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A Straight Dolly Track Setup
If your camera rides a straight dolly track, you can use the length of that track to set the scale, and almost the entire coordinates system if desired. While the camera height measurement setup discussed above is simpler, it is appropriate mainly for a studio environment with a flat floor. The dolly track setup here is useful when a dolly track is set up outdoors in an environment with no clearly-defined ground plane—in front of a hillside, say.
For this setup, you should measure the distance traveled by the camera head down the track, by a consistent point on the camera or tripod. For example, if you have a 20’ track, the camera might travel only 16’ or so because there will be a 2’ dead zone at each end due to the width of the tripod and dolly. Measure the starting/ending position of the right front wheel, say.
Next, clear any solved path (or click View/Show seed path), and animate the camera motion, for example moving from 0,0,0 at the beginning of the shot to 16,0,0 at the end (or wherever it reaches the maximum, if it comes back).
You now have two main options: A) mostly tracker-based coordinate setup, or B) mostly dolly-based coordinate setup, for side-of-hillside shots.
For setup A, turn on only the L/R camera axis constraint checkbox on the first and last frames (only). The X values you have set up for the camera have set up an X positioning for the scene, so when you set up constraints on the trackers, they should constrain rotation fully, plus the front/back and up/down directions—but not the L/R
direction since that would duplicate and conflict with the camera constraint (unless you are careful and lucky).
For setup B, turn on L/R, F/B, and U/D on the first and last frames (only). You should take some more care in deciding exactly what coordinate values you want to use for each axis of the animated camera path, because those will be defining the coordinate system. [By setting keys only at the beginning and end of the shot, you largely avoid problems with the camera tilting up and down—at most it tilts the overall coordinate system from end to end, without causing conflicting constraints.]
If the track is not level from end to end, you can adjust the beginning or ending height coordinate of the tracker as appropriate. But usually we expect the track to have been leveled from end to end.
With X, Y, and Z coordinates keyed at the beginning and end of the shot, you have already completely constrained translation and scale, and have constrained 2 of the 3 rotation axes. The only remaining unconstrained rotation axis is a rotation around the dolly.
To constrain this remaining rotation requires only a single additional tracker, and only its height measurement! On the set, you should measure the relative height of a trackable feature compared to the track (usually this will be to the base of the track, so you should also measure the height of the camera versus the base). You can measure this height using a level line (a string and a clip-on bubble level) and a ruler.
On the Coordinate System Control Panel, select the tracker and set it to Any XY Plane and set the Z coordinate (for Z-up mode), or select Any XZ Plane and set the Y coordinate (for Y-up mode).
Now you’re ready to go! This setup is a valuable one for outdoor shots where a true vertical reference is required, but the features being tracked are not structured (rocks, bushes, etc).
Again, we recommend not trying to constrain the camera to be exactly linear, though you can easily set this up by locking Y and Z to be fixed for the duration of the shot, with single-frame locks on X at the beginning and end of the shot. This setup forces the camera motion to be exactly straight, but moving in an unknown fashion in X. Although the motion will be constrained, the setup will not allow you to use fewer trackers for the solve.
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