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Locked vs Unlocked Joints
Once you have the GeoH objects (pivots) placed properly, you'll need to configure them to specify which joints can move (be tracked), and which will always be
fixed. This is done with the six (seven including Dist.) Active vs Locked buttons on the GeoH panel.
When the button is on (blue), the joint axis locked to the value given by the corresponding joint value spinner. When the button is off, the joint is unlocked and will be determined as a result of tracking.
Warning! Be sure to double-check which rotation axis is which , by wiggling the value spinner! Because they depend on the setup of the pivot, relying on the names is unwise and freeing the wrong joint will produce non-useful results.
The axis lock buttons are animated, so that axes can be unlocked or locked on various frames during the shot. For example, if there are some frames where tracking is problematic, you can lock the joint(s), and hand-animate the spinner values in order to control the object.
Note: Each joint axis has minimum and maximum values for guidance, accessible via the Joint/Locks dropdown. The limits are not enforced. For position values, they range from -100 to +100. If the required range of values greatly exceeds that, you should adjust the minimum and maximum values to more plausible values, otherwise the tracking/solver may decide to do nothing.
For a free-flying root object, you'll want to have all six axes unlocked. You should never have child objects with all six axes unlocked. That's saying that the child's motion is unrestricted—it should be a free-flying root object in its own right.
For child objects, you'll need to think carefully about which to unlock. For a simple elbow, there will be only a single unlocked joint. For a jiggling belly, you may want two unlocked axes, one horizontal, one vertical. They may be strictly translational, or you may want to have virtual pivot points deep inside the body, or even outside the back. There are artistic decisions, based on the type of motion that happening in the shot.
Note: if you will be exporting BVH files, beware of having unlocked translational axes for child objects. They require a non-standard use of the BVH file format which other applications may not be able to read. Use rotational joints with a long pivot radius instead.
You need to avoid having too many unlocked axes on small objects. Consider a cheek: while it might be able to move in three translational directions, that is likely unwise. There will be too little perspective shift due to the cheek moving towards and away from the camera to obtain useful tracking data in that direction. Similarly, small rotations of a small patch may not produce enough change in the image to be tracked. These are fundamental physics limitations, not a software limitation (multiple cameras in a motion capture setup would be required).
Accordingly, you should base your decision to unlock the joints of child objects based on whether useful information can be obtained. Clever placements of the pivots may also help produce data when it might not otherwise be possible, when the object moves in some axes/planes but not others. You can align the pivot to take advantage of that. See the section "A Bouncing Ball" for an example.
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