b = append(b, trns...)
		}
		b = append(b, idat...)
		b = append(b, iend...)

		var want color.Color
		m, err := Decode(bytes.NewReader(b))
		switch i {
		case 0:
			if err != nil {
				t.Errorf("%d tRNS chunks: %v", i, err)
				continue
			}
			want = color.RGBA{0xff, 0x00, 0x00, 0xff}
		case 1:
			if err != nil {
				t.Errorf("%d tRNS chunks: %v", i, err)
				continue

	case cbTC8, cbTC16:
		if length != 6 {
			return FormatError("bad tRNS length")
		}
		n, err := io.ReadFull(d.r, d.tmp[:length])
		if err != nil {
			return err
		}
		d.crc.Write(d.tmp[:n])

		copy(d.transparent[:], d.tmp[:length])
		d.useTransparent = true

	case cbP1, cbP2, cbP4, cbP8:
		if length > 256 {
			return FormatError("bad tRNS length")
		}
		n, err := io.ReadFull(d.r, d.tmp[:length])

		e.tmp[3*i+2] = c1.B
		if c1.A != 0xff {
			last = i
		}
		e.tmp[3*256+i] = c1.A
	}
	e.writeChunk(e.tmp[:3*len(p)], "PLTE")
	if last != -1 {
		e.writeChunk(e.tmp[3*256:3*256+1+last], "tRNS")
	}
}

// An encoder is an io.Writer that satisfies writes by writing PNG IDAT chunks,
// including an 8-byte header and 4-byte CRC checksum per Write call. Such calls