Weird behaviour of tracker

I wanted to get rid of an artifact in a little timelapse.
It’s a sort of road marker that is right in the middle of the frames and sticks out because it’s yellow.
Now I looked al over for tuts and found some real interesting ones on how to proceed in this case. ( removing objects with clean plates )
Problem is that ones the tracking is started the created x-pline starts to behave like a trapezium on the move…
On the movie i created, you can see what i mean.

Anybody has an idea what is causing this, and how to solve it ? ( or is this a normal thing in this case…)

Thanks in advance !

Hi Mary,
Thx for yr reply.
Sorry about the vid. Must be an error.
I up loaded it again. Hopefully works now.
Not the best quality, but you can see what I mean.

Here’s the link : - YouTube

Hope you can figger out what’s the cause…

Thanks ahead !


Ok, Thx MAry !
Will try that out !

Many thx !


I can’t see your video yet, it is still being processed, but it is probably an issue we see many times on objects that are tilting in z space. What sounds like is happening to your shot is that the surface is going behind the “camera” in mocha. That means that the track on that shape has corrupted the shape itself, this happens sometimes when the tracking data heads behind the “camera” in mocha, since the track can’t handle negative values. Make sure your surface stays in frame to avoid this issue. If your surface disappears, you’ll know that means the data has gone bad.

A way to track around this it to track from the flattest area towards the screen to either side of the surface tilt you are tracking, and you can also make the surface smaller.

However, slow moving shadows and reflections can also cause these sorts of issues.

I would be happy to look at the shot further if you need for me to, but I would need the footage in order to see what is going on, so I will wait until it is finished being processed.

OK, so the problem here is you are tracking a small area, and two non-coplanar planes in the same layer, and there’s a slow moving shadow over the object you are trying to track.

Draw a spline around the ground plane if you’re trying to track the ground, and align the grid and surface tool to the ground so you can see what the track is doing, the x spline won’t tell you what your track is doing. It is just where the track looks. If you get bad results, put a garbage matte around the shadow in a layer above your track.

The same thing applies to any planar surface you are tracking. Mountains, ground, whatever.

Larger shapes will also get you better data, as mocha has more to compare with.

Please also check out our fundamentals of mocha to get a solid grip on the basics of the tracker, I think it will improve your workflow: