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

Principal ray — where it appears

The ray from the centre of projection perpendicular to the picture plane, meeting it at the principal point. Rules about a drawn circle's axes hold exactly on it and decay away from it, so a construction's accuracy depends on where in the frame it is carried out.

Named by 4 essays across 2 fields — each of them below, with the objects they name alongside it.

5e-14°2.65°0.59°correct from 16 cm, at 160 mm wideaxle solid, minor axis dashed · 2.65° at the edge

The minor axis is not the axle

A wheel's perspective ellipse is supposed to have its short axis along the axle, and it does — on the principal ray, to 5e-14 degrees, and nowhere else. Off it the two part by 5.95 degrees on an ordinary frame while the drawn curves stay 0.98 px apart. A sphere obeys a rule of exactly the same shape and obeys it everywhere, which is why nobody caught the difference.

wrong · Wheel correct from 16 cm, at 160 mm wide35 px from the major axis

A cylinder has two different ends

The two end circles of a cylinder image as two different ellipses — 14.4° apart in the direction of their major axes and 0.918 against 0.839 in aspect — and the outline's straight sides touch neither of them where its major axis ends, missing by 28° round the ellipse and 35 pixels. Walking the cylinder away removes the difference between the ends and does not remove the offset of the touch, which settles at 7.3° off the principal ray and at nothing on it.

wrong · Wheel horizonthe middle of what is leftthe principal pointcorrect from 22 cm, at 160 mm widecrop 20% · centres 81 px apart

The centre of the picture is not the centre of the paper

A crop translates the image rectangle, so the picture's optical centre leaves the middle of the sheet and the focal length does not move — 81.3 pixels apart at a fifth of the picture, with the horizon at 62.5 per cent of the print. A reader who takes the paper's middle for the picture's stands 2.36 cm out of position, which is 4.9 degrees of the wrong direction.

construction · Crop the axis's vanishing pointthe photographtaper 1° per side · drawn at 0.72×4.2 px missed by the habit

A tapered part meets at its apex

The sides of a turned part that narrows by one degree meet 54 pixels from the vanishing point of its axis, at the image of its apex, and a quarter of a degree already moves them 15. Drawn toward the vanishing point instead, a two-degree part loses nine tenths of its own taper. Flare it the other way by 5.9 degrees and a correct photograph prints its sides parallel; flare it further and they spread with depth.

wrong · Wheel

Named alongside it

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

ForeshorteningImaged circlePicture planeVanishing pointConicContour generatorEllipseMinor axisTangencyTaught and unmeasuredCrop factorfield of view

All concepts