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Cylinder Ends

the far end is rounder — but how much?

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How much rounder the far end is

Each cylinder is drawn for you in wireframe except for one thing: the far end is still a flat line. Drag the dot out along the barrel until that end is as round as it really is, then lock it in. Four cylinders, then a score. It never asks which way to go — the first screen says so — only how much, which is the only part that takes an eye. The true end is drawn over yours after each cylinder, and the round closes by naming which way you leaned.

The far end is always rounder — never flatter, never equal — because a circle set further back is turned more nearly face-on to the eye. It is also smaller, and that half beginners get right; the error survives because the two facts pull opposite ways. A shape that shrinks feels as though it should narrow too, so the far end gets drawn flat or copied off the near one. Copying it across is a miss of about 0.25 on a typical cylinder and scores 11 at best; leaving it flat scores 0 everywhere.

Roundness, and where the score runs out

Roundness is the far end's height over its width, and the number is 100 · (1 − |yours − true| / zero): exact is 100, fading to 0 as you drift, with the reveal's two faint dotted ends marking where it runs out. The zero-point is the wider of two tolerances: a ratio — 0.18 on a pen, 0.36 on a trackpad, 0.27 on a finger — which is scale-free, so a phone and a desktop ask the identical question, and a pixel floor, because a ratio alone hands a small sheet a stricter drill: 0.27 of roundness is 31px of drag on a 780px sheet and 15px on a 360px one. That subtraction is the entire judgement — no model anywhere in it, and the same drag always scores the same.

More form & perspective drills: Cast Shadow · Horizon Read · Wrap the Form · the whole catalogue