Creating the S3D isnt the real problem, because S3D files are only a container, like an AVI. Models (Creatures/Items/Zones) in EQ are WLD for mesh and BMP/TGA/DDS for textures.
So if you want to add your custom item/creature, best is mongrel solution, create your XXX.S3D files and add your files in. Textures isnt the really a problem, real problem is mesh (WLD), i remember Windcatcher made a WLD Doc Reference, link is somewhere on EQEmu forum.
Best thing would be to make a max plugin to create mesh as WLD, anyone ? :p
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