Bringing Linearly Transformed Cosines to Anisotropic GGX

TL;DR

This paper extends Linearly Transformed Cosines (LTCs) to support anisotropic GGX models, enabling artifact-free, real-time area-light shading in graphics by solving challenges in fitting, storage, and interpolation.

Contribution

It introduces robust methods for fitting and interpolating LTCs for anisotropic GGX, resulting in a compact look-up table for artifact-free, real-time shading.

Findings

Achieved artifact-free LTC approximation for anisotropic GGX

Developed a small 8^4 look-up table for real-time shading

Ensured robust LTC operations to prevent rendering artifacts

Abstract

Linearly Transformed Cosines (LTCs) are a family of distributions that are used for real-time area-light shading thanks to their analytic integration properties. Modern game engines use an LTC approximation of the ubiquitous GGX model, but currently this approximation only exists for isotropic GGX and thus anisotropic GGX is not supported. While the higher dimensionality presents a challenge in itself, we show that several additional problems arise when fitting, post-processing, storing, and interpolating LTCs in the anisotropic case. Each of these operations must be done carefully to avoid rendering artifacts. We find robust solutions for each operation by introducing and exploiting invariance properties of LTCs. As a result, we obtain a small $8^4$ look-up table that provides a plausible and artifact-free LTC approximation to anisotropic GGX and brings it to real-time area-light shading.