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 jpeg implements a JPEG image decoder and encoder.
6 //
7 // JPEG is defined in ITU-T T.81: https://www.w3.org/Graphics/JPEG/itu-t81.pdf.
8 package jpeg
9 10 import (
11 "image"
12 "image/color"
13 "image/internal/imageutil"
14 "io"
15 )
16 17 // A FormatError reports that the input is not a valid JPEG.
18 type FormatError []byte
19 20 func (e FormatError) Error() string { return "invalid JPEG format: " + string(e) }
21 22 // An UnsupportedError reports that the input uses a valid but unimplemented JPEG feature.
23 type UnsupportedError []byte
24 25 func (e UnsupportedError) Error() string { return "unsupported JPEG feature: " + string(e) }
26 27 var errUnsupportedSubsamplingRatio = UnsupportedError("luma/chroma subsampling ratio")
28 29 // Component specification, specified in section B.2.2.
30 type component struct {
31 h int // Horizontal sampling factor.
32 v int // Vertical sampling factor.
33 c uint8 // Component identifier.
34 tq uint8 // Quantization table destination selector.
35 }
36 37 const (
38 dcTable = 0
39 acTable = 1
40 maxTc = 1
41 maxTh = 3
42 maxTq = 3
43 44 maxComponents = 4
45 )
46 47 const (
48 sof0Marker = 0xc0 // Start Of Frame (Baseline Sequential).
49 sof1Marker = 0xc1 // Start Of Frame (Extended Sequential).
50 sof2Marker = 0xc2 // Start Of Frame (Progressive).
51 dhtMarker = 0xc4 // Define Huffman Table.
52 rst0Marker = 0xd0 // ReSTart (0).
53 rst7Marker = 0xd7 // ReSTart (7).
54 soiMarker = 0xd8 // Start Of Image.
55 eoiMarker = 0xd9 // End Of Image.
56 sosMarker = 0xda // Start Of Scan.
57 dqtMarker = 0xdb // Define Quantization Table.
58 driMarker = 0xdd // Define Restart Interval.
59 comMarker = 0xfe // COMment.
60 // "APPlication specific" markers aren't part of the JPEG spec per se,
61 // but in practice, their use is described at
62 // https://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html
63 app0Marker = 0xe0
64 app14Marker = 0xee
65 app15Marker = 0xef
66 )
67 68 // See https://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html#Adobe
69 const (
70 adobeTransformUnknown = 0
71 adobeTransformYCbCr = 1
72 adobeTransformYCbCrK = 2
73 )
74 75 // unzig maps from the zig-zag ordering to the natural ordering. For example,
76 // unzig[3] is the column and row of the fourth element in zig-zag order. The
77 // value is 16, which means first column (16%8 == 0) and third row (16/8 == 2).
78 var unzig = [blockSize]int{
79 0, 1, 8, 16, 9, 2, 3, 10,
80 17, 24, 32, 25, 18, 11, 4, 5,
81 12, 19, 26, 33, 40, 48, 41, 34,
82 27, 20, 13, 6, 7, 14, 21, 28,
83 35, 42, 49, 56, 57, 50, 43, 36,
84 29, 22, 15, 23, 30, 37, 44, 51,
85 58, 59, 52, 45, 38, 31, 39, 46,
86 53, 60, 61, 54, 47, 55, 62, 63,
87 }
88 89 // Deprecated: Reader is not used by the [image/jpeg] package and should
90 // not be used by others. It is kept for compatibility.
91 type Reader interface {
92 io.ByteReader
93 io.Reader
94 }
95 96 // bits holds the unprocessed bits that have been taken from the byte-stream.
97 // The n least significant bits of a form the unread bits, to be read in MSB to
98 // LSB order.
99 type bits struct {
100 a uint32 // accumulator.
101 m uint32 // mask. m==1<<(n-1) when n>0, with m==0 when n==0.
102 n int32 // the number of unread bits in a.
103 }
104 105 type decoder struct {
106 r io.Reader
107 bits bits
108 // bytes is a byte buffer, similar to a bufio.Reader, except that it
109 // has to be able to unread more than 1 byte, due to byte stuffing.
110 // Byte stuffing is specified in section F.1.2.3.
