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The first step in fixing a shot like this is to carefully analyze what you have and decide on the most efficient way to deal with the problem. You almost always need multiple copies of your footage so the first step would be to duplicate the footage.
The most obvious first problem is that the camera is moving around. The obvious solution for that is to Motion Stabilize the shot so that it is not moving around anymore. This is easy to do if you open up the Motion Tracking workspace and stabilize motion including rotation and scale. You will want to pick two points that do not move and that are visible during the entire shot.
These two points would work. You run and apply Stabilize Motion and make sure that Position, Rotation, and Scale are checked. To keep things organized rename the stabilized layer "stabilized."
When you remove something from a shot, like the woman that is sneaking through this shot, you need to replace the background somehow. The best way to do this is with a Clean Plate. In this case, a "Clean Plate" is footage or a frame of footage that does not have the woman in the shot. Fortunately, the first frame of the footage is a great clean plate so all you have to do is to Freeze Frame the bottom layer and you have your clean plate.
The next step is to create a mask that hides the woman. Sometimes this is very complicated because there is a lot of action happening behind the thing you are trying to remove from the scene. In your shot, the entire foreground does not move so one big mask is going to work for most of the shot. There are a few frames where the kids are moving right behind the woman so you will have to take care of that problem with, you guessed it, another layer. The easiest way to create that matte is to add a solid layer to the top of the timeline and set the blend mode to Screen so you can easily see through the layer. A light blue works really well for this shot. Then you use the pen tool (g) to draw a mask that includes everything that is moving in the shot except the woman. As you scrub through the timeline you'll find places where the woman in front of one of the kids and they are moving. There are other places in the timeline where the woman is in front of kids and they are not moving. for those frames you can simply animate the mask you are drawing on the track matte layer. In other places, the kids will be moving when she is in front of them. This is where you need another layer. This is the first obvious place where I see this happening.
At this point the easiest thing to do is duplicate the stabilized footage layer, trim the in and out points so the layer is just long enough to cover the frames where the woman is in front of the kids, then add another track matte and then time shift the kid moving layer so you can line up their movement and get the timing right. This is the hardest part of the project.
When you have all of the problems fixed it is time to enable the track mattes and put the motion back in the camera. You do this by adding a null to the timeline, moving to the first frame, then tying the null's Position, Rotation and Scale to the second layer's Anchor Point, Rotation, and Scale. The last step is to parent all of the layers except the clean plate to the null.
This screenshot shows the project before fixing the frames where the hands and iPad are in front of the moving kids. I didn't take the time to fix it. I hope you get the idea.
This is a link to an animation preset you can apply to the Null layer that will write the expressions for you: Dropbox - destabilize Rotation Scale.ffx
Here are the expressions if you want to write them yourself.
Position:
thisComp.layer("stabilized").transform.anchorPoint
Scale:
x = value[0];
y = value[1];
tx = thisComp.layer("stabilized").transform.scale[0];
ty = thisComp.layer("stabilized").transform.scale[1];
nx = x/tx*x;
ny = y/ty*y;
[nx, ny]
Rotation:
- thisComp.layer("stabilized").rotation
The only difficult one is scale.
I hope you understand the process. It's all about layers. Removing someone from a different shot may require an entirely different workflow. I think this is the most efficient way to do this shot. Getting this far took me less time than it did to write this post.
There are really only a few frames that are going to require the extra work of time shifting. With some experience you could pull off this composite in about an hour.
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