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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: http://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  // TODO(nigeltao): fix up the doc comment style so that sentences start with
    18  // the name of the type or function that they annotate.
    19  
    20  // A FormatError reports that the input is not a valid JPEG.
    21  type FormatError string
    22  
    23  func (e FormatError) Error() string { return "invalid JPEG format: " + string(e) }
    24  
    25  // An UnsupportedError reports that the input uses a valid but unimplemented JPEG feature.
    26  type UnsupportedError string
    27  
    28  func (e UnsupportedError) Error() string { return "unsupported JPEG feature: " + string(e) }
    29  
    30  var errUnsupportedSubsamplingRatio = UnsupportedError("luma/chroma subsampling ratio")
    31  
    32  // Component specification, specified in section B.2.2.
    33  type component struct {
    34  	h  int   // Horizontal sampling factor.
    35  	v  int   // Vertical sampling factor.
    36  	c  uint8 // Component identifier.
    37  	tq uint8 // Quantization table destination selector.
    38  }
    39  
    40  const (
    41  	dcTable = 0
    42  	acTable = 1
    43  	maxTc   = 1
    44  	maxTh   = 3
    45  	maxTq   = 3
    46  
    47  	maxComponents = 4
    48  )
    49  
    50  const (
    51  	sof0Marker = 0xc0 // Start Of Frame (Baseline Sequential).
    52  	sof1Marker = 0xc1 // Start Of Frame (Extended Sequential).
    53  	sof2Marker = 0xc2 // Start Of Frame (Progressive).
    54  	dhtMarker  = 0xc4 // Define Huffman Table.
    55  	rst0Marker = 0xd0 // ReSTart (0).
    56  	rst7Marker = 0xd7 // ReSTart (7).
    57  	soiMarker  = 0xd8 // Start Of Image.
    58  	eoiMarker  = 0xd9 // End Of Image.
    59  	sosMarker  = 0xda // Start Of Scan.
    60  	dqtMarker  = 0xdb // Define Quantization Table.
    61  	driMarker  = 0xdd // Define Restart Interval.
    62  	comMarker  = 0xfe // COMment.
    63  	// "APPlication specific" markers aren't part of the JPEG spec per se,
    64  	// but in practice, their use is described at
    65  	// http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html
    66  	app0Marker  = 0xe0
    67  	app14Marker = 0xee
    68  	app15Marker = 0xef
    69  )
    70  
    71  // See http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html#Adobe
    72  const (
    73  	adobeTransformUnknown = 0
    74  	adobeTransformYCbCr   = 1
    75  	adobeTransformYCbCrK  = 2
    76  )
    77  
    78  // unzig maps from the zig-zag ordering to the natural ordering. For example,
    79  // unzig[3] is the column and row of the fourth element in zig-zag order. The
    80  // value is 16, which means first column (16%8 == 0) and third row (16/8 == 2).
    81  var unzig = [blockSize]int{
    82  	0, 1, 8, 16, 9, 2, 3, 10,
    83  	17, 24, 32, 25, 18, 11, 4, 5,
    84  	12, 19, 26, 33, 40, 48, 41, 34,
    85  	27, 20, 13, 6, 7, 14, 21, 28,
    86  	35, 42, 49, 56, 57, 50, 43, 36,
    87  	29, 22, 15, 23, 30, 37, 44, 51,
    88  	58, 59, 52, 45, 38, 31, 39, 46,
    89  	53, 60, 61, 54, 47, 55, 62, 63,
    90  }
    91  
    92  // Deprecated: Reader is deprecated.
    93  type Reader interface {
    94  	io.ByteReader
    95  	io.Reader
    96  }
    97  
    98  // bits holds the unprocessed bits that have been taken from the byte-stream.
    99  // The n least significant bits of a form the unread bits, to be read in MSB to
   100  // LSB order.
   101  type bits struct {
   102  	a uint32 // accumulator.
   103  	m uint32 // mask. m==1<<(n-1) when n>0, with m==0 when n==0.
   104  	n int32  // the number of unread bits in a.
   105  }
   106  
   107  type decoder struct {
   108  	r    io.Reader
   109  	bits bits
   110  	// bytes is a byte buffer, similar to a bufio.Reader, except that it
   111  	// has to be able to unread more than 1 byte, due to byte stuffing.
