Abstract:
The disclosed system and method provide a binary image generation method. Used, for example, in a digital reprographic system, the method is suitable for eliminating printing artifacts for high addressable binary digital output, such as digital copies, using an optimized ordered error diffusion method with dynamically computable quantization error and dynamic binary output calculation.

Description:
The disclosed system and method provide a binary image generation technique, and more particularly, a system and method suitable for eliminating printing artifacts for high addressable binary digital output such as digital copies using an optimized ordered error diffusion method with dynamically computable quantization error and dynamic binary output calculation. 
   BACKGROUND AND SUMMARY 
   When standard error diffusion is applied to scanned documents in the copy image path, many isolated dots are created in the resultant image. Some print engines are not capable of printing/reproducing these isolated pixels accurately and systematically, which leads to unwanted image quality artifacts in the copies, such as uneven and patchy image areas. The image quality of text and line art is also impacted, even in the case of high addressable printing. 
   Rank order error diffusion (ROED), such as disclosed by (i) R. Loce et al., in U.S. application Ser. No. 09/968,651, filed Oct. 1, 2001, (U.S. Publication 20030090729) for, “RANK-ORDER ERROR DIFFUSION IMAGE PROCESSING”; (ii) R. Loce et al., in U.S. application Ser. No. 11/034,057, filed Jan. 13, 2005, for “SYSTEMS AND METHODS FOR CONTROLLING A TONE REPRODUCTION CURVE USING ERROR DIFFUSION”; (iii) B. Xu et al., U.S. application Ser. No. 11/013,787, filed Dec. 17, 2004 for “SYSTEMS AND METHODS FOR RANK-ORDER ERROR DIFFUSION IMAGE PROCESSING”; and (iv) Y. Zhang et al., U.S. application Ser. No. 10/923,116, filed Aug. 20, 2004, for “UNIFORMITY COMPENSATION IN HALFTONED IMAGES”, all of which are hereby incorporated in their entirety for their teachings, has numerous applications and is an excellent starting step for eliminating artifacts in non high addressable output. However, some artifacts are likely to remain. In the high addressable binary image context, rank order error diffusion combined with compact high addressable pixel creation, as described for example in (i) U.S. application Ser. No. 11/034,057; (ii) U.S. application Ser. No. 11/013,787; and (iii) U.S. Pat. No. 6,683,702 to R. Loce et al., for “Compact-Dot Reproduction of Scanned Halftone Screens,” also incorporated by reference in its entirety, generates unwanted patterning artifacts. 
   Implementation of rank order error diffusion as described in (i) U.S. application Ser. No. 09/968,651; or (ii) U.S. application Ser. No. 11/104,758, by R. Loce et al., filed Apr. 13, 2005, for “BLENDED ERROR DIFFUSION AND ADAPTIVE QUANTIZATION”, also hereby incorporated by reference in its entirety, may be prohibitive in high-speed image paths. Rank order error diffusion is also disclosed in: U.S. application Ser. No. 11/218,966, by R. Loce et al., filed Sep. 2, 2005 for “COLOR MANAGEMENT OF HALFTONED IMAGES”, which is also hereby incorporated by reference in its entirety. 
   Accordingly, a modified and less computationally intensive ordered error diffusion method, a different technique of generating the high addressable binary output, and a different technique of calculating the quantization error based on local context, are needed for the copy path to meet increased throughput speed and image quality requirements. The disclosed system and method provide a binary image generation method using ordered error diffusion with optimized and improved image processing speed and image quality for eliminating printing artifacts for digital output such as copies. 
   Disclosed in embodiments herein is a method for processing an input image, comprising: creating binary output by thresholding a current grayscale input image pixel and an associated intermediate predicted pixel value; calculating a pixel quantization error based upon the current image pixel value and the intermediate predicted pixel value; ordering a plurality of neighboring pixels of the current pixel located in a predefined processing window according to their gray values; diffusing a maximum possible amount of quantization error for the current image pixel sequentially to a subset of the plurality of neighboring pixels of the current pixel, where the subset of the plurality of neighbors are selected in accordance with the sign of quantization error; and repeating the steps above for each input pixel in the image to produce an error-diffused output image. 
   Also disclosed in embodiments herein is a method creating a halftoned image, comprising: receiving a pixel value from an input image; selecting an processing window encompassing M neighboring pixels; selecting a threshold; thresholding the pixel value with the selected threshold to make a binary marking decision; calculating a quantization error value based on the pixel value and the binary marking decision; and diffusing quantization error to at least one selected neighboring pixel(s) N within the diffusion mask M, based on the calculated error value, the selection being based on pixel values of a plurality of neighboring pixels within the diffusion window. 
