Hi, I am Mary Poplin with Boris FX and in this video I will show you how to replace
out-of-focus backgrounds with Mocha’s Unlink tracking workflow.
This example is shown running the Mocha Pro plug-in inside Adobe After Effects, but the
same technique can be used with Mocha Pro inside Premiere Pro or other hosts.
Sky Replacements are a common Visual Effects challenge, especially when the sky in the
original footage is out of focus or motion blurred.
These invisible effects shots are actually quite common in films & television - especially
when we see large sunsets or dramatic skies.
To replace a background or sky, we need to use motion tracking to match the camera motion.
This is commonly referred to as “match moving.”
(show before/after - could cut in other sky replacement examples from the past).
Out-of-focus backgrounds or shots with motion blur can make tracking difficult for the native
Adobe point tracker or camera tracker (could show an example of how this fails), but is
quite easy with a few important Mocha planar tracking tips.
So to get started, I will apply the Mocha Pro plugin effect to the layer I want to track.
I can then click the Track in Mocha button in the plugin interface in the effects panel.
This opens the Mocha interface and for this tutorial, I will be using the Classic Workspace
(shown here).
To master Mocha it is important to understand that we are tracking planar surfaces and not
specific pixels.
For match moving we typically draw a shape around an object and track forwards (show
this).
For example I might track this area where the sky meets the mountain.
But when you are tracking the sky in a moving shot, it is very common that our track can
simply move offscreen or get lost due to motion.
So let's focus on one of Mocha’s most powerful features; unlink tracking.
On a shot like this where no noticeable tracking areas that stay in frame, we will use the
unlink workflow to create a track offset relationship between the search area and the data that
we are capturing.
I will draw a search area here on the mountain, making sure to avoid my talent, and then turn
my surface tool on, which is an accurate visual representation of our track.
Now I align the surface to the front face of the mountain; I can use the surface center
point as transform data, the corners as corner pins, and I can use the grid tool to view
the track in more detail.
I could move the shape around the mountain however I like but that wouldn't affect the
surface tool at all because the surface tool always follows my track, not the shape.
I am only going to use translation, scale, and rotation because that’s all the tracking
information I need for this simple camera movement and because of the lack of parallax
in camera.
If the object was close enough to be moving in perspective, I would need to track perspective
as well.
But since I am NOT using a corner pin and am only using transform data for my match
move, this isn’t necessary.
Because the shape defines the texture I am tracking and the surface represents the track
I can unlink the track from the shape by going to the dropdown menu in layer properties called
“link to track” and selecting none.
Now, when I track forwards, my shape stays in one place but my track creates keyframes
as the texture data from the background moves beneath my shape.
Sometimes the fact that the shape can be keyframed confuses users but the shape will not affect
the track other than changing where the tracker is looking.
To get a rock solid track, make sure you track one plane at a time, track only what you need,
and make sure to avoid occlusions, reflections, and shadows.
The surface tool will tell you whether or not this is an accurate track because it will
"stick" to the texture you are tracking when you play through the shot.
Now that my track is complete I can save and close Mocha.
It’s time to apply my track to a new Sky Layer in my AE comp, which is a larger than
HD file because I don’t want uprez issues visible in my final composite.
I can use a layer in my comp or I can import an image into mocha directly using the insert
clip import, but the most nondestructive way to work and the most collaborative workflow
is to use a layer in my comp.
First I will duplicate shot and I can use a Luma key like Continuum’s Chroma Key Studio
to replace the foreground on top but have the new sky visible below.
I will Position and Scale the Sky layer exactly where I want it underneath this keyed layer.
Now I can use the tracking exports in the Mocha Pro Plugin Interface to apply the data
to my layer.
To choose a layer to create tracking data from, click the 'Create Track Data' button
in the Tracking Data section of the plugin.
Then I click the layer I want to read tracking data from in the dialog that appears.
I can only choose one layer at a time.
Once I click 'OK', the plugin will generate keyframes to populate the tracking parameters
in the plugin.
I can then use this data to copy to other layers, or link via expressions, without needing
to export from the Mocha GUI.
I can use any of these keyframes once they're created, so I can copy/paste the data or I
can use expressions to apply them to other layers in your composition.
In this instance, I'd like to apply the transform data to my sky.
The best way to do this is with a null.
So to apply tracking data to a Null, I use the dropdown menu to select an export option
at the bottom of the Tracking Data section.
Because this is a null I will use the Transform option to apply Scale, position and rotation
only.
Clicking 'Apply Export' then applies the tracking data to the specified layer in the dropdown
menu.
And now I can pick-whip my sky to the null and it will move perfectly in my scene.
And to finish the shot I should add motion blur, grain, color correction to match the
original plate in my composite.
This makes a world of difference for “selling” the effect.
Note that you can use a corner pin to place the sky but for oversized images like matte
paintings it becomes a bit more of a process.
You can use the technique in our Megaplate matte painting tutorial for a more complex
oversized compositing workflow.
You can also find tutorials on using the insert module for sky replacements there as well.
Now I can render right back to my timeline and am free to move on to the next clip in
my queue.
Unlink and offset tracking is one of the most powerful concepts users can turn to for tracking
long pans or erratic camera motion, because it captures all of the background information
you need for sky replacements even on “untrackable” skies with zero texture detail.
When in doubt, if you don’t have background objects to track, you can always track the
horizon.
And when clouds are present, Mocha’s planar tracker is sensitive enough to grab that texture
and lock onto it.
Saving time is the main goal of planar tracking, you’re telling the story, you don’t have
time to fiddle with hand animation.
Rock solid match moving is invaluable for making believable effects so you can get that
“in camera” look on every shot regardless of how the weather was cooperating that day
or not.
To learn more about Mocha Planar Tracking and Mocha Pro, please visit us at www.borsifx.com,
where you can find Free Training Videos, Official Documentation, Boris FX news, Artist Interviews,
Case Studies, and so much more.