Observing the way light interacts with an object or group of objects, noticing shadows and the resulting variations in value, is light logic. Think about how light falls on spheres, cylinders, cones, and organic volumes creating a gradual change from light to dark over their surfaces. Alternatively, cubes, pyramids, and other angular forms change abruptly from light to dark at the intersections of their planes. The intensity of shadows and how clean their edges are is directly related to the strength of the light and its proximity to an object.
Light and shadow on a volume can be reduced to six categories, of course each subject requires more specific investigation to be accurate. The six categories are as follows:
1. Highlight (most often the brightest/lightest
part of your object; where the light hits and bounces directly back)2. Light (light is washing over the object, illuminating)
3. Shadow/Half Tone (true color of the object; the object is beginning to turn away from the light and coming into shadow)
4. Core Shadow (typically the darkest part of your object; completely obscured by the light; it's shape contours the volume)
5. Reflected Light (light is also interacting with the surface your object is resting on or near, so reflected light is the light that is bouncing off the surface and reflected on your object; generally happens on the underside of the object; also contours the volume)
6. Cast Shadow (an object is obscuring the light from the surface, or perhaps another object; cast shadows resemble the shape of the object casting the shadow; cast shadows vary in intensity and length)