The Game and cycling sim Zwift uses .ztx files for textures.
They have their own game engine sadly. It must be possible to modify / convert these textures, since there is one replacement mod out there, but the modder won't say how he managed that.
I've attached one file (a road texture) if someone wants to try their luck. I'm not very experienced with hex editors myself:
https://www.mediafire.com/file/7mq87bsu ... e.ztx/file
One information I could get frome someone else:
I looked at hex; I think this works like a standard video game pipeline - let me explain:
(1) create/edit textures in standard tool such as photoshop or the like; save the source as jpg or equivalent
(2) run the source textures through a pipeline tool that converts them into ZTX format to use in the runtime game; typically pipelines will perform actions like compression, combing textures, etc.
(3) the second pipeline tool combines a set of assets needed for a level or part of world; this is the WAD file; these assets can be textures, geometry, audio, etc.
Bottom line is you need the tools or have to do a lot of reverse engineering.