Image processing system and method employing combined black and white and gray scale image data
FIELD AND BACKGROUND OF THE INVENTION 
     This invention relates to the processing of image data, and more particularly to a system and method for producing high resolution gray scale image data representing an image. 
     The system and method of the present invention is especially useful in image processing systems wherein the images from documents, such as bank checks, are stored electronically in a computer system and the images may be subsequently retrieved for display or printing. 
     One such system is disclosed in commonly owned copending U.S. application Ser. No. 134,734, filed concurrently herewith (Attorneys Docket CT9-86-009). In this system, paper documents such as bank checks, are processed through a high speed reader/sorter. As the documents are processed, the magnetic ink characters on the documents are read by an MICR reader. In addition, the documents are directed past an image scanning station which electronically captures the image of both the front and back of the document. 
     Many kinds of documents, including bank checks, contain information in the form of sharp high contrast images, such as numbers and signatures, as well as broader, lower contrast images such as endorsements, printed inforaation, pictures, background designs and the like. Documents of this type cannot adequately be represented by black and white image data, since the thresholding of the image data into discrete black and white values results in loss of low contrast information. A high quality, high resolution gray scale image representation would adequately reproduce both high contrast images and lower contrast images, but would require massive amounts of data storage, which would be impractical and economically infeasible in a system that processes image data for high volumes of documents. 
     SUMMARY OF THE INVENTION 
     In order to reduce data storage and transmission bandwidth requirements, a high resolution gray scale image is represented in two separate image forms, namely, a high resolution, high sampling density black and white image and a lower resolution, lower sampling density gray scale image. Generating separate black and white and gray scale image data allows the use of either image form independently. The high resolution black and white image data and the low resolution gray scale image data may be recombined subsequently to produce reconstructed high resolution gray scale image data representing the image. 
     Thus, broadly the present invention provides a method for producing a high resolution gray scale representation of an image which comprises providing low resolution gray scale image data for the image, providing high resolution black and white image data for the image, and merging the low resolution gray scale image data and the high resolution black and white image data to produce reconstructed high resolution gray scale image data representing the image. The present invention also provides an image processing system for carrying out this method. 
     In merging the low resolution gray scale image data and the high resolution black and white image data, the low resolution gray scale image data is first interpolated by a two dimensional linear interpolation to high sampling density gray scale image data of the same sampling density as the black and white image data. Then the interpolated high sampling density gray scale data and the high resolution black and white image data are combined to produce the reconstructed high resolution gray scale image data. 
     The interpolated high sampling density gray scale image data suffers from loss of definition and detail, especially along high contrast boundaries, as a result of the earlier reduction in resolution. The gray scale values present in the interpolated high sampling density gray scale data represent averages of the gray scale values at the lower resolution. In combining the interpolated gray scale values with the high resolution black and white image data, a convolutional technique is employed to restore the lost definition and detail. In this technique, a sliding convolutional window is employed for generating for each successive data element of the high resolution black and white data a corresponding high resolution enhancement value based upon the sum of weighted values of that data element and a plurality of surrounding data elements. The derived high resolution enhancement value for each data element or pel is added to the interpolated high sampling density gray scale value for the corresponding pel to thereby produce the reconstructed high resolution gray scale image data. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Some of the features and advantages of the invention having been stated, others will become apparent from the detailed description which follows, and from the accompanying drawings, in which 
     FIG. 1A illustrates an image containing both high contrast information and low contrast information; 
     FIG. 1B illustrates the loss of image data and detail resulting from thresholding the image of FIG. 1A into black and white image data at a relatively low threshold level; 
     FIG. 1C illustrates the loss of image data and detail resulting from thresholding the image of FIG. 1A into black and white data at a higher threshold level; 
     FIG. 2 is a schematic diagram illustrating the procedure for producing reconstructed high resolution gray scale image data in accordance with the invention; 
     FIG. 3A is a graph representing a raster scan of an image; 
     FIG. 3B is a graph illustrating a low resolution, low sampling density gray scale representation of the image of FIG. 3A; 
     FIG. 3C is a graph illustrating a high resolution black and white (binary) image of the same image; 
     FIG. 3D is a graph illustrating an interpolated high sampling density gray scale image; 
     FIG. 3E is a graph illustrating on an expanded scale the enhancement values; 
     FIG. 3F is a graph illustrating the reconstructed high resolution gray scale image produced by combining the enhancement values of FIG. 3E with the interpolated high sampling density gray scale image of FIG. 3D; 
     FIG. 4 is a schematic representation of a raster scanned black and white (binary) image, with the picture elements (pels) of the image shown as ones and zeros; 
     FIG. 5 illustrates the sliding convolutional window used to produce the high resolution enhancement values; 
     FIG. 6A-6D illustrate a sequence of steps and the results obtained when the sliding convolutional window is applied to the image of FIG. 4 to produce the enhancement values; 
     FIG. 7 illustrates the partially completed arrangement of enhancement values produced by applying the sliding convolutional window to the image of FIG. 4; and 
     FIG. 8 is a view similar to FIG. 7, but with values along the borders completed. 
