Classifications

• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
  • G06T7/00—Image analysis
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
  • G01N33/0078—Testing material properties on manufactured objects
  • G01N33/0081—Containers; Packages; Bottles
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V5/00—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity
  • G01V5/20—Detecting prohibited goods, e.g. weapons, explosives, hazardous substances, contraband or smuggled objects
• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06V—IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING
  • G06V10/00—Arrangements for image or video recognition or understanding
  • G06V10/20—Image preprocessing
• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06V—IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING
  • G06V10/00—Arrangements for image or video recognition or understanding
  • G06V10/20—Image preprocessing
  • G06V10/28—Quantising the image, e.g. histogram thresholding for discrimination between background and foreground patterns
• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06V—IMAGE OR VIDEO RECOGNITION OR UNDERSTANDING
  • G06V10/00—Arrangements for image or video recognition or understanding
  • G06V10/40—Extraction of image or video features

Definitions

• This invention relates to an image analysis system and more particularly a system for analysing an X-ray image of mail items for the purpose of detecting the pressure of Improvised Explosive Devices (IED's).
• IED's Improvised Explosive Devices
• vapour trace detection Historically there are several techniques and devices employed in the detection of IED's, the principal ones being X-ray fluoroscopy and vapour trace detection.
• the former technique has involved visual examination of an X-ray image, and is dependent on the sustained alertness and interpretative ability of a human operator, whilst the main problem with vapour trace detection techniques is that they can be readily countered by suitable containment of the explosive device, and furthermore they are incapable of detecting some of the types of explosives commonly used.
• the present invention involves identification and therefore detection by image analysis of explosive devices in situations where a number of apparently similar items are present, by utilising the particular characteristic qualities of the radioscopic images produced by such explosive devices.
• the radioscopic image of a lethal IED possesses a minimum proportion of dark area caused by the presence of X-ray absorbing material essential to the operation of the IED, and in particular a lead azide primer charge, present in the most commonly used commercially available detonators required to detonate an explosive charge.
• the present invention involves discerning the presence of such a material as distinct from the radioscopic images produced by other items present, which in the case of mail processing may be various types of paper clips.
• the invention of SYSTEM 1 utilised the characteristic of the fluorescent image produced by the IED by employing a closed circuit television (CCTV) camera which provided video signals from which the extent of dark areas of the images produced may be measured in terms of the duration of any signal whose intensity was less than a predetermined level (threshold level).
• CCTV closed circuit television
• SYSTEM 1 as applied to the detection of IED's, basically rested with an electronic system utilising a CCTV camera to rapidly clear any large number of articles, such as mail, as being safe, whilst if the throughput amount is relatively small a trained operator may be sufficient to clear any items detected as being possibly harmful.
• SYSTEM 1 can be regarded as a "stand-alone" system. Its employment in a mail Registry situation would be directed at ensuring that mail which has been checked by the system does not contain an IED. However, articles with X-ray image densities comparable to that of an IED image are also rejected by the system. Thus, generally, a number of innocuous articles in addition to IED's would be rejected. For a small Registry situation, SYSTEM 1 may, for example, clear as safe 98% of the checked mail. The remaining suspect mail may then be examined fluoroscopically by an observer. Nevertheless, the small percentage of suspect articles could still amount to a large volume of mail in a high throughput situation which would make considerable demands upon an observer.
• the system of the present invention is designed to reduce this high volume by reducing the number of suspect articles, and this is achieved by the use of pattern recognition techniques by which miscellaneous or common articles such as X-ray dense large paper clips are accepted as being innocuous.
• the system of the present invention may be integrated with other mail sorting equipment or procedures. These may include (in the interest of speed) an initial sorting device which diverts mail of thickness equal to or greater than that of a minimum size explosive device capable of causing significant injury.
