Rendering with
Spherical Radiance Transport Maps
Jiaoying Shi
State Key Lab of CAD&CG
Zhejiang University, P.R.China
Background
• Pre-computed Radiance Transfer(PRT) [Sloan02, Ng03]
[Sloan03] is an efficient method for rendering soft shadow
and inter-reflection under low frequency illumination.
• Radiance transfer information – occlusion and reflection
relationship between geometry models, which is the most
important in global illumination rendering
• Problems in traditional methods like Ray Tracing: slow
computation of ray-model intersection for Radiance
transfer information, unpractical for soft shadow/reflection
rendering under complex lighting
Precomputed Radiance Transfer
• PRT aims at transferring the computation burden to
precomputation process
• PRT precompute, compress and save the Radiance Transfer
information to support fast global illumination rendering
• The key to PRT is the constraining mode of scene, the
strategy of precomputation, and the data compress method.
Precomputed Radiance Transfer
• PRT method for single object or static scene, in which the
radiance transfer information is fixed
• Precomputation method
obtain the occlusion, reflection information for each
model vertex at every direction
occlusion
inter-reflection
Precomputed Radiance Transfer
• Analysis for the complex illumination using orthonormal
basis function, such as Spherical Harmonics, Wavelets.
• Analysis for the precomputed Radiance Transfer
information at each vertex with same basis function
• Efficient shading computation using these analysis
coefficients
PRT for dynamical scene
• Radiance Transfer information changes at every frame of
dynamical scene
• Previous PRT for dynamical scene [Sloan02]- not a
practical approach
Spherical Radiance Transport Maps
• SRTMs – Real-time global illumination rendering method for multiple
dynamical objects
• Process each object independently
• Precompute the mutual radiance transfer information, occlusion and
reflection, on a bounding sphere of each dynamical object.
• In rendering, these information can be obtained easily by searching the
precomputed results
Spherical Radiance Transport Maps
• The Radiance Transfer information for each vertex is
dynamical, so the orthonormal analysis cannot be used like
previous PRT method
• The orthonormal analysis cannot be used for illumination
either.
• We decompose the complex illumination into many
directional lights[Agarwla03], and process each light in
rendering independently
Problems of Bounding Sphere Sampling
• Large data amounts of 4D function, compression
needed
• Inconvenient storage and search for data on sphere
• Computation of ray-model intersection is reduced
to that of ray-bounding sphere intersection, but
can this computation be simplified furthest?
Spherical Shadow Map
• Spherical Shadow Map(SSM) for soft shadow rendering.
• Sampling a planar shadow map at every direction, as
rearrangement of sampling on bounding sphere
• Small data amount
utilizing the insensitivity of human eyes to shape of shadow under
complex illumination, reducing the density of sampling directions,
• Uniform resolution of planar shadow map, easy for compression
• Capability to support fast rendering, avoid trigonometric computation
Spherical Shadow Map
• Self shadow map
precompute and store self occlusion of each object vertex
at every sampling direction
• Shadow Rendering Process
• For each vertex, to determine if every light is occluded by the object
itself or by other dynamical object
• Firstly for rendering self shadow,
we determine self occlusion
by searching precomputed
result in self shadow map
Spherical Shadow Map
• Mutual shadow rendering (shadow casted on A by B)
For each vertex of A, if some light is not occluded by A itself, judge if
it is occluded by other objects
Firstly, two cases of unocclusion are examined
Spherical Shadow Map
• Mutual shadow rendering (shadow casted on A by B)
Then, search the planar map in SSM to determine occlusion
 T
x  q x

y  q  yT
Spherical Shadow Map
• Rendering with light clusters – further acceleration
• Some results
Spherical Shadow Map
Results comparison
rendering by SSM
rendering by ray-tracing
Spherical Reflection Map
• Spherical Reflection Map(SRM) for soft inter-reflection
rendering
• Percomputation process
sample a planar map at each direction ( relatively low sampling
resolution on each planar map)
find the direction of reflection of every sampling ray
ignore multi-fold reflection
sample self reflection map
Spherical Reflection Map
Inter-reflection rendering
• Reflection rendering is more complicated than shadow rendering
• Compute self reflection firstly
• Computer mutual inter-reflection with SRM
Spherical Reflection Map
• Occlusion problem in reflection rendering
• Judge occlusion using shadow computation results
Demos
Future Work
• Sampling of Radiance Transfer information on various
bounding shell
• SRTM for transparent objects
• SRTM for deformable objects
• Using SRTM with BRDF and BTF
THANK YOU!