1. UMBC Graphics for Games
Shadows
UMBC Graphics for Games

2. Punctual Shadow Algorithms

3. Shadow Volumes Seldom used now
Find silhouette faces (extend triangle including light & edge)
Increment counter for front-facing
Hardware has a stencil buffer that is perfect for this
Decrement counter for back-facing
Lit if not 0
Problems & Variations
Eye inside volume
Adapt starting count, or count from ∞
Lots of pixels to fill
Clipping & culling helps, but this is shadow volume’s biggest problem -1 +1 +1

4. Shadow Rays Trace rays through some representation of the geometry
Ray tracing’s primary shadow method
Distance Field Shadows
Pre-process 3D grid of distances to closest object
Ray march (like volume shadows)
At each step, can go at least as far as the distance stored there before you hit anything

5. Shadow Map Most common method Render objects from camera at light
Each pixel has 3D position, of the first thing the light sees
from shadow map pixel position
from stored depth value
Render objects (or deferred pass)
Each pixel has 3D position,
from pixel position, from pixel depth
Transform to light space, compare to

6. Shadow Acne & Shadow Bias
Numerical error for closest surface = self shadowing
sometimes says yes when should equal
Appears as random spots of shadow
Bias
Rather than , use
Too little, still get artifacts
Too much, and objects seem to float
Some methods adjust per pixel

7. Shadow Resolution Want about 1 shadow pixel per pixel
Warped shadow space
Non-linear shadow map pixel spacing
Works for lights perpendicular to view, but not head lights
Cascaded shadow maps
Multiple shadow maps at different resolutions for each depth range
This is the choice for most games
UE4 shadow atlas

8. Shadow Aliasing Blending depths doesn’t work
Blending near & far depths gives a hard aliased shadow at a depth between them
Percentage Closer Filtering (PCF)
Evaluate shadow (0) or not (1) at four shadow texels
Blend these intensities
Variance Shadow Maps (VSM), [Moment Shadow Maps (MSM)]
Average Z, Z2, [Z3, Z4]
Estimate variance and shadow coverage
Prone to precision problems & light leaks (assumes Gaussian-distributed depths)
Exponential (& others): like VSM, but different distribution assumptions

9. Soft Shadows Area lights give soft-edged shadows Umbra (full shadow)
Penumbra (partial shadow)
Contact hardening
Penumbra size depends on how close blocker is to surface
Penumbra
Umbra

10. Abuse Shadow Filtering
For any of PCF, VSM, MSM
Lie about the filter size
Usually a fixed amount of softening

11. Percentage Closer Soft Shadows (PCSS)
Realistic penumbra width and contact hardening
Assumes a spherical light
Sample shadow map to estimate blocker distance
Turn to per-pixel filter radius for PCF, VSM, MSM, etc.
Similar triangles:

12. Multiple Point Lights Same lights for all pixels = shadow bands
Different lights for each pixel = noisy shadows
Recent work combines LTC & per-pixel sampling
Basically factors in how much shadows change things