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// 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 encoder.
//
// The PNG specification is at http://www.libpng.org/pub/png/spec/1.2/PNG-Contents.html
package png
import (
"compress/zlib"
"fmt"
"hash"
"hash/crc32"
"image"
"io"
"os"
)
// Color type, as per the PNG spec.
const (
ctGrayscale = 0
ctTrueColor = 2
ctPaletted = 3
ctGrayscaleAlpha = 4
ctTrueColorAlpha = 6
)
// A cb is a combination of color type and bit depth.
const (
cbInvalid = iota
cbG1
cbG2
cbG4
cbG8
cbGA8
cbTC8
cbP1
cbP2
cbP4
cbP8
cbTCA8
cbG16
cbGA16
cbTC16
cbTCA16
)
// Filter type, as per the PNG spec.
const (
ftNone = 0
ftSub = 1
ftUp = 2
ftAverage = 3
ftPaeth = 4
nFilter = 5
)
// 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
)
const pngHeader = "\x89PNG\r\n\x1a\n"
type imgOrErr struct {
img image.Image
err os.Error
}
type decoder struct {
width, height int
depth int
palette image.PalettedColorModel
cb int
stage int
idatWriter io.WriteCloser
idatDone chan imgOrErr
tmp [3 * 256]byte
}
// A FormatError reports that the input is not a valid PNG.
type FormatError string
func (e FormatError) String() string { return "png: invalid format: " + string(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 "png: 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 "png: unsupported feature: " + string(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.tmp[0:13])
if err != nil {
return err
}
crc.Write(d.tmp[0:13])
if d.tmp[10] != 0 || d.tmp[11] != 0 || d.tmp[12] != 0 {
return UnsupportedError("compression, filter or interlace method")
}
w := int32(parseUint32(d.tmp[0:4]))
h := int32(parseUint32(d.tmp[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.cb = cbInvalid
d.depth = int(d.tmp[8])
switch d.depth {
case 1:
switch d.tmp[9] {
case ctGrayscale:
d.cb = cbG1
case ctPaletted:
d.cb = cbP1
}
case 2:
switch d.tmp[9] {
case ctGrayscale:
d.cb = cbG2
case ctPaletted:
d.cb = cbP2
}
case 4:
switch d.tmp[9] {
case ctGrayscale:
d.cb = cbG4
case ctPaletted:
d.cb = cbP4
}
case 8:
switch d.tmp[9] {
case ctGrayscale:
d.cb = cbG8
case ctTrueColor:
d.cb = cbTC8
case ctPaletted:
d.cb = cbP8
case ctGrayscaleAlpha:
d.cb = cbGA8
case ctTrueColorAlpha:
d.cb = cbTCA8
}
case 16:
switch d.tmp[9] {
case ctGrayscale:
d.cb = cbG16
case ctTrueColor:
d.cb = cbTC16
case ctGrayscaleAlpha:
d.cb = cbGA16
case ctTrueColorAlpha:
d.cb = cbTCA16
}
}
if d.cb == cbInvalid {
return UnsupportedError(fmt.Sprintf("bit depth %d, color type %d", d.tmp[8], d.tmp[9]))
}
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 || np > 1<<uint(d.depth) {
return FormatError("bad PLTE length")
}
n, err := io.ReadFull(r, d.tmp[0:3*np])
if err != nil {
return err
}
crc.Write(d.tmp[0:n])
switch d.cb {
case cbP1, cbP2, cbP4, cbP8:
d.palette = image.PalettedColorModel(make([]image.Color, np))
for i := 0; i < np; i++ {
d.palette[i] = image.RGBAColor{d.tmp[3*i+0], d.tmp[3*i+1], d.tmp[3*i+2], 0xff}
}
case cbTC8, cbTCA8, cbTC16, cbTCA16:
// 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).
default:
return FormatError("PLTE, color type mismatch")
}
return nil
}
func (d *decoder) parsetRNS(r io.Reader, crc hash.Hash32, length uint32) os.Error {
if length > 256 {
return FormatError("bad tRNS length")
}
n, err := io.ReadFull(r, d.tmp[0:length])
if err != nil {
return err
}
crc.Write(d.tmp[0:n])
switch d.cb {
case cbG8, cbG16:
return UnsupportedError("grayscale transparency")
case cbTC8, cbTC16:
return UnsupportedError("truecolor transparency")
case cbP1, cbP2, cbP4, cbP8:
if n > len(d.palette) {
return FormatError("bad tRNS length")
}
for i := 0; i < n; i++ {
rgba := d.palette[i].(image.RGBAColor)
d.palette[i] = image.RGBAColor{rgba.R, rgba.G, rgba.B, d.tmp[i]}
}
case cbGA8, cbGA16, cbTCA8, cbTCA16:
return FormatError("tRNS, 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) (image.Image, os.Error) {
r, err := zlib.NewReader(idat)
if err != nil {
return nil, err
}
defer r.Close()
bitsPerPixel := 0
maxPalette := uint8(0)
var (
gray *image.Gray
rgba *image.RGBA
paletted *image.Paletted
nrgba *image.NRGBA
gray16 *image.Gray16
rgba64 *image.RGBA64
nrgba64 *image.NRGBA64
img image.Image
)
switch d.cb {
case cbG1, cbG2, cbG4, cbG8:
bitsPerPixel = d.depth
gray = image.NewGray(d.width, d.height)
img = gray
case cbGA8:
bitsPerPixel = 16
nrgba = image.NewNRGBA(d.width, d.height)
img = nrgba
case cbTC8:
bitsPerPixel = 24
rgba = image.NewRGBA(d.width, d.height)
img = rgba
case cbP1, cbP2, cbP4, cbP8:
bitsPerPixel = d.depth
paletted = image.NewPaletted(d.width, d.height, d.palette)
img = paletted
maxPalette = uint8(len(d.palette) - 1)
case cbTCA8:
bitsPerPixel = 32
nrgba = image.NewNRGBA(d.width, d.height)
img = nrgba
case cbG16:
bitsPerPixel = 16
gray16 = image.NewGray16(d.width, d.height)
img = gray16
case cbGA16:
bitsPerPixel = 32
nrgba64 = image.NewNRGBA64(d.width, d.height)
img = nrgba64
case cbTC16:
bitsPerPixel = 48
rgba64 = image.NewRGBA64(d.width, d.height)
img = rgba64
case cbTCA16:
bitsPerPixel = 64
nrgba64 = image.NewNRGBA64(d.width, d.height)
img = nrgba64
}
bytesPerPixel := (bitsPerPixel + 7) / 8
// cr and pr are the bytes for the current and previous row.
// The +1 is for the per-row filter type, which is at cr[0].
cr := make([]uint8, 1+(bitsPerPixel*d.width+7)/8)
pr := make([]uint8, 1+(bitsPerPixel*d.width+7)/8)
for y := 0; y < d.height; y++ {
// Read the decompressed bytes.
_, err := io.ReadFull(r, cr)
if err != nil {
return nil, err
}
// Apply the filter.
cdat := cr[1:]
pdat := pr[1:]
switch cr[0] {
case ftNone:
// No-op.
case ftSub:
for i := bytesPerPixel; i < len(cdat); i++ {
cdat[i] += cdat[i-bytesPerPixel]
}
case ftUp:
for i := 0; i < len(cdat); i++ {
cdat[i] += pdat[i]
}
case ftAverage:
for i := 0; i < bytesPerPixel; i++ {
cdat[i] += pdat[i] / 2
}
for i := bytesPerPixel; i < len(cdat); i++ {
cdat[i] += uint8((int(cdat[i-bytesPerPixel]) + int(pdat[i])) / 2)
}
case ftPaeth:
for i := 0; i < bytesPerPixel; i++ {
cdat[i] += paeth(0, pdat[i], 0)
}
for i := bytesPerPixel; i < len(cdat); i++ {
cdat[i] += paeth(cdat[i-bytesPerPixel], pdat[i], pdat[i-bytesPerPixel])
}
default:
return nil, FormatError("bad filter type")
}
// Convert from bytes to colors.
