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 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 create_calib_data
When specifying the camera type with set_calib_data_cam_param
Configure the calibration process, e.g., specify the
reference camera, using set_calib_data
The calibration performed by calibrate_cameras
Building a chain of observation poses:
In the first step, the operator calibrate_cameras
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.
First optimization:
In this step, calibrate_cameras
Second optimization:
Based on the so-far calibrated cameras, the algorithm corrects all
observations that contain mark contour information
(see find_calib_object
Compute quality of parameter estimation:
In the last step, calibrate_cameras
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.
| (1) | (2) |
Possible projective area scan cameras are:
'area_scan_division'
'area_scan_polynomial'
'area_scan_tilt_division'
'area_scan_tilt_polynomial'
'area_scan_tilt_image_side_telecentric_division'
'area_scan_tilt_image_side_telecentric_polynomial'
'area_scan_hypercentric_division'
'area_scan_hypercentric_polynomial'
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:
'area_scan_telecentric_division'
'area_scan_telecentric_polynomial'
'area_scan_tilt_bilateral_telecentric_division'
'area_scan_tilt_bilateral_telecentric_polynomial'
'area_scan_tilt_object_side_telecentric_division'
'area_scan_tilt_object_side_telecentric_polynomial'
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.
| (1) | (2) |
Setups with telecentric line scan cameras
('line_scan_telecentric') behave identically to setups
with telecentric area scan cameras and the same restrictions and
ambiguities that are described above apply. For this type of
setup, two possible configurations can be distinguished. In the
first configuration, all cameras are mounted rigidly and
stationary and the object is moved linearly in front of the
cameras. Alternatively, all cameras are mounted rigidly with
respect to each other and are moved across the object by the same
linear actuator. In both cases, all cameras share a common motion
vector, which is modeled in the camera coordinate system of the
reference camera and is transformed to the camera coordinate
systems of all other cameras by the rotation part of the
respective camera's pose. This configuration is assumed by
default. In the second configuration, the cameras are moved by
independent linear actuators in different directions. In this
case, each camera has its own independent motion vector. The type
of configuration can be selected with set_calib_data
| (1) | (2) | (3) |
Note that two different stereo setups are common for telecentric line scan cameras. For both setups, a linear, constant motion is assumed for the observed object or the camera system respectively.
For along-track setups one camera is placed in front, looking in backwards direction, while the second camera is mounted behind, looking forwards, both at an suitable angle in respect to the motion vector.
The cameras in an across-track setup are all directed perpendicular to the motion vector, while the viewing planes are approximately coplanar. Therefore, the depth of field is rather limited. Precise measurements are only possible in areas where the depth of field of the individual cameras overlap.
| (1) | (2) |
For setups with projective line scan cameras
('line_scan'), 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
gen_caltabcreate_caltab
If more than one camera is calibrated simultaneously, the value
of ErrorError
The results of the calibration, i.e., internal camera parameters,
camera poses (external camera parameters), calibration objects poses
etc., can be queried with get_calib_data
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.
calibrate_camerasclear_calib_dataclear_camera_setup_modelcreate_calib_datacreate_camera_setup_modeldeserialize_calib_datadeserialize_camera_setup_modelget_calib_dataget_calib_data_observ_contoursget_calib_data_observ_pointsget_camera_setup_paramquery_calib_data_observ_indicesread_calib_dataread_camera_setup_modelremove_calib_dataremove_calib_data_observserialize_calib_dataserialize_camera_setup_modelset_calib_dataset_calib_data_calib_objectset_calib_data_cam_paramset_calib_data_observ_pointsset_camera_setup_cam_paramset_camera_setup_paramwrite_calib_datawrite_camera_setup_model