This invention relates to video security systems and a method for detecting the presence of an intruder into an area being monitored by the system; and more particularly, to the rejection of false alarms which might otherwise occur because of global or local, natural or manmade lighting changes which occur within a scene observed by the system.
A security system of the invention uses a video camera as the principal sensor and processes a resulting image to determine the presence or non-presence of an intruder. The fundamental process is to establish a reference scene known, or assumed, to have no intruder(s) present. An image of the present scene, as provided by the video camera, is compared with an image of the reference scene and any differences between the two scenes are ascertained. If the contents of the two scenes are markedly different, the interpretation is that an intrusion of some kind has occurred within the scene. Once the possibility of an intrusion is evident, the system and method operate to first eliminate possible sources of false alarms, and to then classify any remaining differences as being the result of a human or non-human intrusion. This application and co-pending, co-assigned U.S. patent application Ser. No. 08/772,731 describe the identification and elimination of various sources of potential false alarms. Co-pending, co-assigned U.S. patent application Ser. No. 08/771,991 describes a classification process by which a human intruder is identified as such and an alarm given.
Processes of the type described in this and the referenced applications are particularly useful in security systems requiring video verification by a security service operator to confirm the presence of an intruder. Previously, security systems have relied upon contact break mechanisms or PID (passive infra red) motion sensors to detect intruder presence. Examples of the use of infrared devices either as a passive single element or as a scanning device, are disclosed in U.S. Pat. Nos. 5,283,551, 5,101,194, 4,967,183, 4,952,911, 4,949,074, 4,939,359, 4,903,009, 4,847,485, 4,364,030, and 4,342,987. More recently, however, the realization that an image processor is required to transmit the video for confirmation purposes has led to the development of using the image processor to actually detect the possible presence of an intruder. Such a system has an economy of hardware and obviates the need for PID sensors or contact breaker devices. A security system of this type and the referenced U.S. patent applications has comparable performance to a PID counterpart. However, there are areas where considerable benefits accrue if false alarms which occur due to the erroneous injection of light into the scene without the presence of an intruder are reduced or eliminated.
The cause of these false alarms stem from the sensor and methodology used to ascertain if an intrusion has occurred. As stated earlier, a past image of the scene being surveyed is compared with the present scene as taken from the camera. The form of comparison is essentially a subtraction of the two scenes on a pixel by pixel basis. Each pixel, however, represents a gray level measure of the scene intensity that is reflected from that part of the scene. Gray level intensity can change for a variety of reasons, the most important being a new physical presence within a particular part of the scene. Additionally, the intensity will change at that location if the overall lighting of the total scene changes (a global change), or the lighting at this particular part of the scene changes (a local change), or the AGC (automatic gain control) of the camera changes, or the ALC (automatic light level) of the camera changes. With respect to global or local lighting changes, these can result from natural lighting changes or manmade lighting changes. Finally, there will be a difference of gray level intensity at a pixel level if there is noise present in the video. Only the situation of a physical presence in the scene is a true alarm; the remainder all comprise false alarms within the system. For a security system to be economically viable and avoid an unduly high load on an operator who has to verify each alarm, the system must process images in a manner which eliminates as many of false alarms as possible without impacting the overall probability of detecting the presence of an intruder.
Earlier efforts have been made in detecting the presence of a person in a scene using a video system. In U.S. Pat. No. 4,697,077, a camera is used to view a scene. An original reference image of the scene is prestored and a frame to frame subtraction process is employed to detect the presence of an object (an anomaly). Once an anomaly is detected, the system automatically transmits any differences, provided they are large enough, to another location over a telephone line. At this location, the image is reviewed by a human. While this patent teaches detection, it does not attempt to use image processing to recognize an anomaly as caused by a human presence or any other source.
U.S. Pat. No. 4,257,063, teaches that a video line from a camera can be compared to the same video line produced at an earlier time to detect the presence of a human. However, what is reviewed is not a whole image, nor is any compensation made for changes in light, nor is any attempt made to automatically recognize the contents of the image as being derived from a human. Similarly, U.S. Pat. No. 4,161,750, teaches that changes in the average value of a video line can be used to detect the presence of an anomalous object.
Overall, the relevant art, while generally teaching some type of detection, does not address the use of image processing as an automatic recognition stage to prevent false alarms, nor the use of image processing to distinguish between a global or local change in lighting caused, for example, by a lightning flash, building lights going on or off, or a flash light being shone over the scene, and the actual presence of an intruder in the scene. As compared to method of the present invention, what these systems produce would, in many instances be false alarms. It therefore will be appreciated that a need exists for a method of removing unwanted detections due to sudden changes in lighting conditions both globally throughout the scene or locally within the scene, whether this change in lighting is positive or negative, temporary or permanent. The system and method of the present invention fulfills this need.