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.

 // The jpeg package implements a decoder for JPEG images, as defined in ITU-T T.81.
 package jpeg

 // See http://www.w3.org/Graphics/JPEG/itu-t81.pdf

 import (
 	"bufio"
 	"image"
 	"io"
 	"os"
 )

 // 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 {
 	c  uint8 // Component identifier.
 	h  uint8 // Horizontal sampling factor.
 	v  uint8 // Vertical sampling factor.
 	tq uint8 // Quantization table destination selector.
 }

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

 	dcTableClass = 0
 	acTableClass = 1
 	maxTc        = 1
 	maxTh        = 3
 	maxTq        = 3

 	// We only support 4:4:4, 4:2:2 and 4:2:0 downsampling, and assume that the components are Y, Cb, Cr.
 	nComponent = 3
 	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
 	image         *image.RGBA
 	ri            int // Restart Interval.
 	comps         [nComponent]component
 	huff          [maxTc + 1][maxTh + 1]huffman
 	quant         [maxTq + 1][blockSize]int
 	b             bits
 	blocks        [nComponent][maxH * maxV][blockSize]int
 	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 {
 	if n != 6+3*nComponent {
 		return UnsupportedError("SOF has wrong length")
 	}
 	_, err := io.ReadFull(d.r, d.tmp[0:6+3*nComponent])
 	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 d.tmp[5] != nComponent {
 		return UnsupportedError("SOF has wrong number of image components")
 	}
 	for i := 0; i < nComponent; i++ {
 		hv := d.tmp[7+3*i]
 		d.comps[i].c = d.tmp[6+3*i]
 		d.comps[i].h = hv >> 4
 		d.comps[i].v = hv & 0x0f
 		d.comps[i].tq = d.tmp[8+3*i]
 		// We only support YCbCr images, and 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
 }

 // Set the Pixel (px, py)'s RGB value, based on its YCbCr value.
 func (d *decoder) calcPixel(px, py, lumaBlock, lumaIndex, chromaIndex int) {
 	y, cb, cr := d.blocks[0][lumaBlock][lumaIndex], d.blocks[1][0][chromaIndex], d.blocks[2][0][chromaIndex]
 	// The JFIF specification (http://www.w3.org/Graphics/JPEG/jfif3.pdf, page 3) gives the formula
 	// for translating YCbCr to RGB as:
 	//   R = Y + 1.402 (Cr-128)
 	//   G = Y - 0.34414 (Cb-128) - 0.71414 (Cr-128)
 	//   B = Y + 1.772 (Cb-128)
 	yPlusHalf := 100000*y + 50000
 	cb -= 128
 	cr -= 128
 	r := (yPlusHalf + 140200*cr) / 100000
 	g := (yPlusHalf - 34414*cb - 71414*cr) / 100000
 	b := (yPlusHalf + 177200*cb) / 100000
 	if r < 0 {
 		r = 0
 	} else if r > 255 {
 		r = 255
 	}
 	if g < 0 {
 		g = 0
 	} else if g > 255 {
 		g = 255
 	}
 	if b < 0 {
 		b = 0
 	} else if b > 255 {
 		b = 255
 	}
 	d.image.Pix[py*d.image.Stride+px] = image.RGBAColor{uint8(r), uint8(g), uint8(b), 0xff}
 }

 // Convert the MCU from YCbCr to RGB.
 func (d *decoder) convertMCU(mx, my, h0, v0 int) {
 	lumaBlock := 0
 	for v := 0; v < v0; v++ {
 		for h := 0; h < h0; h++ {
 			chromaBase := 8*4*v + 4*h
 			py := 8 * (v0*my + v)
 			for y := 0; y < 8 && py < d.height; y++ {
 				px := 8 * (h0*mx + h)
 				lumaIndex := 8 * y
 				chromaIndex := chromaBase + 8*(y/v0)
 				for x := 0; x < 8 && px < d.width; x++ {
 					d.calcPixel(px, py, lumaBlock, lumaIndex, chromaIndex)
 					if h0 == 1 {
 						chromaIndex += 1
 					} else {
 						chromaIndex += x % 2
 					}
 					lumaIndex++
 					px++
 				}
 				py++
 			}
 			lumaBlock++
 		}
 	}
 }

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

 	mcu, expectedRST := 0, uint8(rst0Marker)
 	var allZeroes [blockSize]int
 	var dc [nComponent]int
 	for my := 0; my < myy; my++ {
 		for mx := 0; mx < mxx; mx++ {
 			for i := 0; i < nComponent; i++ {
 				qt := &d.quant[d.comps[i].tq]
 				for j := 0; j < int(d.comps[i].h*d.comps[i].v); j++ {
 					d.blocks[i][j] = allZeroes

 					// Decode the DC coefficient, as specified in section F.2.2.1.
 					value, err := d.decodeHuffman(&d.huff[dcTableClass][scanComps[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
 					d.blocks[i][j][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[acTableClass][scanComps[i].ta])
 						if err != nil {
 							return err
 						}
 						v0 := value >> 4
 						v1 := value & 0x0f
 						if v1 != 0 {
 							k += int(v0)
 							if k > blockSize {
 								return FormatError("bad DCT index")
 							}
 							ac, err := d.receiveExtend(v1)
 							if err != nil {
 								return err
 							}
 							d.blocks[i][j][unzig[k]] = ac * qt[k]
 						} else {
 							if v0 != 0x0f {
 								break
 							}
 							k += 0x0f
 						}
 					}

 					idct(&d.blocks[i][j])
 				} // for j
 			} // for i
 			d.convertMCU(mx, my, int(d.comps[0].h), int(d.comps[0].v))
 			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.
 				for i := 0; i < nComponent; i++ {
 					dc[i] = 0
 				}
 			}
 		} // 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
 		}
 	}
 	return d.image, nil
 }

 // 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
 	}
 	return image.Config{image.RGBAColorModel, d.width, d.height}, nil
 }

 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.

