An application specifies which pels are drawn when it calls GpiPolygons. There are two options:
When the overlap value is POLYGON_INCL, the bottom right is included in the polygon. The value POLYGON_EXCL, the exclusive value, indicates that the bottom right is excluded from the polygon. This value acts in conjunction with the mix attribute in determining the appearance of a polygon, which is especially important for a group of overlapping or adjacent polygons.
For example, GpiPolygons specifies a number of polygons. Two of the polygons share the vertex pair (6,7) and (9,7). When the polygons are drawn, if the overlap value was POLYGON_INCL, there are two distinct lines to be drawn from (6,7) to (9,7). A mix attribute other than FM_OVERPAINT could cause undesirable results as the line and drawing-surface colors mix.
As with the polygon boundary and construction values, the overlap value can be ORed when calling GpiPolygons to create a specific effect.