Re: Working with interlaced footage

Guys, yeah, i think this is getting more complex than it might need to be. Fields can be overwhelming, but they are really pretty simple, you just have to get the "pattern" down.

The original post was "I will work with 1080i 29,97 footage for some effects". The real key here is to figure out the acqusition format, not the delivery format, as 1080i 29.97 can actually be a variety of diff formats (confusing i know, but bear with me) If your trying to match the footage exactly, the goal is to get the CG footage to match up with the "acquisition" footage exactly, meaning you need to work with whole frames. It's really easy to spot footage that has CG thats a diff frame rate than the original source footage or not interlaced the same, and really throws off the whole believability. But how to get whole frames, thats the rub.

Easiest way to tell import the footage into AFX, a frame sequencer, or just look at it in QT (if it's a QT file). Step through frame by frame. For the sake of discussion here W= whole frame, S= strobe frame/interlaced frame. As your stepping through, if you see it do something like WWWSS or SSWWW (or any kind of pattern like that), your footage was actually aquired at 23.976 fps, which means you can do a 3:2 pulldown in your comp program and get whole 24p footage. Then you just animate at 24fps, do your composite, then put the 3:2 pullup in at the last step in the process and now your back to 1080i 29.97

If your stepping through and there are NO strobe/interlace lines, the footage was probably acquired at 30p, so you just need to animate at 30p and your good to go.

If your stepping through and it's strobe/interlace lines on every frame, then it was shot at 60i. Best thing to do here is to take your footage and use a plugin to conver the footage to 60p. What happens is that it takes every frame, extracts the fields to two seperate frames (doubling the framerate) and guessing/interpolating on each frame what the missing information looks like. Then you animate your CG at 60fps, so it will line up with the processed footage. Then to get it back together you can do it two ways:
1.) work at 60fps in the comp all the way till the end. At the end, render out a 60fps sequence. Bring this back in to AFX (or wahtever) and put it in a 29.97 comp. When you render out of this, it will put the fields back in for you and it will all match up.
2.) from the get go, keep your comps at 29.97 and tell your footage that it's 60fps. Then when you render, it will look synced up.

There are some "gotchas" to watch out for. Namely in this case, if there is time remapping going on. If the footage has 3:2 pull up/down in it, and an editor has stretched the clip to make it longer or shorter, then your screwed :) The cadence of the pull up/down will be trashed. If you can get a unmolested peice of footage from the editor, thats the ideal situation, otherwise you'll just have to fudge it the best you can, by matching his time stretch, if you can find out what percentage he slowed it up/down.

This sounds complicated, but it's actually quite simple :) Hopefully this cleared stuff up, as we do this all the time and it's the #1 mistake i see younger compositors make, is not knowing how to work with fields. I really should probably do a tutorial on this at some point :)

cheers
Chad