Can Mocha Track This Difficult Shot?


#1

I have a difficult stereoscopic 360 shot that needs a remove as the camera moves in the z-plane. Aside from the question of getting the stereo to sit properly, the shot itself has a lot of moving texture and no obvious planes. Here’s what it looks like as an equirectangular file:

Here’s how it looks playing back in 360:

At the end of the shot, the filmmaker is hiding behind a tree (he had to hide there because the cable-cam rig wasn’t working when he hid out of camera range.)

We’d like to remove him from the shot and I’ve set up a remove that looks like this:

I’m tracking backwards from the end of the shot to the beginning. While I’m able to track my remove layer shapes just fine, I can’t figure out how to get a stable background track.

I’ve tried various part of the frame, using tree trunks etc., but nothing is working. My background track doesn’t hold no matter what I try.

How would you solve a shot like this? Is this even something Mocha can do, or would I be better off trying this in a feature tracker?

Thanks for your help!


#2

Try tracking the ground plane, using link to track “none” to divorce the track from the layer spline, and use that as your tracking data. You can always relink your track when it is done and adjust the shape to the proper position. That’s the first thing I would try. Can you try that and let me know?

Cheers,
Mary


#3

Hi Mary,

I tried your suggestion and while it seems promising, I’m still having trouble. I tried reverse tracking the ground with link to track set to “none” in both equirectangular and 360 view modes. I made roto holdouts for the trees that come into view as the camera travels back. Still having two problems:

  1. In every try, the I could track about 7-seconds before getting a “failed to track” error. Moving the track shape didn’t help - I get a “please retract layer” error after moving the shape.

  2. For the 7-second section that does track, the track veers off to the right as you can see from the surface in this screen grab:

If I link my background and remove layers to the track, the remove veers off as well.

I’m having a hard time judging my track because of the distortion of the 360 sphere and the shot being on an angled hill. What would a proper track look like in this shot?

I have separate LR cleanplates and the stereo is perfect for the first couple remove frames until the slipping becomes obvious. I feel like this will work if I can just get a good track.

How would you approach tracking the ground and dealing with the problem areas that don’t track?

Thanks!


#4

This is a tricky shot.

Can you try increasing the zoom and angle search area parameters in the track tab? It might hold on better, and you will probably need shear and perspective as well to get it to really match the angle for the remove.

To judge the track, turn the surface and grid tool on and align them to some landmarks. If they more or less stay on those landmarks like they are glued to it, that’s a good indicator that your track is holding on well.

I agree that if you can get a good track here you should get a good remove, but it’s absolutely tricky.

Try using shear and perspective, increasing your min% of pixels to 100, and increasing your angle and zoom for the search area. It will absolutely take longer to track, but it’s your best hope. You also may need to hand animate shapes between trees for the track to grab more ground texture and get better results where your track starts to slip, you can just delete those after the track is complete for those problem areas. You can always change parameters and shape size and location to look at different parts of the ground plane texture to track. As long as they’re on the same plane, they should give you the same data.


#5

Thanks Mary, I’ll give that a try and let you know how it goes.


#6

I’m make progress but still hitting snags.

I turned on shear and perspective and set min% to 100. I set zoom and angle search to 10% each (higher numbers were unreasonably slow)

Using landmarks on the ground plane for the surface corners really helped. I used trees and rocks.

Tracking seemed to hold very well, at least for the first few seconds until the trees come into view. The track jumped at that point.

I did a test remove of the first few seconds to see how well it was working and noticed something odd. If you look at the tree in the area between the background and the remove, you see the tree is slipping upwards:

Is this because the tree is on a different plane than the ground plane? If so, do I need to rotomask the tree out for this to work?

Even though it looks pretty solid, after a few seconds , it’s clear the track is slipping. Even a little slip breaks it. I think adjust track will be able to help with this, but first I want to be sure I understand what’s happening with the upward slipping of the tree.

Thanks!


#7

Because this track is so tricky, you’re going to really have to massage it. And yes, the tree is on a different plane, so if you grab too much of it, it will slip. But you can use the bottom edges of the trees. Expect to hand animate a lot of shapes to get this to work. I wish I had better news for you there. Also, make sure you are tracking the clip at full rez as well if you keep slipping, it may be that if you’re using proxy you don’t have enough data.


#8

Gotcha, thanks. Yes this is full res at 60p so this is the best I have to work with. Will dig in and see see how far I can get.