Tracking and stabilizing motion

After Effects includes Imagineer Systems mocha for After Effects (mocha-AE), a stand-alone planar tracking application that can export tracking data for use in compositions in After Effects. For many tracking tasks, mocha-AE provides superior results with greater convenience than do the native After Effects tracking features. For more information, see the mocha-AE documentation, which is available from the Help menu in the mocha-AE application.

mocha AE is launchable from within After Effects CS6. To use mocha AE in After Effects CS6, choose from the following:

• Animation > Track in mocha AE
• Edit > Paste mocha mask

Todd Kopriva provides a basic introduction to using mocha-AE for motion tracking in "Overview of the mocha-AE interface and workflow" on the video2brain website.

If you have questions and issues regarding mocha-AE, see the FAQ list for mocha for After Effects and the support forum for mocha for After Effects.

The Imagineer website provides several video tutorials and other resources for learning to use mocha-AE with After Effects.

Adobe TV has a mocha-AE channel, which includes several video tutorials about using mocha planar tracking and rotoscoping utilities.

Chris and Trish Meyer provide a video tutorial that introduces mocha for After Effects on the Lynda.com website.

Chris and Trish Meyer provide tips about mocha-AE and mocha shape, including tips about variable-width feather, in an article on the ProVideo Coalition website.

David Torno provides extensive video tutorials that show how to use mocha-AE as part of a workflow to replace one face with another in a movie. Todd Kopriva provides links and details on his After Effects Region of Interest blog.

Mathias Möhl provides the MochaImport script and a set of related tutorials on his website. MochaImport automates common parts of the workflow of using mocha-AE with After Effects.

Jeff Foster provides a tutorial on the ProVideo Coalition website that demonstrates the use of mocha for After Effects to replace a sign on the side of a moving truck in a shaky video clip.

With motion tracking, you can track the movement of an object and then apply the tracking data for that movement to another object—such as another layer or an effect control point—to create compositions in which images and effects follow the motion. You can also stabilize motion, in which case the tracking data is used to animate the tracked layer to compensate for movement of an object in that layer. You can link properties to tracking data using expressions, which opens up a wide variety of uses.

After Effects tracks motion by matching image data from a selected area in a frame to image data in each succeeding frame. You can apply the same tracking data to different layers or effects. You can also track multiple objects in the same layer.

For information about Imagineer Systems mocha for After Effects, see Resources for mocha for After Effects (mocha-AE).

Motion tracking has many uses. Here are some examples:

• Combining elements filmed separately, such as adding video to the side of a moving city bus or a star to the end of a sweeping wand.

• Animating a still image to match the motion of action footage, such as making a cartoon bumblebee sit on a swaying flower.

• Animating effects to follow a moving element, such as making a moving ball glow.

• Linking the position of a tracked object to other properties, such as making stereo audio pan from left to right as a car races across the screen.

• Stabilizing footage to hold a moving object stationary in the frame to examine how a moving object changes over time, which can be useful in scientific imaging work.

• Stabilizing footage to remove the jostling (camera shake) of a handheld camera.

You set up, initiate, and apply motion tracking with the Tracker panel.

As with all properties, you can modify, animate, manage, and link tracking properties in the Timeline panel.

You specify areas to track by setting track points in the Layer panel. Each track point contains a feature region, a search region, and an attach point. A set of track points is a tracker.

After Effects uses one track point to track position, two track points to track scale and rotation, and four points to perform tracking using corner pinning.

Curtis Sponsler provides detailed instructions and explanations for tracking and stabilizing motion in a PDF excerpt from his book The Focal Easy Guide to After Effects.

Chris and Trish Meyer provide a video tutorial on the ProVideo Coalition website that demonstrates and explains the basics of motion tracking.

This video from the After Effects CS5: Learn By Video series shows how to combine motion tracking and the Clone Stamp tool to remove an object from a scene.

Angie Taylor provides a tutorial on the Digital Arts website that shows how to use tracking data and the Clone Stamp tool to apply copies of an object in a scene while matching a camera move.

Michele Yamazaki provides a tutorial on the Toolfarm website that shows how to use motion tracking to obscure a logo in motion footage.

Sean Kennedy provides a set of detailed tutorials on the SimplyCG website that demonstrate advanced motion tracking techniques:

• Basic 2D tracking

• Planar tracking

• Motion tracking and compositing computer-generated elements into a scene

• Screen tracking and replacement

Sean Kennedy provides a free script, TrackerViz, that makes tracking motion and applying tracking data to masks easier. You can get TrackerViz and a series of detailed instructions on the SimplyCG website.

