1 // Copyright 2009 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 png 6 7 import ( 8 "bufio" 9 "compress/zlib" 10 "encoding/binary" 11 "hash/crc32" 12 "image" 13 "image/color" 14 "io" 15 "strconv" 16 ) 17 18 // Encoder configures encoding PNG images. 19 type Encoder struct { 20 CompressionLevel CompressionLevel 21 22 // BufferPool optionally specifies a buffer pool to get temporary 23 // EncoderBuffers when encoding an image. 24 BufferPool EncoderBufferPool 25 } 26 27 // EncoderBufferPool is an interface for getting and returning temporary 28 // instances of the EncoderBuffer struct. This can be used to reuse buffers 29 // when encoding multiple images. 30 type EncoderBufferPool interface { 31 Get() *EncoderBuffer 32 Put(*EncoderBuffer) 33 } 34 35 // EncoderBuffer holds the buffers used for encoding PNG images. 36 type EncoderBuffer encoder 37 38 type encoder struct { 39 enc *Encoder 40 w io.Writer 41 m image.Image 42 cb int 43 err error 44 header [8]byte 45 footer [4]byte 46 tmp [4 * 256]byte 47 cr [nFilter][]uint8 48 pr []uint8 49 zw *zlib.Writer 50 zwLevel int 51 bw *bufio.Writer 52 } 53 54 type CompressionLevel int 55 56 const ( 57 DefaultCompression CompressionLevel = 0 58 NoCompression CompressionLevel = -1 59 BestSpeed CompressionLevel = -2 60 BestCompression CompressionLevel = -3 61 62 // Positive CompressionLevel values are reserved to mean a numeric zlib 63 // compression level, although that is not implemented yet. 64 ) 65 66 type opaquer interface { 67 Opaque() bool 68 } 69 70 // Returns whether or not the image is fully opaque. 71 func opaque(m image.Image) bool { 72 if o, ok := m.(opaquer); ok { 73 return o.Opaque() 74 } 75 b := m.Bounds() 76 for y := b.Min.Y; y < b.Max.Y; y++ { 77 for x := b.Min.X; x < b.Max.X; x++ { 78 _, _, _, a := m.At(x, y).RGBA() 79 if a != 0xffff { 80 return false 81 } 82 } 83 } 84 return true 85 } 86 87 // The absolute value of a byte interpreted as a signed int8. 88 func abs8(d uint8) int { 89 if d < 128 { 90 return int(d) 91 } 92 return 256 - int(d) 93 } 94 95 func (e *encoder) writeChunk(b []byte, name string) { 96 if e.err != nil { 97 return 98 } 99 n := uint32(len(b)) 100 if int(n) != len(b) { 101 e.err = UnsupportedError(name + " chunk is too large: " + strconv.Itoa(len(b))) 102 return 103 } 104 binary.BigEndian.PutUint32(e.header[:4], n) 105 e.header[4] = name[0] 106 e.header[5] = name[1] 107 e.header[6] = name[2] 108 e.header[7] = name[3] 109 crc := crc32.NewIEEE() 110 crc.Write(e.header[4:8]) 111 crc.Write(b) 112 binary.BigEndian.PutUint32(e.footer[:4], crc.Sum32()) 113 114 _, e.err = e.w.Write(e.header[:8]) 115 if e.err != nil { 116 return 117 } 118 _, e.err = e.w.Write(b) 119 if e.err != nil { 120 return 121 } 122 _, e.err = e.w.Write(e.footer[:4]) 123 } 124 125 func (e *encoder) writeIHDR() { 126 b := e.m.Bounds() 127 binary.BigEndian.PutUint32(e.tmp[0:4], uint32(b.Dx())) 128 binary.BigEndian.PutUint32(e.tmp[4:8], uint32(b.Dy())) 129 // Set bit depth and color type. 130 switch e.cb { 131 case cbG8: 132 e.tmp[8] = 8 133 e.tmp[9] = ctGrayscale 134 case cbTC8: 135 e.tmp[8] = 8 136 e.tmp[9] = ctTrueColor 137 case cbP8: 138 e.tmp[8] = 8 139 e.tmp[9] = ctPaletted 