Orthocentre — where it appears
Named by 2 essays across one field — each of them below, with the objects they name alongside it.
Recovering the camera from the picture it drew
Draw a box from a known camera, forget the camera, and get it back from the twelve drawn edges alone. Agreement to one part in 10¹⁵ is a statement about the geometry, because the only thing that crossed between the two halves was a list of line segments.
One conic calibrates the camera
This site has recovered a focal length from two perpendicular vanishing points since its first phase, by an orthocentre construction with a square root in it. There is a second derivation with no construction and no square root — two vanishing points of perpendicular directions must be conjugate with respect to one conic in the picture — and the two agree to the last bit. They are not two methods. The conic is what a calibrated camera is.
Named alongside it
The objects these essays reach for when they reach for this one.
Camera calibrationPrincipal pointVanishing pointAbsolute conicCamera matrixCircular pointsConicDemonstrationFocal lengthFocal recoveryHomogeneous coordinatesHomography