The thread: What a second picture adds — page 4
A false pair is named by its neighbours, not by itself
Read against the rest of a symmetric facade, a pair whose right half has moved six centimetres is explained exactly by two stories: the right half moved, or the left half moved the mirror way from a pair standing six centimetres over. Predicted from left to right and from right to left, the pair gives one residual twice. What names the half is a mark that shares a line with it — the window's upper corner names a move across ninety-three times in a hundred at two centimetres — and what the readings recover is the move as the camera sees it, blind along its ray.
The eye that movesA scroll camera rings with its vehicle's suspension
A vehicle does not pitch with the chord between its wheels; its body follows that chord through springs, lags it, overshoots and swings back. Over a step to six per cent at ten metres a second, a body sprung at 1.3 hertz parts a scroll's rows by up to 2.6 pixels where a rigid vehicle parted them by 2.2, and leaves them parted eight metres past the step. No rigid vehicle can take the difference, and one line of posts read to a tenth of a pixel gives the suspension back: 1.302 ± 0.011 hertz, damped 0.303 ± 0.008.
The second eyeA drift across the lane closes the blind point, slowly
A car that wanders twenty centimetres either side of its lane every four seconds moves its camera twenty-eight centimetres sideways in a second, far more than steering ever swings it. Looking back over that second, the road twenty metres ahead is known to 23 per cent a pixel; over four seconds, to 10.5. The error falls as one over the wander and grows with its period and the car's speed, so a car that keeps its lane well, drifts slowly or drives fast leaves the point it is heading for nearly as blind as before.