12_VisibleSurface

12_VisibleSurface - Visible Visible-Surface Detection...

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Unformatted text preview: Visible Visible-Surface Detection Surface Detection Methods Methods Chapter 9 The Visibility Problem 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 n Remove of hidden parts of an object n Hidden-surface removal : surface rendering n Hidden-line removal : line drawing Techniques Techniques n Visible-surface detection, or Hidden-surface elimination n Object-space methods n Compare objects and parts of objects to each other within the scene n Image-space methods n Visibility is decided point by point at each pixel on the projection plane n Most visible-surface algorithms use image-space methods Techniques Techniques n Most algorithms use sorting and coherence methods to improve performance n Sorting n Facilitate depth comparison by ordering individual surfaces in a scene according to their distance from the view plane n Coherence methods n An individual scan line is expected to contain intervals(runs) of constant pixel intensities n Scan-line patterns often change little from one line to the next n Animation frames contain changes only in the vicinity of moving objects Techniques for Efficient Techniques for Efficient Algorithms Algorithms n Bounding volumes n Approximate complex objects with simple enclosures before making comparisons. n The simplest approximate enclosure is a boundary box Back Back-face Removal face Removal (Back (Back-face Culling) face Culling) n A fast and simple object-space method n For locating the back faces of a polyhedron n Remove entire polygons that face away from the viewer > × N V view : back face Back Back-face Removal face Removal (Back (Back-face Culling) face Culling) n Back-face culling after viewing transformation n simpler culling test n consider only z component of normal vectors because the viewing direction is parallel to the viewing z v axis n more points to transform £ C = + + + D Cz By Ax : back face n For a single convex polyhedron, culling completely identifies all the hidden surfaces. n Vertex order in surface normal calculation ß counterclockwise in the right-handed viewing system n Partially hidden faces cannot be determined by back-face culling Back Back-face Removal face Removal (Back (Back-face Culling) face Culling) Depth Depth-Buffer (Z Buffer (Z-Buffer) Buffer) n A commonly used image-space approach for detecting visible surfaces n The basic idea is to test the depth values ( z value) of each surface to determine the closest (visible) surface n z-buffer method n Use two buffers n frame buffer : surface-color values (illumination or shading) n depth buffer ( z-buffer ): depth values ( z value) for each ( x , y ) position Depth Depth-Buffer (Z Buffer (Z-Buffer) Buffer) n As each surface is processed, its depth from the view plane is compared to previously processed surfaces.compared to previously processed surfaces....
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This note was uploaded on 04/29/2010 for the course CSE 4190.411 taught by Professor Shinyeonggil during the Fall '08 term at Seoul National.

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12_VisibleSurface - Visible Visible-Surface Detection...

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