Source file src/image/color/ycbcr.go
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 // https://www.w3.org/Graphics/JPEG/jfif3.pdf says Y but means Y'. 14 15 r1 := int32(r) 16 g1 := int32(g) 17 b1 := int32(b) 18 19 // yy is in range [0,0xff]. 20 // 21 // Note that 19595 + 38470 + 7471 equals 65536. 22 yy := (19595*r1 + 38470*g1 + 7471*b1 + 1<<15) >> 16 23 24 // The bit twiddling below is equivalent to 25 // 26 // cb := (-11056*r1 - 21712*g1 + 32768*b1 + 257<<15) >> 16 27 // if cb < 0 { 28 // cb = 0 29 // } else if cb > 0xff { 30 // cb = ^int32(0) 31 // } 32 // 33 // but uses fewer branches and is faster. 34 // Note that the uint8 type conversion in the return 35 // statement will convert ^int32(0) to 0xff. 36 // The code below to compute cr uses a similar pattern. 37 // 38 // Note that -11056 - 21712 + 32768 equals 0. 39 cb := -11056*r1 - 21712*g1 + 32768*b1 + 257<<15 40 if uint32(cb)&0xff000000 == 0 { 41 cb >>= 16 42 } else { 43 cb = ^(cb >> 31) 44 } 45 46 // Note that 32768 - 27440 - 5328 equals 0. 47 cr := 32768*r1 - 27440*g1 - 5328*b1 + 257<<15 48 if uint32(cr)&0xff000000 == 0 { 49 cr >>= 16 50 } else { 51 cr = ^(cr >> 31) 52 } 53 54 return uint8(yy), uint8(cb), uint8(cr) 55 } 56 57 // YCbCrToRGB converts a Y'CbCr triple to an RGB triple. 58 func YCbCrToRGB(y, cb, cr uint8) (uint8, uint8, uint8) { 59 // The JFIF specification says: 60 // R = Y' + 1.40200*(Cr-128) 61 // G = Y' - 0.34414*(Cb-128) - 0.71414*(Cr-128) 62 // B = Y' + 1.77200*(Cb-128) 63 // https://www.w3.org/Graphics/JPEG/jfif3.pdf says Y but means Y'. 64 // 65 // Those formulae use non-integer multiplication factors. When computing, 66 // integer math is generally faster than floating point math. We multiply 67 // all of those factors by 1<<16 and round to the nearest integer: 68 // 91881 = roundToNearestInteger(1.40200 * 65536). 69 // 22554 = roundToNearestInteger(0.34414 * 65536). 70 // 46802 = roundToNearestInteger(0.71414 * 65536). 71 // 116130 = roundToNearestInteger(1.77200 * 65536). 72 // 73 // Adding a rounding adjustment in the range [0, 1<<16-1] and then shifting 74 // right by 16 gives us an integer math version of the original formulae. 75 // R = (65536*Y' + 91881 *(Cr-128) + adjustment) >> 16 76 // G = (65536*Y' - 22554 *(Cb-128) - 46802*(Cr-128) + adjustment) >> 16 77 // B = (65536*Y' + 116130 *(Cb-128) + adjustment) >> 16 78 // A constant rounding adjustment of 1<<15, one half of 1<<16, would mean 79 // round-to-nearest when dividing by 65536 (shifting right by 16). 80 // Similarly, a constant rounding adjustment of 0 would mean round-down. 81 // 82 // Defining YY1 = 65536*Y' + adjustment simplifies the formulae and 83 // requires fewer CPU operations: 84 // R = (YY1 + 91881 *(Cr-128) ) >> 16 85 // G = (YY1 - 22554 *(Cb-128) - 46802*(Cr-128)) >> 16 86 // B = (YY1 + 116130 *(Cb-128) ) >> 16 87 // 88 // The inputs (y, cb, cr) are 8 bit color, ranging in [0x00, 0xff]. In this 89 // function, the output is also 8 bit color, but in the related YCbCr.RGBA 90 // method, below, the output is 16 bit color, ranging in [0x0000, 0xffff]. 