src/image/jpeg/reader.go - go - Git at Google

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // Package jpeg implements a JPEG image decoder and encoder.
 //
 // JPEG is defined in ITU-T T.81: http://www.w3.org/Graphics/JPEG/itu-t81.pdf.
 package jpeg

 import (
 	"image"
 	"image/color"
 	"io"
 )

 // TODO(nigeltao): fix up the doc comment style so that sentences start with
 // the name of the type or function that they annotate.

 // A FormatError reports that the input is not a valid JPEG.
 type FormatError string

 func (e FormatError) Error() string { return "invalid JPEG format: " + string(e) }

 // An UnsupportedError reports that the input uses a valid but unimplemented JPEG feature.
 type UnsupportedError string

 func (e UnsupportedError) Error() string { return "unsupported JPEG feature: " + string(e) }

 // Component specification, specified in section B.2.2.
 type component struct {
 	h  int   // Horizontal sampling factor.
 	v  int   // Vertical sampling factor.
 	c  uint8 // Component identifier.
 	tq uint8 // Quantization table destination selector.
 }

 const (
 	dcTable = 0
 	acTable = 1
 	maxTc   = 1
 	maxTh   = 3
 	maxTq   = 3

 	// A grayscale JPEG image has only a Y component.
 	nGrayComponent = 1
 	// A color JPEG image has Y, Cb and Cr components.
 	nColorComponent = 3

 	// We only support 4:4:4, 4:4:0, 4:2:2 and 4:2:0 downsampling, and therefore the
 	// number of luma samples per chroma sample is at most 2 in the horizontal
 	// and 2 in the vertical direction.
 	maxH = 2
 	maxV = 2
 )

 const (
 	soiMarker   = 0xd8 // Start Of Image.
 	eoiMarker   = 0xd9 // End Of Image.
 	sof0Marker  = 0xc0 // Start Of Frame (Baseline).
 	sof2Marker  = 0xc2 // Start Of Frame (Progressive).
 	dhtMarker   = 0xc4 // Define Huffman Table.
 	dqtMarker   = 0xdb // Define Quantization Table.
 	sosMarker   = 0xda // Start Of Scan.
 	driMarker   = 0xdd // Define Restart Interval.
 	rst0Marker  = 0xd0 // ReSTart (0).
 	rst7Marker  = 0xd7 // ReSTart (7).
 	app0Marker  = 0xe0 // APPlication specific (0).
 	app15Marker = 0xef // APPlication specific (15).
 	comMarker   = 0xfe // COMment.
 )

 // unzig maps from the zig-zag ordering to the natural ordering. For example,
 // unzig[3] is the column and row of the fourth element in zig-zag order. The
 // value is 16, which means first column (16%8 == 0) and third row (16/8 == 2).
 var unzig = [blockSize]int{
 	0, 1, 8, 16, 9, 2, 3, 10,
 	17, 24, 32, 25, 18, 11, 4, 5,
 	12, 19, 26, 33, 40, 48, 41, 34,
 	27, 20, 13, 6, 7, 14, 21, 28,
 	35, 42, 49, 56, 57, 50, 43, 36,
 	29, 22, 15, 23, 30, 37, 44, 51,
 	58, 59, 52, 45, 38, 31, 39, 46,
 	53, 60, 61, 54, 47, 55, 62, 63,
 }

 // Reader is deprecated.
 type Reader interface {
 	io.ByteReader
 	io.Reader
 }

 // bits holds the unprocessed bits that have been taken from the byte-stream.
 // The n least significant bits of a form the unread bits, to be read in MSB to
 // LSB order.
 type bits struct {
 	a uint32 // accumulator.
 	m uint32 // mask. m==1<<(n-1) when n>0, with m==0 when n==0.
 	n int32  // the number of unread bits in a.
 }

 type decoder struct {
 	r    io.Reader
 	bits bits
 	// bytes is a byte buffer, similar to a bufio.Reader, except that it
 	// has to be able to unread more than 1 byte, due to byte stuffing.
 	// Byte stuffing is specified in section F.1.2.3.
 	bytes struct {
 		// buf[i:j] are the buffered bytes read from the underlying
 		// io.Reader that haven't yet been passed further on.
 		buf  [4096]byte
 		i, j int
 		// nUnreadable is the number of bytes to back up i after
 		// overshooting. It can be 0, 1 or 2.
 		nUnreadable int
 	}
 	width, height int
 	img1          *image.Gray
 	img3          *image.YCbCr
 	ri            int // Restart Interval.
 	nComp         int
 	progressive   bool
 	eobRun        uint16 // End-of-Band run, specified in section G.1.2.2.
 	comp          [nColorComponent]component
 	progCoeffs    [nColorComponent][]block // Saved state between progressive-mode scans.
 	huff          [maxTc + 1][maxTh + 1]huffman
 	quant         [maxTq + 1]block // Quantization tables, in zig-zag order.
 	tmp           [blockSize + 1]byte
 }

