Piero della Francesca, Leonardo da Vinci, and Albrecht Durer were all theorists as well as practicioners.
Both Piero della Francesca and Albrecht Durer published what were in essence math text books!
Pisarro, In the Wood, 1862 | Monet, Street in Ste Addresse, c. 1869 | Sargent, A Street in Venice, 1882 |
And so pictures got flat - so flat that art critics even argued over the thickness of the paint, and some artists thinned their oils with turpentine to eliminate even that thickness. And perspective was dead, at least for the avante-guarde, although illustrators and other artists used it as before.