Visual Plane
every picture states the point it is correct from
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The thread: What a second picture adds — page 3

One view fixes a ray and a set of ratios. A second fixes a point, the whole three-dimensional shape, and where the other eye was — recovered from the two drawings and nothing else. It does not fix a size, and it does not fix itself when the eyes are close together. Each of those is measured here rather than stated. Essays 49 to 50 of 50.
0102030400102030roll of the head about its line of sight (degrees)vertical disparity at the eyes (arcminutes)15′ fusion limitinfinityten screens backtwo screens backhalf-way outon the glass: 0′ at every rollscreen 2.60 m away · eyes 63 mm The second projection

A stereo picture is drawn for a level head

Every stereo pair is drawn for two eyes level with each other — a point's two images share a row and differ only across it. Tilt the head 10° in front of a television and that difference turns partly vertical, 14.46 arcminutes for anything drawn at infinity, and the two sightlines to a point stop meeting. At a desk monitor the same fifteen-arcminute limit arrives at 2.58°.

 correct from 14 cm, at 160 mm wide5.1e-12 px Mirrors that are not cameras

A symmetric object is its own stereo pair

A building with a plane of symmetry photographed once gives fourteen correspondences whose joining lines meet at one point to 1.9 × 10⁻¹² pixels, a skew-symmetric matrix, and the object's whole shape to fifteen digits — with no mirror anywhere and no second exposure. What it does not give is the size, and the instrument that decides whether any of it applies is the same meeting point, which opens to 12.7 pixels when the symmetry is half a per cent out.

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