// Copyright 2013 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
import (
"bufio"
"bytes"
"compress/lzw"
"errors"
"image"
"image/color"
"image/color/palette"
"image/draw"
"io"
)
// Graphic control extension fields.
const (
gcLabel = 0xF9
gcBlockSize = 0x04
)
var log2Lookup = [8]int{2, 4, 8, 16, 32, 64, 128, 256}
func log2(x int) int {
for i, v := range log2Lookup {
if x <= v {
return i
}
}
return -1
}
// Little-endian.
func writeUint16(b []uint8, u uint16) {
b[0] = uint8(u)
b[1] = uint8(u >> 8)
}
// writer is a buffered writer.
type writer interface {
Flush() error
io.Writer
io.ByteWriter
}
// encoder encodes an image to the GIF format.
type encoder struct {
// w is the writer to write to. err is the first error encountered during
// writing. All attempted writes after the first error become no-ops.
w writer
err error
// g is a reference to the data that is being encoded.
g GIF
// globalCT is the size in bytes of the global color table.
globalCT int
// buf is a scratch buffer. It must be at least 256 for the blockWriter.
buf [256]byte
globalColorTable [3 * 256]byte
localColorTable [3 * 256]byte
}
// blockWriter writes the block structure of GIF image data, which
// comprises (n, (n bytes)) blocks, with 1 <= n <= 255. It is the
// writer given to the LZW encoder, which is thus immune to the
// blocking.
type blockWriter struct {
e *encoder
}
func (b blockWriter) setup() {
b.e.buf[0] = 0
}
func (b blockWriter) Flush() error {
return b.e.err
}
func (b blockWriter) WriteByte(c byte) error {
if b.e.err != nil {
return b.e.err
}
// Append c to buffered sub-block.
b.e.buf[0]++
b.e.buf[b.e.buf[0]] = c
if b.e.buf[0] < 255 {
return nil
}
// Flush block
b.e.write(b.e.buf[:256])
b.e.buf[0] = 0
return b.e.err
}
// blockWriter must be an io.Writer for lzw.NewWriter, but this is never
// actually called.
func (b blockWriter) Write(data []byte) (int, error) {
for i, c := range data {
if err := b.WriteByte(c); err != nil {
return i, err
}
}
return len(data), nil
}
func (b blockWriter) close() {
// Write the block terminator (0x00), either by itself, or along with a
// pending sub-block.
if b.e.buf[0] == 0 {
b.e.writeByte(0)
} else {
n := uint(b.e.buf[0])
b.e.buf[n+1] = 0
b.e.write(b.e.buf[:n+2])
}
b.e.flush()
}
func (e *encoder) flush() {
if e.err != nil {
return
}
e.err = e.w.Flush()
}
func (e *encoder) write(p []byte) {
if e.err != nil {
return
}
_, e.err = e.w.Write(p)
}
func (e *encoder) writeByte(b byte) {
if e.err != nil {
return
}
e.err = e.w.WriteByte(b)
}
func (e *encoder) writeHeader() {
if e.err != nil {
return
}
_, e.err = io.WriteString(e.w, "GIF89a")
if e.err != nil {
return
}
// Logical screen width and height.
writeUint16(e.buf[0:2], uint16(e.g.Config.Width))
writeUint16(e.buf[2:4], uint16(e.g.Config.Height))
e.write(e.buf[:4])
if p, ok := e.g.Config.ColorModel.(color.Palette); ok && len(p) > 0 {
paddedSize := log2(len(p)) // Size of Global Color Table: 2^(1+n).
e.buf[0] = fColorTable | uint8(paddedSize)
e.buf[1] = e.g.BackgroundIndex
e.buf[2] = 0x00 // Pixel Aspect Ratio.
e.write(e.buf[:3])
var err error
e.globalCT, err = encodeColorTable(e.globalColorTable[:], p, paddedSize)
if err != nil && e.err == nil {
e.err = err
return
}
e.write(e.globalColorTable[:e.globalCT])
} else {
// All frames have a local color table, so a global color table
// is not needed.
e.buf[0] = 0x00
e.buf[1] = 0x00 // Background Color Index.
e.buf[2] = 0x00 // Pixel Aspect Ratio.
e.write(e.buf[:3])
}
// Add animation info if necessary.
if len(e.g.Image) > 1 {
e.buf[0] = 0x21 // Extension Introducer.
e.buf[1] = 0xff // Application Label.
e.buf[2] = 0x0b // Block Size.
e.write(e.buf[:3])
_, err := io.WriteString(e.w, "NETSCAPE2.0") // Application Identifier.
if err != nil && e.err == nil {
e.err = err
return
}
e.buf[0] = 0x03 // Block Size.
e.buf[1] = 0x01 // Sub-block Index.
writeUint16(e.buf[2:4], uint16(e.g.LoopCount))
e.buf[4] = 0x00 // Block Terminator.
e.write(e.buf[:5])
}
}
func encodeColorTable(dst []byte, p color.Palette, size int) (int, error) {
if uint(size) >= uint(len(log2Lookup)) {
return 0, errors.New("gif: cannot encode color table with more than 256 entries")
}
for i, c := range p {
if c == nil {
return 0, errors.New("gif: cannot encode color table with nil entries")
}
var r, g, b uint8
// It is most likely that the palette is full of color.RGBAs, so they
// get a fast path.
if rgba, ok := c.(color.RGBA); ok {
r, g, b = rgba.R, rgba.G, rgba.B
} else {
rr, gg, bb, _ := c.RGBA()
r, g, b = uint8(rr>>8), uint8(gg>>8), uint8(bb>>8)
}
dst[3*i+0] = r
dst[3*i+1] = g
dst[3*i+2] = b
}
n := log2Lookup[size]
if n > len(p) {
// Pad with black.
fill := dst[3*len(p) : 3*n]
for i := range fill {
fill[i] = 0
}
}
return 3 * n, nil
}
func (e *encoder) colorTablesMatch(localLen, transparentIndex int) bool {
localSize := 3 * localLen
if transparentIndex >= 0 {
trOff := 3 * transparentIndex
return bytes.Equal(e.globalColorTable[:trOff], e.localColorTable[:trOff]) &&
bytes.Equal(e.globalColorTable[trOff+3:localSize], e.localColorTable[trOff+3:localSize])
}
return bytes.Equal(e.globalColorTable[:localSize], e.localColorTable[:localSize])
}
func (e *encoder) writeImageBlock(pm *image.Paletted, delay int, disposal byte) {
if e.err != nil {
return
}
if len(pm.Palette) == 0 {
e.err = errors.New("gif: cannot encode image block with empty palette")
return
}
b := pm.Bounds()
if b.Min.X < 0 || b.Max.X >= 1<<16 || b.Min.Y < 0 || b.Max.Y >= 1<<16 {
e.err = errors.New("gif: image block is too large to encode")
return
}
if !b.In(image.Rectangle{Max: image.Point{e.g.Config.Width, e.g.Config.Height}}) {
e.err = errors.New("gif: image block is out of bounds")
return
}
transparentIndex := -1
for i, c := range pm.Palette {
if c == nil {
e.err = errors.New("gif: cannot encode color table with nil entries")
return
}
if _, _, _, a := c.RGBA(); a == 0 {
transparentIndex = i
break
}
}
if delay > 0 || disposal != 0 || transparentIndex != -1 {
e.buf[0] = sExtension // Extension Introducer.
e.buf[1] = gcLabel // Graphic Control Label.
e.buf[2] = gcBlockSize // Block Size.
if transparentIndex != -1 {
e.buf[3] = 0x01 | disposal<<2
} else {
e.buf[3] = 0x00 | disposal<<2
}
writeUint16(e.buf[4:6], uint16(delay)) // Delay Time (1/100ths of a second)
// Transparent color index.
if transparentIndex != -1 {
e.buf[6] = uint8(transparentIndex)
} else {
e.buf[6] = 0x00
}
e.buf[7] = 0x00 // Block Terminator.
e.write(e.buf[:8])
}
e.buf[0] = sImageDescriptor
writeUint16(e.buf[1:3], uint16(b.Min.X))
writeUint16(e.buf[3:5], uint16(b.Min.Y))
writeUint16(e.buf[5:7], uint16(b.Dx()))
writeUint16(e.buf[7:9], uint16(b.Dy()))
e.write(e.buf[:9])
// To determine whether or not this frame's palette is the same as the
// global palette, we can check a couple things. First, do they actually
// point to the same []color.Color? If so, they are equal so long as the
// frame's palette is not longer than the global palette...
paddedSize := log2(len(pm.Palette)) // Size of Local Color Table: 2^(1+n).
if gp, ok := e.g.Config.ColorModel.(color.Palette); ok && len(pm.Palette) <= len(gp) && &gp[0] == &pm.Palette[0] {
e.writeByte(0) // Use the global color table.
} else {
ct, err := encodeColorTable(e.localColorTable[:], pm.Palette, paddedSize)
if err != nil {
if e.err == nil {
e.err = err
}
return
}
// This frame's palette is not the very same slice as the global
// palette, but it might be a copy, possibly with one value turned into
// transparency by DecodeAll.
if ct <= e.globalCT && e.colorTablesMatch(len(pm.Palette), transparentIndex) {
e.writeByte(0) // Use the global color table.
