The form principle is the analysis of nature in terms of geometrical solids, which can be rendered according to laws of tonal contrast.
The photograph above shows a sphere in direct sunlight. It has a distinct set of tonal steps from light to shadow, known as modeling factors.
In direct sunlight, there’s a strong division of light and shade. The light side includes the light and dark halftones, the center light, and the highlight. The center light is the point at which the light rays strike the form most vertically. The highlight is the point where, in a shiny surface, we see a reflection of the light source.
Note that the center light and highlight are not at the same location. The Terminator The terminator is the area where the form transitions from light into shadow. It occurs where the light rays from the source are tangent to the edge of the form. If the source is soft and indirect, the transition from light to shadow at the terminator will be more gradual. The form shadow begins just beyond the terminator.
To test which areas are in light and which are in shadow, you can cast a shadow with a pencil on the object. The cast shadow will show up only on the lighted side, not on the shadow side.
Light and Form, Part 2
Light and Form, Part 3
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More about all this in my book: Color and Light: A Guide for the Realist Painter