overlacing

Overlacing is a technique used in video production and display technologies to improve the perceived smoothness of motion without doubling the frame rate. In a traditional interlaced signal, each complete frame of video is divided into two fields. One field contains the odd-numbered scan lines of the image, and the other field contains the even-numbered scan lines. These two fields are then displayed sequentially, one after the other, in rapid succession.

When the display refreshes, it shows the odd lines, followed by the even lines. This process happens at a rate that is effectively twice the rate of a full frame. For example, a standard interlaced television signal might display 60 fields per second, but this corresponds to only 30 complete frames per second. The human eye, due to persistence of vision, tends to perceive these rapidly alternating fields as a single, more fluid image, especially during movement.

Interlacing was originally developed to conserve bandwidth in analog television broadcasting. While it was effective in improving motion rendition with limited broadcast capacity, it can lead to artifacts such as "combing" or "jagged" edges on fast-moving objects when displayed on modern progressive scan displays, which show all scan lines of a frame at once. This is because the two fields capture slightly different moments in time. As a result, many modern video formats and displays utilize progressive scanning, where each frame is transmitted and displayed in its entirety.