Eran Stern provides a video tutorial on the Artbeats website that demonstrates the use of 3D tracking software that solves for camera movement so that additional elements can be composited into the scene and appear to honor the same camera movement. This video tutorial uses Pixel Farm PFHoe, but the techniques can be applied to almost any matchmoving software.

This post on the AE Enhancers forum describes and links to an animation preset from Donat van Bellinghen for scaling a set of Corner Pin effect points.

This post on the AE Enhancers forum describes and links to a script from Paul Tuersley that takes a stabilized layer, precomposes it, and then adds expressions that counter the stabilization.

This post on the AE Enhancers forum describes and links to a script from Paul Tuersley that can make a difficult tracking job easier by averaging multiple sets of tracking data.

Jeff Almasol provides a script on his redefinery website that creates a null layer with an expression that sets the Position property to be the average of the values of motion tracking track points for the selected layer.

Jörgen Persson provides a script on the After Effects Scripts website with which you can import tracking data from Apple Shake into After Effects.

Mathias Möhl provides useful scripts for motion tracking—including MochaImport, KeyTweak, and Tracker2Mask—on his website. Mathias also provides video tutorials explaining the use of the scripts.

The implicit first step of any workflow is to determine the result that you want to achieve before you begin. What type of motion will you track, and what will you apply the tracking data to?

As with many workflows in the real world, you may have to repeat some of these steps. You can track a layer as many times as desired and apply any combination of tracking results.

Set up the shot

For motion tracking to go smoothly, you must have a good feature to track, preferably a distinctive object or region.

For best results, prepare the object or region that you are tracking before you begin shooting. Because After Effects compares image data from one frame to the next to produce an accurate track, attaching high-contrast markers to the object or region lets After Effects more easily follow the motion from frame to frame. Lightweight, brightly colored balls (such as ping-pong balls) placed on the feature work well, in part because their appearance is the same from all angles. The number of markers that you use corresponds to the number of points you are tracking. For example, if you’re tracking four points using the Perspective Corner Pinning option, you’ll track four features, to correspond to the four corners of the layer to attach. The more markers you add to your subject before shooting, the more features you’ll have for tracking—but the more items you may have to remove later from the image with the Clone Stamp tool. You don’t need to add a marker for each feature if a distinctive object or region is already at the appropriate location.

If you’re tracking a large object or the set itself—such as for matchmoving—you can get good results by using a grid of uniformly spaced triangles of a uniform size as tracking markers.

Add the appropriate number of track points

When you choose a mode from the Track Type menu in the Tracker panel, After Effects places the appropriate number of track points in the Layer panel for that mode. You can add more track points to track additional features with one tracker.

Select features to track, and place feature regions

Before you begin tracking, view the entire duration of the shot to determine the best features to track. What is clearly identifiable in the first frame may later blend into the background because the angle, lighting, or surrounding elements have changed. A tracked feature may disappear off the edge of the frame or be obscured by another element at some point in the scene. Though After Effects can extrapolate the motion of a feature, your chances for successful tracking are highest if you step through the entire shot to select the best candidates for tracking.

A good tracked feature has these characteristics:

• Visible for the entire shot

• A contrasting color from the surrounding area in the search region

• A distinct shape within the search region

• A consistent shape and color throughout the shot

Set the attach point offset

The attach point is where the target layer or effect control point will be placed. The default attach point position is in the center of the feature region. You can move the attach point to offset the position of the target relative to the position of the tracked feature by dragging the attach point in the Layer panel before tracking.

For example, to animate a cloud above a person’s head, position the feature region on the head and move the attach point above the head. If you left the attach point centered in the feature region, the cloud would be attached to that point and would obscure the head.

Adjust the feature region, search region, and tracking options

Place each feature region control tightly around its tracked feature, completely enclosing the tracked feature, but including as little of the surrounding image as possible.

The size and position of the search region depend on the movement of the feature you want to track. The search region must accommodate the movement of the tracked feature, but only the frame-to-frame movement, not its movement throughout the shot. As After Effects locates the tracked feature in a frame, both the feature region and search region move to the new location. Therefore, if the frame-to-frame movement of the tracked feature is gradual, then the search region needs to be only slightly larger than the feature region. If the feature changes position and direction quickly, then the search region needs to be big enough to encompass the largest position and direction change in any pair of frames.

