} else {
			e.cb = cbP8
		}
	} else {
		switch m.ColorModel() {
		case color.GrayModel:
			e.cb = cbG8
		case color.Gray16Model:
			e.cb = cbG16
		case color.RGBAModel, color.NRGBAModel, color.AlphaModel:
			if opaque(m) {
				e.cb = cbTC8
			} else {
				e.cb = cbTCA8
			}
		default:
			if opaque(m) {
				e.cb = cbTC16
			} else {
				e.cb = cbTCA16
			}
		}
	}

	Pix []uint8
	// Stride is the Pix stride (in bytes) between vertically adjacent pixels.
	Stride int
	// Rect is the image's bounds.
	Rect Rectangle
}

func (p *Alpha) ColorModel() color.Model { return color.AlphaModel }

func (p *Alpha) Bounds() Rectangle { return p.Rect }

func (p *Alpha) At(x, y int) color.Color {
	return p.AlphaAt(x, y)
}

func (p *Alpha) RGBA64At(x, y int) color.RGBA64 {

func sng(w io.WriteCloser, filename string, png image.Image) {
	defer w.Close()
	bounds := png.Bounds()
	cm := png.ColorModel()
	var bitdepth int
	switch cm {
	case color.RGBAModel, color.NRGBAModel, color.AlphaModel, color.GrayModel:
		bitdepth = 8
	default:
		bitdepth = 16
	}
	cpm, _ := cm.(color.Palette)
	var paletted *image.Paletted
	if cpm != nil {
		switch {
		case len(cpm) <= 2:
			bitdepth = 1