switch d.cb {
case cbG1:
for x := 0; x < d.width; x += 8 {
b := cdat[x/8]
for x2 := 0; x2 < 8 && x+x2 < d.width; x2++ {
gray.Set(x+x2, y, image.GrayColor{(b >> 7) * 0xff})
b <<= 1
}
}
case cbG2:
for x := 0; x < d.width; x += 4 {
b := cdat[x/4]
for x2 := 0; x2 < 4 && x+x2 < d.width; x2++ {
gray.Set(x+x2, y, image.GrayColor{(b >> 6) * 0x55})
b <<= 2
}
}
case cbG4:
for x := 0; x < d.width; x += 2 {
b := cdat[x/2]
for x2 := 0; x2 < 2 && x+x2 < d.width; x2++ {
gray.Set(x+x2, y, image.GrayColor{(b >> 4) * 0x11})
b <<= 4
}
}
case cbG8:
for x := 0; x < d.width; x++ {
gray.Set(x, y, image.GrayColor{cdat[x]})
}
case cbGA8:
for x := 0; x < d.width; x++ {
ycol := cdat[2*x+0]
nrgba.Set(x, y, image.NRGBAColor{ycol, ycol, ycol, cdat[2*x+1]})
}
case cbTC8:
for x := 0; x < d.width; x++ {
rgba.Set(x, y, image.RGBAColor{cdat[3*x+0], cdat[3*x+1], cdat[3*x+2], 0xff})
}
case cbP1:
for x := 0; x < d.width; x += 8 {
b := cdat[x/8]
for x2 := 0; x2 < 8 && x+x2 < d.width; x2++ {
idx := b >> 7
if idx > maxPalette {
return nil, FormatError("palette index out of range")
}
paletted.SetColorIndex(x+x2, y, idx)
b <<= 1
}
}
case cbP2:
for x := 0; x < d.width; x += 4 {
b := cdat[x/4]
for x2 := 0; x2 < 4 && x+x2 < d.width; x2++ {
idx := b >> 6
if idx > maxPalette {
return nil, FormatError("palette index out of range")
}
paletted.SetColorIndex(x+x2, y, idx)
b <<= 2
}
}
case cbP4:
for x := 0; x < d.width; x += 2 {
b := cdat[x/2]
for x2 := 0; x2 < 2 && x+x2 < d.width; x2++ {
idx := b >> 4
if idx > maxPalette {
return nil, FormatError("palette index out of range")
}
paletted.SetColorIndex(x+x2, y, idx)
b <<= 4
}
}
case cbP8:
for x := 0; x < d.width; x++ {
if cdat[x] > maxPalette {
return nil, FormatError("palette index out of range")
}
paletted.SetColorIndex(x, y, cdat[x])
}
case cbTCA8:
for x := 0; x < d.width; x++ {
nrgba.Set(x, y, image.NRGBAColor{cdat[4*x+0], cdat[4*x+1], cdat[4*x+2], cdat[4*x+3]})
}
case cbG16:
for x := 0; x < d.width; x++ {
ycol := uint16(cdat[2*x+0])<<8 | uint16(cdat[2*x+1])
gray16.Set(x, y, image.Gray16Color{ycol})
}
case cbGA16:
for x := 0; x < d.width; x++ {
ycol := uint16(cdat[4*x+0])<<8 | uint16(cdat[4*x+1])
acol := uint16(cdat[4*x+2])<<8 | uint16(cdat[4*x+3])
nrgba64.Set(x, y, image.NRGBA64Color{ycol, ycol, ycol, acol})
}
case cbTC16:
for x := 0; x < d.width; x++ {
rcol := uint16(cdat[6*x+0])<<8 | uint16(cdat[6*x+1])
gcol := uint16(cdat[6*x+2])<<8 | uint16(cdat[6*x+3])
bcol := uint16(cdat[6*x+4])<<8 | uint16(cdat[6*x+5])
rgba64.Set(x, y, image.RGBA64Color{rcol, gcol, bcol, 0xffff})
}
case cbTCA16:
for x := 0; x < d.width; x++ {
rcol := uint16(cdat[8*x+0])<<8 | uint16(cdat[8*x+1])
gcol := uint16(cdat[8*x+2])<<8 | uint16(cdat[8*x+3])
bcol := uint16(cdat[8*x+4])<<8 | uint16(cdat[8*x+5])
acol := uint16(cdat[8*x+6])<<8 | uint16(cdat[8*x+7])
nrgba64.Set(x, y, image.NRGBA64Color{rcol, gcol, bcol, acol})
}
}
// The current row for y is the previous row for y+1.
