Abstract: A system and method is provided for displaying trimmed surfaces on a computer graphics system. The present invention tessellates a three-dimensional surface into triangles and determines whether a trimming curve intersects any triangles. If an intersection occurs, a polygon trimming mask is formed by performing an exclusive OR operation and rendering the polygon into a mask plane. The XOR operation sets the bits in the mask plane corresponding to the trimming mask to logical 1. The rendering hardware then compares the bits in the mask plane with the bits in the frame buffer and draws those pixels which correspond to the bits set to 1 in the mask plane. Since, the trimming mask is a polygon the rendering hardware can perform shading, or lighting calculations using values for points which are contained on the tessellated triangle. In this manner, consistent lighting of a trimmed surface can be achieved.

Abstract: In a data processing system, a method for performing a series of operation sequence results and providing such results including the steps of (1) computing the results for each sequence of operations consecutively and (2) reading the results for the proceeding computations during the computation of a current operation result. This method further includes the use of registers for the temporary storage of the sequence results. During the computation of the operation sequence results, other registers are used in performing the sequence operations. The operations store parameters in a progressive fashion. In other words, the initial operations are performed in one set of registers while the final result from the sequence operation is stored in a different register. The result of a previous operation sequence computation is read from a register that is not being used during the computation of the current sequence operation.