You make a good point Tom. I think a lot does depend on whether you plan to publish or not. I know that publishers insist that artists/authors verify that there are no copyright problems with work.
The "grey" areas I referred to probably also come from things like trademark rights. The two can overlap a bit.
But ultimately, as I say, it's easier when you can see what someone is doing, then it's much easier to judge if it really does constitute a new work of art.
Maybe my mega-caution on this comes from the fact that I spent years working in Corporate Identity and Branding. In that field copyright infringement is enforced vigorously. I'm sure that in other fields it's probably more relaxed.
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edited to say - by the way, I agree with someone else here on the forum who said some time back that copyright and intellectual property issues have become a bit mad. In some cases I think it does hold people back from creativity in an irrational way (did you know that someone tried to "own" the New York skyline - or so I was told, I hope it's an urban myth!) - but in others, well, it seems quite fair to protect copyright. Anyway, as ever, an interesting issue!