Interlace

Interlacing is a method of displaying images on a raster-scanned display, such as a CRT, that results in less visible flickering than non-interlaced methods.

A non-interlaced raster display draws every line of a picture, or frame, in sequence from top to bottom. This takes a finite length of time, during which the image on the CRT begins to decay, resulting in flicker. An interlaced display reduces this effect by drawing first all the even-numbered lines (forming the so-called even field), leaving spaces between them for all the odd-numbered lines (forming the odd field) which it fills in afterwards to complete the frame. This results in the display being refreshed from top to bottom twice as frequently as in the non-interlaced case.

The eye suffers less fatigue (eye-strain) when viewing an interlaced display compared with a non-interlaced display that has the same frame rate.

Interlacing is used by all the analogue TV broadcast systems in current use (mainly NTSC, PAL and SECAM).

In modern monitors and television sets, interlacing is being slowly superseded as the refresh rate of non-interlaced displays increases beyond the level at which flicker can be detected.

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External links:

Video Interlacing / Deinterlacing Websites

• 100FPS.COM (http://www.100fps.com)

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Interlacing is done on some GIF and PNG images so that the viewer on a slow line can see what the image looks like before it is finished loading. GIF is interlaced by sending the lines in the order 0, 8, 16, ..., 4, 12, ..., 2, 6, 10, 14, ..., 1, 3, 5, 7, 9, .... PNG is interlaced by sending pixels in a square lattice before sending the rest.