src/pkg/image/png/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 png package implements a PNG image decoder (and eventually, an encoder).
 //
 // The PNG specification is at http://www.libpng.org/pub/png/spec/1.2/PNG-Contents.html
 package png

 // TODO(nigeltao): Add tests.
 import (
 	"compress/zlib";
 	"hash";
 	"hash/crc32";
 	"image";
 	"io";
 	"os";
 )

 // Color type, as per the PNG spec.
 const (
 	ctGrayscale = 0;
 	ctTrueColor = 2;
 	ctPaletted = 3;
 	ctGrayscaleAlpha = 4;
 	ctTrueColorAlpha = 6;
 )

 // Filter type, as per the PNG spec.
 const (
 	ftNone = 0;
 	ftSub = 1;
 	ftUp = 2;
 	ftAverage = 3;
 	ftPaeth = 4;
 )

 // Decoding stage.
 // The PNG specification says that the IHDR, PLTE (if present), IDAT and IEND
 // chunks must appear in that order. There may be multiple IDAT chunks, and
 // IDAT chunks must be sequential (i.e. they may not have any other chunks
 // between them).
 const (
 	dsStart = iota;
 	dsSeenIHDR;
 	dsSeenPLTE;
 	dsSeenIDAT;
 	dsSeenIEND;
 )

 type decoder struct {
 	width, height int;
 	image image.Image;
 	colorType uint8;
 	stage int;
 	idatWriter io.WriteCloser;
 	idatDone chan os.Error;
 	scratch [3 * 256]byte;
 }

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

 func (e FormatError) String() string {
 	return "invalid PNG format: " + e;
 }

 var chunkOrderError = FormatError("chunk out of order")

 // An IDATDecodingError wraps an inner error (such as a ZLIB decoding error) encountered while processing an IDAT chunk.
 type IDATDecodingError struct {
 	Err os.Error;
 }

 func (e IDATDecodingError) String() string {
 	return "IDAT decoding error: " + e.Err.String();
 }

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

 func (e UnsupportedError) String() string {
 	return "unsupported PNG feature: " + e;
 }

 // Big-endian.
 func parseUint32(b []uint8) uint32 {
 	return uint32(b[0])<<24 | uint32(b[1])<<16 | uint32(b[2])<<8 | uint32(b[3]);
 }

 func abs(x int) int {
 	if x < 0 {
 		return -x;
 	}
 	return x;
 }

 func min(a, b int) int {
 	if a < b {
 		return a;
 	}
 	return b;
 }

 func (d *decoder) parseIHDR(r io.Reader, crc hash.Hash32, length uint32) os.Error {
 	if length != 13 {
 		return FormatError("bad IHDR length");
 	}
 	_, err := io.ReadFull(r, d.scratch[0:13]);
 	if err != nil {
 		return err;
 	}
 	crc.Write(d.scratch[0:13]);
 	if d.scratch[8] != 8 {
 		return UnsupportedError("bit depth");
 	}
 	if d.scratch[10] != 0 || d.scratch[11] != 0 || d.scratch[12] != 0 {
 		return UnsupportedError("compression, filter or interlace method");
 	}
 	w := int32(parseUint32(d.scratch[0:4]));
 	h := int32(parseUint32(d.scratch[4:8]));
 	if w < 0 || h < 0 {
 		return FormatError("negative dimension");
 	}
 	nPixels := int64(w) * int64(h);
 	if nPixels != int64(int(nPixels)) {
 		return UnsupportedError("dimension overflow");
 	}
 	d.colorType = d.scratch[9];
 	switch d.colorType {
 	case ctTrueColor:
 		d.image = image.NewRGBA(int(w), int(h));
 	case ctPaletted:
 		d.image = image.NewPaletted(int(w), int(h), nil);
 	case ctTrueColorAlpha:
 		d.image = image.NewNRGBA(int(w), int(h));
 	default:
 		return UnsupportedError("color type");
 	}
 	d.width, d.height = int(w), int(h);
 	return nil;
 }

