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Using a Camera Height Measurement
Suppose the camera is sitting on a moving dolly in a studio, and you measured the camera (lens’s) height above the floor, and you have some trackers that are (exactly) on that floor. You can use the height measurement to set up the scene size as follows:
1. Show the seed path: View/Show Seed Path menu item
2. At frame 0, position the camera at the desired height above the ground plane: 2 meters, 48 inches, whatever.
3. Turn on the U/D button on frame 0, turn it back off at frame 1 .
4. Set up a main coordinate system using 3 or more trackers on the floor. Make sure to not create a size constraint in the process: if using the *3 button on the Coordinate system panel or the Coord button on the Summary panel, select the 2 nd (on-axis) tracker, and in the Coordinate panel, change it from Lock Point (at 20,0,0) to On X Axis or On Y Axis.
5. Solve with Go! on the Solver panel
Note that you can use whatever more complex setup you like in step 4, as long as it completely constrains both the translation and rotation, but not the size.
WARNING: You might be tempted to think “Hmmm, the camera is on a dolly, so the entire path must be exactly 43 inches off the floor, let me set that up!” (by not turning
U/D back off). But this is almost always a bad idea! The obvious problem is that the dolly track is never really completely flat and free of bumps. If the vertical field of view is 2 meters, and you are shooting 1080i/p HDTV, then roughly your track must be perfectly flat to 1 millimeter or so to have a sub-pixel impact. If your track is that flat, congratulations.
The conceptually more subtle, but bigger impact problem is this: a normal tripod head puts the camera lens very far from the center of rotation of the head—roughly 1 foot or 0.25 meter. As you tilt the head, the position of the camera increases and decreases up to that much in height! Unless your camera does not tilt during the shot, or you have an extra-special nodal-pan head, the camera height will change dramatically during the shot.
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