Mocha Tracking in Silhouette FX

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Silhouette FX is a dedicated rotoscoping program. Rotoscoping is the process of tracing a video image frame by frame creating a matte for later compositing. Essentially, think of of a father and son throwing a football back and forth in the front yard. What if you wanted them playing catch in a more obvious atmosphere – like a warring alien planet! You will need to rotoscope, or trace, around the father, the son, and that darn football in every single frame of that video clip. Once you are done tracing, you will have a series of black and white images called an alpha matte. Other software can then interrupt the image’s black as transparent and white as opaque. Therefore, the background will be removed, leaving you with just a go-lucky father and son playing catch. Now you can add in a new background, like that warring alien planet, underwater Atlantis, or in front of the great Pyramids of Egypt.

In the past I have shown you how to create an alpha using Silhouette FX, and also rotoscoping with Silhouette FX. This time, I am going to break down how to motion track. This is an advanced technique that is required for reducing the workload of rotoscoping by hand each frame of movement. The idea is that if you can mocha track an entire limb, for instance, throughout a shot, you will be able to apply your shapes using that tracked data and greatly reduce, if not fully eliminate, the need for manual frame by frame adjustments. I will now show you how to mocha track in three basic steps:

  • Setting Up Your Track
  • Tracking
  • Filing and Functionality

SETTING UP YOUR TRACK

Mocha Tracking is a partnership in the newest version of Silhouette from the planar tracking program, Mocha. I use this tracking program the most while working, and I find it to be the most accurate in diverse situations. Mocha is a planar tracker, which means that you create a shape (plane) that, when isolated, you can use Mocha to track from similarities in pattern, color, contrast, etc. The tracking shape will then follow along the path of tracking while storing the information in a layer (known as the tracking matrix). By storing the tracking information in a layer, you are able to add limitless shapes under that layer and the tracking data will apply to each of those shapes. Extremely helpful!

In the scene I am using I have a pair of hands with tracking markers on them. Tracking markers are not necessary, but are helpful in certain circumstances and encouraged if you have a savvy VFX supervisor on set to make those calls.

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To Mocha track, I first need to create a layer in the Object list panel.

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From there, using the B spline tool (it doesn’t matter if you use x, b, or bezier. I just prefer using B spline with human anatomy) draw a shape around the “area” you want to track. Now, I say “area” because you might want to track just the thumb, the index finger, wrist, or something that has a consistent movement throughout the clip. Think of a man walking from the profile view – you wouldn’t track his head and expect your shapes to adhere to the legs properly. You will need to track the head separate from shapes on the head (nose, chin, forehead), the thigh separate from the calf, the forearm separate from the shoulder, and so on. Since each section usually takes 5 – 10 shapes to complete, having a track all of those shapes can follow is a huge time saver. So again, I am going to draw a shape around the “area” I want to track.

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Keep the tracking shape tight around the area you want to track without it being a pixel perfect shape to what you need to roto. It needs some data from the surrounding area to differentiate pattern and movement. At this point, let’s go into our tracker controls.

TRACKING

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Looking at the controls (unless I have a scene where the cameraman is moving around a scene while filming) I generally only want to track the TRANSLATION, SCALE, and ROTATION.

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In Pre-Processing, you can check on PREVIEW and play with the Blur, Sharpen, contrast, etc., until you get a high contrasted image that gives nice shapes and patterns for your tracking shape to follow.

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Now go ahead and Track forward.

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FILING AND FUNCTIONALITY 

Back in the Timeline you will notice the LAYER you created now has multiple keyframes under whats called the TRANSFORM MATRIX.

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This is your tracking storage, and now you can create any number of shapes you need under that layer, and that tracking data will now apply to each of those shapes.

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Notice towards the bottom of the list, I labeled that initial b spline I used for tracking as my “tracking shape” and just locked it and turned it off. That way, if I need to adjust the track down the line, I still have it for reference.

For your reference, here is the video that particular sample clip came from. In this example, you can see how rotoscoping became important for us (me and the other artist working on this clip) in order to strategically animate on new skin tones and iron man hand blasters.

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