Abstract:
The disclosed systems and methods pertain to the processing of large format images in a manner to avoid introducing imaging defects, and more particularly to dividing the large image into bands, processing these bands in parallel and then putting them back together in to create the final processed image. The methods include the use of overlapping the image bands, padding of the bands prior to processing and the redefinition of image values at common boundaries of the bands to minimize or eliminate visible defects.

Description:
The disclosed systems and methods pertain to the processing of large format images in a manner to avoid introducing imaging defects, and more particularly to dividing the large image into bands, processing these bands in parallel and then putting them back together in to create the final processed image. 
   BACKGROUND AND SUMMARY 
   Digital image processing of large format images is often done in parallel for cost efficiency, speed, and reuse of systems (hardware as well as software and image processing algorithms) utilized for processing standard size documents. For instance, wide format scanners need to handle input documents that are up to 36 inches wide or more. In order to make use of existing image processing systems that are designed for regular-size documents (smaller sizes usually up to a maximum of 12 inches wide), or to perform parallel processing to achieve performance goals, it is desirable to divide the input documents into several segments, perform image processing on the segments separately and then stitch the results together at the end to produce a resulting image or document. 
   However, simply stitching several processed image segments together results in a visible defect at the common boundary(s) and “depletion” artifacts. These artifacts are especially visible when binary (already rendered) parts of the document are put together directly and/or when scanning and printing resolutions differ. A classic example is stitching of two error-diffused images together, where there arises an obvious artifact at the common boundary, as depicted in  FIG. 12 . There are at least two problems with the image—the artifact at the common image boundary and the “depletion” artifact at the top and bottom of the boundary region, and image boundaries. Artifacts are also visible when anamorphic printing resolutions are used to print the stitched document, such as the case of solid ink printing. 
   Alternatively, if rendering were performed after stitching the individual segments together, then the rendering module would be expensive because of the memory requirements for processing the larger image portions. Hence, the disclosed systems and methods are directed to various alternative embodiments to reduce the visibility of defects when processing large format images, without increasing the cost of the image-processing pipeline or hardware. 
   A straightforward method to avoid boundary artifacts is to keep an error buffer with error values at the right edge of the left image and use these error values for the left side of the right image as taught in U.S. Pat. No. 6,282,325 for “Image Processing Technique for Binarizing an Image Scanned by a Shuttle Type Scanner,” issued Aug. 28, 2001 to Ji-Hoon Han, and U.S. Pat. No. 4,958,236 for an “Image Processing Method and Apparatus Therefor,” by Nao Nagashima, et al., issued Sep. 18, 1990. Such a method could be fairly costly, however, and requires specialized hardware and complicated image manipulation. 
   Another method to avoid or reduce artifacts is to modulate the error diffusion threshold at the boundary as disclosed by Zhen He et al., in “Boundary Stitching Algorithm for Parallel Implementation of Error Diffusion”, pp. 344-355, in Color Imaging IX: Processing, Hardcopy, and Applications, (Reiner Eschbach, Gabriel G. Marcu editors), held in San Jose, Calif., Volume 5293, No. 5293, published by SPIE and IS&amp;T in 2004. This method requires a specialized threshold modulation circuit. 
   Generally, the depletion artifact causes corners to be rounded and dots to be “aligned”. This defect type may be avoided by using a different threshold in highlight regions as taught in U.S. Pat. No. 6,285,464 for an “Apparatus and Method for Producing a Half-tone Image,” by Akihiro Katayama, et al., issued Sep. 4, 2001. U.S. Pat. No. 4,958,236 proposes parallel processing by dividing the image into equally overlapping or overlapping to the left/right only segments, and also passing the error to the next band. This method requires scanline buffers storing video data. 
   The systems and methods disclosed herein tackle all of the noted problems, and further improve the resultant images by manipulation of the large image&#39;s parts (segments). The disclosed systems and method further avoid the difficult processing associated with diffusing error to an adjacent band. In a general sense, the disclosed embodiments include dividing the large image into bands in particular ways, processing these bands in parallel and then putting them back together in an optimal way to create the final processed image. The methods described herein include a combination of some, or all, of the following operations:
         1. Overlapping the image bands;   2. Padding the bands with different values and images;   3. Redefining the binary values at the common boundary of the bands.       

