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152 7. The Rendering Pipeline
should obviously be culled, but the situation is complicated by the presence
of portals (the doors): some objects in distant rooms may still be visible from
the viewpoint. In computer games and other real-time VR applications, spe-
cial pre-processing is sometimes applied to determine the visibility status of
objects in such scenes.
7.2.1 Culling
For 3D graphics, there are three important types of cull:
1. Remove polygons behind the viewpoint. If the x-coordinate (again,
the assumption being that the viewing direction is along the x-axis) of
all vertices for polygon k are such that x ≤ 1thencullpolygonk.
Remember