  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.
