Level 2 Help for ZFILL

 The length in lines of the window used to scan the input image.

 The width in pixels of the window used to scan the input image.

 The value specified by REPL will be replaced with the average of the
 other pixels within the window.

 The value specified by EXCL will be ignored in the calculation of the
 average of the pixels within a given window.

 Two algorithms (MODEs) are available.  Both produce the same results,
 but their speeds differ, depending upon the number of pixels to be
 replaced.  As a rule of thumb, use the dense algorithm if the fraction 
 of pixels to be replaced is less than 2/NSW, and use the sparse algorithm
 otherwise.