Visible - Visible-Surface Detection Methods Chapter 9 Intro...

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Visible-Surface Detection Methods Chapter 9 Intro. to Computer Graphics Spring 2009, Y. G. Shin
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The Visibility Problem [Problem Statement] GIVEN: a set of 3-D surfaces, a projection from 3-D to 2-D screen, DETERMINE: the nearest surface encountered at any point on 2-D screen Remove of hidden parts of an object Hidden-surface removal : surface rendering Hidden-line removal : line drawing
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Techniques Visible-surface algorithms are 3D versions of sorting, i.e., depth comparison Avoid comparing all pairs of objects using the following coherence: object coherence: no comparison between components of objects if objects are separated each other face coherence: surface properties vary smoothly across a face edge coherence: an edge changes its visibility not frequently
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Techniques implied edge coherence: line of intersection of two face can be determined from two intersection points scan-line coherence: little change in visible spans from one scanline to another area coherence: a group of pixels is often covered by the same visible surface span coherence: (special case of area coherence) homogeneous runs in a scanline depth coherence: adjacent parts of the same surface are typically close in depth frame coherence: animation frames contain small changes from the previous frame
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Techniques for Efficient Algorithms Bounding volumes approximate complex objects with simple enclosures before making comparisons. the simplest approximate enclosure is a boundary box
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Backface Removal (Backface Culling) Remove entire polygons that face away from the viewer If we are dealing with a single convex object, culling completely solves the hidden surface problem Geometric test for the visibility surface visible : 0 < N V V N s silhouette : 0 = N V
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Back-face Culling More..
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