111 bytes struct {
112 // buf[i:j] are the buffered bytes read from the underlying
113 // io.Reader that haven't yet been passed further on.
114 buf [4096]byte
115 i, j int
116 // nUnreadable is the number of bytes to back up i after
117 // overshooting. It can be 0, 1 or 2.
118 nUnreadable int
119 }
120 width, height int
121 122 img1 *image.Gray
123 img3 *image.YCbCr
124 blackPix []byte
125 blackStride int
126 127 ri int // Restart Interval.
128 nComp int
129 130 // As per section 4.5, there are four modes of operation (selected by the
131 // SOF? markers): sequential DCT, progressive DCT, lossless and
132 // hierarchical, although this implementation does not support the latter
133 // two non-DCT modes. Sequential DCT is further split into baseline and
134 // extended, as per section 4.11.
135 baseline bool
136 progressive bool
137 138 jfif bool
139 adobeTransformValid bool
140 adobeTransform uint8
141 eobRun uint16 // End-of-Band run, specified in section G.1.2.2.
142 143 comp [maxComponents]component
144 progCoeffs [maxComponents][]block // Saved state between progressive-mode scans.
145 huff [maxTc + 1][maxTh + 1]huffman
146 quant [maxTq + 1]block // Quantization tables, in zig-zag order.
147 tmp [2 * blockSize]byte
148 }
149 150 // fill fills up the d.bytes.buf buffer from the underlying io.Reader. It
151 // should only be called when there are no unread bytes in d.bytes.
152 func (d *decoder) fill() error {
153 if d.bytes.i != d.bytes.j {
154 panic("jpeg: fill called when unread bytes exist")
155 }
156 // Move the last 2 bytes to the start of the buffer, in case we need
157 // to call unreadByteStuffedByte.
158 if d.bytes.j > 2 {
159 d.bytes.buf[0] = d.bytes.buf[d.bytes.j-2]
160 d.bytes.buf[1] = d.bytes.buf[d.bytes.j-1]
161 d.bytes.i, d.bytes.j = 2, 2
162 }
163 // Fill in the rest of the buffer.
164 n, err := d.r.Read(d.bytes.buf[d.bytes.j:])
165 d.bytes.j += n
166 if n > 0 {
167 return nil
168 }
169 if err == io.EOF {
170 err = io.ErrUnexpectedEOF
171 }
172 return err
173 }
174 175 // unreadByteStuffedByte undoes the most recent readByteStuffedByte call,
176 // giving a byte of data back from d.bits to d.bytes. The Huffman look-up table
177 // requires at least 8 bits for look-up, which means that Huffman decoding can
178 // sometimes overshoot and read one or two too many bytes. Two-byte overshoot
179 // can happen when expecting to read a 0xff 0x00 byte-stuffed byte.
180 func (d *decoder) unreadByteStuffedByte() {
181 d.bytes.i -= d.bytes.nUnreadable
182 d.bytes.nUnreadable = 0
183 if d.bits.n >= 8 {
184 d.bits.a >>= 8
185 d.bits.n -= 8
186 d.bits.m >>= 8
187 }
188 }
189 190 // readByte returns the next byte, whether buffered or not buffered. It does
191 // not care about byte stuffing.
192 func (d *decoder) readByte() (x byte, err error) {
193 for d.bytes.i == d.bytes.j {
194 if err = d.fill(); err != nil {
195 return 0, err
196 }
197 }
198 x = d.bytes.buf[d.bytes.i]
199 d.bytes.i++
200 d.bytes.nUnreadable = 0
201 return x, nil
202 }
203 204 // errMissingFF00 means that readByteStuffedByte encountered an 0xff byte (a
205 // marker byte) that wasn't the expected byte-stuffed sequence 0xff, 0x00.
206 var errMissingFF00 = FormatError("missing 0xff00 sequence")
207 208 // readByteStuffedByte is like readByte but is for byte-stuffed Huffman data.
209 func (d *decoder) readByteStuffedByte() (x byte, err error) {
210 // Take the fast path if d.bytes.buf contains at least two bytes.
