Perspectives on lighting simulation DTU influence in the past and perspectives on the future Jeppe Revall Frisvad DTU Compute Technical University of Denmark August 2015

The importance of specular paths • • • •

Light through windows Light in water Glossy materials Translucent materials

Architectural Visualization

© Duncan Howdin

• But occluded specular paths are hard to find

The caustics challenge

[Hachisuka et al. 2013]

• How to find a small light source occluded by a transparent specular object (glass, water)? • Translucent objects carry the same challenge

Path-traced caustics are noisy

Regular path tracing, 100 paths per pixel

Reusing computations • Store light path vertices (photon mapping) – Path reuse vs. memory consumption [Jensen and Christensen 2000]

trace and store

flux density estimation

photon mapping

Reusing computations

• Storing eye path vertices (photon splatting) – Path reuse vs. accuracy in density estimation

Reusing computations

• Store all path vertices and refine progressively (stochastic progressive photon mapping) – Path reuse, little memory, but slow convergence

[Hachisuka et al. 2012]

Unified framework

photon map

data structure

connect

merge

continue [Hachisuka et al. 2013]

• Bidirectional path tracing (vertex connection) • Progressive photon mapping (vertex merging) • Same framework, different integrators

Commercial success

[Keller et al. 2015]

• The path-tracing revolution in the movie industry (SIGGRAPH 2015) •

SolidAngle’s Arnold Renderer –

•

–

•

©Metro-Goldwyn-Mayer Pictures

Gains a VFX market share with physically based rendering

Pixar RenderMan switches from –

•

©Heyday Films

REYES [Rendering Everything You Ever Saw]: micropolygon rendering (rasterization) RIS [Renderman Integration System]: unified path tracing framework

Disney Hyperion – –

Disney’s in-house production path tracer Launched in 2013, used for Big Hero 6

–

Physically based spectral renderer

Weta’s Manuka

©Disney/Pixar

©Disney

• Also used extensively in product vis and architectural vis

– Luxion’s KeyShot, Next Limit’s Maxwell Render, Glare Technologies’ Indigo Renderer, etc.

Volumetric effects

[Jensen 2001]

[Jensen and Christensen 1998]

[Jensen and Christensen 2000]

[Jarosz et al. 2008]

Improving density estimation

• Photon differentials for surfaces and volumes

path tracing photon mapping

mapping

photon differentials

[Frisvad et al. 2014a]

First-order approximation of the photon path derivative

[Jarosz et al. 2011]

splatting

differentials

Photon diffusion

• Diffusion-based analytical models ease computation of subsurface scattering

[Donner and Jensen 2007]

photon (beam) diffusion [Habel et al. 2013]

dipole model

[Jensen et al. 2001]

path tracing (days) still noisy

diffusion models (minutes) [Frisvad et al. 2014b] no noise

directional dipole model

Unified framework with volumes

eye beams

photon beams

path tracing

photon mapping

continue …

[Křivánek et al. 2014]

The input challenge • Light transport simulation has come a long way • How do we get physically plausible materials?

– Measuring (example: diffuse reflectance spectroscopy)

[Abildgaard et al. 2015]

– Modeling (example: light scattering by particles)

Material properties from particles Building models using particle composition and Lorenz-Mie theory

Particles: Refractive index, concentration, size distribution, density

ingredients

Rendering & Simulation

Model Host medium: Refractive index, density, viscosity, surface tension

algae in sea ice

Global parameters: temperature, gravity, pressure

[Frisvad et al. 2007]

organic low fat milk

render

photo

products

Digital Prototype unfiltered apple juice

render

photo

What’s next?

