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# Scan Conversion of Polygons

Scan Conversion of Polygons. Dr. Scott Schaefer. Drawing Rectangles. Which pixels should be filled?. Drawing Rectangles. Is this correct?. Drawing Rectangles. What if two rectangles overlap?. Drawing Rectangles. Is this correct?. Drawing Rectangles. Overlap!!!. Is this correct?.

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## Scan Conversion of Polygons

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1. Scan Conversion of Polygons Dr. Scott Schaefer

2. Drawing Rectangles Which pixels should be filled?

3. Drawing Rectangles Is this correct?

4. Drawing Rectangles What if two rectangles overlap?

5. Drawing Rectangles Is this correct?

6. Drawing Rectangles Overlap!!! Is this correct?

7. Drawing Rectangles Solution: Exclude pixels on top and right

8. Drawing Rectangles Artifacts are possible

9. General Polygons – Basic Idea • Intersect scan lines with edges • Find ranges along x • Fill interior of those ranges Don’t fill top/right Edges may NOT match with line drawing algo

10. General Polygons – Basic Idea Use coherence to speed up • Intersect scan lines with edges • Find ranges along x • Fill interior of those ranges Don’t fill top/right Edges may NOT match with line drawing algo

11. General Polygons – Basic Idea Use coherence to speed up • Intersect scan lines with edges • Find ranges along x • Fill interior of those ranges Edges intersection one scan line are mostly same as those intersecting previous scan line

12. General Polygons – Basic Idea Use coherence to speed up • Intersect scan lines with edges • Find ranges along x • Fill interior of those ranges The x-value of an intersection with one scan line is close to the intersection with the previous one

13. General Polygons – Data Structures • Edge: maxY: currentX: xIncr: Edge maxY currentX xIncr

14. General Polygons – Data Structures • Edge: maxY: currentX: xIncr: Edge maxY currentX xIncr Horizontal edges will not be used!!!

15. General Polygons – Data Structures • Active Edge Table: Store a linked-list per scan-line. Insert edges into table at scan-line associated with lowest end-point. Scan Lines Edges

16. General Polygons – Data Structures • Active Edge List: List of all edges intersecting current scan-line sorted by their x-values Edges

17. General Polygons – Algorithm line = 0 While (line < height ) Add edges to Active Edge List from Active Edge Table starting at line Remove edges that end at line Fill pixels Increment x-values on edges in Active Edge List Increment line

18. General Polygons – Example C Active Edge Table B D G Active Edge List F E A

19. General Polygons – Example C Active Edge Table B D G AB Active Edge List F E A

20. General Polygons – Example C Active Edge Table B BC D G AB Active Edge List F E A

21. General Polygons – Example C Active Edge Table B BC D CD G AB Active Edge List F E A

22. General Polygons – Example C Active Edge Table B BC D CD G ED AB Active Edge List F E A

23. General Polygons – Example C Active Edge Table B BC D CD G ED AB Active Edge List F E A

24. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB Active Edge List F E A

25. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E A

26. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E AB AG A maxY currentX xIncr

27. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E AB AG FG ED A maxY currentX xIncr

28. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E AB AG FG ED A maxY currentX xIncr

29. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E AB AG FG ED A maxY currentX xIncr

30. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E AB AG FG ED A maxY currentX xIncr

31. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E AB AG FG ED A maxY currentX xIncr

32. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E AB ED A maxY currentX xIncr

33. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E AB ED A maxY currentX xIncr

34. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E AB ED A maxY currentX xIncr

35. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E AB ED A maxY currentX xIncr

36. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E AB ED CD A maxY currentX xIncr

37. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E AB ED CD A maxY currentX xIncr

38. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E AB CD A maxY currentX xIncr

39. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E AB CD A maxY currentX xIncr

40. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E AB BC CD A maxY currentX xIncr

41. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E AB BC CD A maxY currentX xIncr

42. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E BC CD A maxY currentX xIncr

43. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E BC CD A maxY currentX xIncr

44. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E BC CD A maxY currentX xIncr

45. General Polygons – Example C Active Edge Table B BC D CD G FG ED AB AG Active Edge List F E BC CD A maxY currentX xIncr

46. General Polygons – Problems • Sliver polygons may not be drawn correctly • No simple solution • Long, thin triangles cause problems • Want triangles with good aspect ratio (close to equilateral)

47. Boundary Fill • Start with drawn outline of a polygon and an interior point • Recursively recolor outward from that point • If neighbor different, then recolor and recur • Everything within the boundary is changed to that color

48. Boundary Fill • How to define a neighbor? 8-connected 4-connected

49. Boundary Fill – Example

50. Boundary Fill – Example

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