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Working with Multiple Scenes
When using AprilTags to ensure common coordinates for several different scenes, typically you will use one overall shot survey that includes an extensive wide view of the set as a reference. You’ll set up the coordinate system for that reference scene and use it to construct 3-D set models and inserted virtual set objects.
Then, you track and solve subsequent shots, then impose the reference coordinate frame upon them. There are at least three approaches to doing so:
1. Export the tags’ 3-D coordinates from the reference SNI file using File/Export/Plain Text/Plain Trackers, then read them into the additional shots using File/Import/Tracker Locations. When exporting the tag data, temporarily turn off the Exportable checkbox (on the 3-D panel) for the non-Tag trackers, so that only the tags are exported (or edit the output file with any text editor).
2. Open the reference scene in SynthEyes, then Shot/Add Shot the new shot. When running the AprilTags Detector, select the “Copy coordinates” (which is the default) option of the Look for links control. It will copy the solved XYZ coordinates from the reference shot tags into the added shot. You can continue adding more shots, or you could delete the reference shot from the scene. Either way, don’t overwrite the original reference scene.
3. Open the reference scene in SynthEyes, then Shot/Add Shot the new shot. When running the AprilTags Detector, select the “Make link” option of the Look for links control. Instead of copying the coordinates to the non-reference tags, each non-reference tag tracker will have its Target point (on the coordinates panel) set to the matching tracker on the reference shot. This links the two trackers so that any change in the reference tag will be reflected in any non-reference trackers linked to it. This is more useful if you load several live shots into the same scene file, in case you change the reference. It is also necessary if you want the reference and live shots to be solved as a group, ie if there is no true gold reference. You can keep the reference scene disabled so that it’s coordinates aren’t changed due to the additional scene.
Tip : If you use option 2 or especially 3, turn on “Prefix name with camera” on the AprilTags Detector control panel, so that each tracker will have a different name, based on the camera. This may help avoid your confusion over exactly which Tag3 we’re talking about, it will be Cam01Tag3 vs Cam02Tag3.
With any of these three methods, you might enhance overall repeatability by setting the lens field of view to the same value as the reference, if they were take with the same camera and lens settings.
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