Interlace, how "fields" are retained through editing
 

Interlace, let's talk about it.......

I know what interlace is and why it exists but I'm confused about how it operates between different hardware and software.

I'm working on my never ending home movies restoration job. All my VHS tapes are caped in HUFFYUV 720x480i. So the files look more or less identical to the original VHS except the rescale. Now when I play the HUFFY files back in, say, VLC and choose the de-interlace option, it does a fairly good job on playback. Also AVISynth does a good job of de-interlaceing.

But... if I want to leave the video interlaced, how does software and hardware understand how to de-interlace video? I derailed my logic when I realized that 720 is a higher resolution than the original VHS frames, therefore I have effectively doubled up on some pixels. How in the world does my playback system know which pixels belong to which fields? If I place a black mat over my frame or resize the footage and encode is to MPEG2 and burn it to DVD, do I get some weird multiple interlace? How does anything in the encode/playback chain keep track of "fields" when footage has been re-scaled?

Keep in mind that interlacing applies only to the vertical resolution (the 480 part). Every other horizontal line in your video is part of a field. Editing software is made aware of interlacing by you telling it or if the file itself is flagged as interlaced.

De-interlacing is far from a perfect science. Different methods have been developed through the years and some work better then others depending on content. Thats why programs like VLC give you multiple options to deinterlace your video.

There's also variations in hardware vs software algorithms, and then the past couple of years have seen some impressive software processing from the command-line video tools like Avisynth and mencoder. Yadif, YadifMod, and the NNEDI family, for example. There's a few more, too. Speed and quality varies highly. There's even been some decent repair tools created, that can fix deinterlacing damage.