Determination of ambient light level changes in visual images
Contents
1. 45 6 3 The magnitude K of the gray scale difference neces sary within a given pixel pair to signify an alarm condition It can be readily appreciated that a comparison of each pixel of the first image with its corresponding pixel of the second image would be extremely time consum ing if all 61 440 pixels of each image were compared Obviously comparison processing time is related to the number of pixels compared For example in a software driven embodiment of the invention a comparison of 20 480 pixels skipping every third pixel takes approxi mately 0 71 seconds of processing time whereas com paring 8 777 pixels skipping every seventh pixel takes approximately 0 43 seconds Fortunately it is unnecessary to compare each and every pixel to achieve a system which functions inde pendently of changes in ambient light intensity More over the number of comparisons needed the percent age of those comparisons which must show a difference the spatial distribution of the pixels compared and the magnitude of the gray difference between pixels being compared are variables dependent upon whether the comparison is being done to determine a change in ambient light intensity or to determine an alarm condi tion The first comparison of the method determines whether a difference in ambient light intensity exists between the first and second visual images In a pre ferred embodiment experiments indicate that for an image comp2. Otherwise control passes to box 26 and the process repeats as described above It is the unique method used to compare old and new images which enables the present invention to operate independently of changes in ambient light intensity This comparison is best understood with reference to FIGS 3A and 3B FIG 3A represents a first digital representation of a first visual image The representation comprises a plu of pixels X 1 1 X 1 2 1 3 X 240256 where each pixel has an associated gray scale indicative of brightness or light intensity For example an individual pixel may have a gray scale G ranging from 0 to 63 where 0 indicates black and 63 indicates white or from 0 to 255 where 0 indicates black and 255 indicates white depending on the interface board used Similarly FIG 3B represents a second digital repre sentation of a second visual image such as that of an image obtained at a later point in time than the first image mentioned previously The second representation comprises a plurality of pixels Y 1 1 1 2 Y 13 gt Y 240 256 Where each pixel has an associated gray scale as discussed above The comparison method of the invention functions by comparing digital representations of corresponding pixels of the first and second images For example XG and are corresponding pixels There are three variables which factor into the comparison pro cess as follows 25 30 35 40
3. ing a present or new image to the old in real time elimi nating the need for one of the memories It is also possi ble to compare two distinct images simultaneously re ceived by two video cameras at the same time eliminat ing both memories For example one camera could be focused on an image such as a fingerprint while a second camera scans other images looking for a match An inverter circuit connected to controller 17 or execu tor 18 would sound an alarm only when a match is found Similarly on an assembly line one camera could be focused on a static image of a workpiece as it should appear at a certain step in assembly while a second camera is timed to monitor workpieces on the actual assembly line If the two images don t match a suitable warning would be given to indicate a possible product defect Adverting once again to the preferred embodiment depicted in FIG 1B it should be noted that VIC 10 may be implemented in apparatus form in one of at least two ways In a first embodiment VIC 10 may comprise software run by a conventional computer such as an PC or compatible computer In a second em bodiment VIC 10 may comprise a dedicated circuit specially designed to implement the method of the in vention In either case the method of comparison is the same and this method is described herebelow The Visual Image Comparison Method The present invention broadly comprises a visual image comparison method comp
4. A method as described in claim 1 wherein said second predetermined amount of difference in gray scale is defined as K2 and K2 is determined automati cally based on an average gray scale value of ambient light within the visual images being compared in said first comparison wherein said average gray scale value is defined as and wherein the average gray scale of the ambient is determined by adding all of the individual gray scales values G s of each considered pixel and then dividing this total by the number of pixels considered wherein K2 is determined according to Table I herebelow TABLE I AGA 1 2 3 4 5 6 7 8 9 10 K2 1 2 4 7 9 12 M 16 16 16 AGA 1 12 1 3 14 15 16 18 19 20 K2 16 16 16 16 16 16 17 17 17 17 AGA 21 22 23 24 25 26 27 28 29 30 K2 18 18 18 18 18 19 19 19 19 19 AGA 31 32 33 34 35 36 37 38 39 40 K2 20 20 20 20 21 21 21 21 21 21 AGA 51 52 53 54 55 56 57 58 59 60 K2 26 26 26 27 2 27 28 28 29 29 AGA 61 62 63 30 3 2 3 A visual comparison apparatus comprising a means for obtaining a first digital representation of a first visual image said first digital representation comprising a first plurality of pixels wherein each pixel has a gray scale indicative of light intensity means for obtaining a second digital representation of a second visual image said second digital repre sentation comprising a second plurality of pixels wherein each pixel has a gray scale indicative of light intensity
