Yes, like for example, the original FXAA algorithm was copyrighted by Nvidia. Even though the creator wrote a paper and released the code on Github, it was bound by Nvidia's proprietary license. Later they decided to open source an older version of the algorithm, but the newest version is still owned by Nvidia.
However, if you read the paper and wrote your own implementation based on the idea, I don't think they own that. They would if they received a software patent, which is possible in the US, but AFAIK Nvidia did not do this in this case with FXAA.
So if you look at the pseudo code in the original paper and write your own shader based on that, it should be 100% legal. Again, I am just talking about FXAA because I researched it. Every project is different, so you have to look for yourself.