   112  	// Byte stuffing is specified in section F.1.2.3.
   113  	bytes struct {
   114  		// buf[i:j] are the buffered bytes read from the underlying
   115  		// io.Reader that haven't yet been passed further on.
   116  		buf  [4096]byte
   117  		i, j int
   118  		// nUnreadable is the number of bytes to back up i after
   119  		// overshooting. It can be 0, 1 or 2.
   120  		nUnreadable int
   121  	}
   122  	width, height int
   123  
   124  	img1        *image.Gray
   125  	img3        *image.YCbCr
   126  	blackPix    []byte
   127  	blackStride int
   128  
   129  	ri    int // Restart Interval.
   130  	nComp int
   131  
   132  	// As per section 4.5, there are four modes of operation (selected by the
   133  	// SOF? markers): sequential DCT, progressive DCT, lossless and
   134  	// hierarchical, although this implementation does not support the latter
   135  	// two non-DCT modes. Sequential DCT is further split into baseline and
   136  	// extended, as per section 4.11.
   137  	baseline    bool
   138  	progressive bool
   139  
   140  	jfif                bool
   141  	adobeTransformValid bool
   142  	adobeTransform      uint8
   143  	eobRun              uint16 // End-of-Band run, specified in section G.1.2.2.
   144  
   145  	comp       [maxComponents]component
   146  	progCoeffs [maxComponents][]block // Saved state between progressive-mode scans.
   147  	huff       [maxTc + 1][maxTh + 1]huffman
   148  	quant      [maxTq + 1]block // Quantization tables, in zig-zag order.
   149  	tmp        [2 * blockSize]byte
   150  }
   151  
   152  // fill fills up the d.bytes.buf buffer from the underlying io.Reader. It
   153  // should only be called when there are no unread bytes in d.bytes.
   154  func (d *decoder) fill() error {
   155  	if d.bytes.i != d.bytes.j {
   156  		panic("jpeg: fill called when unread bytes exist")
   157  	}
   158  	// Move the last 2 bytes to the start of the buffer, in case we need
   159  	// to call unreadByteStuffedByte.
   160  	if d.bytes.j > 2 {
   161  		d.bytes.buf[0] = d.bytes.buf[d.bytes.j-2]
   162  		d.bytes.buf[1] = d.bytes.buf[d.bytes.j-1]
   163  		d.bytes.i, d.bytes.j = 2, 2
   164  	}
   165  	// Fill in the rest of the buffer.
   166  	n, err := d.r.Read(d.bytes.buf[d.bytes.j:])
   167  	d.bytes.j += n
   168  	if n > 0 {
   169  		err = nil
   170  	}
   171  	return err
   172  }
   173  
   174  // unreadByteStuffedByte undoes the most recent readByteStuffedByte call,
   175  // giving a byte of data back from d.bits to d.bytes. The Huffman look-up table
   176  // requires at least 8 bits for look-up, which means that Huffman decoding can
   177  // sometimes overshoot and read one or two too many bytes. Two-byte overshoot
   178  // can happen when expecting to read a 0xff 0x00 byte-stuffed byte.
   179  func (d *decoder) unreadByteStuffedByte() {
   180  	d.bytes.i -= d.bytes.nUnreadable
   181  	d.bytes.nUnreadable = 0
   182  	if d.bits.n >= 8 {
   183  		d.bits.a >>= 8
   184  		d.bits.n -= 8
   185  		d.bits.m >>= 8
   186  	}
   187  }
   188  
   189  // readByte returns the next byte, whether buffered or not buffered. It does
   190  // not care about byte stuffing.
   191  func (d *decoder) readByte() (x byte, err error) {
   192  	for d.bytes.i == d.bytes.j {
   193  		if err = d.fill(); err != nil {
   194  			return 0, err
   195  		}
   196  	}
   197  	x = d.bytes.buf[d.bytes.i]
   198  	d.bytes.i++
   199  	d.bytes.nUnreadable = 0
   200  	return x, nil
   201  }
   202  
   203  // errMissingFF00 means that readByteStuffedByte encountered an 0xff byte (a
   204  // marker byte) that wasn't the expected byte-stuffed sequence 0xff, 0x00.
