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You must set up a complete set of constraints position locks, orientation, and distance (scale) for both the camera and object (a set for each object, if there are several). Frequently, users ask why a second set of constraints for the object is required, when it seems that the camera (background) constraints should be enough.
However, recall a common film-making technique: shooting an actor, who is close to the camera, in front of a set that is much further away. Presto, a giant among mere mortals! Or, in reverse, a sequel featuring yet another group of shrunken relatives, name the variety. The reason this works is that it is impossible to visually tell the difference between a close-up small object moving around slightly, and a larger object moving around dramatically, a greater distance away. This is true for a person or a machine, or by any mathematical means.
This applies independently to the background of a set, and to each object moving around in the set. Each might be large and far, or close and small. Each one requires its own distance constraint, one way or another.
The object’s position and orientation constraints are necessary for a different reason: they define the object’s local coordinate system. When you construct a mesh in your favorite animation package, you can move it around with respect to a local center point, about which the model will rotate when you later begin to animate it. In SynthEyes, the object’s coordinate constraints define this local coordinate system.
Despite the veracity of the above, there are ways that the relative positioning of objects moving around in a scene can be discerned: shadows of an object, improper actor sightlines, occasions where a moving object comes in contact with the background set, or when the moving object temporarily stops. These are assumptions that can be intellectually deduced by the audience, though the images do not require it. Indeed, these assumptions are systematically violated by savvy filmmakers for cinematic effect.
However, SynthEyes is neither smart nor stupid enough to make assumptions, nor to know when they have been violated. Consequently, it must be instructed how to align and size the scenes in the most useful fashion.
The alignment of the camera and object coordinate systems can be determined independently, using the usual kinds of setups for each.
The relative sizing for camera and object must be considered more carefully when the two must interact, for example, to cast shadows from the object onto a stationary object.
When both camera and object move and must be tracked, it is a good idea to take on-set measurements between trackable points on the object and background. These measurements can be used as distance constraints to obtain the correct relative scaling.
If you do not have both scales, you will need to fix either the camera or object scale, then systematically vary the other scale until the relationship between the two looks correct.
If the shot is suitable, you can use the Moving-Obj Path or Moving-Obj Position Phases to set the relative scale of the camera and object. In both cases, the camera must be moving, not locked or on a tripod. Use the Moving-Obj Path phase when the object comes to a stop for some portion of the shot, or less desirably, has a section where it travels in an exactly straight line. Use the Moving-Obj Position phase when trackers on the moving object come very close to, or become aligned with, trackers on the camera.
For further details, see the phase overview and the Phase Reference manual
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