Oppel-Kundt illusion

What can you see? The Oppel-Kundt illusion, named after the German physicists Oppel and Kundt, and first described by them in 1854 and 1863 respectively, is one of the oldest known geometric visual illusions. Nevertheless it is not yet understood. Horizontal lines of equal length are compared (AB = BC = CD). AB and CD are divided by vertical lines (or points). Subjectively, the divided lines AB and CD are perceived to be longer than the line BC. The number of vertical lines probably influences the strength of the perceived effect.

There are many variants of this illusion, with or without a baseline, with the baseline at the bottom or in the middle, with symmetrical line fields left and right, different thickness of lines, and more. Oppel originally showed the effect with a series of dots.

What can you do? Almost all parameters can be changed in the program. So you can reproduce the images above and you can examine the influence of single variables or their combination. What influence has the number of vertical lines or the spacing between them or their height? What is the effect of their thickness (only changeable in the code!)? The baseline can be faded out (0) or faded in (1) or drawn in the middle (2).
An Oppel-Kundt measuring lab: If you want to measure your own or your visitors‘ estimated values, you can use a slightly modified program (without the symmetry case). All variables can be defined as usual. The actual measurement process is started by pressing the [S]-key. You can then move the right line with the mouse until you think you have found the right distance. If you then press the [space bar], the line is fixed and you get the deviation from the correct distance in pixels as well as in %.References:
Pia, L. et al. (2012). The Oppel-Kundt illusion is effective in modulatng horizontal space representation in humans. Perceptual & Motor Skills: Perception 115, 3, 729-742. Available as download.
Mikellidou, K. (2012). Illusions of filled extent: psychophysics and neuroimaging methods. PhD Thesis. Available as download.