Movie S1.

The perception of surface opacity and translucency depends on apparent 3D shape. Two 3D shapes texture mapped with identical luminance gradients. The luminance gradients were created by rendering the snake shape with translucent material properties (depicted in the first half of the movie). The alternative shape interpretation (i.e., the ribbon depicted in the second half of the movie) was designed to suggest that the same luminance gradients are due to shading across an opaque surface. Specifically, the ribbon was engineered to have a distribution of surface normals that covaries with luminance (Fig. 2). Note that the snake shape appears more translucent and the ribbon shape appears more opaque.