	// The jpeg package implements a decoder for JPEG images, as defined in ITU-T T.81.
	package jpeg

	// See http://www.w3.org/Graphics/JPEG/itu-t81.pdf

	import (
	"bufio"
	"image"
	"io"
	"os"
	)

	// 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 {
	c uint8 // Component identifier.
	h uint8 // Horizontal sampling factor.
	v uint8 // Vertical sampling factor.
	tq uint8 // Quantization table destination selector.
	}

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

	dcTableClass = 0
	acTableClass = 1
	maxTc = 1
	maxTh = 3
	maxTq = 3

	// We only support 4:4:4, 4:2:2 and 4:2:0 downsampling, and assume that the components are Y, Cb, Cr.
	nComponent = 3
	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
	image *image.RGBA
	ri int // Restart Interval.
	comps [nComponent]component
	huff [maxTc + 1][maxTh + 1]huffman
	quant [maxTq + 1][blockSize]int
	b bits
	blocks [nComponent][maxH * maxV][blockSize]int
	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 {
	if n != 6+3*nComponent {
	return UnsupportedError("SOF has wrong length")
	}
	_, err := io.ReadFull(d.r, d.tmp[0:6+3*nComponent])
	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 d.tmp[5] != nComponent {
	return UnsupportedError("SOF has wrong number of image components")
	}
	for i := 0; i < nComponent; i++ {
	hv := d.tmp[7+3*i]
	d.comps[i].c = d.tmp[6+3*i]
	d.comps[i].h = hv >> 4
	d.comps[i].v = hv & 0x0f
	d.comps[i].tq = d.tmp[8+3*i]
	// We only support YCbCr images, and 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
	}

	// Set the Pixel (px, py)'s RGB value, based on its YCbCr value.
	func (d *decoder) calcPixel(px, py, lumaBlock, lumaIndex, chromaIndex int) {
	y, cb, cr := d.blocks[0][lumaBlock][lumaIndex], d.blocks[1][0][chromaIndex], d.blocks[2][0][chromaIndex]
	// The JFIF specification (http://www.w3.org/Graphics/JPEG/jfif3.pdf, page 3) gives the formula
	// for translating YCbCr to RGB as:
	// R = Y + 1.402 (Cr-128)
	// G = Y - 0.34414 (Cb-128) - 0.71414 (Cr-128)
	// B = Y + 1.772 (Cb-128)
	yPlusHalf := 100000*y + 50000
	cb -= 128
	cr -= 128
	r := (yPlusHalf + 140200*cr) / 100000
	g := (yPlusHalf - 34414cb - 71414cr) / 100000
	b := (yPlusHalf + 177200*cb) / 100000
	if r < 0 {
	r = 0
	} else if r > 255 {
	r = 255
	}
	if g < 0 {
	g = 0
	} else if g > 255 {
	g = 255
	}
	if b < 0 {
	b = 0
	} else if b > 255 {
	b = 255
	}
	d.image.Pix[py*d.image.Stride+px] = image.RGBAColor{uint8(r), uint8(g), uint8(b), 0xff}
	}

	// Convert the MCU from YCbCr to RGB.
	func (d *decoder) convertMCU(mx, my, h0, v0 int) {
	lumaBlock := 0
	for v := 0; v < v0; v++ {
	for h := 0; h < h0; h++ {
	chromaBase := 84v + 4*h
	py := 8 * (v0*my + v)
	for y := 0; y < 8 && py < d.height; y++ {
	px := 8 * (h0*mx + h)
	lumaIndex := 8 * y
	chromaIndex := chromaBase + 8*(y/v0)
	for x := 0; x < 8 && px < d.width; x++ {
	d.calcPixel(px, py, lumaBlock, lumaIndex, chromaIndex)
	if h0 == 1 {
	chromaIndex += 1
	} else {
	chromaIndex += x % 2
	}
	lumaIndex++
	px++
	}
	py++
	}
	lumaBlock++
	}
	}
	}

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

	mcu, expectedRST := 0, uint8(rst0Marker)
	var allZeroes [blockSize]int
	var dc [nComponent]int
	for my := 0; my < myy; my++ {
	for mx := 0; mx < mxx; mx++ {
	for i := 0; i < nComponent; i++ {
	qt := &d.quant[d.comps[i].tq]
	for j := 0; j < int(d.comps[i].h*d.comps[i].v); j++ {
	d.blocks[i][j] = allZeroes

	// Decode the DC coefficient, as specified in section F.2.2.1.
	value, err := d.decodeHuffman(&d.huff[dcTableClass][scanComps[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
	d.blocks[i][j][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[acTableClass][scanComps[i].ta])
	if err != nil {
	return err
	}
	v0 := value >> 4
	v1 := value & 0x0f
	if v1 != 0 {
	k += int(v0)
	if k > blockSize {
	return FormatError("bad DCT index")
	}
	ac, err := d.receiveExtend(v1)
	if err != nil {
	return err
	}
	d.blocks[i][j][unzig[k]] = ac * qt[k]
	} else {
	if v0 != 0x0f {
	break
	}
	k += 0x0f
	}
	}

	idct(&d.blocks[i][j])
	} // for j
	} // for i
	d.convertMCU(mx, my, int(d.comps[0].h), int(d.comps[0].v))
	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.
	for i := 0; i < nComponent; i++ {
	dc[i] = 0
	}
	}
	} // 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
	}
	}
	return d.image, nil
	}

	// 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
	}
	return image.Config{image.RGBAColorModel, d.width, d.height}, nil
	}

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