Deinterlacing is the process taking a stream of interlaced frames and converting it to a stream of progressive frames. Ideally, each field becomes its own frame of video, so an interlaced NTSC clip at 29.97 frames per second stream becomes a 59.94 frame per second progressive. Since each field is only half the scanlines of a full frame, interpolation must be used to form the missing scanlines. There are various methods of doing the interpolation, ranging from simply doubling scanlines to motion-adaptive methods.
When you make a movie with your digital camcorder, it does the following: records 50 pictures per second, intermixing every 2 consecutive pictures (with half the height) into 1 frame. 2 pictures (fields) are mixed into 1 frame. This mixing is called interlacing.
In many computing applications, deinterlacing is part of a recompression process, for example, converting a DVD of music videos in interlaced MPEG2 format to a progressive MPEG4 stream. For these applications, an output framerate of 59.94fps or 50fps is too high. Instead, most deinterlacer filters convert every interlaced frame to one single progressive frame, effectively halving the framerate of the content.
Deinterlacing methods are:
1) Blend, average, combine fields. Both fields are overlaid together. This gives you good results, when there’s no movement, but results in unnatural low quality movements. There seem to be a ghosty unsharpness when something moves.
2) Discard. You discard every second line (the movie is half the height then) and then resize the picture during play. That is the same as skipping Field2, Field4, Field6… You could call this “Even Fields Only” or “Odd Fields Only”. You lose half of the resolution and the movie becomes kind of stuttering (as mentioned above). That means, that it doesn’t play as fluidly as it could be.
3) Bob (Progressive Scan). Displaying every field (so you don’t lose any information), one after the other (= without interlacing) but with 50 fps. Thus each interlaced frame is split into 2 SMALL frames (= the 2 former fields) half the height.
4) The best method: Bob (Progressive scan) + Intelligent Adaptive Hybrid.
Analyzing the two fields and deinterlace only parts which need to. It gives you a 50fps movie instead of a 25fps movie, thus leaving you with perfect fluidity of motions.
Check out these screenshots to see the difference between these methods:
If the deinterlacing is done at the time of broadcasting, the quality of the deinterlace can vary. It should be managed by professionals, who have powerful processors and tools. On the other hand, it needs to be done in real time, so the effort that the deinterlacer can put in is limited by time.