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

The Mind's Eye | 247ing in color photography. Light from lightbulbs is orange; light fromftuorescents is olive; light from the sun is yellow; light from the sky isblue. Our brain somehow factors out the color of the illumination, just asit factors out the intensity of the illumination, and sees an object in itscorrect color in all those lights. Cameras don't. Unless they send outtheir mvn white light from a flash, they render an indoor scene with athick rusty cast, a shady scene as pasty blue, and so on. A knowledgeablephotographer can buy special film or screw a filter on the lens to compensate,and a good lab technician can correct the color when printingthe photograph, but an instant camera obviously cannot. So Land had apractical interest in how to remove the intensity and color of the illumination,a problem called color constancy.But he was also a self-taught, ingenious perception scientist, curiousabout how the brain solves the problem. He set up a color perception laband developed a clever theory of color constancy. His idea, called theRetinex theory, gave the perceiver several assumptions. One is thatearthly illumination is a rich mixture of wavelengths, (The exception thatproves the rule is the sodium vapor lamp, the energy-saving fixture foundin parking lots. It sends out a narrow range of wavelengths which ourperception system can't factor out; cars and faces are dyed a ghastly yellow.)The second assumption is that gradual changes in brightness andcolor across the visual field probably come from the way the scene is illuminated,whereas abrupt transitions probably come from the boundarywhere one object ends and another begins. To keep things simple, hetested people and his model on artificial worlds composed of 2-D rectangularpatches, which he called Mondrians, after the Dutch painter. In aMondrian lit from the side, a yeilow patch at one edge can reflect verydifferent light from the same yellow patch at the other. But people seechem both as yellow, and the Betinex model, which removes the lightinggradient from edge to edge, does too.The Retinex theory was a good start, hut it turned out to be too simple,One problem is the assumption that the world is a Mondrian, a bigflat plane. Go back to Adelson s drawings on page 242, which are zigzagMondrians. The Retinex model treats all sharp boundaries alike, interpretingEdge I in the left drawing like Edge 1 in the right drawing. But toyou, the left one looks like a border between stripes of different colors,and the right one looks like a single stripe that is folded and partly inshade. The difference comes from your interpretation of 3-D shape. Yourshape analyzer has bent the Mondrians into striped room dividers, but