Source file src/image/jpeg/reader.go

Documentation: image/jpeg

		 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 string
		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 string
		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  		err = nil
	 168  	}
	 169  	return err
	 170  }
	 171  
	 172  // unreadByteStuffedByte undoes the most recent readByteStuffedByte call,
	 173  // giving a byte of data back from d.bits to d.bytes. The Huffman look-up table
	 174  // requires at least 8 bits for look-up, which means that Huffman decoding can
	 175  // sometimes overshoot and read one or two too many bytes. Two-byte overshoot
	 176  // can happen when expecting to read a 0xff 0x00 byte-stuffed byte.
	 177  func (d *decoder) unreadByteStuffedByte() {
	 178  	d.bytes.i -= d.bytes.nUnreadable
	 179  	d.bytes.nUnreadable = 0
	 180  	if d.bits.n >= 8 {
	 181  		d.bits.a >>= 8
	 182  		d.bits.n -= 8
	 183  		d.bits.m >>= 8
	 184  	}
	 185  }
	 186  
	 187  // readByte returns the next byte, whether buffered or not buffered. It does
	 188  // not care about byte stuffing.
	 189  func (d *decoder) readByte() (x byte, err error) {
	 190  	for d.bytes.i == d.bytes.j {
	 191  		if err = d.fill(); err != nil {
	 192  			return 0, err
	 193  		}
	 194  	}
	 195  	x = d.bytes.buf[d.bytes.i]
	 196  	d.bytes.i++
	 197  	d.bytes.nUnreadable = 0
	 198  	return x, nil
	 199  }
	 200  
	 201  // errMissingFF00 means that readByteStuffedByte encountered an 0xff byte (a
	 202  // marker byte) that wasn't the expected byte-stuffed sequence 0xff, 0x00.
	 203  var errMissingFF00 = FormatError("missing 0xff00 sequence")
	 204  
	 205  // readByteStuffedByte is like readByte but is for byte-stuffed Huffman data.
	 206  func (d *decoder) readByteStuffedByte() (x byte, err error) {
	 207  	// Take the fast path if d.bytes.buf contains at least two bytes.
	 208  	if d.bytes.i+2 <= d.bytes.j {
	 209  		x = d.bytes.buf[d.bytes.i]
	 210  		d.bytes.i++
	 211  		d.bytes.nUnreadable = 1
	 212  		if x != 0xff {
	 213  			return x, err
	 214  		}
	 215  		if d.bytes.buf[d.bytes.i] != 0x00 {
	 216  			return 0, errMissingFF00
	 217  		}
	 218  		d.bytes.i++
	 219  		d.bytes.nUnreadable = 2
	 220  		return 0xff, nil
	 221  	}
	 222  
	 223  	d.bytes.nUnreadable = 0
	 224  
	 225  	x, err = d.readByte()
	 226  	if err != nil {
	 227  		return 0, err
	 228  	}
	 229  	d.bytes.nUnreadable = 1
	 230  	if x != 0xff {
	 231  		return x, nil
	 232  	}
	 233  
	 234  	x, err = d.readByte()
	 235  	if err != nil {
	 236  		return 0, err
	 237  	}
	 238  	d.bytes.nUnreadable = 2
	 239  	if x != 0x00 {
	 240  		return 0, errMissingFF00
	 241  	}
	 242  	return 0xff, nil
	 243  }
	 244  
	 245  // readFull reads exactly len(p) bytes into p. It does not care about byte
	 246  // stuffing.
	 247  func (d *decoder) readFull(p []byte) error {
	 248  	// Unread the overshot bytes, if any.
