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Moving-Obj Position* (Coordinates)
This phase allows you to generate an overall scaling for a moving object such that some of its trackers match specified world-coordinate constraints as best as possible, over a specific range of frames ranging from a single frame to the entire shot.
Background: when you have a shot with both moving objects and moving cameras, you need to have accurate size control for both the moving object and camera, otherwise you get the "Honey I Shrunk the Kids" effect—it is impossible to tell the difference between a small object moving a little and a big object moving a lot. Getting both scales to match is often difficult. However, if we know more about the situation, we can determine the relative scale. It's a good trick!
Here's the idea: we select one or more trackers on the moving object and pick the scale to make them match either the 3D position of a specific camera tracker, or some numeric coordinates from the Moving-Obj Position phase's control panel. We can make all three axes match, or maybe just Y or Z.
Example: a flying craft slams into the wall, leaving a dent. We object-track the craft, and camera-track the scene, including the dent. We set up the Moving-Obj Position so that the nose of the flying craft matches the world position of the dent at the instant of the collision. Note that neither has to be visible at the instant of collision!
Example: a car drives on a floor. We object-track the car, using offset tracking to track the point of contact of the wheel and ground. We camera-track the scene, using dirt and oil on the floor. We could set up the Moving-Obj Position phase to match the height of the contact point to the height of the ground, but that will be inaccurate unless the camera view is substantially vertical. Instead, we set it up to match the wheel's Y coordinate to the Y coordinate of an oil spot (Y for Z Up, match Z for Y-Up) right as the wheel drives by that spot. This is a better choice since the uncertainty in object scaling (which we want to eliminate) is towards or away from the camera.
To set the tracker(s) on the moving object, click the Store trackers button. To set the target camera tracker, also click the Store trackers button. The phase looks at the type of the selected trackers to determine what you mean when you click the button. Note that you should generally not try to match X, Y, and Z of several different trackers to the same coordinate.
Multiple object trackers is mainly useful for matching all of them to Z=0, say. Typically X&Y&Z matches match one tracker to another, or to specific coordinates. If you inadvertently set a target tracker, but wish to use numeric coordinates instead, click the Clear target button. The target tracker's coordinates are used whenever a tracker is present, and the numeric coordinates whenever there is not a target tracker.
You must configure the starting and ending frame numbers of the portion of the shot during which the moving object's trackers should match. Typically that's only a few frames.
Note that in some circumstances, a Moving-Obj Position phase can do the work of a Moving-Obj Path phase—ie if the camera is moving while the object is stopped, you can track a single image feature as both camera and object tracks, then use the Moving-Obj Position phase to make them as close as possible in 3D. The Moving-Obj Position phase is more flexible because it does not require that the object ever stop.
Fine Print: You should set up your shot so that its rough initial scaling is within a factor of ten of the correct scaling. The phase considers only solutions that are at most 10x larger or 1/10th smaller than the current size, to avoid spurious crazy solutions.
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