animatedgif.go

package main

import (
	"bufio"
	"fmt"
	"image"
	"image/draw"
	"image/gif"
	"image/png"
	"io"
	"os"
	"path/filepath"
)

func main() {
	const GIF = `some.gif`
	fd, err := os.Open(GIF)
	if err != nil {
		panic(err)
	}
	defer fd.Close()
	frames, err := GetAnimatedGifFrames(bufio.NewReader(fd))
	if err != nil {
		panic(err)
	}
	i := 0
	for frame := range frames {
		fp := filepath.Join("img", fmt.Sprintf("%03d.png", i))
		{
			fd, err := os.OpenFile(fp, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0750)
			if err != nil {
				panic(err)
			}
			bout := bufio.NewWriter(fd)
			png.Encode(bout, frame)
			bout.Flush()
			fd.Close()
		}
		i++
	}
}

func GetAnimatedGifFrames(in io.Reader) (<-chan *image.RGBA, error) {
	g, err := gif.DecodeAll(in)
	if err != nil {
		return nil, err
	}
	imgchan := make(chan *image.RGBA)
	go func() {
		rect := g.Image[0].Rect
		dst := image.NewRGBA(rect)
		noDisposeIndex := -1
		draw.Draw(dst, rect, g.Image[0], image.ZP, draw.Src)
		imgchan <- dst
		for index := 1; index < len(g.Image); index++ {
			switch g.Disposal[index-1] {
			case gif.DisposalNone:
				draw.Draw(dst, rect, g.Image[index], image.ZP, draw.Over)
				noDisposeIndex = index - 1
			case gif.DisposalBackground:
				draw.Draw(dst, rect, g.Image[index], image.ZP, draw.Src)
			case gif.DisposalPrevious:
				if noDisposeIndex >= 0 {
					draw.Draw(dst, rect, g.Image[noDisposeIndex], image.ZP, draw.Src)
					draw.Draw(dst, rect, g.Image[index], image.ZP, draw.Over)
				} else {
					draw.Draw(dst, rect, g.Image[index], image.ZP, draw.Src)
				}
			default:
				draw.Draw(dst, rect, g.Image[index], image.ZP, draw.Over)
			}
			imgchan <- dst
		}
		close(imgchan)
	}()
	return imgchan, nil
}