block portrait

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A degraded image, usually of a face, divided into large rectangles or blocks, the lightness of each block being set to the average of the image in that region, so that a block overlaying an area of dark hair becomes uniformly black, a block overlaying an area of light skin becomes uniformly light, and a block straddling light skin and a dark eyebrow becomes uniformly grey (see illustration). Coarse block portraits, made up of approximately 16 × 16 rectangles using eight or sixteen lightness values or shades of grey, have information of high spatial frequency filtered out and are impossible to recognize when viewed close up in clear detail, but they are easy to recognize when viewed from a distance or blurred by half-closing the eyes or overlaying the portrait with tissue paper or frosted glass, the resulting loss of information paradoxically increasing the recognizability of the image. The first block portrait was constructed by the US biomedical engineer Leon D(avid) Harmon (1922–84) and published in a technical report in 1971, followed by an article in Scientific American magazine in 1973. See also Abraham Lincoln effect.

Block portrait

Subjects: Psychology.

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