The tone used here relies on an observed directional light source, which is then exaggerated to create the effect. The shadow cast makes us believe the sphere has form. Without it the drawing would look flat.
This cube has been constructed using three defined tonal variations - white, grey and black - to give the illusion of form. It is a constructed illusion, of course, and has not come about as a result of observed tonal changes.
The lighting appears to be from the front in this example of constructed tonal form. The light cast at this angle is very intense, gradually fading to complete darkness towards the sides of the cone. Lighting was used by early Renaissance artists such as Giotto.
The cylinder is like the sphere in observation, tone and how light plays across the surface of the form. But the tone we have used to define the form is an expressive gestural tone. It has less form than the previous tonal gradation and is more emotive and responsive to the observation.
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