Automatic Orthophoto Printer And Display Including Position Error Compensation For Photo-positioning Transport

Abstract

An automatic orthophoto printer and display for the preparation of orthophotographs from a pair of stereo aerial photographs is described which includes circuitry for compensating for position errors of the transports holding the photographs. Position sensors provide digital position signals representing the actual position of each transport in X and Y directions to digital comparators which subtractively combine the position signals with digital stage coordinate signals representing desired position of each transport in X and Y. The resultant error signals are used to proportionately shift the rasters of video scanners scanning a portion of each photograph. A specific embodiment of a photo-positioning transport and a position sensor comprising an incremental encoder is also described.

Description

FIELD OF THE INVENTION

This invention generally relates to the field of automatic orthophoto printers and displays and more particularly to an improvement which provides compensation of position errors encountered with mechanical photo-positioning systems used in such printers and displays.

BACKGROUND OF THE INVENTION

An improved system which operates automatically to produce an orthophotograph from one or more pairs of stereo photographs is described in U.S. Pat. Nos. 3,659,939 and 3,674,369, both entitled "Automatic Orthophoto Printer" by Gilbert L. Hobrough, and assigned to the assignee of the present invention. This system greatly reduces the time necessary to produce an orthophotograph and includes first and second photo-scanning devices which are operated in synchronism to provide video signals for each of the two photographs making up a stereo pair. Homologous areas of the photographs are scanned, with the portion under consideration being termed the "correlation zone." In order to reduce X parallax errors and other higher-ordered errors of the video signals within the correlation zone, the system includes a correlation network which operates on the video signals to determine the amount of X parallax error, and image transformation circuitry and raster-shaping circuits controlled thereby for altering the scanning patterns of the two photo-scanning devices. Once X parallax for a center-point in the correlation zone has been reduced by movement of appropriate photo positioning devices, the system alters the scan pattern of one or both photo-scanning devices to reduce the parallax and other errors throughout the correlation zone. One of the resultant video signals, which represents one image of the correlation zone, is supplied to a cathode ray tube for imprinting on the photographic negative. By investigating a number of correlation zones, an orthophotograph can be formed.

In the printing of orthophotographs by such a system, it is imperative that center-point parallax and other errors be reduced to a very low level or to zero in order to obtain a desired degree of accuracy of representation and precision in detail in the resultant orthophotograph. It is therefore very important to have, in such a system, a photo positioning device capable of rapidly and accurately moving a photograph to a position where any selected portion thereof has exactly known coordinates relative to the scanning device.

Such an accurate photo-positioning device is described and claimed in U.S. Pat. No. 3,687,547, by Gilbert L. Hobrough and George A. Wood, entitled "Photo Positioning System," which is also assigned to the assignee of the present invention. In such a device, the photograph is carried by a frame assembly positioned for movement on a flat support surface. A photo-scanning device, such as a flying spot scanner, is aligned with an opening in the support surface and with intermediate optics and is adapted to scan the portion of the photograph aligned with the opening by means of a scanning raster. First and second sides of the photo-holding frame are accurately machined as flat surfaces which intersect at an angle of 90.degree.. A pair of toothed rack elements each carry at one end thereof a frame-abutting end member, with each such end member having a pair of rollers maintained in engagement with an associated one of said flat side surfaces of the frame assembly. The racks extend from the abutting end members in a manner such that the racks extend perpendicular from the edge of the frame. These racks are each driven by a drive gear carried on the ends of the drive shaft by a pair of electric stepping motors. These stepping motors receive the input signals for positioning of the photograph.

In an orthophoto printer system such as previously described, the input signals to the stepping motors may comprise desired signals for effectively moving the optical center of the scanning rasters to a selected pair of coordinates in the photograph to define a correlation zone by the two scanning areas. As the initial portion of the correlation process proceeds, X and Y parallax at the center point of the correlation zone is generally removed by changing the input signals to one or both of the photo-positioning devices associated with the left and right stereo photographs. It can therefore be seen that the resolution of X parallax reduction that can be accomplished, using this technique, is directly related to the accuracy and precision of the photo-positioning devices. Any unknown error in the positioning of the two photos results directly in incorrect parallax measurements and consequently incorrect height calculations by the system.

