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All examples are black and white images, partitioned by lines such that crossing a line switches the background color and the foreground color. (Sometimes it is not clear which is "background" and which is "foreground".) In the space between two dividing lines, there is a black and white scene; the outlines of the shapes are curves dividing black from white. Images sorted left are such that each outline-curve present in a scene that comes in contact non-tangentially with a dividing line continues across the dividing line, across which the black and white sides of it switch.
Examples (especially right) usually have ambiguity to some degree; depending on how a person reads the images, dividing lines may be confused for curves within a scene. | 