140 case cbP4: 141 e.tmp[8] = 4 142 e.tmp[9] = ctPaletted 143 case cbP2: 144 e.tmp[8] = 2 145 e.tmp[9] = ctPaletted 146 case cbP1: 147 e.tmp[8] = 1 148 e.tmp[9] = ctPaletted 149 case cbTCA8: 150 e.tmp[8] = 8 151 e.tmp[9] = ctTrueColorAlpha 152 case cbG16: 153 e.tmp[8] = 16 154 e.tmp[9] = ctGrayscale 155 case cbTC16: 156 e.tmp[8] = 16 157 e.tmp[9] = ctTrueColor 158 case cbTCA16: 159 e.tmp[8] = 16 160 e.tmp[9] = ctTrueColorAlpha 161 } 162 e.tmp[10] = 0 // default compression method 163 e.tmp[11] = 0 // default filter method 164 e.tmp[12] = 0 // non-interlaced 165 e.writeChunk(e.tmp[:13], "IHDR") 166 } 167 168 func (e *encoder) writePLTEAndTRNS(p color.Palette) { 169 if len(p) < 1 || len(p) > 256 { 170 e.err = FormatError("bad palette length: " + strconv.Itoa(len(p))) 171 return 172 } 173 last := -1 174 for i, c := range p { 175 c1 := color.NRGBAModel.Convert(c).(color.NRGBA) 176 e.tmp[3*i+0] = c1.R 177 e.tmp[3*i+1] = c1.G 178 e.tmp[3*i+2] = c1.B 179 if c1.A != 0xff { 180 last = i 181 } 182 e.tmp[3*256+i] = c1.A 183 } 184 e.writeChunk(e.tmp[:3*len(p)], "PLTE") 185 if last != -1 { 186 e.writeChunk(e.tmp[3*256:3*256+1+last], "tRNS") 187 } 188 } 189 190 // An encoder is an io.Writer that satisfies writes by writing PNG IDAT chunks, 191 // including an 8-byte header and 4-byte CRC checksum per Write call. Such calls 192 // should be relatively infrequent, since writeIDATs uses a bufio.Writer. 193 // 194 // This method should only be called from writeIDATs (via writeImage). 195 // No other code should treat an encoder as an io.Writer. 196 func (e *encoder) Write(b []byte) (int, error) { 197 e.writeChunk(b, "IDAT") 198 if e.err != nil { 199 return 0, e.err 200 } 201 return len(b), nil 202 } 203 204 // Chooses the filter to use for encoding the current row, and applies it. 205 // The return value is the index of the filter and also of the row in cr that has had it applied. 206 func filter(cr *[nFilter][]byte, pr []byte, bpp int) int { 207 // We try all five filter types, and pick the one that minimizes the sum of absolute differences. 208 // This is the same heuristic that libpng uses, although the filters are attempted in order of 209 // estimated most likely to be minimal (ftUp, ftPaeth, ftNone, ftSub, ftAverage), rather than 210 // in their enumeration order (ftNone, ftSub, ftUp, ftAverage, ftPaeth). 211 cdat0 := cr[0][1:] 212 cdat1 := cr[1][1:] 213 cdat2 := cr[2][1:] 214 cdat3 := cr[3][1:] 215 cdat4 := cr[4][1:] 216 pdat := pr[1:] 217 n := len(cdat0) 218 219 // The up filter. 220 sum := 0 221 for i := 0; i < n; i++ { 222 cdat2[i] = cdat0[i] - pdat[i] 223 sum += abs8(cdat2[i]) 224 } 225 best := sum 226 filter := ftUp 227 228 // The Paeth filter. 229 sum = 0 230 for i := 0; i < bpp; i++ { 231 cdat4[i] = cdat0[i] - pdat[i] 232 sum += abs8(cdat4[i]) 233 } 234 for i := bpp; i < n; i++ { 235 cdat4[i] = cdat0[i] - paeth(cdat0[i-bpp], pdat[i], pdat[i-bpp]) 236 sum += abs8(cdat4[i]) 237 if sum >= best { 238 break 239 } 240 } 241 if sum < best { 242 best = sum 243 filter = ftPaeth 244 } 245 246 // The none filter. 247 sum = 0 248 for i := 0; i < n; i++ { 249 sum += abs8(cdat0[i]) 250 if sum >= best { 251 break 252 } 253 } 254 if sum < best { 255 best = sum 256 filter = ftNone 257 } 258 259 // The sub filter. 260 sum = 0 261 for i := 0; i < bpp; i++ { 262 cdat1[i] = cdat0[i] 263 sum += abs8(cdat1[i]) 264 } 265 for i := bpp; i < n; i++ { 266 cdat1[i] = cdat0[i] - cdat0[i-bpp] 267 sum += abs8(cdat1[i]) 268 if sum >= best { 269 break 270 } 271 } 272 if sum < best { 273 best = sum 274 filter = ftSub 275 } 276 277 // The average filter. 278 sum = 0 279 for i := 0; i < bpp; i++ { 280 cdat3[i] = cdat0[i] - pdat[i]/2 281 sum += abs8(cdat3[i]) 282 } 283 for i := bpp; i < n; i++ { 284 cdat3[i] = cdat0[i] - uint8((int(cdat0[i-bpp])+int(pdat[i]))/2) 285 sum += abs8(cdat3[i]) 286 if sum >= best { 287 break 288 } 289 } 290 if sum < best { 291 filter = ftAverage 292 } 293 294 return filter 295 } 296 297 func zeroMemory(v []uint8) { 298 for i := range v { 299 v[i] = 0 300 } 301 } 302 303 func (e *encoder) writeImage(w io.Writer, m image.Image, cb int, level int) error { 304 if e.zw == nil || e.zwLevel != level { 305 zw, err := zlib.NewWriterLevel(w, level) 306 if err != nil { 307 return err 308 } 309 e.zw = zw 310 e.zwLevel = level 311 } else { 312 e.zw.Reset(w) 313 } 314 defer e.zw.Close() 315 316 bitsPerPixel := 0 317 318 switch cb { 319 case cbG8: 320 bitsPerPixel = 8 321 case cbTC8: 322 bitsPerPixel = 24 323 case cbP8: 324 bitsPerPixel = 8 325 case cbP4: 326 bitsPerPixel = 4 327 case cbP2: 328 bitsPerPixel = 2 329 case cbP1: 330 bitsPerPixel = 1 331 case cbTCA8: 332 bitsPerPixel = 32 333 case cbTC16: 334 bitsPerPixel = 48 335 case cbTCA16: 336 bitsPerPixel = 64 337 case cbG16: 338 bitsPerPixel = 16 339 } 340 341 // cr[*] and pr are the bytes for the current and previous row. 342 // cr[0] is unfiltered (or equivalently, filtered with the ftNone filter). 343 // cr[ft], for non-zero filter types ft, are buffers for transforming cr[0] under the 344 // other PNG filter types. These buffers are allocated once and re-used for each row. 345 // The +1 is for the per-row filter type, which is at cr[*][0]. 346 b := m.Bounds() 347 sz := 1 + (bitsPerPixel*b.Dx()+7)/8 348 for i := range e.cr { 349 if cap(e.cr[i]) < sz { 350 e.cr[i] = make([]uint8, sz) 351 } else { 352 e.cr[i] = e.cr[i][:sz] 353 } 354 e.cr[i][0] = uint8(i) 355 } 356 cr := e.cr 357 if cap(e.pr) < sz { 358 e.pr = make([]uint8, sz) 359 } else { 360 e.pr = e.pr[:sz] 361 zeroMemory(e.pr) 362 } 363 pr := e.pr 364 365 gray, _ := m.(*image.Gray) 366 rgba, _ := m.(*image.RGBA) 367 paletted, _ := m.(*image.Paletted) 368 nrgba, _ := m.(*image.NRGBA) 369 370 for y := b.Min.Y; y < b.Max.Y; y++ { 371 // Convert from colors to bytes. 372 i := 1 373 switch cb { 374 case cbG8: 375 if gray != nil { 376 offset := (y - b.Min.Y) * gray.Stride 377 copy(cr[0][1:], gray.Pix[offset:offset+b.Dx()]) 378 } else { 379 for x := b.Min.X; x < b.Max.X; x++ { 380 c := color.GrayModel.Convert(m.At(x, y)).(color.Gray) 381 cr[0][i] = c.Y 382 i++ 383 } 384 } 385 case cbTC8: 386 // We have previously verified that the alpha value is fully opaque. 