Progressive scan
From Freepedia
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:
Image:Progressive scan odd.png
Second (even) field:
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:
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.
The standard of the future
Significantly, progressive scan gives the perception of much higher vertical resolution than would be the case with an interlaced image of the same resolution. This explains, for example, why HDTV standards such as 1080i (1920x1080, interlaced) actually deliver a poorer quality image than 720p (1280x720, progressive scan) in most cases. This is because the perceived vertical resolution of an interlaced image is usually equivalent to multiplying the active lines by about 0.6; so 1080 × 0.6 = 648.
Interlacing was originally conceived in the early days of television as a technical compromise between vertical line resolution and reducing "flicker effect", while still managing to fit all the picture information within a reasonable bandwidth. 25Hz(for PAL - 30Hz in the case of NTSC) displays would give unacceptable flicker, whereas 50 (or 60) full-resolution frames per second would have needed too much bandwidth for broadcast purposes. The compromise was to use two half-resolution fields, at a higher refresh rate. However, since the two fields are temporally distinct (they actually represent two frames taken 1/50th or 1/60th of a second apart) the perceived result is seldom anything like the full vertical resolution of the display.
Progressive scan has long been the standard for film. To combat the flicker that would arise from seeing only 24 frames per second (24Hz), film frames are actually projected twice each, i.e. 24fps × 2 = 48Hz, and shown in dark cinemas which further reduces the perception of flicker. Progressive scan has also been the standard for computer monitors and LCD or Plasma displays all along.
Progressive scan also has significant advantages over interlacing when it comes to digital video and television formats. This is because progressive scan images are able to be compressed much more efficiently, and with far fewer artifacts, than interlaced images of the same resolution. This makes progressive scan preferable for applications such as digital HDTV broadcasts, cable television, HD media and compressed digital video acquisition formats (e.g. video cameras that record HD onto Mini-DV cassettes).
