Well, as reported by Hrag Vartanian on his excellent Hyperallergic website, Hungarian new media artist Bence Hadju has created the series Abandoned Paintings to make it easy. According to Hadju's artist's statement . . .
I am a student at the university of fine arts, Hungary. At one of the descriptive geometry classes we had a task to find and draw the perspective and horizon lines of renaissance and other pictures with significant perspective space. I thought it is not that interesting to just draw lines, so I decided to erase all the characters from them and examine how the painter really created the perspective space and how it actually looks. I saw this could be something exciting and continued thinking and working on it. After a while I found myself interested in the new atmosphere and the new thoughts the retouched pieces generated without their main subjects.Hyperallergic has made it even easier by creating animated GIF's - like the one of Sandro Botticelli's The Annunciation, which we cribbed above - that toggle between the original painting and Hadju's depopulated version. And so we have David's Oath of the Horati de-horatied, Lorrain's Seaport with the Embarkation of St. Ursula after everyone had gone home, and Leonardo's Last Supper with Christ and his disciplines all having left the room for a bathroom break.
Leonardo, of course, actually was an architect - along with being a painter, sculptor, scientist, inventor, mathematician and web developer - and we could say that he not only painted the Last Supper, but designed the room in which it takes place. All the artists here, however, were masters of perspective, and the backdrops of their paintings could said also to said to be an architectural analysis of space.
You can see all of Vartanian's now-you-see-them, now-you-don't GIF's here, and Hadju's Abandoned Paintings, with Mantegna's Occulus in the Camera Degli Sposi added, here.