Progressive scan
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Progressive or noninterlaced scanning is a method for displaying, storing or transmitting moving images in which all the lines of each frame are drawn in sequence. This is in contrast to the interlacing used in traditional television systems where only the odd lines, then the even lines of each frame (each image now called a field) are drawn alternately.
The system was originally known as "sequential scanning" when it was used in the Baird 240 line television transmissions from Alexandra Palace, England in 1936. It was also used in Baird's experimental transmissions using 30 lines in the 1920s.
This rough animation compares progressive scan (480p) with interlace scan (480i), also demonstrating the interline twitter effect associated with interlace. The interlaced images use half the bandwidth of the progressive ones. The left-center image precisely duplicates the pixels of the progressive one, but interlace causes details to twitter. Real interlaced video blurs such details to prevent twitter, but as seen on the right-center image, such softening (or anti-aliasing) comes at the cost of image clarity. A line doubler could not restore the previously interlaced image on the right to the full quality of the progressive image on the left.
(Note - Because the refresh rate has been greatly slowed down, and the resolution is much lower than that of typical 480-line interlaced video, the flicker in the simulated interlaced portions and also the visibility of the black lines in this image are exaggerated. Also, the images above are based on what it would look like on a monitor that does not support interlaced scan, such as a PC monitor, or an LCD or plasma-based television set, with the interlaced images displayed using the same 480p mode as the progressive images.)
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Usage in storing or transmitting
Progressive scan is used for scanning and storing film-based material on DVDs, for example, as 480p24 or 576p25 formats.
Usage in TVs, video projectors and monitors
Progressive scan (also known as: P-Scan) is used for most cathode ray tube (CRT) computer monitors, all LCD computer monitors, and most HDTVs as the display resolutions are progressive by nature. (Other CRT-type displays, such as SDTVs, typically display interlaced video only)
Some TVs, and most video projectors have one or more progressive scan inputs. Before HDTV became common, some high end displays supported 480p (480 vertical lines of resolution with progressive scan.) This allowed these displays to be used with devices which output progressive scan like progressive scan DVD players and certain video game consoles. HDTVs support the progressively scanned resolutions of 480p and 720p. 1080p displays are available, but are usually more expensive than the comparable lower resolution HDTV models. Computer monitors can use even greater display resolutions.
Advantages of progressive scan
- Higher vertical resolution than interlaced video with the same frame rate.
- The perceived vertical resolution of displayed video is traditionally adjusted using a Kell factor coefficient. This coefficient has no fixed value and depends on display device. Its value for interlaced video is usually lower than for progressive video, when the same display device is used. When interlaced video is compared to progressive video with the same number of scan lines, interlaced video delivers lower perceived vertical resolution at a lower frame rate.
- Absence of visual artifacts associated with interlaced video of the same line rate, such as interline twitter.
- No necessity in intentional blurring (sometimes referred to as anti-aliasing) of video to reduce interline twitter and eye strain.
- In the case of most media such as DVD movies and video games, the video is blurred during the authoring process itself to mask flicker artifacts when used on interlace displays. As a consequence, recovering the sharpness of the original video is impossible when the video is viewed progressively. An excellent, but rarely employed countermeasure to this is when display hardware and video games come equipped with options to blur the video at will, or to keep it at its original sharpness. This allows the viewer to achieve the desired image sharpness with both interlaced and progressive displays. An example of a video game with such a feature is Super Smash Bros. Melee, where a "Deflicker" option exists. Ideally it would be turned on when played on an interlaced display to reduce interline twitter, and off when played on a progressive display for maximum image clarity.
- Offers much better results for scaling to higher resolutions than equivalent interlaced video, such as upconverting 480p to display on a 1080p HDTV.
- Scaling works well with full frames, therefore interlaced video must be deinterlaced before it is scaled. Deinterlacing can result in severe "combing" artifacts.
- Frames have no interlace artifacts and can be used as still photos.
A disadvantage of progressive scan is that it requires higher bandwidth than interlaced video that has the same frame size and vertical refresh rate. For explanations of why interlacing was originally used, see Interlace. For an in-depth explanation of the fundamentals and advantages/disadvantages of converting interlaced video to a progressive format, see Deinterlacing.
See also
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