// per table B.3.
	zigStart, zigEnd, ah, al := int32(0), int32(blockSize-1), uint32(0), uint32(0)
	if d.progressive {
		zigStart = int32(d.tmp[1+2*nComp])
		zigEnd = int32(d.tmp[2+2*nComp])
		ah = uint32(d.tmp[3+2*nComp] >> 4)
		al = uint32(d.tmp[3+2*nComp] & 0x0f)
		if (zigStart == 0 && zigEnd != 0) || zigStart > zigEnd || blockSize <= zigEnd {
			return FormatError("bad spectral selection bounds")
		}