john9818a on 8/5/2013 at 18:46

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Whenever you see something like this in the readme file of a fan mission it is ok:

Distribution of this level is allowed as long as
it is free and the package is kept intact. You may not include this level
to any map pack without my permission. No one may edit and re-distribute this
mission without my express permission.

This level was not made and is not supported by Looking Glass Studios or Eidos
Interactive.

With other resources I would ask the author for permission first.

The Watcher on 10/5/2013 at 11:36

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It also gets really complicated when you throw in textures created using images from places like CGTextures. That site in particular has a really frustrating license on its images: you can use and distribute them as part of 2D or 3D games, but you are not allowed to "...sell or distribute any of these textures (modified or not) by itself or in a texture pack, material, clipart, website theme or scrapbooking pack!"

So, it's fine to distribute them with fan missions, or even packaged with objects they should be applied to, but if you strictly interpret their license you shouldn't be hosting CGTextures-derived textures or packs containing CDTextures-derived textures, even if they've been so heavily modified as to be nearly unrecognisable as such.

Frustrating, and not a little crazy, but such is the world of copyright.