	 249  	if d.bytes.nUnreadable != 0 {
	 250  		if d.bits.n >= 8 {
	 251  			d.unreadByteStuffedByte()
	 252  		}
	 253  		d.bytes.nUnreadable = 0
	 254  	}
	 255  
	 256  	for {
	 257  		n := copy(p, d.bytes.buf[d.bytes.i:d.bytes.j])
	 258  		p = p[n:]
	 259  		d.bytes.i += n
	 260  		if len(p) == 0 {
	 261  			break
	 262  		}
	 263  		if err := d.fill(); err != nil {
	 264  			if err == io.EOF {
	 265  				err = io.ErrUnexpectedEOF
	 266  			}
	 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  			if err == io.EOF {
	 295  				err = io.ErrUnexpectedEOF
	 296  			}
	 297  			return err
	 298  		}
	 299  	}
	 300  	return nil
	 301  }
	 302  
	 303  // Specified in section B.2.2.
	 304  func (d *decoder) processSOF(n int) error {
	 305  	if d.nComp != 0 {
	 306  		return FormatError("multiple SOF markers")
	 307  	}
	 308  	switch n {
	 309  	case 6 + 3*1: // Grayscale image.
	 310  		d.nComp = 1
	 311  	case 6 + 3*3: // YCbCr or RGB image.
	 312  		d.nComp = 3
	 313  	case 6 + 3*4: // YCbCrK or CMYK image.
	 314  		d.nComp = 4
	 315  	default:
	 316  		return UnsupportedError("number of components")
	 317  	}
	 318  	if err := d.readFull(d.tmp[:n]); err != nil {
	 319  		return err
	 320  	}
	 321  	// We only support 8-bit precision.
	 322  	if d.tmp[0] != 8 {
	 323  		return UnsupportedError("precision")
	 324  	}
	 325  	d.height = int(d.tmp[1])<<8 + int(d.tmp[2])
	 326  	d.width = int(d.tmp[3])<<8 + int(d.tmp[4])
	 327  	if int(d.tmp[5]) != d.nComp {
	 328  		return FormatError("SOF has wrong length")
	 329  	}
	 330  
	 331  	for i := 0; i < d.nComp; i++ {
	 332  		d.comp[i].c = d.tmp[6+3*i]
	 333  		// Section B.2.2 states that "the value of C_i shall be different from
	 334  		// the values of C_1 through C_(i-1)".
	 335  		for j := 0; j < i; j++ {
	 336  			if d.comp[i].c == d.comp[j].c {
	 337  				return FormatError("repeated component identifier")
	 338  			}
	 339  		}
	 340  
	 341  		d.comp[i].tq = d.tmp[8+3*i]
	 342  		if d.comp[i].tq > maxTq {
	 343  			return FormatError("bad Tq value")
	 344  		}
	 345  
	 346  		hv := d.tmp[7+3*i]
	 347  		h, v := int(hv>>4), int(hv&0x0f)
	 348  		if h < 1 || 4 < h || v < 1 || 4 < v {
	 349  			return FormatError("luma/chroma subsampling ratio")
	 350  		}
	 351  		if h == 3 || v == 3 {
	 352  			return errUnsupportedSubsamplingRatio
	 353  		}
	 354  		switch d.nComp {
	 355  		case 1:
	 356  			// If a JPEG image has only one component, section A.2 says "this data
	 357  			// is non-interleaved by definition" and section A.2.2 says "[in this
	 358  			// case...] the order of data units within a scan shall be left-to-right
	 359  			// and top-to-bottom... regardless of the values of H_1 and V_1". Section
	 360  			// 4.8.2 also says "[for non-interleaved data], the MCU is defined to be
	 361  			// one data unit". Similarly, section A.1.1 explains that it is the ratio
	 362  			// of H_i to max_j(H_j) that matters, and similarly for V. For grayscale
	 363  			// images, H_1 is the maximum H_j for all components j, so that ratio is
	 364  			// always 1. The component's (h, v) is effectively always (1, 1): even if
	 365  			// the nominal (h, v) is (2, 1), a 20x5 image is encoded in three 8x8
	 366  			// MCUs, not two 16x8 MCUs.
