Shadows

CSC 476

Zoë Wood / Ian Dunn

Shadows

Reveal spatial relationships

Shadow Terminology

Occluders

Umbra

Penumbra

Real-Time Shadow Techniques

• Planar shadows
• Shadow volumes
• Shadow maps

Andrew Wyeth

Planar shadows

• Projection shadow
• Squash polygons onto plane
• Depth issues

Projective textures

• Using projective textures (for planar shadows) – general idea:

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Done in shader of

affected object

Shadow volumes (skirts)

• Shadow volumes
• Compute shadow volumes
• At render determine if fragment in or out of shadow volumes

Shadow Volumes - Generation

• Determine silhouettes
• Edges of triangles
• Dot geometric normals
• Sign changes

Shadow Volumes - Generation

• Extrude edges towards infinity
• Away from light source
• Where?
• CPU
• Slow if changing per frame!
• GPU - geometry shaders

Shadow maps

• Shadow maps
• Render scene from light’s perspective
• Use depth values to find occluders

Shadow Maps

Shadow Maps - Steps

1. Determine light parameters
2. Render with light camera
3. Render with usual camera
1. Transform to light space (light ndc)
2. Sample depth from shadow map
3. Compare sampled and transformed depth

Shadow Maps

Light Parameters? See everything that casts shadows

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Constrain this!

Shadow Maps

Light matrix = L_p * L_v

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Want: Light NDC - how and why?

Shadow Acne

Bias = peter panning

Shadow Maps - Problems

• Shadowing determined per-fragment
• Shadow map texels =/= fragments

Shadow Maps

Ideally - one shadow texel = one fragment

Projection transform makes this difficult

Projection transform of the viewer

Cascaded Shadow Maps

Percentage Closer Filtering

• Not true/false
• Measure the percentage of closer texels

Trade-off: sample count vs. performance

What about soft shadows?

These are still hard shadows

Percentage Closer Soft Shadows

• Simple method to better approximate soft shadows

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• Increase shadow map�search radius when�far from occluder