The painter and mosaic artist Cimabue is regarded as the teacher of Giotto and, for this reason, is considered a pioneer of Italian Renaissance art. He overcame the statuary style of Byzantine painting in favor of more liveliness and naturalistic details. Giotto refined and perfected this style and was the first artist to experiment with perspective. Vasari reports that Giotto's saints displeased church officials because they appeared too human—thus too worldly. As the chief architect of the cathedral in Florence, Giotto became one of the leading artists of his time during his lifetime. In contrast, Pietro Cavallini helped establish Rome as an important art center. He created frescoes and mosaics in the tradition of the Cosmati; more recently, several previously painted-over frescoes by his hand have been uncovered in Roman churches.