Mocha Tips: How to Track Occluded Objects

In this quick tip, Mary Poplin, Boris FX Product Specialist, will cover some Mocha tracking techniques to solve difficult shots: what to do when the object you are trying to track is partially or fully occluded, the importance of layer order, how to avoid occlusions, and how to spot occlusions before you start tracking. One of the most common problems in tracking occurs when foreground objects cover the object we are trying to track. We call these occlusions. For example, when a finger swipes across a phone or when actors pass in front of monitors we have a tracking occlusion to solve.

How to spot occlusions:
Most occlusions are obvious. Mocha is pretty smart about filtering texture, but it can get confused by moving objects, reflections, shadows, or people that slowly or barely move in front of the area you are trying to track. Mocha is smart enough to ignore most noise and quick inconsistencies like explosions and rain, but slow moving objects give mocha enough texture from one frame to another for mocha to get derailed. (Show explosion example, even though it is used before.

Partial Occlusions:
For partial occlusions, avoiding the occlusion can be as simple as tracking the object as normal and then pausing and moving the shape to another area moving in the same direction. Just keep in mind that animating a shape doesn't animate the track, it simply tells the track to look in a new location. Turn the surface tool on to see what your track is really doing.

Layer Order:
You can also use layer order to hold objects out of your track. Layers at the top of the layer pile are automatically held out of layers below them when tracking. Simply draw a shape around the offending object, and drag that layer to the top of the layer pile in layer properties. If you always track objects from the FG to the BG you will always have holdout shapes for your tracks below.

Full Occlusions:
When an object is not visible or entirely occluded, you can use manual track to jump past them. You will need to turn your surface tool on and then switch to manual track in the track tab, notice how the surface tool becomes a dotted line. Then you can hand keyframe any interruptions in the track. If this looks good, keep tracking forward. If not, track into the occlusion from either side of the timeline and delete any messy tracking keyframes by using the keyframe Minus - button.

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[trailhops]

No other tutorial has mentioned the layer order trick before. That's a lifesaver, thank you!

[thegatewaytoprestige]

Thanks Mary! As always, you're simply the best instructor out there.

[Corn_Pone_Flicks]

Does the technique of tracking foreground objects so that they don't interfere only work in Mocha Pro? I've tried that in Mocha AE, and it doesn't seem to do anything. The moment the object starts to get eclipsed, everything starts to drift.

[PesesitoTony]

but how do i make it modify its mask so the mask of the objects changes based on what its ocluded of the object

[itssarun]

how do we mask out two roto layer from one tracking layer

[OptimalMan]

hi what about objects that go out of frame? there is an error message that comes up.

[itssarun]

i mean is that if unpper layer roto is for masking then if i add one more shape does the 3rd layer will do what ?

[michaelhogsett]

Layer order did nothing for me, it still shows the image im trying to replace above the other layers that should be subtracted. How to fix?

[RobCrusoe]

This isn't working on my Mocha. I put the layers in the correct order but the background layer still shifts.