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// Copyright 2011 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 gif implements a GIF image decoder.
//
// The GIF specification is at http://www.w3.org/Graphics/GIF/spec-gif89a.txt.
package gif
import (
"bufio"
"compress/lzw"
"fmt"
"image"
"io"
"os"
)
// If the io.Reader does not also have ReadByte, then decode will introduce its own buffering.
type reader interface {
io.Reader
io.ByteReader
}
// Masks etc.
const (
// Fields.
fColorMapFollows = 1 << 7
// Image fields.
ifLocalColorTable = 1 << 7
ifInterlace = 1 << 6
ifPixelSizeMask = 7
// Graphic control flags.
gcTransparentColorSet = 1 << 0
)
// Section indicators.
const (
sExtension = 0x21
sImageDescriptor = 0x2C
sTrailer = 0x3B
)
// Extensions.
const (
eText = 0x01 // Plain Text
eGraphicControl = 0xF9 // Graphic Control
eComment = 0xFE // Comment
eApplication = 0xFF // Application
)
// decoder is the type used to decode a GIF file.
type decoder struct {
r reader
// From header.
vers string
width int
height int
flags byte
headerFields byte
backgroundIndex byte
loopCount int
delayTime int
// Unused from header.
aspect byte
// From image descriptor.
imageFields byte
// From graphics control.
transparentIndex byte
// Computed.
pixelSize uint
globalColorMap image.PalettedColorModel
// Used when decoding.
delay []int
image []*image.Paletted
tmp [1024]byte // must be at least 768 so we can read color map
}
// blockReader parses the block structure of GIF image data, which
// comprises (n, (n bytes)) blocks, with 1 <= n <= 255. It is the
// reader given to the LZW decoder, which is thus immune to the
// blocking. After the LZW decoder completes, there will be a 0-byte
// block remaining (0, ()), but under normal execution blockReader
// doesn't consume it, so it is handled in decode.
type blockReader struct {
r reader
slice []byte
tmp [256]byte
}
func (b *blockReader) Read(p []byte) (int, os.Error) {
if len(p) == 0 {
return 0, nil
}
if len(b.slice) == 0 {
blockLen, err := b.r.ReadByte()
if err != nil {
return 0, err
}
if blockLen == 0 {
return 0, os.EOF
}
b.slice = b.tmp[0:blockLen]
if _, err = io.ReadFull(b.r, b.slice); err != nil {
return 0, err
}
}
n := copy(p, b.slice)
b.slice = b.slice[n:]
return n, nil
}
// decode reads a GIF image from r and stores the result in d.
func (d *decoder) decode(r io.Reader, configOnly bool) os.Error {
// Add buffering if r does not provide ReadByte.
if rr, ok := r.(reader); ok {
d.r = rr
} else {
d.r = bufio.NewReader(r)
}
err := d.readHeaderAndScreenDescriptor()
if err != nil {
return err
}
if configOnly {
return nil
}
if d.headerFields&fColorMapFollows != 0 {
if d.globalColorMap, err = d.readColorMap(); err != nil {
return err
}
}
Loop:
for err == nil {
var c byte
c, err = d.r.ReadByte()
if err == os.EOF {
break
}
switch c {
case sExtension:
err = d.readExtension()
case sImageDescriptor:
var m *image.Paletted
m, err = d.newImageFromDescriptor()
if err != nil {
break
}
if d.imageFields&fColorMapFollows != 0 {
m.Palette, err = d.readColorMap()
if err != nil {
break
}
// TODO: do we set transparency in this map too? That would be
// d.setTransparency(m.Palette)
} else {
m.Palette = d.globalColorMap
}
var litWidth uint8
litWidth, err = d.r.ReadByte()
if err != nil {
return err
}
if litWidth < 2 || litWidth > 8 {
return fmt.Errorf("gif: pixel size in decode out of range: %d", litWidth)
}
// A wonderfully Go-like piece of magic.
lzwr := lzw.NewReader(&blockReader{r: d.r}, lzw.LSB, int(litWidth))
if _, err = io.ReadFull(lzwr, m.Pix); err != nil {
break
}
// There should be a "0" block remaining; drain that.
c, err = d.r.ReadByte()
if err != nil {
return err
}
if c != 0 {
return os.NewError("gif: extra data after image")
}
// Undo the interlacing if necessary.
if d.imageFields&ifInterlace != 0 {
uninterlace(m)
}
d.image = append(d.image, m)
d.delay = append(d.delay, d.delayTime)
d.delayTime = 0 // TODO: is this correct, or should we hold on to the value?
case sTrailer:
break Loop
default:
err = fmt.Errorf("gif: unknown block type: 0x%.2x", c)
}
}
if err != nil {
return err
}
if len(d.image) == 0 {
return io.ErrUnexpectedEOF
}
return nil
}
func (d *decoder) readHeaderAndScreenDescriptor() os.Error {
_, err := io.ReadFull(d.r, d.tmp[0:13])
if err != nil {
return err
}
d.vers = string(d.tmp[0:6])
if d.vers != "GIF87a" && d.vers != "GIF89a" {
return fmt.Errorf("gif: can't recognize format %s", d.vers)
}
d.width = int(d.tmp[6]) + int(d.tmp[7])<<8
d.height = int(d.tmp[8]) + int(d.tmp[9])<<8
d.headerFields = d.tmp[10]
d.backgroundIndex = d.tmp[11]
d.aspect = d.tmp[12]
d.loopCount = -1
d.pixelSize = uint(d.headerFields&7) + 1
return nil
}
func (d *decoder) readColorMap() (image.PalettedColorModel, os.Error) {
if d.pixelSize > 8 {
return nil, fmt.Errorf("gif: can't handle %d bits per pixel", d.pixelSize)
}
numColors := 1 << d.pixelSize
if d.imageFields&ifLocalColorTable != 0 {
numColors = 1 << ((d.imageFields & ifPixelSizeMask) + 1)
}
numValues := 3 * numColors
_, err := io.ReadFull(d.r, d.tmp[0:numValues])
if err != nil {
return nil, fmt.Errorf("gif: short read on color map: %s", err)
}
colorMap := make(image.PalettedColorModel, numColors)
j := 0
for i := range colorMap {
colorMap[i] = image.RGBAColor{d.tmp[j+0], d.tmp[j+1], d.tmp[j+2], 0xFF}
j += 3
}
return colorMap, nil
}
func (d *decoder) readExtension() os.Error {
extension, err := d.r.ReadByte()
if err != nil {
return err
}
size := 0
switch extension {
case eText:
size = 13
case eGraphicControl:
return d.readGraphicControl()
case eComment:
// nothing to do but read the data.
