WO1992005668A1

WO1992005668A1 - Scene balance calibration of digital scanner

Info

Publication number: WO1992005668A1
Application number: PCT/US1991/006578
Authority: WO
Inventors: Patricia Andre Cosgrove
Original assignee: Eastman Kodak Company
Priority date: 1990-09-17
Filing date: 1991-09-13
Publication date: 1992-04-02
Also published as: JPH05502570A; EP0503042A1; CA2069332A1

Abstract

A digital imagery capture and storage mechanism controls the operation of a high resolution digital film scanner (12) such that the scanner scans the image twice, the first scan being used to gather data to calibrate the scanner (12), and the second scan being used to capture and store a high resolution image. The scanner carries out a low resolution mode, prescan of the color photographic image frame, thereby obtaining a low spatial resolution digitized image. This low resolution digitized image is then analyzed by a scene balance mechanism (14) to determine how the response characteristic of the scanner'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 used to adjust the sensitivity range of the scanner, 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's imaging pixel array. The high resolution digitized image is then processed by the scene balance mechanism (14) to map the image data into adigitized image having a reduced encoding resolution corresponding to that of an attendant framestore.

Description

SCENE BALANCE CALIBRATION OF DIGITAL SCANNER

FIELD OF THE INVENTION

The present invention relates in general to digitized color imagery photofinishing systems and is particularly directed to a mechanism for using a scene balance mechanism to calibrate the operation of a high resolution digital opto-electronic scanner.

BACKGROUND OF THE INVENTION

Because the dynamic range of photo-imagery input/output devices employed in photofinishing systems is considerably narrower than that of the image capture medium (photographic film), not all of the information contained on the film is reproducible. In order to optimize the presentation of a reproduced (printed) image to the human visual system, the device (opto-electronic scanner) that inputs the film image to the photoprocessing system should be calibrated such that the principal subject matter of the image preferably falls within the linear portion of the response

characteristic of the imaging device. In an analog photofinishing system, calibration is driven not only by the limited dynamic range of the components, but it can be expected that the response range of the film scanner may not match or even substantially overlap the output transfer function of the reproduction medium (print paper). Consequently, calibration can become a difficult and time-consuming trial and error process.

In a digital imagery processing system, on the other hand, where the input device (a digital opto-electronic scanner) and the output device (e.g. high resolution thermal printer) employ spatial arrays of pixels that interface (via DAC circuitry) with digital data signals, the fundamental mismatch problem does not occur. However, with the introduction of very high resolution film scanners (e.g. those having an imaging pixel array of 2028 x 3072 pixels, the response of which is resolved into sixteen bits per color per pixel), the quantity of data produced per image is so large that it must be reduced for storage in a

practical sized framestore. Namely, some of the scene information in the digitized image must be discarded. Thus calibration of the scanner remains a key aspect of quality system performance.

SUMMARY OF THE INVENTION

In accordance with a first embodiment of the present invention, the above-referenced calibration and storage problem of conventional photofinishing systems is obviated by means of a new and improved digital imagery capture and storage mechanism which scans the image twice, the first scan being used to gather data to calibrate the scanner, and the second scan being used to capture and store a high resolution image.

For this purpose, the scanner may be controlled to carry out a low resolution mode, prescan of the color photographic image of interest, thereby obtaining a low spatial resolution digitized image. (Because fewer pixels are scanned during the low resolution scan the low resolution pixel values may be encoded to a greater data precision.) This low

resolution digitized image is then analyzed by a scene balance mechanism to determine how the response characteristic of the scanner's imaging pixel array sees the image and encodes its spatial content. For purposes of the present invention, by 'scene balance mechanism' is meant an adjustment of image color balance based upon the scene content and the sensitometric characteristics (e.g. exposure, light source etc.) of the image being reproduced. 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, 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's imaging pixel array. Adjustment of color balance is defined as adjusting the average red, green and blue image levels, so as to ensure that an image will have the appropriate color and neutral reproduction characteristics. 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. Although some of the information in the captured scene is lost by the reduction in encoding resolution, since the scanner has been

calibrated so as to optimize its sensitivity to the color balance content of the image, the essential information (i.e. that which is necessary to reproduce a high quality image) is captured and stored.

