Qualitative invisibility
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In CAD/CAM, Quantitative Invisibility (QI) is the number of solid bodies obscuring a point in space as projected onto a plane (called [vector] hidden line removal used by the Vector format). Oftentimes, when a CAD engineer creates a projection of his model into a plane [the drawing], he wishes to denote the edges which are visible by a solid segment and those which are hidden by dashed or dimmed segments. The idea of keeping track of the number of obscuring bodies gives rise to an algorithm which propages the quantitative invisibility throughout the model. This technique uses edge coherence to speed up the calculations in the algorithm. However, QI really only works well when bodies are larger solids, non-interpenetrating, and not transparent. A technique like this would fall apart when trying to render soft organic tissue as found in the human body, because there is not always a clear delineation of structures. Also, when images become too cluttered and intertwined, then the contribution of this algorithm is marginal.
Quantitative Invisibility is a term coined by Arthur Appel of the graphics group at IBM Watson Research and used in several of his papers.
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[edit] Reference
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, pp. 214-220.
