Light on form

A single light source lands on a form and divides it into zones. The plane facing the light directly is the light; within it a small spot may be a highlight. As the surface turns away it passes through halftone and reaches the core shadow, the darkest part of the form itself. Beyond the core, light bouncing back from the ground or a nearby wall gives reflected light, which is never as light as the lit side. Where the form meets the ground it blocks the light and casts a shadow, which is darkest right at the contact point.

On a cube the zones are flat planes: top lightest (it faces the sky), one side halftone, one side shadow, cast shadow on the ground. On a sphere they are bands that curve around it. A building is cubes. A tree is a cluster of spheres. Once you see the zones you can shade anything.

Decide where the light comes from before the first mark, and write a small arrow in the corner of the page. Every shadow and every cast shadow in the drawing obeys that arrow. Two shadows that disagree about where the sun is will collapse the picture.