   205  var errMissingFF00 = FormatError("missing 0xff00 sequence")
   206  
   207  // readByteStuffedByte is like readByte but is for byte-stuffed Huffman data.
   208  func (d *decoder) readByteStuffedByte() (x byte, err error) {
   209  	// Take the fast path if d.bytes.buf contains at least two bytes.
   210  	if d.bytes.i+2 <= d.bytes.j {
   211  		x = d.bytes.buf[d.bytes.i]
   212  		d.bytes.i++
   213  		d.bytes.nUnreadable = 1
   214  		if x != 0xff {
   215  			return x, err
   216  		}
   217  		if d.bytes.buf[d.bytes.i] != 0x00 {
   218  			return 0, errMissingFF00
   219  		}
   220  		d.bytes.i++
   221  		d.bytes.nUnreadable = 2
   222  		return 0xff, nil
   223  	}
   224  
   225  	d.bytes.nUnreadable = 0
   226  
   227  	x, err = d.readByte()
   228  	if err != nil {
   229  		return 0, err
   230  	}
   231  	d.bytes.nUnreadable = 1
   232  	if x != 0xff {
   233  		return x, nil
   234  	}
   235  
   236  	x, err = d.readByte()
   237  	if err != nil {
   238  		return 0, err
   239  	}
   240  	d.bytes.nUnreadable = 2
   241  	if x != 0x00 {
   242  		return 0, errMissingFF00
   243  	}
   244  	return 0xff, nil
   245  }
   246  
   247  // readFull reads exactly len(p) bytes into p. It does not care about byte
   248  // stuffing.
   249  func (d *decoder) readFull(p []byte) error {
   250  	// Unread the overshot bytes, if any.
   251  	if d.bytes.nUnreadable != 0 {
   252  		if d.bits.n >= 8 {
   253  			d.unreadByteStuffedByte()
   254  		}
   255  		d.bytes.nUnreadable = 0
   256  	}
   257  
   258  	for {
   259  		n := copy(p, d.bytes.buf[d.bytes.i:d.bytes.j])
   260  		p = p[n:]
   261  		d.bytes.i += n
   262  		if len(p) == 0 {
   263  			break
   264  		}
   265  		if err := d.fill(); err != nil {
   266  			if err == io.EOF {
   267  				err = io.ErrUnexpectedEOF
   268  			}
   269  			return err
   270  		}
   271  	}
   272  	return nil
   273  }
   274  
   275  // ignore ignores the next n bytes.
   276  func (d *decoder) ignore(n int) error {
   277  	// Unread the overshot bytes, if any.
   278  	if d.bytes.nUnreadable != 0 {
   279  		if d.bits.n >= 8 {
   280  			d.unreadByteStuffedByte()
   281  		}
   282  		d.bytes.nUnreadable = 0
   283  	}
   284  
   285  	for {
   286  		m := d.bytes.j - d.bytes.i
   287  		if m > n {
   288  			m = n
   289  		}
   290  		d.bytes.i += m
   291  		n -= m
   292  		if n == 0 {
   293  			break
   294  		}
   295  		if err := d.fill(); err != nil {
   296  			if err == io.EOF {
   297  				err = io.ErrUnexpectedEOF
   298  			}
   299  			return err
   300  		}
   301  	}
   302  	return nil
   303  }
   304  
   305  // Specified in section B.2.2.
   306  func (d *decoder) processSOF(n int) error {
   307  	if d.nComp != 0 {
   308  		return FormatError("multiple SOF markers")
   309  	}
   310  	switch n {
   311  	case 6 + 3*1: // Grayscale image.
   312  		d.nComp = 1
   313  	case 6 + 3*3: // YCbCr or RGB image.
   314  		d.nComp = 3
   315  	case 6 + 3*4: // YCbCrK or CMYK image.
   316  		d.nComp = 4
   317  	default:
   318  		return UnsupportedError("number of components")
   319  	}
   320  	if err := d.readFull(d.tmp[:n]); err != nil {
   321  		return err
   322  	}
   323  	// We only support 8-bit precision.
