It's more complicated than that. For an asset to be derived work from an original, it is not necessary for it to contain anything from the original. If you start from copyrighted assets, and meticulously replace them all with your own art piece by piece, while following the style and constraints of the originals, and while looking at the originals, I'd bet that a court would find your work to be derived from the originals and therefore under their copyright.
A lot of the fan-driven reimplementations of classic games are trivially derived works, because people seem to think that the copyright only covers the pixels in the originals and if you replace them you're fine.
FreeDoom does that with Doom and it has compatible assets but not in the same style altough they are done in such smart way that most PWADs and TC are totally playable without clashes, from Requiem to Back To Saturn.
On game engines, reimplementations are not derivations at all but tools for interoperability, totally legal to create. From Wine to most of the stuff of https://osgameclones.com, to GNUStep against NeXT/OpenStep API (and Cocoa from early OSX) and so on.
If you could sell Cedega back in the day you can totally sell OpenTTD with free assets, period.
The entire PC industry exists today because of cheap IBM BIOS clones from Taiwan.
Yes, the engines are fine. And if the assets are free, then it is fine to sell the engine with them.
What I'm contending is if the assets are actually free. And just because they were all created by volunteers and contain no data from the originals doesn't mean that they are actually free. The rules around derived works are complicated, and too close homages have been found to be derived works, even if there is no actual copying.
If this were to go into court, things that would matter would include both "how visually similar do they look" (the answer is "very"), and "was the artist aware of and did they refer to the originals while doing their work", (given it was done by volunteers who are enthusiasts of the original game, the answers are almost certainly "yes" and "they can't prove they didn't").
And on those facts, the new art is a derived work of the original and falls under its copyright.
Ahem, no. Not the case there. The artwork under OpenTTD fails under fair reimplementation for cohesiveness with the current extensions and modules. Ditto with FreeDoom with Doom: is not inspired but art-compatible so your Strain, Requiem, Back to Saturn and so on PWADs run the same without texture or styling clashes.
Artistically speaking FreeDoom it's closer to Half Life and the like than Doom but here's the catch: playing Strain for instance won't look like a mess, but different, a bit like a demaked Half Life (or a game from its era with the Unreal engine) but not a copy.
> The artwork under OpenTTD fails under fair reimplementation for cohesiveness with the current extensions and modules.
I sincerely doubt that. Unlike the FreeDoom assets, it is too visually similar to the originals, and visual cohesiveness with existing materials (which were created to fit the style of the originals) is a point in favor of it being a derived work, not against.
> The entire PC industry exists today because of cheap IBM BIOS clones from Taiwan.
Forgot to reply to this part: And the reason those clones exist is because multiple companies reimplemented the BIOS in a clean way, where they had one team produce a clean spec of all the interfaces, and then sequestered a different team, who could attest in court that they had never worked with, seen or in other way come in contact with materials related to the IBM PC, to produce a replacement BIOS based on only the given spec. The clone makers that didn't go to all this effort were sued out of existence.
Do you believe that the free assets produced for games generally meet this standard?
A lot of the fan-driven reimplementations of classic games are trivially derived works, because people seem to think that the copyright only covers the pixels in the originals and if you replace them you're fine.