91 // Outputting 16 bit color simply requires changing the 16 to 8 in the "R = 92 // etc >> 16" equation, and likewise for G and B. 93 // 94 // As mentioned above, a constant rounding adjustment of 1<<15 is a natural 95 // choice, but there is an additional constraint: if c0 := YCbCr{Y: y, Cb: 96 // 0x80, Cr: 0x80} and c1 := Gray{Y: y} then c0.RGBA() should equal 97 // c1.RGBA(). Specifically, if y == 0 then "R = etc >> 8" should yield 98 // 0x0000 and if y == 0xff then "R = etc >> 8" should yield 0xffff. If we 99 // used a constant rounding adjustment of 1<<15, then it would yield 0x0080 100 // and 0xff80 respectively. 101 // 102 // Note that when cb == 0x80 and cr == 0x80 then the formulae collapse to: 103 // R = YY1 >> n 104 // G = YY1 >> n 105 // B = YY1 >> n 106 // where n is 16 for this function (8 bit color output) and 8 for the 107 // YCbCr.RGBA method (16 bit color output). 108 // 109 // The solution is to make the rounding adjustment non-constant, and equal 110 // to 257*Y', which ranges over [0, 1<<16-1] as Y' ranges over [0, 255]. 111 // YY1 is then defined as: 112 // YY1 = 65536*Y' + 257*Y' 113 // or equivalently: 114 // YY1 = Y' * 0x10101 115 yy1 := int32(y) * 0x10101 116 cb1 := int32(cb) - 128 117 cr1 := int32(cr) - 128 118 119 // The bit twiddling below is equivalent to 120 // 121 // r := (yy1 + 91881*cr1) >> 16 122 // if r < 0 { 123 // r = 0 124 // } else if r > 0xff { 125 // r = ^int32(0) 126 // } 127 // 128 // but uses fewer branches and is faster. 129 // Note that the uint8 type conversion in the return 130 // statement will convert ^int32(0) to 0xff. 131 // The code below to compute g and b uses a similar pattern. 132 r := yy1 + 91881*cr1 133 if uint32(r)&0xff000000 == 0 { 134 r >>= 16 135 } else { 136 r = ^(r >> 31) 137 } 138 139 g := yy1 - 22554*cb1 - 46802*cr1 140 if uint32(g)&0xff000000 == 0 { 141 g >>= 16 142 } else { 143 g = ^(g >> 31) 144 } 145 146 b := yy1 + 116130*cb1 147 if uint32(b)&0xff000000 == 0 { 148 b >>= 16 149 } else { 150 b = ^(b >> 31) 151 } 152 153 return uint8(r), uint8(g), uint8(b) 154 } 155 156 // YCbCr represents a fully opaque 24-bit Y'CbCr color, having 8 bits each for 157 // one luma and two chroma components. 158 // 159 // JPEG, VP8, the MPEG family and other codecs use this color model. Such 160 // codecs often use the terms YUV and Y'CbCr interchangeably, but strictly 161 // speaking, the term YUV applies only to analog video signals, and Y' (luma) 162 // is Y (luminance) after applying gamma correction. 