Definition: (1) Technique used to selectively obscure or hold back parts of an image while allowing other parts to show. * It may be physical (e.g. sheet of paper, image on film) as used in the darkroom to hold back foreground while burning in skies; chemical (e.g. certain colours or parts of image are bonded with a chemical to render them inactive) as occurs in the Sabattier effect or in photomechanical reproduction; digital (e.g. a selection is laid over another layer or turned into an alpha channel). (2) Array of values that are used by the computer in digital image processing to calculate digital filter effects such as unsharp masking, noise reduction or increase etc. * Array or matrix of coefficients used in convolution of a kernel in image processing. * Sometimes used synonymously with 'kernel'. (3) Screen-like structure drilled with holes to direct electrons from cathode ray tube to appropriate colour phosphor, used in construction of colour monitors.

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