IMPROVED VIDEO PROCESSING USING PREFILTERING

Abstract: A method of image processing reducing or eliminating noise effects in image signals processed by an image processing system (10). An image obtained from a camera (12) comprises an analog signal (A) which is converted to a digital signal (D). A processor (18) processes the images with one image forming a reference image (RI) and another image (SI) being compared to it. As a result of the comparison a difference image (DI) is formed, this image also being a digital image. The difference image is then filtered by a filter (22) to eliminate the effects of noise in the signals.

Description: IMPROVED VIDEO PROCESSING USING PREFILTERING Technical Field This invention relates to the processing of video images; and, more particularly to a method of reducing the effects of noise when images generally of the same scene are compared to determine if there are any differences between the two. Background Art In certain imaging systems, for example, video security systems, a camera is directed at a scene obtains images of the scene at a predetermined frame rate. An initial image of the scene is treated as a reference image and subsequent images are compared to the reference image to determine if there are any changes to the scene. If the images differ in certain predetermined aspects, further action may be warranted. The cameras and image processing equipment operate in a noise environment. The cameras used are necessarily of the best quality, wiring connections may be faulty, etc. As a result, signals generated and processed in the system may include electrical noise. The effect of this noise may be to distort the processed images so when image comparisons are performed, this being done electronically within the system, details which should stand out may be obscured, or images which may not need further review are identified as needing to be. The present invention provides a process for eliminating the effects of noise so to facilitate review of only those images requiring a review. Summary of the Invention Among the several objects of the present invention is a method of image processing to reduce or eliminate the effects of noise in image signals processed within an image processing system. An image obtained from a camera or the like is comprised of an analog signal which is converted to a digital signal. A series of images are processed with one image forming a reference image and the other images being compared to it. Comparison of the two images results in a difference image being formed, this image also being a digital image. The difference image is then filtered to eliminate the effects of noise in the signals. This is done by averaging the pixel values comprising the difference image in a predetermined manner and forming a filtered image matrix whose contents are the averaged pixel values. After filtering, the filtered image is processed to determine if there is sufficient difference between the reference image and the image compared with it as to require further investigation. Other objects and features will be in part apparent and in part pointed out hereinafter. Brief Description of Drawings In the drawings, Fig. 1 is a simplified block diagram of an image processing system; Fig. 2 illustrates a first image processing step performed in accordance with the method of the present invention; Fig. 3 illustrates a second image processing step employing image filtering; and, Fig. 4 illustrates a threshold detection step of the method to determine if the results of the image comparison warrant further investigation of the scene based upon the processed images. Corresponding reference characters indicate corresponding parts throughout the drawings. Best Mode for Carrying Out the Invention Referring to the drawings, an image processing system of the present invention is indicated generally 10. A camera 12 obtains a video image of a scene and provides an output which is an analog signal A. This signal is provided as an input to an analog-to-digital converter 14 which converts signal A to a digital signal D. The digital signal is supplied to a digital video recorder (DVR) 16 in which the digital signal is stored. As indicated in Fig. 1 , the image processing system captures and stores a series of images I-H N over time, and all of these images are stored in DVR 16. One of these images is designated a reference image RI by the system, and subsequent images SI are compared with the reference image. The reference image represents a view of the scene at a given point in time, and comparison of subsequent images SI to the reference is done to determine if there have been any significant changes to the scene. In a security environment, for example, this is done to ascertain if an intrusion has occurred. The image processing is accomplished in an image processor 18. Those skilled in the art will understand that while system 10 is shown to include a video camera, other imaging devices, for example, an infrared (IR) detector can also be used to obtain images which are processed in accordance with the method of the present invention. It will further be understood that the reference image is periodically updated. Referring to Fig. 2, processor 18 includes a comparator 20 to which is provided as inputs reference image RI and a subsequent image SI. Since the two images are of the same size; i.e., they have the same number of pixels arranged in an identical format, comparator 20, can readily compare the two images and determine if there are any