211 if d.bytes.i+2 <= d.bytes.j {
212 x = d.bytes.buf[d.bytes.i]
213 d.bytes.i++
214 d.bytes.nUnreadable = 1
215 if x != 0xff {
216 return x, err
217 }
218 if d.bytes.buf[d.bytes.i] != 0x00 {
219 return 0, errMissingFF00
220 }
221 d.bytes.i++
222 d.bytes.nUnreadable = 2
223 return 0xff, nil
224 }
225 226 d.bytes.nUnreadable = 0
227 228 x, err = d.readByte()
229 if err != nil {
230 return 0, err
231 }
232 d.bytes.nUnreadable = 1
233 if x != 0xff {
234 return x, nil
235 }
236 237 x, err = d.readByte()
238 if err != nil {
239 return 0, err
240 }
241 d.bytes.nUnreadable = 2
242 if x != 0x00 {
243 return 0, errMissingFF00
244 }
245 return 0xff, nil
246 }
247 248 // readFull reads exactly len(p) bytes into p. It does not care about byte
249 // stuffing.
250 func (d *decoder) readFull(p []byte) error {
251 // Unread the overshot bytes, if any.
252 if d.bytes.nUnreadable != 0 {
253 if d.bits.n >= 8 {
254 d.unreadByteStuffedByte()
255 }
256 d.bytes.nUnreadable = 0
257 }
258 259 for {
260 n := copy(p, d.bytes.buf[d.bytes.i:d.bytes.j])
261 p = p[n:]
262 d.bytes.i += n
263 if len(p) == 0 {
264 break
265 }
266 if err := d.fill(); err != nil {
267 return err
268 }
269 }
270 return nil
271 }
272 273 // ignore ignores the next n bytes.
274 func (d *decoder) ignore(n int) error {
275 // Unread the overshot bytes, if any.
276 if d.bytes.nUnreadable != 0 {
277 if d.bits.n >= 8 {
278 d.unreadByteStuffedByte()
279 }
280 d.bytes.nUnreadable = 0
281 }
282 283 for {
284 m := d.bytes.j - d.bytes.i
285 if m > n {
286 m = n
287 }
288 d.bytes.i += m
289 n -= m
290 if n == 0 {
291 break
292 }
293 if err := d.fill(); err != nil {
294 return err
295 }
296 }
297 return nil
298 }
299 300 // Specified in section B.2.2.
301 func (d *decoder) processSOF(n int) error {
302 if d.nComp != 0 {
303 return FormatError("multiple SOF markers")
304 }
305 switch n {
306 case 6 + 3*1: // Grayscale image.
307 d.nComp = 1
308 case 6 + 3*3: // YCbCr or RGB image.
309 d.nComp = 3
310 case 6 + 3*4: // YCbCrK or CMYK image.
311 d.nComp = 4
312 default:
313 return UnsupportedError("number of components")
314 }
315 if err := d.readFull(d.tmp[:n]); err != nil {
316 return err
317 }
318 // We only support 8-bit precision.
319 if d.tmp[0] != 8 {
320 return UnsupportedError("precision")
321 }
322 d.height = int(d.tmp[1])<<8 + int(d.tmp[2])
323 d.width = int(d.tmp[3])<<8 + int(d.tmp[4])
324 if int(d.tmp[5]) != d.nComp {
325 return FormatError("SOF has wrong length")
326 }
327 328 for i := 0; i < d.nComp; i++ {
329 d.comp[i].c = d.tmp[6+3*i]
330 // Section B.2.2 states that "the value of C_i shall be different from
331 // the values of C_1 through C_(i-1)".
332 for j := 0; j < i; j++ {
333 if d.comp[i].c == d.comp[j].c {
334 return FormatError("repeated component identifier")
335 }
336 }
337 338 d.comp[i].tq = d.tmp[8+3*i]
339 if d.comp[i].tq > maxTq {
340 return FormatError("bad Tq value")
341 }
342 343 hv := d.tmp[7+3*i]
344 h, v := int(hv>>4), int(hv&0x0f)
345 if h < 1 || 4 < h || v < 1 || 4 < v {
346 return FormatError("luma/chroma subsampling ratio")
347 }
348 if h == 3 || v == 3 {
349 return errUnsupportedSubsamplingRatio
350 }
351 switch d.nComp {
352 case 1:
353 // If a JPEG image has only one component, section A.2 says "this data
354 // is non-interleaved by definition" and section A.2.2 says "[in this
355 // case...] the order of data units within a scan shall be left-to-right
356 // and top-to-bottom... regardless of the values of H_1 and V_1". Section
357 // 4.8.2 also says "[for non-interleaved data], the MCU is defined to be
358 // one data unit". Similarly, section A.1.1 explains that it is the ratio
359 // of H_i to max_j(H_j) that matters, and similarly for V. For grayscale
360 // images, H_1 is the maximum H_j for all components j, so that ratio is
361 // always 1. The component's (h, v) is effectively always (1, 1): even if
362 // the nominal (h, v) is (2, 1), a 20x5 image is encoded in three 8x8
363 // MCUs, not two 16x8 MCUs.
