计算机图形学研究.pptVIP

Attributes of Geometric Primitives几何图元的属性 Outline (Hearn Baker, P141: Chapter 4) 1. Fill-Area ★ （4.10,4.13 P159-162. P163-167） 2. Antialiasing ★ （4.17 P175-182） 1. Area Fill Algorithms(区域填充算法) ( Hearn Baker, Section 4-10/13, P159-162. P163-167) Area Fill Fill or colorize a bounded area using a color or pattern Examples: bar chart, surfaces, solid objects, etc. Description of Area The description of the area to be filled is given geometrically: 1. by edges, lines, polygons, circles, etc. (边界定义区域, boundary-defined) 2. by a set of pixels defining a boundary (内部定义区域，interior-defined) Comments on Filling Algorithms Techniques for rastering a polygon or an area: Scan-line filling methods (扫描线填充算法) Scan-line methods (扫描线填充算法) Works row by row from top to bottom A pixel inside the current scan line is only drawn if it is located inside the polygon. 如何区分点在多边形的内部与外部？ 射线法 比较常用 从一点p引射线，如果与多边形边界交点数为偶数，则p在多边形外，否则在多边形内部 如果交点为顶点，需要特别处理 通常射线就是扫描线 Scan-line methods Works for geometrically as well as pixel-wise defined areas Greedy approach very slow!!! ? Improvements possible by exploiting coherence! (连贯性): Scan-line methods The approach is based on scan-line coherence (连贯性): Neighboring pixels are very likely to get the same intensity/color values assigned. The pixel characteristics, i.e. color/intensity values, only change where an edge of the polygon intersects the scan line, i.e. the part of the scan-line between two intersection points is either completely inside or completely outside of the polygon. (扫描线的连贯性) Scan-line methods: Example Example: Scan line y=2: - intersection with polygon at x = 1,8 - we can subdivide the scan line into 3 sections: Scan-line methods: Example Example: Scan line y=4: - intersection with polygon  at x = 1,4,6,8 - we can subdivide the scan line into 5 sections: Scan-line methods Singularities are Problematic: i.e. locations where the scan li