   324  	if d.tmp[0] != 8 {
   325  		return UnsupportedError("precision")
   326  	}
   327  	d.height = int(d.tmp[1])<<8 + int(d.tmp[2])
   328  	d.width = int(d.tmp[3])<<8 + int(d.tmp[4])
   329  	if int(d.tmp[5]) != d.nComp {
   330  		return FormatError("SOF has wrong length")
   331  	}
   332  
   333  	for i := 0; i < d.nComp; i++ {
   334  		d.comp[i].c = d.tmp[6+3*i]
   335  		// Section B.2.2 states that "the value of C_i shall be different from
   336  		// the values of C_1 through C_(i-1)".
   337  		for j := 0; j < i; j++ {
   338  			if d.comp[i].c == d.comp[j].c {
   339  				return FormatError("repeated component identifier")
   340  			}
   341  		}
   342  
   343  		d.comp[i].tq = d.tmp[8+3*i]
   344  		if d.comp[i].tq > maxTq {
   345  			return FormatError("bad Tq value")
   346  		}
   347  
   348  		hv := d.tmp[7+3*i]
   349  		h, v := int(hv>>4), int(hv&0x0f)
   350  		if h < 1 || 4 < h || v < 1 || 4 < v {
   351  			return FormatError("luma/chroma subsampling ratio")
   352  		}
   353  		if h == 3 || v == 3 {
   354  			return errUnsupportedSubsamplingRatio
   355  		}
   356  		switch d.nComp {
   357  		case 1:
   358  			// If a JPEG image has only one component, section A.2 says "this data
   359  			// is non-interleaved by definition" and section A.2.2 says "[in this
   360  			// case...] the order of data units within a scan shall be left-to-right
   361  			// and top-to-bottom... regardless of the values of H_1 and V_1". Section
   362  			// 4.8.2 also says "[for non-interleaved data], the MCU is defined to be
   363  			// one data unit". Similarly, section A.1.1 explains that it is the ratio
   364  			// of H_i to max_j(H_j) that matters, and similarly for V. For grayscale
   365  			// images, H_1 is the maximum H_j for all components j, so that ratio is
   366  			// always 1. The component's (h, v) is effectively always (1, 1): even if
   367  			// the nominal (h, v) is (2, 1), a 20x5 image is encoded in three 8x8
   368  			// MCUs, not two 16x8 MCUs.
   369  			h, v = 1, 1
   370  
   371  		case 3:
   372  			// For YCbCr images, we only support 4:4:4, 4:4:0, 4:2:2, 4:2:0,
   373  			// 4:1:1 or 4:1:0 chroma subsampling ratios. This implies that the
   374  			// (h, v) values for the Y component are either (1, 1), (1, 2),
   375  			// (2, 1), (2, 2), (4, 1) or (4, 2), and the Y component's values
   376  			// must be a multiple of the Cb and Cr component's values. We also
   377  			// assume that the two chroma components have the same subsampling
   378  			// ratio.
   379  			switch i {
   380  			case 0: // Y.
   381  				// We have already verified, above, that h and v are both
   382  				// either 1, 2 or 4, so invalid (h, v) combinations are those
   383  				// with v == 4.
   384  				if v == 4 {
   385  					return errUnsupportedSubsamplingRatio
   386  				}
   387  			case 1: // Cb.
   388  				if d.comp[0].h%h != 0 || d.comp[0].v%v != 0 {
   389  					return errUnsupportedSubsamplingRatio
   390  				}
   391  			case 2: // Cr.
   392  				if d.comp[1].h != h || d.comp[1].v != v {
   393  					return errUnsupportedSubsamplingRatio
   394  				}
   395  			}
   396  
   397  		case 4:
   398  			// For 4-component images (either CMYK or YCbCrK), we only support two
   399  			// hv vectors: [0x11 0x11 0x11 0x11] and [0x22 0x11 0x11 0x22].
   400  			// Theoretically, 4-component JPEG images could mix and match hv values
   401  			// but in practice, those two combinations are the only ones in use,
   402  			// and it simplifies the applyBlack code below if we can assume that:
   403  			//	- for CMYK, the C and K channels have full samples, and if the M
   404  			//	  and Y channels subsample, they subsample both horizontally and
   405  			//	  vertically.
