In the computational theory of vision pioneered in the late 1970s by the English psychologist David Courtenay Marr (1945–80), the first stage in the perceptual process, similar to a rough line drawing, its function being to represent the variations in luminance present in the image after eliminating extraneous visual noise (2) such as glare that does not convey useful information about the image. The sketch is constructed by first blurring or smoothing the image with filters (2) of different spatial frequencies, capable of detecting information at different levels of resolution, and then locating regions of maximum change in luminance by finding zero-crossings, the result of this process being a two-dimensional black-and-white representation (2) of contours and shapes present in the image. The raw primal sketch incorporates edges and lines (formed wherever a zero-crossing emerges under a blur filter or pairs of zero-crossings emerge in the same place under two adjacent blur filters), bars (formed wherever a line detected under a filter that transmits only low-frequency information is flanked by two edges detected under a filter that transmits only high-frequency information), terminations (ends of bars), and blobs (areas enclosed in single edges). The full primal sketch joins together lines and edges that belong with one another, groups other items that appear to be associated, and locates the apparent boundaries of objects. Compare 3-D model description, 2½-D sketch.