Pursuant to a second embodiment of the invention, rather than scan the image twice, once at low resolution for purposes of calibration, and then at high resolution for data capture, the image is scanned only once, at high spatial resolution and high digital resolution. The high spatial resolution image is then converted into a high digital, low spatial resolution image, which is processed to calibrate the scene balance mapping function. Namely, the processed data is used to calibrate the mapping of the originally digitized image into a reduced digital resolution (e.g. eight bits per pixel per color) framestore.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 diagrammatically illustrates a photographic color film photofinishing minilab with which the scene balance-based digital imagery capture and storage mechanism of the present invention may be employed;

Figure 2 is an imagery processing flow diagram of the scene balance based calibration and high resolution capture mechanism of a first embodiment of the present invention; and

Figure 3 shows the steps of an alternative calibration mechanism in which only a single (high resolution) scan is carried out. DETAILED DESCRIPTION

Before describing in detail the scene balance-based digital imagery capture and storage mechanism in accordance with the present invention, 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.

Figure 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 Serial

Number ___, filed ___, by S. Kristy, entitled "Multiresolution Digital Imagery Photofinishing System", assigned to the assignee of the present application and the disclosure of which is incorporated herein. 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 x 2048) image scan of a 36mm-by- 24mm image frame of a 35mm 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 x 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 Figure 1, color photographic images, such as a set of twenty-four or thirty-six 36mm-by-24mm image frames of a 35mm 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 3072 x 2048 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. As noted earlier, because of its very large (2048 x3072) 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 a in 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 copending application Serial No. ___, filed ___, by T.

Madden et al, entitled "Extending Dynamic Range of Stored Image Database," 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. As pointed out above, when digitizing an image on the film strip, it is preferred that the film scanner be calibrated such that the principal subject matter of the image falls within the linear portion of the response range of the scanner's imaging pixel array. For this purpose, a first embodiment of the present invention employs a calibration and high resolution capture procedure, diagrammatically

illustrated in the imagery processing flow diagram of Figure 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 seven to twenty-four by ten to thirty-six 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 x 192 image is captured. Through further spatial integration of the imagery data within workstation 14, a captured 128 x 192 pixel version of the image may be reduced to a very small sub-array

(e.g. 24 x 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 x 36) digitized image is then analyzed in step 102 by the scene balance mechanism to determine how the response characteristic of the scanner's imaging pixel array sees the image and encodes its spatial content. The scene balance

mechanism (the image processing 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's imaging pixel array. While the scene balance output values may be employed to effect vernier

adjustments of reference voltages for the scanner'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'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's linear response range), the scanner is controlled in step 104 to execute a high spatial 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. Figure 3 shows the steps of an alternative calibration mechanism in which only a single (high resolution) scan is carried out. Pursuant to this embodiment, rather than scan the image twice, once at low resolution for purposes of calibration, and then at high resolution for data capture, the image is scanned only once, at high spatial resolution and high digital resolution. The high spatial resolution image is then converted into a high digital, low spatial resolution image, which is processed to calibrate the scene balance mapping function. Namely, the processed data is used to calibrate the mapping of the originally

digitized image into a reduced digital resolution (e.g. eight bits per pixel per color) framestore. More particularly, with reference to Figure

3, at step 200, the image is scanned to obtain a high spatial resolution (e.g. 2048 x 3072) image digitized, for example at sixteen bits per color per pixel, just as in the second, calibrated high resolution scan of the first embodiment. In step 201, the high resolution image is spatially down-converted (decimated, filtered) to a relatively low spatial resolution digitized iamge, e.g. on the order of seven to twenty-four by ten to thirty- six pixels per frame, so as to reduce the computational intensity of the application of the high spatial resolution image to the scene balance mechanism through which that image is to be mapped into the framestore.

This very low resolution (e.g. 24 x 36 pixel sub-array) digitized image is then analyzed in step 202 by the scene balance mechanism as in the first

embodiment. Again, the scene balance mechanism outputs three values, one for each color, which represent the color balance content of the digitized image.

In step 203, using these values, the scene balance mapping function is calibrated (shifted), so that, during its application to the originally derived high resolution image, the essential subject matter of the image will be mapped in accordance with the linear portion of the response range of the scanner's imaging pixel array. Again, a respective offset code, one for each of the color values, may be added to the inputs of each scene balance look-up table in order to

effectively shift or translate its mapping function.

