binary.BigEndian.PutUint32(e.tmp[4:8], uint32(b.Dy()))
	// Set bit depth and color type.
	switch e.cb {
	case cbG8:
		e.tmp[8] = 8
		e.tmp[9] = ctGrayscale
	case cbTC8:
		e.tmp[8] = 8
		e.tmp[9] = ctTrueColor
	case cbP8:
		e.tmp[8] = 8
		e.tmp[9] = ctPaletted
	case cbP4:
		e.tmp[8] = 4
		e.tmp[9] = ctPaletted
	case cbP2:
		e.tmp[8] = 2

)

// A cb is a combination of color type and bit depth.
const (
	cbInvalid = iota
	cbG1
	cbG2
	cbG4
	cbG8
	cbGA8
	cbTC8
	cbP1
	cbP2
	cbP4
	cbP8
	cbTCA8
	cbG16
	cbGA16
	cbTC16
	cbTCA16
)

func cbPaletted(cb int) bool {
	return cbP1 <= cb && cb <= cbP8
}

func cbTrueColor(cb int) bool {
	return cb == cbTC8 || cb == cbTC16
}

// Filter type, as per the PNG spec.