   Further disclosed in embodiments herein is a digital reprographic system, comprising: an image source; an image processor, for processing each input pixel of an input image from said image source to produce an error-diffused output image, said image processor including a thresholder for creating binary output and an associated intermediate predicted pixel value as a function of a current image pixel; a pixel error calculator for calculating pixel quantization error based upon the current image pixel value and the intermediate predicted pixel value; a pixel ordering circuit for ordering the current input image pixel&#39;s M neighboring pixels located in a predefined processing window according to their gray values, said ordering circuit further diffusing the quantization error for the current image pixel sequentially to N out of the M neighbors of the current pixel, where N is not greater than M; where the N neighbors are selected depending on the sign of quantization error, and an image output terminal for outputting the error-diffused output image. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a flow diagram illustrating aspects of an embodiment of the method disclosed herein; 
       FIG. 2  is a block diagram of a portion of an image processing system operative to perform the method of  FIG. 1 ; 
       FIG. 3  is an illustration of an exemplary error diffusion window to which the method of  FIG. 1  may be employed; 
       FIGS. 4 and 5  are illustrative representations of the dynamically computable quantization error and dynamic binary output calculation; and 
       FIG. 6  is a flow diagram illustrating an alternative process employed with the method of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   The following disclosure sets forth embodiments for a system and method. However, it will be appreciated that the disclosed features and functions, and alternatives thereof, may be combined into alternative systems or applications. For example, in  FIG. 1 , there is depicted a flowchart that depicts the primary operations of the disclosed method. Similarly,  FIG. 2  depicts a schematic block diagram of a digital reprographic or multifunction system wherein certain functional components are identified as providing an embodiment suitable for operation of the system as described relative to  FIG. 1 . 
   Referring briefly to  FIG. 3 , there is depicted an example of a processing window for use with an error diffusion operation in accordance with an aspect of the disclosure that follows. In particular, window  310 , includes a plurality of pixels (a, b . . . j) located at defined positions ( 0 ,  1 , . . . ,  9 ) adjoining a current pixel “p” ( 320 ). As used herein, the pixels in the window are temporarily stored in an array of size M (e.g., edarray[ 10 ] where edarray[i] corresponds to position i in window  310 ). 
   Referring to  FIG. 1 , there is depicted a method  110  for processing a continuous tone or multi-quantization level input image, high addressable to produce possibly high addressable binary output image representative thereof and without print engine artifacts. Method  110  comprises creating a binary output by thresholding a current grayscale input image pixel and an associated intermediate predicted pixel value. It will be appreciated that the disclosed method finds particular use in high addressable images, and one embodiment hereof contemplates such a possibility as will be further described relative to  FIGS. 4 and 5 . Respectively,  FIGS. 4 and 5  illustrate the manner in which thresholding may be performed for a 2× and a 1× (non) high addressable case. In the 2× case of  FIG. 4 , depending upon the current pixel value (p) and the intermediate predicted value (predval), the possibly binary high addressable output values are 00, 01, 10, or 11, and the pixel error (pixerr) to be propagated is calculated as a function of the current pixel value (p) and the intermediate predicted value predval (e.g., for p=215 and predval=200, pixerr=−40). In other words, a binary output is created by thresholding a current grayscale input image pixel and a intermediate predicted pixel value, producing a high addressable binary representation of the grayscale input image pixel, where the quantization error for the current pixel is a function of the high addressable binary representation. 
   In one embodiment, predval is determined as a function of the current pixel (p) and the next adjacent pixel (a) as depicted in  FIG. 3 , where
 
predval= wa·a+wp·p,  
 
and where wa and wp are respective weighting variables. The thresholding operation depicted in  FIG. 5  is similar, but is directed to a 1× (non high addressable) case, and hence the outputs are simply a single binary value.
 