With the scene balance look-up tables shifted on the basis of the analysis of the low resolution converted image in step 203, the high resolution digitized image is mapped into the framestore, in step 204, such that essential image information (i.e. that which is necessary to obtain a high quality print) is captured and stored.

As will be appreciated from the foregoing description, by subjecting a low resolution prescan of an image to be scanned by a high resolution digital scanner, the present invention is able to successfully ensure that the essential subject matter of the image will fall within the linear portion of the response range of the scanner's imaging pixel array during a high resolution scan. 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. Although some of the information in the captured scene is lost by the reduction in encoding resolution, since the scanner has been 'scene balance' calibrated, essential information (necessary to reproduce a high quality image) will be captured and stored.

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.

Claims

WHAT IS CLAIMED

1. A method of controlling the manner in which a color photographic image that has been captured on a color photographic image recording medium is scanned by an opto-electronic scanning device and processed for storage as a digitized image in a digital imagery data base, comprising the steps of:

(a) causing said opto-electronic scanning device to scan said color photographic image to produce a first digitally encoded image;

(b) processing said first digitally encoded image in accordance with an image adjustment mechanism and producing an output representative of the manner in which said opto-electronic scanning device encodes the color content of said color photographic image; and

(c) causing said opto-electronic scanning device to rescan said color photographic image based upon the output produced in step (b).

2. A method according to claim 1, further including the step of:

(d) processing a second digitally encoded image produced as a result of said opto-electronic scanning device rescanning said color photographic image in step (c), and processing said second digitally encoded image in accordance with said image adjustment mechanism, so as to obtain a color content-balanced digitized image.

3. A method according to claim 1, wherein said image adjustment mechanism comprises a scene balance mechanism.

4. A method according to claim 1, wherein step (a) comprises causing said opto-electronic scanning device to scan said color photographic image to produce a first digitally encoded image having a first spatial resolution and a first encoding

resolution, and step (c) comprises causing said opto- electronic scanning device to rescan said color

photographic image in dependence upon the output produced in step (b) to produce a second digitally encoded image having a second spatial resolution greater than said first spatial resolution and a second encoding resolution less than said first encoding resolution.

5. A method of controlling the manner in which a color photographic image that has been captured on a color photographic image recording medium is scanned by an opto-electronic scanning device and processed for storage as a color-balanced digitized image in a digital imagery data base, comprising the steps of:

(a) scanning said color photographic image by means of said opto-electronic device so as to obtain a first digitally encoded image;

(b) processing said first digitally encoded image in accordance with an image adjustment mechanism and producing an output representative of the manner in which said opto-electronic scanning device encodes the color content of said color photographic image;

(c) rescanning said color photographic image by means of said opto-electronic scanning device so as to obtain a second digitally encoded image; and

(d) processing said second digitally encoded image in accordance with said image adjustment

mechanism, calibrated by the output produced in step

(b), so as to obtain a color content-balanced digitized image for storage in said digital imagery database.

6. A method according to claim 5, wherein step (a) comprises scanning said color photographic image by means of said opto-electronic device so as to obtain a first digitally encoded image having a first spatial resolution, and step (c) comprises rescanning said color photographic image by means of said optoelectronic scanning device so as to obtain a second digitally encoded image having a second spatial resolution, larger than said first spatial resolution.

7. A method according to claim 6 , wherein step (a) comprises scanning said color photographic image by means of said opto-electronic device so as to obtain a first digitally encoded image of said first spatial resolution and a first encoding resolution, and step (c) comprises rescanning said color photographic image by means of said opto-electronic scanning device so as to obtain a second digitally encoded image of said second spatial resolution, and a second encoding resolution less than said first encoding resolution.

