Patent GB-1227579-A (GB)
IPC: G01C11/00
Filed: 1968-07-29
Abstract: 1,227,579. Photogrametry. ITEK CORP. 29 July, 1968 [31 Aug., 1967], No. 36016/68. Heading H4D. Corresponding details in stereo photographs 30 and 31, Fig. 1, are compared, to determine parallex &c. by means 49 and 43 producing first and second analogue signals, respectively representing detail in corresponding paths in the two photographs, and comparison means 61 wherein each signal is analysed into regions of slope sign change and the phase difference between corresponding regions in the signals is determined. Flying spot scanners 33 and 34 are used having a diagonal rectangular scan, Fig. 12, under the control of x and y deflection signals from generator 43. A phase lead of one signal over the other, for an x component scan of left to right, becomes a phase lag when said component scan is from right to left. Correlation between the operation of comparison means 61 and the deflection signal is.thus necessary.. If the x-scale of one photograph is different from the other, i.e. one is stretched horizontally, Fig. 8, a phase lag with the scanning spot in the righthand half of the photograph becomes a phase lead with the spot in the left hand half. Further deflection correlation is thus necessary. Skew correlation, Figs. 10, 11, is also accounted for. Photo-multiplier detectors 51, 52 thus produces respective signals 55, 56, Fig. 6, which are fed to comparison means 61, Fig. 2. The lagging signal 55 feeds a peak/valley detector 65, Fig. 2a (not shown), wherein pulse signals 69 and 71 appear, respectively indicative of peaks and valleys in signal 55. The pulse signals are fed to a bi-stable device 94, whereby a binary signal 95 appears having a zero voltage part commenced by a peak pulse and a positive voltage part commenced by a valley pulse, whilst the complement 96 of signal 95 appears at the other output of device 94. Complementary binary signals 98 and 99 are similarly produced by device 97 from the leading signal 56. Overlap of signals 95 and 99 is detected by AND gate 101 to give signal 102 and-overlap of signals 96 and 98 is detected by AND gate 103 to give signal 104. The two overlap signals being combined in OR gate 105 to give signal 106 indicating by its total duration the delay between the signals 55 and 56. Said total duration is determined by using signal 106 to gate through at 108, clock signals 109 from generator 110. Pulse train 107 is thus produced. To determine the sign of the delay, signals 69 and 71 are combined in OR gate 111  to give signal 112 and signal 112 is fed to gate 113. Signals 92 and 93 are combined in OR gate 116 and resultant signal 117 is used to inhibit gate 113. Similarly signal 117 is fed to gate 115 which is inhibited by signal 112. The outputs 119 and 121 of gates 113 and 115 control bi-stable device 122 whereby enabling signals 123 and 124 appear, for enabling AND gates 125 and 126, respectively. Pulse train 107 is fed to the AND gates and is distributed by the gates whereby if signal 55 lags 56 most of the pulses in the train 107 appear in "positive" signal 63 whilst a leading condition is indicated by a majority of the train pulses appearing in "negative" signal 63. The parallex analyser system 64, fed with the signals 62 and 63 and with the x and y scan deflection signals (the x deflection signal being shown at 42, Fig. 14 and 15), is shown in Fig. 13. A signal 136, Fig. 14, representing the direction of the x scan is produced for signal 42 by means of differentiator 135 and complementary signals 138 and 141 are produced therefrom by bi-stable device 139. Signals 138 and 141, together with signals 62 and 63 are fed to an x parallex correlator 143, shown in detail in Fig. 15, whereby during right to left movement of the scan spot the signals 62 and 63 are switched over to appear in signals 145 and 144, respectively. Thus a signal 62 indicating lead/lag/lag &c. becomes a signal 144 indicating lead/lead/lead &c. independent of the x-scan direction. Signals 144 and 145 are fed to a counter 146 wherein they are subtracted to give a count of the x-parallex. Means 135<SP>1</SP>, 139<SP>1</SP>, 153 and 156 similarly indicate the yparallex. To determine differences in the xscales of the two photographs the switch-over process of means 143 is repeated on signals 144 and 145 in correlator 159. The switch-over signals 158 and 162 indicate in which horizontal half of the picture the scan spot is. y-Skew distortion is detected in correlator 185 by switching the switched y-parallex signals 154 and 155 by the x scan switch signals 158 and 162. The contents of the various counters 146 &c. can be fed to a computer or can be used to register the photographs. Such registration is only with respect to the coarse features of the photographs, the fine detail in the photographs being left unregistered to retain the stereoscopic information.