* example of shading correction by subtraction of
* the background.
* background is calculated by maxmin filtering
*
ton                 * timer on
readf SCHEME.IM,A   * read image
* thresholding is not possible
* with uncorrected image:
thresh A,B,1        
* calculate the background by
* maxmin filtering
lmax A,B,31         * max filter
lmin B,C,31         * min filter
sub A,C,255         * subtract background
* obtain from the corrected image
* a binary image by thresholding
thresh C,B,1,220    * threshold
toff