   Returning to  FIG. 1 , operation  120  of the method  110  further comprises calculating a pixel error based upon the current image pixel value and the intermediate predicted pixel value. As noted above, the function used to calculate quantization error (pixerr) is a function of the current pixel (p) and the intermediate predicted value (predval), as represented in the examples of  FIGS. 4 and 5 . It will be appreciated that the disclosed method of calculating quantization error is different that that suggested by the Loce et al. as noted above. After thresholding, the quantization error for the current image pixel is diffused sequentially to N out of the M neighbors of the current pixel, where N is not greater than M; where the N neighbors are the N biggest or N smallest pixel values in the window of M neighboring pixels depending if the quantization error is positive or negative, respectively, The N neighbors are picked from the edarray[M] in the order corresponding to the sign of the quantization error (increasing if the sign of the quantization error is negative, decreasing if the sign is positive). A saddr[M] memory array or buffer is used as an address buffer, wherein the addresses point to the edarray[i] values in the respective (increasing or decreasing) order. More specifically, the process may be generally characterized by the following code section: 
   
     
       
             
           
             
             
           
             
           
             
             
           
         
             
                 
             
           
           
             
               if (pixerr &gt;= 0) 
             
           
        
         
             
                 
               for all i=0,..,N 
             
             
                 
               { 
             
             
                 
               deltaer = pixerr 
             
             
                 
               if (value at saddr[i] + deltaer&gt; 255) deltaer = 255 − value at saddr[i] 
             
             
                 
               pixerr = pixerr − deltaer 
             
             
                 
               value at saddr[i] = value at saddr[i] + deltaer 
             
             
                 
               } 
             
           
        
         
             
               else 
             
           
        
         
             
                 
               for all i=0,..,N 
             
             
                 
               { 
             
             
                 
               deltaer = pixerr 
             
             
                 
               if (value at saddr[i] + deltaer&lt;0) deltaer = − value at saddr[i] 
             
             
                 
               pixerr = pixerr − deltaer 
             
             
                 
               value at saddr[i] = value at saddr[i] + deltaer 
             
             
                 
               } 
             
             
                 
                 
             
           
        
       
     
   
   Continuing with  FIG. 1 , the ordering of the current input image pixel&#39;s M neighboring pixels in the processing window is accomplished as depicted at  130 —according to the gray values of the neighboring pixels. As indicated in before, a saddr[ 10 ] memory array or buffer is used as an address buffer, wherein the addresses point to the edarray[i] values in the respective (increasing or decreasing) order. Therefore, as the image is scanned from left to right and top to bottom, the ordering can be implemented in parallel in hardware and/or (as the processing window “slides”) only the new pixels have to be ordered with respect to the overlapping old ones. 
   The diffusion operation  140  intentionally diffuses a maximum possible amount of quantization error for the current image pixel, sequentially, to the N neighbors of the current pixel. The diffusion process is applied in this instance only to the first N (e.g., N=4) ordered neighbors in the processing window (out of M neighbors). In one embodiment the following values were used: M=10 and N=4. As will be appreciated, other combinations may be employed depending on the imaging characteristics of the system and the hardware. 
   As the pixels are ordered it is possible to pre-calculate (perhaps using a lookup table) how much error remains to saturate each pixel in the processing window (of size 10 in this embodiment) and at what point the pixel error will be consumed. This index and partial sums may be used in instantly performing the ordered error diffusion operation  140 , by knowing that all the pixels in the processing window with the calculated sum (see code below) less or equal to the pixel error (pixerr) become 0 or 255 (in an 8-bit per pixel imaging system): 
   