   406  			//	- for YCbCrK, the Y and K channels have full samples.
   407  			switch i {
   408  			case 0:
   409  				if hv != 0x11 && hv != 0x22 {
   410  					return errUnsupportedSubsamplingRatio
   411  				}
   412  			case 1, 2:
   413  				if hv != 0x11 {
   414  					return errUnsupportedSubsamplingRatio
   415  				}
   416  			case 3:
   417  				if d.comp[0].h != h || d.comp[0].v != v {
   418  					return errUnsupportedSubsamplingRatio
   419  				}
   420  			}
   421  		}
   422  
   423  		d.comp[i].h = h
   424  		d.comp[i].v = v
   425  	}
   426  	return nil
   427  }
   428  
   429  // Specified in section B.2.4.1.
   430  func (d *decoder) processDQT(n int) error {
   431  loop:
   432  	for n > 0 {
   433  		n--
   434  		x, err := d.readByte()
   435  		if err != nil {
   436  			return err
   437  		}
   438  		tq := x & 0x0f
   439  		if tq > maxTq {
   440  			return FormatError("bad Tq value")
   441  		}
   442  		switch x >> 4 {
   443  		default:
   444  			return FormatError("bad Pq value")
   445  		case 0:
   446  			if n < blockSize {
   447  				break loop
   448  			}
   449  			n -= blockSize
   450  			if err := d.readFull(d.tmp[:blockSize]); err != nil {
   451  				return err
   452  			}
   453  			for i := range d.quant[tq] {
   454  				d.quant[tq][i] = int32(d.tmp[i])
   455  			}
   456  		case 1:
   457  			if n < 2*blockSize {
   458  				break loop
   459  			}
   460  			n -= 2 * blockSize
   461  			if err := d.readFull(d.tmp[:2*blockSize]); err != nil {
   462  				return err
   463  			}
   464  			for i := range d.quant[tq] {
   465  				d.quant[tq][i] = int32(d.tmp[2*i])<<8 | int32(d.tmp[2*i+1])
   466  			}
   467  		}
   468  	}
   469  	if n != 0 {
   470  		return FormatError("DQT has wrong length")
   471  	}
   472  	return nil
   473  }
   474  
   475  // Specified in section B.2.4.4.
   476  func (d *decoder) processDRI(n int) error {
   477  	if n != 2 {
   478  		return FormatError("DRI has wrong length")
   479  	}
   480  	if err := d.readFull(d.tmp[:2]); err != nil {
   481  		return err
   482  	}
   483  	d.ri = int(d.tmp[0])<<8 + int(d.tmp[1])
   484  	return nil
   485  }
   486  
   487  func (d *decoder) processApp0Marker(n int) error {
   488  	if n < 5 {
   489  		return d.ignore(n)
   490  	}
   491  	if err := d.readFull(d.tmp[:5]); err != nil {
   492  		return err
   493  	}
   494  	n -= 5
   495  
   496  	d.jfif = d.tmp[0] == 'J' && d.tmp[1] == 'F' && d.tmp[2] == 'I' && d.tmp[3] == 'F' && d.tmp[4] == '\x00'
   497  
   498  	if n > 0 {
   499  		return d.ignore(n)
   500  	}
   501  	return nil
   502  }
   503  
   504  func (d *decoder) processApp14Marker(n int) error {
   505  	if n < 12 {
   506  		return d.ignore(n)
   507  	}
   508  	if err := d.readFull(d.tmp[:12]); err != nil {
   509  		return err
   510  	}
   511  	n -= 12
   512  
   513  	if d.tmp[0] == 'A' && d.tmp[1] == 'd' && d.tmp[2] == 'o' && d.tmp[3] == 'b' && d.tmp[4] == 'e' {
   514  		d.adobeTransformValid = true
   515  		d.adobeTransform = d.tmp[11]
   516  	}
   517  
   518  	if n > 0 {
   519  		return d.ignore(n)
   520  	}
   521  	return nil
   522  }
   523  
   524  // decode reads a JPEG image from r and returns it as an image.Image.
   525  func (d *decoder) decode(r io.Reader, configOnly bool) (image.Image, error) {
   526  	d.r = r
   527  
   528  	// Check for the Start Of Image marker.
