Rendering with Spherical Radiance Transport Maps

1. Rendering with Spherical Radiance Transport Maps Jiaoying Shi State Key Lab of CAD&CG Zhejiang University, P.R.China

2. 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

3. 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.

4. 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

5. 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

6. PRT for dynamical scene • Radiance Transfer information changes at every frame of dynamical scene • Previous PRT for dynamical scene [Sloan02]- not a practical approach

7. 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

8. 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

9. 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?

10. 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

11. 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

12. 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

13. Spherical Shadow Map • Mutual shadow rendering (shadow casted on A by B) Then, search the planar map in SSM to determine occlusion

14. Spherical Shadow Map • Rendering with light clusters – further acceleration • Some results

15. Spherical Shadow Map Results comparison rendering by SSM rendering by ray-tracing

16. 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

17. Spherical Reflection Map Inter-reflection rendering • Reflection rendering is more complicated than shadow rendering • Compute self reflection firstly • Computer mutual inter-reflection with SRM

18. Spherical Reflection Map • Occlusion problem in reflection rendering • Judge occlusion using shadow computation results

19. Demos

20. 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

21. THANK YOU!