 func (d *decoder) parsePLTE(r io.Reader, crc hash.Hash32, length uint32) os.Error {
 	np := int(length / 3);	// The number of palette entries.
 	if length % 3 != 0 || np <= 0 || np > 256 {
 		return FormatError("bad PLTE length");
 	}
 	n, err := io.ReadFull(r, d.scratch[0:3 * np]);
 	if err != nil {
 		return err;
 	}
 	crc.Write(d.scratch[0:n]);
 	switch d.colorType {
 	case ctPaletted:
 		palette := make([]image.Color, np);
 		for i := 0; i < np; i++ {
 			palette[i] = image.RGBAColor{ d.scratch[3*i+0], d.scratch[3*i+1], d.scratch[3*i+2], 0xff };
 		}
 		d.image.(*image.Paletted).Palette = image.PalettedColorModel(palette);
 	case ctTrueColor, ctTrueColorAlpha:
 		// As per the PNG spec, a PLTE chunk is optional (and for practical purposes,
 		// ignorable) for the ctTrueColor and ctTrueColorAlpha color types (section 4.1.2).
 		return nil;
 	default:
 		return FormatError("PLTE, color type mismatch");
 	}
 	return nil;
 }

 // The Paeth filter function, as per the PNG specification.
 func paeth(a, b, c uint8) uint8 {
 	p := int(a) + int(b) - int(c);
 	pa := abs(p - int(a));
 	pb := abs(p - int(b));
 	pc := abs(p - int(c));
 	if pa <= pb && pa <= pc {
 		return a;
 	} else if pb <= pc {
 		return b;
 	}
 	return c;
 }

 func (d *decoder) idatReader(idat io.Reader) os.Error {
 	r, err := zlib.NewInflater(idat);
 	if err != nil {
 		return err;
 	}
 	defer r.Close();
 	bpp := 0;	// Bytes per pixel.
 	maxPalette := uint8(0);
 	var (
 		rgba *image.RGBA;
 		nrgba *image.NRGBA;
 		paletted *image.Paletted;
 	);
 	switch d.colorType {
 	case ctTrueColor:
 		bpp = 3;
 		rgba = d.image.(*image.RGBA);
 	case ctPaletted:
 		bpp = 1;
 		paletted = d.image.(*image.Paletted);
 		maxPalette = uint8(len(paletted.Palette) - 1);
 	case ctTrueColorAlpha:
 		bpp = 4;
 		nrgba = d.image.(*image.NRGBA);
 	}
 	// cr and pr are the bytes for the current and previous row.
 	cr := make([]uint8, bpp * d.width);
 	pr := make([]uint8, bpp * d.width);

 	var filter [1]uint8;
 	for y := 0; y < d.height; y++ {
 		// Read the decompressed bytes.
 		n, err := io.ReadFull(r, filter[0:1]);
 		if err != nil {
 			return err;
 		}
 		n, err = io.ReadFull(r, cr);
 		if err != nil {
 			return err;
 		}

 		// Apply the filter.
 		switch filter[0] {
 		case ftNone:
 			// No-op.
 		case ftSub:
 			for i := bpp; i < n; i++ {
 				cr[i] += cr[i - bpp];
 			}
 		case ftUp:
 			for i := 0; i < n; i++ {
 				cr[i] += pr[i];
 			}
 		case ftAverage:
 			for i := 0; i < bpp; i++ {
 				cr[i] += pr[i] / 2;
 			}
 			for i := bpp; i < n; i++ {
 				cr[i] += uint8((int(cr[i - bpp]) + int(pr[i])) / 2);
 			}
 		case ftPaeth:
 			for i := 0; i < bpp; i++ {
 				cr[i] += paeth(0, pr[i], 0);
 			}
 			for i := bpp; i < n; i++ {
 				cr[i] += paeth(cr[i - bpp], pr[i], pr[i - bpp]);
 			}
 		default:
 			return FormatError("bad filter type");
 		}

 		// Convert from bytes to colors.
 		switch d.colorType {
 		case ctTrueColor:
 			for x := 0; x < d.width; x++ {
 				rgba.Set(x, y, image.RGBAColor{ cr[3*x+0], cr[3*x+1], cr[3*x+2], 0xff });
 			}
 		case ctPaletted:
 			for x := 0; x < d.width; x++ {
 				if cr[x] > maxPalette {
 					return FormatError("palette index out of range");
 				}
 				paletted.SetColorIndex(x, y, cr[x]);
 			}
 		case ctTrueColorAlpha:
 			for x := 0; x < d.width; x++ {
 				nrgba.Set(x, y, image.NRGBAColor{ cr[4*x+0], cr[4*x+1], cr[4*x+2], cr[4*x+3] });
 			}
 		}

 		// The current row for y is the previous row for y+1.
 		pr, cr = cr, pr;
 	}
 	return nil;
 }

 func (d *decoder) parseIDAT(r io.Reader, crc hash.Hash32, length uint32) os.Error {
 	// There may be more than one IDAT chunk, but their contents must be
 	// treated as if it was one continuous stream (to the zlib decoder).
 	// We bring up an io.Pipe and write the IDAT chunks into the pipe as
 	// we see them, and decode the stream in a separate go-routine, which
 	// signals its completion (successful or not) via a channel.
 	if d.idatWriter == nil {
 		pr, pw := io.Pipe();
 		d.idatWriter = pw;
 		d.idatDone = make(chan os.Error);
 		go func() {
 			err := d.idatReader(pr);
 			if err == os.EOF {
 				err = FormatError("too little IDAT");
 			}
 			pr.CloseWithError(FormatError("too much IDAT"));
 			d.idatDone <- err;
 		}();
 	}
 	var buf [4096]byte;
 	for length > 0 {
 		n, err1 := r.Read(buf[0:min(len(buf), int(length))]);
 		// We delay checking err1. It is possible to get n bytes and an error,
 		// but if the n bytes themselves contain a FormatError, for example, we
 		// want to report that error, and not the one that made the Read stop.
 		n, err2 := d.idatWriter.Write(buf[0:n]);
 		if err2 != nil {
 			return err2;
 		}
 		if err1 != nil {
 			return err1;
 		}
 		crc.Write(buf[0:n]);
 		length -= uint32(n);
 	}
 	return nil;
 }

 func (d *decoder) parseIEND(r io.Reader, crc hash.Hash32, length uint32) os.Error {
 	if length != 0 {
 		return FormatError("bad IEND length");
 	}
 	return nil;
 }

 func (d *decoder) parseChunk(r io.Reader) os.Error {
 	// Read the length.
 	n, err := io.ReadFull(r, d.scratch[0:4]);
 	if err == os.EOF {
 		return io.ErrUnexpectedEOF;
 	}
 	if err != nil {
 		return err;
 	}
 	length := parseUint32(d.scratch[0:4]);

 	// Read the chunk type.
 	n, err = io.ReadFull(r, d.scratch[0:4]);
 	if err == os.EOF {
 		return io.ErrUnexpectedEOF;
 	}
 	if err != nil {
 		return err;
 	}
 	crc := crc32.NewIEEE();
 	crc.Write(d.scratch[0:4]);

