Movie S2.

Identical luminance gradients can appear as either an opaque or translucent surface depending on perceived 3D shape. The luminance gradients were created to covary perfectly with the surface normals of the bump shape (depicted in the first half of the movie); hence, the luminance gradients are consistent with shading across an opaque surface when they appear as the bump shape. The alternative shape interpretation (i.e., the torus depicted in the second half of the movie) was designed to suggest that the same luminance gradients are due to a translucent object. Specifically, the torus has much less covariation of luminance with surface normals than the bump (Fig. 2). Note that the torus shape appears more translucent and the bump shape appears more opaque.