   Disclosed in embodiments herein is a method for processing an image, comprising: dividing the image into bands; adding to each band additional image data not found in the band; processing the bands to produce processed bands; and recombining the processed bands to produce a processed image. 
   Also disclosed in embodiments herein is a method for processing a large-format image, comprising: dividing the large format image into N bands; adding to each of said N bands, along at least an edge additional image data not found in said band; processing each of said N bands to produce N processed bands, wherein said N processed bands each include processed image values derived from the additional image data; and recombining the processed bands while eliminating overlapping regions thereof to produce a processed image. 
   Further disclosed in embodiments herein is a system for processing an image, comprising: an image splitter for dividing the image into bands, each of said bands assigned to a dedicated processing channel; within the channel adding to each band, additional image data not originally found in the band; an image processor for processing all the image data in each band to produce processed bands; and an image stitcher for recombining the processed bands to produce a processed image. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an illustrative example of a system suitable for executing the methods disclosed in various embodiments herein; 
       FIG. 2  is a simplified view of an input image; 
       FIG. 3  is an exemplary view of a method in which an image is processed in accordance with an embodiment disclosed herein; 
       FIG. 4  is an illustrative example of the results of processing an image in accordance with the method illustrated in  FIG. 3 ; 
       FIGS. 5-7  are exemplary views of alternative embodiments of the processing methods disclosed herein; 
       FIG. 8  is an illustrative example of the results of processing an image in accordance with the method illustrated in  FIG. 7 ; 
       FIGS. 9-10  are exemplary views of alternative embodiment of the processing methods disclosed herein; 
       FIG. 11  is an illustrative example of the results of processing an image in accordance with the method illustrated in  FIGS. 9-10 ; and 
       FIG. 12  is an illustrative example of an image processed in accordance with prior art techniques. 
   

   DETAILED DESCRIPTION 
   The methods described herein include a combination of one or more of the following operations designed to eliminate artifacts and defects in a processed image: 
   1. Overlapping the image bands; 
   2. Padding the bands with different values and images; 
   3. Redefining the binary values at the common boundary of the bands. 
   Referring to  FIG. 1 , depicted therein is an exemplary image processing path implementing the system and methods disclosed herein. Considering a first embodiment, there is disclosed a specific method for stitching together error diffused images. The image is first segmented or divided in overlapping areas. These areas can be overlapping or not. More specifically, the big image is divided in a number of unequally or equally overlapping segments called bands. Referring to  FIGS. 2 and 3 , just two image bands will be shown for purposes of illustration for all methods, however, the various embodiments will work for any number of bands. Specifically,  FIG. 2  illustrates an original image  210 , that is P-pixels by S-scanlines, and which, for example, may be split in half along the line  220 . Then two bands may be created. The first band may be the first half of the original image (region  310  in  FIG. 3 ) extended into the second half by amount A (region  312  in  FIG. 3 ), and the second band maybe the second half of the original image (region  320  in  FIG. 3 ) extended into the first half by amount B (region  322  in  FIG. 3 ). Amounts A and B may be equal or unequal in accordance with the various embodiments disclosed herein. 
   Although it will be appreciated that the present embodiments may be applied to various systems suitable for carrying out various image processing operations, the following discussion will be directed, in large part, to a discussion of error-diffusion of large-format images. Accordingly, the particular embodiments characterized will include specific examples of error-diffusion processing and the requisite steps to overcome defects that are prevalent in error-diffused images. Such embodiments are not, however, intended to limit the scope to large format images. 
   Referring also to  FIG. 3 , there is depicted a first method to process image  210 . The system and method, as depicted in  FIGS. 1 and 3 , includes:
         1. Padding the top and left of each band with a pattern to avoid “depletion” artifacts;   2. Render (error diffuse) all padded bands; and   3. Recompose the image (e.g.,  FIG. 4 ).       

