Abstract:
A method for the optimization of optical scanners by automatically determining image processing threshold values using a novel threshold calibration sheet. The calibration sheet image has lines at a forty-five degree angle to the path of the document through the scanner and has a predetermined image dot count. One embodiment comprises the steps of repetitively scanning the sheet until scanner threshold values which satisfy predetermined relationships with said dot count are determined. Another embodiment comprises the steps of a single calibration scan with multiple processing of the gray scale scan data on a host computer. By using this method, two or more scanner cameras can work together on processing document images and consistently give uniform results, independent of their individual technical operating or performance differences.

Description:
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent disclosure, as it appears in the Patent and Trademark Office public patent files or records, but otherwise reserves all rights whatsoever in copyright. 
     TECHNICAL FIELD 
     This invention relates to a method for the optimization of optical scanners by automatically determining image processing threshold values using novel threshold calibration sheets 
     BACKGROUND OF THE INVENTION 
     Optical character scanners are devices for digitizing documents by converting character images on the document into digital data form for subsequent use in analyzing, storing and reprinting a copy of the original so as to eliminate hours of tedious typing. Optical scanners are currently used in a variety of applications to perform steps which were previously executed manually. 
     Optical scanners are well known in the art. In such systems, a light source is typically directed toward the document while an array of photodetectors scans the document. During scanning, the array detects light reflected from the images on the document. Digital data is generated corresponding to the detected light, and thus, to the image on the document from which data is being generated relative to a reference location. 
     The use of optical scanners for reproducing typed form documents and the like has been somewhat limited by the difficulty realized in calibrating the imaging system to differentiate between characters comprising the text to be reproduced via the optical scanner and aberrant marks or poorly formed characters which cannot be recognized by the scanning system. A poorly calibrated system results in a failure of the scanning system to retrieve all of the data required by the operator. 
     The calibration of optical scanners is typically effected by a manual process wherein the scanner user is required to optimize various threshold parameters by visual comparisons of the resulting images in order to ensure that all of the text is converted digitally without causing the scanner to detect &#34;ghost&#34; images or otherwise &#34;misread&#34; the image. 
     Various methods have been proposed in the art for calibrating optical scanners. For example, U.S. Pat. No. 4,408,231 discloses a method for calibrating a photosensitive scanning array wherein a linear image sensor and the video channel associated with the sensor are calibrated to maximize the video signal available. The useable video signal is maximized by calibrating the sensor relative to the range of the analog-to-digital converter. The sensor is calibrated by adjusting its exposure to illumination. The adjustment may be made by adjusting the illumination source or by changing the sampling interval. The gain is adjusted by using a variable gain amplifier. A microprocessor monitors the digitized video signal and controls the exposure and gain to calibrate the sensor and channel. 
     U.S. Pat. No. 4,605,970 discloses a method and apparatus for calibrating an optical document digitizer in which the calibration mechanism is mounted to a rotatable drum of the digitizer for such purpose. The apparatus comprises an elongated strip having an optical pattern of contrasting light and dark areas on the surface thereof, the pattern including plural discrete blocks, each of the blocks having first and second orthogonal edges wherein the first edges are positioned along a scanning reference line. 
     A need in the art exists for a relatively inexpensive and practical method suitable for reliably calibrating an optical scanner which does not utilize the cumbersome and expensive apparatus or the time consuming subjective manual approach of prior art systems. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method for calibrating an optical scanner by determining optimal threshold values which comprises the steps of repetitively scanning a unique calibration sheet with predetermined dot counts until lower, upper and processed threshold final values are determined which satisfy predetermined relationships with said dot counts and setting the lower, upper and processed threshold values to their respective final values. By basing the calibration on a scan of a calibration sheet with predetermined dot counts, two or more scanner cameras can work together on processing document images and consistently give uniform results, independent of their individual technical operating or performance differences. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a preferred embodiment arrangement of scanner, printer, host computer, communication means and switches. 
     FIG. 2 shows one side of a preferred embodiment of a calibration sheet, the reverse side being a duplicate of the side shown. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a method for optimizing or calibrating an optical scanner which utilizes an automated computer controlled scheme. Applicants have discovered that an optical scanner can be conveniently calibrated by utilizing a novel and nonobvious calibration sheet which allows for the calculation of optimum threshold values for maximizing the utility of the optical scanner. The same calibration sheets may be used for calibrating different cameras and scanners resulting in a uniformity of image processing across multiple scanners. 
     Optical scanners known in the art can be calibrated by the method according to the present invention. One example is the Photomatrix Document Scanner. The Photomatrix is a dual sided scanning device with two cameras which can scan, compress, and transmit to a host computer digitized images of paper documents. This operation can either be controlled by an operator entering commands via the operator&#39;s console or via a host control interface. The scanner&#39;s optical system illuminates the document with light of a specified wavelength. The image is reflected by a mirror through a lens onto charge coupled devices (&#34;CCD&#39;s&#34;). The light source is a halogen light. The amount of time that the CCD&#39;s are exposed to the projected image, as well as the brightness intensity of the light affects the quality of raw data that results from a scan. 
     The CCD&#39;s provide the raw data in the form of an analog signal. Initially, the analog signal is converted by analog-to-digital converters to a 256 grey scale digital signal. The grey scale signal is processed and converted to a binary digital, black dot or white dot signal for final use. As the intensity of the light may vary from lamp to lamp as well as over time, in order to minimize inaccuracies or inconsistencies, several user-defined constants, or thresholds are utilized. 
     In the case of the Photomatrix Scanner, an image processor receives 8 bit per pixel data (&#34;P&#34;) and converts it to 1 bit per pixel data. The image processor converts pixels defined on a scale of 256 grey levels to pixels defined as either black (1) or white (0). Each image processor holds in registers three 8 bit threshold values: a low intensity threshold or dark level (&#34;LTH&#34;); a high intensity upper threshold or light level (&#34;UTH&#34;); and a processed video threshold (&#34;PTH&#34;). The image processor for a CCD evaluates each P value in two steps where it compares the pixel value to the three thresholds. 
     The first step is a course adjustment calculation which classifies P as black, white or as requiring further classification. If P is less than the LTH, P is output as black. If P is greater than the UTH, P is output as white. If P is between LTH and UTH then P needs further resolution (fine adjustment) and P is fed into a convolution filter for the image processor for further threshold processing which involves utilization of the PTH. 
     The convolution filter is a group of high-speed adders and multipliers that construct a two-dimensional 5-by-5 matrix of pixels with P as the center pixel. The convolution filter determines whether the pixel data is to be processed as white or black in this fine adjustment by employing additional parameters which comprise PTH and pixel priority values. A pixel&#39;s priority value determines the weight given the pixel&#39;s contribution to the fine adjustment determination. The pixel priority value is the actual output decision device that multiplexes the high, low and processed video choices. It determines their combined value to obtain a final decision and outputs that decision. The final binary data values, representing black or white pixels, are sent to a buffer-write module. 
     The threshold settings in the Photomatrix image processing module determine the levels at which the processed grey level (8 bits per pixel) image is converted to a binary bit per pixel) image. This process of converting from 8 bit pixels to 1 bit pixels is called binarization. The many combinations of the high, low and processed thresholds are grouped into 8 levels which provide a spectrum of binarization from the darker to the lighter. These 8 threshold settings operate in the same manner as the darkness control on a copy machine. The 8 threshold levels can be selected either from the operator&#39;s terminal or the host computer interface. 
     One of the unexpected benefits of the present invention is that there is no need to use or adjust these darkness controls. When the Photomatrix Scanner is calibrated in accordance with the present invention the 50% threshold setting has been found to give accurate, consistent results. This may be due to the fact that the present invention is printer independent. In the manual process, it is necessary to compare the original scanned sheet with the image of the sheet as printed on another sheet of paper. Obviously, the printed image depends on the printer being used and its capabilities. According to the present invention, the data obtained by the scanner is directly analyzed and is therefore much more accurate than the manual process. While the 50% setting has been discussed, any of the other settings may be calibrated to give such results. 
