Composition — where it appears
Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.
A projection of a projection
Photograph a photograph and the composite map is a homography — four marks determine it and every other lands where they say, to 1e-13 px. Roll the print and the same four mispredict the rest by 30.5 px, which is why straightening a page of a thick book never quite works.
Two lamps and one map
A flat object lit by two lamps casts two shadows, and one is the other scaled about a point — ratio 1.2509 here, carrying every point of the first outline onto the second to 1e-15 m. No rotation and no shear is available to it, because a projection between two parallel planes has its axis at infinity. And the ratio is exactly 1 when the two lamps are at the same height, which makes a pair of shadows a measurement of the lamps.
Named alongside it
The objects these essays reach for when they reach for this one.
Projective mapAxis of a homologyCentral collineationCharacteristic ratioCross ratioHomographyHomothetyPicture planePlanar homologypoint at infinityRectificationResidual