What is the "Levels>Auto Input Levels" operation actually doing?

[Ofnuts]

GMIC does CLUT, but CLUT are much older than GMIC. CLUT means "Color Look-Up Table" and maps a RGB triplet of colors to another. A full CLUT has 16M entries (the 2^24 possible triplets), but very often a smaller CLUT and some interpolation is sufficient(*) (smaller CLUTs are 2^3n). When done in software, a CLUT can be represented by an image where pixel coordinates correspond to the input RGB triplet, and the pixel color is the output RGB.

In GMIC you have basically two tools: one that creates a CLUT (HaldCLUT) from before & after images, and one that applies a CLUT to an image. You can find them using the filter search tool.

(*) Interpolation is unavoidable when you deal with high precision images anyway.

[rich2005]

Not just gmic although using Gimp that possibly the only way.

For example, these CLUT (cubic lookup tables) files https://www.shutterstock.com/blog/free-l...og-footage

You can use those .cubic LUT's in gmic. I suspect that they might already be included in the very extensive collections that come with gmic Colours > Color Presets filter.

edit: another large collection of .cube files https://forum.affinity.serif.com/applica...p?id=35861