Method for calibrating the image of a camera

• US 8,964,035 B2
• Filed: 05/04/2010
• Issued: 02/24/2015
• Est. Priority Date: 05/05/2009
• Status: Active Grant

• Current Assignee: Kapsch TrafficCom AG (Datax Handelsgmbh)
• Original Assignee: Kapsch TrafficCom AG (Datax Handelsgmbh)
• Inventors: Kahler, Peter, Janner, Christian, Lhndorf, Maike
• Primary Examiner(s): CHRISTENSEN, SCOTT B
• Application Number:
  US12/773,059
• Publication Number:
  US 20100283856A1
• Time in Patent Office: 1,757 Days
• Field of Search: None
• US Class Current: 348/149
• CPC Class Codes: G06T 7/80 Analysis of captured images to determine intrinsic or extrinsic camera parameters, i.e. camera calibration
• CPC Class Codes: H04N 17/002 for television cameras

Abstract

A method for calibrating the image (3_I, 3_II) of a camera (1), and, in particular, a traffic-monitoring camera, with respect to its viewing space, such that the camera projection parameters (m_k1) of a camera projection of the viewing space onto the image are determined, includes the steps of providing a movable carrier (4) with at least four markings that can be viewed by the camera (1) and that span a volume and that form known first world points (w_i) in the viewing space, recording a first image (3_I) of the viewing space, shifting the carrier (4) in the viewing space by an unknown offset (v) in order to obtain second world points (w_i) dependent on this offset in the viewing space, recording a second image (3_II) of the viewing space, detecting the markings in the first image (3_I) as first image points (p_i) and in the second image (3_II) as second image points (p_i), and determining the camera projection parameters (m_k1), while also determining the offset (v), from the known first world points, the second world points dependent on the offset, the detected first image points, and the detected second image points.

Claims

1. A method for calibrating an image of a stationary camera with respect to a viewing space of the camera, comprising:

providing a movable carrier with at least four markings thereon that can be viewed by the stationary camera and that span a volume, the markings forming known first world points in the viewing space;

recording a first image of the markings on the movable carrier in a first position in the viewing space;

shifting the carrier with the at least four markings thereon to a second position in the viewing space by an unknown offset, the markings forming second world points which are dependent on this offset in the viewing space;

recording a second image of the markings on the movable carrier in the second position in the viewing space;

detecting the markings in the first image as first image points and in the second image as second image points;

determining the offset and camera projection parameters, which define a projection of world points in the viewing space onto image points in the image, from the known first world points, the second world points dependent on the offset, the detected first image points, and the detected second image points; and

calibrating the image of the camera using said camera projection parameters;

wherein the step of determining the camera projection parameters is performed using a direct linear transformation (“

DLT”

) method; and

wherein the step of determining the camera projection parameters is performed by solving a DLT equation

D(s,t)·

m=p

with respect to m, s, and t, wherein;

D(s,t) is a DLT matrix of all world points and image points in which the second world points are expressed by the first world points shifted by the offset (s,t,0);

m is a vector of the camera projection parameters; and

p is a vector of all image points.

2. The method according to claim 1, wherein in order to solve the DLT equation, the minimization task

∥

D(s,t)·

m−

p∥

→

min

is solved by means of a Gauss-Newton method.

3. The method according to claim 1, wherein at least four markings are detected in one image and at least three markings are detected in the other image.

4. The method according to claim 1, wherein more than two images are recorded in order to detect additional image points of the markings that are included in the determination of the camera projection parameters.

5. The method according to claim 1, wherein a vehicle is used as the carrier and a driven distance of the vehicle is used as the offset.

6. The method according to claim 5, wherein distinctive points of the vehicle are used as the markings.

7. The method according to claim 5, wherein lighting means are mounted on a rear carrier, a roof carrier and/or a rearview-mirror carrier of the vehicle as markings.

8. The method according to claim 1, wherein the method is repeated and the determined camera projection parameters are averaged.

9. A method for calibrating an image of a camera with respect to its viewing space, such that camera projection parameters of a camera projection of the viewing space onto the image are determined, comprising the steps of:

providing a movable carrier with at least four markings that can be viewed by the camera and that span a volume and form known first world points in the viewing space;

recording a first image of the viewing space;

shifting the carrier in the viewing space by an unknown offset in order to obtain second world points dependent on this offset in the viewing space;

recording a second image of the viewing space;

detecting the markings in the first image as first image points and in the second image as second image points; and

determining the camera projection parameters, while also determining the offset, from the known first world points, the second world points dependent on the offset, the detected first image points, and the detected second image points, wherein the determining step is performed by solving a DLT equation

D(s,t)·

m=p

with respect to m, s, and t, wherein;

D(s,t) is a DLT matrix of all world points and image points in which the second world points are expressed by the first world points shifted by the offset (s,t,0);

m is a vector of the camera projection parameters; and

p is a vector of all image points.

10. The method according to claim 9, wherein in order to solve the DLT equation, the minimization task

∥

D(s,t)·

m−

p∥

→

min

is solved by means of a Gauss-Newton method.

11. The method according to claim 9, wherein at least four markings are detected in one image and at least three markings are detected in the other image.

12. The method according to claim 9, wherein more than two images are recorded in order to detect additional image points of the markings that are included in the determination of the camera projection parameters.

13. The method according to claim 9, wherein a vehicle is used as the carrier and a driven distance of the vehicle is used as the offset.

14. The method according to claim 13, wherein distinctive points of the vehicle are used as the markings.

15. The method according to claim 13, wherein lighting means are mounted on a rear carrier, a roof carrier and/or a rearview-mirror carrier of the vehicle as markings.

16. The method according to claim 9, wherein the method is repeated and the determined camera projection parameters are averaged.