The image $in$ is sampled into an image $out$ defined by
$$out(x,y) = c\cdot u([delta +x\cdot step],[delta +y\cdot step]),$$
where $[\cdot]$ means the (lower) integer part, and $c=1$ (default) or
$c=step^2$ (if the \verb+-n+ option is selected).
\index{sampling}