364 h, v = 1, 1
365 366 case 3:
367 // For YCbCr images, we only support 4:4:4, 4:4:0, 4:2:2, 4:2:0,
368 // 4:1:1 or 4:1:0 chroma subsampling ratios. This implies that the
369 // (h, v) values for the Y component are either (1, 1), (1, 2),
370 // (2, 1), (2, 2), (4, 1) or (4, 2), and the Y component's values
371 // must be a multiple of the Cb and Cr component's values. We also
372 // assume that the two chroma components have the same subsampling
373 // ratio.
374 switch i {
375 case 0: // Y.
376 // We have already verified, above, that h and v are both
377 // either 1, 2 or 4, so invalid (h, v) combinations are those
378 // with v == 4.
379 if v == 4 {
380 return errUnsupportedSubsamplingRatio
381 }
382 case 1: // Cb.
383 if d.comp[0].h%h != 0 || d.comp[0].v%v != 0 {
384 return errUnsupportedSubsamplingRatio
385 }
386 case 2: // Cr.
387 if d.comp[1].h != h || d.comp[1].v != v {
388 return errUnsupportedSubsamplingRatio
389 }
390 }
391 392 case 4:
393 // For 4-component images (either CMYK or YCbCrK), we only support two
394 // hv vectors: [0x11 0x11 0x11 0x11] and [0x22 0x11 0x11 0x22].
395 // Theoretically, 4-component JPEG images could mix and match hv values
396 // but in practice, those two combinations are the only ones in use,
397 // and it simplifies the applyBlack code below if we can assume that:
398 // - for CMYK, the C and K channels have full samples, and if the M
399 // and Y channels subsample, they subsample both horizontally and
400 // vertically.
401 // - for YCbCrK, the Y and K channels have full samples.
402 switch i {
403 case 0:
404 if hv != 0x11 && hv != 0x22 {
405 return errUnsupportedSubsamplingRatio
406 }
407 case 1, 2:
408 if hv != 0x11 {
409 return errUnsupportedSubsamplingRatio
410 }
411 case 3:
412 if d.comp[0].h != h || d.comp[0].v != v {
413 return errUnsupportedSubsamplingRatio
414 }
415 }
416 }
417 418 d.comp[i].h = h
419 d.comp[i].v = v
420 }
421 return nil
422 }
423 424 // Specified in section B.2.4.1.
425 func (d *decoder) processDQT(n int) error {
426 loop:
427 for n > 0 {
428 n--
429 x, err := d.readByte()
430 if err != nil {
431 return err
432 }
433 tq := x & 0x0f
434 if tq > maxTq {
435 return FormatError("bad Tq value")
436 }
437 switch x >> 4 {
438 default:
439 return FormatError("bad Pq value")
440 case 0:
441 if n < blockSize {
442 break loop
443 }
444 n -= blockSize
445 if err := d.readFull(d.tmp[:blockSize]); err != nil {
446 return err
447 }
448 for i := range d.quant[tq] {
449 d.quant[tq][i] = int32(d.tmp[i])
450 }
451 case 1:
452 if n < 2*blockSize {
453 break loop
454 }
455 n -= 2 * blockSize
456 if err := d.readFull(d.tmp[:2*blockSize]); err != nil {
457 return err
458 }
459 for i := range d.quant[tq] {
460 d.quant[tq][i] = int32(d.tmp[2*i])<<8 | int32(d.tmp[2*i+1])
461 }
462 }
463 }
464 if n != 0 {
465 return FormatError("DQT has wrong length")
466 }
467 return nil
468 }
469 470 // Specified in section B.2.4.4.