 	// Read the chunk data.
 	switch string(d.scratch[0:4]) {
 	case "IHDR":
 		if d.stage != dsStart {
 			return chunkOrderError;
 		}
 		d.stage = dsSeenIHDR;
 		err = d.parseIHDR(r, crc, length);
 	case "PLTE":
 		if d.stage != dsSeenIHDR {
 			return chunkOrderError;
 		}
 		d.stage = dsSeenPLTE;
 		err = d.parsePLTE(r, crc, length);
 	case "IDAT":
 		if d.stage < dsSeenIHDR || d.stage > dsSeenIDAT || (d.colorType == ctPaletted && d.stage == dsSeenIHDR) {
 			return chunkOrderError;
 		}
 		d.stage = dsSeenIDAT;
 		err = d.parseIDAT(r, crc, length);
 	case "IEND":
 		if d.stage != dsSeenIDAT {
 			return chunkOrderError;
 		}
 		d.stage = dsSeenIEND;
 		err = d.parseIEND(r, crc, length);
 	default:
 		// Ignore this chunk (of a known length).
 		var ignored [4096]byte;
 		for length > 0 {
 			n, err = io.ReadFull(r, ignored[0:min(len(ignored), int(length))]);
 			if err != nil {
 				return err;
 			}
 			crc.Write(ignored[0:n]);
 			length -= uint32(n);
 		}
 	}
 	if err != nil {
 		return err;
 	}

 	// Read the checksum.
 	n, err = io.ReadFull(r, d.scratch[0:4]);
 	if err == os.EOF {
 		return io.ErrUnexpectedEOF;
 	}
 	if err != nil {
 		return err;
 	}
 	if parseUint32(d.scratch[0:4]) != crc.Sum32() {
 		return FormatError("invalid checksum");
 	}
 	return nil;
 }

 func (d *decoder) checkHeader(r io.Reader) os.Error {
 	_, err := io.ReadFull(r, d.scratch[0:8]);
 	if err != nil {
 		return err;
 	}
 	if string(d.scratch[0:8]) != "\x89PNG\r\n\x1a\n" {
 		return FormatError("not a PNG file");
 	}
 	return nil;
 }

 func Decode(r io.Reader) (image.Image, os.Error) {
 	var d decoder;
 	err := d.checkHeader(r);
 	if err != nil {
 		return nil, err;
 	}
 	for d.stage = dsStart; d.stage != dsSeenIEND; {
 		err = d.parseChunk(r);
 		if err != nil {
 			break;
 		}
 	}
 	if d.idatWriter != nil {
 		d.idatWriter.Close();
 		err1 := <-d.idatDone;
 		if err == nil {
 			err = err1;
 		}
 	}
 	if err != nil {
 		return nil, err;
 	}
 	return d.image, nil;
 }
	// 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 png package implements a PNG image decoder (and eventually, an encoder).
	//
	// The PNG specification is at http://www.libpng.org/pub/png/spec/1.2/PNG-Contents.html
	package png

	// TODO(nigeltao): Add tests.
	import (
	"compress/zlib";
	"hash";
	"hash/crc32";
	"image";
	"io";
	"os";
	)

	// Color type, as per the PNG spec.
	const (
	ctGrayscale = 0;
	ctTrueColor = 2;
	ctPaletted = 3;
	ctGrayscaleAlpha = 4;
	ctTrueColorAlpha = 6;
	)

	// Filter type, as per the PNG spec.
	const (
	ftNone = 0;
	ftSub = 1;
	ftUp = 2;
	ftAverage = 3;
	ftPaeth = 4;
	)

	// Decoding stage.
	// The PNG specification says that the IHDR, PLTE (if present), IDAT and IEND
	// chunks must appear in that order. There may be multiple IDAT chunks, and
	// IDAT chunks must be sequential (i.e. they may not have any other chunks
	// between them).
	const (
	dsStart = iota;
	dsSeenIHDR;
	dsSeenPLTE;
	dsSeenIDAT;
	dsSeenIEND;
	)

	type decoder struct {
	width, height int;
	image image.Image;
	colorType uint8;
	stage int;
	idatWriter io.WriteCloser;
	idatDone chan os.Error;
	scratch [3 * 256]byte;
	}