• Improvement/exploration of the unified framework • Multi-scale modeling • Physically based scattering models (BxDF) References (chronologically, underlining DTU affiliated authors and DTU alumni dashed): ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶ ̶

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̶

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Jensen, H. W., and Christensen, N. J. Photon maps in bidirectional Monte Carlo ray tracing of complex objects. Computers & Graphics 19(2): 215-224, 1995. Jensen, H. W., and Christensen, P. H. Efficient simulation of light transport in scenes with participating media using photon maps. In Proceedings of ACM SIGGRAPH 1998: 311-320, 1998. Igehy, H. Tracing ray differentials. In Proceedings of ACM SIGGRAPH 1999: 179-186, 1999. Jensen, H. W., and Christensen, N. J. A practical guide to global illumination using photon maps. In Proceedings of ACM SIGGRAPH 2000 Courses, Article 8, 2000. Jensen, H. W. Realistic Image Synthesis Using Photon Mapping. A K Peters, 2001. Jensen, H. W., Marschner, S., Levoy, M., and Hanrahan, P. A practical model for subsurface light transport. In Proceedings of ACM SIGGRAPH 2001: 511-518, 2001. Donner, C., and Jensen, H. W. Rendering translucent materials using photon diffusion. In Proceedings of EGSR 2007 (Rendering Techniques '07): 243-251, 2007. Frisvad, J. R., Christensen, N. J., and Jensen, H. W. Computing the scattering properties of participating media using Lorenz-Mie theory. ACM Transactions on Graphics (Proceedings of ACM SIGGRAPH 2007) 26(3), Article 60, 2007. Schjøth, L., Frisvad, J. R., Erleben, K., and Sporring, J. Photon differentials. In Proceedings of GRAPHITE 2007: 179-186, 2007. Jarosz, W., Donner, C., Zwicker, M., and Jensen, H. W. Radiance caching for participating media. ACM Transactions on Graphics 27(1), Article 7, 2008. Presented at ACM SIGGRAPH 2008. Jarosz, W., Nowrouzezahrai, D., Thomas, R., Sloan, P.-P., and Zwicker, M. Progressive photon beams. ACM Transactions on Graphics (Proceedings of ACM SIGGRAPH Asia 2011) 30(6), Article 181, 2011. Hachisuka, T., Jarosz, W., Bouchard, G., Christensen, P. H., Frisvad, J. R., Jakob, W., Jensen, H. W., Kaschalk, M., Knaus, C., Selle, A., and Spencer, B. State of the art in photon density estimation. Proceedings of ACM SIGGRAPH 2012 Courses, Article 6, 2012. Habel, R., Christensen, P. H., and Jarosz, W. Photon beam diffusion: a hybrid Monte Carlo method for subsurface scattering. Computer Graphics Forum (Proceedings of EGSR 2013) 32(4): 27-37, 2013. Eisenacher, C., Nichols, G., Selle, A. and Burley, B. Sorted deferred shading for production path tracing. Computer Graphics Forum (Proceedings of EGSR 2013) 32(4): 125–132, 2013. Hachisuka, T., Jarosz, W., Georgiev, I., Kaplanyan, A., Nowrouzezahrai, D., and Spencer, B. State of the art in photon density estimation. Proceedings of ACM SIGGRAPH Asia 2013 Courses, Article 15, 2013. Křivánek, J., Georgiev, I., Hachisuka, T., Vévoda, P., Šik, M., Nowrouzezahrai, D., and Jarosz, W. Unifying points, beams, and paths in volumetric light transport simulation. ACM Transactions on Graphics (Proceedings of ACM SIGGRAPH 2014) 33(4), Article 103, 2014. Frisvad, J. R., Schjøth, L., Erleben, K., and Sporring, J. Photon differential splatting for rendering caustics. Computer Graphics Forum 33(6): 252-263, 2014. Frisvad, J. R., Hachisuka, T., and Kjeldsen, T. K. Directional dipole model for subsurface scattering. ACM Transactions on Graphics 34(1), Article 5, 2014. Presented at ACM SIGGRAPH 2015. Keller, A., Fascione, L., Fajardo, M., Georgiev, I., Christensen, P. H., Hanika, J., Eisenacher, C., and Nichols, G. The path tracing revolution in the movie industry. Proceedings of ACM SIGGRAPH 2015 Courses, Article 24, 2015. Abildgaard, O. H. A., Kamran, F., Dahl, A. B., Skytte, J. L., Nielsen, F. D., Thomsen, C. L., Andersen, P. E., Larsen, R., and Frisvad, J. R. Non-invasive assessment of dairy products using spatially resolved diffuse reflectance spectroscopy. Applied Spectroscopy 69(9): 1096-1105, 2015.