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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 and encoder.
//
// The GIF specification is at http://www.w3.org/Graphics/GIF/spec-gif89a.txt.
package gif
import (
"bufio"
"compress/lzw"
"errors"
"fmt"
"image"
"image/color"
"io"
)
var (
errNotEnough = errors.New("gif: not enough image data")
errTooMuch = errors.New("gif: too much image data")
errBadPixel = errors.New("gif: invalid pixel value")
)
// 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.
fColorTable = 1 << 7
fInterlace = 1 << 6
fColorTableBitsMask = 7
// Graphic control flags.
gcTransparentColorSet = 1 << 0
gcDisposalMethodMask = 7 << 2
)
// Disposal Methods.
const (
DisposalNone = 0x01
DisposalBackground = 0x02
DisposalPrevious = 0x03
)
// 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
loopCount int
delayTime int
backgroundIndex byte
disposalMethod byte
// From image descriptor.
imageFields byte
// From graphics control.
transparentIndex byte
hasTransparentIndex bool
// Computed.
globalColorTable color.Palette
// Used when decoding.
delay []int
disposal []byte
image []*image.Paletted
tmp [1024]byte // must be at least 768 so we can read color table
}
// 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, ()), which is consumed when checking that the
// blockReader is exhausted.
type blockReader struct {
r reader
slice []byte
err error
tmp [256]byte
}
func (b *blockReader) Read(p []byte) (int, error) {
if b.err != nil {
return 0, b.err
}
if len(p) == 0 {
return 0, nil
}
if len(b.slice) == 0 {
var blockLen uint8
blockLen, b.err = b.r.ReadByte()
if b.err != nil {
return 0, b.err
}
if blockLen == 0 {
b.err = io.EOF
return 0, b.err
}
b.slice = b.tmp[:blockLen]
if _, b.err = io.ReadFull(b.r, b.slice); b.err != nil {
return 0, b.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) 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
}
for {
c, err := d.r.ReadByte()
if err != nil {
return err
}
switch c {
case sExtension:
if err = d.readExtension(); err != nil {
return err
}
case sImageDescriptor:
m, err := d.newImageFromDescriptor()
if err != nil {
return err
}
useLocalColorTable := d.imageFields&fColorTable != 0
if useLocalColorTable {
m.Palette, err = d.readColorTable(d.imageFields)
if err != nil {
return err
}
} else {
if d.globalColorTable == nil {
return errors.New("gif: no color table")
}
m.Palette = d.globalColorTable
}
if d.hasTransparentIndex && int(d.transparentIndex) < len(m.Palette) {
if !useLocalColorTable {
// Clone the global color table.
m.Palette = append(color.Palette(nil), d.globalColorTable...)
}
m.Palette[d.transparentIndex] = color.RGBA{}
}
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.
br := &blockReader{r: d.r}
lzwr := lzw.NewReader(br, lzw.LSB, int(litWidth))
defer lzwr.Close()
if _, err = io.ReadFull(lzwr, m.Pix); err != nil {
if err != io.ErrUnexpectedEOF {
return err
}
return errNotEnough
}
// Both lzwr and br should be exhausted. Reading from them should
// yield (0, io.EOF).
//
// The spec (Appendix F - Compression), says that "An End of
// Information code... must be the last code output by the encoder
// for an image". In practice, though, giflib (a widely used C
// library) does not enforce this, so we also accept lzwr returning
// io.ErrUnexpectedEOF (meaning that the encoded stream hit io.EOF
// before the LZW decoder saw an explict end code), provided that
// the io.ReadFull call above successfully read len(m.Pix) bytes.
// See https://golang.org/issue/9856 for an example GIF.
if n, err := lzwr.Read(d.tmp[:1]); n != 0 || (err != io.EOF && err != io.ErrUnexpectedEOF) {
if err != nil {
return err
}
return errTooMuch
}
if n, err := br.Read(d.tmp[:1]); n != 0 || err != io.EOF {
if err != nil {
return err
}
return errTooMuch
}
// Check that the color indexes are inside the palette.
if len(m.Palette) < 256 {
for _, pixel := range m.Pix {
if int(pixel) >= len(m.Palette) {
return errBadPixel
}
}
}
// Undo the interlacing if necessary.
if d.imageFields&fInterlace != 0 {
uninterlace(m)
}
d.image = append(d.image, m)
d.delay = append(d.delay, d.delayTime)
d.disposal = append(d.disposal, d.disposalMethod)
// The GIF89a spec, Section 23 (Graphic Control Extension) says:
// "The scope of this extension is the first graphic rendering block
// to follow." We therefore reset the GCE fields to zero.
