e.tmp[3*i+0] = c1.R
		e.tmp[3*i+1] = c1.G
		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,