	 367  			h, v = 1, 1
	 368  
	 369  		case 3:
	 370  			// For YCbCr images, we only support 4:4:4, 4:4:0, 4:2:2, 4:2:0,
	 371  			// 4:1:1 or 4:1:0 chroma subsampling ratios. This implies that the
	 372  			// (h, v) values for the Y component are either (1, 1), (1, 2),
	 373  			// (2, 1), (2, 2), (4, 1) or (4, 2), and the Y component's values
	 374  			// must be a multiple of the Cb and Cr component's values. We also
	 375  			// assume that the two chroma components have the same subsampling
	 376  			// ratio.
	 377  			switch i {
	 378  			case 0: // Y.
	 379  				// We have already verified, above, that h and v are both
	 380  				// either 1, 2 or 4, so invalid (h, v) combinations are those
	 381  				// with v == 4.
	 382  				if v == 4 {
	 383  					return errUnsupportedSubsamplingRatio
	 384  				}
	 385  			case 1: // Cb.
	 386  				if d.comp[0].h%h != 0 || d.comp[0].v%v != 0 {
	 387  					return errUnsupportedSubsamplingRatio
	 388  				}
	 389  			case 2: // Cr.
	 390  				if d.comp[1].h != h || d.comp[1].v != v {
	 391  					return errUnsupportedSubsamplingRatio
	 392  				}
	 393  			}
	 394  
	 395  		case 4:
	 396  			// For 4-component images (either CMYK or YCbCrK), we only support two
	 397  			// hv vectors: [0x11 0x11 0x11 0x11] and [0x22 0x11 0x11 0x22].
	 398  			// Theoretically, 4-component JPEG images could mix and match hv values
	 399  			// but in practice, those two combinations are the only ones in use,
	 400  			// and it simplifies the applyBlack code below if we can assume that:
	 401  			//	- for CMYK, the C and K channels have full samples, and if the M
	 402  			//		and Y channels subsample, they subsample both horizontally and
	 403  			//		vertically.
	 404  			//	- for YCbCrK, the Y and K channels have full samples.
	 405  			switch i {
	 406  			case 0:
	 407  				if hv != 0x11 && hv != 0x22 {
	 408  					return errUnsupportedSubsamplingRatio
	 409  				}
	 410  			case 1, 2:
	 411  				if hv != 0x11 {
	 412  					return errUnsupportedSubsamplingRatio
	 413  				}
	 414  			case 3:
	 415  				if d.comp[0].h != h || d.comp[0].v != v {
	 416  					return errUnsupportedSubsamplingRatio
	 417  				}
	 418  			}
	 419  		}
	 420  
	 421  		d.comp[i].h = h
	 422  		d.comp[i].v = v
	 423  	}
	 424  	return nil
	 425  }
	 426  
	 427  // Specified in section B.2.4.1.
	 428  func (d *decoder) processDQT(n int) error {
	 429  loop:
	 430  	for n > 0 {
	 431  		n--
	 432  		x, err := d.readByte()
	 433  		if err != nil {
	 434  			return err
	 435  		}
	 436  		tq := x & 0x0f
	 437  		if tq > maxTq {
	 438  			return FormatError("bad Tq value")
	 439  		}
	 440  		switch x >> 4 {
	 441  		default:
	 442  			return FormatError("bad Pq value")
	 443  		case 0:
	 444  			if n < blockSize {
	 445  				break loop
	 446  			}
	 447  			n -= blockSize
	 448  			if err := d.readFull(d.tmp[:blockSize]); err != nil {
	 449  				return err
	 450  			}
	 451  			for i := range d.quant[tq] {
	 452  				d.quant[tq][i] = int32(d.tmp[i])
	 453  			}
	 454  		case 1:
	 455  			if n < 2*blockSize {
	 456  				break loop
	 457  			}
	 458  			n -= 2 * blockSize
	 459  			if err := d.readFull(d.tmp[:2*blockSize]); err != nil {
	 460  				return err
	 461  			}
	 462  			for i := range d.quant[tq] {
	 463  				d.quant[tq][i] = int32(d.tmp[2*i])<<8 | int32(d.tmp[2*i+1])
	 464  			}
	 465  		}
	 466  	}
	 467  	if n != 0 {
	 468  		return FormatError("DQT has wrong length")
	 469  	}
	 470  	return nil
	 471  }
	 472  
	 473  // Specified in section B.2.4.4.
