Many examples of illusory contours are available.
The examples below are variants of similarly constructed illusory contours. Illusory squares are perceived in the three examples even though no boundary separates the squares. Also, the white "square"appears even whiter that its ground, and the black square looks darker that its ground.
Notice the yellow in the figure and also the ground.
Although most illusory contour figures are constructed with right looking tools, they can also easily be made by ordinary means. One could draw freehand and the effects can still be striking. For example, you could put a thin piece of cardboard over paper and draw lines with a Sharpie and obtain a nice illusory circle.
And, the forms do not have to be regular.
Stinking illusory-contour effects can result when there are gradual changes, as in the following. This example relates to the Mach Band effect and to Glare effects.
Illusory contours can result from other means. For example, illusory lines are seen throughout the figure below.
The series below show that illusory lines result from angular change. Normally we are aware of it with right angles, but the gradual introduction of curves clarifies where the illusory effect is seen.
Artists have explored illusory contours for many years. The next image is a variant on one of a series if pieces Ellsworth Kelly did the the 1950s. The series is well worth studying.
Finally, the older logo for the Center for Maine Contemporary Art, when it was Maine Coast Artists, shows wonderful use of illusory contours.
