Four points on a conic look the same from anywhere on it
Four marks on a photographed circle subtend the same cross-ratio at every point of the curve — 1.627695, unmoved over twenty-two positions of the fifth point, to 1.1e-13 degrees of projective spread. A fifth point 6.1 pixels off the conic reads anything from 1.00 to 2.52, so the invariant belongs to the curve rather than to the four marks.
part 1 · foundations