Although devices such as described and claimed in the aforementioned U.S. Pat. No. 3,687,547 have proved quite acceptable for the resolution required to make typical orthophotographs, they have proved somewhat inadequate when it is desired to use a similar system to derive and print contour information on the orthophotograph. In such a case, a very high degree of resolution of terrain height, and thus of parallax, is required. Because the photo-positioning devices are mechanical, they produce errors arising from non-linearities in the mechanical surfaces, and additionally arising from wear and slight damage. Moreover, their position resolution capability is limited by the finite step increment afforded by the stepping motors and drive trains coupling those stepping motors to the frame assembly holding the photograph.

It is therefore an object of this invention to provide a highly accurate and precise positioning system, primarily useful in automatic orthophoto display and printing systems.

It is another object of this invention to provide an automatic orthophoto printer having improved error transformation capability due to the reduction of photo-positioning errors therein.

It is yet another object of this invention to provide an apparatus for compensating between errors resulting from the difference between a desired theoretical position for an object carried by an object-positioning device and the actual position thereof.

It is still a further object of this invention to provide an apparatus for accurately and precisely sensing the actual position of an object-positioning device useful in an automatic orthophoto printer.

SUMMARY OF THE INVENTION

These objects and others are achieved, briefly, by accurately and precisely sensing the actual position of a carriage in the transport which bears the photograph, comparing that actual position with a desired position therefor, converting a resultant error signal into an analog signal, and modifying the X or Y position of a raster on an apparatus for scanning the photograph in direct proportion to said analog signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The above as well as additional advantages and objects of this invention will be more clearly understood with reference to the following description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a combined schematic and block diagram showing an automatic orthophoto printing system;

FIG. 2 is a combined pictorial and schematic diagram showing a photo-positioning device and one embodiment of the position sensor of the present invention;

FIG. 3 is a logic signal diagram for illustrating the operation of the position sensor of FIG. 2; and,

FIG. 4 is a block diagram illustrating one embodiment of the raster shaper in the printing system of FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT

In FIG. 1, diapositives 12A and 12B corresponding to the stereo aerial photographs are mounted on the carriages 17 and 18 on the scanner and plate transport assemblies 41 and 42. The carriages 17 and 18 are adapted for movement in the X and Y directions by the X drive motors 19 and 20 and the Y drive motors 21 and 22. Light sources 23 and 24 for this particular scanning technique illuminate the diapositives and provide light for the scanners 25 and 26 which are conventional TV pickup units, however, flying spot scanners can also be used. A scanning printer 43 contains the cathode ray tube 59 and an optical system for printing on sensitive film 43A. A computer 29, which can be any of a number available on the market, solves the basic resection-intersection equations and delivers stage coordinate signals to the transports 41 and 42 and to printer 43, on lines 44X, 44Y, 45X, 45Y and 46X, 46Y, respectively. The electronic viewer 47 enables an operator to observe the images being scanned. A steering control 48 delivers instructions to the computer during manual operations. An electronic correlator 49 generates X and Y parallax error signals in response to timing differences between corresponding elements of the left and right video signals on output lines 50 and 51 from scanners 25 and 26.

The operation of the system illustrated in FIG. 1 is as follows. A sequential program within the computer 29 establishes a pair of model coordinates for examination. These model coordinates are stored in the form of highly precise digital numbers. From the stored model coordinates and from the X and Y parallax signals on lines 52 and 53, the computer 29 calculates stage coordinates, again in the form of highly precise digital numbers, and develops therefrom stage coordinate signals. First, a set of stage coordinate signals is delivered to the printer 43 along lines 46X and 46Y, causing the sensitive film 43A in the printer 43 to assume a position corresponding to the selected model coordinates. Second, a set of stage coordinate signals for the left and right scanners is computed on the basis of an initial or arbitratry terrain height (Z) evaluation for the center of the scanning area. Such coordinate signals are delivered on lines 44X, 44Y and 45X, 45Y to actuate the transports 41, 42, respectively, so that the diapositives 12A, 12B assume a position corresponding to the desired stage coordinates.

The correlator 49 operates on the left and right video signals, on lines 50 and 51, respectively, to determine the X and Y parallax errors of the scanned area. The X parallax error signal from the correlator 49 is delivered to the computer along line 52. On the analysis of this signal, the computer 29 can order a modification of the initial Z value in a direction that will reduce the center X parallax error to zero. The computer reevaluates the stage coordinates of the scanners on the basis of the new Z value and delivers modified stage coordinate signals to the transports 41, 42 on lines 44X, 44Y and 45X, 45Y, respectively. The motions of the transports 41, 42 are electronically compensated for by the simultaneous cancellation from the "memory" of the correlator 49 of the center point Z value. In this manner, the "range" of the correlator's memory is optimized about the center point of the scanned area.

