List of Operators ↓
This chapter describes how to calibrate different multi-view camera setups.
In order to achieve high accuracy for your measuring tasks you need to calibrate your camera setup. In comparison to a single-camera setup, some additional requirements apply to the calibration of a multi-view camera setup. The following paragraphs provide explanations regarding the calibration of multi-view camera setups. For general information on camera calibration please refer to the chapter reference Calibration.
Before the actual calibration can be performed, a calibration data model must be prepared (as described in Calibration). For setups with multiple cameras, these additional aspects should be considered:
The number of cameras in the setup and the number of used
calibration objects can be set when calling
When specifying the camera type with
note that only cameras of the same type can be calibrated in a single
Configure the calibration process, e.g., specify the
reference camera, using
. You can also specify
parameters for the complete setup or just configure parameters of
individual cameras as well as calibration object poses in the setup.
The calibration performed by
depends on the
camera types that are involved in the calibration setup. While different
camera setups require specific conditions when acquiring images, the basic
steps of the calibration procedure for setups including projective and/or
telecentric cameras are similar:
Building a chain of observation poses:
In the first step, the operator
tries to build a
valid chain of observation poses, that connects all cameras and calibration
object poses to the reference camera. Depending on the setup, the conditions
for a valid chain of poses differ. For specific information see the
respective paragraphs below.
If there is a camera that cannot be reached (i.e., it is not observing any calibration object pose that can be connected in the chain), the calibration process is terminated with an error. Otherwise, the algorithm initializes all calibration items' poses by going down this chain.
In this step,
performs the actual
optimization for all optimization parameters that were not explicitly
excluded from the calibration.
Based on the so-far calibrated cameras, the algorithm corrects all
observations that contain mark contour information
Then, the calibration setup is optimized anew for the corrections to take
effect. If no contour information was available, this step is
Compute quality of parameter estimation:
In the last step,
computes the standard
deviations and the covariances of the calibrated internal camera parameters.
The following paragraphs give further information about the conditions specific to the camera setups.
For a setup with projective area scan cameras, the calibration is performed in the four steps listed above. The algorithm tries to build a chain of observation poses that connects all cameras and calibration object poses to the reference camera like in the diagram below.
Possible projective area scan cameras are:
For a setup with telecentric area-scan cameras, similar to projective area scan cameras, the same four steps that are listed above are executed. In the first step (building a chain of observation poses that connects all cameras and calibration objects), additional conditions must hold. Since the pose of an object can only be determined up to a translation along the optical axis, each calibration object must be observed by at least two cameras to determine its relative location. Otherwise, its pose is excluded from the calibration. Also, since a planar calibration object appears the same from two different observation angles, the relative pose of the cameras among each other cannot be determined unambiguously. Therefore, there are always two valid alternative relative poses. Both alternatives result in a consistent camera setup which can be used for measuring. Since the ambiguity cannot be resolved, the first of the alternatives is returned. Note that, if the returned pose is not the real pose but the alternative one, then this will result in a mirrored reconstruction.
Possible telecentric area scan cameras are:
For a mixed setup with projective and telecentric area scan cameras, the algorithm performs the same four steps as enumerated above. Possible ambiguities during the first step (building a chain of observation poses that connects all cameras and calibration objects), as described above for the setup with telecentric cameras, can be resolved as long as there exists a chain of observation poses consisting of all perspective cameras and a sufficient number of calibration objects. Here, sufficient number means that each telecentric camera observes at least two calibration objects of this chain.
For a setup with line scan cameras (
'line_scan_telecentric'), the following restriction exists:
only one camera can be calibrated and only one calibration object
per setup can be used.
Finally, for calibration plates with rectangularly arranged marks (see
) all observations must contain the projection
coordinates of all calibration marks of the calibration object. For
calibration plates with hexagonally arranged marks (see
) this restriction is not applied. You can find
further information about calibration plates and the acquisition of
calibration images in the section “Additional information about the
calibration process” within the chapter Calibration.
If more than one camera is calibrated simultaneously, the value
is more difficult to judge.
As a rule of thumb,
should be as small as
possible and at least smaller than 1.0, thus indicating that a
subpixel precise evaluation of the data is possible with the
calibrated parameters. This value might be difficult to reach in
particular configurations. For further analysis of the quality of
the calibration, refer to the standard deviations and covariances of
the estimated parameters.
The results of the calibration, i.e., internal camera parameters,
camera poses (external camera parameters), calibration objects poses
etc., can be queried with
Note that the poses of telecentric cameras can only be determined up to a displacement along the z-axis of the coordinate system of the respective camera (perpendicular to the image plane). Therefore, all camera poses are moved along this axis until they all lie on a common sphere. The center of the sphere is defined by the pose of the first calibration object. The radius of the sphere depends on the calibration setup. If projective and telecentric area scan cameras are calibrated, the radius is the maximum over all distances from the perspective cameras to the first calibration object. Otherwise, if only telecentric area scan cameras are considered, the radius is equal to 1 m.