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Clipping on a Raster Display

Clipping on a Raster Display. Approaches to Clipping. Analytical Clipping. Clipping Lines Against Rectangles. Clipping Rules. Computing Intersections. Cohen-Sutherland Algorithm. Outcodes. Outcode Computation. typedef unsigned int outcode;

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Clipping on a Raster Display

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  1. Clipping on a Raster Display 168 471 Computer Graphics, KKU. Lecture 8

  2. Approaches to Clipping 168 471 Computer Graphics, KKU. Lecture 8

  3. Analytical Clipping 168 471 Computer Graphics, KKU. Lecture 8

  4. Clipping Lines Against Rectangles 168 471 Computer Graphics, KKU. Lecture 8

  5. Clipping Rules 168 471 Computer Graphics, KKU. Lecture 8

  6. Computing Intersections 168 471 Computer Graphics, KKU. Lecture 8

  7. Cohen-Sutherland Algorithm 168 471 Computer Graphics, KKU. Lecture 8

  8. Outcodes 168 471 Computer Graphics, KKU. Lecture 8

  9. Outcode Computation typedef unsigned int outcode; enum {TOP = 0x1, BOTTOM = 0x2, RIGHT = 0x4, LEFT = 0x8} outcode CompOutCode( double x, double y, double xmin, double xmax, double ymin, double ymax) { outcode code = 0; if ( y > ymax ) code |= TOP; else if ( y < ymin ) code |= BOTTOM; if ( x > xmax ) code |= RIGHT; else if ( x < xmin ) code |= LEFT; return code; } 168 471 Computer Graphics, KKU. Lecture 8

  10. Cohen-Sutherland Procedures 168 471 Computer Graphics, KKU. Lecture 8

  11. Cohen-Sutherland Procedures 168 471 Computer Graphics, KKU. Lecture 8

  12. Cohen-Sutherland Algorithm 168 471 Computer Graphics, KKU. Lecture 8

  13. Cohen-Sutherland Algorithm (cont.) 168 471 Computer Graphics, KKU. Lecture 8

  14. Cohen-Sutherland Procedures 168 471 Computer Graphics, KKU. Lecture 8

  15. Parametric Line-Clipping Algorithm • Introduced by Cyrud and Beck in 1978 • Efficiently improved by Liang and Barsky • Essentially find the parameter t from P(t) = P0 + (P1-P0)t 168 471 Computer Graphics, KKU. Lecture 8

  16. Parametric Line-Clipping Algorithm (cont.) • Formally, intersections can be classified as PE (potentially entering) and PL (potentially leaving) on the basis of the angle between P0P1 and Ni • Determine tE or tL for each intersection • Select the line segment that has maximum tE and minimum tL • If tE > tL, then trivially rejected 168 471 Computer Graphics, KKU. Lecture 8

  17. Parametric Line-Clipping Algorithm (cont.) 168 471 Computer Graphics, KKU. Lecture 8

  18. Cyrus-Beck Algorithm (Pseudocode) 168 471 Computer Graphics, KKU. Lecture 8

  19. Clipping Circles and Ellipses • Firstly, do a trivial accept/reject test by intersecting the circle’s/elleipse’s extent with the clip rectengle. • If intersection occurs, divide it into and do the trivial accept/reject test for each. • If scan conversion is fast or if the circle is not too large, scissoring on a pixel-by-pixel basis would be more efficient. 168 471 Computer Graphics, KKU. Lecture 8

  20. Clipping Polygons Example of polygon clipping, (a) Multiple components. (b) Simple convex case. (c) Concave case. 168 471 Computer Graphics, KKU. Lecture 8

  21. Clipping Polygons (cont.) Polygon clipping, edge by edge. (a) Before clipping. (b) Clip on right. (c) Clip on bottom. (d) Clip on left. (e) Clip on top; polygon is fully clipped 168 471 Computer Graphics, KKU. Lecture 8

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