471 func (d *decoder) processDRI(n int) error {
472 if n != 2 {
473 return FormatError("DRI has wrong length")
474 }
475 if err := d.readFull(d.tmp[:2]); err != nil {
476 return err
477 }
478 d.ri = int(d.tmp[0])<<8 + int(d.tmp[1])
479 return nil
480 }
481 482 func (d *decoder) processApp0Marker(n int) error {
483 if n < 5 {
484 return d.ignore(n)
485 }
486 if err := d.readFull(d.tmp[:5]); err != nil {
487 return err
488 }
489 n -= 5
490 491 d.jfif = d.tmp[0] == 'J' && d.tmp[1] == 'F' && d.tmp[2] == 'I' && d.tmp[3] == 'F' && d.tmp[4] == '\x00'
492 493 if n > 0 {
494 return d.ignore(n)
495 }
496 return nil
497 }
498 499 func (d *decoder) processApp14Marker(n int) error {
500 if n < 12 {
501 return d.ignore(n)
502 }
503 if err := d.readFull(d.tmp[:12]); err != nil {
504 return err
505 }
506 n -= 12
507 508 if d.tmp[0] == 'A' && d.tmp[1] == 'd' && d.tmp[2] == 'o' && d.tmp[3] == 'b' && d.tmp[4] == 'e' {
509 d.adobeTransformValid = true
510 d.adobeTransform = d.tmp[11]
511 }
512 513 if n > 0 {
514 return d.ignore(n)
515 }
516 return nil
517 }
518 519 // decode reads a JPEG image from r and returns it as an image.Image.
520 func (d *decoder) decode(r io.Reader, configOnly bool) (image.Image, error) {
521 d.r = r
522 523 // Check for the Start Of Image marker.
524 if err := d.readFull(d.tmp[:2]); err != nil {
525 return nil, err
526 }
527 if d.tmp[0] != 0xff || d.tmp[1] != soiMarker {
528 return nil, FormatError("missing SOI marker")
529 }
530 531 // Process the remaining segments until the End Of Image marker.
532 for {
533 err := d.readFull(d.tmp[:2])
534 if err != nil {
535 return nil, err
536 }
537 for d.tmp[0] != 0xff {
538 // Strictly speaking, this is a format error. However, libjpeg is
539 // liberal in what it accepts. As of version 9, next_marker in
540 // jdmarker.c treats this as a warning (JWRN_EXTRANEOUS_DATA) and
541 // continues to decode the stream. Even before next_marker sees
542 // extraneous data, jpeg_fill_bit_buffer in jdhuff.c reads as many
543 // bytes as it can, possibly past the end of a scan's data. It
544 // effectively puts back any markers that it overscanned (e.g. an
545 // "\xff\xd9" EOI marker), but it does not put back non-marker data,
546 // and thus it can silently ignore a small number of extraneous
547 // non-marker bytes before next_marker has a chance to see them (and
548 // print a warning).
549 //
550 // We are therefore also liberal in what we accept. Extraneous data
551 // is silently ignored.
552 //
553 // This is similar to, but not exactly the same as, the restart
554 // mechanism within a scan (the RST[0-7] markers).
555 //
556 // Note that extraneous 0xff bytes in e.g. SOS data are escaped as
557 // "\xff\x00", and so are detected a little further down below.
558 d.tmp[0] = d.tmp[1]
559 d.tmp[1], err = d.readByte()
560 if err != nil {
561 return nil, err
562 }
563 }
564 marker := d.tmp[1]
565 if marker == 0 {
566 // Treat "\xff\x00" as extraneous data.
567 continue
568 }
569 for marker == 0xff {
570 // Section B.1.1.2 says, "Any marker may optionally be preceded by any
571 // number of fill bytes, which are bytes assigned code X'FF'".
572 marker, err = d.readByte()
573 if err != nil {
574 return nil, err
575 }
576 }
577 if marker == eoiMarker { // End Of Image.
578 break
579 }
580 if rst0Marker <= marker && marker <= rst7Marker {
581 // Figures B.2 and B.16 of the specification suggest that restart markers should
582 // only occur between Entropy Coded Segments and not after the final ECS.
583 // However, some encoders may generate incorrect JPEGs with a final restart
584 // marker. That restart marker will be seen here instead of inside the processSOS
585 // method, and is ignored as a harmless error. Restart markers have no extra data,
586 // so we check for this before we read the 16-bit length of the segment.
