several hatchings such as cross lines and dots are used to represent the depth within this portrait. as seen in this image the darkness of an area is shown by doing a much more of a closer/tight hatchings.
Paris based artist Felice Varini. At first glance I assumed these were just geometric designs digitally layered over a photograph… I was wrong. They are in fact geometric designs painted directly onto actual buildings, but, the trick is, you must be standing at an exact point to see them, otherwise they appear as random broken lines (as seen above).