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I've managed to get into the .HOG files and extracted quite a few different files from Syphon Filter: The Omega Strain. Since these archives are simple to repack, I wanted to try retexturing some of the objects and clothing in the game. The .TDX files seem to be the textures, as they're the only ones found in the .HOG files that seem to be for textures, but I may be wrong.
Here's the first bit of one of the .TDX files opened in a hex editor. Hopefully this could be of some use to someone more knowledgeable about texture file types since I'm clueless at this point. I'll attach the file here as well so someone can take a better look at it.
It doesn't seem to be the same TDX used in other games, such as Tony Hawk's Pro Skater 3, so none of the tools I've tried seem to open it. Thanks for reading, hope someone can come up with something!