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Upside-down Cameras: Selecting the Desired Solution
Many coordinate system setups can be satisfied in two or more different ways: completely different camera and tracker positions that are camera matches, and satisfy the constraints. This applies to object tracks as well as camera tracks.
To review, the most basic 3-point setup consists of a point locked to the origin and a point locked to a specific coordinate on the X axis, plus a third point locked to be somewhere on the ground plane (XY plane for Z-up). This setup can be satisfied two different ways. If you start from one solution, you can get the other by rotating the entire scene 180 degrees around the X axis. If the camera is upright in the first solution, it will be upside-down in the second. The third point will have a Y coordinate that is positive in one, and negative in the other (for Z-Up coordinates).
If you take this basic 3-point setup, change the setup of the second point from a lock to (20,0,0) to “On X Axis,” and add a separate distance (scale) constraint, there are now four different possible solutions: the different combinations of the second point’s X being positive or negative, and the third point’s Y coordinate being positive or negative.
SynthEyes offers two ways to control which solution is used. Without specific instructions, SynthEyes uses the solution where the camera is upright, not upside-down.
That handles the most common case, but if you need the camera upside-down, have a setup with four solutions, have a specific object orientation setup, or generally need one of the other solutions, you need to be more specific about what you want.
SynthEyes lets you specify whether a coordinate should be positive or negative (a polarity), for each coordinate of each constrained tracker. The Coordinate System Control panel has buttons next to the X, Y, and Z spinners. The X button, for example, sequences from X to X+ to X-, meaning that X can have either polarity, that X must be positive, or that X must be negative.
If there are two solutions, you should set up a polarity for an axis of one point; if there are four solutions, set the polarity for one axis of two points. For example, set a polarity for Y of the 3 rd tracker, and an X polarity for the 2 nd (on-axis) tracker.
As SynthEyes considers different possible orientations, it examines whether the resulting coordinates will satisfy the polarity constraints you have set up, and disallowed solutions are ignored.
You can put a polarity constraint on any tracker, whether or not it is otherwise used in the coordinate system setup.
If you put a polarity constraint on a tracker that has a target point, the polarity constraint is based not directly on the tracker's coordinate, but on the difference between the tracker's coordinate and its target's coordinate. This is true whether or not any axes of the tracker are constrained to the target.
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