1. Parallax-Interpolated Shadow Map Occlusion
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 sufficient quality for visual effects
Want speed vs. quality control
Area Light
Penumbra
Umbra

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
128 Shadow Maps
Pismo with 4 Shadow Maps
64 Shadow Maps
256 Shadow Maps
16 Shadow Maps
4 Shadow Maps
8 Shadow Maps
32 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 the triangle 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: Solution Shadow Map #1
Solution: 2) Project O into screen space of Shadow Map #1
Use depth value at this pixel to compute ray occlusion
Using O instead of P accounts for parallax shift between view from Shadow Map camera and view from L P d O L

9. Pismo Shadow Estimation: Solution Shadow Map #2
Likewise, project O into screen space of Shadow Map #2 P d O L

10. Pismo Shadow Estimation: Solution Shadow Map #3
Likewise, project O into 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 results to 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 P and other light positions on the area light P

13. Estimating Initial Depth d: Preprocess Shadow Maps
For each Shadow Map, we generate a Bled Shadow Map: an image-processed copy where we
bleed out silhouette pixels
apply a slight blur
One-time preprocess per shadow map
Regular Shadow Map stores multiple occluder 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 P to L, subtract and we’re done; else, compute following average:
Sample multiple jittered pixels in (layered) Shadow Map, centered about original pixel
For each sample: If queried depth is less than depth from P to L, add difference 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 d2 as 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 d3 as 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 d value 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 from Shadow Map D to P (DP)
Need to compensate depth estimates that use D in place of L P D DP LP L

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

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
Pismo Off
Pismo On
128 rays / pixel
Area Light

23. Results: Area Light with 5 Shadow Maps
Pismo Off
Pismo On
128 rays / pixel
Area Light

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

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

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

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

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

29. Quality: Pismo@9 vs. Ray Tracer
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