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
		e.tmp[9] = ctPaletted
	case cbP1:
		e.tmp[8] = 1
		e.tmp[9] = ctPaletted
	case cbTCA8:
		e.tmp[8] = 8

	ctPaletted       = 3
	ctGrayscaleAlpha = 4
	ctTrueColorAlpha = 6
)

// 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 {

		containing color.NRGBA{0xff, 0x00, 0x00, 0x7f}:
			0000000: 8950 4e47 0d0a 1a0a 0000 000d 4948 4452  .PNG........IHDR
			0000010: 0000 0001 0000 0001 0803 0000 0028 cb34  .............(.4
			0000020: bb00 0000 0350 4c54 45ff 0000 19e2 0937  .....PLTE......7
			0000030: 0000 0001 7452 4e53 7f80 5cb4 cb00 0000  ....tRNS..\.....
			0000040: 0e49 4441 5478 9c62 6200 0400 00ff ff00  .IDATx.bb.......