Bi-Scale Radiance Transfer Peter-Pike Sloan Xinguo Liu Heung-Yeung Shum John Snyder Microsoft

Radiance Transfer Techniques Bidirectional Texture Functions (BTFs) local transport directional light basis fine spatial scale (meso-scale)

Radiance Transfer Techniques Precomputed Radiance Transfer (PRT) global transport low frequency light basis coarse spatial scale (macro-scale)

Radiance Transfer Techniques Bi-Scale Radiance Transfer local + global transport low frequency light basis two spatial scales

Our Goal add meso-scale texture to PRT extend BTF to handle –global transport –soft (area) lighting dynamic light/view in real-time

Bidirectional Texture Functions (BTFs) b(xp,vp,s)b(xp,vp,s) 6D scalar function x p : texture index at point p s : light direction v p : view direction at p get exit radiance e by integrating over directions s : [Dana99] s vpvp xpxp

Masking (View Dependence) different viewing angles image different surface points BTF models parallax/occlusion xpxp

Precomputed Radiance Transfer (PRT) l p object

Precomputed Radiance Transfer (PRT) p l'l' l

[Kautz02] p vpvp e(p,v p ) l'l'

Related Interactive Rendering Techniques TechniqueTransportLightingReflection PRT [Sloan02]… MacroLow FreqGlossy BTF [Liu02] [Suykens03] MesoDirectionalGlossy PTM [Malzbender01] MesoDirectionalDiffuse SIT [Ashikmin02] Meso*SteerableDiffuse SLF [Miller98]… MacroFrozenGlossy

Problems with BTFs and PRT BTF: hard to add global transport and soft lighting –already unwieldy 6D function –expensive integration for area lighting PRT: hard to add meso-scale effects –lengthy simulation: meso-scale = many surface points –huge storage: PRT matrix (625D) at each surface point –cant render in real-time at meso-scale resolutions

Radiance Transfer Texture (RTT) 4D vector-valued function (25-vector) computed (BTF RTT) by integration:

RTT Demo

Radiance Transfer Texture (RTT) just a spatially-varying BRDF vector from PRT: tabulated over small patch provides inexpensive area lighting but still no global transport

Bi-scale Radiance Transfer

Bi-Scale Radiance Transfer l : vector = source radiance spherical function M p : 25x25 transfer matrix at point p (source transferred incident) q(x p ) : ID map (2D 2D, maps RTT patch over surface) b(x,v) : RTT (4D 25D, tabulated over small spatial patch) applies macro-scale transferred radiance to meso-scale RTT

Preprocessing compute transfer matrix M p at mesh vertices [Sloan02] generate BTF over small patch b(x p,v p,d) [Liu01] convert BTF to RTT by integration: b(x p,v p ) build ID map between surface and RTT patch: q(x p )

ID Map: q ( x p ) RTT texel: 25x3 components, 8x8 views impractical to store/synthesize unique RTT texel per surface point (2k x 2k) instead synthesize index into a small patch (64 x 64) –2D rather than 4,800D –but cant interpolate indices

ID Map q(p)q(p) p RTT value

Preprocessing - ID map create atlas [Sander01] map RTT ID into 2D texture synthesize RTT on meso-mesh [Tong02] mesh RTT

Preprocessing - Examples 64×64×8×8 bunny model: 10k vertices ID map: 2048× ×64×8×8 128×128 (view-independent) 64×64×8×8

Run-Time Rendering Lighting Vertex Info CPU Vertex Shader To Pixel Shader…

Run-Time Rendering: Pixel Shader Pixel Shader Exit Radiance RTT ID mapV map

Results: View Dependent Effects (Masking)

Results: Demo ATI Radeon 9800

Performance Statistics computing PRT –8 minutes for 10k vertices (25x25 transfer matrices) generating synthetic RTT –6-27 hours for 64x64 patch building ID map –20 minutes for creating atlas –4 hours for texture synthesis run time performance –14.5 frames per second –2.2Ghz Pentium IV with ATI Radeon 9700.

Limitations of Bi-Scale Radiance Transfer static geometry distant, low-frequency lighting coarse view dependence: –shallow meso-scale texture –no highly specular materials

Contributions propose a bi-scale representation for radiance transfer parameterize BTF by spherical harmonics –RTT (Radiance Transfer Texture) –for fast area lighting generalize PRT via RTT rather than BRDF –practical, meso-scale transport effects access RTT using an id map –high spatial resolution using small patch

Future Work better spatial filtering of RTT robust sampling of PRT surface signal combine with CPCA handling meso-scale silhouettes general LOD control

Questions? Thanks to Dan Ling, John Hart, Jingdan Zhang, Paul Debevec, Stanford, and MPI.

Previous Work TechniqueTransportLightingReflection Bi-scaleMacro+MesoLow FreqGlossy RTTMesoLow FreqGlossy PRT [Sloan02]… MacroLow FreqGlossy BTF [Dana99]… MesoDirectionalGlossy PTM [Malzbender01] MesoDirectionalDiffuse SIT [Ashikmin02] Meso*SteerableDiffuse SLF [Miller98]… MacroFrozenGlossy

View Dependent Parameterization 8x8 samples on unit square mapped to hemisphere [Shirley98] Texture is created that maps from hemisphere (in parabolic parameterization) to indices in unit square