    
    
     DESCRIPTION OF ILLUSTRATED EMBODIMENTS 
     Many paper documents, including checks, contain sharp, distinct markings such as machine printing and handwritten information. It is recognized that high resolution black and white (or binary) image form produces the best reconstructed image quality for the least amount of data quantity for these types of markings. However, those same documents may also contain less distinct, lower contrast markings such as scenic backgrounds and colored endorsements. 
     FIGS. 1A-1C illustrate the problems of using only black and white images. FIG. 1A shows the original image with the word FIRST printed in a relatively low contrast color, while in the background is a series of digits printed in high contrast black and white. FIG. 1B illustrates the result of attempting to capture the low contrast information represented by the word FIRST by using a relatively low black and white threshold value. It can be seen that high contrast data, represented by the digits underlying the letters of the word FIRST, are lost. FIG. 1C illustrates the black and white result of using a higher threshold value. In this instance, the lower contrast information represented by the word FIRST is lost. In instances where all of the information on a document is important, the best image representation is a high resolution gray scale image employing multiple levels of gray, e.g. 256 gray levels. However, this generally results in an unmanageable amount of image data, since gray scale image representation requires data on the level of grayness and thus inherently requires more storage than black and white information, which is represented by a single bit per picture element. 
     By representing the image separately by high resolution black and white image data (e.g. 240 pels per inch) and low resolution gray scale image data (e.g. 80 pels per inch) a reduction in the amount of data required to represent the image is realized. Either form of representation can be separately and independently utilized to recall and display the image. Another advantage of two separate representations is the availability of redundant information for certain parts of the document in the case of data errors. Still another advantage is the availability of a partial image for different applications. For example, signature verification and OCR applications may suitably use the black and white high resolution data, while in other applications, such as endorsement verification for example, only the gray scale data may be required. Both forms of image representation are kept separate through any image processing steps such as data reduction and compression and subsequent storage. When it is desired to recall and display or print the image in a high resolution gray scale form, the two image forms are decompressed and merged analytically into one image representation. 
     As shown in FIG. 2, high resolution black and white raster scanned image data at a resolution of 240 pels per inch is stored in compressed form as indicated at 10. Any of a number of well known data compression techniques, such as run length coding for example, can be employed to reduce the volume of data required to represent an image. Similarly, low resolution gray scale raster scanned image data at a resolution of 80 pels per inch is stored in compressed form as indicated at 11. Any of a number of well known data compression techniques can be utilized for reducing the volume of the gray scale data. In producing a reconstructed image, the compressed high resolution black and white image data is decompressed by a suitable decompression means 12 appropriate for the compression technique employed, and a separate decompression means 13 is used for decompressing the low resolution gray scale data. 
     In forming a reconstructed high resolution gray scale image, the decompressed low resolution gray scale image data is first expanded into high sampling density gray scale image data of the same sampling density as the black and white image data, as indicated at 14. This is achieved by suitable interpolation techniques, such as by two dimensional linear interpolation. Then the interpolated high sampling density gray scale is merged as indicated at 15 with the high resolution black and white data to produce reconstructed high resolution gray scale image data, as indicated at 16. This reconstructed data can be displayed on a suitable display device 17. More particularly, the merging of the interpolated high resolution gray scale data with the high sampling density black and white data employs a convolutional technique designed to restore lost definition and detail to the low resolution gray scale data. This technique involves generating high resolution enhancement values from weighted sums of the high resolution black and white image data to compensate for the loss of definition which results from the low resolution representation of the gray scale image data. These enhancement values are then added to the interpolated high sampling density gray scale data to thereby produce reconstructed high resolution gray scale image data. 
     The effect of the enhancement values is to subtract out the &#34;smearing&#34; or &#34;averaging&#34; effect of the low resolution gray scale representation. This will be better understood by referring to FIGS. 3A-3F, where FIG. 3A graphically represents the raster scan of an image. 
