Cross-ratio (Doppelverhältnis) We are looking for invariants with respect to projective transformations. Definition: Let $ P_{i} =[v_i]= \ssqvector{x_{i} \\ y_{i}}$, $i=1,\ldots,4$, be four distinct points on a projective line $ \RP^{1} $. Then the cross-ratio of these points is \begin{align*} … Continue reading Lecture 6