differences between them. Comparator 20 can do this in a number of ways in accordance with the invention. For example, in Fig. 2, the pixels comprising the two images are arranged in a matrix having M rows and N columns. Accordingly, comparator 20 first compares the pixel values represented by the data bits at locations RI, pι,ι and SI, p 1t1 . It then compares the pixel values represented by the data bits at RI, p 1 2 and SI, pi ,2 , RI, pι, 3 and SI, p 1|3 , etc., through the pixel values represented by the data bits at RI, P M,N and SI, PM , N* The comparison is accomplished by subtracting the one pixel value from the other, with the results being stored in a difference image DI. If there is no change in the pixel values for a given location between the reference and subsequent images, a value of 0 is stored in the appropriate address of the difference image. Otherwise a value representing the difference is entered at the matrix location for the pixel values which have been compared. Were all the difference values zero, the scene viewed in the reference and subsequent images would be identical and no further action would typically need to be taken. However, if there is any differences between the two images, as represented by the difference values, then the question becomes of if the differences are significant enough that further investigation is required. Before making this determination, and in accordance with the method of the present invention, a filter or prefilter operation is performed on the contents of difference image DI to eliminate the effect of noise. As shown in Fig. 3, processor 18 further includes a filter 22 to which the contents of the difference image are supplied. Filter 22 operates by averaging a predetermined number of adjacent pixels in the matrix comprising the difference. Consider, for example, a difference images in which the contents of the first row of the image matrix contains the following pixel values: Pl,1 P1,2 Pl,3 P1,4 Pl,5 Pl,6 Pl,7 Pl,8 Pl,9 Pl,10 Filter 22 will take the first three pixel values and average the value of their contents. The result will be entered into a new matrix for a filtered image Fl as shown in Fig. 3. Filter 22 then averages the next three adjacent pixel values and enters this resultant value in the next position in the filtered image Fl matrix, and so forth. The values entered in the filtered image Fl matrix thus appear as follows: (Pι,ι + Pι, 2 + Pι,3)/3, (pι,2 + pι, 3 + pι, 4 )/3, (pi, 3 + pι, 4 + pι, 5 )/3, etc. This process is repeated until all the pixel value contents in difference image DI have been averaged together. For the last two positions in each row, the average values are computed as (pι, 9 + pι,ιo)/2, and (pι,ιo)/1. It will be understood by those skilled in the art that other averaging formulas can be used without departing from the method of the invention. For example, the number of adjacent pixel values averaged together can be other than three. Also, the row end value contents can be averaged differently than as set forth above. For example, the averaging could be done as (Pι,9 + Pι,9 + pι,ιo)/3 and (p 1ι9 + pι,ιo+ Pι,ιo)/3 Regardless, the prefiltering operation performed by filter 22 results in a filtered image Fl in which the effects of noise have eliminated. After the prefiltering operation is completed, filtered image Fl is supplied to a threshold detector 24 of processor 18. In detector 24, each averaged pixel value is compared against a predetermined threshold to determine whether or not the average value exceeds the threshold. A total is kept of the number of averaged pixel values which exceed the threshold. After all the averaged pixel values have been examined, a determination is made as to whether this number exceeds a predetermined count value. If it does, that is indicative the difference image represents a "significant" change in the scene viewed by camera 10 and this change should be further investigated. Further investigation of the scene can involve a number of activities. For example, an alarm may be "sounded". Video images of the scene may be displayed at a monitoring site for someone to observe what is happening. Images obtained by the camera may be stored for further review. However, if the processing of the filtered image indicates no significant change between the reference image and subsequent image, processor 18 moves on to the next subsequent image and the method is repeated. What has been described is a method of processing images obtained of a scene and comparing a reference image of the scene with subsequently acquired images. To ensure that the comparison is not influenced by the effect of noise in the image processing system, a difference image formed by a comparison of the reference and a subsequent image is prefiltered. Prefiltering involves averaging the values of adjacent pixels in the difference image and entering the result in a second difference image matrix. The averaged values in this second difference image matrix are then compared to a threshold value and if the number of values exceeding the threshold exceed a predetermined count value, the subsequent image is deemed to show a significant change from the reference image. In view of the foregoing, it will be seen that the several objects of the invention are achieved and other advantageous results are obtained. As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims: Claims: 1. A method of image processing to reduce or eliminate the effects of noise in image signals processed within an image processing system comprising: viewing a scene with an imaging means to obtain images of the scene at different times, each image obtained being represented by an analog signal; converting each analog signal to a digital signal; comparing images of scene at different times by comparing one of the digital signals with another of the digital signals to produce a difference image representing any changes in the scene between the two images; filtering the difference image to eliminate the effects of noise in the imaging means and image processing system, filtering the difference image producing a filtered image; and, processing the filtered image to determine if there is sufficient difference between the two compared images as to require further investigation. 