{
    my ($ix, $iy, @rgb);     # preallocate and reuse
    for $ix (0 .. $size[0]-1) {
        for $iy (0 .. $size[1]-1) {
            @rgb = $image->rgb(image->getPixel($ix,$iy)));
            @rgb = $reference_image->rgb( $reference_image->colorClosestHWB( @rgb ));
            $new_image->setPixel($ix, $iy, getindex($new_image, @rgb));
        }
    }
}