Projective Invariants & the Cross Ratio

Following Klein's idea of studying geometry by its transformations and
invariants, this chapter focuses on the invariants of projective
geometry. These form the basis of many other results and help to provide
intuition about the structure of projective space. The simplest
projective invariant is the *cross ratio*, which generates a scalar
from four points of any 1D projective space (*e.g.* a projective
line).