Progressive video

Progressive scanning (alternatively referred to as noninterlaced scanning, and not to be confused with progressive download) is a way of displaying, storing, or transmitting moving images in which all the lines of each frame are drawn in sequence. This is in contrast to interlaced video used in traditional analog television systems where only the odd lines, then the even lines of each frame (each image called a video field) are drawn alternately, so that only half the number of actual image frames are used to produce video. The system was originally known as "sequential scanning" when it was used in the Baird 240 line television transmissions from Alexandra Palace, United Kingdom in 1936. It was also used in Baird's experimental transmissions using 30 lines in the 1920s. Progressive scanning is universally used in computer screens in the 2000s.

This rough animation compares progressive scan with interlace scan, also demonstrating the interline twitter effect associated with interlacing. On the left there are two progressive scan images. In the middle there are two interlaced images and on the right there are two images with line doublers. The original resolutions are above and the ones with spatial anti-aliasing are below. The interlaced images use half the bandwidth of the progressive ones. The images in the center column precisely duplicate the pixels of the ones on the left, but interlacing causes details to twitter. Real interlaced video blurs such details to prevent twittering, but as seen in the pictures of the lower row, such softening (or anti-aliasing) comes at the cost of image clarity. A line doubler shown in the bottom center picture cannot restore the previously interlaced image to the full quality of the progressive image shown in the top left one.

Note: Because the refresh rate has been slowed down by a factor of three, and the resolution is less than half a resolution of a typical interlaced video, the flicker in the simulated interlaced portions and also the visibility of the black lines in these examples 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 mode as the progressive images.

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Wikipedia