- Hidden line removal
**Hidden line removal**is an extension of wireframe rendering where lines (or segments of lines) covered by surfaces are not drawn.This is not the same as

hidden face removal since this involves depth and occlusion while the other involves normals.A commonly used algorithm to implement it is Arthur Appel's algorithm (Appel, A., "The Notion of Quantitative Invisibility and the Machine Rendering of Solids," Proceedings ACM National Conference, Thompson Books, Washington, DC, 1967, pp. 387-393.). This algorithms works by propagating the visibility from a segment with a known visibility to a segment whose visibility is yet to be determined. Certain pathological cases exist that can make this algorithm difficult to implement. Those cases are (i) vertices on edges and (ii) edges on vertices and (iii) edges on edges. This algorithm is unstable because an error in visibility will be propagated to subsequent nodes (although there are ways to compensate for this problem). Also see James Blinn's "Fractional Invisibility," IEEE Computer Graphics and Applications, Nov. 1988, pp. 77-84.

**External links*** [

*http://www.chez.com/pmaillot Patrick-Gilles Maillot's Thesis*] an extension of the Bresenham line drawing algorithm to perform 3D hidden lines removal; also published in MICAD '87 proceedings on CAD/CAM and Computer Graphics, page 591 - ISBN 2-86601-084-1.* [

*http://wheger.tripod.com/vhl/vhl.htm Vector Hidden Line Removal*] An article by Walter Heger with a further description (of the pathological cases) and more citations.

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