387 cr0 := cr[0] 388 stride, pix := 0, []byte(nil) 389 if rgba != nil { 390 stride, pix = rgba.Stride, rgba.Pix 391 } else if nrgba != nil { 392 stride, pix = nrgba.Stride, nrgba.Pix 393 } 394 if stride != 0 { 395 j0 := (y - b.Min.Y) * stride 396 j1 := j0 + b.Dx()*4 397 for j := j0; j < j1; j += 4 { 398 cr0[i+0] = pix[j+0] 399 cr0[i+1] = pix[j+1] 400 cr0[i+2] = pix[j+2] 401 i += 3 402 } 403 } else { 404 for x := b.Min.X; x < b.Max.X; x++ { 405 r, g, b, _ := m.At(x, y).RGBA() 406 cr0[i+0] = uint8(r >> 8) 407 cr0[i+1] = uint8(g >> 8) 408 cr0[i+2] = uint8(b >> 8) 409 i += 3 410 } 411 } 412 case cbP8: 413 if paletted != nil { 414 offset := (y - b.Min.Y) * paletted.Stride 415 copy(cr[0][1:], paletted.Pix[offset:offset+b.Dx()]) 416 } else { 417 pi := m.(image.PalettedImage) 418 for x := b.Min.X; x < b.Max.X; x++ { 419 cr[0][i] = pi.ColorIndexAt(x, y) 420 i += 1 421 } 422 } 423 424 case cbP4, cbP2, cbP1: 425 pi := m.(image.PalettedImage) 426 427 var a uint8 428 var c int 429 for x := b.Min.X; x < b.Max.X; x++ { 430 a = a<<uint(bitsPerPixel) | pi.ColorIndexAt(x, y) 431 c++ 432 if c == 8/bitsPerPixel { 433 cr[0][i] = a 434 i += 1 435 a = 0 436 c = 0 437 } 438 } 439 if c != 0 { 440 for c != 8/bitsPerPixel { 441 a = a << uint(bitsPerPixel) 442 c++ 443 } 444 cr[0][i] = a 445 } 446 447 case cbTCA8: 448 if nrgba != nil { 449 offset := (y - b.Min.Y) * nrgba.Stride 450 copy(cr[0][1:], nrgba.Pix[offset:offset+b.Dx()*4]) 451 } else { 452 // Convert from image.Image (which is alpha-premultiplied) to PNG's non-alpha-premultiplied. 453 for x := b.Min.X; x < b.Max.X; x++ { 454 c := color.NRGBAModel.Convert(m.At(x, y)).(color.NRGBA) 455 cr[0][i+0] = c.R 456 cr[0][i+1] = c.G 457 cr[0][i+2] = c.B 458 cr[0][i+3] = c.A 459 i += 4 460 } 461 } 462 case cbG16: 463 for x := b.Min.X; x < b.Max.X; x++ { 464 c := color.Gray16Model.Convert(m.At(x, y)).(color.Gray16) 465 cr[0][i+0] = uint8(c.Y >> 8) 466 cr[0][i+1] = uint8(c.Y) 467 i += 2 468 } 469 case cbTC16: 470 // We have previously verified that the alpha value is fully opaque. 471 for x := b.Min.X; x < b.Max.X; x++ { 472 r, g, b, _ := m.At(x, y).RGBA() 473 cr[0][i+0] = uint8(r >> 8) 474 cr[0][i+1] = uint8(r) 475 cr[0][i+2] = uint8(g >> 8) 476 cr[0][i+3] = uint8(g) 477 cr[0][i+4] = uint8(b >> 8) 478 cr[0][i+5] = uint8(b) 479 i += 6 480 } 481 case cbTCA16: 482 // Convert from image.Image (which is alpha-premultiplied) to PNG's non-alpha-premultiplied. 483 for x := b.Min.X; x < b.Max.X; x++ { 484 c := color.NRGBA64Model.Convert(m.At(x, y)).(color.NRGBA64) 485 cr[0][i+0] = uint8(c.R >> 8) 486 cr[0][i+1] = uint8(c.R) 487 cr[0][i+2] = uint8(c.G >> 8) 488 cr[0][i+3] = uint8(c.G) 489 cr[0][i+4] = uint8(c.B >> 8) 490 cr[0][i+5] = uint8(c.B) 491 cr[0][i+6] = uint8(c.A >> 8) 492 cr[0][i+7] = uint8(c.A) 493 i += 8 494 } 495 } 496 497 // Apply the filter. 498 // Skip filter for NoCompression and paletted images (cbP8) as 499 // "filters are rarely useful on palette images" and will result 500 // in larger files (see http://www.libpng.org/pub/png/book/chapter09.html). 501 f := ftNone 502 if level != zlib.NoCompression && cb != cbP8 && cb != cbP4 && cb != cbP2 && cb != cbP1 { 503 // Since we skip paletted images we don't have to worry about 504 // bitsPerPixel not being a multiple of 8 505 bpp := bitsPerPixel / 8 506 f = filter(&cr, pr, bpp) 507 } 508 509 // Write the compressed bytes. 