     
       
             
             
           
             
             
           
             
             
           
             
             
           
             
             
           
             
             
           
             
             
           
             
             
           
             
             
           
             
             
           
             
             
           
             
             
           
             
             
           
         
             
                 
                 
             
           
           
             
                 
               satsum = 0; 
             
             
                 
               index=0; 
             
             
                 
               if (pixerr &gt;= 0) 
             
             
                 
               { 
             
           
        
         
             
                 
               for (i=0; i&lt;9; i++) 
             
             
                 
               { 
             
           
        
         
             
                 
               satsum=satsum + 255 − edarray[i]; 
             
             
                 
               if( satsum &gt; pixerr ) 
             
             
                 
               { 
             
           
        
         
             
                 
               index = i; 
             
             
                 
               break; 
             
           
        
         
             
                 
               } 
             
           
        
         
             
                 
               } 
             
           
        
         
             
                 
               else 
             
             
                 
               { 
             
             
                 
               for (i=0; i&lt;9; i++) 
             
           
        
         
             
                 
               { 
             
           
        
         
             
                 
               satsum=satsum−edarray[i]; 
             
             
                 
               if( satsum &gt; pixerr ) 
             
             
                 
               { 
             
           
        
         
             
                 
               index = i; 
             
             
                 
               break; 
             
           
        
         
             
                 
               } 
             
           
        
         
             
                 
               } 
             
           
        
         
             
                 
               } 
             
             
                 
                 
             
           
        
       
     
