// 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 https://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
)
func readFull(r io.Reader, b []byte) error {
_, err := io.ReadFull(r, b)
if err == io.EOF {
err = io.ErrUnexpectedEOF
}
return err
}
func readByte(r io.ByteReader) (byte, error) {
b, err := r.ReadByte()
if err == io.EOF {
err = io.ErrUnexpectedEOF
}
return b, err
}
// 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.
//
// To avoid the allocation of a bufio.Reader for the lzw Reader, blockReader
// implements io.ReadByte and buffers blocks into the decoder's "tmp" buffer.
type blockReader struct {
d *decoder
i, j uint8 // d.tmp[i:j] contains the buffered bytes
err error
}
func (b *blockReader) fill() {
if b.err != nil {
return
}
b.j, b.err = readByte(b.d.r)
if b.j == 0 && b.err == nil {
b.err = io.EOF
}
if b.err != nil {
return
}
b.i = 0
b.err = readFull(b.d.r, b.d.tmp[:b.j])
if b.err != nil {
b.j = 0
}
}
func (b *blockReader) ReadByte() (byte, error) {
if b.i == b.j {
b.fill()
if b.err != nil {
return 0, b.err
}
}
c := b.d.tmp[b.i]
b.i++
return c, nil
}
// blockReader must implement io.Reader, but its Read shouldn't ever actually
// be called in practice. The compress/lzw package will only call ReadByte.
func (b *blockReader) Read(p []byte) (int, error) {
if len(p) == 0 || b.err != nil {
return 0, b.err
}
if b.i == b.j {
b.fill()
if b.err != nil {
return 0, b.err
}
}
n := copy(p, b.d.tmp[b.i:b.j])
b.i += uint8(n)
return n, nil
}
// close primarily detects whether or not a block terminator was encountered
// after reading a sequence of data sub-blocks. It allows at most one trailing
// sub-block worth of data. I.e., if some number of bytes exist in one sub-block
// following the end of LZW data, the very next sub-block must be the block
// terminator. If the very end of LZW data happened to fill one sub-block, at
// most one more sub-block of length 1 may exist before the block-terminator.
// These accommodations allow us to support GIFs created by less strict encoders.
// See https://golang.org/issue/16146.
func (b *blockReader) close() error {
if b.err == io.EOF {
// A clean block-sequence terminator was encountered while reading.
return nil
} else if b.err != nil {
// Some other error was encountered while reading.
return b.err
}
if b.i == b.j {
// We reached the end of a sub block reading LZW data. We'll allow at
// most one more sub block of data with a length of 1 byte.
b.fill()
if b.err == io.EOF {
return nil
} else if b.err != nil {
return b.err
} else if b.j > 1 {
return errTooMuch
}
}
// Part of a sub-block remains buffered. We expect that the next attempt to
// buffer a sub-block will reach the block terminator.
b.fill()
if b.err == io.EOF {
return nil
} else if b.err != nil {
return b.err
}
return errTooMuch
}
// decode reads a GIF image from r and stores the result in d.
func (d *decoder) decode(r io.Reader, configOnly, keepAllFrames 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)
}
d.loopCount = -1
err := d.readHeaderAndScreenDescriptor()
if err != nil {
return err
}
if configOnly {
return nil
}
for {
c, err := readByte(d.r)
if err != nil {
return fmt.Errorf("gif: reading frames: %v", err)
}
switch c {
case sExtension:
if err = d.readExtension(); err != nil {
return err
}
case sImageDescriptor:
if err = d.readImageDescriptor(keepAllFrames); err != nil {
return err
}
case sTrailer:
if len(d.image) == 0 {
return fmt.Errorf("gif: missing image data")
}
return nil
default:
return fmt.Errorf("gif: unknown block type: 0x%.2x", c)
}
}
}
func (d *decoder) readHeaderAndScreenDescriptor() error {
err := readFull(d.r, d.tmp[:13])
if err != nil {
return fmt.Errorf("gif: reading header: %v", err)
}
d.vers = string(d.tmp[:6])
if d.vers != "GIF87a" && d.vers != "GIF89a" {
return fmt.Errorf("gif: can't recognize format %q", 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 := readFull(d.r, d.tmp[:3*n])
if err != nil {
return nil, fmt.Errorf("gif: reading 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 := readByte(d.r)
if err != nil {
return fmt.Errorf("gif: reading extension: %v", 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 := readByte(d.r)
if err != nil {
return fmt.Errorf("gif: reading extension: %v", 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 := readFull(d.r, d.tmp[:size]); err != nil {
return fmt.Errorf("gif: reading extension: %v", 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 err != nil {
return fmt.Errorf("gif: reading extension: %v", err)
}
if n == 0 {
return nil
}
if n == 3 && d.tmp[0] == 1 {
d.loopCount = int(d.tmp[1]) | int(d.tmp[2])<<8
}
}
for {
n, err := d.readBlock()
if err != nil {
return fmt.Errorf("gif: reading extension: %v", err)
}
if n == 0 {
return nil
}
}
}
func (d *decoder) readGraphicControl() error {
if err := readFull(d.r, d.tmp[:6]); err != nil {
return fmt.Errorf("gif: can't read graphic control: %s", err)
}
if d.tmp[0] != 4 {
return fmt.Errorf("gif: invalid graphic control extension block size: %d", d.tmp[0])
}
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
}
if d.tmp[5] != 0 {
return fmt.Errorf("gif: invalid graphic control extension block terminator: %d", d.tmp[5])
}
return nil
}
func (d *decoder) readImageDescriptor(keepAllFrames bool) error {
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 {
if !useLocalColorTable {
// Clone the global color table.