8. A method according to claim 5, wherein said image adjustment mechanism comprises a scene balance mechanism.

9. For use with a digitized image processing system, in which a color photographic image that has been captured on a color photographic image recording medium is scanned by an opto-electronic device and processed by means of an image adjustment mechanism to derive a content-corrected output

digitized image for storage in a digital imagery database, a method of controlling the manner in which a scanned color photographic image is scanned and

digitized for storage in said digital imagery database comprising the steps of: (a) causing said opto-electronic device to conduct a first scan of a color photographic image, so as to obtain a first digitized image of a first spatial resolution and a first encoding resolution;

(b) processing said first digitized image in accordance with said image adjustment mechanism and producing an output representative of the color balance content of said first digitized image; and

(c) causing said opto-electronic device to conduct a second scan of said color photographic image and processing the digitized image obtained thereby in accordance with said image adjustment mechanism, as calibrated by the output produced in step (b), so as to obtain a second digitized image of a second spatial resolution higher than said first spatial resolution and a second encoding resolution lower than said first encoding resolution.

10. A method according to claim 9, wherein said image adjustment mechanism comprises a scene balance mechanism.

11. For use with a digitized image processing system, in which a color photographic image that has been captured on a color photographic image recording medium is scanned by an opto-electronic scanning device to produce a digitally encoded image, said digitally encoded image being processed by means of a image adjustment mechanism to derive a content-corrected digitized image for storage in a digital database, a method of controlling the manner in which a color photographic image is scanned and processed into a color-balanced digitized image comprising the steps of:

(a) causing said opto-electronic scanning device to scan a color photographic image to produce a first digitally encoded image having a first spatial resolution which is less than the image resolving capability of said scanning device and at a first encoding resolution;

(b) processing said first digitally encoded image in accordance with said image adjustment

mechanism and producing an output representative of the manner in which said opto-electronic scanning device encodes the color content of said color photographic image;

(c) causing said opto-electronic scanning device to rescan said color photographic image so as to obtain a second digitally encoded image having a second spatial resolution corresponding to that of the image resolving capability of said scanning device; and

mechanism, modified in accordance with the output produced in step (b), and producing a color content- balanced digitized image having a second encoding resolution less than said first encoding resolution.

12. A method according to claim 11, wherein said image adjustment mechanism comprises a scene balance mechanism.

13. A method of controlling the manner in which a color photographic image that has been captured on a color photographic image recording medium is scanned by an opto-electronic scanning device and processed for storage as a digitized image in a digital imagery data base, comprising the steps of:

(b) processing said first digitally encoded image in accordance with an image adjustment mechanism and producing an output representative of the manner in which said opto-electronic scanning device encodes the content of said color photographic image; and

(c) processing said first digitally encoded image in accordance with said image adjustment

mechanism, so as to obtain a content-adjusted digitized image.

14. A method according to claim 13, wherein step (b) comprises converting said first digitally encoded image to a second, reduced spatial resolution digital image, and processing said reduced spatial resolution digital image in accordance with said image adjustment mechanism and producing an output

representative of the manner in which said opto- electronic scanning device encodes the content of said color photographic image, and step (c) comprises processing said first digitally encoded image in accordance with said image adjustment mechanism, modified by the output produced in step (b), so as to obtain a content-adjusted digitized image.

15. A method according to claim 14, wherein said image adjustment mechanism comprises a scene balance mechanism

16. A method of controlling the manner in which a color photographic image that has been captured on a color photographic image recording medium is scanned by an opto-electronic scanning device and processed for storage as a color-balanced digitized image in a digital imagery data base, comprising the steps of:

(a) scanning said color photographic image by means of said opto-electronic device so as to obtain a first high spatial resolution digitally encoded image;

(b) processing said first digitally encoded image in accordance with a scene balance mechanism and producing an output representative of the manner in which said opto-electronic scanning device encodes the color content of said color photographic image;

(c) processing said first digitally encoded image in accordance with said scene balance mechanism, calibrated by the output produced in step (b) , so as to obtain a color content-balanced digitized image for storage in said digital imagery database.

17. A method according to claim 16, wherein step (a) comprises scanning said color photographic image by means of said opto-electronic device so as to obtain said first digitally encoded image having a high spatial resolution and a high digital encoding

resolution, and step (b) comprises converting said high spatial resolution image to a low spatial resolution image and processing said converted low spatial

resolution image in accordance with said scene balance mechanism and producing an output representative of the manner in which said opto- electronic scanning device encodes the color content of said color photographic image.

18. A method according to claim 17, wherein step (c) comprises mapping said first digitally encoded image into said digital imagery database in accordance with said scene balance mechanism, calibrated by the output produced in step (b) .