 // fill fills up the d.bytes.buf buffer from the underlying io.Reader. It
 // should only be called when there are no unread bytes in d.bytes.
 func (d *decoder) fill() error {
 	if d.bytes.i != d.bytes.j {
 		panic("jpeg: fill called when unread bytes exist")
 	}
 	// Move the last 2 bytes to the start of the buffer, in case we need
 	// to call unreadByteStuffedByte.
 	if d.bytes.j > 2 {
 		d.bytes.buf[0] = d.bytes.buf[d.bytes.j-2]
 		d.bytes.buf[1] = d.bytes.buf[d.bytes.j-1]
 		d.bytes.i, d.bytes.j = 2, 2
 	}
 	// Fill in the rest of the buffer.
 	n, err := d.r.Read(d.bytes.buf[d.bytes.j:])
 	d.bytes.j += n
 	if n > 0 {
 		err = nil
 	}
 	return err
 }

 // unreadByteStuffedByte undoes the most recent readByteStuffedByte call,
 // giving a byte of data back from d.bits to d.bytes. The Huffman look-up table
 // requires at least 8 bits for look-up, which means that Huffman decoding can
 // sometimes overshoot and read one or two too many bytes. Two-byte overshoot
 // can happen when expecting to read a 0xff 0x00 byte-stuffed byte.
 func (d *decoder) unreadByteStuffedByte() {
 	if d.bytes.nUnreadable == 0 {
 		panic("jpeg: unreadByteStuffedByte call cannot be fulfilled")
 	}
 	d.bytes.i -= d.bytes.nUnreadable
 	d.bytes.nUnreadable = 0
 	if d.bits.n >= 8 {
 		d.bits.a >>= 8
 		d.bits.n -= 8
 		d.bits.m >>= 8
 	}
 }

 // readByte returns the next byte, whether buffered or not buffered. It does
 // not care about byte stuffing.
 func (d *decoder) readByte() (x byte, err error) {
 	for d.bytes.i == d.bytes.j {
 		if err = d.fill(); err != nil {
 			return 0, err
 		}
 	}
 	x = d.bytes.buf[d.bytes.i]
 	d.bytes.i++
 	d.bytes.nUnreadable = 0
 	return x, nil
 }

 // errMissingFF00 means that readByteStuffedByte encountered an 0xff byte (a
 // marker byte) that wasn't the expected byte-stuffed sequence 0xff, 0x00.
 var errMissingFF00 = FormatError("missing 0xff00 sequence")

 // readByteStuffedByte is like readByte but is for byte-stuffed Huffman data.
 func (d *decoder) readByteStuffedByte() (x byte, err error) {
 	// Take the fast path if d.bytes.buf contains at least two bytes.
 	if d.bytes.i+2 <= d.bytes.j {
 		x = d.bytes.buf[d.bytes.i]
 		d.bytes.i++
 		d.bytes.nUnreadable = 1
 		if x != 0xff {
 			return x, err
 		}
 		if d.bytes.buf[d.bytes.i] != 0x00 {
 			return 0, errMissingFF00
 		}
 		d.bytes.i++
 		d.bytes.nUnreadable = 2
 		return 0xff, nil
 	}

 	x, err = d.readByte()
 	if err != nil {
 		return 0, err
 	}
 	if x != 0xff {
 		d.bytes.nUnreadable = 1
 		return x, nil
 	}

 	x, err = d.readByte()
 	if err != nil {
 		d.bytes.nUnreadable = 1
 		return 0, err
 	}
 	d.bytes.nUnreadable = 2
 	if x != 0x00 {
 		return 0, errMissingFF00
 	}
 	return 0xff, nil
 }

 // readFull reads exactly len(p) bytes into p. It does not care about byte
 // stuffing.
 func (d *decoder) readFull(p []byte) error {
 	// Unread the overshot bytes, if any.
 	if d.bytes.nUnreadable != 0 {
 		if d.bits.n >= 8 {
 			d.unreadByteStuffedByte()
 		}
 		d.bytes.nUnreadable = 0
 	}

 	for {
 		n := copy(p, d.bytes.buf[d.bytes.i:d.bytes.j])
 		p = p[n:]
 		d.bytes.i += n
 		if len(p) == 0 {
 			break
 		}
 		if err := d.fill(); err != nil {
 			if err == io.EOF {
 				err = io.ErrUnexpectedEOF
 			}
 			return err
 		}
 	}
 	return nil
 }

