[afavour]

IIRC if you make entirely new assets you're good to go. OpenTTD (Open source version of Transport Tycoon Deluxe) has its own custom made assets, but can also be used with the original if you own them.

https://www.openttd.org

Not sure it's ever been proven definitively in court, though. And if you "made" custom assets that were exactly like the original ones only with a 1px color difference or something I'm sure you'd fall foul of it. What counts as different "enough" is always debatable.

[mikepurvis]

It would probably be the usual clean room reverse engineering rules: one guy describes the assets to be cloned, and then another guy who has never seen the originals uses that documentation to create the replacements.

Once you've seen the originals, you're contaminated and no longer suitable for the role of doing the replacement work.

[catapart]

Oh, awesome! Yeah, this is a great example of something that I would have guessed would kind of be "over the line" being that it looks similar enough to be an issue. I'm glad it's not, though! But, either way, it's a perfect practical example of what I was wondering about, so thanks!

[afavour]

Unfortunately I think definitive answers are difficult to come by. No one cares that much about Transport Tycoon so no one is motivated to enforce anything. But if you made a clone of Call of Duty that had models as similar as OpenTTDs are to the original you might find yourself in hot water.