   
   Furthermore, as represented by operation  150  of  FIG. 1 , the process is repeated using the steps above until the input pixels in the image have been processed to produce an error-diffused output image. The processing is likely done in raster order, but other processing orders (e.g., serpentine) are feasible. 
   Referring to  FIG. 2 , a portion of an image processing system  204  operative to perform the method disclosed above, including performing ordered error diffusion, includes an image source such as a digital or digitizing scanner  208  to produce an image  210 , which may be stored in memory  212  (e.g., magnetic media, RAM, etc.). The image  210  is passed to a pixel and neighborhood selector  214 , a pixel value sorter  218 , a thresholder  222 , a marker  226 , an ordered error diffuser  230 , and an output device such as an image output terminal  234  (e.g., a laser printer, photoprinter, etc.). 
   The pixel and neighborhood selector  214  selects a processing path or space-filling curve for processing the pixels of an image  210 . Additionally, the pixel and neighborhood selector  214  selects or generates a diffusion mask or set of diffusion masks that is compatible with a selected processing path. The processing path selection can be based on input received from a system operator or based on an analysis of the image and known or default rendering preferences, as described for example in U.S. Patent Application 2003/0090729 by Loce et al., published May 15, 2003, for a “Rank-Order Error Diffusion Image Processing.” 
   Selection of a preferred rendering characteristic is interpreted by the pixel and neighborhood selector  214  as a request for the use of a related processing path. Selection of a diffusion mask or set of masks may be based on a default, selected space-filling curve or may be based on operator input. The operator input may be an explicit mask selection, or may again be implied through the selection of a preferred rendering characteristic. Alternatively, the pixel and neighborhood selector  214  may analyze the image  210  and automatically select a space filling curve and diffusion mask(s) based on characteristics of the image. 
   Once the processing path and diffusion mask(s) have been selected, the pixel and neighborhood selector  214  follows the processing path working sequentially through the input image and selecting pixels of interest and neighboring pixels from the image and delivers information about them to the pixel value sorter  218  and the thresholder  222 . The pixel value sorter  218  receives information about the current pixel and the neighboring pixels. The thresholder  222  simply receives information about the current pixel and its right neighbor. 
   For example, the pixel value sorter  218  simply receives pixel value information. Optionally, the pixel ranker  218  also receives pixel position information. The value sorter  218  orders the neighboring pixels based on their values. Optionally, the pixel value sorter  218  applies spatial weights to the pixel values of the neighboring pixels before sorting the neighboring pixels. Appropriate spatial weights may be either predetermined default values, indicated by an operator selected rendering preference, or are based on an analysis of the image performed by the pixel value sorter  218 . Alternatively, the spatial weights include consideration of lightness or darkness variance of the neighboring pixels (i.e., processing generally uniform light or dark regions of the image preferentially. The pixel sorter  218  passes pixel information including, for example, pixel values, pixel ranks and pixel position information to the error diffuser  230 . 
   The thresholder  222  receives pixel value and pixel location information about a current pixel. The thresholder  222  compares the pixel value of the current pixel to a threshold value. The threshold value may be a constant value (e.g., binth=127), used to compare to all the pixel values of the image or the threshold value may vary. For example, the threshold value may be taken from a halftone screen. The thresholder, however, operates in a manner consistent with  FIGS. 4 and 5 , where the output is a function of not only the threshold applied relative to the pixel value (p), but also as a function of the level of the intermediate predicted pixel value (predval) as described above. The thresholder then passes the results of the comparison to the marker  226 . For example, the thresholder  222  tells the marker  226  whether or not the pixel value of the current pixel is above or below the threshold value and whether or not the intermediate predicted pixel value (predval) is above or below the threshold value (output=0 or 1 in the 1× case as described in  FIG. 4 ; output=00, 01, 10 or 11 in the 2× case as described in  FIG. 5 ), thereby permitting marker to record a binary indication of a mark. The thresholder also passes the pixel value of the current pixel on to the error diffuser  230 . The threshold value for the current pixel and the threshold value for the predicted pixel value do not need to be the same. 
   The marker  226  makes a marking decision based on the results received from the thresholder. For example, if the pixel value is above the threshold value the marker decides to place a mark in a position in an output image corresponding to the current pixel. The output image may be an electronic image stored in an electronic memory or buffer. Alternatively, the output image may be an image formed by an outputting device such as an image output terminal  234 . The marker  226  also passes a value related to the marking decision to the error diffuser  230 . For example, the marker passes to the error diffuser  230  a value corresponding to a fully saturated mark, such as, for example, 255 (in an eight-bit system) or a value corresponding to a fully unsaturated mark (white or no mark), such as, for example zero. 
   The error diffuser  230  compares the value received from the marker to the pixel value of the current pixel and calculates an error value. The error diffuser then transfers or distributes error to one or more neighboring pixels N (from within the diffusion window  310 ). The error transfer or distribution is based on the sorting performed by the pixel value sorter  218 . Therefore, the error transfer or distribution is based on lightness or darkness of the neighboring pixels, as indicated by one or more pixel values of the pixels. Optionally, the sorting, and therefore the error transfer or distribution, is additionally based on the spatial position of the neighboring pixels. For example, a distance of neighboring pixels from a current pixel may be taken into account during the ranking. Alternatively or additionally, the positions of the neighboring pixels relative to an associated halftone dot are accounted for during the sorting. As explained above, all the error may be transferred to a plurality of pixels N, where N≦M. Error can also be transferred or distributed in a weighted or non-weighted manner. Whether weighted or not, error can further be transferred or distributed in a clipped or limited manner or in an unlimited manner. 
   By transferring or distributing error, the error diffuser  230  modifies or updates pixel values of some of the neighboring pixels from the image  210 . The pixel and neighborhood selector  214  then selects a new current pixel and a new related set of neighboring pixels based on the previously selected window. The functional blocks  214 ,  218 ,  222 ,  226 ,  230  then repeat their functions for the new current pixel. 
   While the image is being processed, or when the entire image has been processed, the marking decisions made by the marker  226  can be delivered to the outputting device  234 , or temporarily stored in a memory such as memory device  212 . As noted previously, the outputting device can be a binary outputting device or a multi-quantization level outputting device. In a xerographic environment the outputting device  234  is a xerographic printer. Xerographic printers are known to include a fuser, a developer, and an imaging member for receiving an exposure source (e.g., light) in response to the marks for the image. In other environments the outputting device  234  may be a display or other printing device such as an ink jet, lithographic or ionographic printer. 
   The above-described portion of an image processing system  204 , operative to perform the method of  FIG. 2  can be implemented in many ways. For example, the pixel and neighboring pixel selector  214 , pixel value sorter  218 , thresholder  222 , marker  226 , and error diffuser  230  may be implemented in software or in dedicated hardware components operating under software control. The software is stored in a computer memory and run on one or more computational devices such as, for example, a microprocessor(s) or digital signal processor(s). The functional blocks  214 ,  218 ,  222 ,  226 ,  230  can be physically located in, or performed by, a single device or can be distributed over an interconnected computer network (not shown). 
   Having described the general steps in accordance with the method of  FIG. 1  and the system depicted in  FIG. 1 , attention is now turned to further alternatives to the disclosed embodiments. More specifically, the method described above may further comprise resetting the quantization error to zero upon detecting a region of the input image having pixel values consistently near one extreme (all light or all dark) of the gray scale. The error is reset each time the pixels in the window are all light or all dark and the error is in a certain range to avoid generating dots/holes in very light/dark regions as controlled by the following code section: 
   
