Is it legal to say that MyGame supports other (specific) games' textures, sounds and models?
I'd like to get involved into an open source game project and implement support for Quake's weapon models (for example), but the owners of the project don't know about the legal aspect of it, so I'm asking here.
It depends on the third party. Valve's SDK, for example, prohibits use of it for anything other than mods running on the Half Life engines, so if you were to use their formats you would be in violation. Other third parties may have different or similar terms.
I believe the information provided by ID software with their code releases would be the closest thing to actually asking ID yourself
ID Software github - readme.txt for Quake
All of the Quake data files remain copyrighted and licensed under the original terms, so you cannot redistribute data from the original game, but if you do a true total conversion, you can create a standalone game based on this code.
--- snip ---
You can still download one of the original quake demos and use that data with the code, but there are restrictions on the redistribution of the demo data.
If you never actually bought a complete version of Quake, you might want to rummage around in a local software bargain bin for one of the originals, or perhaps find a copy of the "Quake: the offering" boxed set with both mission packs.
The only company that can truly answer your question would be ID software, if they consider your game to be in violation of their copyrights then they will most likely take legal action regardless of what anyone here tells you. On the other hand, they have released the source code of Quake in a manner that allows you to modify it as long as you don't distribute the other files with your modifications.
You may wish to consider that by making the game "borrow" models from another game installed on the users computer you will never be able to make a full installation that does not require that game. In short, you will artificially shrink your potential user base, all to save yourself the relatively short time it would take to make a suitable weapon (and projectile) model in a 3D program.
Personally I would use Blender and make a quick weapon myself, even if it looks crude it would be safe to use and could be upgraded in the future without any legal dilemmas.
I've seen people doing this by not including the models, textures etc in game.
IANAL but I don't see how supporting other people's textures etc can be illegal.
Shipping it with your game without prior permission probably is illegal (unless otherwise stated).
It's a faff for users to have to provide them, so having a tool to make import easy would be a winner.
I don't think you're going to get the answer you want because, as mh01 said, the different companies Valve, ID etc don't share a common licence regarding usage of their models.
You're either going to need to evaluate them on a company by company basis. Or otherwise implement support / compatibility without including models.
Again IANAL but it would make sense to me that the responsibility transfers to the user of the app / game if they find models themselves and put them in the right place.
A couple of examples:
You should be safe if you claim support for other product's asset formats. Directly claiming support for other game's data might be ok too, but the user could be in violation depending on the EULA and such - you yourself commented that many allow personal use, so check it for each one in your mind. Examples of open-source games that depend on the original data are Arx Libertatis and OpenMW.
Regarding implementation, I think the main issue is whether the file formats contain patented features in which case it would be illegal to implement in some countries. This is most frequently encountered in formats that attempt some kind of compression, such as audio/video files.
