GeoH Track with 3D Motion Capture Trackers

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‌GeoH Track with 3D Motion Capture Trackers

In a 2D hybrid setup, the GeoH tracker adjusts the rig so that the 3D tracker seed locations on the mesh rig match up with 2D locations of the trackers in the image.

A 3D hybrid setup takes this one step further, starting with tracker paths that are already 3D as a result of a multi-camera motion capture (mocap) setup. It adjusts the GeoH rig so that the 3D tracker seed locations on the mesh match up in 3D on each frame with the 3D world coordinate path from motion capture solve.

Why is this useful? The SynthEyes mocap solve produces an animated point cloud, with many points and a path for each individual point. That cloud can be exported and used by some other software. (See "Motion Capture and Face Tracking" in the SynthEyes User Manual for information.)

But often we'd like to know how to animate a complete specific rig, to be able to do a BVH export with joint angles. The 3D hybrid setup bridges the point cloud and rig, producing the rig configuration that best matches the tracked point cloud.

First, a quick review of what comprises a motion capture shot and solved scene.

 Two or more cameras that have been precalibrated, so that the exact field of view and relative position of each is known. Potentially they can move, or be a 3D stereo pair, but that relative positioning is always known, for example via camera solves.

 On each camera, a "moving object" placeholder, which doesn't actually move. The moving object is configured in the "Individual mocap" solving mode.

 On each moving object, one or more, typically many, trackers. Trackers on all but the first moving object are linked back to the corresponding tracker on the first moving object.

 Each group of trackers that are linked together have a single 3D path, ie a position on each frame of the shot.

To expand on the tracker linking, we might have a tracker LTipOfNose on the left camera, and RTipOfNose on the right camera, with RTipOfNose linked to LTipOfNose, identifying that they track the same point. (The linking is seen on the Coordinates panel or Constrained Points view.) There's a single path in world coordinates for LTipOfNose/RTipOfNose.

We're going to take those paths and use them to drive the rig. The workflow proceeds as follows:

 Starting with the solved mocap scene, add the GeoH rig, either by creating it from scratch, or by using File/Merge to bring it in.

 Move the trackers from the mocap moving objects to be children of the desired objects in the rig (via the Hierarchy View or the parentage selector on the Coordinates panel).

 Use "Place" mode in the Perspective view to drop each tracker onto its corresponding location on the mesh, which creates Lock coordinates for it. (It's easy to use the up and down arrows to sequence through the trackers and then drop them.) The usual initial alignment is not required.

 Enable solving for the rig, ie by unlocking joints and/or removing Keys ... no keys or locks should be necessary on any frames.

 Track through the entire shot.

At this point, you'll have the full set of tracking data, and can export deformed meshes, BVH files, etc as desired.

If you need to refine the tracking data, you can move the trackers back to the original mocap moving objects, re-solve, then move them back. It would also be possible to change the mode of all the GeoH objects, but that is likely more error-prone.

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