Global Shape Mediates the Perception of the Parts of a Picture,Not Vice Versa: Comments on “Ellipses on the Surface of a Picture” by Hammad et al (2008)

Hammad et al (2008, Perception 37 504–510) argue that the apparent shape of a drawn ellipse in the context of a drawn cylinder is mediated by the presence of local ‘depth cues’. I attempt to show that such features are incapable of constraining the shape of the whole pattern—and thus of its parts—in the manner claimed by the authors, and offer alternative explanations for their experimental outcome.