   529  	if err := d.readFull(d.tmp[:2]); err != nil {
   530  		return nil, err
   531  	}
   532  	if d.tmp[0] != 0xff || d.tmp[1] != soiMarker {
   533  		return nil, FormatError("missing SOI marker")
   534  	}
   535  
   536  	// Process the remaining segments until the End Of Image marker.
   537  	for {
   538  		err := d.readFull(d.tmp[:2])
   539  		if err != nil {
   540  			return nil, err
   541  		}
   542  		for d.tmp[0] != 0xff {
   543  			// Strictly speaking, this is a format error. However, libjpeg is
   544  			// liberal in what it accepts. As of version 9, next_marker in
   545  			// jdmarker.c treats this as a warning (JWRN_EXTRANEOUS_DATA) and
   546  			// continues to decode the stream. Even before next_marker sees
   547  			// extraneous data, jpeg_fill_bit_buffer in jdhuff.c reads as many
   548  			// bytes as it can, possibly past the end of a scan's data. It
   549  			// effectively puts back any markers that it overscanned (e.g. an
   550  			// "\xff\xd9" EOI marker), but it does not put back non-marker data,
   551  			// and thus it can silently ignore a small number of extraneous
   552  			// non-marker bytes before next_marker has a chance to see them (and
   553  			// print a warning).
   554  			//
   555  			// We are therefore also liberal in what we accept. Extraneous data
   556  			// is silently ignored.
   557  			//
   558  			// This is similar to, but not exactly the same as, the restart
   559  			// mechanism within a scan (the RST[0-7] markers).
   560  			//
   561  			// Note that extraneous 0xff bytes in e.g. SOS data are escaped as
   562  			// "\xff\x00", and so are detected a little further down below.
   563  			d.tmp[0] = d.tmp[1]
   564  			d.tmp[1], err = d.readByte()
   565  			if err != nil {
   566  				return nil, err
   567  			}
   568  		}
   569  		marker := d.tmp[1]
   570  		if marker == 0 {
   571  			// Treat "\xff\x00" as extraneous data.
   572  			continue
   573  		}
   574  		for marker == 0xff {
   575  			// Section B.1.1.2 says, "Any marker may optionally be preceded by any
   576  			// number of fill bytes, which are bytes assigned code X'FF'".
   577  			marker, err = d.readByte()
   578  			if err != nil {
   579  				return nil, err
   580  			}
   581  		}
   582  		if marker == eoiMarker { // End Of Image.
   583  			break
   584  		}
   585  		if rst0Marker <= marker && marker <= rst7Marker {
   586  			// Figures B.2 and B.16 of the specification suggest that restart markers should
   587  			// only occur between Entropy Coded Segments and not after the final ECS.
   588  			// However, some encoders may generate incorrect JPEGs with a final restart
   589  			// marker. That restart marker will be seen here instead of inside the processSOS
   590  			// method, and is ignored as a harmless error. Restart markers have no extra data,
   591  			// so we check for this before we read the 16-bit length of the segment.
   592  			continue
   593  		}
   594  
   595  		// Read the 16-bit length of the segment. The value includes the 2 bytes for the
   596  		// length itself, so we subtract 2 to get the number of remaining bytes.
   597  		if err = d.readFull(d.tmp[:2]); err != nil {
   598  			return nil, err
   599  		}
   600  		n := int(d.tmp[0])<<8 + int(d.tmp[1]) - 2
   601  		if n < 0 {
   602  			return nil, FormatError("short segment length")
   603  		}
   604  
   605  		switch marker {
   606  		case sof0Marker, sof1Marker, sof2Marker:
   607  			d.baseline = marker == sof0Marker
   608  			d.progressive = marker == sof2Marker
   609  			err = d.processSOF(n)
   610  			if configOnly && d.jfif {
   611  				return nil, err
   612  			}
   613  		case dhtMarker:
   614  			if configOnly {
   615  				err = d.ignore(n)
   616  			} else {
   617  				err = d.processDHT(n)
   618  			}
   619  		case dqtMarker:
   620  			if configOnly {
   621  				err = d.ignore(n)
   622  			} else {
   623  				err = d.processDQT(n)
   624  			}
   625  		case sosMarker:
   626  			if configOnly {
   627  				return nil, nil
   628  			}
   629  			err = d.processSOS(n)
   630  		case driMarker:
   631  			if configOnly {
   632  				err = d.ignore(n)
   633  			} else {
   634  				err = d.processDRI(n)
   635  			}
   636  		case app0Marker:
   637  			err = d.processApp0Marker(n)
   638  		case app14Marker:
   639  			err = d.processApp14Marker(n)
   640  		default:
   641  			if app0Marker <= marker && marker <= app15Marker || marker == comMarker {
   642  				err = d.ignore(n)
   643  			} else if marker < 0xc0 { // See Table B.1 "Marker code assignments".
