|The highest contrast of values, and if working in color, the brightest colors, also tend to advance toward the viewer, while midtones and duller colors tend to recede. Purposefully muting the lights and darks will cause that part of the subject to appear farther away, and purposefully heightening the black-white value contrast and brightening the colors of the near portions will advance the closer part of the subject.
The degree of development also creates the illusion of depth. Highly developed areas advance, whereas silhouetted shapes with perhaps hazy edges, recede.
This is the second week Heather mentioned Mach bands, an optical illusion causing forward edges to appear lighter against darker values behind. This optical illusion occurs even though the local value does not change — our eyes do it for us. If the artist will exaggerate Mach bands, that too will help give the illusion of depth.
The examples in this post exaggerate depth.