     The manual process to determine the LTH, UTH and PTH values is tedious, time-consuming and requires numerous subjective judgments by the operator. The following procedure is generally followed in manually determining these values for the Photomatrix Scanner. 
     LTH is determined by initially setting LTH=UTH=PTH=0, scanning a document and printing the image scanned. If the printed image is &#34;all&#34; white, 10 is added to LTH and UTH while keeping PTH=0. The scanner is set to the new values, the document is scanned and the image is printed. This process is continued until the document becomes &#34;visible&#34; in the printed image. The value of LTH at this point is the final LTH value and is recorded. 
     UTH is determined by initially setting LTH=UTH=255 and PTH=0, scanning a document and printing the image scanned. If the printed image is &#34;all&#34; black, 10 is subtracted from LTH and UTH while keeping PTH=0. The scanner is set to the new values, the document scanned and the image printed. This process is continued until the document becomes visible in the printed image. The value of UTH at this point is the final UTH value and is recorded. 
     PTH is determined by initially setting LTH to the final LTH value, UTH to the final UTH value and PTH=0, scanning a document and printing the image scanned. If the image is &#34;too&#34; light, 10 is added to PTH while keeping both LTH and UTH at their set values. This process is continued until the printed image becomes &#34;too&#34; dark, at which point the last two PTH values are averaged and the average value is the set PTH value and is recorded. 
     In the prior art, thresholds for each camera were determined by the tedious, complicated and lengthy manual process described above. The manual process depended upon many subjective judgments by the operator who had to be particularly trained to make what were hopefully consistent judgments. Obviously, judgments as to &#34;all&#34; white or black, a &#34;visible&#34; image and &#34;too&#34; light or dark can easily vary from scan to scan and operator to operator. In addition to a lack of uniformity between judgments by a single individual there was evident an even greater lack of uniformity among different operators and different scanners. Moreover, because the manual process can take upwards of eight hours per camera, sixteen hours for both cameras, it could not be practically performed on a daily basis. Using the present invention complete calibration of both camera can be accomplished in about fifteen minutes. 
     According to the present invention, a computer program running on a digital computer is used to carry out the procedure of setting the threshold values. All decisions relating to image evaluation are performed by a computer. The operator merely loads the automatic sheet feeder with a stack of calibration sheets and runs the program. When the program finishes the scanner is loaded with the computer determined final threshold values. The novel and non-obvious calibration sheets are critical to the practice of the present invention. 
     In a preferred embodiment of the present invention, a Photomatrix Document Scanner, manufactured by Photomatrix of Phoenix, Ariz., utilizing a Terminal Data Corporation DS-4100 Document Transport, communicates with a host SUN 3-280 computer, manufactured by Sun Microsystems Inc. of Mountain View, Calif. via a Small Computer Systems Interface for the transmission of images and via an RS-232 serial link for the transmission of commands. 
     As shown in FIG. 1, the scanner communicates with its maintenance terminal via an RS-232 serial connection with a first intervening switch 1, while the host computer communicates with a printer also via an RS-232 serial connection with a second intervening switch 2. The switches are joined with an RS-232 compatible link. Both switches ca be controlled by the host computer. 
     During normal scanning operation, switch 1 is set to the scanner maintenance terminal and switch 2 is set to the printer. While running the threshold setting program of the present invention, the host computer sets switch 2 to pass data to switch 1 and switch 1 to accept data from switch 2. During termination of the program the switches are reset to normal operating mode, with switch 1 reset to accept data from the scanner maintenance terminal and switch 2 reset to pass data to the printer. 
     The threshold determinative portion of the calibration sheet image should comprise lines at a forty-five degree angle to the path of the document through the scanner (i.e. the vertical direction). For a resolution of 150 dots per inch (&#34;dpi&#34;) the lines are preferably spaced 1/2 inch apart. A resolution of 150 dpi generally implies a resolution along the scan line (i.e., horizontal direction) of 150 pixels per inch and a resolution in the cross-scan (i.e., vertical direction) of 150 lines per inch. The lines should be the same color as the printing on the documents to be scanned and should have the thickness of 2 dots at whatever resolution is being used. The background should be the same color as the background of the document to be scanned. Images consisting of characters, figures, horizontal or vertical lines should be avoided. The count of the number of black dots on a scanned calibration sheet image is the criterion used to evaluate image quality. It is not necessary for the same calibration sheet to be reused, duplicate originals may be used. 