• SYSTEM 1 the system of the present invention, and fluoroscopic viewing facilities may be incorporated into one unit employing one assembly of X-ray source and electro- optical equipment. Alternatively the system of the present invention may be utilised in a sequential checking system at the cost of extra assemblies.
• the present invention therefore envisages an apparatus for the detection of IED's comprising means to produce an X-ray image of the article under investigation, means to scan said image and provide signals indicative of the intensity of the image at any particular point, means to process said signals so as to provide an indication of at least when any signal makes a transition through at least one predetermined (threshold) level, and means activated in response thereto to process said signals and apply a pattern recognition technique to the image.
• the means to scan said image is a CCTV camera.
• said means to process said signals is in accordance with that the subject of our aforementioned International Patent Application, that is, such as to provide an indication of not only when the signal makes a transition through a predetermined intensity level but also the duration of any signal whose (threshold level) varies from the, or each, said predetermined level as a measure of the extent of dark or light areas on the image, and said means for processing said signals to apply said pattern recognition technique is automatically activated in response thereto.
• said means to process said signals merely provides an indication of when any signal makes a transition through the, or each, predetermined threshold level, and said means for processing said signals to apply said pattern recognition technique is activated in response thereto.
• the invention also envisages a method for analysing images, comprising the steps of scanning said image and providing signals indicative of the intensity of the image at any particular point, processing said signals so as to provide an indication when any signal makes a transition through at least one predetermined threshold level and, in response thereto, applying a pattern recognition technique to the image.
• the pattern recognition technique may be automatically activated to directly apply the pattern recognition technique to the image of the suspect article. Alternatively the suspect article may be dive'rted for separate and subsequent application of the pattern recognition technique.
• Figure 1 is a schematic diagram showing the basic arrangement of equipment in accordance with one preferred form of the present invention, and in particular the manner of linking the equipment to the multibus of the Single Board Computer utilised for the purposes of carrying out the pattern recognition technique of- the present invention,
• FIG. 2 is a block diagram of the basic circuitry of the processing system as employed in this preferred form of the invention
• Figure 3 is the procedure as carried out in a processor to provide recognition or identification of radiographic images
• Figure 4 is a flow chart of the algorithm employed to carry out part of the processing technique of the present invention
• Figure 5 is a flow chart of an example of an algorithm employed specifically for the identification of paper clips and therefore the clearance of mail containing such items.
• the article under investigation is positioned before an X-ray emitter, and the X-ray image produced is displayed on a fluorescent screen where it is scanned by a CCTV camera to produce a digitised signal.
• the digitised signal may then be processed in accordance with SYSTEM 1 for the purposes of detecting the presence of a possible IED.
• the "Processed Video” image which would be seen on the TV monitor with SYSTEM 1 is a single level discriminated image of the complete TV picture.
• the only information in the "Processed Video” image is the perimeter where the video picture makes a transition through a fixed (threshold) level.
• Run end coding is employed in the storage of the perimeter data in order to reduce the quantity of data and data rate.
• An Intel Single Board Computer 86/12A is employed. It has an 8086 CPU which has a 16 bit, one megabyte addressable microprocessor.
• Figures 1 and 2 consists of four boards fitted into the Intel SBC 604 cardcage.
• the four boards are:- (a) Video Encoder Board which takes the Video signal from the TV camera and processes it for two level (i.e. above and below the threshold level), run end coded data output.
• DMA Direct Memory Access
• Data transfer rates within the system determine in one complete scan of the TV camera (odd and even lines), 23 thousand transitions which can be stored in the 86/12A memory. For a 512 line picture this is an average of 45 transitions per line. The maximum number of transitions for anv one line is 512.
• any transition in the output of the comparator in SYSTEM 1 causes the position of the transition in the TV image to be encoded into a sixteen bit word, together with the direction of transition.
• the sixteen bit word is sent to the data acquisition and storage circuit described below.
• the encoding of the position of any pixel in a 512 x 512 matrix in a sixteen bit word is performed by run end coding.
