These are anamorphics. Part of what may be confusing you is that Diddi is so good his images look like full constructions. Just like all anamorphics, Diddi's images require viewing from a certain angle to appear to rise out of the page. Tried to find you a vid of a rotation one of his drawings, but no luck.
Anyhow, unlike trompe l'oeil (which mimics reality and wants to drop back into the frame), anamorphics seek to fake the eye into believing the image has real depth through application of complex perspective. They can appear to both fall into and rise from the frame, but only look "real" when viewed either from the right position or with the tool.
Here's a good article about them, including how to design them.
Anamorphics are commonly seen as chalk art.
You may also be familiar with them in conjunction with viewing tools, like mirrors or metal balls. Here's a page that shows examples of those.