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Many render engines implement the microfacet model for rendering surfaces that are rough, metallic, and/or transparent, a concept first published in "A Reflectance Model for Computer Graphics" This model implies that surfaces comprise a large number of microfacets, each with a random slope. The slopes of all the microfacets on a surface can be categorized with a facet slope distribution function.

Some render engines like EEVEE and Cycles allow users to select such function (called a "light scattering distribution function" in Blender), and the default is GGX.

Blender nodes with GGX

GGX is presented in the paper "Microfacet Models for Refraction through Rough Surfaces."

In this paper's introduction:

We also introduce a new microfacet distribution, which we call GGX...

...but they never explain why they call it GGX.

Perhaps it's mentioned somewhere else?

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According to E. Heitz (Sampling the GGX Distribution of Visible Normals) it stands for "ground glass unknown". I am not sure what the word unknown means though.

By the way, it is equivalent to Trowbridge-Reitz distribution, introduced much earlier.

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