case eApplication:
b, err := d.r.ReadByte()
if err != nil {
return err
}
// The spec requires size be 11, but Adobe sometimes uses 10.
size = int(b)
default:
return fmt.Errorf("gif: unknown extension 0x%.2x", extension)
}
if size > 0 {
if _, err := io.ReadFull(d.r, d.tmp[0:size]); err != nil {
return err
}
}
// Application Extension with "NETSCAPE2.0" as string and 1 in data means
// this extension defines a loop count.
if extension == eApplication && string(d.tmp[:size]) == "NETSCAPE2.0" {
n, err := d.readBlock()
if n == 0 || err != nil {
return err
}
if n == 3 && d.tmp[0] == 1 {
d.loopCount = int(d.tmp[1]) | int(d.tmp[2])<<8
}
}
for {
n, err := d.readBlock()
if n == 0 || err != nil {
return err
}
}
panic("unreachable")
}
func (d *decoder) readGraphicControl() os.Error {
if _, err := io.ReadFull(d.r, d.tmp[0:6]); err != nil {
return fmt.Errorf("gif: can't read graphic control: %s", err)
}
d.flags = d.tmp[1]
d.delayTime = int(d.tmp[2]) | int(d.tmp[3])<<8
if d.flags&gcTransparentColorSet != 0 {
d.transparentIndex = d.tmp[4]
d.setTransparency(d.globalColorMap)
}
return nil
}
func (d *decoder) setTransparency(colorMap image.PalettedColorModel) {
if int(d.transparentIndex) < len(colorMap) {
colorMap[d.transparentIndex] = image.RGBAColor{}
}
}
func (d *decoder) newImageFromDescriptor() (*image.Paletted, os.Error) {
if _, err := io.ReadFull(d.r, d.tmp[0:9]); err != nil {
return nil, fmt.Errorf("gif: can't read image descriptor: %s", err)
}
left := int(d.tmp[0]) + int(d.tmp[1])<<8
top := int(d.tmp[2]) + int(d.tmp[3])<<8
width := int(d.tmp[4]) + int(d.tmp[5])<<8
height := int(d.tmp[6]) + int(d.tmp[7])<<8
d.imageFields = d.tmp[8]
m := image.NewPaletted(width, height, nil)
// Overwrite the rectangle to take account of left and top.
m.Rect = image.Rect(left, top, left+width, top+height)
return m, nil
}
func (d *decoder) readBlock() (int, os.Error) {
n, err := d.r.ReadByte()
if n == 0 || err != nil {
return 0, err
}
return io.ReadFull(d.r, d.tmp[0:n])
}
// interlaceScan defines the ordering for a pass of the interlace algorithm.
type interlaceScan struct {
skip, start int
}
// interlacing represents the set of scans in an interlaced GIF image.
var interlacing = []interlaceScan{
{8, 0}, // Group 1 : Every 8th. row, starting with row 0.
{8, 4}, // Group 2 : Every 8th. row, starting with row 4.
{4, 2}, // Group 3 : Every 4th. row, starting with row 2.
{2, 1}, // Group 4 : Every 2nd. row, starting with row 1.
}
// uninterlace rearranges the pixels in m to account for interlaced input.
func uninterlace(m *image.Paletted) {
var nPix []uint8
dx := m.Bounds().Dx()
dy := m.Bounds().Dy()
nPix = make([]uint8, dx*dy)
offset := 0 // steps through the input by sequential scan lines.
for _, pass := range interlacing {
nOffset := pass.start * dx // steps through the output as defined by pass.
for y := pass.start; y < dy; y += pass.skip {
copy(nPix[nOffset:nOffset+dx], m.Pix[offset:offset+dx])
offset += dx
nOffset += dx * pass.skip
}
}
m.Pix = nPix
}
// Decode reads a GIF image from r and returns the first embedded
// image as an image.Image.
func Decode(r io.Reader) (image.Image, os.Error) {
var d decoder
if err := d.decode(r, false); err != nil {
return nil, err
}
return d.image[0], nil
}
// GIF represents the possibly multiple images stored in a GIF file.
type GIF struct {
Image []*image.Paletted // The successive images.
Delay []int // The successive delay times, one per frame, in 100ths of a second.
LoopCount int // The loop count.
}
// DecodeAll reads a GIF image from r and returns the sequential frames
// and timing information.
func DecodeAll(r io.Reader) (*GIF, os.Error) {
var d decoder
if err := d.decode(r, false); err != nil {
return nil, err
}
gif := &GIF{
Image: d.image,
LoopCount: d.loopCount,
Delay: d.delay,
}
return gif, nil
}
// DecodeConfig returns the global color model and dimensions of a GIF 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{d.globalColorMap, d.width, d.height}, nil
}
func init() {
image.RegisterFormat("gif", "GIF8?a", Decode, DecodeConfig)
}