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

	func (e FormatError) String() string {
	return "invalid PNG format: " + e;
	}

	var chunkOrderError = FormatError("chunk out of order")

	// An IDATDecodingError wraps an inner error (such as a ZLIB decoding error) encountered while processing an IDAT chunk.
	type IDATDecodingError struct {
	Err os.Error;
	}

	func (e IDATDecodingError) String() string {
	return "IDAT decoding error: " + e.Err.String();
	}

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

	func (e UnsupportedError) String() string {
	return "unsupported PNG feature: " + e;
	}

	// Big-endian.
	func parseUint32(b []uint8) uint32 {
	return uint32(b[0])<<24 \| uint32(b[1])<<16 \| uint32(b[2])<<8 \| uint32(b[3]);
	}

	func abs(x int) int {
	if x < 0 {
	return -x;
	}
	return x;
	}

	func min(a, b int) int {
	if a < b {
	return a;
	}
	return b;
	}

	func (d *decoder) parseIHDR(r io.Reader, crc hash.Hash32, length uint32) os.Error {
	if length != 13 {
	return FormatError("bad IHDR length");
	}
	_, err := io.ReadFull(r, d.scratch[0:13]);
	if err != nil {
	return err;
	}
	crc.Write(d.scratch[0:13]);
	if d.scratch[8] != 8 {
	return UnsupportedError("bit depth");
	}
	if d.scratch[10] != 0 \|\| d.scratch[11] != 0 \|\| d.scratch[12] != 0 {
	return UnsupportedError("compression, filter or interlace method");
	}
	w := int32(parseUint32(d.scratch[0:4]));
	h := int32(parseUint32(d.scratch[4:8]));
	if w < 0 \|\| h < 0 {
	return FormatError("negative dimension");
	}
	nPixels := int64(w) * int64(h);
	if nPixels != int64(int(nPixels)) {
	return UnsupportedError("dimension overflow");
	}
	d.colorType = d.scratch[9];
	switch d.colorType {
	case ctTrueColor:
	d.image = image.NewRGBA(int(w), int(h));
	case ctPaletted:
	d.image = image.NewPaletted(int(w), int(h), nil);
	case ctTrueColorAlpha:
	d.image = image.NewNRGBA(int(w), int(h));
	default:
	return UnsupportedError("color type");
	}
	d.width, d.height = int(w), int(h);
	return nil;
	}

	func (d *decoder) parsePLTE(r io.Reader, crc hash.Hash32, length uint32) os.Error {
	np := int(length / 3); // The number of palette entries.
	if length % 3 != 0 \|\| np <= 0 \|\| np > 256 {
	return FormatError("bad PLTE length");
	}
	n, err := io.ReadFull(r, d.scratch[0:3 * np]);
	if err != nil {
	return err;
	}
	crc.Write(d.scratch[0:n]);
	switch d.colorType {
	case ctPaletted:
	palette := make([]image.Color, np);
	for i := 0; i < np; i++ {
	palette[i] = image.RGBAColor{ d.scratch[3i+0], d.scratch[3i+1], d.scratch[3*i+2], 0xff };
	}
	d.image.(*image.Paletted).Palette = image.PalettedColorModel(palette);
	case ctTrueColor, ctTrueColorAlpha:
	// As per the PNG spec, a PLTE chunk is optional (and for practical purposes,
	// ignorable) for the ctTrueColor and ctTrueColorAlpha color types (section 4.1.2).
	return nil;
	default:
	return FormatError("PLTE, color type mismatch");
	}
	return nil;
	}

