Use of Polygon() Versus PolyPolygon()

The information in this article applies to:

• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT versions 3.1 and 3.5
  

SUMMARY

Polygon() draws a polygon, while PolyPolygon() draws a series of polygons. Using multiple calls to Polygon() can offer better performance than using a single call to PolyPolygon(); this is because PolyPolygon() does not consider the polygons to be independent, but considers them to be one polygon with multiple disjointed edges. However, there are times when PolyPolygon() is preferable, particularly if the number of polygons is small.

MORE INFORMATION

PolyPolygon() batches polygons in a single call, so there is less call overhead than there is for multiple calls to Polygon(). However, to perform one combined fill, PolyPolygon() has to work with all the edges in all of the polygons simultaneously, resulting in sorting overhead. The overhead involved in sorting becomes quite expensive when there are a lot of polygons, causing a net loss of performance in comparison to Polygon().

GDI batches multiple Polygon() calls to be more efficient. Setting the batch limit higher than the default of 10 with GdiSetBatchLimit() improves performance even further. GDI and some drivers optimize convex polygons, but will only optimize a single polygon drawn with either Polygon() or PolyPolygon().

Because PolyPolygon() treats all edges as part of one big polygon, it also draws every pixel to be filled exactly once; this may be a performance advantage if a lot of overlapping polygons are drawn, because Polygon() draws every pixel in each polygon only once, even where there is an overlap.

PolyPolygon() considers all the polygons when applying the current fill mode, as set by calling SetPolyFillMode(). Consequently, if any polygons overlap, the result of one PolyPolygon() call may be different than the result of the equivalent multiple Polygon() calls. If the polygons overlap and the raster operation takes the destination pixel values into account, or if you want the fill rule to be applied to overlapping areas, then it is preferable to use PolyPolygon().