Patent GB-963644-A (GB)
Title: Improvements in automatic image matching apparatus
IPC: G01C11/00
Filed: 1961-07-28
Abstract: 963,644. Photo-electric cartographic devices. AUTOMETRIC CORPORATION. July 28, 1961 [Sept. 9, 1960], No. 27480/61. Heading G1A. A pair of images A, B having portions of substantially identical contents, e.g. adjacent film transparencies in a series of partially overlapping aerial photographs, are examined over their overlapping portions by means of a common cathoderay tube flying-spot scanner 6 and individual photomultipliers-type photo-cells 14,16 to derive electrical signals which are compared to determine whether one image is misaligned with respect to the other, an error signal being produced which operates a servo system including motors 52 and 54 which function to reposition image A so as to reduce the error signal to a minimum or zero. The apparatus provides co-ordinate information whereby one transparency may be positioned correctly with respect to the other in a process of building up a map. The invention is characterized by the method of comparing the two image signals. Positive and negative versions of the signals are first derived by phase splitters 18, 20, the positive versions are delayed in lines 30, 32, and the signals are then combined in networks 34 to obtain signals of the form A (delayed) -B and B (delayed) -A. By means of further phase splitters 36, 38, clamps and cathode followers 40 and pushpull rectifiers 42, 44 there are derived the unipolar signals |A (delayed) -B| and |B (delayed) -A| representive of the absolute magnitudes of the signals. The two signals are obtained with opposite polarity by reversing the connection of rectifier 44 with respect to 42. Finally the two signals are combined to obtain the resultant |A (delayed) -B| - |B (delayed) -A|. This signal has the characteristic that it varies in sign and magnitude in accordance with the direction and extent respectively of the misalignment of one image with respect to the other and thus constitutes a suitable signal for control of a servo system. The scan effected by flying- spot scanner 6 is in the form of a raster formed of parallel horizontal lines and the apparatus as so far described functions to determine the horizontal misalignment of the images, delay lines 30, 32 having delays of the order of the time of scanning typical small image details, so that in effect the apparatus determines the horizontal misalignment by comparing elemental picture areas closely adjacent to corresponding points in the two image. To determine vertical misalignment, the apparatus includes a second flying spot scanner 6<1> whose raster lines are orientated at right-angles to those of scanner 6 and which is controlled to function alternately with scanner 6 by means of a scanner control unit 80 which applies blanking signals alternately to the two tube grids. The unit 80 also controls a switching unit 56 by means of which the error signal is routed alternately to horizontal and vertical servo channels. Each channel includes an integrating network 48, 51 and an amplifier 50, 53 from which the signal is applied to a motor 52, 54 by which horizontal and vertical movements may be imparted to image A (see Fig. 6, not shown). An alternative arrangement, Fig. 2 (not shown), employs a single flying-spot scanner which is scanned in a raster of parallel horizontal lines as in Fig. 1, but is also subjected to a rapid "wiggle" deflection transverse the lines and to an extent . sufficient to cover adjacent lines. The outputs from phase splitters 118, 120 are each applied through two delay lines, one of short delay sufficient to produce comparison of picture elements in adjacent lines and the other of a longer delay corresponding to the period of the "wiggle" deflection so as to produce comparison of picture elements occupying adjacent elements along a line. The signals from the first delay lines form the input for a circuit for deriving the vertical alignment error, and the signals from the second delay lines form the input for a circuit for deriving the horizontal alignment error. The circuits which are substantially identical with those in Fig. 1 then function to derive the error signals and control motors 52 and 54 simultaneously. In addition to vertical and horizontal misalignment image A may be rotated with respect to image B. This gives rise to a characteristic amplitude modulation of the error signals occurring at the scanning raster or frame rate. According to a further feature of the invention, Fig. 8 (not shown), one of the error signals, before it is integrated, may be applied to a synchronous detector, together with a reference signal, and the characteristic modulation detected to produce a control signal which operates a servomotor controlling the angular position of image A. Specification 853,794 is referred to.