There are many images demonstrating illusory contours – our perception of a contour or edge where in fact there is only an area of uniform colour. These illusory edges are usually induced by real edges that are aligned but some distance apart, or a number of lines all ending along an imaginary line, which our brain then interprets as the edge of a shape obscuring the lines.
With this interactive applet, we can explore this effect and see how far we can push it. With the right alignments, we can also create illusory colours: When the illusory contours define a square, the background colour appears to be different inside the square from outside. (This effect seems to be strongest in the 'colour outside' mode: click on the applet and type 'o1'.)