Use of Silhouette Edges and Ambient Occlusion in Particle Visualization

1. Use of Silhouette Edges and Ambient Occlusion in Particle Visualization Oral defense of James L. Bigler School of Computing August 16, 2004

2. Outline • Motivation and Introduction • Ambient Occlusion Shading • Silhouette Edges • Conclusions and Future Work

3. Phong Shaded

4. With Silhouettes

5. With Ambient Occlusion

6. With Ambient Occlusion and Silhouettes

7. Why Particle Visualization? Macro Micro Crop by value

8. Material Point Method A B C D

9. How are Particles Visualized?

10. Local Lighting Models Good for local (micro) structure, bad for global (macro) structure.

11. Shadows

12. Global Illumination • Variation in ambient regions • Soft shadows • Interreflection of light between surfaces

13. Acceleration Schemes Greger et al. Ward et al.

14. Wyman Global Illumination for Interactive Isosurfaces Wyman et al. cached global illumination values on a grid. Goal was to maintain interactivity during rendering.

15. Ambient Occlusion or Obscurances Zhukov et al. Iones et al. Precomputed Stored as textures Geometric property

16. Vicinity Shading • James Stewart • Similar to Wyman, precomputes and stores in a texture volume for later use in interactive applications.

17. Silhouette Edges • Gooch et al. (“Interactive technical illustration) • OpenGL based method (polygonal based) • Environment map (angle between normal and eye) • Polygon by polygon software method • Object based methods not appropriate for particles

18. Silhouette Edges from Depth Buffer • Usually black, emphasizes view dependent hull of objects • Saito and Takahashi (“Comprehensible Rendering of 3-D Shapes”) • Cache various aspects of the rendered image • Use depth and convolution to find silhouette edges

19. Particle Ray Tracing • Parker et al. show in “Interactive ray tracing” that large numbers of particles can interactively be rendered using a parallel ray tracer.

20. Outline • Motivation and Introduction • Ambient Occlusion Shading • Silhouette Edges • Conclusions and Future Work

21. Ambient Occlusion

22. Texture Mapping • Common globe uv mapping

23. Texture Generation cosine distribution

24. Texture Resolution • 16x16 provides a nice compromise • Fidelity • Memory • Computation time

25. Dilation

26. How Does This Happen? Outside Inside Linear interpolation is the culprit!

27. How Is This Fixed? Outside Inside Dilation based on value only would results in lightening all dark areas.

28. Inside or Outside Outside Inside Only the “inside” texels should be changed. A way to determine if a texel is “inside” is needed.

29. Inside Texel Detection

30. Proper Dilation Outside Inside Now only “inside” texels are dilated.

31. Dilation Performed

32. Dilation Performed

33. Render Phase • Texture and sphere data loaded in • Sphere ID used to lookup corresponding texture • Removing textures seams

34. Precomputation Time and Memory Bullet Fireball Foam • Using 20 R14K processors on an SGI Origin 3800 (muse.sci.utah.edu). Textures were 16x16 with 49 samples per texel. 543,088 33 min. 132 MB 955,000 66 min. 233 MB 952,755 261 min. 232 MB 7,157,720 12 hours 1,747 MB

35. Impact on Performance • 10% slower than direct lighting alone. • However, using only the ambient occlusion values can yield as good as or better performance than direct lighting alone.

36. Direct Lighting only Images Direct lighting with ambient occlusion textures Ambient occlusion textures only Cylinder 22 Bullet 6 Fireball 11

37. Impact on Performance • 10% slower than direct lighting alone. • However, using only the ambient occlusion values can yield as good as or better performance than direct lighting alone.

38. Results • Movie 1 (show off some data sets) • Movie 2 (use with direct lighting and shadows)

39. Outline • Motivation and Introduction • Ambient Occlusion Shading • Silhouette Edges • Conclusions and Future Work

40. Silhouette Edges • Two options • Precomputation (object based) • Run time • Object based • Image based

41. Ingredients for Edges • Image buffer • Depth buffer • Edge detection kernel • Threshold for zero crossings Laplacian kernel

42. Threshold Edge Response

43. Depth Buffer • Anatomy of a ray • If a and |b| are the same for each pixel we can use the collection of t as a depth buffer. p(t) = a + tb t

44. Movies • Movie 1 (Varying the threshold and changing the view point and field of view) • Movie 2 (Time varying data)

45. Performance A B C D E

46. Outline • Motivation and Introduction • Ambient Occlusion Shading • Silhouette Edges • Conclusions and Future Work

47. Ambient Occlusion • Shows macroscopic structure well • Renderings are still interactive • Precomputation time is reasonable, but still expensive. • Issues with Time-Dependent Visualization

48. Future Work for Ambient Occlusion • Compress the textures to save memory • Reduce the texture generation time • Smaller textures • Better acceleration structures for ray intersections • Look for occlusions in a predefined radius, rather than the whole volume • View dependent texture generation • Update textures during cropping

49. Silhouette Edges • No precomputation time required • Image based method developed has little impact on rendering time • Intuitive user control for selection of how many silhouettes to view • Improved visualization of structure

50. Future Work for Silhouette Edges • Edges of silhouettes are aliased. • Gray levels or varying thickness to indicate degrees of discontinuities in depth. • How to appropriately apply silhouette edges to multi-sampled renderings.