Collinearity — where it appears
Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.
A shadow across an edge
A straight rod's shadow crossing the crease between floor and wall is two straight pieces, each dead straight to 1e-15 m, meeting at 35.08°. The corner is a fact about the room and not about the rod. Fit the floor's map from four marks and apply it across the whole shadow and the part on the wall comes back up to 78.9 cm from the object — the wrong map, applied confidently.
The line every nosing is on
Nothing in a staircase points up the pitch. Every surface in it is level or vertical, and its picture has a vanishing point off the horizon all the same — belonging to the line the front edges of the treads lie on. That point is not free: it is collinear with the travel point and the vertical point, and the angle it makes says the rise-to-run the builder chose.
Named alongside it
The objects these essays reach for when they reach for this one.
Axis of a homologyDemonstrationFocal lengthForeshorteningHomographyHorizonIncidenceInclined planeOrthocentrePiecewise mapPlanar homologyProjective map