src/pkg/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 (
 	"bufio"
 	"image"
 	"image/ycbcr"
 	"io"
 	"os"
 )

 // 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) String() 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) String() 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.
 }

 type block [blockSize]int

 const (
 	blockSize = 64 // A DCT block is 8x8.

 	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, 4:2:2 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.
 )

 // Maps from the zig-zag ordering to the natural ordering.
 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,
 }

 // If the passed in io.Reader does not also have ReadByte, then Decode will introduce its own buffering.
 type Reader interface {
 	io.Reader
 	ReadByte() (c byte, err os.Error)
 }

 type decoder struct {
 	r             Reader
 	width, height int
 	img1          *image.Gray
 	img3          *ycbcr.YCbCr
 	ri            int // Restart Interval.
 	nComp         int
 	comp          [nColorComponent]component
 	huff          [maxTc + 1][maxTh + 1]huffman
 	quant         [maxTq + 1]block
 	b             bits
 	tmp           [1024]byte
 }

 // Reads and ignores the next n bytes.
 func (d *decoder) ignore(n int) os.Error {
 	for n > 0 {
 		m := len(d.tmp)
 		if m > n {
 			m = n
 		}
 		_, err := io.ReadFull(d.r, d.tmp[0:m])
 		if err != nil {
 			return err
 		}
 		n -= m
 	}
 	return nil
 }

 // Specified in section B.2.2.
 func (d *decoder) processSOF(n int) os.Error {
 	switch n {
 	case 6 + 3*nGrayComponent:
 		d.nComp = nGrayComponent
 	case 6 + 3*nColorComponent:
 		d.nComp = nColorComponent
 	default:
 		return UnsupportedError("SOF has wrong length")
 	}
 	_, err := io.ReadFull(d.r, d.tmp[:n])
 	if 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++ {
 		hv := d.tmp[7+3*i]
 		d.comp[i].h = int(hv >> 4)
 		d.comp[i].v = int(hv & 0x0f)
 		d.comp[i].c = d.tmp[6+3*i]
 		d.comp[i].tq = d.tmp[8+3*i]
 		if d.nComp == nGrayComponent {
 			continue
 		}
 		// For color images, we only support 4:4:4, 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), (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 {
 				return UnsupportedError("luma downsample ratio")
 			}
 		} else if hv != 0x11 {
 			return UnsupportedError("chroma downsample ratio")
 		}
 	}
 	return nil
 }

 // Specified in section B.2.4.1.
 func (d *decoder) processDQT(n int) os.Error {
 	const qtLength = 1 + blockSize
 	for ; n >= qtLength; n -= qtLength {
 		_, err := io.ReadFull(d.r, d.tmp[0:qtLength])
 		if 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] = int(d.tmp[i+1])
 		}
 	}
 	if n != 0 {
 		return FormatError("DQT has wrong length")
 	}
 	return nil
 }

 // makeImg allocates and initializes the destination image.
 func (d *decoder) makeImg(h0, v0, mxx, myy int) {
 	if d.nComp == nGrayComponent {
 		m := image.NewGray(8*mxx, 8*myy)
 		d.img1 = m.SubImage(image.Rect(0, 0, d.width, d.height)).(*image.Gray)
 		return
 	}
 	var subsampleRatio ycbcr.SubsampleRatio
 	n := h0 * v0
 	switch n {
 	case 1:
 		subsampleRatio = ycbcr.SubsampleRatio444
 	case 2:
 		subsampleRatio = ycbcr.SubsampleRatio422
 	case 4:
 		subsampleRatio = ycbcr.SubsampleRatio420
 	default:
 		panic("unreachable")
 	}
 	b := make([]byte, mxx*myy*(1*8*8*n+2*8*8))
 	d.img3 = &ycbcr.YCbCr{
 		Y:              b[mxx*myy*(0*8*8*n+0*8*8) : mxx*myy*(1*8*8*n+0*8*8)],
 		Cb:             b[mxx*myy*(1*8*8*n+0*8*8) : mxx*myy*(1*8*8*n+1*8*8)],
 		Cr:             b[mxx*myy*(1*8*8*n+1*8*8) : mxx*myy*(1*8*8*n+2*8*8)],
 		SubsampleRatio: subsampleRatio,
 		YStride:        mxx * 8 * h0,
 		CStride:        mxx * 8,
 		Rect:           image.Rect(0, 0, d.width, d.height),
 	}
 }