 // ignore ignores the next n bytes.
 func (d *decoder) ignore(n int) error {
 	// Unread the overshot bytes, if any.
 	if d.bytes.nUnreadable != 0 {
 		if d.bits.n >= 8 {
 			d.unreadByteStuffedByte()
 		}
 		d.bytes.nUnreadable = 0
 	}

 	for {
 		m := d.bytes.j - d.bytes.i
 		if m > n {
 			m = n
 		}
 		d.bytes.i += m
 		n -= m
 		if n == 0 {
 			break
 		}
 		if err := d.fill(); err != nil {
 			if err == io.EOF {
 				err = io.ErrUnexpectedEOF
 			}
 			return err
 		}
 	}
 	return nil
 }

 // Specified in section B.2.2.
 func (d *decoder) processSOF(n int) error {
 	switch n {
 	case 6 + 3*nGrayComponent:
 		d.nComp = nGrayComponent
 	case 6 + 3*nColorComponent:
 		d.nComp = nColorComponent
 	default:
 		return UnsupportedError("SOF has wrong length")
 	}
 	if err := d.readFull(d.tmp[:n]); err != nil {
 		return err
 	}
 	// We only support 8-bit precision.
 	if d.tmp[0] != 8 {
 		return UnsupportedError("precision")
 	}
 	d.height = int(d.tmp[1])<<8 + int(d.tmp[2])
 	d.width = int(d.tmp[3])<<8 + int(d.tmp[4])
 	if int(d.tmp[5]) != d.nComp {
 		return UnsupportedError("SOF has wrong number of image components")
 	}
 	for i := 0; i < d.nComp; i++ {
 		d.comp[i].c = d.tmp[6+3*i]
 		d.comp[i].tq = d.tmp[8+3*i]
 		if d.nComp == nGrayComponent {
 			// If a JPEG image has only one component, section A.2 says "this data
 			// is non-interleaved by definition" and section A.2.2 says "[in this
 			// case...] the order of data units within a scan shall be left-to-right
 			// and top-to-bottom... regardless of the values of H_1 and V_1". Section
 			// 4.8.2 also says "[for non-interleaved data], the MCU is defined to be
 			// one data unit". Similarly, section A.1.1 explains that it is the ratio
 			// of H_i to max_j(H_j) that matters, and similarly for V. For grayscale
 			// images, H_1 is the maximum H_j for all components j, so that ratio is
 			// always 1. The component's (h, v) is effectively always (1, 1): even if
 			// the nominal (h, v) is (2, 1), a 20x5 image is encoded in three 8x8
 			// MCUs, not two 16x8 MCUs.
 			d.comp[i].h = 1
 			d.comp[i].v = 1
 			continue
 		}
 		hv := d.tmp[7+3*i]
 		d.comp[i].h = int(hv >> 4)
 		d.comp[i].v = int(hv & 0x0f)
 		// For color images, we only support 4:4:4, 4:4:0, 4:2:2 or 4:2:0 chroma
 		// downsampling ratios. This implies that the (h, v) values for the Y
 		// component are either (1, 1), (1, 2), (2, 1) or (2, 2), and the (h, v)
 		// values for the Cr and Cb components must be (1, 1).
 		if i == 0 {
 			if hv != 0x11 && hv != 0x21 && hv != 0x22 && hv != 0x12 {
 				return UnsupportedError("luma/chroma downsample ratio")
 			}
 		} else if hv != 0x11 {
 			return UnsupportedError("luma/chroma downsample ratio")
 		}
 	}
 	return nil
 }

 // Specified in section B.2.4.1.
 func (d *decoder) processDQT(n int) error {
 	const qtLength = 1 + blockSize
 	for ; n >= qtLength; n -= qtLength {
 		if err := d.readFull(d.tmp[:qtLength]); err != nil {
 			return err
 		}
 		pq := d.tmp[0] >> 4
 		if pq != 0 {
 			return UnsupportedError("bad Pq value")
 		}
 		tq := d.tmp[0] & 0x0f
 		if tq > maxTq {
 			return FormatError("bad Tq value")
 		}
 		for i := range d.quant[tq] {
 			d.quant[tq][i] = int32(d.tmp[i+1])
 		}
 	}
 	if n != 0 {
 		return FormatError("DQT has wrong length")
 	}
 	return nil
 }

 // Specified in section B.2.4.4.
 func (d *decoder) processDRI(n int) error {
 	if n != 2 {
 		return FormatError("DRI has wrong length")
 	}
 	if err := d.readFull(d.tmp[:2]); err != nil {
 		return err
 	}
 	d.ri = int(d.tmp[0])<<8 + int(d.tmp[1])
 	return nil
 }

 // decode reads a JPEG image from r and returns it as an image.Image.
 func (d *decoder) decode(r io.Reader, configOnly bool) (image.Image, error) {
 	d.r = r

 	// Check for the Start Of Image marker.
 	if err := d.readFull(d.tmp[:2]); err != nil {
 		return nil, err
 	}
 	if d.tmp[0] != 0xff || d.tmp[1] != soiMarker {
 		return nil, FormatError("missing SOI marker")
 	}

 	// Process the remaining segments until the End Of Image marker.
 	for {
 		err := d.readFull(d.tmp[:2])
 		if err != nil {
 			return nil, err
 		}
 		for d.tmp[0] != 0xff {
 			// Strictly speaking, this is a format error. However, libjpeg is
 			// liberal in what it accepts. As of version 9, next_marker in
 			// jdmarker.c treats this as a warning (JWRN_EXTRANEOUS_DATA) and
 			// continues to decode the stream. Even before next_marker sees
 			// extraneous data, jpeg_fill_bit_buffer in jdhuff.c reads as many
 			// bytes as it can, possibly past the end of a scan's data. It
 			// effectively puts back any markers that it overscanned (e.g. an
 			// "\xff\xd9" EOI marker), but it does not put back non-marker data,
 			// and thus it can silently ignore a small number of extraneous
 			// non-marker bytes before next_marker has a chance to see them (and
 			// print a warning).
 			//
 			// We are therefore also liberal in what we accept. Extraneous data
 			// is silently ignored.
 			//
 			// This is similar to, but not exactly the same as, the restart
 			// mechanism within a scan (the RST[0-7] markers).
 			//
 			// Note that extraneous 0xff bytes in e.g. SOS data are escaped as
 			// "\xff\x00", and so are detected a little further down below.
 			d.tmp[0] = d.tmp[1]
 			d.tmp[1], err = d.readByte()
 			if err != nil {
 				return nil, err
 			}
 		}
 		marker := d.tmp[1]
 		if marker == 0 {
 			// Treat "\xff\x00" as extraneous data.
 			continue
 		}
 		for marker == 0xff {
 			// Section B.1.1.2 says, "Any marker may optionally be preceded by any
 			// number of fill bytes, which are bytes assigned code X'FF'".
 			marker, err = d.readByte()
 			if err != nil {
 				return nil, err
 			}
 		}
 		if marker == eoiMarker { // End Of Image.
 			break
 		}
 		if rst0Marker <= marker && marker <= rst7Marker {
 			// Figures B.2 and B.16 of the specification suggest that restart markers should
 			// only occur between Entropy Coded Segments and not after the final ECS.
 			// However, some encoders may generate incorrect JPEGs with a final restart
 			// marker. That restart marker will be seen here instead of inside the processSOS
 			// method, and is ignored as a harmless error. Restart markers have no extra data,
 			// so we check for this before we read the 16-bit length of the segment.
 			continue
 		}

 		// Read the 16-bit length of the segment. The value includes the 2 bytes for the
 		// length itself, so we subtract 2 to get the number of remaining bytes.
 		if err = d.readFull(d.tmp[:2]); err != nil {
 			return nil, err
 		}
 		n := int(d.tmp[0])<<8 + int(d.tmp[1]) - 2
 		if n < 0 {
 			return nil, FormatError("short segment length")
 		}

 		switch {
 		case marker == sof0Marker || marker == sof2Marker: // Start Of Frame.
 			d.progressive = marker == sof2Marker
 			err = d.processSOF(n)
 			if configOnly {
 				return nil, err
 			}
 		case marker == dhtMarker: // Define Huffman Table.
 			err = d.processDHT(n)
 		case marker == dqtMarker: // Define Quantization Table.
 			err = d.processDQT(n)
 		case marker == sosMarker: // Start Of Scan.
 			err = d.processSOS(n)
 		case marker == driMarker: // Define Restart Interval.
 			err = d.processDRI(n)
 		case app0Marker <= marker && marker <= app15Marker || marker == comMarker: // APPlication specific, or COMment.
 			err = d.ignore(n)
 		default:
 			err = UnsupportedError("unknown marker")
 		}
 		if err != nil {
 			return nil, err
 		}
 	}
 	if d.img1 != nil {
 		return d.img1, nil
 	}
 	if d.img3 != nil {
 		return d.img3, nil
 	}
 	return nil, FormatError("missing SOS marker")
 }

 // Decode reads a JPEG image from r and returns it as an image.Image.
 func Decode(r io.Reader) (image.Image, error) {
 	var d decoder
 	return d.decode(r, false)
 }

 // DecodeConfig returns the color model and dimensions of a JPEG image without
 // decoding the entire image.
 func DecodeConfig(r io.Reader) (image.Config, error) {
 	var d decoder
 	if _, err := d.decode(r, true); err != nil {
 		return image.Config{}, err
 	}
 	switch d.nComp {
 	case nGrayComponent:
 		return image.Config{
 			ColorModel: color.GrayModel,
 			Width:      d.width,
 			Height:     d.height,
 		}, nil
 	case nColorComponent:
 		return image.Config{
 			ColorModel: color.YCbCrModel,
 			Width:      d.width,
 			Height:     d.height,
 		}, nil
 	}
 	return image.Config{}, FormatError("missing SOF marker")
 }

 func init() {
 	image.RegisterFormat("jpeg", "\xff\xd8", Decode, DecodeConfig)
 }
	// Copyright 2009 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// Package jpeg implements a JPEG image decoder and encoder.
	//
	// JPEG is defined in ITU-T T.81: http://www.w3.org/Graphics/JPEG/itu-t81.pdf.
	package jpeg

	import (
	"image"
	"image/color"
	"io"
	)

	// TODO(nigeltao): fix up the doc comment style so that sentences start with
	// the name of the type or function that they annotate.

	// A FormatError reports that the input is not a valid JPEG.
	type FormatError string

	func (e FormatError) Error() string { return "invalid JPEG format: " + string(e) }

	// An UnsupportedError reports that the input uses a valid but unimplemented JPEG feature.
	type UnsupportedError string

	func (e UnsupportedError) Error() string { return "unsupported JPEG feature: " + string(e) }

	// Component specification, specified in section B.2.2.
	type component struct {
	h int // Horizontal sampling factor.
	v int // Vertical sampling factor.
	c uint8 // Component identifier.
	tq uint8 // Quantization table destination selector.
	}

	const (
	dcTable = 0
	acTable = 1
	maxTc = 1
	maxTh = 3
	maxTq = 3

	// A grayscale JPEG image has only a Y component.
	nGrayComponent = 1
	// A color JPEG image has Y, Cb and Cr components.
	nColorComponent = 3

	// We only support 4:4:4, 4:4:0, 4:2:2 and 4:2:0 downsampling, and therefore the
	// number of luma samples per chroma sample is at most 2 in the horizontal
	// and 2 in the vertical direction.
	maxH = 2
	maxV = 2
	)

	const (
	soiMarker = 0xd8 // Start Of Image.
	eoiMarker = 0xd9 // End Of Image.
	sof0Marker = 0xc0 // Start Of Frame (Baseline).
	sof2Marker = 0xc2 // Start Of Frame (Progressive).
	dhtMarker = 0xc4 // Define Huffman Table.
	dqtMarker = 0xdb // Define Quantization Table.
	sosMarker = 0xda // Start Of Scan.
	driMarker = 0xdd // Define Restart Interval.
	rst0Marker = 0xd0 // ReSTart (0).
	rst7Marker = 0xd7 // ReSTart (7).
	app0Marker = 0xe0 // APPlication specific (0).
	app15Marker = 0xef // APPlication specific (15).
	comMarker = 0xfe // COMment.
	)

	// unzig maps from the zig-zag ordering to the natural ordering. For example,
	// unzig[3] is the column and row of the fourth element in zig-zag order. The
	// value is 16, which means first column (16%8 == 0) and third row (16/8 == 2).
	var unzig = [blockSize]int{
	0, 1, 8, 16, 9, 2, 3, 10,
	17, 24, 32, 25, 18, 11, 4, 5,
	12, 19, 26, 33, 40, 48, 41, 34,
	27, 20, 13, 6, 7, 14, 21, 28,
	35, 42, 49, 56, 57, 50, 43, 36,
	29, 22, 15, 23, 30, 37, 44, 51,
	58, 59, 52, 45, 38, 31, 39, 46,
	53, 60, 61, 54, 47, 55, 62, 63,
	}

	// Reader is deprecated.
	type Reader interface {
	io.ByteReader
	io.Reader
	}

	// bits holds the unprocessed bits that have been taken from the byte-stream.
	// The n least significant bits of a form the unread bits, to be read in MSB to
	// LSB order.
	type bits struct {
	a uint32 // accumulator.
	m uint32 // mask. m==1<<(n-1) when n>0, with m==0 when n==0.
	n int32 // the number of unread bits in a.
	}

	type decoder struct {
	r io.Reader
	bits bits
	// bytes is a byte buffer, similar to a bufio.Reader, except that it
	// has to be able to unread more than 1 byte, due to byte stuffing.
	// Byte stuffing is specified in section F.1.2.3.
	bytes struct {
	// buf[i:j] are the buffered bytes read from the underlying
	// io.Reader that haven't yet been passed further on.
	buf [4096]byte
	i, j int
	// nUnreadable is the number of bytes to back up i after
	// overshooting. It can be 0, 1 or 2.
	nUnreadable int
	}
	width, height int
	img1 *image.Gray
	img3 *image.YCbCr
	ri int // Restart Interval.
	nComp int
	progressive bool
	eobRun uint16 // End-of-Band run, specified in section G.1.2.2.
	comp [nColorComponent]component
	progCoeffs [nColorComponent][]block // Saved state between progressive-mode scans.
	huff [maxTc + 1][maxTh + 1]huffman
	quant [maxTq + 1]block // Quantization tables, in zig-zag order.
	tmp [blockSize + 1]byte
	}

