If you were to step through the frames now (or, particularly, in a more complicated animation where the pivot moved multiple times), you would probably see some jiggle.  This is because the movement of the bones is controlled by motion splines, and the influence of the two keyframes extends past the time between them.  If the interpolation is set to Spline, then the movement doesn't just linearly proceed between the two points: it bends to try to adjust to curvature.  While that is what we want in general animation, it certainly isn't what we want in this case (since the bends will only make is harder to get the movement of the Patriarch and the Compensator to cancel out precisely).

To repair this problem, double click on the Patriarch channel in the Action.  This will bring up a channel window with the Patriarch's channels (presumably only X-Translate, Y-Translate and Z-Translate).  Drag the Patriarch Compensator channel into the channel window, and it will appear along with the Patriarch.

Select each channel and change its interpolation on all point to be linear.  This will guarantee that the movement of the two bones will be simple, and cancel out perfectly.

Close the channel window, and you'll be set to finish up the pivot step.

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