Medieval scholars such as Roger Bacon and Robert Grosseteste were keenly interested in the relationship between the viewer and the objects observed at any given moment, and how this relationship changed with every shift of distance or angle.
Beginning with Giotto and Duccio di Buoninsegna, it became a problem of great interest to artists as well. Both scholars and painters sought to learn how to see the world, trusting their own eyes, and not old doctrines or pictures. The man who made the great breakthrough in the discovery of the principles of visual perspective was Filippo Brunelleschi, and he did it in a way that was as empirical and untheoritical as a kitchen sink.
He gave two public demonstrations in Florence of his discovery, once while standing in the central door of the Duomo. Using the doorway as his frame of vision of the Baptistry, and stretching strings across it to create a grid, he sketched and then painted the contents as he saw them, keeping a fixed distance at all times to accurately render geometry into two dimensions. It changed the history of painting and visual theory, which Leon Battista Alberti would later describe in his landmark Della Pittura.
When he was finished, Brunelleschi showed viewers his completed picture of the Baptistry: he cut a peephole in the canvas at eye level and set a small mirror in front of it, to control the fixed distance of the spectator to the work and making it appear perfectly to scale.
Images by: aparences.net