DiffRgn
DiffRgn Subtract a Region from an other, yielding difference
#include <Quickdraw.h> Quickdraw
void DiffRgn(srcRgnA, srcRgnB, destRgn );
RgnHandle srcRgnA ; the two existing ...
RgnHandle srcRgnB ; ... source Regions
RgnHandle destRgn ; handle of existing region to hold result
DiffRgn calculates the difference between two regions (the area at which one
region does not overlap the other) and stores the result into a third region.
srcRgnA and . . .
srcRgnB are handles of existing regions in the current GrafPort.
destRgn is the handle of an existing Region. Its current structure is
destroyed and it is given the structure defining the difference of
srcRgnB subtracted from srcRgnA. It is OK if this parameter is the
same as either of the source regions.
Returns: none

Notes: The destRgn must already exist (see NewRgn). The difference of srcRgnA
minus srcRgnB is calculated, and the result is stored as destRgn . Note that
the order of the parameters is significant:
srcRgnA ˆsrcRgnB
srcRgnA-srcRgnB srcRgnB-srcRgnA
If srcRgnA is empty, destRgn is set to a region defined by the empty
rectangle (0,0)(0,0). You can test for this via EmptyRgn.