Absolute conic — where it appears
Named by 2 essays across one field — each of them below, with the objects they name alongside it.
The two points a picture hides
The stratification rung says a photograph of a plane is projective, becomes affine when the vanishing line is known, and becomes metric with one more fact. The one more fact has a size and a shape: it is two points, complex conjugates on the vanishing line, and a rectification built from those two and nothing else returns every world angle to a tenth of a picodegree and every ratio of lengths to five parts in a quadrillion — and refuses to name a length.
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.
Circular pointsConicDemonstrationline at infinityMetric rectificationCamera calibrationCamera matrixConic fitdegrees of freedomFocal recoveryGround plane rectificationHomogeneous coordinates