     Before the calibration method can be put into automatic mode an initial determination is made to determine the optimal dot counts of the calibration sheet being used to calibrate the scanners. Once determined, the optimal dot counts for a calibration sheet remain constant for all calibrations for all cameras and for all scanners used. Thereby, beneficially increasing the consistency of results across all cameras and scanners. 
     A scan tolerance dot count (&#34;TDC&#34;) is determined experimentally by examining several scans of a particular calibration sheet, noting the variations in dot counts and choosing a TDC value large enough so that the results of a significant number of rescans fall within it. A maximum dot count (&#34;MDC&#34;) for the calibration image is determined, it is based on the maximum possible dot count for the camera for the calibration scan image. A minimum dot count set equal to is zero is also used. 
     In the case of the Photomatrix Scanner it is necessary to determine the processed video target dot count (&#34;PTDC&#34;), the lower target dot count (&#34;LTDC&#34;) and the upper target dot count (&#34;UTDC&#34;) for the calibration sheet. The PTDC is used in the fine processing that occurs in the convolution filter. In accordance with the present invention, LTDC is set equal to TDC+1, while the UTDC is set equal to MDC-(TDC+1). 
     According to one embodiment the MDC is simply the area of the calibration sheet multiplied by the resolution of the scan squared. For example, if the scan area is 8.5 inches by 1 inches, TDC=200 and the resolution is 150 dots per inch, MDC=(8.5)×(11)×(150)×(150)=2,103,750, the UTDC=2,103,549  and the LTDC=201. 
     Another embodiment adjusts the calibration scan area in order to minimize image processing time and errors due to misalignment of the calibration sheet and take into account scanner resolution limitations. The height of the scan area is symmetrically reduced to minimize processing time. The width is symmetrically reduced to minimize errors caused by sheet misallignment. The number of dots expected to be counted are based on the outer limits of scanner resolution. 
     In this later embodiment the scan area is effectively a rectangle located in the center of the calibration sheet. The height of the rectangle has been reduced to about two and one third inches, while the width of the rectangle is about two inches less than the width of the calibration sheet. At a resolution of 150 dots per inch, the Photomatrix Document Scanner uses between 16 pixels to 1728 pixels per line and between 16 lines to 3000 lines per image. The number of pixels per line must be an even multiple of 16 pixels. The resolution of 150 dots per inch is considered accurate to within ±2 per cent. 
     Subtracting 160 pixels, about one inch from each side of the scan line, results in a calibration scan width of I408 pixels. Reducing the number of calibration lines scanned to 358 lines reduced the processing time to under second per calibration image. The MDC is then (1408)×(358)×(1+0.02) 2  or about 524,428. Since TDC is still 201, UTDC=MDC+201=524,629 and LTDC=201. 
     As long as the threshold determinative portion of the calibration sheet remains the same, TDC, MDC, UTDC and LTDC values need not change. 
     PTDC is determined by experimentation. A set of calibration sheets are prepared, final LTH and UTH values are determined by the manual method set forth above. During determination of the final UTH and LTH values the PTH value is set to and maintained at a value equal to zero. The calibration sheets are scanned and re-scanned while the PTH is varied until the operators agree that the particular PTH setting is resulting in the best image. The PTDC is set to the dot count at that point. As with the UTDC and the LTDC, as long as the threshold determinative portion of the calibration sheet remains the same, the PTDC will remain constant. 
     Next an example of a preferred embodiment of the present invention which uses LTDC, UTDC and PTDC as determined above is described in further detail. For this example, in determining the final threshold values for the 50% level, we start with the following values: LTDC=201, UTDC=508344, PTDC=35,000 and TDC=200 dots. 
     In determining the final LTH, a bisection method is used to find a dot count that matches a count of LTDC. While a bisection method is currently used, any analytic methodology may be used to find the desired match. At each step of the bisection process the LTH and UTH values are set to the average of the bounding values for LTH. PTH remains set to zero. This corresponds to the appearance of an image from the &#34;all&#34; white state in the manual process. 
     For the first scan, LTH --  1 is set equal to LTH(0)=0 plus LTH(255)=255 divided by two, that is: 
     
