Trademark: if the game is trademarked, you can't re-use the name. Anything with a trademark needs to be replaced with a new name/logo that is sufficiently different as to not induce confusion in the consumer.
Copyright: all the source code is rewritten so there is no copyright there. The game assets like textures and levels are under copyright and needs to be re-created from scratch as well. Or require the player to own the original game and provide an import method. A lot of Doom engines for example would require you to provide the WAD files.
Patents: this might be more tricky to work around. Patents protect algorithms and methods of production which might be integral to the game. I am not aware of games who are protected by patents.
These are the 3 legal pillars of legal protection
Given how much money is in the games industry as a whole it is kind of weird actually that we haven’t seen any publicly known patent trolling targeting game development studios yet.
Perhaps they haven’t been targeted. Perhaps they’ve paid up in silence. Or perhaps it has happened and the public knows — it’s just that I haven’t heard of it.
Hence why when PlayerUnknown's Battlegrounds tried to sue Epic, they did it in South Korea.
Also, FWIW, I found this online: https://www.gamasutra.com/view/news/273935/Texas_court_affir...
(admittedly, that ruling makes sense, game mechanics would need to be patented, not something you copyright)
https://www.x-plane.com/x-world/lawsuit/
https://www.clickerheroes2.com/patent_libel.php
https://www.gamedev.net/forums/topic/645267-patent-trolls-at...
...there is no copyright infringement. You own copyright on the things you create (or your employer does when hired by them to do such work.) You infringe others' copyright by using their work in a way that they didn't approve (or goes against how the law says you can use their work.)
By creating your own implementation of $THING, you now hold copyright in your implementation. This does not alleviate the patent and trademark concerns.
Not so fast. It's very likely that a court would find that they are derivative works, and are therefore protected.
https://en.wikipedia.org/wiki/Software_patent
You might be able to in some places but in Europe they will laugh at you if you try and get your software patented.
This has been the situation for a long time, so things like the Amazon 1 click patent are freak occurrences rather than the norm.
Short version, can they sue you? Yes, just like anyone in the US can sue anyone else for whatever reasons valid or not. In Hasbro v Scrabulous, Hasbro dropped the suit given that they were losing and Scrabulous had run out of money. Had they continued the suit they would have had a judgement on the record regarding the legality of cloning the game experience and possibly been ordered to pay legal costs had they actually lost. As it happened they didn't need to win because they just buried their competitor with legal fees.
[1]: https://www.quora.com/Did-Scrabble-sue-Words-with-Friends-Wh...
that's pretty fucked. I wonder if there's any lending mechanism which would front the legal fees, and then recoupe it back from the judgement (if won).
It seems that the method of burying a competitor with a suite that lacks real merit would only work if the fees become unpayable. So closing this loophole would fix the problem!
If your lawyer thinks you have a strong case, they will work for you for free, in exchange for a share of the bounty.
Unfortunately, you can't always tell if you have a strong case.
Unfortunately if you are being sued winning just means you don't own anyone money. It is a separate thing to collect lawyers fees from the other party.
I truly wish there was some sort of legal money parity rule, so the burden of paying for the case in terms of lawyers and court costs was shared by both sides in proportion to their available assets.
Maybe then this sort of behaviour would stop, as large companies would realise any attempt at legal action on spurious/questionable grounds would result in a battle with billion dollar opposition, and their intended target not paying them a penny regardless.
edit: The "legal aspects" section of this wikipedia article mention a case in 2012 that may have changed that https://en.wikipedia.org/wiki/Video_game_clone
This is why projects like Freedoom or Hedgewars recreate all game assets to make them look quite different. Just making minor changes that make it obvious that the original artwork was used as a guideline is not enough of a defense against an accusation of infringement.
What is a defense, however, is when copyright infringement is not willful. If you and I happen to make the same artwork but we can prove that neither of us looked at each other's work and the similarity is just a coincidence, then it's unlikely that we can claim copyright infringement.
Note again that if you clone Tetris, YOU WILL BE SUED and you will probably lose. Tetris is the most vigorously defended game IP in the world, by a company that, like Oracle, is mostly legal department, and judges have ruled that the shapes of the tetrominoes, the manner in which they spin and fall, and the dimensions of the game board are all copyrightable game elements.
Looks like their current strategy is to collect a few heads (e.g., Xio) and post them on pikes outside the village gates pour décourager les autres. They may not be able to nail everybody, but you run substantial risk of being targeted.
https://www.debian.org/security/2002/dsa-205
And of course I only started poking around because it was a game I played very very often in the office at the time.
I don't know how they acquire theses licenses, but if they did, maybe you can too and if you can't, maybe they already have it made, or needs a developer to implement it (which can be you!).