1. Robust Shadow Maps for Large Environments
Daniel Scherzer
Hi! I‘m Daniel Scherzer from the Vienna Univerersity of Technology and today I want to talk about: „Robust shadow maps for large environments“
Institute of Computer Graphics and Algorithms
Vienna University of Technology

2. Motivation: The Challenge
Huge and dynamic environments
More than 100,000 visible triangles
Automatic shadow generation
No artifacts?
Let’s think of a real challenge for shadow generation.
We may think of an environment which has huge extents and thousends of objects.
As a result of this we may have 100,000 or more visible triangles.
We want every object to be a possible dynamic shadow caster or shadow receiver.
And we want to do this automatic without any need for a user intervention.
And of course we want to achive this without noticable artifacts.
Daniel Scherzer

3. Motivation: Why Shadow Maps?
Independent of scene complexity
Not as fill-rate limited with many polygons as shadow volumes
Only one additional (depth only) render pass
Handle self-shadowing correctly
Handle arbitrary caster/receiver constellations
As the title of the talk already suggests we used shadow maps to conquer the given challenge.
Shadow maps where choosen because they offer great independence of the scene complexity
And as a result are not as easily fill-rate limited as shadow volumes for the case of many polygons.
They also need only one additional depth only render pass.
The also can handle self-shadowing and arbitrary caster/receiver constellations.
Daniel Scherzer

4. Shadow Map Algorithm: Second Pass
Shadow Map Algorithm: First Pass
Light
Eye
Eye-view
Shadow map
Render scene from light-view and save depth values
Render scene from eye-view
Transform each fragment to light source space
Compare zeye with zlight value stored in shadow map
zeye > zlight fragment is in shadow
In the second pass you render the scene from the eyes point-of-view and for each fragment you transform this fragment into light space
and in light space you compare the depth values of the eye-fragment and the shadow map texel at this position.
and if the depth of the eye fragment is bigger than the fragment is in shadow because its farther away than the texel seen from the light source.
Daniel Scherzer

5. Problem: Perspective aliasing
Sufficient resolution far from the observer
Insufficient resolution near the observer
okay
aliased
Daniel Scherzer

6. Problem: Projection aliasing
Receivers ~ perpendicular to shadow plane
With receivers ruffly perpendicular to the shadow plane we have stored little information in the shadow map. From the observers point of view this can lead to very noticable artefacts caused by projection aliasing.
Daniel Scherzer

7. Problem: Self-(un)shadowing
Polygon
Observers‘s distance > shadow depth
Incorrect self-shadowing
Daniel Scherzer

8. Problems of Shadow Maps
Cause
Sample
Error
Perspective aliasing
Insufficient resolution near the observer
Projection aliasing
Insufficient resolution on polygons almost parallel to the light direction
Self-(un) shadowing
Moiré-patterns
Daniel Scherzer

9. Solution: Perspective aliasing
Insufficient resolution near the observer
aliased
okay
Daniel Scherzer

10. Solution: Perspective aliasing
Insufficient resolution near the observer
Redistribute shadow map samples
Daniel Scherzer

11. Solution: Perspective aliasing
Sufficient resolution near the observer
Redistribute shadow map samples
still okay
okay now
Daniel Scherzer

12. Solution: Perspective aliasing
How do we redistribute the shadow map samples?
Using a perspective transformation
Just another perspective matrix
During shadow map generation
During rendering
For further details see [WSP2004]
[WSP2004] M. Wimmer, D. Scherzer, and W. Purgathofer; Light space perspective shadow maps; In Proceedings of Eurographics Symposium on Rendering 2004
Daniel Scherzer

13. Solution: Projection aliasing
Receivers ~ perpendicular to shadow plane
Redistribution doesn‘t work
But!
With receivers ruffly perpendicular to the shadow plane we have stored little information in the shadow map. From the observers point of view this can lead to very noticable artefacts caused by projection aliasing.
Daniel Scherzer

14. Solution: Projection aliasing
Diffuse lighting: I = IL max( dot( L, N ), 0 )
~ perpendicular receivers have small I
Dark Hides artefacts! L N
Daniel Scherzer

15. Solution: Projection aliasing
Guidelines for the light- source
Small ambient term
Diffuse is good for hiding artefacts
Specular is no problem
Light direction and view direction nearly the same
Resolution in shadow map suffices
Daniel Scherzer

16. Solution: Projection aliasing
Screen-space blur of shadows
Hides artefacts
Shadows get softer
Daniel Scherzer

17. Problem: Self-(un)shadowing
Polygon
Biased polygon
Observers‘s distance > shadow depth
Incorrect self-shadowing
Observer‘s distance < shadow depth
Self-shadowing eliminated
Daniel Scherzer

18. Solution: Self-(un)shadowing
How do we choose the bias?
No biasing
Constant biasing
Slope-scale biasing
Daniel Scherzer

19. Solution: Self-(un)shadowing
How do we choose the bias?
Perspective Z is hyperbolic, not linear!
Normal Slope-scale doesn’t work
Do slope-scale biasing
On the post-projective Z-slope
Or calculate linear Z with vertex shader
Another problem occurs for spot lights and for techniques to reduce the perspective aliasing, because here we have a perspective Z.
Daniel Scherzer

20. Solution: Self-(un)shadowing
Other possibility to avoid self-shadowing:
Normally we use the front-side polygons
Now we use the back-side polygons
Only works with watertight models
Daniel Scherzer

21. Conclusions Cause Sample Solution Perspective aliasing
Perspective Transformation (LispSM)
Projection aliasing
Blurring, light-model
Self-(un) shadowing
Biasing, back-side rendering
Daniel Scherzer

22. Putting It All Together
Daniel Scherzer