 // Specified in section B.2.3.
 func (d *decoder) processSOS(n int) os.Error {
 	if d.nComp == 0 {
 		return FormatError("missing SOF marker")
 	}
 	if n != 4+2*d.nComp {
 		return UnsupportedError("SOS has wrong length")
 	}
 	_, err := io.ReadFull(d.r, d.tmp[0:4+2*d.nComp])
 	if err != nil {
 		return err
 	}
 	if int(d.tmp[0]) != d.nComp {
 		return UnsupportedError("SOS has wrong number of image components")
 	}
 	var scan [nColorComponent]struct {
 		td uint8 // DC table selector.
 		ta uint8 // AC table selector.
 	}
 	for i := 0; i < d.nComp; i++ {
 		cs := d.tmp[1+2*i] // Component selector.
 		if cs != d.comp[i].c {
 			return UnsupportedError("scan components out of order")
 		}
 		scan[i].td = d.tmp[2+2*i] >> 4
 		scan[i].ta = d.tmp[2+2*i] & 0x0f
 	}
 	// mxx and myy are the number of MCUs (Minimum Coded Units) in the image.
 	h0, v0 := d.comp[0].h, d.comp[0].v // The h and v values from the Y components.
 	mxx := (d.width + 8*h0 - 1) / (8 * h0)
 	myy := (d.height + 8*v0 - 1) / (8 * v0)
 	if d.img1 == nil && d.img3 == nil {
 		d.makeImg(h0, v0, mxx, myy)
 	}

 	mcu, expectedRST := 0, uint8(rst0Marker)
 	var (
 		b  block
 		dc [nColorComponent]int
 	)
 	for my := 0; my < myy; my++ {
 		for mx := 0; mx < mxx; mx++ {
 			for i := 0; i < d.nComp; i++ {
 				qt := &d.quant[d.comp[i].tq]
 				for j := 0; j < d.comp[i].h*d.comp[i].v; j++ {
 					// TODO(nigeltao): make this a "var b block" once the compiler's escape
 					// analysis is good enough to allocate it on the stack, not the heap.
 					b = block{}

 					// Decode the DC coefficient, as specified in section F.2.2.1.
 					value, err := d.decodeHuffman(&d.huff[dcTable][scan[i].td])
 					if err != nil {
 						return err
 					}
 					if value > 16 {
 						return UnsupportedError("excessive DC component")
 					}
 					dcDelta, err := d.receiveExtend(value)
 					if err != nil {
 						return err
 					}
 					dc[i] += dcDelta
 					b[0] = dc[i] * qt[0]

 					// Decode the AC coefficients, as specified in section F.2.2.2.
 					for k := 1; k < blockSize; k++ {
 						value, err := d.decodeHuffman(&d.huff[acTable][scan[i].ta])
 						if err != nil {
 							return err
 						}
 						val0 := value >> 4
 						val1 := value & 0x0f
 						if val1 != 0 {
 							k += int(val0)
 							if k > blockSize {
 								return FormatError("bad DCT index")
 							}
 							ac, err := d.receiveExtend(val1)
 							if err != nil {
 								return err
 							}
 							b[unzig[k]] = ac * qt[k]
 						} else {
 							if val0 != 0x0f {
 								break
 							}
 							k += 0x0f
 						}
 					}

