There are a few thoughts I'd like to add. Nearly all painters working before 1500 had something to learn as compared to later artists. Hockney's use (and abuse) of them could have involved more than his tiresome harping on lines. He could have talked about their weak color and weird compositions. He could have mentioned the fact that the fingers and hands were stiff and posed looking - many hands were too small or too large. Some hands were stuck in the corners of the canvas right against the frame. He could have discussed aerial perspective. (I think landscape painting was one of the slowest types of painting to mature. Few landscapes done before 1750 look credible. Those that do are nearly all grey.)
He plainly chose to pick on some of the arts' weaker links. None of these things point to the use of "artist aids" as he contends, but rather to a general lack of knowledge. Painting matured slowly as did other fields of study. It matured at a faster rate than did other fields. Many doctors didn't start washing their hands until the U.S. Civil War and then not because of germs.
The truth is, I see most of the actual errors that Hockney lists are simply artistic human errors. Some other presumptions of later painters are simply presumptions on his part. His statements are being refuted the world over by numbers of real experts in many realms.
One silly thing he speaks of is dying fruit in still life painting. Ann Massey speaks of that correctly when she explains that anyway.
I paint fruit from life. I simply paint those most fleeting alla prima until they are complete. The slice of lime was cut, placed and painted. Later I broke open the pomegranate and it was so painted. No secret - no camera - no big deal.
Hockney suffers from ignorance just like the painters of 1400's lacked understanding on some points. Plain old human failings are sometimes not all that deep. Sometimes bad drawing is only bad drawing.
