Generates a portable graymap array of size
width x
height (or
width x
width if
height is not specified) to be used as a convolution file by
pnmconvol. The data in the convolution array K are computed according to the
formula:

The computation time is proportional to
width *
height. This increases rapidly with the increase of the kernel size.
A better approach could be using a FFT in these cases.