587 continue
588 }
589 590 // Read the 16-bit length of the segment. The value includes the 2 bytes for the
591 // length itself, so we subtract 2 to get the number of remaining bytes.
592 if err = d.readFull(d.tmp[:2]); err != nil {
593 return nil, err
594 }
595 n := int(d.tmp[0])<<8 + int(d.tmp[1]) - 2
596 if n < 0 {
597 return nil, FormatError("short segment length")
598 }
599 600 switch marker {
601 case sof0Marker, sof1Marker, sof2Marker:
602 d.baseline = marker == sof0Marker
603 d.progressive = marker == sof2Marker
604 err = d.processSOF(n)
605 if configOnly && d.jfif {
606 return nil, err
607 }
608 case dhtMarker:
609 if configOnly {
610 err = d.ignore(n)
611 } else {
612 err = d.processDHT(n)
613 }
614 case dqtMarker:
615 if configOnly {
616 err = d.ignore(n)
617 } else {
618 err = d.processDQT(n)
619 }
620 case sosMarker:
621 if configOnly {
622 return nil, nil
623 }
624 err = d.processSOS(n)
625 case driMarker:
626 if configOnly {
627 err = d.ignore(n)
628 } else {
629 err = d.processDRI(n)
630 }
631 case app0Marker:
632 err = d.processApp0Marker(n)
633 case app14Marker:
634 err = d.processApp14Marker(n)
635 default:
636 if app0Marker <= marker && marker <= app15Marker || marker == comMarker {
637 err = d.ignore(n)
638 } else if marker < 0xc0 { // See Table B.1 "Marker code assignments".
639 err = FormatError("unknown marker")
640 } else {
641 err = UnsupportedError("unknown marker")
642 }
643 }
644 if err != nil {
645 return nil, err
646 }
647 }
648 649 if d.progressive {
650 if err := d.reconstructProgressiveImage(); err != nil {
651 return nil, err
652 }
653 }
654 if d.img1 != nil {
655 return d.img1, nil
656 }
657 if d.img3 != nil {
658 if d.blackPix != nil {
659 return d.applyBlack()
660 } else if d.isRGB() {
661 return d.convertToRGB()
662 }
663 return d.img3, nil
664 }
665 return nil, FormatError("missing SOS marker")
666 }
667 668 // applyBlack combines d.img3 and d.blackPix into a CMYK image. The formula
669 // used depends on whether the JPEG image is stored as CMYK or YCbCrK,
670 // indicated by the APP14 (Adobe) metadata.
671 //
672 // Adobe CMYK JPEG images are inverted, where 255 means no ink instead of full
673 // ink, so we apply "v = 255 - v" at various points. Note that a double
674 // inversion is a no-op, so inversions might be implicit in the code below.
675 func (d *decoder) applyBlack() (image.Image, error) {
676 if !d.adobeTransformValid {
677 return nil, UnsupportedError("unknown color model: 4-component JPEG doesn't have Adobe APP14 metadata")
678 }
679 680 // If the 4-component JPEG image isn't explicitly marked as "Unknown (RGB
681 // or CMYK)" as per
682 // https://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html#Adobe
683 // we assume that it is YCbCrK. This matches libjpeg's jdapimin.c.
684 if d.adobeTransform != adobeTransformUnknown {
685 // Convert the YCbCr part of the YCbCrK to RGB, invert the RGB to get
686 // CMY, and patch in the original K. The RGB to CMY inversion cancels
687 // out the 'Adobe inversion' described in the applyBlack doc comment
688 // above, so in practice, only the fourth channel (black) is inverted.
689 bounds := d.img3.Bounds()
690 img := image.NewRGBA(bounds)
691 imageutil.DrawYCbCr(img, bounds, d.img3, bounds.Min)
692 for iBase, y := 0, bounds.Min.Y; y < bounds.Max.Y; iBase, y = iBase+img.Stride, y+1 {
693 for i, x := iBase+3, bounds.Min.X; x < bounds.Max.X; i, x = i+4, x+1 {
694 img.Pix[i] = 255 - d.blackPix[(y-bounds.Min.Y)*d.blackStride+(x-bounds.Min.X)]
695 }
696 }
697 return &image.CMYK{
698 Pix: img.Pix,
699 Stride: img.Stride,
700 Rect: img.Rect,
701 }, nil
702 }
703 704 // The first three channels (cyan, magenta, yellow) of the CMYK
705 // were decoded into d.img3, but each channel was decoded into a separate
706 // []byte slice, and some channels may be subsampled. We interleave the
707 // separate channels into an image.CMYK's single []byte slice containing 4
708 // contiguous bytes per pixel.
