ConeCulled Soft Shadows

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Cone-Culled Soft Shadows Louis Bavoil, Claudio
T. Silva University of Utah
Cone Culling Assuming that the light are
spherical, the shadow rays between a surface
element and the light describe a cone. The idea
is to first find all the shadow-map photons
blocking the light from a surface point. An
octree is built from the 3D points of the shadow
map. On a CPU, for every pixel, the octree is
culled hierarchically using sphere-cone tests,
and the resulting set of points is culled using
point-cone tests. Solid Angles Each culled
shadow map sample contributes a solid angle to
the shadow at a surface element. The solid angle
of a cone is SolidAngle(Cone)
2p.(1-cos(HalfAngle)) Assuming that all the
shadow rays have the same intensity, the exact
contribution of a photon is the solid angle of
the piece of surface it represents. This piece
of surface is approximated by the bounding sphere
of the photon, which is the sphere passing
through the corners of the shadow map pixel, in
world-space. The solid angle of a photon is the
solid angle of its associated cone normalized by
the solid angle of the light.
Introduction
Solution
Ray Tracing In high-quality rendering,
soft shadows are usually rendered using
stochastic ray tracing. For each visible surface
point, hundreds of rays are shot to each light,
which is slow. Shadow Mapping In interactive
rendering on GPU, hard shadows are usually
rendered using shadow mapping. A shadow map is a
picture of the scene from the view of a point
light. Each photon (shadow-map pixel) stores a
distance from the light to the nearest surface
point along a ray. Percentage closer filtering
(PCF) is used to soften the shadow edges. PCF
shadows have non-physics-based penumbra and need
to be tuned to avoid self-shadowing artifacts.
Soft Shadows Soft shadows are important for
realistic computer graphics.
Hard shadows
Soft shadows
Light
Ray Tracing
Results
Scene 7,000 triangles 1 spherical light. CPU
Opteron _at_ 2.2 Ghz 1MB cache Soft Shadows
Shadow Mapping
Problem
Ray Tracing 800x600, 2000spp, 10.3 min
Cone Culling 800x600, 10242 photons, 22 s
Shadow Extent
World-Space Shadow Map (green) Octree (black)
Ray Tracing 800x600, 2000spp, 5.2 min
Cone Culling 800x600, 10242 photons, 12 s
Input Data Instead of sampling the light like in
ray-tracing, a set of hit points (shadow map) is
computed using rasterization from the light. The
shadow map is in the form of an RGBA float
texture, storing world coordinates of
photons. Problem For each surface element, what
are the photons blocking the light, and what are
their shadow contribution?

• The artifacts in the cone-culled shadows are a
  combination of the following issues
• Gaps between shadow map pixels ? holes in the
  shadows
• Overlap between bounding spheres ? larger shadows
• Partial inclusion of shadow map pixels in the
  light cone ? larger shadows

This work was started at Sony Computer
Entertainment America Research Development in
collaboration with Alan Heirich and Gabor Nagy.