Proposition 1: The image of a
straight line is always a straight line (or, in the unlikely case that this
line is an orthogonal that passes through the center of projection, a point).

This means that scenes which only contain straight lines are easiest to
deal with since the only tool they require is the straight-edge. The representation
of curved lines are another matter, which poses difficult problems for artists.
We will take this up later.
The second property of perspective concerns the representation of sets of
parallel lines (such as when you draw a box). For the moment, let us assume
that the picture plane is vertical.