🍥 Wrap the Form

$ man cross-contour

One line is already drawn around the solid. Draw four more at other heights: press the dot on one edge, pull a single curve across the form, finish at the ring on the other edge. Press near the dot and you get snapped onto it; lift and carry on from where you stopped as often as your hand needs, or press “clear” to start that one over.

Scored against the circle the object really makes at that height (it is a real 3D form, drawn with a real camera): 60 how close your line runs to it, 40 how deeply you bowed the curve, and the whole thing scaled by how much of the wrap you actually crossed. A straight line across fails on every device — the curve is the answer.

These are called cross-contour lines: lines that ride over a surface instead of across the picture. They are how a drawing says “this is round” without a drop of shading.

Tip: every wrap on one form bends the same way as the given one — copy its direction before you worry about its depth. The end of the form leaning away from you is the one the curves belly toward, and slices nearer your eye open up rounder than the far ones.

Each wrap's reveal moves on by itself after a pause — but the pause is a floor, not a clock. A quick tap jumps ahead, and a press you keep down holds the reveal for as long as your finger does.