2. The method of claim 1 wherein the imaging means comprises a camera operating in the visual range of the light spectrum. 3. The method of claim 2 wherein the imaging means further includes an infrared detector. 4. The method of claim 1 wherein filtering the difference image to eliminate the effects of noise includes averaging the contents of the difference image in a predetermined manner. 5. The method of claim 4 including forming a filtered image formed by combining all the averaged values for the contents of the difference image in a predetermined format. 6. The method of claim 5 wherein processing the filtered image includes comparing the averaged values comprising the filtered image against a predetermined threshold value and counting the number of averaged values which exceed the threshold. 7. The method of claim 6 wherein further investigation of the scene is determined by whether or not the threshold count value exceeds a predetermined number. 8. The method of claim 5 wherein a predetermined number of adjacent pixel values in each row of the difference image are averaged together with the result being used to form the filtered image. 9. A method of image processing to reduce or eliminate the effects of noise in image signals processed within an image processing system comprising: obtaining an image of scene with a camera at different points in time with each image obtained being represented by an analog signal; converting each analog signal to a digital signal; establishing one of the images as a reference image and comparing another image of the scene with the reference image, comparing the reference image and the other image producing a difference image representing changes in the scene between the two images; filtering the difference image to eliminate the effects of noise in the camera and image processing system, filtering the difference image including averaging the contents of the difference image in a predetermined manner to produce a filtered image; and, processing the filtered image to determine if there is sufficient difference between the two compared images as to warrant investigation of the scene. 10. The method of claim 9 wherein the filtered image is formed by combining all the averaged values for the contents of the difference image in a predetermined format. 11. The method of claim 10 wherein processing the filtered image includes comparing the averaged values comprising the filtered image against a predetermined threshold value and counting the number of averaged values which exceed the threshold. 12. The method of claim 11 wherein further investigation of the scene is determined by whether or not the threshold count value exceeds a predetermined number. AMENDED CLAIMS [received by the International Bureau on 18 December 2001 (18 12 01)), new claims 13-17 added, remaining claims unchanged (1 page)] 13. A method of image processing to reduce or eliminate the effects of noise in image signals processed within an image processing system comprising- viewing a scene with an imaging means to obtain images of the scene at different times, each image obtained being represented by an analog signal, converting each analog signal to a digital signal; comparing images of scene at different times by comparing one of the digital signals with another of the digital signals to produce a difference image representing any changes in the scene between the two images; filtering the difference image to eliminate the effects of noise in the imaging means and image processing system, filtering the difference image producing a filtered image and the step of filtering including averaging the contents of the difference image in a predetermined manner; and, processing the filtered image to determine if there is sufficient difference between the two compared images as to require further investigation. 14. The method of claim 13 including forming a filtered image formed by combining all the averaged values for the contents of the difference image in a predetermined format. 15. The method of claim 13 wherein processing the filtered image includes comparing the averaged values comprising the filtered image against a predetermined threshold value and counting the number of averaged values which exceed the threshold. 16. The method of claim 15 wherein further investigation of the scene is determined by whether or not the threshold count value exceeds a predetermined number. 17. The method of claim 15 wherein a predetermined number of adjacent pixel values in each row of the difference image are averaged together with the result being used to form the filtered image.

Inventors: MYERS JAMES CARROL, MYERS, JAMES, CARROL
Applicants: COMTRAK TECHNOLOGIES LLC, COMTRAK TECHNOLOGIES, LLC
Application year: 2001
Main classification: H04N5
IPC codes: H04N5