The process of analysis of the X parallax error on line 52 and the determination of the new Z value continues iteratively until the center-point X error (a zero-order signal) has been reduced to an acceptable level that has made optimum use of the correlator's memory.

An average Y parallax signal on line 53 is also delivered to the computer 29 and is used during setup and orientation of the model to generate new stage coordinate signals for the scanners in the Y direction. After orientation, the Y parallax signal should be zero. During compilation of the model, the computer 29 responds to Y parallax error signals on line 53 to compensate for the effects of film shrinkage, optical distortions, and so forth.

The printer 43 shown in FIG. 1 produces an orthophotograph on sensitive film 43A therein. The cathode ray tube in the printer 43 is scanned synchronously with the scanner 25 and 26 and one of the video signals from the scanners is used to modulate the light intensity of the scanning spot in the printer. In FIG. 1, the video signal from a left or right scanner is delivered along line 50 or 51 through the scanner selector switch 54 and along line 55 to printer 43. Normally, the left video signal is selected for printing areas towards the left of the model, and the right video signal is selected for printing areas toward the right of the model. For this purpose, a left-right signal from computer 29 is delivered to selected switch 54 along line 55A. The computer also delivers an inhibit signal on line 56A that is combined in summing circuit 57A with the video signal from switch 54. The inhibit signal blanks the light output from cathode ray tube 59 to zero except during the desired printing period.

The scan generator 56 produces deflection waveforms required for scanning the diapositives and the sensitive film. The scanning pattern or "raster" is normally square, but, as discussed below, the "raster" signals for scanners 25 and 26 are shaped as required for registration. In FIG. 1, the deflection waveforms from scan generator 56 are delivered along line 58 to the printing cathode ray tube 59, and via lines 57 and 59A to the raster shaper 62 for the left scanner camera, and via lines 57 and 60 to the raster shaper 63 for the right scanner camera. Scanning reference signals via lines 57 and 61 are delivered to the correlator 49.

The raster shapers 62 and 63 both receive .DELTA.Z, or height, signals from the correlator 49 along lines 64 and 65. Raster shapers 62 and 63 also receive signals (K1, K2, K3, K4, K5 and K6, FIG. 4) from computer 29 along lines 66 and 67, respectively.

The raster shapers 62 and 63 modify the square raster waveforms from the scan generator 56 delivered on lines 59A and 60, in a manner more fully detailed hereinafter, to produce raster waveforms on lines 66A and 67A that produce in scanners 25 and 26 rasters that are distorted by high order transformations from their normal square shape. By this means, the left and right stereo images are transformed in such a manner that the video signals on lines 50 and 51 become more similar, and the image in the scanning printer 43 reflects the corrections for scale, parallax and other distortions arising out of non-orthogonal conditions when the pictures were taken.

To summarize the operation of the system as it has been described so far, the orthophotograph is constructed by first establishing a pair of model coordinates within the computer 29, then calculating stage coordinates therefrom and delivering representative stage coordinate signals via lines 46X, 46Y to the motors of the printer 43, so as to cause the film 43A to assume a position corresponding to the selected model coordinates. At this point in time, the cathode ray tube 59 is blanked by the inhibit signal appearing on line 56A. Subsequently, the computer causes the scanners 25 and 26 to reduce the center-point X parallax error to an acceptable level or to zero by causing the transports 41, 42 to move in iterative steps in response to successive sets of stage coordinate signals delivered on lines 44X, 44Y, 45X, 45Y. Thereafter, higher order parallax and other errors are reduced to zero in response to the signals from the correlator 49 and computer 29, with simultaneous operation of the distortion of the rasters of scanners 25 and 26 by the raster-shapers 62 and 63.

When X parallax and other errors are eliminated, the inhibit signal is removed from line 56A and the video output of either scanner 25 or 26, as determined by the left-right signal on line 55A, is coupled via line 55 to the cathode ray tube 59. Accordingly, the corrected video signal appears as an image on the face of the scanner 59. This image will thereafter expose a portion of the film 43A. After exposure is complete, the computer 29 selects new model coordinates and delivers appropriate center-point stage coordinate signals via lines 46X, 46Y so that the printer 43 assumes a new position. Thereafter, the cycle is repeated.