163 // 164 // Conversion between RGB and Y'CbCr is lossy and there are multiple, slightly 165 // different formulae for converting between the two. This package follows 166 // the JFIF specification at https://www.w3.org/Graphics/JPEG/jfif3.pdf. 167 type YCbCr struct { 168 Y, Cb, Cr uint8 169 } 170 171 func (c YCbCr) RGBA() (uint32, uint32, uint32, uint32) { 172 // This code is a copy of the YCbCrToRGB function above, except that it 173 // returns values in the range [0, 0xffff] instead of [0, 0xff]. There is a 174 // subtle difference between doing this and having YCbCr satisfy the Color 175 // interface by first converting to an RGBA. The latter loses some 176 // information by going to and from 8 bits per channel. 177 // 178 // For example, this code: 179 // const y, cb, cr = 0x7f, 0x7f, 0x7f 180 // r, g, b := color.YCbCrToRGB(y, cb, cr) 181 // r0, g0, b0, _ := color.YCbCr{y, cb, cr}.RGBA() 182 // r1, g1, b1, _ := color.RGBA{r, g, b, 0xff}.RGBA() 183 // fmt.Printf("0x%04x 0x%04x 0x%04x\n", r0, g0, b0) 184 // fmt.Printf("0x%04x 0x%04x 0x%04x\n", r1, g1, b1) 185 // prints: 186 // 0x7e18 0x808d 0x7db9 187 // 0x7e7e 0x8080 0x7d7d 188 189 yy1 := int32(c.Y) * 0x10101 190 cb1 := int32(c.Cb) - 128 191 cr1 := int32(c.Cr) - 128 192 193 // The bit twiddling below is equivalent to 194 // 195 // r := (yy1 + 91881*cr1) >> 8 196 // if r < 0 { 197 // r = 0 198 // } else if r > 0xff { 199 // r = 0xffff 200 // } 201 // 202 // but uses fewer branches and is faster. 203 // The code below to compute g and b uses a similar pattern. 204 r := yy1 + 91881*cr1 205 if uint32(r)&0xff000000 == 0 { 206 r >>= 8 207 } else { 208 r = ^(r >> 31) & 0xffff 209 } 210 211 g := yy1 - 22554*cb1 - 46802*cr1 212 if uint32(g)&0xff000000 == 0 { 213 g >>= 8 214 } else { 215 g = ^(g >> 31) & 0xffff 216 } 217 218 b := yy1 + 116130*cb1 219 if uint32(b)&0xff000000 == 0 { 220 b >>= 8 221 } else { 222 b = ^(b >> 31) & 0xffff 223 } 224 225 return uint32(r), uint32(g), uint32(b), 0xffff 226 } 227 228 // YCbCrModel is the Model for Y'CbCr colors. 229 var YCbCrModel Model = ModelFunc(yCbCrModel) 230 231 func yCbCrModel(c Color) Color { 232 if _, ok := c.(YCbCr); ok { 233 return c 234 } 235 r, g, b, _ := c.RGBA() 236 y, u, v := RGBToYCbCr(uint8(r>>8), uint8(g>>8), uint8(b>>8)) 237 return YCbCr{y, u, v} 238 } 239 240 // NYCbCrA represents a non-alpha-premultiplied Y'CbCr-with-alpha color, having 241 // 8 bits each for one luma, two chroma and one alpha component. 242 type NYCbCrA struct { 243 YCbCr 244 A uint8 245 } 246 247 func (c NYCbCrA) RGBA() (uint32, uint32, uint32, uint32) { 248 // The first part of this method is the same as YCbCr.RGBA. 249 yy1 := int32(c.Y) * 0x10101 250 cb1 := int32(c.Cb) - 128 251 cr1 := int32(c.Cr) - 128 252 253 // The bit twiddling below is equivalent to 254 // 255 // r := (yy1 + 91881*cr1) >> 8 256 // if r < 0 { 257 // r = 0 258 // } else if r > 0xff { 259 // r = 0xffff 260 // } 261 // 262 // but uses fewer branches and is faster. 263 // The code below to compute g and b uses a similar pattern. 