Interlacing is a method of displaying images on a raster-scanned display device, such as a cathode ray tube (CRT). The method causes less visible flickering than non-interlaced methods. The display alternates between drawing the even-numbered lines and the odd-numbered lines of each picture. In the PAL and NTSC standards, the lower (even) field is always drawn first.
A non-interlaced raster display draws every line of a picture, or frame, in sequence from top to bottom. This takes a finite length of time, during which the image on the CRT begins to decay, resulting in flicker. An interlaced display reduces this effect by drawing first all the even-numbered lines (forming the even field), leaving spaces between them for all the odd-numbered lines (forming the odd field) which it fills in afterwards to complete the frame. This results in the display being refreshed from top to bottom twice as frequently as in the non-interlaced case.
The odd field is the pattern created by tracing the odd-numbered scanning lines. It is sometimes called the top field (since the top line is numbered one, which is odd). The pattern created by tracing the even-numbered scanning lines is called the even field. Each field contains half the information content, i.e., pixels, of the complete video frame.
The eye suffers less fatigue (eye-strain) when viewing an interlaced display compared with a non-interlaced display that has the same frame rate. One reason is that the image flicker is less apparent, because the rate at which fields are refreshed is twice the rate at which frames would occur in a progressive display containing the same number of scanning lines and having the same frame rate.
In modern monitors and television sets, interlacing is being slowly superseded as the refresh rate of non-interlaced displays increases beyond the level at which flicker can be detected.
The early televisions in the 1920s were unable to refresh at high speeds. Instead they were limited to 30 or 25 frames per second. This could potentially cause flickering on the screen. Interlacing solved this. Each frame is split into an odd and even field. When played back on a television, the TV would project the odd field followed by the even field. When an image was moving, then one of the fields would be offset from the other. The image, however, would look smoother than if it had merely shown a single frame.
For example, the PAL television system is based on displaying 50 fields per second, and as two fields contain the full resolution, it corresponds to 25 full video frames per second. Interlaced video is often used as a tradeoff between bandwidth requirements and smooth reproduction of movement.
The major disadvantage of interlacing is the reduction in vertical display resolution. Consider a TV image made of alternating white and black horizontal lines, each line being only one TV line high. Instead of a fine black and white striped image, an interlaced display would show a black and white flashing image at the frame rate of the video. This is the most dramatic example of an interference effect called twitter, caused when an interlaced image attempts to display vertical information at too high a frequency.
Interlacing is done on some GIF and PNG images so that the viewer on a slow line can see what the image looks like before it is finished loading. GIF is interlaced by sending the lines in the order 0, 8, 16, ..., 4, 12, ..., 2, 6, 10, 14, ..., 1, 3, 5, 7, 9, .... PNG is interlaced by sending pixels in a square lattice before sending the rest.