	 474  func (d *decoder) processDRI(n int) error {
	 475  	if n != 2 {
	 476  		return FormatError("DRI has wrong length")
	 477  	}
	 478  	if err := d.readFull(d.tmp[:2]); err != nil {
	 479  		return err
	 480  	}
	 481  	d.ri = int(d.tmp[0])<<8 + int(d.tmp[1])
	 482  	return nil
	 483  }
	 484  
	 485  func (d *decoder) processApp0Marker(n int) error {
	 486  	if n < 5 {
	 487  		return d.ignore(n)
	 488  	}
	 489  	if err := d.readFull(d.tmp[:5]); err != nil {
	 490  		return err
	 491  	}
	 492  	n -= 5
	 493  
	 494  	d.jfif = d.tmp[0] == 'J' && d.tmp[1] == 'F' && d.tmp[2] == 'I' && d.tmp[3] == 'F' && d.tmp[4] == '\x00'
	 495  
	 496  	if n > 0 {
	 497  		return d.ignore(n)
	 498  	}
	 499  	return nil
	 500  }
	 501  
	 502  func (d *decoder) processApp14Marker(n int) error {
	 503  	if n < 12 {
	 504  		return d.ignore(n)
	 505  	}
	 506  	if err := d.readFull(d.tmp[:12]); err != nil {
	 507  		return err
	 508  	}
	 509  	n -= 12
	 510  
	 511  	if d.tmp[0] == 'A' && d.tmp[1] == 'd' && d.tmp[2] == 'o' && d.tmp[3] == 'b' && d.tmp[4] == 'e' {
	 512  		d.adobeTransformValid = true
	 513  		d.adobeTransform = d.tmp[11]
	 514  	}
	 515  
	 516  	if n > 0 {
	 517  		return d.ignore(n)
	 518  	}
	 519  	return nil
	 520  }
	 521  
	 522  // decode reads a JPEG image from r and returns it as an image.Image.
	 523  func (d *decoder) decode(r io.Reader, configOnly bool) (image.Image, error) {
	 524  	d.r = r
	 525  
	 526  	// Check for the Start Of Image marker.
	 527  	if err := d.readFull(d.tmp[:2]); err != nil {
	 528  		return nil, err
	 529  	}
	 530  	if d.tmp[0] != 0xff || d.tmp[1] != soiMarker {
	 531  		return nil, FormatError("missing SOI marker")
	 532  	}
	 533  
	 534  	// Process the remaining segments until the End Of Image marker.
	 535  	for {
	 536  		err := d.readFull(d.tmp[:2])
	 537  		if err != nil {
	 538  			return nil, err
	 539  		}
	 540  		for d.tmp[0] != 0xff {
	 541  			// Strictly speaking, this is a format error. However, libjpeg is
	 542  			// liberal in what it accepts. As of version 9, next_marker in
	 543  			// jdmarker.c treats this as a warning (JWRN_EXTRANEOUS_DATA) and
	 544  			// continues to decode the stream. Even before next_marker sees
	 545  			// extraneous data, jpeg_fill_bit_buffer in jdhuff.c reads as many
	 546  			// bytes as it can, possibly past the end of a scan's data. It
	 547  			// effectively puts back any markers that it overscanned (e.g. an
	 548  			// "\xff\xd9" EOI marker), but it does not put back non-marker data,
	 549  			// and thus it can silently ignore a small number of extraneous
	 550  			// non-marker bytes before next_marker has a chance to see them (and
	 551  			// print a warning).
	 552  			//
	 553  			// We are therefore also liberal in what we accept. Extraneous data
	 554  			// is silently ignored.