} else {
// Use a local color table.
e.writeByte(fColorTable | uint8(paddedSize))
e.write(e.localColorTable[:ct])
}
}
litWidth := paddedSize + 1
if litWidth < 2 {
litWidth = 2
}
e.writeByte(uint8(litWidth)) // LZW Minimum Code Size.
bw := blockWriter{e: e}
bw.setup()
lzww := lzw.NewWriter(bw, lzw.LSB, litWidth)
if dx := b.Dx(); dx == pm.Stride {
_, e.err = lzww.Write(pm.Pix[:dx*b.Dy()])
if e.err != nil {
lzww.Close()
return
}
} else {
for i, y := 0, b.Min.Y; y < b.Max.Y; i, y = i+pm.Stride, y+1 {
_, e.err = lzww.Write(pm.Pix[i : i+dx])
if e.err != nil {
lzww.Close()
return
}
}
}
lzww.Close() // flush to bw
bw.close() // flush to e.w
}
// Options are the encoding parameters.
type Options struct {
// NumColors is the maximum number of colors used in the image.
// It ranges from 1 to 256.
NumColors int
// Quantizer is used to produce a palette with size NumColors.
// palette.Plan9 is used in place of a nil Quantizer.
Quantizer draw.Quantizer
// Drawer is used to convert the source image to the desired palette.
// draw.FloydSteinberg is used in place of a nil Drawer.
Drawer draw.Drawer
}
// EncodeAll writes the images in g to w in GIF format with the
// given loop count and delay between frames.
func EncodeAll(w io.Writer, g *GIF) error {
if len(g.Image) == 0 {
return errors.New("gif: must provide at least one image")
}
if len(g.Image) != len(g.Delay) {
return errors.New("gif: mismatched image and delay lengths")
}
if g.LoopCount < 0 {
g.LoopCount = 0
}
e := encoder{g: *g}
// The GIF.Disposal, GIF.Config and GIF.BackgroundIndex fields were added
// in Go 1.5. Valid Go 1.4 code, such as when the Disposal field is omitted
// in a GIF struct literal, should still produce valid GIFs.
if e.g.Disposal != nil && len(e.g.Image) != len(e.g.Disposal) {
return errors.New("gif: mismatched image and disposal lengths")
}
if e.g.Config == (image.Config{}) {
p := g.Image[0].Bounds().Max
e.g.Config.Width = p.X
e.g.Config.Height = p.Y
} else if e.g.Config.ColorModel != nil {
if _, ok := e.g.Config.ColorModel.(color.Palette); !ok {
return errors.New("gif: GIF color model must be a color.Palette")
}
}
if ww, ok := w.(writer); ok {
e.w = ww
} else {
e.w = bufio.NewWriter(w)
}
e.writeHeader()
for i, pm := range g.Image {
disposal := uint8(0)
if g.Disposal != nil {
disposal = g.Disposal[i]
}
e.writeImageBlock(pm, g.Delay[i], disposal)
}
e.writeByte(sTrailer)
e.flush()
return e.err
}
// Encode writes the Image m to w in GIF format.
func Encode(w io.Writer, m image.Image, o *Options) error {
// Check for bounds and size restrictions.
b := m.Bounds()
if b.Dx() >= 1<<16 || b.Dy() >= 1<<16 {
return errors.New("gif: image is too large to encode")
}
opts := Options{}
if o != nil {
opts = *o
}
if opts.NumColors < 1 || 256 < opts.NumColors {
opts.NumColors = 256
}
if opts.Drawer == nil {
opts.Drawer = draw.FloydSteinberg
}
pm, ok := m.(*image.Paletted)
if !ok || len(pm.Palette) > opts.NumColors {
// TODO: Pick a better sub-sample of the Plan 9 palette.
pm = image.NewPaletted(b, palette.Plan9[:opts.NumColors])
if opts.Quantizer != nil {
pm.Palette = opts.Quantizer.Quantize(make(color.Palette, 0, opts.NumColors), m)
}
opts.Drawer.Draw(pm, b, m, image.ZP)
}
// When calling Encode instead of EncodeAll, the single-frame image is
// translated such that its top-left corner is (0, 0), so that the single
// frame completely fills the overall GIF's bounds.
if pm.Rect.Min != (image.Point{}) {
dup := *pm
dup.Rect = dup.Rect.Sub(dup.Rect.Min)
pm = &dup
}
return EncodeAll(w, &GIF{
Image: []*image.Paletted{pm},
Delay: []int{0},
Config: image.Config{
ColorModel: pm.Palette,
Width: b.Dx(),
Height: b.Dy(),
},
})
}