510 if _, err := e.zw.Write(cr[f]); err != nil { 511 return err 512 } 513 514 // The current row for y is the previous row for y+1. 515 pr, cr[0] = cr[0], pr 516 } 517 return nil 518 } 519 520 // Write the actual image data to one or more IDAT chunks. 521 func (e *encoder) writeIDATs() { 522 if e.err != nil { 523 return 524 } 525 if e.bw == nil { 526 e.bw = bufio.NewWriterSize(e, 1<<15) 527 } else { 528 e.bw.Reset(e) 529 } 530 e.err = e.writeImage(e.bw, e.m, e.cb, levelToZlib(e.enc.CompressionLevel)) 531 if e.err != nil { 532 return 533 } 534 e.err = e.bw.Flush() 535 } 536 537 // This function is required because we want the zero value of 538 // Encoder.CompressionLevel to map to zlib.DefaultCompression. 539 func levelToZlib(l CompressionLevel) int { 540 switch l { 541 case DefaultCompression: 542 return zlib.DefaultCompression 543 case NoCompression: 544 return zlib.NoCompression 545 case BestSpeed: 546 return zlib.BestSpeed 547 case BestCompression: 548 return zlib.BestCompression 549 default: 550 return zlib.DefaultCompression 551 } 552 } 553 554 func (e *encoder) writeIEND() { e.writeChunk(nil, "IEND") } 555 556 // Encode writes the Image m to w in PNG format. Any Image may be 557 // encoded, but images that are not image.NRGBA might be encoded lossily. 558 func Encode(w io.Writer, m image.Image) error { 559 var e Encoder 560 return e.Encode(w, m) 561 } 562 563 // Encode writes the Image m to w in PNG format. 564 func (enc *Encoder) Encode(w io.Writer, m image.Image) error { 565 // Obviously, negative widths and heights are invalid. Furthermore, the PNG 566 // spec section 11.2.2 says that zero is invalid. Excessively large images are 567 // also rejected. 568 mw, mh := int64(m.Bounds().Dx()), int64(m.Bounds().Dy()) 569 if mw <= 0 || mh <= 0 || mw >= 1<<32 || mh >= 1<<32 { 570 return FormatError("invalid image size: " + strconv.FormatInt(mw, 10) + "x" + strconv.FormatInt(mh, 10)) 571 } 572 573 var e *encoder 574 if enc.BufferPool != nil { 575 buffer := enc.BufferPool.Get() 576 e = (*encoder)(buffer) 577 578 } 579 if e == nil { 580 e = &encoder{} 581 } 582 if enc.BufferPool != nil { 583 defer enc.BufferPool.Put((*EncoderBuffer)(e)) 584 } 585 586 e.enc = enc 587 e.w = w 588 e.m = m 589 590 var pal color.Palette 591 // cbP8 encoding needs PalettedImage's ColorIndexAt method. 592 if _, ok := m.(image.PalettedImage); ok { 593 pal, _ = m.ColorModel().(color.Palette) 594 } 595 if pal != nil { 596 if len(pal) <= 2 { 597 e.cb = cbP1 598 } else if len(pal) <= 4 { 599 e.cb = cbP2 600 } else if len(pal) <= 16 { 601 e.cb = cbP4 602 } else { 603 e.cb = cbP8 604 } 605 } else { 606 switch m.ColorModel() { 607 case color.GrayModel: 608 e.cb = cbG8 609 case color.Gray16Model: 610 e.cb = cbG16 611 case color.RGBAModel, color.NRGBAModel, color.AlphaModel: 612 if opaque(m) { 613 e.cb = cbTC8 614 } else { 615 e.cb = cbTCA8 616 } 617 default: 618 if opaque(m) { 619 e.cb = cbTC16 620 } else { 621 e.cb = cbTCA16 622 } 623 } 624 } 625 626 _, e.err = io.WriteString(w, pngHeader) 627 e.writeIHDR() 628 if pal != nil { 629 e.writePLTEAndTRNS(pal) 630 } 631 e.writeIDATs() 632 e.writeIEND() 633 return e.err 634 } 635
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