Use of pre-scanned low resolution imagery data for synchronizing application of respective scene balance mapping mechanisms during high resolution rescan of successive images frames on a continuous film strip
FIELD OF THE INVENTION 
     The present invention relates in general to digital imagery processing systems and is particularly directed to a scheme for using image frame identification information, derived during a low resolution scan of successive image frames on a continuous film strip, to spatially synchronize the control of a subsequent reverse-direction, high resolution scan of each of previously scanned image frames on a reel of color photographic film containing that film strip. 
     BACKGROUND OF THE INVENTION 
     Photofinishing systems customarily prescan a strip of (35 mm) film to examine the quality and color content of the respective image frames prior to transferring the images to print paper. On the basis of this prescan examination, exposure control parameters of the imaging optics components are defined so that, during a subsequent pass of the film, the projection of each image onto the print medium will be properly adjusted to yield an acceptable quality hard copy print. 
     In order to demarcate where each image is located on the film strip, respective notches are formed along the edge of the film, for example between successive frames, or centrally of each frame. During rescan, these notches are used to identify successive frames and thereby call up previously defined exposure control parameters for adjusting the projection optics. 
     A shortcoming of such an exposure control procedure is the fact that a notch is sometimes missed. When this happens, there is a mismatch between the current image frame and prescan-derived exposure control parameters. The result is a poor quality set of prints, making it necessary for the photofinisher to reprocess the film strip, which entails additional time, and costs associated with the wasted print materials. Now although a minor lag in processing can be adequately managed, when a large reel of film is to be scanned continuously on a single machine and then printed on the same (or other) machine, it is too complex to track and detect possible sequence errors. As a result, almost no equipment currently available attempts to track such errors over multiple film strips. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, the above-described spatial synchronization problem is effectively obviated by using supplemental or auxiliary information derived during the prescan of the image, thereby augmenting the function of each notch so that even if a notch is missed during rescan, spatial synchronization between each frame and its associated prescan-derived control information will be maintained. The present invention is particularly applicable to a highly automated digital imagery processing system, wherein successive image frames of a continuous reel of color photographic film, such as a spliced-together sequence of 35 mm film strips, are prescanned in a first order at low spatial resolution and then rescanned in reverse order to derive a color balanced, high spatial resolution digitized image for each frame. 
     In such a digital imagery processing system, each frame on the film is pre-scanned at low resolution, with each digitized image being processed by a scene balance mechanism for deriving control information to be used for optimizing the manner in which a subsequently scanned high resolution digitized image is to be mapped into memory. The scene balance mechanism determines how the response characteristic of the scanner&#39;s imaging pixel array sees the image and encodes its spatial content. The output of this analysis, which represents the color balance content of the digitized image, is then used to adjust, or calibrate, the sensitivity parameters of the scanner during the high resolution scan, so that the essential subject matter of the image (i.e. that which is necessary to reproduce a high quality image) will fall within the linear portion of the response range of the scanner&#39;s imaging pixel array. The high resolution digitized image is then processed by the scene balance mechanism to map the image data into a digitized image having a reduced encoding resolution corresponding to that of an attendant framestore. 
     In such a system, should there be a mismatch between prescan imagery data and a high resolution frame, the wrong prescan-based calibration information would be used during the high resolution scan, and an unacceptable image would be mapped into memory. To prevent this from happening, the present invention does not rely exclusively on detecting frame location notches as in conventional (analog) systems, but uses other detected frame identification information that may be used independently of, or in combination with, the notches to improve the reliability of the scanner calibration for each processed image. 
     Pursuant to the invention, a reel of color photographic film (comprised of one or more spliced together film strips) is initially translated in a first direction past a selectable resolution opto-electronic film scanner which scans the images on the film at a first, low resolution, thereby producing a plurality of first digitally encoded images. For each pre-scanned image a first signal, in the form of a first digital code representative of a prescribed attribute of that scanned image, is generated and stored. This first code may represent a statistical measure of the contents of the image, e.g. a summation of the image contents of each of the columns of the low spatial resolution pixel sub-array. Alternatively, or in addition to the first code, a second code, representative of the location of a respective interframe gap, namely, a separation between successive images on the film strip, may be generated. The location of interframe gaps is readily accomplished by observing what is effectively a stepwise variation in the output of the scanner as non-image regions of the film are scanned during the film&#39;s movement past the scanner&#39;s imaging optics. Each of the plurality of first digitally encoded images is then processed in accordance with a scene balance mechanism to derive a calibration code representative of the color content of the image. 
