Patent GB-1363207-A (GB)
Title: Stereoplotting apparatus for correlating image points disposed along epipolar lines
IPC: G01C11/12
Filed: 1972-11-30
Abstract: 1363207 Photogrammetry BENDIX CORP 30 Nov 1972 [8 Dec 1971] 55367/72 Heading H4D In photogrammetric stereoplotters the encountering by the scanning means of areas in the respective stereophotos having no correlatable detail, e.g. deserts, can cause a random, timeconsuming scan search for correlatable detail. If the stereo-model represented in the stereophotos 100, 102, Fig. 2, is represented by a surface 104, and the vantage points, i.e. positions of the aircraft when the photos were taken, are represented by the positions 106, 108 of projection lens, a plane 114 containing position 106, 108 and any point 116 on the surface 104 will intercept the photos in lines 110, 112 which should contain correlatable detail. Such planes are epipolar planes and the lines 110, 112 and 117, 118 pairs of epipolar lines. An infinite number of epipolar planes and lines are produced by rotating plane 114 about line 106-108. Where the vantage points are at the same height, the epipolar lines are parallel with each other, whilst if the vantage points are at different height, Fig. 3, the epipolar lines on any photo form a set of radials converging on the point where the line 124-126 passes through the plane of the photo, e.g. points 137, 138. By causing the scanning means to scan only epipolar pairs of lines of the two photos, the maximum probability of detecting correlatable detail after passing through a detailless area one is achieved. In the stereoplotter of Fig. 1, light from a laser 12 is passed via lenses 46 at position 106, 108, to scan respective photos 18, 20. The lenses 46 and the photos are adjustable in position and attitude to simulate the positions of the aircraft when the photos were taken, The laser 12 is carried on a system of worm drives 26, 32, 34, the former being driven by motor 36 to give the X co-ordinate of the scanned spot, and the drives 32, 34 being driven by motors 38, 40 at different speeds when one of the parallel epipolar lines is to be scanned. The motors are controlled by means 66 which is supplied for checking purposes with feedback position information from Y left counter 76, X counter 74 and Y right counter 78, fed with pulses from tachometers 70, 68, 72, respectively. Photomultipliers 58, 60 detect the modulated light from the photos and corresponding digitized video signals from A/D converters 84, 86 pass into registers 88, 90, together with the positions of the scanned areas from counters 74, 76. The registers 88, 90 store the video signals of corresponding lines in the photos for a search in controller 92 and correlator 94 for corresponding areas of maximum correlation. If the photos 18, 20 are as shown in Fig. 4, the scan along line 142 will detect a detail 150 and means 92, 94 will detect the corresponding detail 152 in the epipolar line 146 in the other photo. Correlator 94 thus compares n consecutive units of detail 150 with a group of n consecutive units of line 146, the group "walking through" the line, until means 96 determines that the degree or correlation determined by 94 has reached a maximum. The locations in the two epipolar lines of the corresponding details 150, 152, is stored at 98 and forms the basis in calculator 64 for a readout of X, Y and height Z of the surface 104. When the next detail 154 is detected on line 142, after a measured X interval, the "walking through" search for the corresponding detail 158 on line 146 is made along a length 156 centred on a point at the X interval distance from detail 152. A detail 160 on line 144 is similarly searched for in epipolar line 148 along a length 162 centred on the position of the detail 160 on line 144. The search time for new correlatable details is thus substantially reduced. The epipolar lines may be curved if the film is warped or the taking optics have introduced distortion. X worm and drive 26, 36 may be replaced by a system of light fibres having their input ends formed into a circle and scanned by a rotating crank-handle shaped light fibre carrying the laser light, and having their output ends formed into a straight line in the X direction and carried by the Y worms and drives (Fig. 5, not shown).