gift.go

	in, err := os.Open(img)
	if err != nil {
		log.Println("error open file", err)
		return
	}
	defer in.Close()

	src, err := png.Decode(in)
	if err != nil {
		log.Println("error open file", err)
		return
	}

	g := gift.New(gift.FlipHorizontal())
	dst := image.NewRGBA(g.Bounds(src.Bounds()))
	g.Draw(dst, src)

	outFile := strings.Replace(img, ".png", "_flipH.png", 1)

	out, err := os.OpenFile(outFile, os.O_WRONLY|os.O_CREATE, 0600)
	if err != nil {
		log.Println("error open file", err)
		return
	}
	defer out.Close()

	err = png.Encode(out, dst)
	if err != nil {
		log.Println("error encode file", err)
		return
	}