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

::find_marks_and_pose is used to determine the necessary input data for the subsequent camera calibration (see ::camera_calibration): On the one hand the 2D center points [RCoord,CCoord] of the calibration marks within the region CalTabRegion of the input image Image are extracted and ordered. On the other hand a rough estimate for the external camera parameters (StartPose) is computed, i.e., the 3D pose of the camera in the calibration table coordinate system.

In the input image Image an edge detector is applied (see ::edges_image, mode 'lanser2') to the region CalTabRegion, which may have been 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 table 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 table).

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 table 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 external camera parameters is computed finally. For that purpose the fixed correspondences between extracted image marks and known model marks are used. The estimate StartPose describes the pose of the camera in the calibration table coordinate system (see ::create_pose) as desired by the operator ::camera_calibration.

HTuple StartCamPar,RCoord,CCoord,StartPose;
// read calibration image
HImage Image("calib-01.tiff");
// find calibration pattern
HRegion Caltab = Image.FindCaltab("caltab.descr",3,112,5);
// read internal camera parameters from file
::read_cam_par("campar.dat",&StartCamPar);  
// find calibration marks and start pose
RCoord = Image.FindMarksAndPose(Caltab,"caltab.descr",StartCamPar,
                                128,10,18,0.9,15.0,100.0,
                                &CCoord,&StartPose);

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

::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