5.1 Fields, frames and interlacing

The main, fundamental difference between computer graphics systems and video systems is the way they store images.

Computer graphics systems store frames, where a frame is all the data required for a single image. However, video systems store fields, where a field is image data sampled at a (single) point in time; this is usually half the data in a frame. The field rate of a video signal is the rate of field sampling, and determines the temporal difference of the fields.

Fields are usually even interlaced, such that the half of the frame they store is initially the even scanlines of the frame , followed by the odd scanlines, and so on⁹.

Video systems do not support any real notion of frames. The idea that a video frame is two adjacent fields is wrong, and is a result of trying to apply computer graphic frame concepts to video, where they don't apply. However, it is occasionally useful to think of a frame as two adjacent fields, ignoring the temporal difference between them.

As a result, there is no way to recover the data for a full frame at a given time instant from a video signal. Simply half the visual data is missing from each time instant recorded.

Figure 10: The 10 frames and fields of the moving ball, as taken by a graphics system and video system, respectively.

[Graphics frames]

[Video fields]

Figure 11: Examples of how the video recording will not look like.

[Frames, at half the frame rate]

[Fields, each pair of fields making up a frame]

Reference [4] has an excellent illustration of the difference between fields and frames. The scenario is a moving ball being recorded by a film camera (ie. as frames), shown in Figure 10(a), and by a video camera (ie. as fields), shown in Figure 10(b). Notice that in Figure 10(b) each field has a different spatial location, that is, each field is of a different instant in time. By contrast, Figure 11 shows two ways that the video camera will not record the moving ball. Figure 11(a) shows full frames at half the field rate, this is incorrect because video cameras record fields at their field rate. Figure 11(b) shows fields where each pair of fields together make up a single frame, this is incorrect because the field pairs are of the same instant in time, whereas the video camera will capture each frame at a different instant in time, as in Figure 10(b).