When the entire overlap area of the stereo photographs has been treated, the orthophotograph appears to comprise a plurality of adjacent patches, one patch for each set of model coordinates that has been selected.

As can be readily appreciated from the foregoing description, which generally corresponds to that of the systems described in U.S. Pat. Nos. 3,659,939 and 3,674,369, previously referred to, position errors encountered with the transports 41, 42 establish a lower resolution limit for reduction of X and Y parallax errors. To compensate for these errors, the system of the present invention includes position sensors 116X, 118X, associated with transports 42, 41, respectively, and coupled to carriages 18, 17 so as to measure the actual carriage displacement in the X direction from a reference position. Similarly, position sensors 116Y, 118Y are provided and are associated with transports 42, 41 and are coupled to carriages 18, 17, respectively, to sense the actual carriage displacement in the Y direction from a reference position. Position sensors 116X, 116Y provide corresponding position signals on lines 122X, 122Y, to a comparator 124, and position sensors 118X, 118Y supply corresponding position signals on lines 123X, 123Y to a comparator 126.

As previously discussed, computer 29 has stored therein calculated stage coordinates corresponding to the desired actual location of the carriages 18, 17. Digital signals representing the desired X and Y coordinates are accordingly supplied on lines 120X and 120Y to comparator 126, for transport 41, and on ines 121X and 121Y to comparator 124, for transport 42. The digital signals provided on lines 120X, 120Y, and 121X, 121Y differ from the stage coordinate signals provided on lines 44X, 44Y, 45X, and 45Y, in that the latter are driving signals necessary to cause the motors 19, 20, 21 and 22 of the transports 41, 42 to move carriages 17, 18 to the nearest "motor-step" positions, whereas the former are accurate and precise digital representations of the desired positions.

Comparators 124, 126 subtractively combine the desired stage coordinate positions with the actual positions which reflect all the mechanical errors of the system as well as the quantitation effects of the stepper motors and provide corresponding output signals on lines 125X, 125Y, 127X, 127Y, which represent the difference therebetween. The signals on lines 125X, 125Y, 127X, 127Y, instead of being applied directly to the motors 19, 20, 21 and 22, as would be customary in servo control systems, are applied to the raster shapers 62, 63 in such a manner so as to produce raster waveforms on lines 66A and 67A that produce in scanners 25 and 26 rasters that are distorted from that normally produced by a DC shift in either the X or the Y directions, or both, to compensate for the position errors encountered with the transports 41 and 42.

To more fully understand the invention, reference will now be made to a specific embodiment of a transport, such as transport 42, and a specific embodiment of the position sensors, such as sensors 116X and 116Y, associated therewith. In FIG. 2, a frame assembly 70 is adapted to hold the diapositive 12B which is to be scanned by the scanner 26. The frame assembly 70 rests upon a flat base assembly, which includes a plate 71 and is provided with a suitable surface to provide easy movement of the frame 70 across the plate 71. Bearing feet or pads have been used on the underside of one frame assembly 70 and were found to work well. A pair of geared driving racks 72 and 73 each carry at one end thereof a frame-engaging end member 74, 75, respectively. Members 74 and 75 each carry a pair of rollers 76 which rotate about horizontal axes and ride on the upper surface of the plate 71. Second pairs of rollers 77, supported for rotation about vertical axes on each of the members 74 and 75, engage the edges 102, 103 of the frame assembly 70. The outer ends of the racks 72, 73 are supported by rollers 78. The edges 102, 103 of the frame assembly 70 are flat surfaces which intersect at an angle of 90.degree. for the particular system illustrated herein. The racks 72 and 73 are maintained perpendicular to the surfaces 102 and 103, and therefore the racks, if extended, would intersect at an angle of 90.degree..

The frame 70 is maintained in engagement with each of the rollers 77 by any suitable assembly. In the embodiment illustrated, the opposite corners 79, 80 of the frame 70 have the ends of cables 81 and 82 secured thereto. Cable 82 passes around a guide 83 supported on plate 71, and then around guide 84 also supported on plate 71. In a similar manner, the cable 81 passes around the guide 84. Two cables extend from guide 84 around a guide 84 supported for rotation about a horizontal support located in the plane of the plate 71, through an opening in the plate 71 adjacent guide 85, and around a pulley 86 having a weight 87 secured thereto. Through pulley 86, the cables extend through an opening, not illustrated, in plate 71, around an adjustment eccentric 88, and are secured by their respective ends to clamps 89 attached to the plate 71. The arrangement is such that the weight 87 acting through the cables 81 and 82 provides a yielding force to the frame 70, which arches the edges 102 and 103 into engagement with the rollers 77 associated with drive racks 72 and 73. Rotation of the eccentric 88 is used for initial adjustment of the cables.