 					// Perform the inverse DCT and store the MCU component to the image.
 					if d.nComp == nGrayComponent {
 						idct(d.img1.Pix[8*(my*d.img1.Stride+mx):], d.img1.Stride, &b)
 					} else {
 						switch i {
 						case 0:
 							mx0 := h0*mx + (j % 2)
 							my0 := v0*my + (j / 2)
 							idct(d.img3.Y[8*(my0*d.img3.YStride+mx0):], d.img3.YStride, &b)
 						case 1:
 							idct(d.img3.Cb[8*(my*d.img3.CStride+mx):], d.img3.CStride, &b)
 						case 2:
 							idct(d.img3.Cr[8*(my*d.img3.CStride+mx):], d.img3.CStride, &b)
 						}
 					}
 				} // for j
 			} // for i
 			mcu++
 			if d.ri > 0 && mcu%d.ri == 0 && mcu < mxx*myy {
 				// A more sophisticated decoder could use RST[0-7] markers to resynchronize from corrupt input,
 				// but this one assumes well-formed input, and hence the restart marker follows immediately.
 				_, err := io.ReadFull(d.r, d.tmp[0:2])
 				if err != nil {
 					return err
 				}
 				if d.tmp[0] != 0xff || d.tmp[1] != expectedRST {
 					return FormatError("bad RST marker")
 				}
 				expectedRST++
 				if expectedRST == rst7Marker+1 {
 					expectedRST = rst0Marker
 				}
 				// Reset the Huffman decoder.
 				d.b = bits{}
 				// Reset the DC components, as per section F.2.1.3.1.
 				dc = [nColorComponent]int{}
 			}
 		} // for mx
 	} // for my

 	return nil
 }

 // Specified in section B.2.4.4.
 func (d *decoder) processDRI(n int) os.Error {
 	if n != 2 {
 		return FormatError("DRI has wrong length")
 	}
 	_, err := io.ReadFull(d.r, d.tmp[0:2])
 	if 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, os.Error) {
 	if rr, ok := r.(Reader); ok {
 		d.r = rr
 	} else {
 		d.r = bufio.NewReader(r)
 	}

 	// Check for the Start Of Image marker.
 	_, err := io.ReadFull(d.r, d.tmp[0:2])
 	if 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 := io.ReadFull(d.r, d.tmp[0:2])
 		if err != nil {
 			return nil, err
 		}
 		if d.tmp[0] != 0xff {
 			return nil, FormatError("missing 0xff marker start")
 		}
 		marker := d.tmp[1]
 		if marker == eoiMarker { // End Of Image.
 			break
 		}

 		// 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.
 		_, err = io.ReadFull(d.r, d.tmp[0:2])
 		if 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: // Start Of Frame (Baseline).
 			err = d.processSOF(n)
 			if configOnly {
 				return nil, err
 			}
 		case marker == sof2Marker: // Start Of Frame (Progressive).
 			err = UnsupportedError("progressive mode")
 		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 marker >= app0Marker && 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, os.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, os.Error) {
 	var d decoder
 	if _, err := d.decode(r, true); err != nil {
 		return image.Config{}, err
 	}
 	switch d.nComp {
 	case nGrayComponent:
 		return image.Config{image.GrayColorModel, d.width, d.height}, nil
 	case nColorComponent:
 		return image.Config{ycbcr.YCbCrColorModel, d.width, 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 (
	"bufio"
	"image"
	"image/ycbcr"
	"io"
	"os"
	)

	// 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) String() 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) String() 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.
	}

	type block [blockSize]int

	const (
	blockSize = 64 // A DCT block is 8x8.

	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, 4:2:2 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.
	)

	// Maps from the zig-zag ordering to the natural ordering.
	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,
	}

	// If the passed in io.Reader does not also have ReadByte, then Decode will introduce its own buffering.
	type Reader interface {
	io.Reader
	ReadByte() (c byte, err os.Error)
	}

	type decoder struct {
	r Reader
	width, height int
	img1 *image.Gray
	img3 *ycbcr.YCbCr
	ri int // Restart Interval.
	nComp int
	comp [nColorComponent]component
	huff [maxTc + 1][maxTh + 1]huffman
	quant [maxTq + 1]block
	b bits
	tmp [1024]byte
	}

	// Reads and ignores the next n bytes.
	func (d *decoder) ignore(n int) os.Error {
	for n > 0 {
	m := len(d.tmp)
	if m > n {
	m = n
	}
	_, err := io.ReadFull(d.r, d.tmp[0:m])
	if err != nil {
	return err
	}
	n -= m
	}
	return nil
	}

