Thursday, December 28, 2006

Field Dominance (or Why Interlaced Sucks)

In a previous post, I said HD 1080i sucks because interlaced, in general, sucks. I went on to explain a few things there. Recently I edited a home movie from old footage of my niece (for her birthday), and tackled a nasty interlaced issue called "Field Dominance". Here's one more reason why interlaced video should be abandoned...

Let's skip the definition on interlaced video (You can always pick it up from Wikipedia), I want to concentrate on working with interlaced video while editing movies - I've used Pinnacle Studio and tried Adobe Premiere. If the content was progressive, 3 consecutive frames might have looked something like this:



But the video is interlaced, so you know how it looks like - the infamous comb lines. So allow me to demonstrate this as if interlacing was not done line by line, but rather in chunks of lines:



Wait, that's not the only possible way the video could look like, there's another option:



Do you see the difference? It's quite subtle - in the first image, the car is a bit backwards on the even lines. This means that the even lines were captured first, and then the odd lines. This is called bottom field first. In the second image, the odd lines were captured first and then the even lines. This is called top field first. Which field is first? This is called field dominance, and it's one more thing you need to worry about when using interlaced video.

Where's the problem? When you're showing the interlaced movie, you have to know which field to show first (and send it first to the TV). If you get it wrong, you're sending the data at the wrong time. In our case the car would move a bit backwards, and then more forwards, then a bit backwards, and then forwards, etc. If you don't understand if completely, just believe me, it looks really bad.

How did I end up having a movie with field dominance issues? I used old video materials for my video, from multiple sources. I captured most of the video using a VCR connected to an Analog-toDV converter, but some of the video was already converted to DVD. The problem was that all my captured material was bottom field first (as is standard in DV), and the DVD originated video was top field first. When you combine TFF and BFF videos, you're bound to have jittery video in the scenes where the field dominance is wrong.

The real problem is that you don't see this problem until you actually see the video on an interlaced device. You see the regular comb lines, but have no way of knowing if the video is BFF or TFF.

Fortunately, I still had my original assets (video material), so I just had to reverse the field dominance on these specific video files. I originally thought of writing a VirtualDub plugin for this, but (of course), there's an excellent plugin called Reverse Field Dominance already available written by Donald A. Graft.

Of course, reversing the field dominance comes with a price:
  • You have to reencode the video, so if, for example, you're working with DV material, you loose something.
  • You either bring everything one pixel up (best solution in my mind) or shift the content one field earlier or later. In any case you introduce either a spatial or temporal change to the video.
What can I say, just use progressive video if you can.

17 comments:

Bond007 said...

Given the rate @ which the frames change, i.e 25-30 Hz with an effective field rate of 50-60 Hz, you wouldnt notice the problem would you?

Also this problem would be more apparent given the fields lines were more than 1 pixel high.

Please comment?

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Jänis Policy said...

We have a children's TV channel here which often show cartoons with incorrect field order. They flicker very much and are difficult to watch.

I believe you could deinterlace (to full rate) and preview any video in a computer player as well. Yadif could deliver realtime performance on most material. The video will either be jittery, worse or about the same as film, or silk smooth.

Interlacing sucks because to watch any video one must always fiddle with these controls to get best quality.

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