# lec7 - Hidden Surface Removal Computer graphics Lecture 7...

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Hidden Surface Removal Computer graphics Lecture 7

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Problem Description Input: Set of polygons in three-dimensional space + a viewpoint Output: A two-dimensional image of projected polygons, containing only visible portions
3D Rendering pipeline Somewhere in here we need to decide which objects are visible, and which objects are hidden.

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Hidden Surface Removal A major research topic in computer graphics A whole variety of algorithms. We only cover a few: Hidden Line Removal Back-face detection Quantitative Visibility Hidden Surface Removal Depth Sort/Painter/BSP (will covered in next lecture) Space Subdivision (Warmock) Z-Buffer Advanced methods (will covered in next lectures) Freeform Hidden Surface/Line Removal Ray Tracing
Hidden Surface Removal Hidden Surface Removal algorithms are broadly classified as: Object Space Methods: Compares objects and parts of objects to each other within the scene definition to determine which surfaces are visible Image Space Methods: Visibility is decided point- by-point at each pixel position on the projection plane Image space methods are by far the more common

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Back-Face detection Fast and simple Based on “inside-outside” tests Object space based Back-Face – every face that cant be seen from a viewer point.
Back-Face detection The simplest thing we can do is find the faces on the backs of polyhedra and discard them

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Back-Face detection We know from before that a point ( x, y, z ) is behind a polygon surface if: where A, B, C D are the plane parameters for the surface This can actually be made even easier if we organise things to suit ourselves 0 D Cz By Ax
Back-Face detection Ensure we have a right handed system with the viewing direction along the negative z -axis Can be simplified by V.N > 0 (inner product) If , object descriptions have been converted to projection coordinates and our viewing direction is parallel to the viewing Zv axis, then V = (0,0,Vz) and V.N=Vz.C. Now we can simply say that if the z component of the polygon’s normal (C) is less than zero the surface cannot be seen.

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lec7 - Hidden Surface Removal Computer graphics Lecture 7...

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