Illusions of bent line

A recent paper offered an explanation of the Poggendorf illusion, assuming that it results from a failure of the visual system to make a perfect resolution of acute angles. Variations of the illusion were drawn eliminating all acute angles, and the illusion was shown to remain in full strength. A variation of the figure preserving the acute angle was drawn, and the illusion was found to reverse in direction. Several theoretical interpretations are considered but none accepted, and it is concluded that the illusion remains unexplained.