {
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if _, ok := c.(CMYK); ok {
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return c
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}
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r, g, b, _ := c.RGBA()
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cc, mm, yy, kk := RGBToCMYK(uint8(r>>8), uint8(g>>8), uint8(b>>8))
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return CMYK{cc, mm, yy, kk}
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}
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