Method and a system for calibrating a camera

1 . A method for calibrating a camera which is arranged on an ego vessel, the method comprising:

receiving image data of an image captured by the camera, the image showing at least one imaged solid object and at least one imaged solid object interface between the imaged solid object and the imaged sky, or between the imaged solid object and an imaged water surface of a waterbody, on which the ego vessel sails;

extracting the imaged solid object interface from the image data;

determining a pose of the ego vessel at the time at which the image is captured;

receiving reference data, wherein the reference data is representative of at least one real solid object interface corresponding to the imaged solid object interface in the image, and wherein the real solid object interface is between the corresponding real solid object and the real sky or, respectively, between the corresponding real solid object and the corresponding real water surface of the waterbody;

extracting the real solid object interface from the reference data;

determining a difference between the extracted imaged solid object interface and the extracted real solid object interface based on the determined pose of the ego vessel; and

determining calibration values for the camera based on the difference such that the difference is reduced.

2 . The method in accordance with claim 1 , wherein:

the extracting of the imaged solid object interface from the image comprises extracting a pathway of the imaged solid object interface within the image;

the extracting of the real solid object interface from the reference data comprises extracting a pathway of the real solid object interface from the reference data; and

the difference between the imaged solid object interface and the real solid object interface corresponds to a difference between the pathway of the imaged solid object interface and the pathway of the real solid object interface.

3 . The method in accordance with claim 1 , wherein:

when extracting the real solid object interface from the reference data, only those parts of the real solid object interface which are visible from the camera are extracted; and

the determining of the difference between the imaged solid object interface and the real solid object interface comprises determining the difference between the imaged solid object interface with the extracted visible parts of the real solid object interface only.

4 . The method in accordance with claim 1 , wherein;

the reference data comprise two-dimensional (2D) map data representing a 2D map of surroundings of the ego vessel.

5 . The method in accordance with claim 1 , wherein:

the reference data comprise Automatic Identification System (AIS) data relating to at least one other vessel in the waterbody.

6 . The method in accordance with claim 1 , wherein:

the reference data comprise elevation map data of the real land abutting on the waterbody.

7 . The method in accordance with claim 1 , wherein:

the calibration values are adjusted until a predetermined condition is fulfilled.

8 . The method in accordance with claim 1 , wherein;

the calibration values are determined by adjusting predetermined calibration values of the camera such that the difference is reduced.

9 . The method in accordance with claim 2 , wherein:

the determining of the difference between the pathway of the imaged solid object interface and the pathway of the real solid object interface comprises transferring the pathway of the imaged solid object interface into the same coordinate system, and determining a misfit function between the imaged solid object interface and the real solid object interface within the coordinate system as the difference; and

the determining of the calibration values of the camera such that the difference is reduced comprises determining the calibration values such that the misfit function is reduced.

10 . The method in accordance with claim 2 , wherein:

the determining of the difference between the pathway of the imaged solid object interface and the pathway of the real solid object interface comprises transferring the pathway of the real solid object interface into the same coordinate system, and determining a misfit function between the imaged solid object interface and the real solid object interface within the coordinate system as the difference; and

the determining of the calibration values of the camera such that the difference is reduced comprises determining the calibration values such that the misfit function is reduced.

11 . The method in accordance with claim 9 , wherein:

the predetermined calibration values are varied such that the misfit function is minimized.

12 . The method in accordance with claim 3 , comprising:

determining a positional relationship between the camera and the real solid object interface from the reference data; and

determining the parts of the real solid object interface which are visible from the camera based on the determined positional relationship.

13 . The method in accordance with claim 12 , wherein:

the AIS data comprise a geo-location of a Global Positioning System (GPS) receiver of the other vessel and an extent of the other vessel in relation to the geo-location of the GPS receiver of the other vessel;

a real vessel-water interface between the real water surface and the other vessel is estimated from the extent and the geo-location of the GPS receiver of the other vessel;

the parts of the real vessel-water interface which are visible from the camera are determined based on the estimated real vessel-water interface and a position of the camera; and

the visible parts of the real vessel-water interface are used as the real solid object interface to compare to the corresponding imaged solid object interface.

14 . The method in accordance with claim 12 , wherein:

the parts of the real solid object interface which are visible from the camera are determined from the elevation map data by generating a textureless rendered image and by extracting the parts from the textureless rendered image.

15 . The method in accordance with claim 4 , wherein;

the parts of the real solid object interface which are visible from the camera are determined from the 2D map data based on a determined positional relationship between the camera and the real solid object interface by ray casting.

16 . A system for calibrating a camera which is arranged on an ego vessel, the system comprising a memory configured to store reference data and/or calibration values and a processing unit which is configured to:

receive image data of an image captured by the camera, the image showing at least one imaged solid object and at least one imaged solid object interface between the imaged solid object and the imaged sky, or between the imaged solid object and an imaged water surface of a waterbody, on which the ego vessel sails;

extract the imaged solid object interface from the image data;

determine a pose of the ego vessel at the time at which the image is captured;

receive reference data, wherein the reference data is representative of at least one real solid object interface corresponding to the imaged solid object interface in the image, and wherein the real solid object interface is between the corresponding real solid object and the real sky or, respectively, between the corresponding real solid object and the corresponding real water surface of the waterbody;

extract the real solid object interface from the reference data;

determine a difference between the extracted imaged solid object interface and the extracted real solid object interface based on the determined pose of the ego vessel; and

determine calibration values for the camera based on the difference such that the difference is reduced.