Multiview — where it appears
Named by 5 essays across 3 fields — each of them below, with the objects they name alongside it.
A third eye sees the water the pair cannot
A level stereo pair looking at a stick in water gets two rays that meet exactly and a stick 285 mm too short, with nothing to warn it. A third eye raised ten centimetres above the pair sees a disagreement of 2.6 pixels at the stick's tip. Asked to agree with the other two as if there were no water, it hides nearly all of that and makes the stick shorter still. Told only that there is a flat surface, the same three eyes find its refractive index and put the metre back.
Split the track and the needles turn
A point triangulated from a track of cameras is not known equally well in every direction: its error is a needle, long along the direction its lines of sight barely constrain. That direction is not the mean line of sight but the lines of sight weighted by how near each camera is — and it can be turned. Two groups of cameras more than a right angle apart swing every needle across, and the same six pictures of a facade then measure its depth four times better.
A water surface's height trades against its index
Three eyes that must find the water's surface as well as its index find the two along a valley: a surface assumed higher bends every ray sooner, and an index assumed weaker makes up for it. From one stick at a quarter pixel the index is known to ±0.14 and the surface to ±48 mm, and the fits do worse than that promises — they slide down the valley, and the metre-long stick comes back 881 mm. A second stick eight metres out narrows the valley to ±7.6 mm. Holding water's own index at 1.333 closes it: the surface to ±23 mm, the stick to 994.
A drawn fold has a phantom
A Necker cube has two readings and so does a drawn fold, and the fold's second reading is not the supplement of the first. The drawing fixes each plate edge's component in the picture and leaves its component along the ray free up to a sign; a reflection identifies two of the four sign pairs, so there are exactly two plates — and a hundred-degree fold reads as eighty-seven as well.
Two outlines are two curves
The whole method of multiview drawing is the transfer line — a feature at a position in the front view is at the same position along that axis in the top view. On a flat-faced solid the feature is a vertex and the rule is exact. On a ball the two views draw two different great circles, meeting in exactly two points, and the transfer line joins places that are √2 radii apart.
Named alongside it
The objects these essays reach for when they reach for this one.
TriangulationDemonstrationDrawing systeminstrument limitOrthographicreconstruction ambiguityRefractionRefractive indexReprojection errorcamera trackConditioningContour generator