Progressive scan

Progressive scanning (alternatively referred to as noninterlaced scanning) 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 computing.

Example of interline twitter

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.