	 555  			//
	 556  			// This is similar to, but not exactly the same as, the restart
	 557  			// mechanism within a scan (the RST[0-7] markers).
	 558  			//
	 559  			// Note that extraneous 0xff bytes in e.g. SOS data are escaped as
	 560  			// "\xff\x00", and so are detected a little further down below.
	 561  			d.tmp[0] = d.tmp[1]
	 562  			d.tmp[1], err = d.readByte()
	 563  			if err != nil {
	 564  				return nil, err
	 565  			}
	 566  		}
	 567  		marker := d.tmp[1]
	 568  		if marker == 0 {
	 569  			// Treat "\xff\x00" as extraneous data.
	 570  			continue
	 571  		}
	 572  		for marker == 0xff {
	 573  			// Section B.1.1.2 says, "Any marker may optionally be preceded by any
	 574  			// number of fill bytes, which are bytes assigned code X'FF'".
	 575  			marker, err = d.readByte()
	 576  			if err != nil {
	 577  				return nil, err
	 578  			}
	 579  		}
	 580  		if marker == eoiMarker { // End Of Image.
	 581  			break
	 582  		}
	 583  		if rst0Marker <= marker && marker <= rst7Marker {
	 584  			// Figures B.2 and B.16 of the specification suggest that restart markers should
	 585  			// only occur between Entropy Coded Segments and not after the final ECS.
	 586  			// However, some encoders may generate incorrect JPEGs with a final restart
	 587  			// marker. That restart marker will be seen here instead of inside the processSOS
	 588  			// method, and is ignored as a harmless error. Restart markers have no extra data,
	 589  			// so we check for this before we read the 16-bit length of the segment.
	 590  			continue
	 591  		}
	 592  
	 593  		// Read the 16-bit length of the segment. The value includes the 2 bytes for the
	 594  		// length itself, so we subtract 2 to get the number of remaining bytes.
	 595  		if err = d.readFull(d.tmp[:2]); err != nil {
	 596  			return nil, err
	 597  		}
	 598  		n := int(d.tmp[0])<<8 + int(d.tmp[1]) - 2
	 599  		if n < 0 {
	 600  			return nil, FormatError("short segment length")
	 601  		}
	 602  
	 603  		switch marker {
	 604  		case sof0Marker, sof1Marker, sof2Marker:
	 605  			d.baseline = marker == sof0Marker
	 606  			d.progressive = marker == sof2Marker
	 607  			err = d.processSOF(n)
	 608  			if configOnly && d.jfif {
	 609  				return nil, err
	 610  			}
	 611  		case dhtMarker:
	 612  			if configOnly {
	 613  				err = d.ignore(n)
	 614  			} else {
	 615  				err = d.processDHT(n)
	 616  			}
	 617  		case dqtMarker:
	 618  			if configOnly {
	 619  				err = d.ignore(n)
	 620  			} else {
	 621  				err = d.processDQT(n)
	 622  			}
	 623  		case sosMarker:
	 624  			if configOnly {
	 625  				return nil, nil
	 626  			}
	 627  			err = d.processSOS(n)
	 628  		case driMarker:
	 629  			if configOnly {
	 630  				err = d.ignore(n)
	 631  			} else {
	 632  				err = d.processDRI(n)
	 633  			}
	 634  		case app0Marker:
	 635  			err = d.processApp0Marker(n)
	 636  		case app14Marker:
	 637  			err = d.processApp14Marker(n)
	 638  		default:
	 639  			if app0Marker <= marker && marker <= app15Marker || marker == comMarker {
	 640  				err = d.ignore(n)
	 641  			} else if marker < 0xc0 { // See Table B.1 "Marker code assignments".
