internal/graphics/copy.go
func CopyImage(img image.Image) []byte {
	size := img.Bounds().Size()
	w, h := size.X, size.Y
	bs := make([]byte, 4*w*h)

	switch img := img.(type) {
	case *image.Paletted:
		b := img.Bounds()
		x0 := b.Min.X
		y0 := b.Min.Y
		x1 := b.Max.X
		y1 := b.Max.Y

		palette := make([]uint8, len(img.Palette)*4)
		for i, c := range img.Palette {
			rgba := color.RGBAModel.Convert(c).(color.RGBA)
			palette[4*i] = rgba.R
			palette[4*i+1] = rgba.G
			palette[4*i+2] = rgba.B
			palette[4*i+3] = rgba.A
		}
		// Even img is a subimage of another image, Pix starts with 0-th index.
		idx0 := 0
		idx1 := 0
		d := img.Stride - (x1 - x0)
		for j := 0; j < y1-y0; j++ {
			for i := 0; i < x1-x0; i++ {
				p := int(img.Pix[idx0])
				bs[idx1] = palette[4*p]
				bs[idx1+1] = palette[4*p+1]
				bs[idx1+2] = palette[4*p+2]
				bs[idx1+3] = palette[4*p+3]
				idx0++
				idx1 += 4
			}
			idx0 += d
		}
	default:
		dstImg := &image.RGBA{
			Pix:    bs,
			Stride: 4 * w,
			Rect:   image.Rect(0, 0, w, h),
		}
		draw.Draw(dstImg, image.Rect(0, 0, w, h), img, img.Bounds().Min, draw.Src)
	}
	return bs
}
func CopyImage(img image.Image) []byte {
	size := img.Bounds().Size()
	w, h := size.X, size.Y
	bs := make([]byte, 4*w*h)

	switch img := img.(type) {
	case *image.Paletted:
		b := img.Bounds()
		x0 := b.Min.X
		y0 := b.Min.Y
		x1 := b.Max.X
		y1 := b.Max.Y

		palette := make([]uint8, len(img.Palette)*4)
		for i, c := range img.Palette {
			rgba := color.RGBAModel.Convert(c).(color.RGBA)
			palette[4*i] = rgba.R
			palette[4*i+1] = rgba.G
			palette[4*i+2] = rgba.B
			palette[4*i+3] = rgba.A
		}
		// Even img is a subimage of another image, Pix starts with 0-th index.
		idx0 := 0
		idx1 := 0
		d := img.Stride - (x1 - x0)
		for j := 0; j < y1-y0; j++ {
			for i := 0; i < x1-x0; i++ {
				p := int(img.Pix[idx0])
				bs[idx1] = palette[4*p]
				bs[idx1+1] = palette[4*p+1]
				bs[idx1+2] = palette[4*p+2]
				bs[idx1+3] = palette[4*p+3]
				idx0++
				idx1 += 4
			}
			idx0 += d
		}
	default:
		dstImg := &image.RGBA{
			Pix:    bs,
			Stride: 4 * w,
			Rect:   image.Rect(0, 0, w, h),
		}
		draw.Draw(dstImg, image.Rect(0, 0, w, h), img, img.Bounds().Min, draw.Src)
	}
	return bs
}
// CopyImage copies img to a new RGBA image.
//
// Basically CopyImage just calls draw.Draw.
// If img is a paletted image, an optimized copying method is used.
//
// CopyImage is used only internally but it is exposed for testing.