1. Fast Cascade VSM
By Zhang Jian

2. Dynamic Shadow Tech Shadow Volume Shadow Map PCF VSM CSM ESM PSM LiPSM
Basic Algorithm
PCF
VSM
CSM
ESM
Soft Shadow
PSM
LiPSM
TSM
CSM
Perspective Alias
Convolution Shadow
Cascaded Shadow
trap·e·zoid
PSSM
2D CSM
3D CSM
Cascade Selection
Per Object Shadow
Deferred Shadow
Rendering Stage

3. Outline Basic Shadow Map and problems Variance Shadow Map
Cascade Shadow Map In Our Engine
Shadow Mask

4. 1. Shadow Map
Basic Idea: If a light can see a point, the point will be lit by the light.

5. 1. Shadow Map
1.Render a texture by using light as a camera, that’s shadow map, the pixel value is depth value.
2. Transform the current position into shadow map space (light space), test if it’s occluded by corresponding shadow map pixel.
3. Done! ?

6. 1.Shadow Map
Peter panning
Perspective Alias

7. 1. Shadow Map Self-shadow Acne Peter panning ac·ne, ai kene
discrete buffer
Orthographic projection
Self-shadow Acne

8. 1.Shadow Map
Peter Panning

9. 1.Shadow Map
Hard Edge

10. 1. Shadow Map Dynamic Alias: Unstable resolution( PSM)
Edge swimming effect
There are bunch of shadow techniques has unstable problem, like PSM
unstable is very annoying, sometimes, it is even worse than low quality.

11. 2.Variance Shadow Map
Soft Shadow

12. 2.Variance Shadow Map Probability theory
dr is the current receiver depth
d0 is the random variable to represent the occluder depth.
VSM is based on Probability theory.
Treat Shadow map depth as a random variable , Represents the occluder depth distribution function
Our goal : define some states between shadow and lit.

13. 2.Variance Shadow Map Chebyshev’s inequality (when dr > μ）
moments M 1 and M 2
μis mean
σ2 is variance, sigma squre

14. 2.Variance Shadow Map Understand VSM by your way.
float d = ( vLightSpacePos.z ) - Depth;
float p_Shadow = 1;
if (d > 0)
{ float variance = ( DepthSquare ) - ( Depth * Depth ); p_Shadow = variance / ( variance + d*d );}
moments M 1 and M 2
μis mean
σ2 is variance

15. VSM In Our Engine Render Z and Z2 into a 2 channels render target
G16R16 ( hardware filter, 1 cycle)
4x MSAA on Xenon ( Free, No predict-tilling)
RGBA8 on PC ( may provide multiple solutions for difference GPU)
Prefilter: 2 pass Gaussian blur
Generate Mipmaps on Xenon.
VSM shadow testing.

16. 2.Variance Shadow Map The good:
Very cheap Soft shadow: Support Pre-filter, Blur == PCF.
Much Better self shadow quality ( solve the shadow bias perfectly)
The ability to use high quality texture filter( trilinear, anistropic filter)
Remember the dilemma between shadow bias and peter panning .

17. 2.Variance Shadow Map The Bad: Light leaking.
Remember the dilemma of shadow bias and peter panning .

18. 2.Variance Shadow Map Light leaking.
Reduce Geometry complexity, use Low Lod for shadow caster.
Tune parameters.(remapping the result)

19. 3. Cascaded Shadow Map
Resolution problem

20. Cascaded Shadow Map Fix resolution problem:
if we fix shadow map resolution for conventional shadow mapping
The far objects require SM cover an area as big as possible, so the far object can cast shadow.
The near objects require SM cover an area as small as possible, so the near object can cover enough SM Texel's

21. Cacade Alignment
What will happen if the camera direction changes. See video EdgeShrim.avi

22. Parallel Split Shadow Map

23. 2D CSM

24. Z precision problem
3D CSM
Side view of ViewFrustum and Light Frustum

25. Cacade Coverage Optimization
PushAway

26. Cacade Transition

27. Interlace updating

28. 4. Shadow Mask
Just an optimization

29. 4.Shadow Mask
Basic Idea

30. Shadow Mask
Why?
Shadow calculation can increase shader complexity a lot.
For most case, it can avoid overdraw.
More aggressive, Postprocess shadow multiply
Why shadow mask

31. Shadow solution

32. Result Memory: ~5M( 3M Vsm + 2M shadow mask)
Performance(ShadowMap+shadowMask):
4 cascade: ms
3 cascade: ms
2 cascade: ms
1 cascade: 1.0~2.0 ms

34. Future Works Optimization Special Mesh LOD for shadow.
Unshadow sky Early out.
Shader optimization
Transparency shadow (e.g.. Smoke shadow)

35. Q&A

36. Declaration
This is from one of my internal presentations. I implemented the shadow tech in our game engine, and shared tips and explanations here. It’s not an official paper, So I don’t have a reference docs list here,