PV227 GPU programming
Marek Vinkler
Department of Computer Graphics and Design
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Shadows
“just” shadow mapping,
hard shadows only.
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Shadow mapping
projective shadowing,
visibility by depth comparison.
Figure: Shadow mapping.
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Basic Theory
scene rendered from the light (depth saved),
scene rendered from the camera (depth compared).
Figure: Taken from nvidia.com
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More Theory
separate shadow map for each light,
only objects casting shadows need to be rendered,
recomputed each time the light or the scene changes.
Figure: Taken from github.io
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Artifacts
perspective aliasing,
projective aliasing,
shadow acne,
Peter panning,
animation artifacts (shimmering).
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Perspective aliasing
caused by mapping of pixels to shadow map texels,
worse near the camera (many-to-one),
solved by remapping the texture:
Perspective Shadow Maps (PSMs),
Logarithmic Perspective Shadow Maps (LogPSMs),
Cascaded Shadow Maps (CSMs),
. . .
Figure: Taken from microsoft.com
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Projective aliasing
caused by orientation of geometry to light (parallel rays),
same solution as in the previous slide.
Figure: Taken from microsoft.com
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Shadow acne
errorneous self shadowing,
caused by quantizing depth over entire texel,
caused by floating point imprecission,
solved by adding bias, making near and far planes as close
as possible.
Figure: Taken from microsoft.com
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Peter panning
shadow detached from object,
caused by too high bias for solving shadow acne,
solved by limiting bias, making near and far planes as
close as possible.
Figure: Taken from microsoft.com
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Slope scaled bias
adding bias depending on the orientation of the geometry,
high for parallel, small for orthogonal directions,
glPolygonOffset.
Figure: Taken from microsoft.com
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Shimmering shadow edges
animation artifact,
brightening and darkening of shadow edges,
caused by recalculation of shadow matrix,
Figure: Taken from microsoft.com
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Sources
http://msdn.microsoft.com/en-us/library/
windows/desktop/ee416324%28v=vs.85%29.aspx
more discussion, other artifact elimination techniques.
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