Desargues’s theorem

This is an illustration of Desargues’s theorem. It says that two triangles are in perspective axially if and only if they are in perspective centrally. \documentclass[tikz,border=10pt]{standalone} \usetikzlibrary{calc,intersections} \begin{document} \begin{tikzpicture} \coordinate [label=above:$p$] (p) at (2,9); \coordinate [label=left:$u$] (u) at (0,3); \coordinate [label=left:$r$] (r) at…