Paolo Uccello, who was commonly regarded as the first great master of perspective, worked in Ghiberti's workshop for several years, making it difficult to determine the extent to which Uccello's innovations in perspective were due to Ghiberti's instruction. Donatello, known for one of the first examples of central-point perspective in sculpture, also worked briefly in Ghiberti's workshop. It was also about this time that Paolo began his lifelong friendship with Donatello. In about 1413 one of Ghiberti's contemporaries, Filippo Brunelleschi, demonstrated the geometrical method of perspective used today by artists, by painting the outlines of various Florentine buildings onto a mirror. When the building's outline was continued, he noticed that all of the lines converged on the horizon line.