	// Specified in section B.2.2.
	func (d *decoder) processSOF(n int) os.Error {
	switch n {
	case 6 + 3*nGrayComponent:
	d.nComp = nGrayComponent
	case 6 + 3*nColorComponent:
	d.nComp = nColorComponent
	default:
	return UnsupportedError("SOF has wrong length")
	}
	_, err := io.ReadFull(d.r, d.tmp[:n])
	if 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++ {
	hv := d.tmp[7+3*i]
	d.comp[i].h = int(hv >> 4)
	d.comp[i].v = int(hv & 0x0f)
	d.comp[i].c = d.tmp[6+3*i]
	d.comp[i].tq = d.tmp[8+3*i]
	if d.nComp == nGrayComponent {
	continue
	}
	// For color images, we only support 4:4:4, 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), (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 {
	return UnsupportedError("luma downsample ratio")
	}
	} else if hv != 0x11 {
	return UnsupportedError("chroma downsample ratio")
	}
	}
	return nil
	}

	// Specified in section B.2.4.1.
	func (d *decoder) processDQT(n int) os.Error {
	const qtLength = 1 + blockSize
	for ; n >= qtLength; n -= qtLength {
	_, err := io.ReadFull(d.r, d.tmp[0:qtLength])
	if 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] = int(d.tmp[i+1])
	}
	}
	if n != 0 {
	return FormatError("DQT has wrong length")
	}
	return nil
	}

	// makeImg allocates and initializes the destination image.
	func (d *decoder) makeImg(h0, v0, mxx, myy int) {
	if d.nComp == nGrayComponent {
	m := image.NewGray(8mxx, 8myy)
	d.img1 = m.SubImage(image.Rect(0, 0, d.width, d.height)).(*image.Gray)
	return
	}
	var subsampleRatio ycbcr.SubsampleRatio
	n := h0 * v0
	switch n {
	case 1:
	subsampleRatio = ycbcr.SubsampleRatio444
	case 2:
	subsampleRatio = ycbcr.SubsampleRatio422
	case 4:
	subsampleRatio = ycbcr.SubsampleRatio420
	default:
	panic("unreachable")
	}
	b := make([]byte, mxxmyy(188n+28*8))
	d.img3 = &ycbcr.YCbCr{
	Y: b[mxxmyy(088n+088) : mxxmyy(188n+088)],
	Cb: b[mxxmyy(188n+088) : mxxmyy(188n+188)],
	Cr: b[mxxmyy(188n+188) : mxxmyy(188n+288)],
	SubsampleRatio: subsampleRatio,
	YStride: mxx * 8 * h0,
	CStride: mxx * 8,
	Rect: image.Rect(0, 0, d.width, d.height),
	}
	}

	// Specified in section B.2.3.
	func (d *decoder) processSOS(n int) os.Error {
	if d.nComp == 0 {
	return FormatError("missing SOF marker")
	}
	if n != 4+2*d.nComp {
	return UnsupportedError("SOS has wrong length")
	}
	_, err := io.ReadFull(d.r, d.tmp[0:4+2*d.nComp])
	if err != nil {
	return err
	}
	if int(d.tmp[0]) != d.nComp {
	return UnsupportedError("SOS has wrong number of image components")
	}
	var scan [nColorComponent]struct {
	td uint8 // DC table selector.
	ta uint8 // AC table selector.
	}
	for i := 0; i < d.nComp; i++ {
	cs := d.tmp[1+2*i] // Component selector.
	if cs != d.comp[i].c {
	return UnsupportedError("scan components out of order")
	}
	scan[i].td = d.tmp[2+2*i] >> 4
	scan[i].ta = d.tmp[2+2*i] & 0x0f
	}
	// mxx and myy are the number of MCUs (Minimum Coded Units) in the image.
	h0, v0 := d.comp[0].h, d.comp[0].v // The h and v values from the Y components.
	mxx := (d.width + 8h0 - 1) / (8 h0)
	myy := (d.height + 8v0 - 1) / (8 v0)
	if d.img1 == nil && d.img3 == nil {
	d.makeImg(h0, v0, mxx, myy)
	}

	mcu, expectedRST := 0, uint8(rst0Marker)
	var (
	b block
	dc [nColorComponent]int
	)
	for my := 0; my < myy; my++ {
	for mx := 0; mx < mxx; mx++ {
	for i := 0; i < d.nComp; i++ {
	qt := &d.quant[d.comp[i].tq]
	for j := 0; j < d.comp[i].h*d.comp[i].v; j++ {
	// TODO(nigeltao): make this a "var b block" once the compiler's escape
	// analysis is good enough to allocate it on the stack, not the heap.
	b = block{}

