Visual Plane
every picture states the point it is correct from
Concept

Motion blur — where it appears

The streak a moving point leaves over an exposure, which is the projection of its world path rather than a smear applied to its image. Its length goes as one over the depth, so two objects at different distances blur by different amounts under one shutter.

Named by 3 essays across 2 fields — each of them below, with the objects they name alongside it.

at one depth — a stadium, to 1.8e-5receding — tapered, 4% outone point, one exposure, the whole pupil1.8e-5 against 4%

The disc and the streak

A frame integrates over the pupil and over the exposure at once. Hold the point's depth and the patch is exactly the streak of its centres with one disc slid along it, to 1.8 × 10⁻⁵ of its own width. Let it recede over the same exposure and the disc's radius falls by 3.7 along the streak, and the patch departs from any single kernel by 16 pixels.

sensor · Aperture 00.2500.5000.7501-10-50510across the edge, in pixels of the picturehow much of the background reaches the sensorwhere a pinhole puts the edgethe pupilone depth per samplethe same edge, two ways of blurring it70% apart

One depth per sample is not enough

A depth buffer keeps a single distance at each sample, so a post-process blur can only ask how far away the thing at this pixel is. Across an occluding edge that answer is two depths and an occlusion, and the gather it produces differs from the pupil's own integral by 70 per cent of full scale over a band eleven pixels wide.

pipeline · Zbuffer 23468121632640.010.1110depth (m, log scale)streak length over the exposure (px, log scale)turning: still worldturning: moving with ittravelling: still worldtravelling: everything moving with the subject 0e+0 px4 m/s at 8 m · 33.3 ms

Turning and travelling blur different worlds

A subject 8 m away crosses the frame at 4 m/s, and the camera keeps it sharp over a thirtieth of a second. Turn to follow it and every still thing blurs by the same 13.5 px, whatever its depth. Travel beside it and the still world blurs as one over its depth — 49 px at 2 m, 1.5 px at 64 m — while everything moving with the subject is sharp at every depth.

sensor · Exposure

Named alongside it

The objects these essays reach for when they reach for this one.

ApertureCircle of confusionConvolutionEntrance pupilinstrument limitcentre of projectionDepth bufferDepth cuedepth of fieldExposureForeshorteningHyperfocal distance

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