// check above that width != 0, and so len(cdat) != 0.
			for i := 0; i < bytesPerPixel; i++ {
				cdat[i] += pdat[i] / 2
			}
			for i := bytesPerPixel; i < len(cdat); i++ {
				cdat[i] += uint8((int(cdat[i-bytesPerPixel]) + int(pdat[i])) / 2)
			}
		case ftPaeth:
			filterPaeth(cdat, pdat, bytesPerPixel)
		default:
			return nil, FormatError("bad filter type")
		}

//
// This panics instead of returning an error because of backwards
// compatibility. The NewXxx functions do not return an error.
func pixelBufferLength(bytesPerPixel int, r Rectangle, imageTypeName string) int {
	totalLength := mul3NonNeg(bytesPerPixel, r.Dx(), r.Dy())
	if totalLength < 0 {
		panic("image: New" + imageTypeName + " Rectangle has huge or negative dimensions")
	}
	return totalLength
}