“A scan line algorithm for computer display of curved surfaces” by Whitted

  • ©John Turner Whitted




    A scan line algorithm for computer display of curved surfaces



    The conventional procedure for generating shaded images of curved surfaces is to approximate each surface element by a mosaic of polygons and to then apply one of several established polygon display algorithms. The method described here extends these polygon based techniques to produce an excellent approximation of bi-cubic parametric surfaces in scan line order. Each surface patch is described in terms of cubic edge curves, including parametric curves on the interior of the patch as well as the patch boundaries. Specifying interior edges has the effect of subdividing the patch and generally results in a more accurate image. The silhouette, approximated by a piecewise cubic interpolant, further divides the patch into front facing and rear facing portions. The edge curves are intersected by successive scanning planes to form the endpoints of scan line segments. Depth and surface normal are linearly interpolated between the endpoint values. Visibility is calculated for each segment by a hybrid priority/z-buffer scheme. Shading is computed using Phong’s illumination model with the interpolated surface normal. The algorithm is currently used for display of B-spline surfaces as part of an experimental display processor.

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