     
       
             
             
           
             
             
           
         
             
                 
                 
             
           
           
             
                 
               ( if (edarray[i]&lt;=lowth for all i=0,..,9) OR if (edarray[i]&gt;=highth 
             
             
                 
               for all i=0,..,9) ) 
             
           
        
         
             
                 
               AND 
             
             
                 
               if errth1 &lt;= pixerr &lt;=errth2 
             
             
                 
               then 
             
             
                 
               pixerr=0 
             
             
                 
                 
             
           
        
       
     
   
   As will be appreciated, the lower threshold (lowth) and higher threshold (highth) used to determine whether a region of pixels lies near an extreme of the continuous tone range, may be predefined. (i.e., lowth defining the range of light areas whereas highth defines the range of dark areas). In one embodiment, the lowth variable was set equal to 5, and the highth threshold was set equal to 250. The additional thresholds, errth 1  and errth 2 , control the magnitude of error that may be reset. In one embodiment, the errth 1  variable was set equal to −127, and the errth 2  threshold was set equal to 5. If errth 1 =−127 and errth 2 =127 then any amount of quantization error may be reset. 
   Further contemplated in accordance with an aspect of the disclosed method is the use of statistical features simultaneously, when ordering the pixels in the processing window (max, min, mean, etc.), so that if the window is in a continuous tone area then the process switches to standard error diffusion. The advantage of such an approach is that typically standard error diffusion performs better on contone regions of scanned images. 
   Alternatively, if segmentation tags are already available in the image processing pipeline (e.g., text &amp; line, halftone (any frequency), and contone/pictorial/photo) a choice may be made between the method described herein and standard error diffusion. An illustration of processing in accordance with this alternative embodiment is depicted in  FIG. 6 . Referring to  FIG. 6 , there is illustrated a segmentation pre-processing where the input image is first segmented ( 620 ) and processed in accordance with other image processing operations (e.g., enhancement, filtering, etc.)  630 , to produce an output image. If the image, or a region thereof, is a continuous tone (contone) region, as detected at  640 , then standard error diffusion is applied as represented by  650 . Otherwise, in the event that a region is not identified as a contone segment, then processing continues in accordance with one of the embodiments described above. Although the flowchart of  FIG. 6  indicates a dynamic or real-time segmentation and tagging operation, it is further contemplated that the pipeline may temporarily store segmentation information relative to an image and that the method, therefore, includes retrieving stored knowledge relating to the input image (such as segmentation tags); and based upon such knowledge, processing portions of the image using only standard error diffusion. 
   It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.