Extract the 2D calibration marks from the image and calculate initial values for the exterior camera parameters.

find_marks_and_pose is used to determine the necessary input data for the subsequent camera calibration (see camera_calibration): First, the 2D center points [RCoord,CCoord] of the calibration marks within the region CalTabRegion of the input image Image are extracted and ordered. Secondly, a rough estimate for the exterior camera parameters (StartPose) is computed, i.e., the 3D pose (= position and orientation) of the calibration plate relative to the camera coordinate system (see create_pose for more information about 3D poses).

In the input image Image an edge detector is applied (see edges_image, mode 'lanser2') to the region CalTabRegion, which can be found by applying the operator find_caltab. The filter parameter for this edge detection can be tuned via Alpha. In the edge image closed contours are searched for: The number of closed contours must correspond to the number of calibration marks as described in the calibration plate description file CalTabDescrFile and the contours have to be ellipticly shaped. Contours shorter than MinContLength are discarded, just as contours enclosing regions with a diameter larger than MaxDiamMarks (e.g., the border of the calibration plate).

For the detection of contours a threshold operator is applied to amplitude of the edge detector. All points with a high amplitude (i.e., borders of marks) are selected.

First, the threshold value is set to StartThresh. If the search for the closed contours or the successive pose estimate fails, this threshold value is successively decreased by DeltaThresh down to a minimum value of MinThresh.

Each of the found contours is refined with subpixel accuracy (see edges_sub_pix) and subsequently approximated by an ellipse. The center points of these ellipses represent a good approximation of the desired 2D image coordinates [RCoord,CCoord] of the calibration mark center points. The order of the values within these two tuples is in row-major order beginning at the upper left in the image. This order must correspond to the order of the 3D coordinates of the calibration marks in the calibration plate description file CalTabDescrFile, since this fixes the correspondences between extracted image marks and known model marks!

Based on the ellipse parameters for each calibration mark, a rough estimate for the exterior camera parameters is finally computed. For this purpose the fixed correspondences between extracted image marks and known model marks are used. The estimate StartPose describes the pose of the calibration plate in the camera coordinate system as required by the operator camera_calibration.

*  read calibration image
read_image(Image, 'calib-01') 
*  find calibration pattern
find_caltab(Image,Caltab1, 'caltab.descr', 3, 112, 5) 
*  find calibration marks and start pose
find_marks_and_pose(Image, Caltab, 'caltab.descr' ,
                    [0.008, 0.0, 0.000011, 0.000011, 384, 288, 768, 576],
                    128, 10, 18, 0.9, 15.0, 100.0, RCoord, CCoord, StartPose)

find_marks_and_pose returns 2 (H_MSG_TRUE) if all parameter values are correct and an estimation for the exterior camera parameters has been determined successfully. If necessary, an exception handling is raised.

find_marks_and_pose is reentrant and processed without parallelization.

find_caltab

camera_calibration

find_caltab, camera_calibration, disp_caltab, sim_caltab, read_cam_par, read_pose, create_pose, pose_to_hom_mat3d, caltab_points, create_caltab, edges_sub_pix, edges_image

Camera calibration