Image-based Remapping of Spatially-varying Material Appearance

Alejandro Sztrajman
University College London
Jaroslav Křivánek
Charles University, Prague
Alexander Wilkie
Charles University, Prague
Tim Weyrich
University College London

Cross-renderer remapping of spatially varying material. Mesh (left); Blender-Ward (center) remapped to Cycles-GGX (right).

Abstract

BRDF models are ubiquitous tools for the representation of material appearance. However, an astonishingly large number of different models are now in practical use. Both a lack of BRDF model standardization across implementations found in different renderers, as well as the often semantically different capabilities of various models, have become a major hindrance to the interchange of production assets between different rendering systems. Current attempts to solve this problem rely on manually finding visual similarities between models, or mathematical similarities between their functional shapes, which requires access to the shader implementation, usually unavailable in commercial renderers. We present a method for automatic translation of material appearance between different BRDF models that uses an image-based metric for appearance comparison and that delegates the interaction with the model to the renderer. We analyze the performance of the method, both with respect to robustness and also visual differences of the fits for multiple combinations of BRDF models. While it is effective for individual BRDFs, the computational cost does not scale well for spatially varying BRDFs. Therefore, we also present two regression schemes that approximate the shape of the transformation function and generate a reduced representation that evaluates instantly and without further interaction with the renderer. We present respective visual comparisons of the remapped SVBRDF models for commonly used renderers and shading models and show that our approach is able to extrapolate transformed BRDF parameters better than other complex regression schemes. Finally, we analyze the transformation between specular and metallic workflows, comparing our results with two analytic conversions.

Publication

A. Sztrajman, J. Křivánek, A. Wilkie, and T. Weyrich. Image-based Remapping of Spatially-varying Material Appearance. Journal of Computer Graphics Techniques (JCGT), vol. 8, no. 4, 1-30, 2019 ... publisher | BibTeX

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Acknowledgments

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 642841 (DISTRO). We would like to thank Cyrille Damez from Allegorithmic for his contribution to the project.