A point is the entity that two incident lines have in common.
A line is the entity that two incident surfaces have in common.
In particular, a straight line is the entity that two
incident planes have in common.
The projection is the fundamental operation that allows you to generate an image of the Model and to control, by means of this image, any geometrical features of the Model.
The projection needs a point, which will be assumed as Centre of projection, and a surface (normally a plane), which will be assumed as Picture, normally called Picture Plane.
In these lessons, and unless otherwise noted, the picture plane is always vertical and therefore perpendicular to the horizontal reference plane.
When you look at a perspective, placing your eye in the centre of projection, we call this condition Restricted Sight.
In Restricted Sight, any point of the Perspective corresponds to a point of the Model. E.g. point A’ is the perspective
of point A, point B’ is the perspective of point B … and so on.
Now, imagine displacing your eye, for instance, lower. In this case any point of the Perspective corresponds to a point of another, and different, Model.
If you want to explore the Restricted Sight, please read tha article Perspective, Theories and Experiments on the “veduta vincolata” (restricted sight), by Riccardo Migliari and Jessica Romor.