A drive gear 91 secured to the drive shaft of the reversible electric stepping motor 22 engages the rack 72. In a similar manner, a drive gear 110 secured to the drive shaft of the second reversible electric stepping motor 20 engages the drive rack 73. The operation of the reversible electric stepping motors 22, 20 is interrupted by operation of an associated control switch by arms 93, 111, having rollers 94 and 112 engaged with the non-toothed edge of the associated racks 72 or 73. The arms 93 and 111 are pivoted on pins secured to the plate 71. The switches 97 and 113 associated with the arms 93 and 111 are adapted to be operated whenever the respective arm is moved clockwise. The end members 74 and 75 on the racks 72 and 73 have beveled surfaces, such as surface 92 on member 74, which engage the rollers 94 and 112 whenever the racks reach their maximum extent of outward movement. Therefore, the associated switch 97 or 113 will be actuated to interrupt further drive of the rack. Corresponding beveled surfaces, such as surface 92A on rack 72, on the outer ends of racks 72 and 73 serve the same purpose when the maximum extent of inward travel has been reached.

An optical system, including a lens 101, is aligned with the intersection of the lines of travel of racks 72 and 73 so that the center of scanning corresponds to this intersection. More specifically, this intersection occurs at the intersection of the pitch lines of the drive racks 72 and 73.

The diapositive 12B can be held in position in the frame 70 by various means. For purposes of illustration, the diapositive 12B is secured to a backing plate 70A. A corner stay 98 is shown as being in engagement with one corner of the plate 70A with springs 99, 100 urging the corner stay 98 and plate 70A toward the opposite corner of the frame 70.

Though a photo-positioning device, such as illustrated in FIG. 2, is capable of very accurate and precise positioning of the diapositive 12B, it is apparent that position errors may yet arrive from several sources. First, the fact that the motors 22 and 20 operate in an incremental stepping mode places a basic limitation on position resolution as being determined by the smallest increment of movement provided by motors 22, 20 as coupled through the drive gears 91, 110. Second, wear of the gears 91, 110 and the corresponding toothed portions of racks 72 and 73 provides a variable position error with time. In addition, the manufacturing tolerances of these elements and the necessity for some play so as to avoid binding introduces other position errors.

In order to compensate for position errors, the position sensors 116Y, 116X include, respectively, incremental rotary encoders 130, 140, which are mounted on supporting brackets 131, 141 attached to a portion of the support plate 71. The encoders 130, 140 have shafts 133, 142 which extend through and are supported by brackets 131, 141. Pulleys 132, 143 are attached to shafts 132, 142 and rotate therewith. Cables 134, 144 attached at one end to the ends 135, 145 of racks 72, 73, respectively, pass over pulleys 133, 143, pass back over pulleys 136, 146 rotatably journalled in mounting brackets 131, 141, and are attached at their other end to weights 137, 147. The function of pulleys 136, 146 and weights 137, 147 is to provide a constant tension on the cables 134, 144, so as to prevent any slack therein. As a result, the pulleys 133, 143 rotate by an amount directly proportional to the translative movement of racks 72, 73 in their respective Y and X directions.

The encoders 130, 140 can be of a type which provides an output signal representing the actual displacement of the encoder shaft from a reference position, better known as an absolute encoder, or of a type which provides an output signal proportional to each predetermined increment of displacement of the encoder's shaft, better known as an incremental encoder. In a preferred embodiment, the encoders 130, 140 comprise an incremental encoder, each operative to provide two digital pulse train outputs on lines 130', 130" and 140', 140", respectively. Each of the pulse train outputs changes its logic state for every predetermined increment of rotation of the associated shaft 132 or 142. In addition, the two pulse trains provided by each encoder are displaced in phase by 90.degree.. The outputs from encoder 130 are seen in FIG. 3.

Encoders of this type are commercially available from Sequential Information Systems, Inc., Elmsford, N.Y., as their model 5000ID:PA1.

