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When a tracking target is partially obscured by something moving in font, the offending object can be subtracted from the tracking mask to restore a clean track. I am using the coffee cup clip from the Lesson_03_Midea folder. First, let me give you a demo to see how it works, then we will work it on a real example. I am gong to start by drawing a simple shape around this coffee mug. I will turn on the Mattes so we can watch the action while we track it.
And even though it's not a planar surface, the Planar Tracker does a pretty darn good job at locking on. Well done. And we have some tracking data here that we can play with. So I will rename this, cup track. Next, I will draw a new shape up here in the chest and I am going to turn off the tracking cog so the tracker doesn't accidentally track it. The two key points are the new layer is above my cup track in the list and second of all it's right in the path of my tracking.
So let's watch what happens. I will select the cup and retrack it. I'll stop the track here to show you how the cup track shape has been erased for the Layer 2 above it in the Layers list. Coming up here to the Mattes overlay pop-up I will be enable these selected track Mattes options, which reveals how track mattes are subtracting from each other. You can see that the top of the coffee cup shape has been erased. I'll set the Mattes overlay back to Selected matte, then come down here to resume tracking to see how far it gets.
It only gets -- bummer! All right! So because Layer 2 was above the cup track it was subtracted from the cup track below. So when the tracker got behind the Layer 2 mask it broke track. So let's go back to frame one and now we will delete the cup track data. The way to delete tracking data for our layer is first make sure the layer is selected and then come down and click on manual track and there is all your tracking data keyframes.
Go over here to the Delete All, confirm Yes. Now all the tracking data is gone. Be sure to put the tracker back to large emotion. I will show you it's gone by moving the playhead and my tracker shape goes nowhere. Now if I go up to Layer 2 and deactivated it, it essentially disappears from the universe. So now I can reselect my cup track and retract it and I'll get a good track, even the Layer 2 is above it in the list it's interactive. No harm.
But let's see what happens backing frame one. If I active Layer 2 and make it invisible. Now let's see what happens. So I'll select my cup tracker and retrack the shot and sure enough it was invisible my cup track broke lock. So it's not a question of visibility. It's a question of being an active or an inactive layer.
So if your tracking layer passes over a layer that's above and active it's going to block the tracker from seeing it. Let's see a real example. So with the cup track selected we will put that in the trash and also delete Layer 2. So I need to do a track on the chest and unfortunately this arm and cup are just swinging right through the middle of my target. So I need to subtract that out. So first I am going to draw a roto around my coffee mug and arm.
I'll track forward from here. I chose that frame because it kind of gave me a good view of the arm and the mug. My tracker went a little bit off the target right here, but that's no problem. The planar tracker is very forgiving of problems like that. So we will go back here where I started and I'll track it backwards. Again, a little exposed coffee mug isn't going to hurt us. So we now have a tracked mask.
So I'll rename this arm mask. I will turn off the tracking cog so we don't actually track it again later. In fact, I will even lock it. Now I will draw my body tracking mask here. I will rename it body tracker and I am going to set all of the overlay masks to be on so that we can see what's going on. I will set the arm mask at the top of the stack so it's above it.
Now I am going to set the arm layer blend mode to Subtract. Now you can see how the arm mask is being subtracted from the body track. So this subtracts arm mask from the body tracker so the tracker won't even see the arm and the coffee cup. The only part that tracker will see is the green overlay area. We will set the playhead back to frame one. Make sure the body track is selected, and we will track that.
Now the coffee cup and the arm are being subtracted and the tracker doesn't get confused. Okay, tracking is done. Put the playhead back to frame one. I am going to turn of those Mattes, turn on the planar surface body tracker. We don't need the splines. We will hide those. So here is my planar surface for the body track and that shows that we've got some decent tracking data that we can use to do some rotos on the body. We will stop that.
I will turn off the Planar Tracker, turn on the Mattes. Now let's look at the white mattes, the alpha channel, if you will. You can see the arm mask is being subtracted from the body tracker, and that's because the blend mode for the arm mask here was Subtract. If I put it to the default of Add, the two are put together. So when layer is a tracker they're going to be subtracted automatically, but the layer is a roto, then you will need to turn on Subtract if you really want the output masks to be contracted from each other.
So remember any acting layers below tracking layers will be subtracted from them. So always keep the current tracking later at the top of the stack to avoid subtraction.
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