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The motion of the black ball you are tracking is very consistent. Personally I would have just animated the position of the mask by hand. You probably could do it with three or four keyframes. Mask tracking seems like a waste of time. If you must use mask tracking read on.
I'm afraid I don't understand your composition very well because I'm not sure what is in the shot you are tracking. You are tracking an image sequence but I'm not sure if the blue ball in the background is part of the footage. One of the problems with the mask tracker is that it needs some detail to work. If layer 1 contains the black object and the blue background you are going to have problems because the tracker is going to catch the edges with detail and lock on that detail and the track will fail. If the layer is only the black stand then the tracker is also going to have a hard time because there is no detail in the ball. I would suggest that you try making the mask quite a bit bigger and give some edge detail to track. This may make it lock on a little better. Your other option would be to use Motion Tracking and apply that motion to a track matte. Here again, you would include a fair amount of extra pixels around the object you are tracking.
If the blue ball is not part of the shot you are tracking you might want to turn off the other layers. if it is part of the shot you might want to try adjusting the color of the layer to blow out the details of the blue ball, then pre-composing the shot so you have something that is easier to track.
Here's another hint. In it's current version the mask colors are pretty horrible. I would click the color square in front of the mask you are working on and pick another color that is easier to see. It does not improve tracking, but it makes it a lot easier to see what you are doing.
Apologies for the late reply. This is a passion project so I can't work on it full time. I've applied your suggestions however even with a much larger mask it does't track. My main goal is to remove the wire rig that animates the objects orbiting the giant blue star.
Some more detailed notes:
Everything you see is one layer and the image is static. The blue sphere is a star and the object that moves in front of it is a planet with an orbiting moon. I want to remove the robotic rig that animates the planet/moon. I have a freeze frame of the blue star in the BG will mask the rig out on a duplicated layer (in motion) in front of it. And add a plasma layer between the two afterwards.
As Rick says, this would probably be done more easily by just key framing the mask by hand.
The mask tracker sometimes doesn't do so well when it has no pattern to latch onto (the black silhouette isn't much for it to go on...)
Yeah. Definitely a learning lesson. Would love to hear any recommendations on how to shoot these kind of shots in the future. Maybe a crosshair on the prop? Or shoot it as a separate plate (paint sphere black, and paint rigging in a chroma something color). Thanks again
Either one of those things would help.
A crosshair on the prop would you let you use AE's point tracker to get your tracking data (which is a lot more flexible and useable than the mask tracker).
Having everything you don't want painted in a color that's different from everything else in the scene would let you simply key out the stuff you don't want and replace it with anything else.
Well, "simply key" is kinda funny. Keying is never as simple as it seems it should be!
Yeah I get what you're saying. Definitely had a few challenging keys before however shooting a black object against a bright background is the simplest key I've had to do.
The movement of the planet is linear and easy to track but the moon complicates it a bit. The other thing working against me is the fact that there will be a plasma layer between the two layers that were shot. So if I have a large mask (cover both planet and moon) it will be seen as the plasma layer will be absent within the mask.
Might reshoot this.