Robust Shadow Maps for Large Environments

Daniel Scherzer

Institute of Computer Graphics and Algorithms

Vienna University of Technology

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Motivation: The Challenge

Huge and dynamic environments

More than 100,000 visible triangles

Automatic shadow generation

No artifacts?

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Motivation: Why Shadow Maps?

Independent of scene complexityNot 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

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Shadow Map Algorithm: Second Pass

EyeLight

Render scene from light-view and save depth values

Render scene from eye-viewTransform each fragment to light source space

Compare zeye with zlight value stored in shadow map

zeye > zlight fragment is in shadow

Shadow Map Algorithm: First Pass

Shadow mapEye-view

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Problem: Perspective aliasing

Sufficient resolution far from the observer

Insufficient resolution near the observer

aliasedokay

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Problem: Projection aliasing

Receivers ~ perpendicular to shadow plane

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Problem: Self-(un)shadowing

Observers‘s distance > shadow depthIncorrect self-shadowing

Polygon

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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

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Solution: Perspective aliasing

Insufficient resolution near the observer

aliasedokay

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Solution: Perspective aliasing

Insufficient resolution near the observerRedistribute shadow map samples

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Solution: Perspective aliasing

Sufficient resolution near the observerRedistribute shadow map samples

still okay okay now

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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

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Solution: Projection aliasing

Receivers ~ perpendicular to shadow plane

Redistribution doesn‘t work

But!

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Solution: Projection aliasing

Diffuse lighting: I = IL max( dot( L, N ), 0 )

~ perpendicular receivers have small I

Dark Hides artefacts!

LN

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Solution: Projection aliasing

Guidelines for the light- source

Small ambient term

Diffuse is good for hiding artefacts

Specular is no problemLight direction and view direction nearly the same

Resolution in shadow map suffices

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Solution: Projection aliasing

Screen-space blur of shadows

Hides artefacts

Shadows get softer

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Problem: Self-(un)shadowing

Observers‘s distance > shadow depthIncorrect self-shadowing

Polygon

Biased polygon

Observer‘s distance < shadow depthSelf-shadowing eliminated

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Solution: Self-(un)shadowing

How do we choose the bias?

Slope-scale biasing

Constant biasing

No biasing

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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 biasingOn the post-projective Z-slope

Or calculate linear Z with vertex shader

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Solution: Self-(un)shadowing

Other possibility to avoid self-shadowing:

Normally we use the front-side polygons

Now we use the back-side polygons

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Conclusions

Cause Sample Solution Sample

Perspective aliasing

Perspective Transformation (LispSM)

Projection aliasing

Blurring, light-model

Self-(un) shadowing

Biasing, back-side rendering

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Putting It All Together