A Scanline Algorithm for Displaying Trimmed Surfaces by Using Bézier Clipping

Displaying objects with high accuracy is necessary for CAGD (Computer Aided Geometric Design) and for the synthesis of photo-realistic images. Traditionally, polygonal approximation methods have been employed to display free-form surfaces. They bring on low accuracy of display not only in shape but also in intensity of objects. In this paper a scanline algorithm to directly display parametric surface patches, expressed by trimmed Bézier surfaces, without polygonal approximation is proposed. In the method proposed here, curved surfaces are subdivided into subpatches with curved edges intersecting with a scanline, and the intersections of every subpatch and the scanline are calculated. This method is extremely robust for calculation of the intersections, which can be obtained with only a few iterations; the Bézier clipping method is used for the iteration. Antialiased images with shadows and texture mapping are given to show the effectiveness of the method proposed