Abstract: M idpoint subdivision algorithm uses the line end point codes and associated tests to immediately identify totally visible lines and trivially invisible lines．In order to fully use this simple algorithm to reject most invisible lines，two sets of inclined parallel orthogonal lines passing through four corners of the original window are constructed，forming a filtering window to carry out the midpoint subdivision algorithm．Repeatedly tilting the filtering window ，i．e．changing the inclined angle of its bounding edges in certain way，trivially invisible lines are rejected successively．Theoretical analysis and running test of the new algorithm verified its high efficiency．Parallel architecture could further raise the speed of clipping greatly．