	// The Paeth filter function, as per the PNG specification.
	func paeth(a, b, c uint8) uint8 {
	p := int(a) + int(b) - int(c);
	pa := abs(p - int(a));
	pb := abs(p - int(b));
	pc := abs(p - int(c));
	if pa <= pb && pa <= pc {
	return a;
	} else if pb <= pc {
	return b;
	}
	return c;
	}

	func (d *decoder) idatReader(idat io.Reader) os.Error {
	r, err := zlib.NewInflater(idat);
	if err != nil {
	return err;
	}
	defer r.Close();
	bpp := 0; // Bytes per pixel.
	maxPalette := uint8(0);
	var (
	rgba *image.RGBA;
	nrgba *image.NRGBA;
	paletted *image.Paletted;
	);
	switch d.colorType {
	case ctTrueColor:
	bpp = 3;
	rgba = d.image.(*image.RGBA);
	case ctPaletted:
	bpp = 1;
	paletted = d.image.(*image.Paletted);
	maxPalette = uint8(len(paletted.Palette) - 1);
	case ctTrueColorAlpha:
	bpp = 4;
	nrgba = d.image.(*image.NRGBA);
	}
	// cr and pr are the bytes for the current and previous row.
	cr := make([]uint8, bpp * d.width);
	pr := make([]uint8, bpp * d.width);

	var filter [1]uint8;
	for y := 0; y < d.height; y++ {
	// Read the decompressed bytes.
	n, err := io.ReadFull(r, filter[0:1]);
	if err != nil {
	return err;
	}
	n, err = io.ReadFull(r, cr);
	if err != nil {
	return err;
	}

	// Apply the filter.
	switch filter[0] {
	case ftNone:
	// No-op.
	case ftSub:
	for i := bpp; i < n; i++ {
	cr[i] += cr[i - bpp];
	}
	case ftUp:
	for i := 0; i < n; i++ {
	cr[i] += pr[i];
	}
	case ftAverage:
	for i := 0; i < bpp; i++ {
	cr[i] += pr[i] / 2;
	}
	for i := bpp; i < n; i++ {
	cr[i] += uint8((int(cr[i - bpp]) + int(pr[i])) / 2);
	}
	case ftPaeth:
	for i := 0; i < bpp; i++ {
	cr[i] += paeth(0, pr[i], 0);
	}
	for i := bpp; i < n; i++ {
	cr[i] += paeth(cr[i - bpp], pr[i], pr[i - bpp]);
	}
	default:
	return FormatError("bad filter type");
	}

	// Convert from bytes to colors.
	switch d.colorType {
	case ctTrueColor:
	for x := 0; x < d.width; x++ {
	rgba.Set(x, y, image.RGBAColor{ cr[3x+0], cr[3x+1], cr[3*x+2], 0xff });
	}
	case ctPaletted:
	for x := 0; x < d.width; x++ {
	if cr[x] > maxPalette {
	return FormatError("palette index out of range");
	}
	paletted.SetColorIndex(x, y, cr[x]);
	}
	case ctTrueColorAlpha:
	for x := 0; x < d.width; x++ {
	nrgba.Set(x, y, image.NRGBAColor{ cr[4x+0], cr[4x+1], cr[4x+2], cr[4x+3] });
	}
	}

	// The current row for y is the previous row for y+1.
	pr, cr = cr, pr;
	}
	return nil;
	}

	func (d *decoder) parseIDAT(r io.Reader, crc hash.Hash32, length uint32) os.Error {
	// There may be more than one IDAT chunk, but their contents must be
	// treated as if it was one continuous stream (to the zlib decoder).
	// We bring up an io.Pipe and write the IDAT chunks into the pipe as
	// we see them, and decode the stream in a separate go-routine, which
	// signals its completion (successful or not) via a channel.
	if d.idatWriter == nil {
	pr, pw := io.Pipe();
	d.idatWriter = pw;
	d.idatDone = make(chan os.Error);
	go func() {
	err := d.idatReader(pr);
	if err == os.EOF {
	err = FormatError("too little IDAT");
	}
	pr.CloseWithError(FormatError("too much IDAT"));
	d.idatDone <- err;
	}();
	}
	var buf [4096]byte;
	for length > 0 {
	n, err1 := r.Read(buf[0:min(len(buf), int(length))]);
	// We delay checking err1. It is possible to get n bytes and an error,
	// but if the n bytes themselves contain a FormatError, for example, we
	// want to report that error, and not the one that made the Read stop.
	n, err2 := d.idatWriter.Write(buf[0:n]);
	if err2 != nil {
	return err2;
	}
	if err1 != nil {
	return err1;
	}
	crc.Write(buf[0:n]);
	length -= uint32(n);
	}
	return nil;
	}

