university of montreal & imagis a light hierarchy for fast rendering of scenes with many lights...
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University of Montreal & iMAGIS
A Light Hierarchy for Fast Rendering of Scenes with Many Lights
E. Paquette, P. Poulin, and G. Drettakis
University of Montreal & iMAGIS
Motivation• Scenes with many light sources are expensive to
render– Shading and shadowing cost dominates
• Typical scenes: city lights, chandelier, Christmas tree ornaments, flame simulation
University of Montreal & iMAGIS
Introduction
• Goal: Accelerate the shading calculation for scenes with many light sources
• Our solution: Hierarchical multi-resolution representation of the light sources
• Context: Ray-tracing, point light sources• Currently restricted to unoccluded light
sources (no shadows)
University of Montreal & iMAGIS
Outline
Previous work• Hierarchical representation of light sources• Diffuse and specular shading• Treating visibility• Conclusion & future work
University of Montreal & iMAGIS
Previous Work
• Adaptive shadow testing [Ward91]– Problem with many similar contributions
• Monte Carlo [Shirley et al. 96]– Noise component can be very high
• Hierarchical radiosity with clustering [Smits et al. 94] [Sillion95]– Problems with clusters containing light
sources
University of Montreal & iMAGIS
Outline
• Previous work Hierarchical representation of light sources• Diffuse and specular shading• Treating visibility• Conclusion & future work
University of Montreal & iMAGIS
Hierarchical Representation
• Distant lights need less accurate treatment
virtual light source
VLthreshold)Shade(lights) Shade(all LV
p
University of Montreal & iMAGIS
Construction
• Light hierarchy (octree)
University of Montreal & iMAGIS
Construction
intensity of VL= intensities
position of VL= weighted
average of positions
• Light hierarchy (octree)
level 1 VL
University of Montreal & iMAGIS
Construction
level 0 VL
intensity of VL= intensities
position of VL= weighted
average of positions
• Light hierarchy (octree)
level 1 VL
• Separate object hierarchy for ray-tracing acceleration
University of Montreal & iMAGIS
Outline
• Previous work• Hierarchical representation of light sources Diffuse and specular shading• Treating visibility• Conclusion & future work
University of Montreal & iMAGIS
Diffuse Reflection Approximation
dmax
max
N
2v
vdvapprox
2i
idi
d
cosk II
d
cosk II
University of Montreal & iMAGIS
Diffuse Reflection Error Bound• Derive an error bound on the virtual light usage
2maxv
maxvdvmin
2i
idi
)d(d
)cos(k II
d
cosk II
dmax
max
N
Similarly for Imax
University of Montreal & iMAGIS
Diffuse Reflection Error Bound• Derive an error bound on the virtual light usage
II
,IImaxError
approxmax
minapprox
dmax
max
N
University of Montreal & iMAGIS
Diffuse Reflection Shading
HierShading(Point p, Voxel v)if Error(p, v) > Tdiffuse then
ShadeAtLowerLevel(p,v)else
AddErrorList(v, Error(p, v))p
University of Montreal & iMAGIS
Error List
• Error bound for replacing one cluster by VL
• Using lower levels replaces many clusters with corresponding VLs
while Sum(errorList) > Tdiffuse
v = VoxelWithGreatestError( errorList )
ShadeAtLowerLevel(p,v)
University of Montreal & iMAGIS
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1
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4
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0
20
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100
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5
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Diffuse Reflection ResultsSpeed U
p(v
s st
andard
ray-
traci
ng)
Number of Light Sources Our method[Ward91]
University of Montreal & iMAGIS
Specular Reflection• Select important voxels and light sources in the
hierarchy based on maximal potential contribution
N2
maxv
nmaxv
svmax
2i
in
si
)d(d
))(cos(k II
d
cosk II
max
University of Montreal & iMAGIS
Specular Reflection Shading
HierShading(Point p, Voxel v)
if Imax(v) > Tspecular then
ShadeAtLowerLevel(p,v) else
AddErrorList(v, Imax)p
University of Montreal & iMAGIS
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60
80
100
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Specular Reflection Results
Number of Light Sources Our method[Ward91]
Speed U
p(v
s st
andard
ray-
traci
ng)
University of Montreal & iMAGIS
Artifacts
• Artifacts that result from increasing the threshold
Rendered image
Difference image
(enhanced contrast)
Discontinuities from discrete
choice of hierarchy level
Solution:Interpolation
University of Montreal & iMAGIS
Outline
• Previous work• Hierarchical representation of light sources• Diffuse and specular shading Treating visibility• Conclusion & future work
University of Montreal & iMAGIS
Treating Visibility
• Hierarchical algorithm gives appropriate nodes of the light sources hierarchy
• Warnock style decision– Full occlusion, full visibility, partial
occlusion• Use lower level nodes when partial visibility• Approximate volumetric visibility method
similar to [Sillion95]• Need error bounds or estimates
University of Montreal & iMAGIS
Outline
• Previous work• Hierarchical representation of light sources• Diffuse and specular shading• Treating visibility Conclusion & future work
University of Montreal & iMAGIS
Conclusion
• Hierarchical multi-resolution representation of the light sources
• Logarithmic rendering time in the number of light sources (diffuse algorithm)
• Accelerates rendering in the presence of many light sources (up to 90 times faster than standard ray-tracing)
University of Montreal & iMAGIS
Future Work
• Shadows– Visibility classification (full, partial or no
occlusion)– Approximate visibility
• Non point light sources– Area, canned light sources, etc.
• General BRDF– Bounds on BRDF value