         LTH.sub.-- 1=UTH=(0+255)÷2≈127. 
    
     The calibration sheet is scanned and Scan 1 results in a black dot count (&#34;L --  B --  D --  C --  1&#34;) of 26505. Since the difference between LTDC and L --  B --  D --  C --  1 is greater than the tolerance of 200 dots the procedure is repeated, but only after the LTH setting is adjusted again by the bisection method. LTH --  2 is set equal to LTH(0) plus LTH --  1 divided by 2, that is: 
     
         LTH.sub.-- 2=UTH=(0+127)÷2≈63. 
    
     A calibration sheet is scanned and Scan 2 results in L --  B --  D --  C --  2=662. Again 662 is not within the 200 dot tolerance of LTDC the process continues. The LTH is adjusted with LTH --  3 set equal to LTH(0) plus LTH --  2 divided by 2, that is: 
     
         LTH.sub.-- 3=UTH=(0+63)÷2≈31. 
    
     A calibration sheet is scanned and Scan 3 results in L --  B --  D --  C --  3=2. Since 2 is within the 200 dot tolerance of LTDC, a final LTH setting has been found and LTH FINAL is set equal to 31. 
     The UTH is determined in a similar manner. A bisection method is used to find to find a dot count that matches a count of UTDC. At each step of the bisection process the UTH and LTH values are set to the average of the bounding values for UTH, while PTH remains at zero. This corresponds to the appearance of an image from the &#34;all&#34; black state in the manual process. 
     For the first scan, UTH --  1 is set equal to UTH(0)=0  plus UTH(255)=255 divided by two, that is: 
     
         UTH.sub.-- 1=LTH=(0+255)÷2≈127. 
    
     The calibration sheet is scanned and Scan 1 results in a black dot count (&#34;U --  B --  D --  C --  1&#34;) of 26507. In a preferred embodiment of the present invention, the UTH --  1 is immediately set equal to the results of the scan used to determine LTH --  1, which values had been stored in the host computer&#39;s memory during the earlier scan. Since the difference between UTDC and U --  B --  D --  C --  1 is greater than the 200 dot tolerance the procedure is repeated, but only after the UTH setting is adjusted again by the bisection method. UTH --  2 is set equal to UTH(255) plus UTH --  1 divided by 2, that is: 
     
         UTH.sub.-- 2=LTH=(255+127)÷2≈191. 
    
     A calibration sheet is scanned and Scan 2 results in U --  B --  D --  C --  2=100,847. Again as 100,847 is not within the 200 dot tolerance of the UTDC the process continues. The UTH is adjusted with UTH --  3 set equal to UTH(255) plus UTH --  2 divided by 2, that is: 
     
         UTH.sub.-- 3=LTH=(255+191)÷2≈223. 
    
     A calibration sheet is scanned and Scan 3 results in U --  B --  D --  C --  3=508,316. Since 508,316 is within the 200 dot tolerance of UTDC, a final UTH setting has been found and UTH --  FINAL is set equal to 223. 
     The PTH is determined by setting the scanner LTH=LTH --  FINAL, UTH=UTH --  FINAL and PTH=0. Additionally, the final PTH value is not determined until two successive scans results in both a dot count within the 200 dot tolerance of PTDC as well as a threshold value falling within an acceptable difference from the previous threshold value. Ten has been used successfully as an acceptable threshold value difference. Other than the preceding, PTH is determined in the manner described above for LTH and UTH. A bisection method is used to find a dot count that matches a count of PTDC. At each step of the bisection process the PTH values are set to the average of the bounding values for PTH. This corresponds to the appearance of the &#34;best&#34; image in the manual process. 
     For the first scan, PTH --  1 is set equal to PTH(0)=0 plus PTH(255)=255 divided by two, that is: 
     
         PTH.sub.-- 1=(0+255)÷2≈127. 
    
