[Tuna-Fish]

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.

[anthk]

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.

[Tuna-Fish]

> 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.