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ACM Transactions on Graphics (Proceedings of SIGGRAPH 2022)

Towards Practical Physical-Optics Rendering

Shlomi Steinberg iD , Pradeep Sen , Ling-Qi Yan

First published 24 July 2022DOI: 10.1145/3528223.3530119

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Abstract

Physical light transport (PLT) algorithms can represent the wave nature of light globally in a scene, and are consistent with Maxwell’s theory of electromagnetism. As such, they are able to reproduce the wave-interference and diffraction effects of real physical optics. However, the recent works that have proposed PLT are too expensive to apply to real-world scenes with complex geometry and materials. To address this problem, we propose a novel framework for physical light transport based on several key ideas that actually makes PLT practical for complex scenes. First, we restrict the spatial coherence shape of light to an anisotropic Gaussian and justify this restriction with general arguments based on entropy. This restriction serves to simplify the rest of the derivations, without practical loss of generality. To describe partially-coherent light, we present new rendering primitives that generalize the radiometric radiance and irradiance, and are based on the well-known Stokes parameters. We are able to represent light of arbitrary spectral content and states of polarization, and with any coherence volume and anisotropy. We also present the wave BSDF to accurately render diffractions and wave-interference effects. Furthermore, we present an approach to importance sample this wave BSDF to facilitate bi-directional path tracing, which has been previously impossible. We show good agreement with state-of-the-art methods, but unlike them we are able to render complex scenes where all the materials are new, coherence-aware physical optics materials, and with performance approaching that of “classical” rendering methods.
Scene rendered with our framework, viewed through a polarization filter (e.g., sunglasses) and lit by sunlight and afternoon sky light.
Scene rendered with our framework, viewed through a polarization filter (e.g., sunglasses) and lit by sunlight and afternoon sky light. Multiple diffraction optical effects are visible: (a) the glass window and (b) moulded plastic spoke guard admit stress birefringence, which results in iridescence depending on viewing direction; and (c) the metal brake surface on the bicycle's wheels acts as an imperfect diffraction grating, dispersing scattered light. Unlike the state-of-the-art which still depends on classical materials for performance, all the materials in this scene are coherence-aware, physical optics materials, nevertheless, our rendering performance is close to classical radiometric renderers. The appearance of these materials depends on the radiometric, polarimetric, and coherence properties of light.

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Cite this work

Shlomi Steinberg, Pradeep Sen, Ling-Qi Yan. 2022. Towards Practical Physical-Optics Rendering. ACM Transactions on Graphics, 41(4), 1–24. https://doi.org/10.1145/3528223.3530119

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@article{Steinberg_practical_plt_2022,
  author = {Shlomi Steinberg and Pradeep Sen and Ling-Qi Yan},
  doi = {10.1145/3528223.3530119},
  journal = {ACM Transactions on Graphics},
  keywords = {wave, optics, coherent, partially-coherent, light transport, path tracing, optical coherence, radiometry, electromagnetism, interference, diffraction, scattering, rendering equation, spectral, polarization, mueller, stokes, generalized stokes parameters, plt},
  number = {4},
  pages = {1--24},
  publisher = {Association for Computing Machinery (ACM)},
  title = {{Towards Practical Physical-Optics Rendering}},
  url = {https://ssteinberg.xyz/2022/04/03/practical_plt/},
  volume = {41},
  year = {2022},
}