Multiple backgrounds

• US 20020039135A1
• Filed: 04/23/2001
• Published: 04/04/2002
• Est. Priority Date: 12/23/1999
• Status: Active Grant

• Current Assignee: Secumanagement B.V.
• Original Assignee: Wespot AB (OPTEX Technologies BV)
• Inventors: Heyden, Anders
• Granted Patent: US 6,819,353 B2
• Time in Patent Office: Days
• US Class Current: 348/143
• CPC Class Codes: G06T 7/0002 Inspection of images, e.g. flaw detection
• CPC Class Codes: G06T 7/254 involving subtraction of images
• CPC Class Codes: G06V 20/52 Surveillance or monitoring of activities, e.g. for recognising suspicious objects recognising microscopic objects G06V20/69
• CPC Class Codes: G08B 13/19602 Image analysis to detect motion of the intruder, e.g. by frame subtraction
• CPC Class Codes: G08B 13/19604 involving reference image or background adaptation with time to compensate for changing conditions, e.g. reference image update on detection of light level change
• CPC Class Codes: G08B 13/19667 Details realated to data compression, encryption or encoding, e.g. resolution modes for reducing data volume to lower transmission bandwidth or memory requirements
• CPC Class Codes: H04N 7/188 Capturing isolated or intermittent images triggered by the occurrence of a predetermined event, e.g. an object reaching a predetermined position signal generation from motion picture films H04N5/253

Abstract

A method for monitoring a monitored area comprising the steps of recording an image of the monitored area and of comparing the recorded image with a reference image in order to determine whether there is an alarm situation. The method further comprises the step of creating the reference image by combining at least two previously stored images from a set of images, so that the lighting conditions in the reference image are adjusted to the lighting conditions in the recorded image.

Claims

1. A method for monitoring a monitored area comprising the steps of recording an image of the monitored area and of comparing the recorded image with a reference image in order to determine whether there is an alarm situation, characterized by the step of creating the reference image by combining at least two previously stored images from a set of images, so that the lighting conditions in the reference image are adjusted to the lighting conditions in the recorded image.

2. A method according to claim 1, in which the reference image is created by linear combination of said at least two previously stored images.

3. A method according to claim 1, further comprising the steps

of calculating a difference image as the difference between the reference image and the recorded image,

of determining at least one divergence area in the difference image, in which at least certain pixels exceed a particular level,

of determining whether the divergence area represents a scene change, and

if the divergence area represents a scene change, of removing the pixels that correspond to the divergence area in the recorded image for the creation of the next reference image.

4. A method according to claim 3, further comprising the steps of determining whether the divergence area represents a light change and, if the divergence area represents a light change, of updating the set of images.

5. A method according to claim 3, further comprising the step of updating the set of images if the divergence area represents a scene change and if the same scene change is detected in a predetermined number of consecutively recorded images.

6. A method according to any one of claims 3, 4 or 5, in which the step of determining whether the divergence area represents a scene change comprises the step of calculating the variance of the divergence area, and, if the variance exceeds a predetermined value, the divergence area is judged to represent a scene change.

7. A method according to claim 1, in which at least the outermost edge of pixels in the recorded image are excluded when creating the reference image.

8. A method according to claim 1, in which the stored images in the set of images are stored at different resolutions.

9. A method according to claim 1, further comprising the step of creating the set of images by recording images of the monitored area under different lighting conditions and of storing one or more of these recorded images.

10. A method according to any one of claims 4 or 5, in which the step of updating the set of images is carried out at predetermined intervals.

11. A method according to claim 1, in which at least two images in the set of images are stored orthogonally.

12. A method according to claim 1, in which the set of images comprises an image in which all the pixels have the same intensity.

15. A computer program for image processing for monitoring a monitored area, which computer program is stored on a computer-readable memory medium which comprises instructions for carrying out one or more of the method steps according to claim 3.

13. A monitoring unit for monitoring a monitored area comprising a light-sensitive sensor which is arranged to record an image of the monitored area, and a central processing unit which is arranged to compare the recorded image with a reference image in order to determine whether there is an alarm situation, characterized in that the central processing unit is further arranged to create a reference image by combining at least two previously stored images from a set of images, so that the lighting conditions in the reference image are adjusted to the lighting conditions in the recorded image.

14. A computer program for image processing for monitoring a monitored area, which computer program is stored on a computer-readable memory medium which comprises instructions to cause the computer to record an image of the monitored area and to compare the recorded image with a reference image in order to determine whether there is an alarm situation, characterized by causing the computer to create a reference image by combining at least two previously stored images from a set of images, so that the lighting conditions in the reference image are adjusted to the lighting conditions in the recorded image.