When you are standing in front of a thing and are painting that thing, you are performing a minor miracle: the transference of the optical appearance of the 3D world onto the flat 2D surface of a painting. This is otherwise known as painting from observation.
It is often the case that we need to transfer the schematics of a 2D image (a drawing or photograph) to a canvas. There are a number of ways to do this. Traditionally, something called ‘gridding up’ was done.