🌑 Shade a Sphere

$ man sphere-shade

Two spheres, light given. Draw the line across the sphere that runs at a right angle (90°) to the light arrow — that is the terminator, where the light stops. The drill only reads the direction of your line, so a wobbly one costs nothing, and lifting does not end it: press again near where you stopped and the two halves count as one line. Then drag the three dots: c the darkest band (core shadow), b the light bouncing back off the floor, o where the ball touches the ground (the contact shadow). Hit done. Keys 1–4 pick a mark (1 is the line), arrows nudge it, enter is done. Press done before you have drawn anything and it asks first, so a stray press never costs you a sphere.

Scored on placement only — this is shading anatomy, not rendering. The grace zone around each answer is set for the hardware you are on (the HUD says which: pen, mouse or trackpad, finger), because the same hand movement means different things on different devices and your score is only ever compared with your own. Press near a dot rather than on it and it still picks it up. On reveal your ink sits next to the lilac answer with a dashed line from each mark to where it belonged, and a flat-value plan sphere shows the whole anatomy named. The ticks read ✓ for 75+, ~ for 45+, ✗ below, followed by how far off you were and which way to turn (the legend under them spells the ↻ ↺ out). The line above the sheet names whichever of the four marks was weakest.

The light is a real 3D vector: the sun on the sheet gives its direction, the little plan view top-right gives its depth — coming toward you, or from behind the sphere. That depth is what fattens the terminator ellipse, swings the core off centre and stretches the cast shadow.

Studio tip: the terminator is geometry, not mood — it is the great circle 90° from the light, so its axis never depends on the tilt. Find it first and everything else hangs off it.