Visual Plane
every picture states the point it is correct from
Ladder

Height — the ladder

2 distinct arguments against one idea, from the one that introduces it to the one that assumes the rest.
  1. horizon — the eye's own heightbase — 0 mhorizon crossing — 1.62 mtop — 3.40 m recoveredthe vertical vanishing point is 8586 px above this framerecovered 3.400 m · true 3.400 m1.8 cm per pixel of click error

    A height, out of one photograph

    Four points on a vertical, one cross-ratio, and the height of something nobody measured. The only metric input is the photographer's own eye height, because the horizon is at eye level and that is the one piece of perspective folklore that is exactly true.

    rung 1 · metrology
  2. 05101550100150200distance from the camera (metres)error in the recovered height, per pixel of click error (%)5 m — 0.46%25 m — 2.27%100 m — 9.04%190 m — 17.16%one pixel, on a 690 px picturelinear in distance

    How wrong a measurement from one picture can be

    The formula divides by a difference of two nearly equal numbers when the object is tall, which looked like the instability and is not. Measured, a taller object is recovered better and a more distant one worse — half a per cent per pixel at six metres and eleven per cent at a hundred and twenty. The argument that was wrong is as much the finding as the curve that replaced it.

    rung 2 · metrology

All ladders