	// fill fills up the d.bytes.buf buffer from the underlying io.Reader. It
	// should only be called when there are no unread bytes in d.bytes.
	func (d *decoder) fill() error {
	if d.bytes.i != d.bytes.j {
	panic("jpeg: fill called when unread bytes exist")
	}
	// Move the last 2 bytes to the start of the buffer, in case we need
	// to call unreadByteStuffedByte.
	if d.bytes.j > 2 {
	d.bytes.buf[0] = d.bytes.buf[d.bytes.j-2]
	d.bytes.buf[1] = d.bytes.buf[d.bytes.j-1]
	d.bytes.i, d.bytes.j = 2, 2
	}
	// Fill in the rest of the buffer.
	n, err := d.r.Read(d.bytes.buf[d.bytes.j:])
	d.bytes.j += n
	if n > 0 {
	err = nil
	}
	return err
	}

	// unreadByteStuffedByte undoes the most recent readByteStuffedByte call,
	// giving a byte of data back from d.bits to d.bytes. The Huffman look-up table
	// requires at least 8 bits for look-up, which means that Huffman decoding can
	// sometimes overshoot and read one or two too many bytes. Two-byte overshoot
	// can happen when expecting to read a 0xff 0x00 byte-stuffed byte.
	func (d *decoder) unreadByteStuffedByte() {
	if d.bytes.nUnreadable == 0 {
	panic("jpeg: unreadByteStuffedByte call cannot be fulfilled")
	}
	d.bytes.i -= d.bytes.nUnreadable
	d.bytes.nUnreadable = 0
	if d.bits.n >= 8 {
	d.bits.a >>= 8
	d.bits.n -= 8
	d.bits.m >>= 8
	}
	}

	// readByte returns the next byte, whether buffered or not buffered. It does
	// not care about byte stuffing.
	func (d *decoder) readByte() (x byte, err error) {
	for d.bytes.i == d.bytes.j {
	if err = d.fill(); err != nil {
	return 0, err
	}
	}
	x = d.bytes.buf[d.bytes.i]
	d.bytes.i++
	d.bytes.nUnreadable = 0
	return x, nil
	}

	// errMissingFF00 means that readByteStuffedByte encountered an 0xff byte (a
	// marker byte) that wasn't the expected byte-stuffed sequence 0xff, 0x00.
	var errMissingFF00 = FormatError("missing 0xff00 sequence")

	// readByteStuffedByte is like readByte but is for byte-stuffed Huffman data.
	func (d *decoder) readByteStuffedByte() (x byte, err error) {
	// Take the fast path if d.bytes.buf contains at least two bytes.
	if d.bytes.i+2 <= d.bytes.j {
	x = d.bytes.buf[d.bytes.i]
	d.bytes.i++
	d.bytes.nUnreadable = 1
	if x != 0xff {
	return x, err
	}
	if d.bytes.buf[d.bytes.i] != 0x00 {
	return 0, errMissingFF00
	}
	d.bytes.i++
	d.bytes.nUnreadable = 2
	return 0xff, nil
	}

	x, err = d.readByte()
	if err != nil {
	return 0, err
	}
	if x != 0xff {
	d.bytes.nUnreadable = 1
	return x, nil
	}

	x, err = d.readByte()
	if err != nil {
	d.bytes.nUnreadable = 1
	return 0, err
	}
	d.bytes.nUnreadable = 2
	if x != 0x00 {
	return 0, errMissingFF00
	}
	return 0xff, nil
	}

	// readFull reads exactly len(p) bytes into p. It does not care about byte
	// stuffing.
	func (d *decoder) readFull(p []byte) error {
	// Unread the overshot bytes, if any.
	if d.bytes.nUnreadable != 0 {
	if d.bits.n >= 8 {
	d.unreadByteStuffedByte()
	}
	d.bytes.nUnreadable = 0
	}

	for {
	n := copy(p, d.bytes.buf[d.bytes.i:d.bytes.j])
	p = p[n:]
	d.bytes.i += n
	if len(p) == 0 {
	break
	}
	if err := d.fill(); err != nil {
	if err == io.EOF {
	err = io.ErrUnexpectedEOF
	}
	return err
	}
	}
	return nil
	}

