From what I’ve read in threads here before (although excuse me for not searching and finding them right now) use of this wording is telling and deserves emphasis.
Yes, the use of perspective as a technique during the Renaissance impacted mightily what was considered “realistic”, but probably more so what we consider “realistic” in a two-dimensional artwork today is heavily biased by our current constant exposure to photography and the artificiality that medium imposes on our perceptions. The reality that was important to capture, and the way to represent that reality, was not always concordant with how a photograph approaches the task.
I’m a TV and film producer. I know nothing about the development of film and video technology. The fact that Hockney paints doesn’t make him an art historian.
The camera obscura under whatever name had been known long before the Renaissance. Hockney’s argument is that it came into use as an aid to more realistic portraiture during the Renaissance is what art historians dispute.
An interesting side note on the realism of medieval portraiture is the way people are depicted as walking, with toes pointed down. I had noticed this and assumed it was some stylistic convention, but according to this video it is actually an accurate representation of the medieval gait:
Depends too on style. For example - the “walk like an Egyptian” pose was drawn by early Egyptian artists because it was the style, but the purpose of the style was to show the characteristics that made the object recognizable. Legs were most recognizable in side view, shoulders and chest in front view, face in silhouette. The goal was to show the person or god, not to create the equivalent of a photograph, or “exactly what we see”. A natural pose frozen in time was a fairly modern concept. Instead, the idea was to get the most impact out of a specific scene - the more costly a work of art in local terms, the more important that the message be apparent. Exactly - it was illustration. In poorer times they didn’t have art for art sake so much as to illustrate religion, stories and legends, allegories, or historic events. Even the Greeks who had amazingly life-like statues had moderately stylistic flat illustrations on their vases, for example.
Similarly, before the renaissance you don’t see a lot of simple portraiture of anyone other than kings - and even then, most pictures are of important events like battles or coronations, not just “The royal family in their garden”. So again, the story not the portrait exactitude is the underlying impetus.