709 bounds := d.img3.Bounds()
710 img := image.NewCMYK(bounds)
711 712 translations := [4]struct {
713 src []byte
714 stride int
715 }{
716 {d.img3.Y, d.img3.YStride},
717 {d.img3.Cb, d.img3.CStride},
718 {d.img3.Cr, d.img3.CStride},
719 {d.blackPix, d.blackStride},
720 }
721 for t, translation := range translations {
722 subsample := d.comp[t].h != d.comp[0].h || d.comp[t].v != d.comp[0].v
723 for iBase, y := 0, bounds.Min.Y; y < bounds.Max.Y; iBase, y = iBase+img.Stride, y+1 {
724 sy := y - bounds.Min.Y
725 if subsample {
726 sy /= 2
727 }
728 for i, x := iBase+t, bounds.Min.X; x < bounds.Max.X; i, x = i+4, x+1 {
729 sx := x - bounds.Min.X
730 if subsample {
731 sx /= 2
732 }
733 img.Pix[i] = 255 - translation.src[sy*translation.stride+sx]
734 }
735 }
736 }
737 return img, nil
738 }
739 740 func (d *decoder) isRGB() bool {
741 if d.jfif {
742 return false
743 }
744 if d.adobeTransformValid && d.adobeTransform == adobeTransformUnknown {
745 // https://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html#Adobe
746 // says that 0 means Unknown (and in practice RGB) and 1 means YCbCr.
747 return true
748 }
749 return d.comp[0].c == 'R' && d.comp[1].c == 'G' && d.comp[2].c == 'B'
750 }
751 752 func (d *decoder) convertToRGB() (image.Image, error) {
753 cScale := d.comp[0].h / d.comp[1].h
754 bounds := d.img3.Bounds()
755 img := image.NewRGBA(bounds)
756 for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
757 po := img.PixOffset(bounds.Min.X, y)
758 yo := d.img3.YOffset(bounds.Min.X, y)
759 co := d.img3.COffset(bounds.Min.X, y)
760 for i, iMax := 0, bounds.Max.X-bounds.Min.X; i < iMax; i++ {
761 img.Pix[po+4*i+0] = d.img3.Y[yo+i]
762 img.Pix[po+4*i+1] = d.img3.Cb[co+i/cScale]
763 img.Pix[po+4*i+2] = d.img3.Cr[co+i/cScale]
764 img.Pix[po+4*i+3] = 255
765 }
766 }
767 return img, nil
768 }
769 770 // Decode reads a JPEG image from r and returns it as an [image.Image].
771 func Decode(r io.Reader) (image.Image, error) {
772 var d decoder
773 return d.decode(r, false)
774 }
775 776 // DecodeConfig returns the color model and dimensions of a JPEG image without
777 // decoding the entire image.
778 func DecodeConfig(r io.Reader) (image.Config, error) {
779 var d decoder
780 if _, err := d.decode(r, true); err != nil {
781 return image.Config{}, err
782 }
783 switch d.nComp {
784 case 1:
785 return image.Config{
786 ColorModel: color.GrayModel,
787 Width: d.width,
788 Height: d.height,
789 }, nil
790 case 3:
791 cm := color.YCbCrModel
792 if d.isRGB() {
793 cm = color.RGBAModel
794 }
795 return image.Config{
796 ColorModel: cm,
797 Width: d.width,
798 Height: d.height,
799 }, nil
800 case 4:
801 return image.Config{
802 ColorModel: color.CMYKModel,
803 Width: d.width,
804 Height: d.height,
805 }, nil
806 }
807 return image.Config{}, FormatError("missing SOF marker")
808 }
809 810 func init() {
811 image.RegisterFormat("jpeg", "\xff\xd8", Decode, DecodeConfig)
812 }
813