The MASK parameter specifies the area within the input channel which
will be processed. Only area under mask will be filtered and the
rest of the image will be unchanged. If a single value is specified,
then this value points to a bitmap segment, which define the area to
be filtered. When four values are specified, these values define the
x,y offsets and x,y dimensions of rectangular window within the
image to be filtered. If defaulted, then entire database is
processed.