   644  				err = FormatError("unknown marker")
   645  			} else {
   646  				err = UnsupportedError("unknown marker")
   647  			}
   648  		}
   649  		if err != nil {
   650  			return nil, err
   651  		}
   652  	}
   653  
   654  	if d.progressive {
   655  		if err := d.reconstructProgressiveImage(); err != nil {
   656  			return nil, err
   657  		}
   658  	}
   659  	if d.img1 != nil {
   660  		return d.img1, nil
   661  	}
   662  	if d.img3 != nil {
   663  		if d.blackPix != nil {
   664  			return d.applyBlack()
   665  		} else if d.isRGB() {
   666  			return d.convertToRGB()
   667  		}
   668  		return d.img3, nil
   669  	}
   670  	return nil, FormatError("missing SOS marker")
   671  }
   672  
   673  // applyBlack combines d.img3 and d.blackPix into a CMYK image. The formula
   674  // used depends on whether the JPEG image is stored as CMYK or YCbCrK,
   675  // indicated by the APP14 (Adobe) metadata.
   676  //
   677  // Adobe CMYK JPEG images are inverted, where 255 means no ink instead of full
   678  // ink, so we apply "v = 255 - v" at various points. Note that a double
   679  // inversion is a no-op, so inversions might be implicit in the code below.
   680  func (d *decoder) applyBlack() (image.Image, error) {
   681  	if !d.adobeTransformValid {
   682  		return nil, UnsupportedError("unknown color model: 4-component JPEG doesn't have Adobe APP14 metadata")
   683  	}
   684  
   685  	// If the 4-component JPEG image isn't explicitly marked as "Unknown (RGB
   686  	// or CMYK)" as per
   687  	// http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html#Adobe
   688  	// we assume that it is YCbCrK. This matches libjpeg's jdapimin.c.
   689  	if d.adobeTransform != adobeTransformUnknown {
   690  		// Convert the YCbCr part of the YCbCrK to RGB, invert the RGB to get
   691  		// CMY, and patch in the original K. The RGB to CMY inversion cancels
   692  		// out the 'Adobe inversion' described in the applyBlack doc comment
   693  		// above, so in practice, only the fourth channel (black) is inverted.
   694  		bounds := d.img3.Bounds()
   695  		img := image.NewRGBA(bounds)
   696  		imageutil.DrawYCbCr(img, bounds, d.img3, bounds.Min)
   697  		for iBase, y := 0, bounds.Min.Y; y < bounds.Max.Y; iBase, y = iBase+img.Stride, y+1 {
   698  			for i, x := iBase+3, bounds.Min.X; x < bounds.Max.X; i, x = i+4, x+1 {
   699  				img.Pix[i] = 255 - d.blackPix[(y-bounds.Min.Y)*d.blackStride+(x-bounds.Min.X)]
   700  			}
   701  		}
   702  		return &image.CMYK{
   703  			Pix:    img.Pix,
   704  			Stride: img.Stride,
   705  			Rect:   img.Rect,
   706  		}, nil
   707  	}
   708  
   709  	// The first three channels (cyan, magenta, yellow) of the CMYK
   710  	// were decoded into d.img3, but each channel was decoded into a separate
   711  	// []byte slice, and some channels may be subsampled. We interleave the
   712  	// separate channels into an image.CMYK's single []byte slice containing 4
   713  	// contiguous bytes per pixel.
