1. The edge buffer : A data structure for easy silhouette rendering by John W. Buchanan and Mario C. Sousa

2. Goal Render edges within relatively low end PC apps –obstacle: Z-buffer reading is expensive if it exists All polygons are processed Artists can define edges in the model which must be displayed

3. Edge Buffer (EB) uses: Closed polyhedral objects Open polyhedral objects There are times when the Artist MUST define edges in order to produce accurate results

4. EB Method Attributes Polygonal mesh stored in indexed vertex mesh representation Front/back facing polygons determined Buffer takes up little additional storage space

5. Closed polyhedral objects 1 2 4 3 5 V- Vertex # F- Front B- Back Vertex VFB 1 200300400500 2 300500x00 3 400500x00 4 500x00 5 Edges passed during pre-polygonal set-up Original Object

6. Closed polyhedral objects 1 2 4 3 5 Vertex VFB 1 210310400500 2 310500x00 3 400500x00 4 500x00 5 F bits have 1 added to them in the corresponding fields

7. Closed polyhedral objects 1 2 4 3 5 Vertex VFB 1 210300410500 2 310500x00 3 410500x00 4 500x00 5 Adding 1 to the F bit twice results in a 0 representing a front-facing, absolute edge.

8. Closed polyhedral objects 1 2 4 3 5 Vertex VFB 1 210300410500 2 310500x00 3 411501x00 4 501x00 5 Back-facing bits have 1’s added to them, the first silhouette is found.

9. Closed polyhedral objects 1 2 4 3 5 Vertex VFB 1 210300410500 2 311501x00 3 411500x00 4 501x00 5

10. Closed polyhedral objects 1 2 4 3 5 Vertex VFB 1 211300410501 2 311500x00 3 411500x00 4 501x00 5

11. Closed polyhedral objects 1 2 4 3 5 Vertex VFB 1 211300411500 2 311500x00 3 411500x00 4 500x00 5

12. Closed polyhedral objects 1 2 4 3 5 Vertex VFB 1 211300411500 2 311500x00 3 411500x00 4 500x00 5 Silhouettes are found and rendered

13. Open polyhedral objects Original Open Object Result of edge buffer technique on Open Objects 1 2 3 4 5 6 7 8

14. Open polyhedral objects Original ObjectRendering edges which do not end in 00 1 2 3 4 5 6 7 8

15. Open polyhedral objects Original objectAllow Artist to define edges to be rendered 1 2 3 4 5 6 7 8

16. Determining back-facing artist edges Three new flags are added to bit field: A, F~ and B~. –Artist defined edge. –Front or Back Absolute. --------->

17. RESULTS 11111 10011 00010 11111 00001 A F B F~ B~ 0 0 0 0 1 = Back-Facing Absolute Edge not to be rendered

18. RESULTS 11111 10011 00010 11111 00001 A F B F~ B~ 0 0 0 1 0 = Front-Facing Absolute Edge not to be rendered

19. RESULTS 11111 10011 00010 11111 00001 A F B F~ B~ 1 0 0 1 1 = Artist edge sharing vertices with both back-facing and front- facing polygons will be rendered

20. RESULTS 11111 10011 00010 11111 00001 A F B F~ B~ 1 1 1 1 1 = Artist Edge sharing vertices with both back-facing and front- facing polygons will be rendered 11111 10011 00010 11111 00001

21. RESULTS 11111 10011 00010 11111 00001 A F B F~ B~ 1 1 1 1 1 = Artist edge sharing vertices with both back-facing and front- facing polygons

22. Conclusions Little additional storage space –At least two bits per edge. Front-and back computations being done Objects stored in indexed polygonal mesh representation Artist Edge can be defined during pre- processing. Increased computation adds less than 1% to rendering time