5. c means for selectively making a first predetermined number of first remotely displaced comparisons of corresponding pixels from said first and second digital representations to determine whether a dif ference in ambient light intensity exists between said first and second visual images wherein said difference in ambient light intensity is defined to exist when a first percentage of said first predeter mined number of first remotely displaced compari sons result in pixels having a difference in gray scale of a first predetermined amount and d means for selectively making a second predeter mined number of second comparisons of corre sponding near neighboring pixels from said first and second digital representations if and only if no 5 151 945 11 12 significant difference in ambient light intensity is of second near neighboring comparisons result in found to exist pixels having a difference in gray scale of a second e and indicating an alarm condition when a second predetermined amount percentage of said second predetermined number DLE E 10 15 20 25 30 35 45 50 55 65
6. a difference in ambient light intensity exists between the first and second visual images and selectively making a second predetermined number of comparisons of corresponding pixels from the first and second digital representations if and only if no difference in ambient light intensity exists between the first and second visual images and indicating an alarm condition when a percentage of the second prede termined number of comparisons result in pixels having a difference in gray scale of a predetermined amount The first and second visual images can be images ob tained at different locations simultaneously images taken of a single location at different times or images obtained at different locations at different times An apparatus is also provided to implement the method of the invention A primary object of the invention is to provide a visual image comparison method which functions inde pendently of ambient changes in light intensity between the visual images being compared A secondary object is to provide a visual image com parison method which functions independently of the time rate of change of ambient light intensity between a first and second visual image being compared BRIEF DESCRIPTION OF THE DRAWINGS FIG 1A is a general block diagram of a video surveil lance apparatus which utilizes the present invention 65 FIG 1B is a general block diagram similar to FIG 1A but expanded to show the major elements of the
7. has a gray scale of 21 and its corresponding pixel has a gray scale of 27 then the gray scale differ ence is said to be 6 27 21 The value of K selected affects the sensitivity of the system The lower the value of K the more sensitive the system In a system such as the preferred embodiment where each pixel has a gray scale range from 0 to 63 a K value of 2 to 3 is a differ ence equivalent to 3 5 of the total gray scale Once again experimental data suggest that values of K 2 or 3 ensure reliable operation in nearly all ambient light conditions This is not to say that other values of K wil not be suitable only that values of K 22 or 3 are preferred In fact other values of K have been proven to achieve suitable results depending upon the ambient light conditions Obviously the method will still work 5 151 945 7 satisfactorily with larger K values but will simply be less sensitive to changes in ambient light intensity Also it is noted that in a preferred embodiment the pixels compared are uniformly distributed throughout the images i e every 25th pixel every 40th pixel etc This is not to imply that uniform distribution is abso lutely necessary although it is preferred In checking for ambient light differences it is necessary however that compared pixels be distributed widely throughout the images Summarizing then in a preferred embodiment the first comparison to determine if a difference in ambient
