Mixing is controlled only by the area foreground mix. GrePaintRegion returns an error when the region is currently selected as a clip region.

If the preprocessor in the graphics engine receives a GrePaintRegion call for a null region, the preprocessor returns GPI_OK and does not forward the call through the dispatch table to the handling routine. Presentation drivers for devices that do not support BitBlt operations (for example, vector devices such as plotters) do not hook this function. Such drivers hook only the function to return an error code.