The pulse trains on lines 130', 130" and 140', 140" are supplied to synchronous phase detectors 150, 154, respectively, and the signals on lines 130', 140' are additionally supplied to the input of reversible counters 152, 156. The direction of counting of counters 152, 156 is controlled by output signals from the phase detectors 150, 154 present on lines 151A, 151B, and 155A, 155B. The outputs of the reversible counters 152, 156 serve as the position sensor outputs 122Y, 122X, respectively.

The phase detectors 150, 154 may be of any type well known to the art for providing binary output signals whose logic state represents the dynamic phase relationship between the input pulse trains thereto. That is, the logic state of the outputs on lines 151 and 155 represents which of the two input pulse trains is leading in phase at a particular point in time for a given direction of movement of racks 72 and 73.

In the embodiment shown in FIG. 2, the phase detectors each comprise a J-K flip-flop with the pulse train on lines 130', 140' being applied to the J steering inputs, and the pulse train outputs on lines 130", 140" being applied to the clock [C] inputs. The K steering inputs are grounded. The Q outputs are connected to lines 155A, and the Q outputs are connected to lines 151B, 155B. The Q and Q outputs on lines 151A, 155A are seen in FIG. 3.

The operation of the phase detectors can be understood from a consideration of FIG. 3 taken in conjunction with the specific connections in FIG. 2 for encoder 130 and phase detector 150.

At the reference positions y.sub.0, the Q and Q outputs of the phase detector 150 are logic 0 and logic 1, respectively. If the rack 72 now is moved in a first direction D.sub.1, indicated by the arrow in FIG. 3, the signal on line 130", or the clock signal, goes from logic 1 to logic 0 at distance y.sub.1. At this transition, the signal on lines 130', or the position signal, is a logic 1. Therefore, the flip-flop within phase detector 150 switches so that the Q output is a logic 1 and the Q output is a logic 0.

If the rack 72 now is moved in an opposite direction D.sub.2, the clock signal goes from logic 0 to logic 1, and then from logic 1 to logic 0 at distance y.sub.2. At this transition, the position signal is a logic 0, and therefore the flip-flop within phase detector 150 switches so that the Q output is a logic 0 and the Q output is a logic 1.

It can be recognized that at every logic 1 to logic 0 transition of the clock signal, the relative phase of the position signal is investigated by phase detector 150. If the position signal is a logic 1 at the transition, the rack 72 is displaced in the first direction, and if a logic 0, the rack 72 is displaced in the second, opposite direction. The first direction is denoted by a logic 1 signal on the Q output, and the second direction is denoted by a logic 1 signal on the Q output.

The reversible counters 152 and 156 are set at zero, or some other established count, when the associated racks 72, 73 are positioned at the reference position. For displacement of the racks 72 and 73 from those reference positions, counters 152 and 156 are incremented by the pulses on lines 130' and 140' in the direction controlled by the Q and Q outputs from phase detectors 150 and 154. Accordingly, counters 152 and 156 contain digital numbers whose magnitudes represent the displacement of the associated rack 72 or 73 from the reference position, and whose signs represent the directions of displacement.

The signals on lines 122Y, 122X are in turn supplied to comparator 124, as heretofore described. In FIG. 2, comparator 124 is seen to comprise two summing junctions 124Y, 124X, for receiving the digital numbers present on lines 122Y, 122X representing the actual positions of the associated racks 72 and 73, and for receiving, on lines 121Y and 121X, the digital numbers supplied from computer 29 representing the desired stage coordinate positions for racks 72 and 73. Summing junctions 124Y and 124X can be any digital comparators, and preferably are within computer 29.

The signals on lines 124Y, 121Y, and 124X and 121X are subtractively combined so that the output error signals, on lines 125Y, 125X comprise digital numbers representing the difference between the desired and actual positions of the racks 72, 73. As previously described, these signals are supplied to raster shaper 63.

Now turning to FIG. 4, which is a block diagram of a preferred embodiment of the raster shaper 63 (which is identical to raster shaper 62), it can be seen that the X and Y deflection waveforms for a square raster delivered from the scan generator 56 on lines 170 and 171, respectively, are modified by multiplier circuits 172 and 173, respectively, to provide deflection signals at different amplitudes on lines 174 and 175. Such signals appear on output lines 176 and 177 after passing through the summing networks 178 and 179, respectively.