m.Palette = append(color.Palette(nil), d.globalColorTable...)
}
if ti := int(d.transparentIndex); ti < len(m.Palette) {
m.Palette[ti] = color.RGBA{}
} else {
// The transparentIndex is out of range, which is an error
// according to the spec, but Firefox and Google Chrome
// seem OK with this, so we enlarge the palette with
// transparent colors. See golang.org/issue/15059.
p := make(color.Palette, ti+1)
copy(p, m.Palette)
for i := len(m.Palette); i < len(p); i++ {
p[i] = color.RGBA{}
}
m.Palette = p
}
}
litWidth, err := readByte(d.r)
if err != nil {
return fmt.Errorf("gif: reading image data: %v", 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{d: d}
lzwr := lzw.NewReader(br, lzw.LSB, int(litWidth))
defer lzwr.Close()
if err = readFull(lzwr, m.Pix); err != nil {
if err != io.ErrUnexpectedEOF {
return fmt.Errorf("gif: reading image data: %v", err)
}
return errNotEnough
}
// In theory, 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 explicit 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[256:257]); n != 0 || (err != io.EOF && err != io.ErrUnexpectedEOF) {
if err != nil {
return fmt.Errorf("gif: reading image data: %v", err)
}
return errTooMuch
}
// In practice, some GIFs have an extra byte in the data sub-block
// stream, which we ignore. See https://golang.org/issue/16146.
if err := br.close(); err == errTooMuch {
return errTooMuch
} else if err != nil {
return fmt.Errorf("gif: reading image data: %v", err)
}
// 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)
}
if keepAllFrames || len(d.image) == 0 {
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
return nil
}
func (d *decoder) newImageFromDescriptor() (*image.Paletted, error) {
if err := 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."
//
// This is conceptually similar to testing
// frameBounds := image.Rect(left, top, left+width, top+height)
// imageBounds := image.Rect(0, 0, d.width, d.height)
// if !frameBounds.In(imageBounds) { etc }
// but the semantics of the Go image.Rectangle type is that r.In(s) is true
// whenever r is an empty rectangle, even if r.Min.X > s.Max.X. Here, we
// want something stricter.
//
// Note that, by construction, left >= 0 && top >= 0, so we only have to
// explicitly compare frameBounds.Max (left+width, top+height) against
// imageBounds.Max (d.width, d.height) and not frameBounds.Min (left, top)
// against imageBounds.Min (0, 0).
if left+width > d.width || top+height > d.height {
return nil, errors.New("gif: frame bounds larger than image bounds")
}
return image.NewPaletted(image.Rectangle{
Min: image.Point{left, top},
Max: image.Point{left + width, top + height},
}, nil), nil
}
func (d *decoder) readBlock() (int, error) {
n, err := readByte(d.r)
if n == 0 || err != nil {
return 0, err
}
if err := readFull(d.r, d.tmp[:n]); err != nil {
return 0, err
}
return int(n), nil
}
// 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, 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 controls the number of times an animation will be
// restarted during display.
// A LoopCount of 0 means to loop forever.
// A LoopCount of -1 means to show each frame only once.
// Otherwise, the animation is looped LoopCount+1 times.
LoopCount int
// 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, true); 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, false); 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)
}
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