Steven Pinker -- How the Mind Works - Hampshire High Italian ...

248 | HOW THE MIND WORKSthe Retinex model sees them as the same old checkerboard. Obviously, itis missing something.That something is the effect of slant on shading, the third taw that turnsa scene into an image. A surface facing a light source head-on reflectsback a lot of light, because the light smacks into the surface andrebounds right back, A surface angled almost parallel to the sourcereflects much less, because most of the light grazes off it and continueson its way. If you are positioned near the light source, your eye picks upmore light when the surface faces you than when it faces almost sideways.You may be able to see the difference by shining a flashlight at apiece of gray cardboard and tilting the cardboard.How might our shading analyzer run the law backwards and figureout how a surface is slanted based on how much light it reflects? Thebenefits go beyond estimating the slant of a panel. Many objects, likecubes and gems, are composed of slanted faces, so recovering the slantsis a way to ascertain their shape. In fact, any shape can be thought of asa carving made up of millions of tiny facets. Even when the surface issmoothly curved so the "facets" shrink to points, the shading law appliesto the light coming off each point. If the law could be run backwards, ourshading analyzer could apprehend the shape of a surface by registeringthe slant of the tangent plane resting on each point,Unfortunately, a given amount of light reflecting off a patch couldhave come from a dark surface angled toward the light or from a lightsurface angled away. So there is no foolproof way to recover a surface'sangle from the light it reflects without making additional assumptions.A first assumption is that surface lightness is uniform: the world ismade of plaster. When surfaces are unevenly pigmented, the assumptionis violated, and our shading analyzer should be fooled. It is. Paintings andphotographs are the most obvious example. A less obvious one is countershadingin animal camouflage. The hides of many animals lightenfrom hack to belly in a gradient that cancels out the effects of light ontheir 3-D shapes. This flattens the animal, making it harder to detect bythe assumption-making, shape-from-shading analyzer in the brain of apredator. Makeup is another example. When applied in sub-Tammy FayeBakker amounts, pigment on the skin can fool the beholder into seeing