A descriptor tells TVCal how to make the jump from detector to object space. In other words: once TVCal finds the features its looking for, how can it translate that into the information it needs to proceed with calibration? Descriptors provide that translation.
Compatible With: Checkerboards, Markerboards, and AprilGrids
Some detectors are constructed with enough information that the descriptors can be inferred. Take checkerboards as an example: the length between every corner is regular and consistent. If TVCal can detect the whole board, it can infer the metric coordinates of every corner in the board.
In this case, a Descriptor isn't needed; the Detector gets the job done on its own.
Compatible With: Markers
Auto-generate Target List with Tangram Vision Basic
If your Markers are in a single plane, or part of a printed board that isn't part of our common Detector types, you can use the Object Space Generator program to create your target list for you.
If regularity isn't guaranteed in a marker detector, then a target list is needed. This list gives additional metric information on every corner of a detected marker that relates it back to the location of a 3D point.
Note the ordering of corners for your marker dictionary: