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Wide Gamut Spectral Upsampling with Fluorescence

Alisa Jung, Alexander Wilkie, Johannes Hanika, Wenzel Jakob, Carsten Dachsbacher

In Computer Graphics Forum 38:4

Wide Gamut Spectral Upsampling with Fluorescence
A spectrally rendered scene with standard RGB to spectrum upsampling (left) cannot achieve reflectance spectra with arbitrary colour saturation and brightness due to energy conservation constraints. Our technique (centre) converts bright and deeply saturated input RGB colours to regular reflectance spectra and enhances them with a fluorescent component, reproducing colours from a considerably wider gamut. The input colours were specified in ACEScg---for instance, the top ring of the swimming pool has color (0.9, 0, 0.9) in ACEScg. To visualise colours of such high saturation, we interpret the rec2020 data of the rendered images as sRGB for display, and we also provide a display-independent difference image in CIE74 Delta E on the right (stopped down by 7ev, i.e. a saturated pixel value of 1.0 corresponds to Delta E = 128). Similarly, the squares to the left visualise the ACEScg input colours reinterpreted as sRGB.

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.

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Bibtex

@article {2019_FluoUpsampling,
journal = {Computer Graphics Forum},
title = {{Wide Gamut Spectral Upsampling with Fluorescence}},
author = {Jung, Alisa and Wilkie, Alexander and Hanika, Johannes and Jakob, Wenzel and Dachsbacher, Carsten},
year = {2019},
publisher = {The Eurographics Association and John Wiley & Sons Ltd.},
ISSN = {1467-8659},
DOI = {10.1111/cgf.13773}
}