	 642  				err = FormatError("unknown marker")
	 643  			} else {
	 644  				err = UnsupportedError("unknown marker")
	 645  			}
	 646  		}
	 647  		if err != nil {
	 648  			return nil, err
	 649  		}
	 650  	}
	 651  
	 652  	if d.progressive {
	 653  		if err := d.reconstructProgressiveImage(); err != nil {
	 654  			return nil, err
	 655  		}
	 656  	}
	 657  	if d.img1 != nil {
	 658  		return d.img1, nil
	 659  	}
	 660  	if d.img3 != nil {
	 661  		if d.blackPix != nil {
	 662  			return d.applyBlack()
	 663  		} else if d.isRGB() {
	 664  			return d.convertToRGB()
	 665  		}
	 666  		return d.img3, nil
	 667  	}
	 668  	return nil, FormatError("missing SOS marker")
	 669  }
	 670  
	 671  // applyBlack combines d.img3 and d.blackPix into a CMYK image. The formula
	 672  // used depends on whether the JPEG image is stored as CMYK or YCbCrK,
	 673  // indicated by the APP14 (Adobe) metadata.
	 674  //
	 675  // Adobe CMYK JPEG images are inverted, where 255 means no ink instead of full
	 676  // ink, so we apply "v = 255 - v" at various points. Note that a double
	 677  // inversion is a no-op, so inversions might be implicit in the code below.
	 678  func (d *decoder) applyBlack() (image.Image, error) {
	 679  	if !d.adobeTransformValid {
	 680  		return nil, UnsupportedError("unknown color model: 4-component JPEG doesn't have Adobe APP14 metadata")
	 681  	}
	 682  
	 683  	// If the 4-component JPEG image isn't explicitly marked as "Unknown (RGB
	 684  	// or CMYK)" as per
	 685  	// https://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html#Adobe
	 686  	// we assume that it is YCbCrK. This matches libjpeg's jdapimin.c.
	 687  	if d.adobeTransform != adobeTransformUnknown {
	 688  		// Convert the YCbCr part of the YCbCrK to RGB, invert the RGB to get
	 689  		// CMY, and patch in the original K. The RGB to CMY inversion cancels
	 690  		// out the 'Adobe inversion' described in the applyBlack doc comment
	 691  		// above, so in practice, only the fourth channel (black) is inverted.
	 692  		bounds := d.img3.Bounds()
	 693  		img := image.NewRGBA(bounds)
	 694  		imageutil.DrawYCbCr(img, bounds, d.img3, bounds.Min)
	 695  		for iBase, y := 0, bounds.Min.Y; y < bounds.Max.Y; iBase, y = iBase+img.Stride, y+1 {
	 696  			for i, x := iBase+3, bounds.Min.X; x < bounds.Max.X; i, x = i+4, x+1 {
	 697  				img.Pix[i] = 255 - d.blackPix[(y-bounds.Min.Y)*d.blackStride+(x-bounds.Min.X)]
	 698  			}
	 699  		}
	 700  		return &image.CMYK{
	 701  			Pix:		img.Pix,
	 702  			Stride: img.Stride,
	 703  			Rect:	 img.Rect,
	 704  		}, nil
	 705  	}
	 706  
	 707  	// The first three channels (cyan, magenta, yellow) of the CMYK
	 708  	// were decoded into d.img3, but each channel was decoded into a separate
	 709  	// []byte slice, and some channels may be subsampled. We interleave the
	 710  	// separate channels into an image.CMYK's single []byte slice containing 4
	 711  	// contiguous bytes per pixel.
