Six to One

In 1930, orienting a pair of aerial photographs meant touching six points by hand, in a fixed order, and watching the parallax close. A century later a single photograph gives up its depth on its own, with no point at all. Six to One walks down that staircase, one step at a time. Five chapters, one real stereo pair. 1. The ritual. The six von Gruber stations on a genuine stereo pair. Touch them in order and watch the residual parallax fall, pass after pass, the way it fell on a plotting instrument. 2. Eight points. The essential matrix asks for eight correspondences and no operator at all. Choose 8, 16, 32 or all of them and see what the algebra does with the extra ones. 3. Five points, and a consensus. Nister's minimal solver plus RANSAC: five points at a time, drawn again and again, until the majority agrees. The inliers appear, the outliers stay out. 4. The single image. No pair, no correspondence. What is left when only the prior remains. 5. The tool. Your own photograph, its depth map, computed on the device in about a second on a recent phone. Choose a photograph or turn on camera mode and watch the relief follow the lens. Light is near, dark is far. Scale, which nothing here ever recovered, stays unknown. Everything runs on the device. The application asks for no account, has no advertising, collects nothing and has no network permission at all: you can check that yourself, and it works in airplane mode. Four languages: French, English, Italian, Spanish. Credits. Depth model MiDaS v2.1 small, Intel ISL, MIT licence. Chapter 1 photograph, United States Geological Survey, public domain. Stereo pair photographed by Pierre Drap in 2011 with two synchronised Nikon D700 on a bar, with the consent of the two people in the frame. IMAGE4D, image4d.fr