What Interlace Isn't
Be warned - this is a mini-rant.A common mistake is to assume that (say) a 50Hz interlaced video clip can be the same thing as a 25Hz progressively-scanned video clip of the same subject, but with the lines reordered a bit. With notable exceptions (films presented on interlaced television, for example), this is not the case! Here's the biggest difference: when you're shooting with a TV camera at 50 fields* per second, that means that successive fields are recorded every 1/50th of a second. It doesn't mean that successive frames are recorded every 1/25th of a second and then sliced up to form two independent fields.
That's the fundamental reason why deinterlacing is hard, because if you simply take two successive fields and combine lines from them alternately to create a full image (a technique often called "weaving"), anything moving within the picture ends up with "comb" artefacts all over it, as seen in this picture.
Like I said, interlace is plain nasty. :-)
* a field is the name given to the image produced by a single vertical sweep of an interlaced camera. Sometimes people claim that two fields can be combined to make a single frame of a moving image, but that's not generally true, for the reasons I just explained...
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