pr, cr = cr, pr
}
return img, 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 imgOrErr)
go func() {
img, err := d.idatReader(pr)
if err == os.EOF {
err = FormatError("too little IDAT")
}
pr.CloseWithError(FormatError("too much IDAT"))
d.idatDone <- imgOrErr{img, 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.tmp[0:4])
if err == os.EOF {
return io.ErrUnexpectedEOF
}
if err != nil {
return err
}
length := parseUint32(d.tmp[0:4])
// Read the chunk type.
n, err = io.ReadFull(r, d.tmp[0:4])
if err == os.EOF {
return io.ErrUnexpectedEOF
}
if err != nil {
return err
}
crc := crc32.NewIEEE()
crc.Write(d.tmp[0:4])
// Read the chunk data.
switch string(d.tmp[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 "tRNS":
if d.stage != dsSeenPLTE {
return chunkOrderError
}
err = d.parsetRNS(r, crc, length)
case "IDAT":
if d.stage < dsSeenIHDR || d.stage > dsSeenIDAT || (d.cb == cbP8 && 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.tmp[0:4])
if err == os.EOF {
return io.ErrUnexpectedEOF
}
if err != nil {
return err
}
if parseUint32(d.tmp[0:4]) != crc.Sum32() {
return FormatError("invalid checksum")
}
return nil
}
func (d *decoder) checkHeader(r io.Reader) os.Error {
_, err := io.ReadFull(r, d.tmp[0:8])
if err != nil {
return err
}
if string(d.tmp[0:8]) != pngHeader {
return FormatError("not a PNG file")
}
return nil
}
// Decode reads a PNG image from r and returns it as an image.Image.
// The type of Image returned depends on the PNG contents.
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 != dsSeenIEND {
err = d.parseChunk(r)
if err != nil {
break
}
}
var img image.Image
if d.idatWriter != nil {
d.idatWriter.Close()
ie := <-d.idatDone
if err == nil {
img, err = ie.img, ie.err
}
}
if err != nil {
return nil, err
}
return img, nil
}
// DecodeConfig returns the color model and dimensions of a PNG image without
// decoding the entire image.
func DecodeConfig(r io.Reader) (image.Config, os.Error) {
var d decoder
err := d.checkHeader(r)
if err != nil {
return image.Config{}, err
}
for {
err = d.parseChunk(r)
if err != nil {
return image.Config{}, err
}
if d.stage == dsSeenIHDR && d.cb != cbP8 {
break
}
if d.stage == dsSeenPLTE && d.cb == cbP8 {
break
}
}
var cm image.ColorModel
switch d.cb {
case cbG1, cbG2, cbG4, cbG8:
cm = image.GrayColorModel
case cbGA8:
cm = image.NRGBAColorModel
case cbTC8:
cm = image.RGBAColorModel
case cbP1, cbP2, cbP4, cbP8:
cm = d.palette
case cbTCA8:
cm = image.NRGBAColorModel
case cbG16:
cm = image.Gray16ColorModel
case cbGA16:
cm = image.NRGBA64ColorModel
case cbTC16:
cm = image.RGBA64ColorModel
case cbTCA16:
cm = image.NRGBA64ColorModel
}
return image.Config{cm, d.width, d.height}, nil
}
func init() {
image.RegisterFormat("png", pngHeader, Decode, DecodeConfig)
}