10.5.1 Selection of the Membership FunctionThe assignment of the membership function may be performed by severalways. Membership based on visual model: The membership function may be assigned in accordance with the human visual perceptual model. We may model the variation of the membership values of the pixels in a linear fashion as the pixel gray value changes from 0 to L - 1 (for an L level image). The human visual response to illumination, however,is not linear but closely exponential. When a shade is too dark or too light (or bright), the human eye fails to notice any slight change around it. Thus the membership values of the gray levels toward the extreme low or extreme high end of the entire scale should not change appreciably. However, the response of the human eye to different degrees of