	// Decode the DC coefficient, as specified in section F.2.2.1.
	value, err := d.decodeHuffman(&d.huff[dcTable][scan[i].td])
	if err != nil {
	return err
	}
	if value > 16 {
	return UnsupportedError("excessive DC component")
	}
	dcDelta, err := d.receiveExtend(value)
	if err != nil {
	return err
	}
	dc[i] += dcDelta
	b[0] = dc[i] * qt[0]

	// Decode the AC coefficients, as specified in section F.2.2.2.
	for k := 1; k < blockSize; k++ {
	value, err := d.decodeHuffman(&d.huff[acTable][scan[i].ta])
	if err != nil {
	return err
	}
	val0 := value >> 4
	val1 := value & 0x0f
	if val1 != 0 {
	k += int(val0)
	if k > blockSize {
	return FormatError("bad DCT index")
	}
	ac, err := d.receiveExtend(val1)
	if err != nil {
	return err
	}
	b[unzig[k]] = ac * qt[k]
	} else {
	if val0 != 0x0f {
	break
	}
	k += 0x0f
	}
	}

	// Perform the inverse DCT and store the MCU component to the image.
	if d.nComp == nGrayComponent {
	idct(d.img1.Pix[8(myd.img1.Stride+mx):], d.img1.Stride, &b)
	} else {
	switch i {
	case 0:
	mx0 := h0*mx + (j % 2)
	my0 := v0*my + (j / 2)
	idct(d.img3.Y[8(my0d.img3.YStride+mx0):], d.img3.YStride, &b)
	case 1:
	idct(d.img3.Cb[8(myd.img3.CStride+mx):], d.img3.CStride, &b)
	case 2:
	idct(d.img3.Cr[8(myd.img3.CStride+mx):], d.img3.CStride, &b)
	}
	}
	} // for j
	} // for i
	mcu++
	if d.ri > 0 && mcu%d.ri == 0 && mcu < mxx*myy {
	// A more sophisticated decoder could use RST[0-7] markers to resynchronize from corrupt input,
	// but this one assumes well-formed input, and hence the restart marker follows immediately.
	_, err := io.ReadFull(d.r, d.tmp[0:2])
	if err != nil {
	return err
	}
	if d.tmp[0] != 0xff \|\| d.tmp[1] != expectedRST {
	return FormatError("bad RST marker")
	}
	expectedRST++
	if expectedRST == rst7Marker+1 {
	expectedRST = rst0Marker
	}
	// Reset the Huffman decoder.
	d.b = bits{}
	// Reset the DC components, as per section F.2.1.3.1.
	dc = [nColorComponent]int{}
	}
	} // for mx
	} // for my

	return nil
	}

	// Specified in section B.2.4.4.
	func (d *decoder) processDRI(n int) os.Error {
	if n != 2 {
	return FormatError("DRI has wrong length")
	}
	_, err := io.ReadFull(d.r, d.tmp[0:2])
	if 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, os.Error) {
	if rr, ok := r.(Reader); ok {
	d.r = rr
	} else {
	d.r = bufio.NewReader(r)
	}

	// Check for the Start Of Image marker.
	_, err := io.ReadFull(d.r, d.tmp[0:2])
	if 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 := io.ReadFull(d.r, d.tmp[0:2])
	if err != nil {
	return nil, err
	}
	if d.tmp[0] != 0xff {
	return nil, FormatError("missing 0xff marker start")
	}
	marker := d.tmp[1]
	if marker == eoiMarker { // End Of Image.
	break
	}

	// 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.
	_, err = io.ReadFull(d.r, d.tmp[0:2])
	if 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: // Start Of Frame (Baseline).
	err = d.processSOF(n)
	if configOnly {
	return nil, err
	}
	case marker == sof2Marker: // Start Of Frame (Progressive).
	err = UnsupportedError("progressive mode")
	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 marker >= app0Marker && 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, os.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, os.Error) {
	var d decoder
	if _, err := d.decode(r, true); err != nil {
	return image.Config{}, err
	}
	switch d.nComp {
	case nGrayComponent:
	return image.Config{image.GrayColorModel, d.width, d.height}, nil
	case nColorComponent:
	return image.Config{ycbcr.YCbCrColorModel, d.width, d.height}, nil
	}
	return image.Config{}, FormatError("missing SOF marker")
	}

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