gif2png/main.go

// main.go
package main

import (
	"errors"
	"flag"
	"fmt"
	"image"
	"image/color"
	"image/draw"
	"image/gif"
	"image/png"
	"os"
	"path/filepath"
)

func main() {
	inPath := stringFlagAliases(
		[]string{"in", "i"},
		"",
		"Input GIF path (required)",
	)

	outDir := stringFlagAliases(
		[]string{"out", "o"},
		"frames",
		"Output directory",
	)

	prefix := stringFlagAliases(
		[]string{"prefix", "p"},
		"frame_",
		"Output filename prefix",
	)

	compose := boolFlagAliases(
		[]string{"compose", "c"},
		true,
		"Compose frames (honor disposal); false = raw stored frames",
	)

	flag.Parse()

	if *inPath == "" {
		fatal(errors.New("missing -in (or -i)"))
	}

	if err := os.MkdirAll(*outDir, 0o755); err != nil {
		fatal(fmt.Errorf("create out dir: %w", err))
	}

	f, err := os.Open(*inPath)
	if err != nil {
		fatal(fmt.Errorf("open input: %w", err))
	}
	defer f.Close()

	g, err := gif.DecodeAll(f)
	if err != nil {
		fatal(fmt.Errorf("decode gif: %w", err))
	}
	if len(g.Image) == 0 {
		fatal(errors.New("gif has no frames"))
	}

	if *compose {
		if err := writeCompositedFrames(g, *outDir, *prefix); err != nil {
			fatal(err)
		}
	} else {
		if err := writeRawFrames(g, *outDir, *prefix); err != nil {
			fatal(err)
		}
	}

	fmt.Printf("Wrote %d frames to %s\n", len(g.Image), *outDir)
}

// Registers multiple flag names that all write to the same *string variable.
func stringFlagAliases(names []string, def, usage string) *string {
	if len(names) == 0 {
		panic("stringFlagAliases: no names provided")
	}
	v := new(string)
	*v = def
	for _, n := range names {
		flag.StringVar(v, n, def, usage)
	}
	return v
}

// Registers multiple flag names that all write to the same *bool variable.
func boolFlagAliases(names []string, def bool, usage string) *bool {
	if len(names) == 0 {
		panic("boolFlagAliases: no names provided")
	}
	v := new(bool)
	*v = def
	for _, n := range names {
		flag.BoolVar(v, n, def, usage)
	}
	return v
}

func writeRawFrames(g *gif.GIF, outDir, prefix string) error {
	for i, frame := range g.Image {
		outPath := filepath.Join(outDir, fmt.Sprintf("%s%04d.png", prefix, i))
		if err := writePNG(outPath, frame); err != nil {
			return fmt.Errorf("write frame %d: %w", i, err)
		}
	}
	return nil
}

// Composites frames to match what you'd visually see when playing the GIF.
func writeCompositedFrames(g *gif.GIF, outDir, prefix string) error {
	canvasRect := image.Rect(0, 0, g.Config.Width, g.Config.Height)
	canvas := image.NewRGBA(canvasRect)

	// Fill initial background.
	bg := gifBackgroundColor(g)
	draw.Draw(canvas, canvasRect, image.NewUniform(bg), image.Point{}, draw.Src)

	var prevCanvas *image.RGBA

	for i, palFrame := range g.Image {
		disposal := byte(0)
		if len(g.Disposal) > i {
			disposal = g.Disposal[i]
		}

		// Save for "restore to previous"
		if disposal == gif.DisposalPrevious {
			prevCanvas = cloneRGBA(canvas)
		} else {
			prevCanvas = nil
		}

		// Draw this frame onto the canvas.
		draw.Draw(canvas, palFrame.Bounds(), palFrame, palFrame.Bounds().Min, draw.Over)

		// Output the composited image for this frame.
		outPath := filepath.Join(outDir, fmt.Sprintf("%s%04d.png", prefix, i))
		if err := writePNG(outPath, canvas); err != nil {
			return fmt.Errorf("write composited frame %d: %w", i, err)
		}

		// Apply disposal *after* output.
		switch disposal {
		case gif.DisposalBackground:
			draw.Draw(canvas, palFrame.Bounds(), image.NewUniform(bg), image.Point{}, draw.Src)
		case gif.DisposalPrevious:
			if prevCanvas != nil {
				copy(canvas.Pix, prevCanvas.Pix)
			}
		default:
			// DisposalNone / Unspecified: keep canvas as-is
		}
	}

	return nil
}

func gifBackgroundColor(g *gif.GIF) color.Color {
	// BackgroundIndex refers to the GIF's global palette if present.
	// In practice, DecodeAll gives each frame a palette; we can use frame 0.
	if len(g.Image) == 0 || g.Image[0] == nil {
		return color.Transparent
	}
	pal := g.Image[0].Palette
	idx := int(g.BackgroundIndex)
	if idx >= 0 && idx < len(pal) {
		return pal[idx]
	}
	return color.Transparent
}

func writePNG(path string, img image.Image) error {
	tmp := path + ".tmp"
	f, err := os.Create(tmp)
	if err != nil {
		return err
	}

	if err := png.Encode(f, img); err != nil {
		_ = f.Close()
		_ = os.Remove(tmp)
		return err
	}

	if err := f.Close(); err != nil {
		_ = os.Remove(tmp)
		return err
	}

	return os.Rename(tmp, path)
}

func cloneRGBA(src *image.RGBA) *image.RGBA {
	dst := image.NewRGBA(src.Rect)
	copy(dst.Pix, src.Pix)
	return dst
}

func fatal(err error) {
	fmt.Fprintln(os.Stderr, "error:", err)
	os.Exit(1)
}