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Concept
Edge enhancement
Résumé
Edge enhancement is an filter that enhances the edge contrast of an or video in an attempt to improve its acutance (apparent sharpness).
The filter works by identifying sharp edge boundaries in the image, such as the edge between a subject and a background of a contrasting color, and increasing the image contrast in the area immediately around the edge. This has the effect of creating subtle bright and dark highlights on either side of any edges in the image, called overshoot and undershoot, leading the edge to look more defined when viewed from a typical viewing distance.
The process is prevalent in the video field, appearing to some degree in the majority of TV broadcasts and DVDs. A modern television set's "sharpness" control is an example of edge enhancement. It is also widely used in computer printers especially for font or/and graphics to get a better printing quality. Most digital cameras also perform some edge enhancement, which in some cases cannot be adjusted.
Edge enhancement can be either an analog or a digital process. Analog edge enhancement may be used, for example, in all-analog video equipment such as modern CRT televisions.
Edge enhancement applied to an image can vary according to a number of properties; the most common algorithm is unsharp masking, which has the following parameters:
Amount. This controls the extent to which contrast in the edge detected area is enhanced.
Radius or aperture. This affects the size of the edges to be detected or enhanced, and the size of the area surrounding the edge that will be altered by the enhancement. A smaller radius will result in enhancement being applied only to sharper, finer edges, and the enhancement being confined to a smaller area around the edge.
Threshold. Where available, this adjusts the sensitivity of the edge detection mechanism. A lower threshold results in more subtle boundaries of colour being identified as edges. A threshold that is too low may result in some small parts of surface textures, film grain or noise being incorrectly identified as being an edge.
Source officielle
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