You can also set tracking options that determine such things as which color channels are compared to find a match to the feature region.

Analyze

You perform the actual motion tracking step by clicking one of the Analyze buttons in the Tracker panel. When tracking a tricky set of features, you may want to analyze a frame at a time.

Repeat as necessary

Because of the changing nature of an image in motion, automatic tracking is rarely perfect. In moving footage, the shape of a feature changes, along with the lighting and surrounding objects. Even with careful preparation, a feature generally changes during a shot and at some point no longer matches the original feature. If the change is too great, After Effects may not be able to track the feature, and the track point will wander or drift.

When the analysis begins to fail, return to the frame where tracking was still accurate and repeat steps 5 and 6: adjust and analyze.

Apply tracking data

If you’re using any Track Type setting other than Raw, you apply tracking data by clicking Apply, after making sure that the correct target is shown for Motion Target. You apply tracking data from a Raw tracking operation by copying keyframes from the trackers to other properties or by linking properties with expressions.

You can also adjust the Attach Point or Attach Point Offset property after tracking in the Timeline panel, which can be useful when applying the same tracking data to multiple targets that you want to distribute around the tracked feature.

Tracking motion and stabilizing motion are essentially the same process, only with a different target and result. Use Track Motion to track motion and apply the results to a different layer or effect control point. Use Stabilize Motion to track motion and apply the results to the tracked layer to compensate for that motion (for example, to remove camera shake).

To stabilize a layer, After Effects tracks the motion of a feature in the layer that should be stationary in the frame, and then uses the tracking data to set keyframes to perform the opposite motion. You can stabilize to remove any combination of changes in position, rotation, and scale, while leaving desired motion unaffected. For example, if the camera is panning, deselect Position but select Scale and Rotation as the properties to stabilize.

When you select Rotation or Scale in the Tracker panel, you set two track points in the Layer panel. A line connects the attach points; an arrow points from the first attach point (the base) to the second. If possible, place the feature regions on opposite sides of the same object, or at least on objects that are the same distance from the camera. The farther apart the regions, the more accurate the calculations and the better the result.

After Effects calculates rotation by measuring the change of angle of the line between the attach points. When you apply the tracking data to the target, After Effects creates keyframes for the Rotation property.

After Effects calculates scale by comparing the distance between attach points on each frame with the distance between the attach points on the start frame. When you apply the tracking data to the target, After Effects creates keyframes for the Scale property.

When you track motion using either parallel or perspective corner pinning, After Effects applies keyframes for the Corner Pin effect to the layer to scale and skew the target layer as necessary to fit the four-sided area defined by the feature regions. The feature regions should lie in a single plane in the real world—for example, on the side of a bus, on the same wall, or on the floor. The attach points should also all lie in a single plane, but not necessarily the same plane as the feature regions.

When you stabilize a layer, the compensating motion may itself cause the layer to move too far in one direction, exposing the background in the composition or moving action out of the action-safe zone. You can correct this with a small change in scale for the layer. Find the frame where the problem is most severe, and then increase or decrease the scale of the layer until the problem is resolved. This technique adjusts the scale for the duration of the layer; you can also animate scale to correct this problem by zooming in and out at different times.

You set up, initiate, and apply motion tracking with the Tracker panel.

These settings apply to a tracker, a group of track points that is generated in one tracking session. You can modify these settings by clicking Options in the Tracker panel.

• Select Continue Tracking to ignore the Confidence value. This behavior is the default behavior.
  
• Select Stop Tracking to stop the motion tracking.
  
• Select Extrapolate Motion to estimate the position of the feature region. Attach-point keyframes aren’t created for low-confidence frames, and attach-point keyframes for the low-confidence frames from previous tracks are deleted.
  
• Select Adapt Feature to use the original tracked feature until the confidence level falls below the specified threshold. At that point, After Effects adapts the tracked feature to be the contents of the feature region in the frame preceding the one that has low confidence and continues tracking. This option isn’t available if Adapt Feature On Every Frame is selected in the Motion Tracker Options dialog box; enabling feature adaptiveness causes After Effects to adapt the feature region with every frame regardless of the confidence level.
  

Each time you click Track Motion or Stabilize Motion in the Tracker panel (or choose Animation > Track Motion or Animation > Stabilize Motion), a new tracker is created for the layer in the Timeline panel. Each tracker contains track points, which are property groups that store the tracking data after tracking has been performed. Trackers are grouped in the Motion Trackers property group for each layer in the Timeline panel.