     FIG. 3B graphically represents a low resolution, low sampling density digitized gray scale form of the same image. Notice how the sharp peaks are reduced in amplitude, and in fact, the twin peaks are run together in the low resolution FIG. 3B. 
     FIG. 3C shows the binary representation of FIG. 3A at high resolution, high sampling density. (The upper line in the curve is the &#34;0&#34;, or white value, the lower line is the &#34;1&#34;, or black value, consistent with the polarity of the gray-scale curves which are shown to range from 0 to 255.) Note that the high resolution binary image captures the fine details, but does not reproduce the overall gray shadings. 
     FIG. 3D shows the &#34;smeared out&#34; high sampling density gray scale which is derived by interpolation from the low resolution, low sampling density gray scale values of FIG. 3B. Although the image is now at the same high sampling density as the black and white image of FIG. 3C, it suffers from the same loss of definition and detail as FIG. 3B. The twin peaks of FIG. 3A are lost. 
     FIG. 3E represents the enhancement values, the output of the weighted window convolution process described in FIGS. 4 to 8. The exact shape of this curve depends on the binary values of the preceding and following lines, but what is shown is a typical type of curve. The vertical scale of this curve is increased to better demonstrate the process. 
     FIG. 3F is the sum of the two previous curves. Note that the &#34;mixed&#34; or reconstructed high resolution curve of FIG. 3F contains the high resolution twin peaks of FIG. 3A and the overall grayscale value trends. 
     The method of recombining the interpolated high sampling density gray scale image data and the high scale black and white image data will be understood more fully from the detailed example which follows. 
     The matrix shown in FIG. 4 consists of the black/white high resolution pel values, of &#34;1&#34;&#39;s and &#34;0&#34;&#39;s which collectively define an image. FIG. 5 shows the convolutional weighting &#34;window&#34; used in generating the output values in FIG. 8. FIGS. 6A-6D show examples of how the window is used. In FIG. 6A, the window overlays the upper left-hand corner of the matrix of FIG. 4. The values shown in FIG. 6A are derived by multiplying, for each of the nine locations, the value of the weighting window by the value of the underlying pel (1 or 0). Thus, the corresponding right edge and lower right corner of the window have weighting values of +8, while the remaining values are zero. Adding up the products a net value of 16. Since the window is centered over location &lt;2,2&gt; of the matrix, the value of 16 is placed in the corresponding position &lt;2,2&gt; of the output matrix in FIG. 7. 
     When the window is moved one position to the right, now centered over position &lt;2,3&gt;, as indicated in FIG. 6B, the contribution to the sum of products from elements &lt;1,2&gt;, &lt;1,3&gt;, &lt;2,2&gt;, and &lt;3,2&gt; is 0, the contribution from the other four perimeter squares is 8 each, for a total of 32, but now the central window weight of -64 is multiplied by the value &#34;1&#34; of element &lt;2,3&gt;, so that the entire total for this window position is -32. FIGS. 6C and 6D give another couple of examples. FIG. 7 has the result for all locations which can be calculated from the combination of the data in FIG. 4 and the window of FIG. 5. By assuming a border of &#34;0&#34;&#39;s outside the perimeter of FIG. 4, we can then fill to the edges, i.e., extend FIG. 7 to FIG. 8. 
     The negative values in FIG. 8 represent a &#34;blackening&#34; of the gray scale values, which occurs on the &#34;black side&#34; of a character stroke in an image. The positive values represent a &#34;whitening&#34; of the gray scale which occurs on the &#34;white side of the boundary. Note that for a large uniform area of all white or all black, the output would be zero since, if every element under the window is &#34;0&#34;, then the sum of products would be zero. On the other hand, if every element under the window is &#34;1&#34;, then the eight +8 terms around the perimeter of the window would exactly cancel the single -64 value in the center. This is a desirable result since the reconstructed high resolution gray scale values of uniform areas should be determined solely by the low-resolution gray scale data. Only at edges of areas do we desire to &#34;sharpen&#34; the contrast of the image. 
     The final step in the process is to add together the interpolated gray scale with the &#34;windowed&#34; black/white weighting values. 
     The weights in the window, FIG. 5, were selected to be used in a system where the whitest gray scale value is 255, and the blackest is 0. The signs of the elements in the window would be changed for reversed video, which has the blacker gray values numerically larger than the whiter gray values. The amplitudes of the windows may be increased or decreased proportionally to increase or decrease the effect of this &#34;sharpness&#34; process. The number of elements in the window (its size) may also be changed, but the 3 by 3 works well for a 3 times relationship between low resolution and high resolution images. For the reasons stated earlier, the sum of the window elements should be zero.