GB1475116A - Image comparator - Google Patents

Abstract

1475116 Image comparison E J BOLSEY 19 June 1974 27146/74 Heading G1A [Also in Divisions G4 and H4] Two images or patterns, one of which may be a stored reference, are compared by scanning a first image to develop a video signal, filtering the signal to cut out low-frequency components, amplifying the signal, quantizing it to obtain binary signals, comparing these with a sequence of reference binary signals representing a second reference, image, and producing an output representing differences, degree of coincidence, there between, means being provided for blocking video signals below a predetermined threshold. Surveillance system, Fig. 1. An area to be protected is scanned by means including e.g. a silicon junction or barrier photo-cell, or a photoresistor of calmium sulphide, solenide or sulphosolenide. A suitable scanner may utilize a doublefaced rotating mirror co-operating with a lens and a mash for the photo-cell, Fig. 5 (not shown). Alternatively a four or more sided mirror drum, Fig. 6 (not shown) is used. A photo-resistor may be connected into circuit with a varistor &c. to obtain a logarithmic detector response for wide brightness variation application Fig. 4 (not shown), alternatively, a solar cell feeding a high inputimpedance amplifier maybe used. A variety of scan configurations are possible including circular or sector shaped. Low frequency components of the video signal are eliminated in filter 103 and quartization effected by flip-flop 104, which produces an output every time the video signal is above the zero threshold. A reference image of the area to be protected is stored in a magnetic, drum or disc, memory 105 and used to supply comparator 106, a logic multiplier or exclusive OR gate, with reference binary signals against which a current video scan is compared. Each time there is a mismatch between there, due e.g. to entry of a person, change in objects in the area, fire, smoke, water leak &c., a signal is fed to integrator 107 which produces a ramp waveform for the duration of the mismatch. If this goes above a threshold 108 an alarm is energized at 110. At the same time a signal to gate 113 allows sawtooth generator 112 output to reach stretcher 114 which indicates the co-ordinate position of the alarm causing event. A number of locations can be simultaneously monitored under the control of a central memory and processor Fig. 2 (not shown). The comparator is connected to a gate 313, Fig. 3, allowing an output only above a predetermined threshold to reach the alarm circuit, to prevent spurious noise signals triggering the alarm. A further magnetic stone track may be provided, for use in different illumination conditions. Alternatively, illumination conditions may be monitored by a low pass filter, Fig. 3, to cause energization of a relay to cause updating of the store at the prevailing illumination condition. An alarm may be generated if the frequency of updating of the store exceeds a pre-set value. Alignment system. The investigation of alignment is appropriate for photo-grammetry when misalignment is indicated on meters, for two coordinate directions, angular rotation with respect to a recorded reference. Two aerial stereophotographs may be analysed simultaneously. Alignment of a photographic mask and a photosensitized substrate in semi-conductor manufacture is also possible, Figs. 8, 9 (not shown). Character recognition, Fig. 10 (not shown). A plurality of reference patterns are stored as binary signal sequences in memory 161, having been produced by scanning known characters using a deflectable photo-multiplier image dissector tube. Unknown characters are scanned and binary signals from 155 are delayed at 162 and compared in gates 168-170 with the stored references to identify the character ultimately producing a single output on one of the channels, outputting at 165-7. If there is slight misalignment of an unknown character with the original location of the stored reference character, no output is produced in any channel, owing to threshold sensors 174-6 not being stimulated. This is detected by control logic 180 to initiate a search scan e.g. an expanding spiral until a significant output occurs. This stops the search and causes the reference binary sequence to be compared with the undelayed unknown character binary sequence and with a double delayed such signal in respective exclusive OR-gates 191, 192, in order to generate the necessary orientation connection signals.