8. plurality of pixels and each pixel has a gray scale indicative of light intensity and selectively mak ing a first predetermined number of comparisons of corresponding pixels from the first and second digital representations to determine whether a difference in ambient light intensity exists between the first and sec ond visual images and selectively making a second predetermined number of comparisons of correspond ing pixels from the first and second digital representa tions if and only if no difference in ambient light inten sity exists between the first and second visual images and indicating an alarm condition when a percentage of the second predetermined number of comparisons result in pixels having a difference in gray scale of a predeter mined amount Apparatus is also described for the pur pose of implementing the method 3 Claims 7 Drawing Sheets Microfiche Appendix Included 1 Microfiche 21 Pages 15 CONTROLLED DEVICES U S Patent Sep 29 1992 Sheet 1 of 7 5 151 945 13 VISUAL CONTROLLED IMAGE DEVICES COMPARATOR RAM OLD CONTROLLED DEVICES FIG 1B 19 U S Patent Sep 29 1992 Sheet 2 of 7 5 151 945 INPUT DIGITAL IMAGE INTO SECOND MEMORY OLD IMAGE INPUT DIGITAL IMAGE INTO FIRST MEMORY NEW IMAGE CHANGE IN ABIENT LIGHT INTENSITY SUFFICIENT DIFFERENCE TWEEN IMAGES SIGNAL ALARM TRANSFER DIGITAL IMAGE FROM FIRST MEMO
9. suitable to overcome the effects of gradually changing brightness Unfortunately this attempt to solve the problem is limited in its usefulness in that it is dependent upon the rate of change of the ambient light intensity While one time setting may be suitable for slowly changing intensity levels such as might occur at sunset dawn etc this same time setting may be unsuit able for rapid changes such as clouds passing overhead during a thunderstorm or sudden dimming of lights in an office etc What is needed then is a surveillance system which is not only immune to false alarms caused by changes in ambient light conditions but also functions indepen dently of the speed with which these ambient changes occur SUMMARY OF THE INVENTION A visual image comparison method is provided in cluding the steps of obtaining a first digital representa tion of a first visual image where the first digital repre sentation comprises a first matrix having a plurality of pixels and each pixel has a gray scale value indicative of light intensity obtaining a second digital representa tion of a second visual image where the second digital representation comprises a second matrix having a plu rality of pixels and each pixel has a gray scale value indicative of light intensity and selectively making a first predetermined number of comparisons of corre sponding pixels from the first and second digital repre sentations to determine whether
10. visual image comparator FIG 2 is a flow diagram illustrating the general method of the invention 5 151 945 3 FIG 3A represents a first digital representation of a first visual image and FIG 3B represents a second digital representation of a second visual image which two images are compared by the present invention FIGS 4A 4E illustrate a schematic circuit diagram of an alternative embodiment of the invention which utilizes a dedicated hard circuit DETAILED DESCRIPTION OF THE INVENTION The present invention is a visual image comparison system which enables automatic surveillance of a loca tion over time or simultaneous monitoring of two or more identical or nearly identical objects The system described herein may be used in homes museums stores offices and other commercial establishments as well as in hotels airports and other special places re quiring security measures The invention may also find applications in industry such as for example the moni toring of a workpiece on an assembly line In addition to security surveillance applications the system may also be used to compare two visual images fingerprints etc simultaneously to determine if the images are the same or nearly the same The system is used in conjunction with a video cam era or other means of providing an analog visual image A visual image comparator method and apparatus ana lyzes the analog visual images and sounds an alarm if
11. 16 1 17 17 17 AGA 21 22 23 24 25 26 27 28 29 30 K 18 18 18 18 18 19 19 19 19 19 31 32 33 34 35 36 37 38 39 40 20 20 20 20 21 21 21 22 22 22 AGA 41 42 43 44 45 46 47 48 49 50 K 23 23 23 24 24 24 24 25 25 25 AGA 51 52 53 54 55 56 57 58 59 60 26 26 26 27 27 27 28 28 29 29 AGA 61 62 63 K 3 31 32 Although is determined automatically in the pre ferred embodiment a less sophisticated embodiment is possible where K is determined manually and by trial and error by the system operator For example if sur veillance of an indoor room having a constant ambient light intensity is desired the operator may manually set the sensitivity level and experiment with persons or objects moving in and out of the visual field until satis factory results are obtained Obviously increasing the number of pixels compared is one way of increasing the reliability of the system but at the cost of increasing processing time Another method of improving reliability without necessarily increasing processing time is in the selection of the number of comparisons which must indicate a differ ence in gray scale in order to signify an alarm condition 10 20 25 30 35 45 55 60 65 8 Moreover it is not necessary to actually make all the planned comparisons if for example a sequence of early comparisons indicate a problem For instance if 2 458 comparisons are to be made but only 90 compari sons indicating a difference
