[pvaldes]

> This has always been both legal, and encouraged.

Not always. The copy must be easily identifiable as copy. An exact reproduction can't have the same dimensions as the original for example.

Drawing just a person or a detail of the picture, or redoing the picture in a different context or style, is encouraged.

Selling a full scale photo of the picture is forbidden. The copyright of famous art belongs to the museum.

[samatman]

The second example is better than the first, yes. I was thinking about the process more than the fact that painting a study produces a work, and a derived one at that, so more normal copyright considerations apply to the work itself.

> An exact reproduction can't have the same dimensions as the original

This is a rule, not a law, and a traditional and widespread one. Museums don't want to be involved in someone selling a forgery, so that rule is a way of making it unlikely. But the difference between "if you do this a museum will kick you out" and "this is illegal" is fairly sharp.

> The copyright of famous art belongs to the museum.

Not in a great number of cases it doesn't, most famous art is long out of copyright and belongs to the public domain. Museums will have copyright on photos of those works, and have been known to fraudulently claim that photos taken by others owe a license fee to the museum, but in the US at least this isn't true. https://www.huffpost.com/entry/museum-paintings-copyright_b_...