It will also be seen that the X and Y waveforms will be modified further by the addition at summing circuits 178 and 179, respectively, of other signals as described below. In particular, the X output signal will be the resultant of the signal on line 174, already described, and a Y deflection signal delivered to summing point 178 on line 180 from multiplier 181 on line 182. Similarly, the Y output signal will be the resultant of the signal on line 175, already described, and the X deflection signal delivered to summing point 179 on line 183 from multiplier 184. The multiplier units can conveniently be conventional digital-to-analog converters with a variably controlled reference being the analog input. Thus, they are labeled as "D/A."

As a result of the action of the elements of the raster shaper so far described, the scale and shape of a raster will be altered in response to computer signals K1, K2, K4 and K5, in FIG. 3, to compensate for the first order effects of irregularities in the flight line of the survey aircraft and orientation of the photographic cameras at the moment of exposure.

Referring again to FIG. 3, it will be seen that the .DELTA.Z signal from the correlator 49 represents variation of the terrain height in the model area being scanned. The multipliers 186 and 187 distribute the .DELTA.Z signal to the X and Y axes of the camera so as to accommodate transformation in accordance with the photogrammetric of geometric aspects of the model at the moment of exposure in the aircraft.

The computer 29 provides the raster-shaping coefficients K1-K6, on lines 66, 67 of FIG. 1, in addition to the center-point coordinates for the transports 41, 42 on lines 44X, 44Y, 45X, 45Y, in accordance with techniques which per se are known in the art.

In addition, the X and Y waveforms are modified further by the signals from the comparator 124. Specifically, the X error signal present on line 125X is first converted into analog form by a digital-to-analog converter 190 and then applied to summing junction 178, and the Y error signal on line 125Y is converted into analog form by a digital-to-analog converter 191 and applied to summing junction 179. As a result, the X and Y waveforms are modified so that the raster produced on scanner 26 is shifted in both X and Y in a direction controlled by the sign of the X and Y error signals and in an amount proportional to the magnitude thereof.

It will thus be apparent that a highly accurate and precise automatic orthophoto display and printing system has been described, which may find additional utility in the orthomapping of contour information, as well as improved resolution of X and Y parallax, due to the accurate and precise positioning of the diapositives 12A and 12B.

Though the invention has been described with respect to a preferred embodiment thereof, it is to be understood by those skilled in the art that the invention is not limited thereto. For example, the position sensors, such as sensor 116X, 116Y can comprise linear absolute or incremental encoders requiring no mechanical connection to the racks 72, 73. In one embodiment, optical sensing equipment can be used comprising an optical sensor disposed adjacent each of the racks 72, 73, with a dual track of a plurality of finely divided black and white coding marks being placed on racks 72 and 73. In such a case, signals derived from the transition from one track would be used to increment a reversible counter, as previously described, and the transitions from the other would control the direction of counting of the counter. Other incremental and absolute encoders and position sensors will be readily apparent to those skilled in the art. Therefore, the limits of the present invention are intended to be bounded only by the appended claims.

Claims

I claim:

1. In an automatic orthophoto display system including first and second scanning means, each of said scanning means establishing a raster, said first and said second scanning means providing video output signals representing the information in a common area of first and second photographs making up a stereo pair, respectively; means providing coordinate signals for establishing desired positions for each of said first and second photographs relative to said first and said second scanning means, respectively; first and second photo-positioning means responsive to said coordinate signals for moving said first and second photographs relative to said first and second scanning means, respectively; means responsive to said video output signals for modifying said coordinate signals for at least one of said photo-positioning means to cause the associated one of said photographs to be moved in a direction relative to the associated scanning means to reduce parallax error at one point in the common area of said photographs scanned by said raster; signal correlating means responsive to said video output signals to derive an error signal proportional to timing differences between homologous components of the video output signals throughout the common area of said photographs scanned by said rasters; and raster-shaping means coupled to said signal correlating means and to said first and to said second scanning means and responsive to said error signal to distort the scan pattern of at least one of said scanning means, said distortion being in a direction to reduce parallax: the improvement comprising means sensing the actual position of at least one of said photo-positioning means relative to its associated scanning means, means comparing the actual position so sensed with the desired position of said photo-positioning means to derive a position error signal proportional to the difference therebetween, and means coupling said comparator to said raster-shaping means so that said raster-shaping means shifts the raster of said associated scanning means in response to said position error signal.

