1. Gabriel.fournier@bat710.univ-lyon1.fr - http://liris.cnrs.fr/~fournier Laboratoire d'InfoRmatique en Image et Systèmes d'information http://liris.cnrs.fr FRE 2672 WSCGWSCG 2005 WSCG - 2/01/2005 Multi-mesh caching and hardware sampling for progressive and interactive rendering Gabriel Fournier and Bernard Péroche

2. WSCG - 2/01/2005 2 Introduction Our Goal: Interactive rendering  a few images/sec  walkthrough of  triangulated scenes  area light sources on a single PC with  soft shadows  indirect lighting  no long preprocessing

3. WSCG - 2/01/2005 3 The problem Full lighting computations for each pixel  Too long for interactive time Solutions :  Lighting computations for only a few pixels  Speed up and adaptation of light sampling  Progressive improvement of the image quality

4. WSCG - 2/01/2005 4 Outline of the presentation  Previous work  Our method  Overview  Multi mesh  Mesh subdivision  Hardware sampling  Results and discussions  Future work

5. WSCG - 2/01/2005 5 Cache of samples  Lighting cached  Re-used from frame to frame - Object space partition cache : - Irradiance caching (Ward et al. – 1988), Light vectors (Zaninetti and Péroche – 1998)  Complex interpolations - Image cache : - Render cache (Walter et al. 1998),  Missing pixels (Render cache), - Tapestry (Simmons and Séquin - 2000)  First frames with inexact geometry - Object space cache : - On the geometry : Shading cache (Tole et al – 2002)

6. WSCG - 2/01/2005 6 Lighting sampling  What is sampled and interpolated ? - Single global lighting value : - Shading cache (Tole et al. - 2002)  Limited re-use of cached value  Sampling density - Multiple lighting values (direct, indirect, caustic radiance): - Light vectors (Zaninetti and Péroche - 1998)  No method with multiple values cached on the geometry

7. WSCG - 2/01/2005 7 Indirect lighting sampling  Direct irradiance required on CPU  Costly if many area lights (100 rays per light)  Direct irradiance should be cached to be re-used during indirect irradiance sampling

8. WSCG - 2/01/2005 8 Overview  Object space cache : triangle mesh over the geometry  hardware rendering, fast subdivisions and interpolations  Multiple meshes : separate storage of  direct irradiance from each light source  indirect irradiance  Re-use of already computed values  Direct diffuse irradiance for the indirect one  Diffuse radiance from the previous frames  Multi-pass rendering : color / direct radiance / indirect radiance  interactivity, progressiveness

9. WSCG - 2/01/2005 9 Framework Radiance and Irradiance meshes Geometrical Mesh CPUGPU Rendering of material reflectance properties Rendering of the final image Refinement Visible triangles set construction Rendering of radiance Rendering of ID Irradiance sampling User interaction

10. WSCG - 2/01/2005 10 Multiple meshes  Triangular mesh  Simple interpolations  Fast subdivision  Fast ray tracing  OpenGL primitive Geometry mesh Direct irradiance meshes for each light source Direct diffuse radiance mesh Indirect irradiance mesh

11. WSCG - 2/01/2005 11 Mesh example

12. WSCG - 2/01/2005 12 Direct irradiance caching  Each light source sampled and cached separately  Reduce the number of samples

13. WSCG - 2/01/2005 13 Priority = max(sons’ priority) … Geometry mesh Radiance mesh … Element to subdivide Subdivision : element choice  Priority stored in the mesh tree leaves  visible size of the triangle  maximum radiance contrast on its edge  0 if the triangle should not be subdivided

14. WSCG - 2/01/2005 14 Subdivision of a triangle if  it has more than 3 visible pixels and  it is too big or  it has at least 2 required vertices : sampled != interpolated or  it has one edge with visible T-vertices Subdivision : criteria Radiance A M : required vertex B Interpolated radiance Sampled radiance T-vertex A B M C D N

15. WSCG - 2/01/2005 15 Subdivision : radiance difference  Subdivision only if there might be a visible difference  Tone mapping of the values  on screen color  Use of an experimentally built map to get the maximum authorized difference Maximum unnoticeable color difference On screen color Delta max 0 8 0255

16. WSCG - 2/01/2005 16 Sampling density

17. WSCG - 2/01/2005 17 Area light source sampling Ray tracing Hardware sampling Nb samples Time  Small light source bounding frustum  Hardware rendering on a small image  Texture mapping to weight each pixel  Multiple passes to sum the image until it is small enough to be read back  Faster than ray tracing  Frees up the CPU Object to shade Occluder Light source Rendering surface View frustum

18. WSCG - 2/01/2005 18 Indirect lighting sampling  Only 1 bounce indirect diffuse radiance being sampled  Rendering of a low detail version of the diffuse direct radiance mesh  Small part of the incoming irradiance is missing  160° field of view Object to shade Indirect lighting emitters Rendering surface View frustum

19. WSCG - 2/01/2005 19 Final image generation Irradiance of each light sources Indirect diffuse irradiance Scene description CPU ScreenGPU 1x /frame Updated 20x /s Updated 3x /s Rendered 3x~15x /s

20. WSCG - 2/01/2005 20 Results

21. WSCG - 2/01/2005 21 Discussion  Pros  Interactivity with exact geometry  Exact soft shadows  We save some computations compared to the Shading cache method  Cons  Indirect lighting quite limited, still slow  High memory cost 200 MB for the 8000 triangles scene  Non real time

22. WSCG - 2/01/2005 22 Conclusion and future work  Include missing part of radiance (indirect specular, caustic and indirect with more bounces)  Include a visual model to better tune the subdivision of the mesh (priority, precision)  Optimize and increase the quality of indirect lighting computations

23. WSCG - 2/01/2005 23 Questions ?

24. WSCG - 2/01/2005 24 References  Ward, Rubinstein, Clear. 1988. A ray tracing solution for diffuse interreflection. In Computer Graphics (Proceedings of SIGGRAPH 88), vol. 22, 85–92.  Zaninetti, Serpaggi, Péroche. 1998. A vector approach for global illumination in ray tracing. Computer GRaphics Forum, 17(3):149-158, 1998  Simmons, Séquin. 2000. Tapestry: A dynamic mesh-based display representation for interactive rendering. In Rendering Techniques 2000: 11th Eurographics Workshop on Rendering, 329–340.  Tole, Pellacini, Walter, P. Greenberg. 2002. Interactive global illumination in dynamic scenes. ACM transactions on graphics, 21(3) : 537-546, July 2002