d.delayTime = 0
d.hasTransparentIndex = false
case sTrailer:
if len(d.image) == 0 {
return io.ErrUnexpectedEOF
}
return nil
default:
return fmt.Errorf("gif: unknown block type: 0x%.2x", c)
}
}
}
func (d *decoder) readHeaderAndScreenDescriptor() error {
_, err := io.ReadFull(d.r, d.tmp[:13])
if err != nil {
return err
}
d.vers = string(d.tmp[: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
if fields := d.tmp[10]; fields&fColorTable != 0 {
d.backgroundIndex = d.tmp[11]
// readColorTable overwrites the contents of d.tmp, but that's OK.
if d.globalColorTable, err = d.readColorTable(fields); err != nil {
return err
}
}
// d.tmp[12] is the Pixel Aspect Ratio, which is ignored.
return nil
}
func (d *decoder) readColorTable(fields byte) (color.Palette, error) {
n := 1 << (1 + uint(fields&fColorTableBitsMask))
_, err := io.ReadFull(d.r, d.tmp[:3*n])
if err != nil {
return nil, fmt.Errorf("gif: short read on color table: %s", err)
}
j, p := 0, make(color.Palette, n)
for i := range p {
p[i] = color.RGBA{d.tmp[j+0], d.tmp[j+1], d.tmp[j+2], 0xFF}
j += 3
}
return p, nil
}
func (d *decoder) readExtension() 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[: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
}
}
}
func (d *decoder) readGraphicControl() error {
if _, err := io.ReadFull(d.r, d.tmp[:6]); err != nil {
return fmt.Errorf("gif: can't read graphic control: %s", err)
}
flags := d.tmp[1]
d.disposalMethod = (flags & gcDisposalMethodMask) >> 2
d.delayTime = int(d.tmp[2]) | int(d.tmp[3])<<8
if flags&gcTransparentColorSet != 0 {
d.transparentIndex = d.tmp[4]
d.hasTransparentIndex = true
}
return nil
}
func (d *decoder) newImageFromDescriptor() (*image.Paletted, error) {
if _, err := io.ReadFull(d.r, d.tmp[: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]
// The GIF89a spec, Section 20 (Image Descriptor) says:
// "Each image must fit within the boundaries of the Logical
// Screen, as defined in the Logical Screen Descriptor."
bounds := image.Rect(left, top, left+width, top+height)
if bounds != bounds.Intersect(image.Rect(0, 0, d.width, d.height)) {
return nil, errors.New("gif: frame bounds larger than image bounds")
}
return image.NewPaletted(bounds, nil), nil
}
func (d *decoder) readBlock() (int, error) {
n, err := d.r.ReadByte()
if n == 0 || err != nil {
return 0, err
}
return io.ReadFull(d.r, d.tmp[: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, 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.
// Disposal is the successive disposal methods, one per frame. For
// backwards compatibility, a nil Disposal is valid to pass to EncodeAll,
// and implies that each frame's disposal method is 0 (no disposal
// specified).
Disposal []byte
// Config is the global color table (palette), width and height. A nil or
// empty-color.Palette Config.ColorModel means that each frame has its own
// color table and there is no global color table. Each frame's bounds must
// be within the rectangle defined by the two points (0, 0) and
// (Config.Width, Config.Height).
//
// For backwards compatibility, a zero-valued Config is valid to pass to
// EncodeAll, and implies that the overall GIF's width and height equals
// the first frame's bounds' Rectangle.Max point.
Config image.Config
// BackgroundIndex is the background index in the global color table, for
// use with the DisposalBackground disposal method.
BackgroundIndex byte
}
// DecodeAll reads a GIF image from r and returns the sequential frames
// and timing information.
func DecodeAll(r io.Reader) (*GIF, 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,
Disposal: d.disposal,
Config: image.Config{
ColorModel: d.globalColorTable,
Width: d.width,
Height: d.height,
},
BackgroundIndex: d.backgroundIndex,
}
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, error) {
var d decoder
if err := d.decode(r, true); err != nil {
return image.Config{}, err
}
return image.Config{
ColorModel: d.globalColorTable,
Width: d.width,
Height: d.height,
}, nil
}
func init() {
image.RegisterFormat("gif", "GIF8?a", Decode, DecodeConfig)
}