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A planar tracker is a rectangle, but you can refine its shape using In-plane Masks, created using controls at the bottom of the Planar control panel. You draw the in-plane mask on the 2-D or 3-D planar tracker itself: if you move or re-shape the tracker, the in-plane masks are carried along and reshaped to match.
Create a mask by clicking the create button in the In-Plane Masks area of the Planar Options panel. Then begin clicking within the planar rectangle in the camera view to create control points for the mask. Points are corners by default; to make a point smooth, hold down control while clicking to create it. After you've created the last one, right-click to exit that mode.
After you finish creating a mask, you can change control points from smooth to corner or vice versa by double-clicking them. You can move a control point, though since the in-plane masks correspond to static features on the plane itself, movements are not animated. You can add more control points by shift-left-clicking the curve (again, control for smooth curves), or delete a control point by shift-right-clicking it. A mask is deleted when it has less than two points, or you click the red X in the In-Plane Masks area.
You can create positive or negative in-plane masks: positive masks say that the interior pixels should be considered in the tracking region, while negative masks are excluded from tracking. Use a positive mask to track only within an irregular area. Use a negative mask where you need hole, for example over a window in the side of a building, if the action happening within the window would compromise the track.
In-plane masks are layered: the order matters and can be adjusted with buttons.
This should be familiar to users of compositing software such as Photoshop, AfterEffects, Illustrator, etc.
The bottom-most mask (ie the first one you create, unless you have changed the order) is treated specially: if the bottom mask is positive, the exterior of the mask is untracked and only the interior is tracked, subject to additional layers added on top. If the bottom mask is negative, then the entire exterior of the mask (planar rectangle) is tracked and the interior is untracked, subject to additional layers added on top. This control is really what you expect and require, though it may surprise you initially as you are setting up and configuring the masks.
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