   714  	bounds := d.img3.Bounds()
   715  	img := image.NewCMYK(bounds)
   716  
   717  	translations := [4]struct {
   718  		src    []byte
   719  		stride int
   720  	}{
   721  		{d.img3.Y, d.img3.YStride},
   722  		{d.img3.Cb, d.img3.CStride},
   723  		{d.img3.Cr, d.img3.CStride},
   724  		{d.blackPix, d.blackStride},
   725  	}
   726  	for t, translation := range translations {
   727  		subsample := d.comp[t].h != d.comp[0].h || d.comp[t].v != d.comp[0].v
   728  		for iBase, y := 0, bounds.Min.Y; y < bounds.Max.Y; iBase, y = iBase+img.Stride, y+1 {
   729  			sy := y - bounds.Min.Y
   730  			if subsample {
   731  				sy /= 2
   732  			}
   733  			for i, x := iBase+t, bounds.Min.X; x < bounds.Max.X; i, x = i+4, x+1 {
   734  				sx := x - bounds.Min.X
   735  				if subsample {
   736  					sx /= 2
   737  				}
   738  				img.Pix[i] = 255 - translation.src[sy*translation.stride+sx]
   739  			}
   740  		}
   741  	}
   742  	return img, nil
   743  }
   744  
   745  func (d *decoder) isRGB() bool {
   746  	if d.jfif {
   747  		return false
   748  	}
   749  	if d.adobeTransformValid && d.adobeTransform == adobeTransformUnknown {
   750  		// http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html#Adobe
   751  		// says that 0 means Unknown (and in practice RGB) and 1 means YCbCr.
   752  		return true
   753  	}
   754  	return d.comp[0].c == 'R' && d.comp[1].c == 'G' && d.comp[2].c == 'B'
   755  }
   756  
   757  func (d *decoder) convertToRGB() (image.Image, error) {
   758  	cScale := d.comp[0].h / d.comp[1].h
   759  	bounds := d.img3.Bounds()
   760  	img := image.NewRGBA(bounds)
   761  	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
   762  		po := img.PixOffset(bounds.Min.X, y)
   763  		yo := d.img3.YOffset(bounds.Min.X, y)
   764  		co := d.img3.COffset(bounds.Min.X, y)
   765  		for i, iMax := 0, bounds.Max.X-bounds.Min.X; i < iMax; i++ {
   766  			img.Pix[po+4*i+0] = d.img3.Y[yo+i]
   767  			img.Pix[po+4*i+1] = d.img3.Cb[co+i/cScale]
   768  			img.Pix[po+4*i+2] = d.img3.Cr[co+i/cScale]
   769  			img.Pix[po+4*i+3] = 255
   770  		}
   771  	}
   772  	return img, nil
   773  }
   774  
   775  // Decode reads a JPEG image from r and returns it as an image.Image.
   776  func Decode(r io.Reader) (image.Image, error) {
   777  	var d decoder
   778  	return d.decode(r, false)
   779  }
   780  
   781  // DecodeConfig returns the color model and dimensions of a JPEG image without
   782  // decoding the entire image.
   783  func DecodeConfig(r io.Reader) (image.Config, error) {
   784  	var d decoder
   785  	if _, err := d.decode(r, true); err != nil {
   786  		return image.Config{}, err
   787  	}
   788  	switch d.nComp {
   789  	case 1:
   790  		return image.Config{
   791  			ColorModel: color.GrayModel,
   792  			Width:      d.width,
   793  			Height:     d.height,
   794  		}, nil
   795  	case 3:
   796  		cm := color.YCbCrModel
   797  		if d.isRGB() {
   798  			cm = color.RGBAModel
   799  		}
   800  		return image.Config{
   801  			ColorModel: cm,
   802  			Width:      d.width,
   803  			Height:     d.height,
   804  		}, nil
   805  	case 4:
   806  		return image.Config{
   807  			ColorModel: color.CMYKModel,
   808  			Width:      d.width,
   809  			Height:     d.height,
   810  		}, nil
   811  	}
   812  	return image.Config{}, FormatError("missing SOF marker")
   813  }
   814  
   815  func init() {
   816  	image.RegisterFormat("jpeg", "\xff\xd8", Decode, DecodeConfig)
   817  }
   818  

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