You can rename trackers and track points and modify and animate their property values in the Timeline panel just as you do for other layer properties and property groups. You must click Apply in the Tracker panel to apply the property changes to the target.

When you set up motion tracking, it’s often necessary to refine your track point by adjusting the feature region, search region, and attach point. You can resize or move these items independently or in groups by dragging using the Selection tool. To help you define the area to be tracked, the image area within the feature region is magnified to 400% while you move the region.

• To turn on or off feature region magnification, choose Magnify Feature When Dragging from the Tracker panel menu.
• To move the feature region, search region, and attach point together, drag inside the track point area (avoiding the region edges and the attach point), or press the Up, Down, Left, or Right Arrow key. Hold Shift while pressing an arrow key to move by an increment 10 times as large.
• To move only the feature and search regions together, drag the edge of the feature region, or Alt-drag (Windows) or Option-drag (Mac OS) with the Selection tool inside the feature or search region. You can also hold Alt (Windows) or Option (Mac OS) while pressing the Up, Down, Left, or Right Arrow key. Hold Alt+Shift (Windows) or Option+Shift (Mac OS) while pressing an arrow key to move by an increment 10 times as large.
• To move only the search region, drag the edge of the search region.

• To move only the attach point, drag the attach point.
• To resize the feature or search region, drag a corner handle.
• To make all of the sides of the region match the length of the longest side, and to resize the region relative to the original center point of the region, Shift-drag a corner handle.
• To make all of the sides of the region match the length of the longest side, and to resize the region relative to a particular corner handle, Ctrl+Shift-drag (Windows) or Command+Shift-drag (Mac OS) the opposite corner handle.
  

After you’ve tracked a motion source layer, you can apply the tracking data stored on that layer to any number of other target layers and effect control points. For example, you can apply the track to the position of a light bulb and to the effect control point of the Lens Flare effect.

As an image moves in a shot, the lighting, surrounding objects, and angle of the object can all change, making the once distinct feature no longer identifiable at the subpixel level. Also, if the search region is too small, the tracked feature may leave its bounds from one frame to the next.

Learning to choose a trackable feature takes time. Even with careful planning and practice, the feature region can drift away from the desired feature. Re-adjusting the feature and search regions, changing the tracking settings, and trying again is a standard part of automatic tracking. It’s not necessary to get a single good track in one try. You may need to track the shot in sections, redefining the feature region in places where the feature changes and the region drifts. You may even need to choose a different feature to track, one with movement that closely matches that of the feature to track, and use the attach point offset to place the target.

After you’ve tracked motion, each track point has a motion path in the Layer panel that shows the position of the center of the feature region. You can fine-tune the keyframes of the motion path in the Layer panel as you would any other motion path. Modifying the motion path is most useful when you want to manually change the motion tracking data before applying it to a target. In some cases, it may be easier to manually modify the motion path created by the motion tracker than to get a perfect track.

You can stabilize motion with the Warp Stabilizer effect. It removes jitter caused by camera movement, making it possible to transform shaky, handheld footage into steady, smooth shots. See Tracking and stabilizing motion for more information about using the point tracker for stabilizing motion.

For video tutorials, details, and resources about the Warp Stabilizer effect, see this article on the Adobe website.

To stabilize motion using the Warp Stabilizer effect, do the following:

Though the Warp Stabilizer will serve most of your needs for stabilization, there are times when you may want to use the legacy stabilizer. You can access the stabilizer in the following way:

In After Effects CS5.5, access the legacy stabilizer in the following way:

Analyze

There is no need to press this button when you first apply Warp Stabilizer, it is pressed for you automatically. The Analyze button remains dimmed until some change takes place. For example, if you adjust a layer’s In or Out points, or there is an upstream change to the layer source. Click the button to reanalyze the footage.

Cancel

Cancels an analysis in progress. During analysis, status information appears next to the Cancel button.

Stabilization

Stabilization settings allow for adjusting the stabilization process.

Borders

Borders settings adjust how borders (the moving edges) are treated for footage that is stabilized.

Advanced

When Framing is set to one of the cropping options and the cropping gets extreme, a red banner appears saying, “To avoid extreme cropping set Framing to Stabilize Only or adjust other parameters”. In this situation, you can either set Framing to Stabilize Only, or Stabilize, Synthesize Edges. Other options include reducing the value of Crop Less <-> Smooth More, or reducing Smoothness. Or, if you are satisfied with the results, enable the Hide Warning Banner option.