Extending dynamic range of stored image database

United States Patent m

Madden et al.

[54] EXTENDING DYNAMIC RANGE OF STORED IMAGE DATABASE

[75] Inventors: Thomas E. Madden, E. Rochester;

Edward J. Giorgianni, Rochester,
both of N.Y.

[73] Assignee: Eastman Kodak Company,

Rochester, N.Y.

[21] Appl. No.: 582,306

[22] Filed: Sep. 14, 1990

[51] Int. CI.' G06K 9/38

[52] U.S. CI 382/50; 358/426;

358/466; 382/54; 382/56

[58] Field of Search 382/50, 51, 52, 53,

382/54, 6, 41, 56; 358/262.1, 448, 461, 465, 466,

426

[56] References Cited

U.S. PATENT DOCUMENTS

4,371,260 2/1983 Yoshimoto et al 355/77

4,384,336 5/1983 Frankle et al 382/49

4,402,015 8/1983 Yamada 358/280

4,410,909 10/1983 Ueda et al 358/75

4,472,736 9/1984 Ushio et al 382/50

4,491,961 1/1985 Sutton et al 382/50

4,731,662 3/1992 Udagawa et al 358/75

4,731,862 3/1988 Tsuda et al 382/50

4,758,885 7/1988 Sasaki et al 358/80

4,845,761 7/1989 Cate et al 382/50

4,887,305 12/1989 Shimura 382/6

4,903,310 2/1990 Takeo et al 382/6

4,953,227 8/1990 Katsuma et al 382/50

[ii] Patent Number: 5,224,178 [45] Date of Patent: Jun. 29, 1993

4,972,500 11/1990 Ishii et al 382/50

5,057,913 10/1991 Nagata et al 358/302

Primary Examiner—Joseph Mancuso
Attorney, Agent, or Firm—Edward Dugas

[57] ABSTRACT

The dynamic range of a digitized image database is extended to permit shifting of encoded pixel values without 'clipping', and to provide limited windows of values into which specular highlights and unusually low reflectances or areas of objects in shadow light may be encoded and stored. Digital codes into which an image scanner output has been mapped by a scene balance mechanism are converted into a set of reduced-range digital codes of the same resolution but having a smaller range of basic 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 allow for the placement of specular highlights that are beyond the 100% diffuse white reflectance maximum, to convert a defined minimum value of low reflectance to an encoded value that is greater than the lower limit of the dynamic range of the database to allow for the placement of unusually low reflectances or areas of objects in shadow light, and to accommodate shifts in the digitized imagery data at both the high and low ends of the range.

14 Claims, 1 Drawing Sheet

5,224,

EXTENDING DYNAMIC RANGE OF STORED
IMAGE DATABASE

FIELD OF THE INVENTION 5

The present invention relates in general to digitized image data processing systems and is particularly directed to a mechanism for extending the dynamic range of a database which stores digitally encoded color im

10

ages. 1U BACKGROUND OF THE INVENTION

Digital imagery processing systems, such as those employed for processing digitize color photographic images, customarily digitized images by way of an opto- 15 electronic scanner, the output of which is encoded to some prescribed digital encoding resolution (or digital code width) that encompasses a range of values over which the contents of a scene, such as that captured on a (color) photographic recording medium, may vary. 20 As diagrammatically illustrated in FIG. 1, for a typical color photographic negative film, this range of values R is less than the density vs. exposure latitude of the film, but is sufficiently wide to encompass those film density values that can be expected to be encountered for a 25 typical scene. Then, by means of a preliminary image operator, such as a scene balancing mechanism, the digitized image is mapped into a set of digital codes, each of which has a digital resolution corresponding to the dynamic range of a digitized image data base (e.g. 30 frame store), the contents of which may be adjusted in the course of driving an output device, for example enabling a print engine to output a high quality color print.

