In order to reconstruct the relationship of an x-ray's focal spot to the calibration object, a framespec (or calibration specification) file is needed. The framespec file itself is a set of xyz coordinates describing the location of the points in your calibration object relative to a single point (the origin). Therefore, each different calibration object has its own, different, framespec file.
Together with the calibration object, the framespec is needed to create DLT coefficients and mayaCams, and the framespec file can also be used to check your calibration.
1. Number the points on your calibration object and designate an origin point and the orientation of the axes relative to this origin.
2. Measure the distance (along each axis) between adjacent points--if this information is not already known from building the calibration cube.
3. Determine the xyz coordinates of the calibration cube points (in cm) relative the origin point.