The calling function uses the driver's CAPS_ADDITIONAL_GRAPHICS / CAPS_CLIP_FILLS field, which is queried during the first instance of DevOpenDC. If it is set, the data delivered to GrePolyScanline has already been clipped and the COM_PRECLIP flag will be set in the flags WORD of the function number; otherwise, they will be passed as unclipped screen coordinates. Filling is inclusive at the left boundaries and exclusive at the right boundaries. The scan lines are ordered from bottom to top and from left to right.

The handling routine can assume that the two polyshortlines do not cross and both polyshortlines have the same height.