Interlacing is another haunting feature of analogue video. It exists for good reason, yet it has several disadvantages. Before we go into the what and the why, let’s have a look at a picture with interlacing artifacts so we can examine some of their characteristics:
This enlarged picture that is part of the landscape seems to be unnatural; it looks like alternate lines of pixels have moved towards the right while their counterparts stand still. Don’t lose trust in your vision: that is exactly what it is! When a video is interlaced, it is split into fields; a field is simply a set of alternate lines of pixels. Consequently, we have odd- and even- numbered fields in an interlaced frame. The effect you see is because the odd lines are a field from a different progressive (non-interlaced) frame than the even ones (see below). Why would anyone do this to our lovely picture I hear you ask?
Well, mainly for two reasons and both of them go back to when TV was introduced...