Usually the measurement programme starts with a group of commands to align the measurement coordinate system with the workpiece. The advantage is that the coordinates read on the machine will correspond to the coordinates show in a drawing or CAD model. In this way it is more clear to the operator where the various measurement points are and if the results are reasonable and therefore it avoids mistakes.
Alignment with mechanical probes can be done in many ways. E.g a common way is to probe a plane, a line (aqxis) and a point (origin). These do not have to be in the same plane as long as the line and the point are projected to the plane. Object suitable for probing with the camera system more likely have the reference marks for the definition of the coordinate system in one plane. E.g. the surface of the object is the reference plane while markers on that surface indicate the origin and the direction of an axis. In that case the alignment procedure can be reduced to the measurements of three or four points. Since focussing is needed for each measurement, as long as the workpiece coordinate system is not yet defined, the procedure is much faster if less points need to be measured
Example 1, Alignment with three points in a plane: If the origin of the workpiece coordinate system is marked in such a ways that the vision software (QVP) can measure and the direection of one of the axes in the plane can be defined by another point then only three points need to be measured.
Example 2, Alignment with four points in a plane: