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Progressive scan

From Academic Kids

Progressive scanning is a method for representing moving images on a display screen, where every pixel is represented in each frame. This is in contrast to the interlacing used in traditional television systems (progressive-scanning devices are sometimes referred to as non-interlaced).

Progressive scan is used in computer monitors. Usually video monitors use a faster scan to order the placement of pixels of the frame of video from left to right and from top to bottom at a given frame rate (e.g. every 1/60 of a second).


In more detail, the visual example below shows what a standard television displays. The standard televisions are "interlaced", meaning each frame is drawn in two "fields", each field consisting of half the lines in the image (odd lines first, then even lines).

First (odd) field:

Missing image
Progressive_scan_odd.png
Image:Progressive_scan_odd.png


Second (even) field:

Missing image
Progressive_scan_even.png
Image:Progressive_scan_even.png


Every 1/60th of a second, a new field is drawn. Because of a theory called persistence of vision, the human eye sees it as a smooth motion, not a series of half-images. However, there is an almost imperceptible flicker. This flicker will be most noticeable on a television displaying an image with contrasting horizontal lines or stripes.

Progressive scan, on the other hand, draws a full frame in the same time it takes interlaced mode to draw a single field, like so:

Missing image
Progressive_scan_full.png
Image:Progressive_scan_full.png

It is because progressive scan draws 60 full frames a second instead of 60 half-frames that the eye sees a much smoother motion. This motion therefore shows less flicker than interlaced scan, while also significantly reducing the visibility of the individual scan lines.

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See also: line doublerde:Progressive Scan

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