	// ignore ignores the next n bytes.
	func (d *decoder) ignore(n int) error {
	// Unread the overshot bytes, if any.
	if d.bytes.nUnreadable != 0 {
	if d.bits.n >= 8 {
	d.unreadByteStuffedByte()
	}
	d.bytes.nUnreadable = 0
	}

	for {
	m := d.bytes.j - d.bytes.i
	if m > n {
	m = n
	}
	d.bytes.i += m
	n -= m
	if n == 0 {
	break
	}
	if err := d.fill(); err != nil {
	if err == io.EOF {
	err = io.ErrUnexpectedEOF
	}
	return err
	}
	}
	return nil
	}

	// Specified in section B.2.2.
	func (d *decoder) processSOF(n int) error {
	switch n {
	case 6 + 3*nGrayComponent:
	d.nComp = nGrayComponent
	case 6 + 3*nColorComponent:
	d.nComp = nColorComponent
	default:
	return UnsupportedError("SOF has wrong length")
	}
	if err := d.readFull(d.tmp[:n]); err != nil {
	return err
	}
	// We only support 8-bit precision.
	if d.tmp[0] != 8 {
	return UnsupportedError("precision")
	}
	d.height = int(d.tmp[1])<<8 + int(d.tmp[2])
	d.width = int(d.tmp[3])<<8 + int(d.tmp[4])
	if int(d.tmp[5]) != d.nComp {
	return UnsupportedError("SOF has wrong number of image components")
	}
	for i := 0; i < d.nComp; i++ {
	d.comp[i].c = d.tmp[6+3*i]
	d.comp[i].tq = d.tmp[8+3*i]
	if d.nComp == nGrayComponent {
	// If a JPEG image has only one component, section A.2 says "this data
	// is non-interleaved by definition" and section A.2.2 says "[in this
	// case...] the order of data units within a scan shall be left-to-right
	// and top-to-bottom... regardless of the values of H_1 and V_1". Section
	// 4.8.2 also says "[for non-interleaved data], the MCU is defined to be
	// one data unit". Similarly, section A.1.1 explains that it is the ratio
	// of H_i to max_j(H_j) that matters, and similarly for V. For grayscale
	// images, H_1 is the maximum H_j for all components j, so that ratio is
	// always 1. The component's (h, v) is effectively always (1, 1): even if
	// the nominal (h, v) is (2, 1), a 20x5 image is encoded in three 8x8
	// MCUs, not two 16x8 MCUs.
	d.comp[i].h = 1
	d.comp[i].v = 1
	continue
	}
	hv := d.tmp[7+3*i]
	d.comp[i].h = int(hv >> 4)
	d.comp[i].v = int(hv & 0x0f)
	// For color images, we only support 4:4:4, 4:4:0, 4:2:2 or 4:2:0 chroma
	// downsampling ratios. This implies that the (h, v) values for the Y
	// component are either (1, 1), (1, 2), (2, 1) or (2, 2), and the (h, v)
	// values for the Cr and Cb components must be (1, 1).
	if i == 0 {
	if hv != 0x11 && hv != 0x21 && hv != 0x22 && hv != 0x12 {
	return UnsupportedError("luma/chroma downsample ratio")
	}
	} else if hv != 0x11 {
	return UnsupportedError("luma/chroma downsample ratio")
	}
	}
	return nil
	}

	// Specified in section B.2.4.1.
	func (d *decoder) processDQT(n int) error {
	const qtLength = 1 + blockSize
	for ; n >= qtLength; n -= qtLength {
	if err := d.readFull(d.tmp[:qtLength]); err != nil {
	return err
	}
	pq := d.tmp[0] >> 4
	if pq != 0 {
	return UnsupportedError("bad Pq value")
	}
	tq := d.tmp[0] & 0x0f
	if tq > maxTq {
	return FormatError("bad Tq value")
	}
	for i := range d.quant[tq] {
	d.quant[tq][i] = int32(d.tmp[i+1])
	}
	}
	if n != 0 {
	return FormatError("DQT has wrong length")
	}
	return nil
	}

	// Specified in section B.2.4.4.
	func (d *decoder) processDRI(n int) error {
	if n != 2 {
	return FormatError("DRI has wrong length")
	}
	if err := d.readFull(d.tmp[:2]); err != nil {
	return err
	}
	d.ri = int(d.tmp[0])<<8 + int(d.tmp[1])
	return nil
	}

	// decode reads a JPEG image from r and returns it as an image.Image.
	func (d *decoder) decode(r io.Reader, configOnly bool) (image.Image, error) {
	d.r = r

	// Check for the Start Of Image marker.
	if err := d.readFull(d.tmp[:2]); err != nil {
	return nil, err
	}
	if d.tmp[0] != 0xff \|\| d.tmp[1] != soiMarker {
	return nil, FormatError("missing SOI marker")
	}

