Summary

Leonardo Da Vinci called the depiction of volume “the supreme mark of skill”. One of the reasons for this is that few people ever see the very subtle flow of tone that occurs within light areas and dark areas. However, mastering the ability to capture the nuanced tonal changes across a volumetric form gives a huge amount of importance to the object as well as authority to the piece as a whole.

Tone changes because reflected light loses its intensity as distance increases. It is so specific that under uniform lighting conditions the speed of tonal change exactly matches the speed with which the plane slants away from the eye. Visualize a sphere by matching the speed of tonal change to the angle of the plane moving away from you in space. The faster it slants away the faster the tone must change. Since on a sphere the tone never stops changing you need to be aware, when rendering, of where you are on the plane and be continually adjusting for what the angle is doing.