1. Single Pass Point Rendering and Transparent Shading Paper by Yanci Zhang and Renato Pajarola Presentation by Harmen de Weerd and Hedde Bosman

2. The standard 2+1 algorithm Visibility Pass Smooth point interpolation and shading Pass Normalization/shading Pass Well known from the lab sessions

3. The standard 2+1 algorithm cons 2 passes over the dataset with computational intensive shaders Transparency hard to achieve  Back to front alpha blending, z-buffer is turned off  Interpolation between overlapping splats in one layer uses the z-buffer to cull fragments that do not belong in this one layer (i.e. are not visible).  Possible with depth-peeling seen in previous presentation, but uses multiple geometry passes  One other algorithm proposed for alpha blending which cannot use the GPU

4. A new algorithm Idea: Create multiple groups of the point set that do not need a separate visibility pass Render an image for each group postpone  -z-buffer test and smooth point interpolation to an image composition pass

5. 1+1 algorithm

6. Deferred Blending Divide point set S in K Groups

7. 1+1 Algorithm: Grouping

9. Largest First grouping algorithm

10. 1 3 2 4 5 7 6 1 2 3

11. 1 3 2 4 5 7 6 1 2 3

12. 1 3 2 4 5 7 6 1 2 3

13. 1 3 2 4 5 7 6 1 2 3

14. 1 3 2 4 5 7 6 1 2 3

15. 1 3 2 4 5 7 6 1 2 3

16. 1+1 Algorithm: Images... Create K images from K groups Add depth info to images. Since splats do not overlap in object space (because of grouping) we do not need to worry about visibility culling Do use z-buffers since splats might overlap in image space

17. 1+1 Algorithm: Image composition For each resulting fragment:  Determine the minimum depth of the fragment  Determine the sum of the color components if the depth is within  of the minimum depth  Determine the sum of all alpha components if the depth is within  of the minimum depth  Normalize the colors using the alpha: RGB=RGB/A

18. 1+1 Algorithm: Transparency Previous algorithm does not account for transparency, so... Adapt grouping algorithm to make sure no holes exists in surfaces of one group. Render each group image using alpha blending

19. Optimal grouping

20. Transparency Problems cannot account weight and alpha simultaneously Need full surface coverage to look 'through' a material layer Should not have overlap between splats

21. Transparency Problems 2,3

22. Transparency Solutions cannot account weight and alpha simultaneously Solution: Do not account the weight.  Artifacts are reduced dramatically by multiple transparent surface layers  Small errors aren't even visible because color is only 8bit.

23. Grouping algorithm extensions Relaxation of some edge definitions  Virtually co-planer overlapping splats can go in the same group  If normals of splats are in opposite directions (n1. N2 < 0) the splats can be in the same group.  Relax overlap condition with user defined parameter Add points to multiple groups

24. Grouping algorithm extensions Grouping results. a) Splats have smaller overlaps but less surface coverage for K = 8. b) Splats have bigger overlaps but better surface cover for K = 4.

25. Fragment culling Optimally in each transparent surface layer there is exactly one fragment contributing to alpha-blending per pixel Two methods to create this situation:  Reduce z-buffer precision  increase surface coverage in a group image: Increased splat radius Add splats to multiple groups Decrease number of groups

26. Basic 1+1 Algorithm (1)‏ For each group  Render splat with color, depth and kernel weight  Voronoi enhanced depth buffer (lower kernel weight is culled)‏ For each pixel  Determine the minimum depth  For each group image Sum the color and weight attributes if the fragment is in the nearest layer.  Average the sum of color by the sum of weights.

27. Voronoi enhance

28. Basic transparency algorithm (2)‏ Use BSP-tree for efficient back-to-front ordering Transparency blending pass (using  -blending):  Render all splats p i of each group S k using modified radii r into separate target images I k  Perform back-to-front  -blending using material transparency  frag : c new =  frag c old + (1 -  frag ) c frag Compositing pass (PBR blending):  Average color of all images I k into final framebuffer

29. Reduction in z-buffer accuracy Overlapping splats cause too much attenuation Solution: cull fragments that are too close to rendered fragments Report the depth of each fragment in steps of size 

30. High quality transparency algorithm (3)‏ Geometry pass for nearest layer  Use basic algorithm 1 to get depth and kernel information about the nearest layer Geometry pass for other layers  Use transparency algorithm 2, but cull all fragments from nearest layer using depth mask the first pass Composition pass  Perform smooth point interpolation for nearest layer  Average the color over all images for other layers  Combine the nearest layer with the other layers

31. Reflections and refractions Refraction effects and specular reflection improve rendering realism Both are derived from incident viewing vector, surface normal, and environment mapping Can be added to the nearest layer pass of algorithm 3  Downside: only reflections and refractions for nearest visible layer

32. Multi-layer reflection/refraction Accumulate opacity over all layers for each group S k separately Assuming constant material opacity , approximate number of layers l =  total /  Ratio of light absorption: (1 –  ) l Transmitted total refraction: sin  T =  l sin  I

33. Single vs Multi-layer refraction

34. Per fragment shading As with our lab sessions, all needed attributes per fragment are interpolated Deferred shading approach can be used for any attribute other than color:  Phong lighting  Environment map reflection  Multi-layer refraction  Attenuation

35. Visual results Combining opaque and transparent objects Single-pass (a) versus two-pass (b) algorithm

36. Visual results Depth Peeling vs Algorithm 3 vs Algorithm 2

37. Speed Comparison Standard 2+1 PBR algorithm vs the novel 1+1 pass PBR algorithm

38. Visual Comparison

39. Ball Joint

40. David Head

41. Female model

42. Q / A