Shadows
David Luebke
University of Virginia
Shadows
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An important visual cue, traditionally hard
to do in real-time rendering
Outline:
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Notation
Planar shadows
Soft shadows
Projective shadows
Shadow volumes
Shadow maps
Shadow optimizations
Notation
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Light source
– Point vs area
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Occluders & receivers
– Identify ahead of time?
– Self-shadowing?
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Shadow
– Umbra
– Penumbra
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Soft vs hard shadows
Planar Shadows
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Old trick: project the occluder geometry to
a plane and render over ground plane
– Can do with a matrix
– Z-bias issues
– Semiopaque shadows harder
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Stencil and Z-buffer tricks
– Another option: generate textured rectangle
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Problems
– Light source inside object (Antishadows)
– Only planar receivers  no self-shadowing
Planar Soft Shadows
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Basic idea:
– Sample light source multiple times, average
results (accumulation buffer) into a texture
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Gooch et al: move plane up and down
– Nested shadows fewer passes
Projective Shadows
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Render scene from light’s point of view
– Render all occluders as black
– Can turn off depth buffer etc
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Project onto receiver polygons using
projective texture mapping
Works for curved surfaces
Designer separates occluders and
receivers  no self-shadowing
Shadow Volumes
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Basic idea:
– Create polygonal objects to represent
shadowed volumes
– Make clever use of stencil buffer so that these
objects affect what lighting is done
Stencil Buffer
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The stencil buffer has been around since
OpenGL 1.0
– Basic idea: provide a per-pixel flag to indicate
whether pixels are drawn or not
– But…
– Let that flag be an integer (usually 8 bits)
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Usually shared with depth buffer
– And let drawing operations increment or
decrement the stencil buffer based on
different events
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Always, depth-pass, depth-fail, etc.
Shadow Volumes
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Details of the basic algorithm:
– Compute shadow volumes
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View-independent!
– Clear stencil buffer
– Render the scene without diffuse/spec lighting
– “Render” front faces of shadow volumes
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Turn off color, depth updates (but leave depth test on)
Visible polygons increment pixel stencil buffer count
– “Render” back faces of shadow volumes
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Turn off color, depth updates (but leave depth test on)
Visible polygons decrement pixel stencil buffer count
– Render scene with only diffuse/spec lighting
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Only update pixels where stencil = 0
Shadow Volumes
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Refinements (see book, next slides)
– NV30, XBox supports signed stencil addition
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Two-sided lighting determines whether polygon
adds or subtracts 1 from stencil buffer
One-pass algorithm! But a little slower in practice?
– What if you’re inside a shadow volume?
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Invert meaning of stencil test
– What if near clip intersects shadow plane?
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Carmack, others: use z-fail test
Clever extensions in NV2X help this idea out
– Creating shadow volumes: vertex program!
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ATI: clever degenerate-edge trick again
Shadow Volumes
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Advantages:
– Robust
– Self-shadowing
– GPU
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Disadvantages:
– Can be geometry limited
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Stencil polys
Multi-pass scene geometry
– Can be fill limited
– Stencil test – per pixel expense
– Hard shadows
Shadow Volumes
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Will return to the gruesome details shortly
Shadow Maps
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Idea:
– Render scene from light source, read Z-buffer
– Result: discretized image (shadow map)
telling distance of objects to light source
– Render scene normally
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At each pixel, calculate distance D to light
Compare to distance S stored in shadow map
If D=S, surface lit by light, else in shadow
Shadow Maps
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The basic algorithm (w/o hardware)
– Render scene with ambient lighting only
– Read Z-buffer, transform values into
coordinate system of light
– Use comparison to set alpha buffer
– Render w/ diffuse and specular components,
multiplying by alpha
Shadow Maps
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Advantages:
– Hardware-accelerated general shadow
algorithm
– Supports self-shadowing
– Cost is linear in # lights and # polygons
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Disadvantages:
– Self-shadow aliasing
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Biasing and other techniques can help, but not fix
– Shadow map resolution critical!
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Solution: perspective shadow maps