Yes, Artists Build On The Works Of Others... So Why Is It Sometimes Infringement?

Techdirt: "Following on our story the other day about copyright questions concerning the 'appropriated art' that became the iconic Obama campaign poster, the Wall Street Journal has an interesting article exploring the fine line between derivative works and transformative works in the art world. As you probably know, derivative works (e.g., making a movie out of a book) are considered copyright infringement, but transformative works are not."