Method for increasing reliability in monitoring systems

1. A method for detecting an object crossing event at a predetermined first line in a scene captured by a motion video camera, the method comprising:

determining from images of the scene captured by the motion video camera if an object image crosses the predetermined first line;

calculating a size value relating to the size of the object image crossing the predetermined first line;

setting a line distance value to a value calculated from the shortest distance between a contact point of the object with the predetermined first line and a predetermined second line; and

generating an object crossing event signal if a relation between the calculated size value and the line distance is within a predetermined range.

2. The method according to claim 1 , wherein the size value relating to the size of the object image is related to the size of an area occupied by the object in the captured image.

3. The method according to claim 2 , wherein the size of the area related to the size of the object image is calculated from a number of image elements the object image occupies in the captured image.

4. The method according to claim 1 , wherein the determining if an object crosses the predetermined first line includes detecting in images captured of the scene if image element representing the object is positioned on an image element included in the predetermined first line.

5. The method according to claim 1 , wherein the predetermined first line and the predetermined second line are virtual lines, wherein each line is defined by at least two coordinates representing positions within the scene.

6. The method according to claim 1 , wherein the first and second predetermined lines are defined in the scene so that they are closer to each other at image elements in the scene representing positions further away from the motion video camera.

7. The method according to claim 1 , wherein the setting a line distance value includes calculating a position along the predetermined second line having the shortest distance to the contact point of the object image with the predetermined first line.

8. The method according to claim 1 , wherein the predetermined first line and the predetermined second line is a detection line pair and a plurality of detection line pairs may be defined for a scene.

9. The method according to claim 1 , wherein the object crossing event signal is generated when the relation between calculated size value and the line distance squared is within the predetermined range.

10. The method according to claim 1 , wherein the object crossing event signal is generated as soon as a relation

L d 2 k≤A o ≤L d 2 K

is valid, wherein L d is a value of the line distance, A o is a value of an area related to the size of the object, k is a predetermined lower limit constant, and K is a predetermined upper limit constant.

11. A system for detecting objects crossing predetermined lines in a captured scene, the system comprising:

a monitoring camera configured to capture images of a scene;

a processing unit configured to:

determine from images of the captured scene if an object image crosses a predetermined first line;

calculate a size value relating to the size of the object image crossing the predetermined first line;

setting a line distance value to a value calculated from the shortest distance between a contact point of the object with the predetermined first line and a predetermined second line; and

generate an object crossing event signal if a relation between the calculated size value and the line distance is within a predetermined range.