On gradients and texture "gradients".

Texture gradients, systematic variations in image texture due to perspective projection of surface texture, provide information about the shape and disposition of surfaces relative to a viewer. Although this three-dimensional information is usually described in terms of mathematical relationships involving gradients of image properties (Cutting & Millard, 1984), the perceptual processes that derive this information need not necessarily measure or register these gradients explicitly (Stevens, 1981). It is difficult to conclude, therefore, that the perception of, say, flatness or curvature derives from processes that capitalize on these gradient relationships, per se. The difficulty becomes most apparent when attempting to formalize the perceptual computations and the spatial information that is made explicit.