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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).


Bill Triggs