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The phase rotates the entire scene so that two specified trackers fall on the horizon—the ground plane at infinity—on a given frame. The direction towards one or the other of the two trackers, or any direction between them, is made to fall along the Y axis(Z-up phase) or Z axis (Y-up coordinates). This completely determines the 3 rotational degrees of freedom.
In a typical use, consider a beach scene with some trackers on the beach, and a few far away features, such as boats way out on the water. If you want to add additional boats that are not near the camera, the nearby trackers may not be define a sea level sufficiently accurate to specify accurately specify the sea level all the way out to the horizon. Typically the horizon will be somewhat tilted and inaccurately placed. If you can track two features well out to sea, you can use this phase to set the sea level precisely.
The two selected trackers can be Far, and are essentially considered to be, but do not have to be configured as Far trackers. If the camera is well above the ground level/horizon, such as an aircraft, you may have to use the Pitch Adjust to compensate.
This phase sets the overall heading, putting the two trackers at the "back" of the scene, as well as leveling them. Though the direction adjust (below) can be used for small adjustments to that, for major adjustments to the scene heading, connect this phase to a Set Heading phase to spin the scene about the vertical axis if you have some trackers in the foreground you wish to align.
This phase does not set up any coordinate constraints to re-establish the same coordinate system if the phase's output is re-solved. If you need to re-solve, you may need another copy of this phase, or configure some other phases to do what you want.
The Direction Adjust (Dir. Adjust) is a value that nominally ranges from 0 to 1, pointing towards the left tracker if the value is zero, or the right tracker if the value is one. At the default
0.5 value, the coordinate axis aims in between the two. The Direction Adjust value can range anywhere between 0 and 1, or even outside that range as well.
The Roll Adjust and Pitch Adjust values make small tweaks to the alignment. The roll adjust is useful if the two available trackers are not exactly on the horizon. Similarly for the pitch adjust, though the pitch adjust can also be used to compensate if the camera is above the ground, and the horizon trackers are not all that far away.
Important note: if your scene's camera move has a substantial change in direction over the length of a shot, you might be tempted to use two or more Set Horizons, configured for different frame numbers during the shot. For example, if the camera begins by shooting towards the north, and later shoots towards the west, you might want to set the horizon in both places.
However, this is fraught with peril. If your scene is accurately and consistently solved, it may work OK. However, if the scene has systematic errors due to lens distortion or rolling shutter, the horizon levels will not be self-consistent, and the two Set Horizon's will interfere with one another. There will be no possible way to align both simultaneously. Such scenes must be addressed by correcting the underlying problem, or applying constraints to the camera roll.
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