12. RY TO SECOND MEMORY FIG 2 U S Patent Sep 29 1992 Sheet 3 of 7 5 151 945 COLUMNS ROWS FIG 3A COLUMNS U S Patent Sep 29 1992 Sheet 4 of 7 5 151 945 5 30pF 14 3I8MHz 38 Kl ERGO N FIG 4A U S Patent Sep 29 1992 Sheet 5 of 7 5 151 945 mal PPPPP Ppp X06 H ppp ts OE wt 8 AIT A AB as A Al AO FIG 4B U S Patent Sep 29 1992 Sheet 6 of 7 5 151 945 n 53 54 ri LI T RESET SWITCH Ss 4 4 MANUAL xT AUTOMATIC LS38 FIG 4C U S Patent Sep 29 1992 Sheet 7 of 7 5 151 945 J CONTROLLED REI SWITCH ROLLED RE2 AC POWER SUPPLY HOV AC FIG 4E 5 151 945 1 DETERMINATION OF AMBIENT LIGHT LEVEL CHANGES IN VISUAL IMAGES In accordance with 37 C F R 1 96 a microfiche ap pendix is to be considered a portion of the entire writ ten description of this invention in conformance with 35 U S C 112 The appendix includes one microfiche having 21 data frames BACKGROUND OF THE INVENTION The present invention relates generally to video sur veillance methods and apparatus and more particularly to automatic surveillance systems which detect changes in a field of view over time and indicate an alarm condi tion accordingly The use of video cameras at remote locations for surveillance by video monitors is well known In some circumstances constant hum
13. United States Patent 4 Lee et al 54 DETERMINATION OF AMBIENT LIGHT LEVEL CHANGES IN VISUAL IMAGES George C Lee Williamsville N Y Xianyi Sun Beijing Taiwan 75 Inventors The Research Foundation of State Univ of N Y Albany N Y 21 Appl No 580 629 73 Assignee 22 Filed Sep 11 1990 51 Int CUS sse GO6K 9 00 52 Ss Chu u ttes 382 1 382 37 382 4 358 105 58 Field of Search 382 1 4 2 37 38 382 39 42 358 105 108 109 101 56 References Cited U S PATENT DOCUMENTS 4 337 481 6 1982 Mick et al 358 105 4 408 224 10 1983 Yoshida 358 108 4 455 550 6 1984 Iguchi 340 525 4 679 077 7 1987 Yuasa et al 358 108 4 737 847 4 1988 Araki et 358 108 Primary Examiner Michael Razavi 57 ABSTRACT A visual image comparison method is provided includ 12 VISUAL IMAGE COMPARATOR HAUT LM US005151945A 11 Patent Number 4 Date of Patent 5 151 945 Sep 29 1992 ing the steps of obtaining a first digital representation of a first visual image where the first digital representation comprises a first plurality of pixels and each pixel has a gray scale indicative of light intensity obtaining a sec ond digital representation of a second visual image where the second digital representation comprises a second
14. a sufficient difference exists between two different im ages f The apparatus of the invention is outlined in block form in FIG 1 to show how the invention interacts with auxiliary equipment Visual image comparator VIC 10 represents the present invention which oper ates on visual images provided by video camera 11 or alternatively by optional video monitor 12 When a change in non ambient light intensity or when motion occurs within the field of view of camera 11 or within monitor 12 VIC 10 signals an alarm represented by controlled devices 13 in FIG 1A Controlled devices 13 may be any device capable of indicating an alarm bell whistle buzzer light etc or it may even comprise a video monitor which automatically displays the chang ing video image when a change occurs For example in one application contemplated by the inventors a video camera is focused on the entrance to a residence As someone approaches the entrance the motion is de tected by the system The system may be programmed to display the video camera image on a television set or to display the image as a picture within a picture on the television to indicate the arrival of a visitor When the television set is turned off the system may be pro grammed to turn on lights or sound audible alarms etc or to turn the television on to display the changing video image In yet another application the camera might be focused on an infant s crib to mon
15. an supervision or monitor ing is required A typical example of this manual sur veillance method would be the remote placement of cameras in a retail store to detect shoplifting another example would be a camera in a bank activated during working hours to monitor a robbery attempt Many security systems typically employ a plurality of video cameras situated throughout a facility with a central monitoring location where a human guard keeps watch These manual systems are predecessors within the field of the present invention Human interaction in surveillance is extremely expen sive In some circumstances then it is economical and desirable to replace the human observer with an auto matic surveillance system or at least to alleviate the guard from the burden of constant supervision freeing him to perform other useful work Automatic surveil lance systems have evolved therefore to handle situa tions which do not require constant human supervision An example of this application would be the monitoring of an empty room at night where an automatic system would sense the entry of an intruder and sound an alarm It may be desired to monitor an outdoor parking lot or perhaps the entrance or exit of a building Other applications include monitoring products or workpieces on an assembly line etc In a multitude of applications automatic surveillance methods and systems are more economical and even more reliable than systems requir ing co
