Spectral Mollification for Bidirectional Fluorescence

We present Optimised Path Space Regularisation (OPSR), a novel regularisation technique for forward path tracing algorithms. Our regularisation controls the amount of roughness added to materials depending on the type of sampled paths and trades a small error in the estimator for a drastic reduction of variance in difficult paths, including indirectly visible caustics. We formulate the problem as a joint bias‐variance minimisation problem and use differentiable rendering to optimise our model. The learnt parameters generalise to a large variety of scenes irrespective of their geometric complexity. The regularisation added to the underlying light transport algorithm naturally allows us to handle the problem of near‐specular and glossy path chains robustly. Our method consistently improves the convergence of path tracing estimators, including state‐of‐the‐art path guiding techniques where it enables finding otherwise hard‐to‐sample paths and thus, in turn, can significantly speed up the learning of guiding distributions.

Section: Methodsmentioning

confidence: 99%

Optimised Path Space Regularisation

Weier¹,

Droske²,

Hanika³

et al. 2021

“…incoming) direction, and ω n the shading normal. Due to this formulation, fluorescence is restricted to spectral renderers, such as uni‐directional path tracers [WTP01; BHD*08; MFW18], and bi‐directional path tracers thanks to wavelength mollification [JHD20]. To the best of our knowledge, reproducing fluorescence in non‐spectral renderers hasn't received much attention.…”

Section: Related Workmentioning

confidence: 99%

Non‐Orthogonal Reduction for Rendering Fluorescent Materials in Non‐Spectral Engines

Fichet,

Belcour,

Barla

2024

We propose a method to accurately handle fluorescence in a non‐spectral (e.g., tristimulus) rendering engine, showcasing color‐shifting and increased luminance effects. Core to our method is a principled reduction technique that encodes the reradiation into a low‐dimensional matrix working in the space of the renderer's Color Matching Functions (CMFs). Our process is independent of a specific CMF set and allows for the addition of a non‐visible ultraviolet band during light transport. Our representation visually matches full spectral light transport for measured fluorescent materials even for challenging illuminants.

“…Based on this work, Jung et al . [JHMD18] derived a new BRRDF. They exploited Kasha's rule to represent their distribution with three 1D distributions (absorption and emission spectra and non‐fluorescent reflectance) and two ratios (emitted to absorbed energy and re‐radiation to non‐fluorescent reflectance).…”

Section: Previous Workmentioning

confidence: 99%

Efficient Storage and Importance Sampling for Fluorescent Reflectance

Hua

Tázlar

Fichet

et al. 2022

Figure 1: In this scene, we use a near-UV light for illumination. While non-fluorescent materials only reflect shades of the illumination colour (left), fluorescent surfaces also reemit a portion of the absorbed energy from incident light as a light at additional wavelengths, leading to the colourful appearance of other objects in the scene. To represent fluorescent materials in a renderer, we typically use reradiation matrices (centre), which have a significant memory overhead. Instead, we propose a more efficient representation of these matrices using Gaussian mixture models (right -eight Gaussians). Although compact, this representation also provides compelling results even with such challenging illumination.