	 712  	bounds := d.img3.Bounds()
	 713  	img := image.NewCMYK(bounds)
	 714  
	 715  	translations := [4]struct {
	 716  		src		[]byte
	 717  		stride int
	 718  	}{
	 719  		{d.img3.Y, d.img3.YStride},
	 720  		{d.img3.Cb, d.img3.CStride},
	 721  		{d.img3.Cr, d.img3.CStride},
	 722  		{d.blackPix, d.blackStride},
	 723  	}
	 724  	for t, translation := range translations {
	 725  		subsample := d.comp[t].h != d.comp[0].h || d.comp[t].v != d.comp[0].v
	 726  		for iBase, y := 0, bounds.Min.Y; y < bounds.Max.Y; iBase, y = iBase+img.Stride, y+1 {
	 727  			sy := y - bounds.Min.Y
	 728  			if subsample {
	 729  				sy /= 2
	 730  			}
	 731  			for i, x := iBase+t, bounds.Min.X; x < bounds.Max.X; i, x = i+4, x+1 {
	 732  				sx := x - bounds.Min.X
	 733  				if subsample {
	 734  					sx /= 2
	 735  				}
	 736  				img.Pix[i] = 255 - translation.src[sy*translation.stride+sx]
	 737  			}
	 738  		}
	 739  	}
	 740  	return img, nil
	 741  }
	 742  
	 743  func (d *decoder) isRGB() bool {
	 744  	if d.jfif {
	 745  		return false
	 746  	}
	 747  	if d.adobeTransformValid && d.adobeTransform == adobeTransformUnknown {
	 748  		// https://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html#Adobe
	 749  		// says that 0 means Unknown (and in practice RGB) and 1 means YCbCr.
	 750  		return true
	 751  	}
	 752  	return d.comp[0].c == 'R' && d.comp[1].c == 'G' && d.comp[2].c == 'B'
	 753  }
	 754  
	 755  func (d *decoder) convertToRGB() (image.Image, error) {
	 756  	cScale := d.comp[0].h / d.comp[1].h
	 757  	bounds := d.img3.Bounds()
	 758  	img := image.NewRGBA(bounds)
	 759  	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
	 760  		po := img.PixOffset(bounds.Min.X, y)
	 761  		yo := d.img3.YOffset(bounds.Min.X, y)
	 762  		co := d.img3.COffset(bounds.Min.X, y)
	 763  		for i, iMax := 0, bounds.Max.X-bounds.Min.X; i < iMax; i++ {
	 764  			img.Pix[po+4*i+0] = d.img3.Y[yo+i]
	 765  			img.Pix[po+4*i+1] = d.img3.Cb[co+i/cScale]
	 766  			img.Pix[po+4*i+2] = d.img3.Cr[co+i/cScale]
	 767  			img.Pix[po+4*i+3] = 255
	 768  		}
	 769  	}
	 770  	return img, nil
	 771  }
	 772  
	 773  // Decode reads a JPEG image from r and returns it as an image.Image.
	 774  func Decode(r io.Reader) (image.Image, error) {
	 775  	var d decoder
	 776  	return d.decode(r, false)
	 777  }
	 778  
	 779  // DecodeConfig returns the color model and dimensions of a JPEG image without
	 780  // decoding the entire image.
	 781  func DecodeConfig(r io.Reader) (image.Config, error) {
	 782  	var d decoder
	 783  	if _, err := d.decode(r, true); err != nil {
	 784  		return image.Config{}, err
	 785  	}
	 786  	switch d.nComp {
	 787  	case 1:
	 788  		return image.Config{
	 789  			ColorModel: color.GrayModel,
	 790  			Width:			d.width,
	 791  			Height:		 d.height,
	 792  		}, nil
	 793  	case 3:
	 794  		cm := color.YCbCrModel
	 795  		if d.isRGB() {
	 796  			cm = color.RGBAModel
	 797  		}
	 798  		return image.Config{
	 799  			ColorModel: cm,
	 800  			Width:			d.width,
	 801  			Height:		 d.height,
	 802  		}, nil
	 803  	case 4:
	 804  		return image.Config{
	 805  			ColorModel: color.CMYKModel,
	 806  			Width:			d.width,
	 807  			Height:		 d.height,
	 808  		}, nil
	 809  	}
	 810  	return image.Config{}, FormatError("missing SOF marker")
	 811  }
	 812  
	 813  func init() {
	 814  	image.RegisterFormat("jpeg", "\xff\xd8", Decode, DecodeConfig)
	 815  }
	 816  

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