16. are required to signal an alarm then the processing can stop as soon as the 90th comparison indicating a difference is reached This may occur at any time 1 6 on the 2 450th comparison even on the 90th comparison etc This manner of processing ensures reliability by preventing false alarms while minimizing processing time A First Physical Embodiment For Implementing The Method In a first embodiment the method of the invention may be implemented using a software driven system of a persona computer such as an IBM PC or equivalent The software necessary to implement the system is included in the microfiche appendix Also required in a video imaging interface for converting the analog video signal to digital signals for processing by the computer In a preferred software driven embodiment a Model DT2803 Frame Grabber was used as the video inter face available from Data Translation Inc 100 Locke Drive Marlborough Massachusetts 01752 1192 Of course any commercially available equivalent video imaging interface could be used in lieu of the DT2803 The DT2803 Frame Grabber is a single board micro processor based video imaging interface suitable for use with the IBM personal computer series IBM PC AT XT and functionally IBM compatible per sonal computers This video interface provides real time 6 bit digitization of an RS 170 RS 330 NTSC or CCIR PAL compatible input signal The DT2803 plugs into the PC backpla
17. itor the baby at sleep If the infant awakens moves or becomes dis tressed the parents can be alerted accordingly A preferred embodiment of the invention which shows VIC 10 in more detail is shown conceptually in FIG 1B VIC 10 includes A D converter 14 RAM NEW 15 RAM or 16 controller 17 and executor 18 A D converter 14 converts the analog video signal provided by camera 14 into digitized signals The digital representation of a first visual image so obtained is then stored RAM wzp 15 which is a random access mem Ory At a subsequent instant in time a new image signal is obtained The digital representation stored in 10 20 25 30 35 40 45 50 55 60 65 4 NEW 15 is transferred to 16 and the new digital image is stored in RAM ven 15 Controller 17 controls the transferring of the old image from RAM NEW 15 to 16 and then compares the old and new images If a sufficient difference exists between the images controller 17 sends an alarm signal to executor 18 Executor 18 activates various auxiliary alarm de vices as discussed previously In the preferred embodiment depicted in FIG 1B both the old and new digitally represented images are stored in separate memories prior to comparison It should be readily apparent however that it is also pos sible to compare temporally displaced images by only storing the earlier image in memory and then compar
18. l images respectively are then com pared box 23 A decision is made as to whether a difference in ambient light intensity exists between the two images Such a difference would occur for exam ple if the sun suddenly disappeared behind a cloud if a tree branch was moved by the wind in front of the camera lens at sunset or sunrise or if the lights in a room were turned on or off etc In other words such a change would likely be distributed somewhat through out the entire visual field The essence of the invention is to distinguish between such ambient intensity changes and other changes such as might be caused by someone or something entering or leaving the visual field The invention will indicate an alarm condition for the latter condition but will not sound a false alarm for the former condition If an ambient intensity difference is found to exist in box 23 control is passed to box 26 where the digital image previously stored in the first memory is trans ferred to the second memory and the image previously stored in the second memory is erased The method then proceeds back to box 22 where a new image is obtained and stored and the process repeated If on the other hand no ambient intensity change or difference is detected control passes directly to box 24 where a second comparison is done between the first and second visual images If a sufficient difference exists between the two images an alarm is indicated box 25
19. light intensity exists between the first and second im ages is preferably made with K 2 or 3 N 90 110 and J 25 40 These ranges are intended to be guidelines and approximations and it is not intended or implied that other combinations of J K and N will not work satisfactorily only that satisfactory results have been obtained when the variables are selected within these ranges In the second comparison or so called common sur veillance mode a different range of the variables are utilized to determine if an alarm condition exists It has been determined experimentally that N 2 or 3 achieves satisfactory results when J 3 7 While other values of N may also work it has been found that N 1 often results in false alarms and high values of N result in low surveillance sensitivities which may not detect small moving objects within the visual field Similarly although larger values of J may work satisfactorily as J increases small moving objects may not be detected In a preferred embodiment the K value in the second comparison is determined by the average gray scale value of the ambient AGA according to Table I be low The average gray scale value of the ambient is determined by adding all of the individual gray scales values G s of each considered pixel and then dividing this total by the number of pixels considered TABLEI AGA 1 2 3 4 5 6 7 8 9 10 K 1 2 4 7 9 12 14 16 16 16 AGA l 12 13 14 15 1 6 18 19 20 16 16 16 16 16