     After the entire reel has been scanned at low resolution and first, second and scanner calibration codes have been stored for each image, the photographic film strip is translated past the opto-electronic scanning device in a reverse direction, so as to effect a rescanning of the plurality of photographic images, but in an order opposite to that of the original scan. The rescan of each image (which may be conducted on the same or a different film scanner) is conducted at a second, high spatial scanning resolution to produce a plurality of second digitally encoded images. During the rescan of each image, a third signal, in the form of a third digital code, representative of the prescribed attribute of the scanned image, is generated and stored. As in the case of the first code, the third signal may represent a statistical measure of the contents of the image, such as a summation of the image contents of those columns of the image array corresponding to the columns of the low spatial resolution pixel sub-array. Each second digitally encoded image is then mapped into memory using a scene balance mechanism that has been calibrated in accordance with a respective scene balance calibration signal. 
     To ensure that the correct calibrated scene balance mechanism is employed during the high resolution scan, its selection may be based upon a combination (e.g. correlation) of the first and third statistical measure representative codes, or by looking for the presence of an interframe gap, as represented by the second code, in addition to a frame demarcating notch. Namely, notch information is augmented by other data that is derived during pre-scan of the film. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 diagrammatically illustrates a photographic color film photofinishing minilab for continuous processing of a reel of spliced-together color photographic film strips; 
     FIG. 2 is an imagery processing flow diagram of a low resolution scene balance based calibration and high resolution imagery digitizing mechanism; 
     FIG. 3 diagrammatically illustrates an exemplary output of an opto-electronic scanner; and 
     FIG. 4 diagrammatically illustrates a sequence of image frames on a color photographic film strip. 
    
    
     DETAILED DESCRIPTION 
     Before describing, in detail, the inventive use of prescanned low resolution imagery data to synchronize the application of a plurality of scene balance mapping mechanisms during high resolution scanning of successive images frames on a continuous film strip, it should be observed that the present invention resides primarily in a novel structural combination of conventional digital imagery processing circuits and components and not in the particular detailed configurations thereof. Accordingly, the structure, control and arrangement of these conventional circuits and components have been illustrated in the drawings by readily understandable block diagrams which show only those specific details that are pertinent to the present invention, so as not to obscure the disclosure with structural details which will be readily apparent to those skilled in the art having the benefit of the description herein. Thus, the block diagram illustrations of the Figures do not necessarily represent the mechanical structural arrangement of the exemplary system, but are primarily intended to illustrate the major structural components of the system in a convenient functional grouping, whereby the present invention may be more readily understood. 
     FIG. 1 diagrammatically illustrates a photographic color film processing system (e.g. photofinishing minilab) with which the present invention may be employed and, for purposes of the present description, such a system may be of the type described in co-pending patent application Ser. No. 582,305, filed Sep. 14, 1990, by S. Kristy, entitled &#34;Multi-resolution Digital Imagery Photofinishing System&#34;, assigned to the assignee of the present application and the disclosure of which is herein incorporated. It should be observed, however, that the system described in the above-referenced co-pending Kristy application is merely an example of one type of system in which the invention may be used and is not to be considered limitative of the invention. In general, the invention may be incorporated in any digitized imagery processing and reproduction system. 
     In accordance with the imagery data processing system of the above referenced co-pending Kristy application, each high resolution captured image is preferably formatted and stored as a respective image data file containing a low, or base, resolution image bit map file and a plurality of higher resolution residual images associated with respectively increasing degrees of image resolution. By iteratively combining these higher resolution residual images with the base resolution image, successively increased resolution images may be recovered from the base resolution image. As an example, spatial data values representative of a high resolution (3072×2048) image scan of a 36 mm-by-24 mm image frame of a 35 mm film strip may be stored as a respective image data file including a base resolution image bit map file containing data values associated with a spatial image array or matrix of 512 rows and 768 columns of pixels and an associated set of residual image files to be stored on the disc. Within a photofinishing workstation, the base resolution image may be further sub-sampled to derive an even lower resolution sub-array of image values (e.g. on the order of 128×192 pixels) for use by the photofinishing operator in the course of formatting and storing a digitized image file. 
     Thus, in the digital image processing system of FIG. 1, color photographic images, such as a set of twenty-four or thirty-six 36 mm-by-24 mm image frames of a 35 mm color film strip 10, are scanned by a high resolution opto-electronic color film scanner 12, such as a commercially available Eikonix Model 1435 scanner. High resolution film scanner 12 outputs digitally encoded data representative of the response of its imaging sensor pixel array (e.g. a 2048×3072 pixel matrix) onto which a respective photographic image frame of film strip 10 has been projected by an input imaging lens system. This digitally encoded data, or `digitized` image, is encoded to some prescribed resolution (e.g. sixteen bits per color per pixel) that encompasses a range of values over which the contents of the scene on the color film may vary. For a typical color photographic negative, the range of values is less than the density vs. exposure latitude of the film, but is sufficiently wide to encompass those density values that can be expected to be encountered for a particular scene. Also shown in FIG. 1 is a notch detector 17 which detects frame location notches in film strip 10 for synchronization purposes, as will be described below. 
