Shadow Silhouette Maps

1. Shadow Silhouette Maps Eric Chan Massachusetts Institute of Technology

2. Game Plan Motivation Algorithm Implementation Examples Comparison

3. Motivation Why another shadow algorithm? Why not use perspective shadow maps? Stamminger and Drettakis, SIGGRAPH 2002

4. Perspective Shadow Maps Addresses perspective aliasing Optimizes distribution of depth samples Difficulties: • Does not handle projection aliasing • Dueling frusta problem

5. Dueling Frusta Problem

6. Shadow Silhouette Maps • Research at Stanford University • P. Sen, M. Cammarano, and P. Hanrahan • Proceedings of SIGGRAPH 2003 • See course notes • Also available online

7. Observation Shadow maps • undersampling can occur anywhere • artifacts visible only at shadow edges

8. silhouette map depth map How To Fix Silhouettes? One solution: • use a better silhouette approximation

9. Shadow Map (Review) light source blocker receiver

10. Shadow Map (Review) depth map

11. Shadow Map (Review) depth map

12. Shadow Map (Review) depth map

13. Depth Mesh depth mesh (sampling grid)

14. Depth Mesh original grid (blue) dual grid (red) depth mesh + dual mesh

15. Depth Mesh original grid (blue) dual grid (red) discrete silhouette boundary depth mesh + dual mesh

16. Depth Mesh original grid (blue) dual grid (red) discrete silhouette boundary continuous silhouette boundary (green) depth mesh + dual mesh

17. Depth Mesh original grid (blue) dual grid (red) discrete silhouette boundary continuous silhouette boundary (green) silhouette map pixels depth mesh + dual mesh

18. Depth Mesh Deformation Move depth samples to lie on silhouette curve deformed depth mesh

19. Depth Mesh Deformation adjusted depth samples deformed depth mesh

20. Depth Mesh Deformation adjusted depth samples deformed depth mesh

21. Better Approximation piecewise-linear approximation

22. silhouette map Silhouette Map deformed depth map Decomposition of deformed depth map depth map

23. What is a Silhouette Map? Many ways to think about it: • Edge representation • 2D image, same resolution as depth map • Offset from depth map by ½ pixel in x, y • Stores xy-coordinates of silhouette points • Stores only one silhouette point per texel • Piecewise-linear approximation

24. Algorithm

25. Algorithm Properties • Scalable • Treats perspective and projection aliasing • Supports dynamic scenes • Maps to graphics hardware

26. Algorithm Overview Image-space algorithm

27. Algorithm Overview Step 1 Create depth map

28. Algorithm Overview Step 2 Create silhouette map

29. Algorithm Overview Step 3 Render scene and shadows

30. Algorithm Details • Focus now on concepts • Worry later about implementation ?

31. Create Depth Map Same as in regular shadow maps

32. Identify Silhouette Edges Find object-space silhouettes (light’s view)

33. Create Silhouette Map • Rasterize silhouette edges (light’s view) • Find points that lie on silhouette edges • Store one such point per texel silhouette edges silhouette points

34. Compute Silhouette Points Example: point of view of light silhouette edges

35. Compute Silhouette Points silhouette map (dual grid)

36. Compute Silhouette Points rasterization of silhouettes

37. Compute Silhouette Points rasterization of silhouettes pick an edge

38. Compute Silhouette Points rasterization of silhouettes rasterize edge conservatively: be sure to generate fragments for silhouette pixels

39. Compute Silhouette Points rasterization of silhouettes for each fragment: pick a point on the edge

40. Compute Silhouette Points rasterization of silhouettes silhouette points

41. Compute Silhouette Points rasterization of silhouettes do the same for other edges

42. Compute Silhouette Points rasterization of silhouettes completed silhouette map • subtle issues: • only one point per texel • new values overwrite old ones how to pick silhouette points?

43. Picking Silhouette Points Pick a point on the line that lies inside the texel ?

44. Silhouette Point Algorithm Case 1: vertex inside

45. Silhouette Point Algorithm Case 1: vertex inside pick the vertex itself

46. Silhouette Point Algorithm Case 1: Case 2: one intersection vertex inside test for intersection against two diagonals

47. Silhouette Point Algorithm Case 1: Case 2: one intersection vertex inside pick the intersection point itself

48. Silhouette Point Algorithm Case 1: Case 2: one intersection vertex inside Case 3: two intersections

49. Silhouette Point Algorithm Case 1: Case 2: one intersection vertex inside Case 3: use midpoint two intersections

50. Silhouette Point Algorithm Case 1: Case 2: one intersection vertex inside Case 3: Case 4: two intersections no intersections