The thread: What a second picture adds — page 5
A mirror corridor shows the room once, and the room's lines are a reference by where they are
Between two facing barber's mirrors the corridor of self-images goes on for a dozen round trips, but the shop around it does not: the floor never appears in a level photograph, and the walls appear in the first reflection only, because every later path has to start and end on glass and only what stands between the two panes can be reached. That one sight of the wall behind the sitter is enough. A rail and a door jamb on it, fitted with the self-images, read each mirror's own lean and turn to about a twentieth of a degree with the camera's heading, pitch and roll unknown — better than a camera believed level and a tenth of a degree out. The reference is not the lines' angles, which can be 0.2° off at almost no cost, but their places: a rail 5 mm higher than believed reads as a lean of 0.14°.
The second eyeThe road under a car ahead gives its growth a distance, while the road stays flat
One camera that knows its own height reads the row where the car in front meets the road, and that row is the camera's height seen from the car: a baseline of 1.3 metres standing on the road, read in every picture. With the car's growth it ranges a car 20 metres ahead to 0.80 per cent a pixel, six times better than a stereo pair's disparity, and reads the car's width to 0.74. A camera height a few centimetres out scales the distance and leaves the time untouched. A road that starts rising by one degree between the camera and the car moves the distance by up to 24 per cent, and neither horizon puts it right. The lane's own edges bend where the road does and read the rise back.
Through water and glassA crooked stick hides in a quarter pixel, and its residual finds it only below a tenth
Eight sticks stand in still water, one of them bowed three millimetres and the rest one, and a refraction fit holds every stick to a straight line. Read exactly, the bad stick carries the largest share of the fit's residual in about seven sets of ten. Read to a quarter pixel, as a careful photograph is, it carries it in one set of five, barely more than chance: a millimetre of bow moves the index more than the reading error does and leaves its rays missing by a twelfth of a quarter pixel. The residual stays blind until the bad stick is bowed five or ten millimetres, and dropping the stick it ranks worst does nothing measurable for the index.