• the y-co-ordinate, or vertical position is not encoded into each word but a word is generated at the end of each scan line and a one bit flag inserted to indicate this.
• the x-co-ordinate or horizontal position of the transition is encoded into the 16 bit word as a value between 0 and 512.
• the 16 bit data words which are generated by the video processing circuit in a minimum time period of 120 nanoseconds, must be stored within the time interval of 120 nanoseconds.
• the high speed memory capable of this performance, consists of two identical memory buffers, each 16 bits wide by 1024 words deep.
• the first buffer is filled by the asynchronously generated 16 bit data words from the video signal processing circuit.
• the 16 bit data path is now switched from the first buffer to input to the second buffer.
• the data in the first buffer is now transferred by a direct memory access circuit to the memory of the central processing unit.
• the second buffer is filled by the data output from the video processing circuit.
• the direct imemory access circuit is designed to interface with a commercial "backplane"' or bussing system marketed by the Intel Corporation as Multibus.
• the DMA circuit When initiated to do so by the filling of one high speed memory buffer, the DMA circuit gains control of the multibus to become the bus "master". In this mode it can write into the memory of the CPU.
• the DMA circuit transfers the data in the high speed memory buffer to the memory of the CPU.
• the DMA circuit relinquishes control of the multibus and hence its ability to access CPU memory. This allows the CPU to immediately begin processing the data which has just been transferred into it by the DMA circuit.
• the CPU is thus processing the video data simultaneously with the acquisition of data by the high speed memory buffers. Analysis of the image can be partly complete before the end of the picture scan.
• the central processing unit is a commercial Single Board Computer 86/12 as supplied by Intel
• the algorithm used to recognise the processed video image is stored in electrically programmable read only memory integrated circuits and can thus be changed by the replacement or reprogramming of those integrated circuits containing the program.
• a standard time-saving technique for analysing an optical image is to couple a TV camera through a suitable interface to a computer.
• the image on the target of the camera tube is scanned by an electron beam which creates a potential difference and produces on a collector a signal proportional to the input brightness pattern.
• a digital image is obtained by quantizing the signal through an analogue to digital converter at fixed points along the scanning beam
• the transfer of fully digitised information at the standard TV scanning rate is difficult to achieve.
• One solution is to use a slow-scan camera. With this preferred embodiment of the invention a Hamamatsu C1000-12 SIT camera is used coupled to the minicomputer as the image acquisition and analysis system.
• a standard TV camera may be used with the amount of video information to be transferred minimised.
• a large reduction in the quantity of video information may be achieved by classifying the video signals into two states according to predetermined intensity levels.
• the two states are, the black, for signals lying between intensity levels deemed to represent useful information, and, the white, for signals outside the range, deemed to be irrelevant.
• a further reduction in the amount of data transfer may be achieved by the application of run-end coding technique to the binary image.
• a run denotes a succession of image elements on a line of scan.
• Run-end coding describes a scheme whereby only the positions of the two ends of each run along the line of scan are stored.
• the system of the present invention is designed to store images in this form.
• the component labelling algorithm is also designed to handle the run-end coding data. Thresholding
• Thresholding involves conversion of the digital image into binary form, based on its densitometric information; thus eliminating irrelevant data from the image as per SYSTEM 1.
• a defect usually associated with the video signal generated by a TV camera is the baseline droop. Applying thresholding techniques to this type of signal would not produce satisfactory results, however this effect can be corrected electronically by devices available commercially. The effect can also be corrected utilising suitable software, by simply subtracting a suitable background from the image.
• TV image is the presence of random noise. If the original image is too noisy near the threshold, the resultant binary picture would contain scattered white and black spots. This noise poses little problem in the present system because the threshold level is usually set well outside the region affected by the noise. In cases where the noise would affect the outcome of thresholding, it may be suppressed by applying a digital filtering technique.
• Component Labelling is the major step in the image analysis of the present invention in that it groups information extracted by thresholding into individual objects.