	func (d *decoder) parseIEND(r io.Reader, crc hash.Hash32, length uint32) os.Error {
	if length != 0 {
	return FormatError("bad IEND length");
	}
	return nil;
	}

	func (d *decoder) parseChunk(r io.Reader) os.Error {
	// Read the length.
	n, err := io.ReadFull(r, d.scratch[0:4]);
	if err == os.EOF {
	return io.ErrUnexpectedEOF;
	}
	if err != nil {
	return err;
	}
	length := parseUint32(d.scratch[0:4]);

	// Read the chunk type.
	n, err = io.ReadFull(r, d.scratch[0:4]);
	if err == os.EOF {
	return io.ErrUnexpectedEOF;
	}
	if err != nil {
	return err;
	}
	crc := crc32.NewIEEE();
	crc.Write(d.scratch[0:4]);

	// Read the chunk data.
	switch string(d.scratch[0:4]) {
	case "IHDR":
	if d.stage != dsStart {
	return chunkOrderError;
	}
	d.stage = dsSeenIHDR;
	err = d.parseIHDR(r, crc, length);
	case "PLTE":
	if d.stage != dsSeenIHDR {
	return chunkOrderError;
	}
	d.stage = dsSeenPLTE;
	err = d.parsePLTE(r, crc, length);
	case "IDAT":
	if d.stage < dsSeenIHDR \|\| d.stage > dsSeenIDAT \|\| (d.colorType == ctPaletted && d.stage == dsSeenIHDR) {
	return chunkOrderError;
	}
	d.stage = dsSeenIDAT;
	err = d.parseIDAT(r, crc, length);
	case "IEND":
	if d.stage != dsSeenIDAT {
	return chunkOrderError;
	}
	d.stage = dsSeenIEND;
	err = d.parseIEND(r, crc, length);
	default:
	// Ignore this chunk (of a known length).
	var ignored [4096]byte;
	for length > 0 {
	n, err = io.ReadFull(r, ignored[0:min(len(ignored), int(length))]);
	if err != nil {
	return err;
	}
	crc.Write(ignored[0:n]);
	length -= uint32(n);
	}
	}
	if err != nil {
	return err;
	}

	// Read the checksum.
	n, err = io.ReadFull(r, d.scratch[0:4]);
	if err == os.EOF {
	return io.ErrUnexpectedEOF;
	}
	if err != nil {
	return err;
	}
	if parseUint32(d.scratch[0:4]) != crc.Sum32() {
	return FormatError("invalid checksum");
	}
	return nil;
	}

	func (d *decoder) checkHeader(r io.Reader) os.Error {
	_, err := io.ReadFull(r, d.scratch[0:8]);
	if err != nil {
	return err;
	}
	if string(d.scratch[0:8]) != "\x89PNG\r\n\x1a\n" {
	return FormatError("not a PNG file");
	}
	return nil;
	}

	func Decode(r io.Reader) (image.Image, os.Error) {
	var d decoder;
	err := d.checkHeader(r);
	if err != nil {
	return nil, err;
	}
	for d.stage = dsStart; d.stage != dsSeenIEND; {
	err = d.parseChunk(r);
	if err != nil {
	break;
	}
	}
	if d.idatWriter != nil {
	d.idatWriter.Close();
	err1 := <-d.idatDone;
	if err == nil {
	err = err1;
	}
	}
	if err != nil {
	return nil, err;
	}
	return d.image, nil;
	}