20. mmed to sound an external alarm For this purpose an addi tional output port interface board is required such as DT2801 also available from Data Translation Inc 5 151 945 9 Commercially available equivalent interfaces are also suitable A Second Physical Embodiment For Implementing The Method FIGS 4A 4E represent a second physical embodi ment for implementing the method of the invention This second embodiment is a hard or dedicated circuit designed specifically to implement the method of the invention Dedicated circuit 30 comprises I O channel socket 49 which interconnects with an image processing inter face board e g DT 2803 available from Data Transla tion Inc or equivalent to accept digitized video sig nals which are stored in the 64 k random access memory RAM on the board RAMs 35 and 36 are used to store the old frame image signals for later comparison with the new signals stored in memory on the interface board RAMs 35 and 36 comprise 64 k of memory 43256 10L or equivalent Erasable programmable read only memory EPROM 34 27C256 20 or equiva lent is used to store the control program similar to ALARM U or ALARM S used in the first embodi ment Central processing unit 32 8088 or equivalent controls the operations of the entire circuit Latches 38 39 and 40 74L S373 or equivalent are used to latch the address signals AQ Al A19 and send them to the appropriate chips as required Bus
21. ne and includes a video imag ing input analog to digital converter and look up tables a 64 kilobyte frame store memory a video imaging output digital to analog converter look up tables and microprocessor and control logic For a more com plete description of the capabilities and operation of the video imaging interface the reader is referred to the User Manual for the DT2803 Low Cost Frame Grab ber available as Document UM 03286A copyright 1985 by Data Translation Inc This document is incor porated herein by reference as representative of the general state of the art with respect to video imaging interfaces The software included in the microfiche appendix is self executing After booting up the computer and load ing the software the user merely types the word ALARMS on the keyboard and then follows the self explanatory menu driven instructions for setting the system sensitivity If desired the user can omit setting sensitivity levels in which case the levels are automati cally set by the software In the software driven embodiment video camera 11 provides analog video signals to analog to digital con verter 14 DT2803 and the digital signals are then processed by the computer as previously described When an abnormal or alarm condition is detected by the system the software sounds an audible alarm through the internal speaker of the computer With minor soft ware modification the system can also be progra
22. nstant human interaction A common problem encountered by all automatic surveillance systems both indoor and outdoor involves false alarms triggered by changes in ambient light inten sity For example in monitoring an outdoor scene such as a parking lot a cloud passing overhead may substan tially affect ambient light conditions and trigger a false alarm Even in indoor applications many offices em ploy automatic light dimming circuits which dim the lights in the evening causing problems for automatic surveillance systems Attempts to solve the false triggering problem are well documented in the art One well known technique involves the use of automatic exposure lenses or cam eras to compensate for ambient light intensity varia tions Unfortunately this method is limited to only small variations in intensity Another alleged solution is proposed by Yoshida in U S Pat No 4 408 224 Oct 4 1983 Yoshida broadly discloses a surveillance method which includes the comparison of two digitized video image signals taken of a place scenery at different points in time To solve the problem caused by changes 10 20 25 30 40 45 50 60 2 in ambient light Yoshida suggests displacing the cap turing of the two video images in time by an amount which is negligible with respect to the ambient changes in brightness For example Yoshida suggests that dis placing the capture of the images by 15 seconds to 1 minute is