• a flow chart of a suitable labelling algorithm is shown in Figure 4. This algorithm has several useful features:-
• each run of picture elements along a line of scan is specified by the coordinates of its ends on the line.
• the most appropriate algorithm for labelling is that of run-tracking in accordance with Figure 4.
• the runs are located sequentially from left to right along each line, and line by line from the top to the bottom.
• On the first line of the image each run is treated as belonging to a new object, thus given a new label.
• the labels which are usually whole numbers start with 1 then continue in ascending order according to the order in which the objects are encountered.
• each run is subjected to an overlap search to determine whether it overlaps with any run on the preceding line.
• the run is assigned a new, hitherto unused label, if there is no overlap. But, if there is overlap, it is given the same label as the run it overlapped.
• the overlap search is continued to determine whether it also overlaps with the next run on the preceding line. If there is more overlap, and the runs are of different labels, the run with the higher number is relabelled with the lower number.
• a run is denoted by R (X-, Y ⁇ ) where X and Y are the coordinates of the left- and right-end of the run
• the condition implies the concept of 4- adjacency for connected elements. Generally, both inequalities must be tested to determine if there is overlap. However, the procedure is so designed that, in most cases, it would be sufficient to test one of the inequalities only. In addition, it is only necessary to test P. (X i , Y i ) for overlap with R (x ' r , Y ' r ) which satisfies the condition Y r > Y i - 1 , and the search is terminated as soon as the condition Y r > Y i is fulfilled.
• shape description should be based on information which is independent of magnification and orientation. Preferably the information should be readily extracted from the image.
• simple geometrical features which may be computed in a single pass curing the labelling process are used to characterise shapes.
• the more exotic shape descriptors such as the Fourier descriptor and differential chain code are not preferred because they would require images with well defined boundaries and would involve greater computational effort.
• the features we have computed include area, perimeter, moments about a fixed point, vertical and horizontal Feret diameter, and secondary features such as centroid, circularity and central moments:- (1) Area. Area definition is trivial simply the total number of elements of an object.
• Horizontal and vertical diameter are respectively the maximum distance between pairs of vertical and horizontal targets of an object. They are determined by recording the left and right, upper and lower extremities of an object during labelling. These parameters may provide information about whether an object is compact or dispersed.
• Centroid The Centroid is computed at the completion of labelling from the first moments and area.
• the centroids are particularly useful as they provide a means of deriving a relational structure among the various objects in a picture.
• Circularity This is defined as 4" X Area/Perimeter. 2 Its magnitude tends to reflect the complexity of the boundary.
• a 1 Objects below a minimum size, may be disregarded. It is designed to eliminate noise and fragments of image close to the intensity threshold.
• the minimum size is chosen to be well below the size of items such as the priming charge of a detonator, blob of solder in electronic circuits, electric wire junctions etc. Criterion 2.
• a 2 is set to be the size of the largest paper clip designed to be cleared.
• the system described above is designed to meet the throughput requirement of a mail exchange. It is capable of converting an X-ray fluoroscopic image into the run-end coding form and transferring it into the CPU of a microprocessor (Intel 80/86) in one twenty-fifth of a second. The entire image analysis using the labelling procedure described may be achieved in a fraction of a second.
• a set of algorithms can be designed to clear various common stationery items in addition, to that described previously for paper clips.
• clearance criteria may be varied according to demand. For example, if there is a demand to automatically clear mail containing audio cassette tape. one may design a set of clearance criteria based on the image of its five screws, appeared as five small circles of approximately equal size, and the sum of the distances from each screw to their centroid. Similarly, if the threat of a particular bomb design were made aware to the postal authority, the system may be programmed to search for this particular design.

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• 1982
  • 1982-09-08 EP EP19820902666 patent/EP0087442A4/fr not_active Withdrawn
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  • 1982-09-08 JP JP57502674A patent/JPS58501445A/ja active Pending
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