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Using Create-circle to align to circular features
You may need to track a circular object, such as a manhole cover in a road or a dinner plate on a table. If you know that the object is truly circular, that is useful information that can be used to enhance 3-D accuracy.
Immediately after you create the planar tracker, change the planar motion type to "3D Known Aspect" and the 3-D Aspect (on the Planar Options panel) to exactly one (1.0).
Next, click the circle icon in the In-Plane Masks section of the control panel. This will create an exactly-circular in-plane mask. Depending on the situation, you can keep it as positive or negative polarity—and you can delete it after initial alignment.
With the aspect at 1.0 and the circular reference mask, adjust the four corners of the planar tracker to exactly position the circular mask over your circular image feature in the main camera view. (Do not adjust the mask handles on the circle itself!)
You can use ALT/command drag of the corners to help position them, in either the camera view or tracker mini-view. The pyramid apex can also be helpful.
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