     Because of its very large (2048×3072) spatial resolution, with the output of each pixel being resolved to sixteen bits, the quantity of data per image produced by such high resolution film scanners is so large that it must be reduced for storage and reasonably fast access in a practical sized framestore, which necessarily implies that some of the scene information in the digitized image will be discarded. For this purpose, a scene balancing mechanism is used to map the digitized image into a set of lower resolution digital codes (e.g. eight bits per color per pixel), each of which has a resolution corresponding to the dynamic range of a digitized image data base (framestore). The database may be resident in a photofinishing workstation 14, which contains imagery application software through which the digitized image may be processed to achieve a desired base image appearance and configuration in the course of driving a high resolution thermal printer 16 to output a high quality color print. 
     Preferably, in the course of being mapped into memory, the digitized imagery data output by the high resolution film scanner is subjected to a code conversion mechanism of the type described in co-pending application Ser. No. 582,306, filed Sep. 14, 1990, by T. Madden et al, entitled &#34;Extending Dynamic Range of Stored Image Database,&#34; assigned to the assignee of the present application and the disclosure of which is herein incorporated. Pursuant to this code conversion scheme, the dynamic range of the digitized image database may be extended to permit shifting of encoded pixel values without `clipping`, and to provide a limited window of values into which extremely high reflectance image points may be encoded and stored. To this end, digital codes, into which the high resolution imagery data output by the image scanner are mapped by the scene balance mechanism, are converted into a set of reduced-range digital codes of the same resolution as, but having a smaller range of image content values than the dynamic range of the digitized image data base. The code conversion mechanism operates to convert a maximum value of 100% white reflectance to an encoded value that is less than the upper limit of the dynamic range of the database to accommodate shifts in the digitized imagery data and allow for the placement of specular highlights that are beyond the 100% white reflectance maximum. 
     When digitizing an image during a high resolution scan, the response of the scanner is calibrated such that the principal subject matter of the image falls within the linear portion of the response range of the scanner&#39;s imaging pixel array. For this purpose, as described in my co-pending application Ser. No. 583,414, filed Sep. 17, 1990 entitled &#34;Scene Balance Calibration of Digital Scanner&#34;, assigned to the assignee of the present application and the disclosure of which is herein incorporated, the digital imagery processing system employs a calibration and high resolution capture procedure, diagrammatically illustrated in the imagery processing flow diagram of FIG. 2, whereby the image is scanned twice, once at low resolution for purposes of calibration, and then at high resolution, for data capture. 
     More particularly, as shown at step 101, image scanner 12 is controlled to carry out a low resolution mode, prescan of an image 10 of interest. Where the scanner has multiple resolution scan capability, it is controlled so as to scan the image at a relatively low spatial resolution, e.g. on the order of 24×36 pixels per frame. Depending upon the size of the low resolution image, it may be necessary to perform a further spatial compression of the captured image, in order to reduce the computational intensity (and thereby achieve a reasonably rapid throughput) of the application of the low resolution image to a scene balance mechanism. In accordance with the multiple mode operation of the above-referenced high resolution scanner, during low resolution scan, a 128×192 image is captured. Through further spatial integration of the imagery data within workstation 14, the captured 128×192 pixel version of the image may be reduced to a very small sub-array (e.g. 24×36 pixels, each encoded at sixteen bits per color) for application to the scene balance mechanism through which high resolution imagery data is mapped into the framestore. 
     This very low resolution (24×36) digitized image is then analyzed in step 102 by the scene balance mechanism to determine how the response characteristic of the scanner&#39;s imaging pixel array sees the image and encodes its spatial content. The scene balance mechanism (the processed result of which may be implemented as a set of look-up tables (LUTs), one for each RGB color) outputs three values, one for each color, which represent the color balance content of the digitized image. 
     In step 103, using these values, the sensitivity of the scanner is calibrated, so that, during a subsequent high resolution scan of the image, the essential subject matter of the image will fall within the linear portion of the response range of the scanner&#39;s imaging pixel array. While the scene balance output values may be employed to effect vernier adjustments of reference voltages for the scanner&#39;s imaging array, in accordance with a preferred mode of the present invention, a respective offset code, one for each of the color values, is added to the inputs of each scene balance look-up table in order to effectively shift or translate its mapping function that brings the essential subject matter of the image into the linear portion of the response range of the scanner&#39;s imaging pixel array. 
     With the scanner now calibrated, (e.g. scene balance look-up tables shifted to optimize the use of the imaging array&#39;s linear response range), the scanner is controlled in step 104 to execute a high resolution scan of the image. Since the scene balance LUTs have been translated in accordance with the output of the low resolution prescan, the high resolution digitized image will be mapped into the framestore such that essential image information (i.e. that which is necessary to obtain a high quality print) is captured and stored. 
