Shape information from shading: a theory about human perception

I present evidence that people assume a simple, linear reflectance function when interpreting shading information. Using this reflectance function I derive a closed-form solution to the problem of extracting shape information from image shading. The solution does not employ an assumption about surface smoothness and so is directly applicable to complex natural surfaces such as hair or cloth. A simple biological mechanism is proposed to implement this recovery of shape. It is shown that this simple mechanism can also extract significant shape information from line drawings.