     The calibration sheet is scanned and Scan 1 results in a black dot count (&#34;P --  B --  D --  C --  1&#34;) of 43,293. Since the difference between PTDC and P --  B --  D --  C --  1 is greater than the tolerance of 200 dots the procedure is repeated, but only after the PTH setting is adjusted again by the bisection method. PTH --  2 is set equal to PTH(0) plus PTH --  1 divided by 2, that is: 
     
         PTH.sub.-- 2=(0+127)÷2≈63. 
    
     A calibration sheet is scanned and Scan 2 results in P --  B --  D --  C --  2=37,510. Again as 37,510 is not within the 200 dot tolerance of the PTDC the process continues. The PTH is adjusted with PTH --  3 set equal to PTH(0) plus PTH --  2 divided by 2, that is: 
     
         PTH.sub.-- 3=(0+63)÷2≈31. 
    
     A calibration sheet is scanned and Scan 3 results in P --  B --  D --  C --  3=35,195. Since 35,195 is within the 200 dot tolerance of PTDC, a final PTH setting has been found and PTH --  FINAL is set equal to 31. As noted above, by retaining values from earlier scans, for example, in determining LTH --  FINAL, the procedure for determining UTH --  FINAL starts with the knowledge of such retained values. In practice, after setting the threshold values using this method, applicants have found the 50% scanner level to be the only one necessary for accurate scanning. 
     In yet another preferred embodiment of the present invention, generally only one calibration scan is required for calibrating a camera. This embodiment uses the signal generated by the analog-to-digital convertors (&#34;ADCs&#34;). The ADCs convert the analog video signal from the CCDs to an 8-bit-per-pixel 256 grey scale digitized video signal. In normal operation the output of the ADCs will be sent to the image processor where the threshold settings determine the levels at which the processed grey scale image is converted to a binary, black and white scale. In this embodiment, the results of the ADCs&#39; output for a complete scan are retained in memory, initial threshold values are utilized in processing the image from the ADCs&#39; output, the dot count is determined and the threshold value adjusted according to the present invention. The adjusted threshold values are then utilized in processing the retained ADCs&#39; output, the dot count is determined and the threshold value adjusted accordingly. A minor difference between this gray scale method and the earlier method is that the black and white concept of the gray scale pixel is the opposite of the black or white pixel. Therefore, the dot counts obtained by the gray scale image is in the reverse order of the black or white threshold dot counts. That is, where the maximum dot count is obtained when LTH=UTH=255 in the earlier method, with a gray scale image the maximum dot count is obtained when LTH=UTH=0. Hence, it can be seen that with just one scan of the calibration sheet, by retaining and using the original grey scale image the scanner camera can be calibrated. 
     The source code listing example, which follows, is an example of a preferred embodiment of the present invention, where by using the gray scale data only one calibration scan is required for calibrating a camera. In this embodiment, the host computer processes the stored results of the ADCs&#39; scan by executing the compiled binary of the source code listing set forth below. The listing is written in C, has been run under Sun Unix 4.2 Release 4.1 on a Sun computer and has been successfully used in determining final LTH, UTH and PTH threshold values for a Photomatrix Document Scanner. This listing enables the host computer to determine the threshold settings for three levels of sensitivity, 10%, 50% and 80% for each camera. It also determines, by interpolation, the threshold settings for the other levels of sensitivity. ##SPC1## 
     Although several embodiments of the present invention have been described, it is desired to emphasize that this has been for the purpose of illustrating and describing the invention, and should not be considered as necessarily limitative of the invention, it being understood that many modifications can be made by those skilled in the art while still practicing the invention claimed herein.