23. rising 61 440 pixels as few as 1 536 pixels need be compared skipping every 40th pixel i e J 40 and yet still achieve accurate and reliable results In other words it is only necessary to compare approxi mately 2 to about 4 of the total image assuming that the compared pixels are distributed throughout the image Of course more pixels could be compared but this would increase processing time The processing time required to compare 1 536 pixels is less than 0 3 seconds Experiments also indicate that for J 25 40 N may be in the approximate range of N 90 110 to achieve accurate results In other words approximately 3 to about 8 of the compared pixel pairs must exceed the predetermined K value magnitude of gray difference in order to indicate a difference in ambient light inten sity Finaly in making the ambient light comparison experiments indicate that a low K value is preferred 55 60 1 The total number of comparisons of corresponding pixels from each image Determined by J the num ber of pixels to jump over when making comparisons from each image 2 The number of comparisons N which must yield a difference in gray scale in order to signify an alarm condition 65 e g K 2 or 3 since the change in ambient light may be very small This is the magnitude K of the gray scale difference within a given pixel pair comparison necessary to signify an alarm condition For example if one pixel
24. rising the steps of obtaining a first digital representation of a first visual image comprising a first plurality of pixels where each pixel has a gray scale indicative of light intensity ob taining a second digital representation of a second visual image comprising a second plurality of pixels where each pixel has a gray scale indicative of light intensity selectively making a first predetermined number of comparisons of corresponding pixels from the first and second digital representations to determine whether a difference in ambient light intensity exists between the first and second visual images and selectively making a second predetermined number of comparisons of corre sponding pixels from the first and second digital repre sentations if and only if no difference in ambient light intensity exists between the first and second visual im ages and indicating an alarm condition when a percent age of the second predetermined number of compari sons result in pixels having a difference in gray scale of a predetermined amount FIG 2 illustrates by flow diagram the general method of the invention To begin the process it is assumed that a frame of digitized data representative of a second visual image is already stored in a second memory box 21 A new image is then digitized and a 5 151 945 5 new frame of digitized data is stored in a first memory box 22 The new and old digital representations of first and second visua
25. said first digital representation com prising a first plurality of pixels wherein each pixel has a gray scale indicative of light intensity Obtaining a second digital representation of a sec ond visual image said second digital representation comprising a second plurality of pixels wherein each pixel has a gray scale indicative of light inten sity 20 25 30 35 45 50 55 c selectively making a first predetermined number of 65 first comparisons of corresponding remotely dis placed pixels from said first and second digital representations to determine whether a difference 10 in ambient light intensity exists between said first and second visual images wherein said difference in ambient light intensity is defined to exist when a first percentage of said first predetermined number of first remotely displaced comparisons result in pixels having a difference in gray scale of a first predetermined amount and d selectively making a second predetermined number of second comparisons of corresponding near neighboring pixels from said first and second digi tal representations 1f and only if no significant dif ference in ambient light intensity exists between said first and second visual images e and indicating an alarm condition when a second percentage of said second predetermined number of second near neighboring comparisons result in pixels having a difference in gray scale of a second predetermined amount 2
26. transceiver 41 74L S245 or equivalent is used to transmit and receive data signals Clock generator 31 8284 or equivalent produces CLK and OSC signals to synchronize the whole system Programmable input output interface chip 42 8255A or equivalent is used to input the man ual sensitivity setting using BCD switches 51 and 52 and octad driver 43 74LS244 or equivalent and to output the sensitivity display via 7 segment decoder drivers 44 and 45 741547 or equivalent to displays 53 and 54 respectively and to send out alarm signals AN1 and AN2 which can drive buzzer 55 via driver 46 74563 or equivalent or other alarm devices via relays RE1 and RE2 driven by driver 48 74563 or equivalent The reset switch is used to reset the relays and sensitiv ity switch is used to select either manual sensitivity selected by BCD switches 51 or 52 or automatic sensi tivity selected by the software While the form of the apparatus used to implement the method of the invention as described herein consti tutes a preferred embodiment of this invention it is to be understood that the invention is not limited to this precise form of apparatus and that changes may be made therein without departing from the scope of the invention as defined in the claims What is claimed is 1 A visual image comparison method for automated video surveillance comprising the steps of a obtaining a first digital representation of a first visual image
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