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Elsewhere in this course I've described this MRI computer footage as being a progressive scan. In other words, not being interlaced or not exhibiting fields. Well, that was a little white lie, but it gives me an opportunity to talk to you about tracking footage that is interlaced. Let's move in time to a point where we have the fast motion in this shot, probably when this woman is coming on screen like right around here. If I Zoom in on this shot and pan over to her, I do indeed to see the telltale comb teeth effect of interlacing.
These horizontal lines being slightly offset from each other indicate that this frame contains lines that occurred at two different points in time and therefore are freezing the motion at two different positions. Normally, I would advice separating fields on any interlaced source. However, through experimentation I found that on this particular clip it does not help, and I will explain to you why. I'll go back to 100% Magnification and open up the Interpret Footage dialog for this clip.
When you separate fields, and by the way, all DV footage as well as most D1 and DC footage is Lower Field First, you are doubling the Time Resolution of the clip, but you're cutting in half the Vertical Resolution of the clip. I'll move this Interpret Footage dialog out of the way and watch what happens to the quality of lines, such as around this monitor or this window into the MRI room. When I switch to Off, you will see those lines are nice and clean. When I indeed Separate Fields, you will suddenly see a lot of aliasing in those lines.
Now, whenever I Separate Fields, I do turn on Preserve Edges, and that will help this aliasing phenomenon a little bit, but not a lot to be honest. I'll click OK. While in the Footage panel, after you've separated the fields for a clip, you can now step through it one field at a time. So I will select this panel to bring it forward, press Page Up, and be moving one field at a time earlier in time. However, look how much the display is changing on this computer. I will Zoom in larger so you can see the effect.
We are seeing the shift in position of the Upper and Lower Fields as I step through this footage. This degree of jitter in the display is something that can throw off the Motion Tracker in After Effects. I'll add this piece of footage to a New Composition, go back to 100%. When you're inside a Composition, by default, you're actually looking at one field of every frame of your interlaced source. So the advantage is, you won't be seeing that jumping up and down in the display, and again, I'll Zoom up and step through.
The disadvantage is you have only half the resolution to track. So if I decide to go ahead and track this shot and open up the options, I am left with two bad choices. If I Track Fields, I'll be tracking an image that appears to move up and down slightly in-between fields, which may throw off my track. If I don't Track Fields, I'll be tracking an image with only half the resolution. Therefore, if I have a shot that is interlaced, but which has a minimal motion to it, I don't see very obvious comb teeth effect of interlaced lines, I will try tracking it both with and without the Fields being separated to see which gives me a more reliable track.
In the case of this shot I got a more reliable track with Separate Fields turned off, and that's why I treated this particular shot as progressive scan for the sake of this course. After you've performed a Motion Track, if you have interlaced footage, you will probably want to go ahead and Separate the Fields again, in case you're doing any transformations to the footage. But this is one thing to be aware of whenever you're trying to Motion Track interlaced footage; sometimes you're going to get a better track with Separate Fields turned off. Now, again, I want to stress, this is a case of a shot with a very minimal motion in it.
If I had an interlaced shot with a lot of motion, I need to Separate Fields. Now, I've gone ahead and created an artificial example of taking this Europa footage we played with earlier and creating an interlaced version of it. As I get to the point in time where there is much more motion and Zoom up to 100% so you can see these fields, you can really see strong signs of interlacing in this shot. Again, you need to be viewing at 100% to truly see these lines. If you're viewing at 50%, you're only going to be seeing one field. So I will go back to 100%.
This is a case of a shot where you would need to Separate Fields before tracking. It would be near impossible to track this without the Fields being Separated, because the artifacts of interlacing would create an image that's just too variable and changes too much from frame to frame. Also, in the case of this shot, I'll add it to a New Composition again, and click Track Motion, this is a case where I would Track Fields. I obviously have a lot of motion going on from field to field during the course of this shot.
I don't want to miss out on that motion that is happening on each field. So in short, interlaced footage with a lot of motion, definitely Separate those Fields and Track those Fields. However, if you have an interlaced shot with minimal motion and minimal visible artifacts from interlacing, try tracking it without Separating Fields to see if you get better results.
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