2. The improvement for an automatic orthophoto display system as recited in claim 1, wherein:

a. said sensing means senses the actual position of said first and said second photo-positioning means;

b. said comparing means derives first and second position error signals for said first and said second photo-positioning means; and,

c. said raster-shaping means shifts the raster of each of said first and said second scanning means in response to said first and said second position error signals, respectively.

3. The improvement for an automatic orthophoto display system as recited in claim 1, wherein:

a. said sensing means senses the actual position of said one photo-positioning means in orthogonal coordinate directions;

b. said comparing means includes means for deriving position error signals for each of said coordinate directions; and,

c. said raster-shaping means includes means operative to shift said raster in either of said coordinate directions in response to said position error signals.

4. The improvement for an automatic orthophoto display system as recited in claim 1, wherein said sensing means includes an encoder means for providing an output pulse for every predetermined increment of movement of said one photo positioning means along a given coordinate axis, a direction detection means providing an output direction signal representing the direction of said movement along said coordinate axis, and bidirectional, reversible counter means operative to accumulate said output pulses in a given direction in response to said output direction signal to provide a digital signal representing said actual position.

5. An automatic orthophoto display system comprising:

a. a photo scanning system including first and second scanning means relatively aligned with first and second photographs making up a stereo pair, and a scan generator providing deflection output signals to said first and said second scanning means to establish a scanning raster for each of said scanning means, each of said first and said second scanning means thereby providing a video output signal representing the information on the portion of the corresponding photograph scanned by said raster;

b. means providing a plurality of first signals establishing desired positions of said first and said second photographs relative to said first and said second scanning means;

c. means positioning said first and said second photographs in response to said plurality of first signals;

d. parallax correction means responsive to said video output signals including means for developing therefrom a parallax correction signal, and means summing said parallax correction signal with those of said deflection output signals which are supplied to at least one of said first or second scanning means to thereby perturb the scan pattern of said one of said first and second scanning means in a direction so as to reduce parallax between said first and said second photographs;

e. sensor means providing a second signal proportional to the actual position of at least one of said photographs relative to the associated one of said scanning means;

f. comparator means providing an output signal proportional to the difference between said first signal establishing the desired position of said one of said photographs and said second signal; and,

g. means summing said output signal with those of said deflection output signals which are supplied to said scanning means associated with said one of said photographs.

6. An apparatus for providing accurate and precise scanning of an object, comprising:

a. scanning means including means establishing a scan pattern;

b. frame means relatively aligned with said scanning means for holding the object;

c. means providing a first signal establishing a desired position of said object relative to the scan pattern of said scanning means;

d. drive means coupled to said frame means for moving said object in response to said first signal;

e. sensor means providing a second signal proportional to the actual position of said object relative to said scan pattern;

f. comparator means for subtractively combining said first and said second signals to obtain a position error signal; and,

g. means coupled to said comparator and to said scanning means for shifting the scan pattern of said scanning means in proportion to said position error signal.

7. An apparatus as recited in claim 6, wherein said sensor means includes:

a. an incremental encoder having an input member;

b. means coupling said input member to said frame means;

c. said incremental encoder including means providing first and second output pulse trains in response to movement of said input member, said pulse trains being displaced in phase by substantially 90.degree., a pulse in each of the pulse trains being provided for every predetermined increment of movement of said input member;

d. bidirectional, reversible counter means having an input terminal, an output terminal on which appears a signal representing the accumulated pulses presented to said input terminal, and a control terminal controlling the direction of counting thereof;

e. means coupling one of said first and second output pulse trains to said input terminal;

f. synchronous phase detection means having both said first and said second output pulse trains coupled thereto and operative in response to the relative phase of said first and second output pulse trains to provide a direction control signal representing the direction of movement of said input member, and therefore of said frame member, along said axis; and,

g. means coupling said direction control signal to said control terminal of said counter.

8. An apparatus as recited in claim 7, wherein said incremental encoder comprises a rotary incremental encoder in which said input member comprises a rotatable shaft, and wherein said means coupling said input member comprises a pulley rigidly rotatable with said shaft, a cable attached at one end to said frame member, and passing over said pulley, and tensioning means attached to the other end of said cable.

9. An apparatus as recited in claim 8, wherein said tensioning means comprises a second pulley spaced apart from said first pulley and supported for independent rotation, said cable passing around said second pulley, and a weight member suspended from the portion of said cable which is passed around said first and said second pulleys.

10. An apparatus as recited in claim 7, wherein said synchronous phase detection means comprises a J-K flip-flop.