From: The World In Your Head by Steven Lehar

Fig. 3.3 (A) The Kanizsa triangle. (B) Tse’s volumetric worm. (C) Idesawa’s spiky sphere. (D) Tse’s “sea monster.” These illusions reveal the constructive or generative function of perception, for the percept encodes more explicit spatial information than the retinal stimulus on which the percept is based.