264 r := yy1 + 91881*cr1 265 if uint32(r)&0xff000000 == 0 { 266 r >>= 8 267 } else { 268 r = ^(r >> 31) & 0xffff 269 } 270 271 g := yy1 - 22554*cb1 - 46802*cr1 272 if uint32(g)&0xff000000 == 0 { 273 g >>= 8 274 } else { 275 g = ^(g >> 31) & 0xffff 276 } 277 278 b := yy1 + 116130*cb1 279 if uint32(b)&0xff000000 == 0 { 280 b >>= 8 281 } else { 282 b = ^(b >> 31) & 0xffff 283 } 284 285 // The second part of this method applies the alpha. 286 a := uint32(c.A) * 0x101 287 return uint32(r) * a / 0xffff, uint32(g) * a / 0xffff, uint32(b) * a / 0xffff, a 288 } 289 290 // NYCbCrAModel is the Model for non-alpha-premultiplied Y'CbCr-with-alpha 291 // colors. 292 var NYCbCrAModel Model = ModelFunc(nYCbCrAModel) 293 294 func nYCbCrAModel(c Color) Color { 295 switch c := c.(type) { 296 case NYCbCrA: 297 return c 298 case YCbCr: 299 return NYCbCrA{c, 0xff} 300 } 301 r, g, b, a := c.RGBA() 302 303 // Convert from alpha-premultiplied to non-alpha-premultiplied. 304 if a != 0 { 305 r = (r * 0xffff) / a 306 g = (g * 0xffff) / a 307 b = (b * 0xffff) / a 308 } 309 310 y, u, v := RGBToYCbCr(uint8(r>>8), uint8(g>>8), uint8(b>>8)) 311 return NYCbCrA{YCbCr{Y: y, Cb: u, Cr: v}, uint8(a >> 8)} 312 } 313 314 // RGBToCMYK converts an RGB triple to a CMYK quadruple. 315 func RGBToCMYK(r, g, b uint8) (uint8, uint8, uint8, uint8) { 316 rr := uint32(r) 317 gg := uint32(g) 318 bb := uint32(b) 319 w := rr 320 if w < gg { 321 w = gg 322 } 323 if w < bb { 324 w = bb 325 } 326 if w == 0 { 327 return 0, 0, 0, 0xff 328 } 329 c := (w - rr) * 0xff / w 330 m := (w - gg) * 0xff / w 331 y := (w - bb) * 0xff / w 332 return uint8(c), uint8(m), uint8(y), uint8(0xff - w) 333 } 334 335 // CMYKToRGB converts a CMYK quadruple to an RGB triple. 336 func CMYKToRGB(c, m, y, k uint8) (uint8, uint8, uint8) { 337 w := 0xffff - uint32(k)*0x101 338 r := (0xffff - uint32(c)*0x101) * w / 0xffff 339 g := (0xffff - uint32(m)*0x101) * w / 0xffff 340 b := (0xffff - uint32(y)*0x101) * w / 0xffff 341 return uint8(r >> 8), uint8(g >> 8), uint8(b >> 8) 342 } 343 344 // CMYK represents a fully opaque CMYK color, having 8 bits for each of cyan, 345 // magenta, yellow and black. 346 // 347 // It is not associated with any particular color profile. 348 type CMYK struct { 349 C, M, Y, K uint8 350 } 351 352 func (c CMYK) RGBA() (uint32, uint32, uint32, uint32) { 353 // This code is a copy of the CMYKToRGB function above, except that it 354 // returns values in the range [0, 0xffff] instead of [0, 0xff]. 355 356 w := 0xffff - uint32(c.K)*0x101 357 r := (0xffff - uint32(c.C)*0x101) * w / 0xffff 358 g := (0xffff - uint32(c.M)*0x101) * w / 0xffff 359 b := (0xffff - uint32(c.Y)*0x101) * w / 0xffff 360 return r, g, b, 0xffff 361 } 362 363 // CMYKModel is the Model for CMYK colors. 364 var CMYKModel Model = ModelFunc(cmykModel) 365 366 func cmykModel(c Color) Color { 367 if _, ok := c.(CMYK); ok { 368 return c 369 } 370 r, g, b, _ := c.RGBA() 371 cc, mm, yy, kk := RGBToCMYK(uint8(r>>8), uint8(g>>8), uint8(b>>8)) 372 return CMYK{cc, mm, yy, kk} 373 } 374