Pismo

1. Pismo Parallax-Interpolated Shadow Map Occlusion Ivan Neulander

2. Introduction:Soft Shadows • Problem: Render soft shadows from an area light source • Light source can be any polygonal model • Want fast solution, of sufficientquality for visual effects • Want speed vs. quality control Area Light Umbra Penumbra

3. Introduction:Soft Shadows • Some Solutions: • Ray tracing • Very high quality, physically accurate but usually slow • “Shadow Map” ray tracing: project ray to map • Lower quality, relatively fast • Samples shadow map many times per ray • Soft Shadows by Ray Tracing Multilayer Transparent Shadow Maps: Xie, Tabellion, Pearce • Efficient Image-Based Methods for Rendering Soft Shadows: Agarwala et al • Use many shadow maps: overlapping shadows • Fast at low quality but very slow at high • Use many shadow maps: Pismo • Almost as fast as (c), but much higher quality • Samples shadow map few times per ray 4 Shadow Maps 8 Shadow Maps 16 Shadow Maps 32 Shadow Maps 64 Shadow Maps 128 Shadow Maps 256 Shadow Maps Pismo with 4 Shadow Maps

4. Soft Shadow Example Area Light

5. Hard Shadows from 3 Point Lights with Shadow Maps

6. Pismo Shadow Estimation: Problem • Problem: Estimate shadow ray PL, extending from P to arbitrary point L on area light • Use the 3 shadow maps rendered from thetriangle vertices, all fixated on a common focal point F P L

7. Pismo Shadow Estimation: Solution • Solution: 1) Compute position of putative occluder O • Displace P toward L by distance d • How to compute d ? P d O L

8. Pismo Shadow Estimation: SolutionShadow Map #1 • Solution: 2) Project Ointo screen space of Shadow Map #1 • Use depth value at this pixel to compute ray occlusion • Using O instead of P accounts for parallaxshift between view from Shadow Mapcamera and view from L P d O L

9. Pismo Shadow Estimation: SolutionShadow Map #2 • Likewise, project Ointo screen space of Shadow Map #2 P d O L

10. Pismo Shadow Estimation: Solution Shadow Map #3 • Likewise, project Ointo screen space of Shadow Map #3 P d O L

11. Pismo Shadow Estimation: Solution Blending the Three Shadow Maps • Barycentrically blend the 3 shadow maps’ occlusion resultsto get the final occlusion estimate for the shadow ray PL • Uses 3 shadow map queries per ray P d O L

12. Pismo Shadow Estimation: Solution Compute Multiple Shadow Rays • Repeat the process for multiple shadow rays between Pand other light positions on the area light P

13. Estimating Initial Depth d:Preprocess Shadow Maps • For each Shadow Map, we generate aBled Shadow Map: an image-processedcopy where we • bleed out silhouette pixels • apply a slight blur • One-time preprocess per shadow map • Regular Shadow Map stores multipleoccluder depths per pixel (layered) • Bled Shadow Map stores max depth only

14. Estimating Initial Depth d:Compute d1 • Project P to a pixel in Bled Shadow Map #1 • If queried depth is less than depth from Pto L, subtract and we’re done; else, compute following average: • Sample multiple jittered pixels in(layered) Shadow Map, centeredabout original pixel • For each sample:If queried depth is less thandepth from Pto L,adddifference to average • Computed once per shading sample (not per ray) P d1 L

15. Estimating Initial Depth d:Compute d2 • Project P to a pixel in Bled Shadow Map #2 • Estimate value for d2as above • Computed once per shading sample P d2 L

16. Estimating Initial Depth d:Compute d3 • Project P to a pixel in Bled Shadow Map #3 • Estimate value for d3as above • Computed once per shading sample P d3 L

17. Estimating Initial Depth d:Blend d1, d2, d3 • Barycentrically blend d1,d2,d3 to get d for given L • Use blended dvalue to compute O as shown • Computed per-ray (but no shadow map queries) P d L

18. Depth Compensation:Problem • Depth from L to P (LP) differs from depth fromShadow Map D to P (DP) • Need to compensate depth estimatesthat use D in place of L P DP D LP L

19. Depth Compensation:Solution • Adjust initial estimates d1,d2,d3by LP – DP • Blend d1,d2,d3 and compute O • Adjust depth queried from D (based on O)by LO – DO D (Shadow Map) L LO LP O DO P DP F (Focal Point)

20. Contact Shadows • Problem: Pismo requires a higher shadow bias than regular shadowing to avoid self-shadow artifacts • This attenuates crisp short-range shadows • Solution: Compute contact shadows at close range and blend them in with Pismo Shadows

21. Contact Shadows:Implementation • A Contact Shadow: • is based on hard shadows from discrete shadow maps • consists of 2 or more such shadows that overlap • fades out beyond Pismo shadow bias distance • is blended with Pismo shadow over bias distance blend =

22. Results:Area Light with 4 Shadow Maps PismoOff PismoOn 128 rays / pixel Area Light

23. Results:Area Light with 5 Shadow Maps PismoOff PismoOn 128 rays / pixel Area Light

24. 1600x1200 resolution • 128 rays / pixel • Athlon64 @ 2800 MHz Performance:Pismo vs. Ray Tracer Pismo@4 5 DMs: 33 s Final: 263 s Total: 296 s Pismo@5 5 DMs: 42 s Final: 246 s Total: 288 s Ray-Traced Total: 1073 s

25. 4 Blurred Shadows 4 Pismo Shadows 5 Blurred Shadows 5 Pismo Shadows 9 Blurred Shadows 9 Pismo Shadows Quality:Large Area Light Ray Tracer 4 Hard Shadows 5 Hard Shadows 9 Hard Shadows

26. 4 Blurred Shadows 4 Pismo Shadows 5 Blurred Shadows 5 Pismo Shadows 9 Blurred Shadows 9 Pismo Shadows 4 Hard Shadows 5 Hard Shadows 9 Hard Shadows

27. Quality:Pismo@4 vs. Ray Tracer Pismo@4 Pixel Difference Ray Tracer

28. Quality:Pismo@5 vs. Ray Tracer Pismo@5 Pixel Difference Ray Tracer

29. Quality:Pismo@9 vs. Ray Tracer Pismo@9 Pixel Difference Ray Tracer

30. Conclusions Pismo is a useful enhancement to rendering soft shadows with multiple shadow maps • Conceptually simple & easy to implement • integrates easily into an existing rendering pipeline • Provides an intuitive control over speed/quality • Creation of multiple shadow maps easily accelerated: • Multiprocessing • GPU rendering • Morphing

31. Future Work • Accelerate creation of shadow maps • Automate tessellation of light polygons • Improve blending of contact shadows • Parallelize Pismo sampling

32. Ivan Neulander www.rhythm.com/~ivan/pubs.html ivan@rhythm.com