Art is illusionistic by nature. In the case of a painting, a three-dimensional scene is depicted by a two-dimensional image. The artist uses illusions that we will accept in order to imply a realistic scene.
One of the first techniques employed was the use of outline to suggest form. From the earliest cave paintings to modern day cartoon, artists have intuitively discovered which information is crucial for recognition. Modern science has found that such outline drawings can actually be recognized by the brain faster than a photograph of the object.
Greco-Roman fresco painters and mosaic artists used dark tones to suggest shadows and form. The use of dark tones to suggest form was not obvious; until recently, shadows were not used in Chinese and Japanese art.
Before the discovery of perspective, Asian and medieval European painters used the technique of raising figures in the picture plane to suggest depth. They did not, however, decrease the size of distant objects consistently. This caused the scene to look unrealistic and flat. It wasn't until the discovery of perspective in the Renaissance that this problem was solved. For the first time, pictures suggested depth.
Color and contrast were also used by artists to suggest depth. Warm tones (reds and yellows) did not appear in the distance. This was because artists had observed the blue cast of distant objects. Scientists later discovered that this blue cast is due to the scattering of light through a greater distance of atmosphere.
Artists would depict a mountain in the distance by making it lighter and lower in contrast. Illuminated objects in the foreground are always pained with the highest contrast. Areas in the foreground that were in shadow were painted in a less defined way. This simulates the inability of the eye to fully adjust to dark areas in a light scene. For example, in a dim room, your eye accommodates to the dark and you can see well. In an area of bright illumination, your eye accomodates to the light and you can't see in the shadows as well. In a bright light, the eye is better able to focus.
Edges were painted softly to suggest gradual curvature, as on a cheek, and sharply to suggest a sudden curve, as along the bridge of a nose. Hard edges could be made to look even sharper by increasing the contrast.
In the 19th century the Impressionists began to look at color relationships and to recognize that an image isn't necessarily perceived as an integrated whole, as in classical depictions; what we really perceive is changing patterns of color and tone. Thus they began to explore the properties of color and the suggestion of form. It was found that the use of cool tones makes an object appear farther away than a similar object painted in warm tones. A neutral tone painted next to a color would assume the appearance of the complementary color.
With the advent of abstract art, many of these illusionistic techniques were explored more systematically, and were used to evoke the perception of form without suggesting a literal meaning. Op Art in the 1960's was dedicated to the exploration of discernible illusions.