General problems: Problems tracing moving and imaginary subjects...

There are numerous Renaissance paintings depicting subjects that cannot be recorded by the Hockney/Falco method and this strongly suggests that Renaissance painters had the drafting talent and visual memory to paint "optically" without mirror projections.

1. Objects in motion cannot be recorded: Running animals, rearing horses, prancing dogs, flying birds, monkeysÊ peeking up women's dresses, people on swings, restless putti, crashing waves, and so on cannot be recorded by means of the Hockney/Falco method because the image moves before the artist can trace or paint over it. Moreover, the human visual system is quite insensitive to dim moving images, as projected in the Hockney/Falco method.
2. Non-existent objects cannot be recorded: Of course dragons, gryphons, minotaurs, wings of angels, and other imaginary objects cannot be recorded through the Hockney/Falco method.

Summary: It is very difficult or even impossible to make images of running animals, flying birds, restless putti, dragons or angel wings by the methods proposed by Hockney and Falco. The fact that extremely realistic "optical" representations of these subjects appear throughout Renaissance painting shows that many painters of that time had the requisite drafting talent and visual memory and did not need to employ optical devices.