Importance

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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. Additionally, the present invention provides for the formation of multiple trimming masks when multiple trimming curves intersect a tessellated triangle. The multiple trimming masks are sequentially XORed into the mask plane and the corresponding pixels are then drawn.

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First Claim