Points in Space

In the physical world (the "real world"), the endpoints of the two spikes are at definite locations. Later on in these notes, locations will be specified using a coordinate frame. But for now let's just think of points in space. In 3D graphics programs you put points in space, and lines in space, and other things in space. And then project them into a 2D picture.

This mimics what has happened when the picture was produced with a camera. The ends of the spikes in the physical world define two 3D points. The whole scene was projected by a lens onto a 2D sheet of film. The ends of the spikes in the 2D picture define two 2D points.