As an example, as further illustrated in FIG. 1, the 35 mapping of the quantized output of a digital image scanning device may translate the contents of a given portion of the density vs. log exposure characteristic of a color photographic negative film into a database digital resolution of eights bits per color per pixel (twenty-four 40 bits per pixel), with a value of 255 corresponding to maximum 100% white reflectance (normally defined as a perfect (100%) non-fluorescent white reflecting diffuser). Other density values representing lesser reflectances are encoded relative to this maximum down to a 45 code value of zero, corresponding to a low reflectance value (e.g. black).

As a consequence, if, in addition to basic content of the scene, an image contains specular highlights (e.g. a reflection from a car bumper, identified at exposure line 50 SH in FIG. 1), their associated pixel values will be maximally encoded or 'clipped' at 255—the same as that for the above-referenced 100% white reflectance, so that a portion of their reflectance characteristics is lost. In addition, supplemental scene balance image process- 55 ing, as may be necessary to accommodate the parameters of a particular output device, may operate so to as adjust one or more pixel values upwardly, causing a further increase in the number of pixel values whose encoded values are maximal. Unfortunately, once a data 60 value has been maximized it cannot be shifted to a lower value without similarly affecting other like valued data, so that the content of an image reproduced (printed or displayed) from the digitized image is degraded.

As a further consequence, if, in addition to basic 65 content of the scene, an image contains unusually low reflectances or areas of objects in shadow light (e.g. shadow object identified at exposure line SS in FIG. 1),

178

2

their associated pixel values will be minimally encoded or 'clipped' at 0, so that a portion of their reflectance characteristics is lost. In addition, supplemental scene balance image processing, as may be necessary to accommodate the parameters of a particular output device, may operate so to as adjust one or more pixel values downwardly, causing a further increase in the number of pixel values whose encoded values are minimal. Unfortunately, once a data value has been minimized it cannot be shifted to a higher value without similarly affecting other like valued data, so that the content of an image reproduced (printed or displayed) from the digitized image is degraded.

SUMMARY OF THE INVENTION

In accordance with the present invention, the above discussed problem is solved by extending the dynamic range of the digitized image database, so as to permit a variation or shifting of the encoded pixel values without 'clipping', to provide a limited window or range of values into which specular reflectance image points, such as a reflection from a car bumper or a specular reflection of sunlight off a water surface, may be encoded and stored, and to provide a limited window, or range of values into which unusually low reflectances or areas of objects in shadow light may be encoded and stored. In particular, the present invention is directed to a method of enabling the dynamic range of the digitized image data base to be effectively extended beyond the range of values into which the digital codes output by the image scanner corresponding to the basip scene content are mapped by an image processing (scene balance) mechanism.

For this purpose, those digital codes into which the scanner output has been mapped by the image processing operator are converted into a set of 'reduced-range' digital codes of the same digital resolution but having a smaller range of basic 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. For the foregoing example of an eight bit encoding and storage resolution, such a value may be somewhat less than the maximum of 255 (e.g. 225), so as to leave a limited range or window of values (here 30 values) at the upper end of the encoding range, to allow for the placement of specular highlights that are beyond the 100% white reflectance maximum, and to accommodate shifts in the digitized imagery data. The code conversion mechanism also operates to convert a defined minimum value of low reflectance to an encoded value that is greater than the lower limit of the dynamic range of the database. For the foregoing example of an eight bit encoding and storage resolution, such a value may be somewhat greater than the minimum of 0 (e.g. 10), so as to leave a limited range or window of values (here 10 values) at the lower end of the encoding range, to allow for the placement of unusually low reflectances or areas of objects in shadow light, and to accommodate shifts in the digitized imagery data.

In effect, what is achieved in accordance with the present invention is a slight or delimited compression of the encoded imagery data values in order to 'fit' the basic scene content into a reduced portion of the dynamic range of the database and allow for the encoding or translation of extended data values.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagrammatically illustrates the variation of density vs. log exposure for a color photographic negative, upon which is superimposed a range of values R 5 less than the density vs. exposure latitude of the film;

FIG. 2 diagrammatically illustrates a photographic color film processing system; and

FIG. 3 diagrammatically illustrates the manner in which the present invention extends the dynamic range 10 of a digitized image database, to permit a variation of encoded pixel values.