	// Process the remaining segments until the End Of Image marker.
	for {
	err := d.readFull(d.tmp[:2])
	if err != nil {
	return nil, err
	}
	for d.tmp[0] != 0xff {
	// Strictly speaking, this is a format error. However, libjpeg is
	// liberal in what it accepts. As of version 9, next_marker in
	// jdmarker.c treats this as a warning (JWRN_EXTRANEOUS_DATA) and
	// continues to decode the stream. Even before next_marker sees
	// extraneous data, jpeg_fill_bit_buffer in jdhuff.c reads as many
	// bytes as it can, possibly past the end of a scan's data. It
	// effectively puts back any markers that it overscanned (e.g. an
	// "\xff\xd9" EOI marker), but it does not put back non-marker data,
	// and thus it can silently ignore a small number of extraneous
	// non-marker bytes before next_marker has a chance to see them (and
	// print a warning).
	//
	// We are therefore also liberal in what we accept. Extraneous data
	// is silently ignored.
	//
	// This is similar to, but not exactly the same as, the restart
	// mechanism within a scan (the RST[0-7] markers).
	//
	// Note that extraneous 0xff bytes in e.g. SOS data are escaped as
	// "\xff\x00", and so are detected a little further down below.
	d.tmp[0] = d.tmp[1]
	d.tmp[1], err = d.readByte()
	if err != nil {
	return nil, err
	}
	}
	marker := d.tmp[1]
	if marker == 0 {
	// Treat "\xff\x00" as extraneous data.
	continue
	}
	for marker == 0xff {
	// Section B.1.1.2 says, "Any marker may optionally be preceded by any
	// number of fill bytes, which are bytes assigned code X'FF'".
	marker, err = d.readByte()
	if err != nil {
	return nil, err
	}
	}
	if marker == eoiMarker { // End Of Image.
	break
	}
	if rst0Marker <= marker && marker <= rst7Marker {
	// Figures B.2 and B.16 of the specification suggest that restart markers should
	// only occur between Entropy Coded Segments and not after the final ECS.
	// However, some encoders may generate incorrect JPEGs with a final restart
	// marker. That restart marker will be seen here instead of inside the processSOS
	// method, and is ignored as a harmless error. Restart markers have no extra data,
	// so we check for this before we read the 16-bit length of the segment.
	continue
	}

	// Read the 16-bit length of the segment. The value includes the 2 bytes for the
	// length itself, so we subtract 2 to get the number of remaining bytes.
	if err = d.readFull(d.tmp[:2]); err != nil {
	return nil, err
	}
	n := int(d.tmp[0])<<8 + int(d.tmp[1]) - 2
	if n < 0 {
	return nil, FormatError("short segment length")
	}

	switch {
	case marker == sof0Marker \|\| marker == sof2Marker: // Start Of Frame.
	d.progressive = marker == sof2Marker
	err = d.processSOF(n)
	if configOnly {
	return nil, err
	}
	case marker == dhtMarker: // Define Huffman Table.
	err = d.processDHT(n)
	case marker == dqtMarker: // Define Quantization Table.
	err = d.processDQT(n)
	case marker == sosMarker: // Start Of Scan.
	err = d.processSOS(n)
	case marker == driMarker: // Define Restart Interval.
	err = d.processDRI(n)
	case app0Marker <= marker && marker <= app15Marker \|\| marker == comMarker: // APPlication specific, or COMment.
	err = d.ignore(n)
	default:
	err = UnsupportedError("unknown marker")
	}
	if err != nil {
	return nil, err
	}
	}
	if d.img1 != nil {
	return d.img1, nil
	}
	if d.img3 != nil {
	return d.img3, nil
	}
	return nil, FormatError("missing SOS marker")
	}

	// Decode reads a JPEG image from r and returns it as an image.Image.
	func Decode(r io.Reader) (image.Image, error) {
	var d decoder
	return d.decode(r, false)
	}

	// DecodeConfig returns the color model and dimensions of a JPEG image without
	// decoding the entire image.
	func DecodeConfig(r io.Reader) (image.Config, error) {
	var d decoder
	if _, err := d.decode(r, true); err != nil {
	return image.Config{}, err
	}
	switch d.nComp {
	case nGrayComponent:
	return image.Config{
	ColorModel: color.GrayModel,
	Width: d.width,
	Height: d.height,
	}, nil
	case nColorComponent:
	return image.Config{
	ColorModel: color.YCbCrModel,
	Width: d.width,
	Height: d.height,
	}, nil
	}
	return image.Config{}, FormatError("missing SOF marker")
	}

	func init() {
	image.RegisterFormat("jpeg", "\xff\xd8", Decode, DecodeConfig)
	}