What Is Geometric Hierarchy Tracking?

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‌What Is Geometric Hierarchy Tracking?

Here's the quick breakdown of the words Geometric Hierarchy (GeoH) tracking:

 Geometric : one or more known meshes. The number and placement of vertices matters little to function or performance, even with 100s of thousands of facets, to be able to accommodate large character models .

 Hierarchy : a specific description of how the parts of the overall object can move with respect to one another.

 Tracking : where does everything go, across the duration of the shot, while maintaining the relationships described by the hierarchy?

But here's a breakdown of the major feature sets of GeoH tracking:

 Geometry tracking: tracking based directly on meshes

 Hierarchical tracking: a rig is animated, maintaining specific relationships between the parts, based on geometry or regular (ie supervised) tracking.

 Deformation : a mesh is deformed, based on a hierarchy.

So while GeoH tracking can involve tracking geometry directly, that may not be the most common case. GeoH tracking using supervised trackers doesn't require that you have an existing mesh structure, and can be used under very adverse conditions.

Here are some examples where we might apply geometric hierarchy tracking:

 a person, with a body connected to each upper arm connected to a forearm connected to the hand then to fingers, etc while separately the body connects to each thigh then to a calf then to a foot;

 facial parts (eyebrows, chin, cheeks, lips) deforming into a sequence of expressions;

 secondary animations such as jiggly body parts;

 similarly for animals: cats, horses, birds, etc;

 machines, such as a car with doors and hood that open and wheels that turn and bounce up and down;

 objects moving with limited degrees of freedom, such as rolling balls;

 perhaps even a hierarchy of a tree blowing in the wind.

It's a key point for GeoH tracking that the tracking preserves the exact dimensions and interrelationships of the hierarchy: tracking jitter or errors won't cause a hand to detach from an arm, or the car door to twist out of the plane of its hinge. Those would be physically impossible, and GeoH tracking won't let them happen.

Those geometric constraints can help tracking as well: tracking interrelated parts can produce more data than tracking either part individually would, or allow less data to be used than would otherwise be required. GeoH tracking allows us to take better advantage of what we know.

The result of GeoH tracking is a 3-D path for each of the component parts of the hierarchy. If the hierarchy is composed of individual mesh pieces, those pieces will be carried along.

If a mesh is affected by multiple parts of the hierarchy, the mesh will be deformed to match the motion of the hierarchy. For example, a single mesh model of an actor may be deformed by a complete hierarchy to match the live action shot. Self-occlusion and occlusion by other objects is taken into account. All of this takes place inside SynthEyes as part of GeoH tracking.

But back to the tracking itself. GeoH tracking itself is based on the shape of the mesh; the mesh is repositioned and deformed continually over the duration of the shot, to match the imagery. In that way it's an elaboration on planar tracking, except that it works for any kind of mesh (potentially very large meshes).

As we'll see, GeoH tracking can also combine with regular object tracking, regular 2D trackers, or already-solved 3D mocap trackers for powerful hybrid tracking capabilities that are very useful for secondary animation.

You can also use image-based GeoH tracking for camera tracking with a reference mesh, by first treating it as an object track, then use the Move Object Path to Camera script.

Once GeoH tracking is complete, the scene can be exported, including individual component paths, positions and joint angles between the parts of the hierarchy, and deformed meshes, typically as point cache files. The exports can take the form of motion-capture BVH files, Filmbox FBX files, Alembic ABC files, or application-specific exports such as Blender.

We think you'll find the GeoH tracker to be enormously powerful and flexible, and let you do some pretty amazing things! So please read on.

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