DETAILED DESCRIPTION

Before describing in detail the particular image data- 15 base dynamic range extension mechanism in accordance with the present invention, it should be observed that the present invention resides primarily in a novel structural combination of conventional imagery data processing circuits and components and not in the particu- 20 lar 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 25 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 drawings do not necessarily represent the me- 30 chanical 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. 35 FIG. 2 diagrammatically illustrates a photographic film processing system in which the present invention may be employed. For purposes of the present description, such a system may be of the type described, for example, in copending patent application Ser. No. 40 582,305, filed Sep. 14, 1990, by Steven Kristy, entitled Multiresolution Digital Imagery Photofinishing System, assigned to the assignee of the present application and the disclosure of which is incorporated herein. However, it should be observed that the system de- 45 scribed in the above-referenced copending 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 sys- 50 tern.

In accordance with the digitized image photo processing system of FIG. 2, photographic images, such as those captured on 35 mm color film 10, are scanned by an opto-electronic film scanner 12, such as a commer- 55 cially available Eikonix, Model 1345 high spatial resolution digital scanner, which outputs digitally encoded data representative of the response of its imaging sensor pixel array onto which a photographic image contained in a respective color film frame is projected. This digi- 60 tally encoded data, or 'digitized', image is coupled in the form of an imaging pixel array-representative bit map to an attendant image processing workstation 14, which contains a frame store and image processing application software through which the digitized image may be 65 processed (e.g. enlarged, rotated, cropped, subjected to a scene balance correction mechanism, etc.) to achieve a desired base image appearance and configuration.

Once the base image has been prepared, it is written onto a transportable medium, such as a compact disc 16, for subsequent playback on a reproduction device 20, for example a relatively moderate resolution consumer television set 22, or output as a hardcopy print, as by way of a high resolution thermal color printer 24.

In accordance with the imagery data processing system described in the above referenced copending application, each captured image is stored in the form of a low resolution image and a plurality of residual images to enhance data processing speed. Regardless of the particular encoding and storage mechanism employed for digitizing the captured photographic image, the format of the data is that of a digitized image bit map 22, each low resolution pixel value of which has some prescribed code width (e.g. twenty-four bits or eight bits per color) corresponding to the dynamic range of the database (digital memory) in which the digitized image is stored.

Because the output device to which the disc-resident base image data may be delivered is not necessarily identified at the time that the base image is prepared and stored on the disc, it may be necessary to perform further processing of the stored image in the course of producing an image in a 'finalized' or 'finished' output form. This further processing may involve scene balance mechanism that tailors the image in accordance with the parameters of the output display or print engine and typically involves a shift or translation of the encoded data values of the scene. (In addition, the image may contain specular highlights that cannot be adequately represented by the maximal encoding value specified by the scene balance mechanism, and the image may contain unusually low reflectances or areas of objects in shadow light that cannot be adequately represented by the minimal encoding value specified by the scene balance mechanism).

FIG. 3 diagrammatically illustrates the manner in which the present invention solves this limited dynamic range problem by extending the dynamic range of the digitized image database, so as to permit a variation or shifting of the encoded pixel values without 'clipping', and to provide a limited window or range of values into which specular reflectance image points, such as a reflection from a car bumper or specular reflection of sunlight off a water surface, may be encoded and stored. In the Figure, trace 30 represents the range of values obtained by the image processing operator corresponding to dynamic range of the database of interest (eight bits in the present example), with a maximum available value of 255 representing a pixel value of 100% white reflectance and a minimum available value of 0 representing a pixel value of low reflectance.

Pursuant to the invention, rather than store the encoded values such that the upper end of the dynamic range of the storage database coincides with the abovereferenced 100% white reflectance, each of the imagerepresentative codes output by the image processing operator (scene balance mechanism), is subjected to a code conversion operator resident within the image processing software of workstation 14, so that a value of 100% white reflectance has an encoded value that is somewhat less than the maximum of 255, for example a value of 225 as shown in trace 32 in FIG. 3. (The conversion mechanism may also provide for a shift at the low end of the range, as shown by offset 33. This limited 'compression' of the encoded imagery data values effectively fits the encoded data into a reduced portion 34 of