Wide Gamut Spectral Upsampling with Fluorescence

Abstract

Physically based spectral rendering has become increasingly important in recent years. However, asset textures in such systems are usually still drawn or acquired as RGB tristimulus values. While a number of RGB to spectrum upsampling techniques are available, none of them support upsampling of all colours in the full spectral locus, as it is intrinsically bigger than the gamut of physically valid reflectance spectra. But with display technology moving to increasingly wider gamuts, the ability to achieve highly saturated colours becomes an increasingly important feature. Real materials usually exhibit smooth reflectance spectra, while computationally generated spectra become more blocky as they represent increasingly bright and saturated colours. In print media, plastic or textile design, fluorescent dyes are added to extend the boundaries of the gamut of reflectance spectra. We follow the same approach for rendering: we provide a method which, given an input RGB tristimulus value, automatically provides a mixture of a regular, smooth reflectance spectrum plus a fluorescent part. For highly saturated input colours, the combination yields an improved reconstruction compared to what would be possible relying on a reflectance spectrum alone. At the core of our technique is a simple parametric spectral model for reflectance, excitation, and emission that allows for compact storage and is compatible with texture mapping. The model can then be used as a fluorescent diffuse component in an existing more complex BRDF model. We also provide importance sampling routines for practical application in a path tracer.

Wide Gamut Moment‐based Constrained Spectral Uplifting

“…Jung et al . [JWH*19] further improve this technique for wide gamut spectral uplifting by introducing new parameters for fluorescence.…”

Spectral Mollification for Bidirectional Fluorescence

“…This has been extended to microfacet and Phong models [WWLP06]. Jung et al [JHD18] model diffuse re-emission and parameterize the effect with continuous spectra instead of a reradiation matrix.…”

“…Acquisition of fluorescence is performed in biology and chemistry by measuring an absorption and emission spectrum [Lak10]. Modeling fluorescent surfaces A fluorescent BBRRDF can be based on existing emission and absorption spectra [JHD18], or on a twodimensional reradiation matrix [Gla95] [HHA * 10], which describes how much light is converted from incident to exitant wavelengths in discrete steps. In order for a BBRRDF to be energy con-serving, it has to fulfill the condition…”

“…We use the continuous BBRRDF by [JHD18], which covers both elastic and inelastic scattering. It is parameterized by an absorption spectrum a(λ i ), an emission spectrum e(λo), a non-fluorescent reflectance spectrum r(λ) and two scalars c, Q ∈ [0, 1]:…”

“…the diagonal of the reradiation matrix. In our implementation, we chose to work with an analytic BBRRDF model [JHD18] which allows us to mollify only the non-fluorescent part.…”