Identifying Spurious Regions In A Video Frame

• US 20080170751A1
• Filed: 02/06/2006
• Published: 07/17/2008
• Est. Priority Date: 02/04/2005
• Status: Active Grant

• Current Assignee: British Telecommunications PLC (BT Group PLC)
• Original Assignee: British Telecommunications PLC (BT Group PLC)
• Inventors: Xu, Li-Qun, Lei, Bangjun
• Granted Patent: US 8,041,075 B2
• Time in Patent Office: Days
• US Class Current: 382/103
• CPC Class Codes: G06T 7/215 Motion-based segmentation

Abstract

In a digital video surveillance system, a number of processing stages are employed to identify foreground regions representing moving objects in a video sequence. An object tracking stage 5 is also provided in order to identify a correspondence between candidate objects in a current frame and those that have already been identified in one or more previous frames. In this way, it is possible to calculate the path taken by the or each foreground object and to record this path information in a trajectory database. In order to improve tracking performance, the object tracking stage 5 employs a state transitional object management scheme which determines whether or not a particular object is to be tracked. As part of the object management scheme, spurious objects, which are identified on the basis of their motion characteristics, are deleted from the system. This ensures that valuable processing resources are not wasted tracking unwanted artefacts which may represent, for example, noise or random motion.

Claims

1. A method of tracking an object appearing in a video sequence comprising a plurality of frames, each frame comprising a plurality of pixels, the method comprising:

(i) comparing first and second frames of the video sequence to identify a region of pixels therein representing an object having inter-frame motion;

(ii) determining whether said region appears in a predetermined number of subsequent frames, and, if so, assigning a motion parameter to said region indicative of the change in position thereof over said predetermined number of frames;

(iii) comparing said motion parameter with a threshold value to determine whether or not said region is to be tracked; and

(iv) if the region is to be tracked, recording the frame position of said region for subsequent frames in which said region is identified.

2. A method according to claim 1, wherein the predetermined number of subsequent frames in (ii) are consecutive frames.

3. A method according to claim 1, wherein, if said region is to be tracked, the method further comprises modifying the appearance of the subsequent frames in (iv) to provide a visual indication that said region is being tracked.

4. A method according to claim 1, wherein the motion parameter is representative of the positional variance of a reference point on said region, said region being tracked if the motion parameter indicates the positional variance to be below the predetermined threshold.

5. A method according to claim 1, wherein first and second motion parameters are assigned to said region, the first motion parameter being representative of the positional variance of a reference point on said region, and the second motion parameter being representative of the velocity of frame movement of said region, said region being tracked if the first motion parameter is above a predetermined first threshold and the second motion parameter is above a predetermined second threshold.

6. A method according to claim 5, wherein the second threshold is greater than the first threshold.

7. A method according to claim 4, wherein the reference point is the approximate centre of said foreground region.

12. A computer program stored on a computer-readable medium and comprising a set of instructions to cause a computer to perform the method according to claim 1.

8. A method of tracking an object appearing in a video sequence comprising a plurality of frames, each frame comprising a plurality of pixels, the method comprising:

(i) comparing first and second frames of the video sequence to identify a region of pixels therein representing an object having inter-frame motion;

(ii) assigning a motion parameter ζ

_m to said region based on its motion characteristics over the plurality of video frames; and

(iii) recording, for subsequent frames of the video sequence in which said region is identified, the frame position of said region only if its motion parameter is below a predetermined threshold T_ζ

.

9. A method according to claim 8, wherein, prior to step (iii), data representing the position of said region is stored in an object queue, and wherein said data is erased in the event that the motion parameter ζ

_m is above the predetermined threshold T_ζ

.

10. A method according to claim 8, wherein the motion factor ζ

_m is defined as;

11. A method according to claim 10, wherein the predetermined threshold T_ζ

is set in the region of 2.5.

13. A video processing system for selectively tracking an object appearing in a video sequence comprising a plurality of frames, each frame comprising a plurality of pixels, the system being arranged in use to:

(i) compare first and second frames of the video sequence to identify a region of pixels therein representing an object having inter-frame motion;

(ii) determine whether said region appears in a predetermined number of subsequent frames, and, if so, assigning a motion parameter to said region representing the change in position thereof over said predetermined number of frames;

(iii) compare said motion parameter with a threshold value to determine whether or not said region is to be tracked; and

(iv) if the region is to be tracked, record the intra-frame position of said region for subsequent frames in which said region is identified.

14. A video surveillance system comprising a video processing system according to claim 13.