     As pointed out above, in a highly automated photofinishing system, wherein successive image frames of a continuous reel of color photographic film, such as a spliced-together sequence of 35 mm film strips, are prescanned at low spatial resolution and then rescanned to derive a color balanced, high spatial resolution digitized image, there is a potential problem of image misalignment or lack of image synchronization between the low and high resolution scans. Namely, should there be a mismatch between prescan imagery data and a high resolution frame, the wrong prescan-based calibration information would be used during the high resolution scan, and an unacceptable image mapped into memory. 
     To prevent such misalignment, the present invention employs a synchronization scheme that does not rely exclusively on detecting frame location notches, but uses other detected frame identification information independently or in combination with the notches to improve the reliability of the scanner calibration for each processed image. Pursuant to the invention, when the reel of color photographic film (comprised of one or more spliced together film strips) is initially scanned at low resolution by the opto-electronic film scanner, a plurality of first digital codes, each of which representative of a prescribed attribute of a respective scanned image, is generated and stored. 
     As diagrammatically illustrated in FIG. 3, which shows an exemplary output of an opto-electronic scan of film strip 10, a portion of which is shown in FIG. 4, this first code may represent a statistical measure of the contents of an image frame, e.g. a summation of the image contents of each of the columns 201 of the low spatial resolution pixel sub-array. Alternatively, or in addition to the first code, a second code, representative of the location of a respective interframe gap 205, for example, a separation between successive image frames 210, 211 on the film strip 10, may be generated. The location of interframe gaps 205, whereat frame registration notches 206 along the edge of the film should be placed, is readily accomplished by observing what is effectively a stepwise variation 207 in the output of the scanner as non-image regions 209 of the film are scanned during the film&#39;s movement past the scanner&#39;s imaging optics. Each of the plurality of first digitally encoded images is then processed in accordance with the scene balance mechanism, as described above, to derive a calibration code representative of the color content of the image. 
     After the entire reel has been scanned at low resolution and first and second codes stored for each image, photographic film strip 10 is translated past the opto-electronic scanning device in a reverse direction, so as to effect a rescanning of the plurality of photographic images, but in an order opposite to that of the original scan. The rescan of each image is conducted at a second, high spatial scanning resolution to produce a plurality of second digitally encoded images. During the rescan of each image, a third signal, in the form of a third digital code, representative of the prescribed attribute of the scanned image, is generated and stored. As in the case of the first code, the third signal may represent a statistical measure of the contents of the image, such as a summation of the image contents of those columns of the image array corresponding to the columns of the low spatial resolution pixel sub-array. Each second digitally encoded image is then mapped into memory using a scene balance mechanism that has been calibrated in accordance with a respective scene balance calibration signal, as described above with reference to FIG. 2. 
     To ensure that the correct calibrated scene balance mechanism is employed during the high resolution scan, its selection may be based upon a combination (correlation) of the first and third statistical measure representative codes, or by looking for the presence of interframe gap 205, as represented by the second code, in addition to frame-demarcating notch 206. Namely, the output of a notch detector 17, which looks for the presence of notches during the rescan, is augmented by other data that is derived during pre-scan of the film. For this purpose, the output of notch detector 17 and the second code signal may be logically combined (e.g. ORed) to identify the location of the successive image frames. Notch detection may also be supplemented by comparing how closely (within a prescribed window), the first and third statistical measure codes match. In effect, each first and third code may be considered to be a `fingerprint` of the image. If these `prints` match, it is inferred that the correct scene balance calibration has been selected for the image of interest, so that the manner in which each successive high resolution digitized image is mapped into memory will be optimized, so that the essential subject matter of each image will fall within the linear portion of the response range of the scanner&#39;s imaging pixel array. The high resolution digitized image is then mapped by the scene balance mechanism into a reduced encoding resolution framestore. 
     As will be appreciated from the foregoing description, the `missed notch` spatial synchronization problem encountered in multiscan photofinishing operations is effectively obviated in accordance with the present invention by the use of auxiliary information derived during the prescan of the image. This auxiliary information is used to supplement the frame-locating function of each notch, so that even if a notch is missed during the high resolution scan, spatial synchronization between each frame and its associated prescan-derived control information will be maintained. As a consequence, the present invention is particularly applicable to a highly automated digital imagery processing system, wherein successive image frames of a continuous reel of color photographic film, such as a spliced-together sequence of 35 mm film strips, are prescanned at low spatial resolution and then rescanned to derive a color balanced, high spatial resolution digitized image. 
     While I have shown and described an embodiment in accordance with the present invention, it is to be understood that the same is not limited thereto but is susceptible to numerous changes and modifications as known to a person skilled in the art, and I therefore do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.