   More specifically, a wide format scanner  110  is used to generate an image. Scanner  110  operates as an image source, but it will be appreciated that various alternative devices may be used to produce such images, including cameras and other digital technologies. It is also contemplated that the source of images may include a storage device or network upon which an image file is placed prior to processing/rendering. 
   Subsequent to receiving the image, which may be stored temporarily in a memory, the image is split at  120  into a plurality of N overlapping or nonoverlapping bands, each of which may be processed in a dedicated channel within the system. If overlapping, the bands may be equally or unequally overlapping. Subsequent to splitting the image into bands, the bands are padded or otherwise prepared for processing in accordance with one or the embodiments set forth herein. For example, as illustrated in  FIG. 3 , the adjacent bands  310  and  320 , are padded along regions  316  and  326 , respectively (Padded Bands,  1  and  2 , respectively). One skillful in the art of image processing will further appreciate that one aspect of the disclosed systems and methods contemplates that the amount of overlap/padding is different for adjacent bands so as to further avoid the introduction of defects. 
   Returning to  FIG. 1 , once the bands  130 - 138  (bands  1 -N) are divided from the image, then the individual bands are operated upon by the image processing stages ( 140 - 148 ). Here again, it should be appreciated that the image processing may be carried out in dedicated hardware, either in a series process, where each band is passed through the hardware, or as a parallel set of processes operating on equivalent hardware devices (e.g., multiple image processing systems to carry out stages  140 - 148 ). In one embodiment, a system capable of performing the described function would include an image splitter for dividing the image into overlapping or non-overlapping bands, each of said bands assigned to a dedicated processing channel, and within the channel circuitry for adding or injecting into each band, additional image data not originally found in the band. The system further includes a dedicated image processor or image processing circuitry for processing the image data in each band to output processed bands, followed by logic (and registers or similar memory devices), for recombining or stitching the processed bands together to produce a processed image. The present invention contemplates the various operations being carried out with conventional hardware and systems that are currently used for the storage, processing and output of images, including systems such as digital scanners, copiers, and multifunction devices, as made or distributed by Xerox Corporation. Although characterized as image processing circuitry or hardware, the disclosed system is not limited to dedicated image processing hardware or circuitry, and may include serial or parallel image processing operations carried out by conventional computational resources under software control. 
   Subsequent to the image processing operations being completed for each of the several bands N, the processed bands, which include the processed image data, are then recombined to produce a processed image. The recombination is reflected by the stitching operation  150  in  FIG. 1 . A simplified view of the recombination is depicted in  FIG. 3 , where the processed bands  310  and  320  are recombined into image  350 . For example, at the left of the split line  220 , image  350  has the values of processed band  310 . At the right of the split line  220 , image  350  has the values of processed band  320 . The processed image may then be stored and/or rendered by a wide format image output terminal (IOT) or similar rendering device  160 . 
   In one embodiment, acceptable results using the system/method of  FIG. 1  are obtained by using for padding (in regions  316  and  326  of  FIG. 2 ) either a uniform gray value (e.g., close to black), or an on/off pattern of mid-gray values. The padding may be done on the fly in a preprocessing module ( 130 - 138 ) before the band data gets into any of the image processing modules ( 140 - 148 ). 
   Processing an image in the manner characterized in  FIG. 3 , results in an image as depicted in  FIG. 4 . As one will observe by comparison, the depletion found in the prior art of  FIG. 12  processing method disappears. Although the disclosed method does not totally eliminate the artifacts in the midtone regions, it visibly improves the boundary artifacts overall and reduces depletions. 
   In an alternative embodiment, as depicted in  FIG. 5 , the method described above is further modified. More specifically, the process includes:
         1. Padding the top and left of each band, including edges where adjacent bands are present, with a pattern to avoid “depletion” artifacts;   2. Render (error diffuse) all padded bands; and   3. Recompose the image, as represented by image  550  the bottom of  FIG. 5 , but redefine the values in an area around the common boundary line  220  by randomly selecting from one of the corresponding left side band values (region  516  or  526 ) or one of the corresponding right side band values (regions  517  or  527 .
 
This approach produces a resultant image having a level of quality similar to that described with respect to the first embodiment above.
       

   Yet another embodiment is set forth in  FIGS. 6 and 7 , where two variations are set forth. Moreover, both variants of this embodiment may also be combined with padding as set forth above, or padding may be done all around the bands. More specifically, the processes include:
         1. Rendering (e.g., error diffusing) the first few scan lines (e.g., two scan lines) of a first band;   2. Storing, in memory (or delay register/pipeline), at least a portion of the scan lines processed in the previous step, where the stored information may be multi-bit error diffused data;   3. Padding the next band, with selected rendered values of the previous band as stored (as soon as they become available from processing in the prior step);   4. Further padding the top and left of each band with a pattern to avoid “depletion” artifacts (for simplicity not shown in  FIGS. 6 and 7 );   5. Rendering (e.g., error diffusing) all of the image bands to completion; and   6. Recomposing the image as shown in  FIGS. 6 and 7 .       

   More specifically, the alternatives shown in  FIGS. 6 and 7  respectively illustrate processing in accordance with the steps described. In  FIG. 6 , the padded variation includes a first padding region  618  that is comprised of stored data from the prior band and that is further added to the data of Band  2  (comprised of regions  322  and  320 ).  FIG. 7  illustrates the variation where the right side does not overlap to the left (Band  2  is comprised only of region  320 ) and padding includes previously processed data from the prior band, for example, the first few scan lines of data are used to produce stored information relating to region  712 . Then, the processed (multi-bit) data from the first band (region  712 ) is provided as an input (padding region  722 ) for the processing of the second band. In accordance with the variation of  FIG. 7  padding may be done all the way around the first band and padding all sides but the left one for the second band. It will be appreciated that padding size and overlap size may be limited in size, for example, maintained relatively small to accommodate hardware that can only process scanlines of limited length. In one embodiment the pad size was on the order of  32  pixels and overlap size was about  128  pixels. The resulting image obtained in accordance with the embodiment and variation illustrated in  FIG. 7  is illustrated in  FIG. 8 . 
   The following embodiment represents a further improvement over the embodiments set forth above. In what follows, a specific embodiment for stitching together error diffused images is described in detail relative to  FIGS. 9 and 10 . The image is assumed to be divided in equally or unequally overlapping areas as previously set forth in the other embodiments described above. Once again, a large image is divided into a number of unequally, or equally, overlapping segments called bands as previously illustrated in  FIG. 2 . The following method may also be employed with padding as in the aforementioned systems and methods or any other combination of padding. 
   With reference to the figures, the embodiment includes the following processes:
         1. Error diffusing the two overlapping bands (Bands  1  and  2  of  FIG. 9 ); and   2. Selecting an area common to both error diffused bands (for example region  962  in Band land region  972  in Band  2 ) and, as illustrated in more detail in  FIG. 10 , processing regions  962  and  972  to produce the corresponding resultant area (region  992 ) of the resulting image  950 . The image area corresponding to regions  962 ,  972 , and  992  is marked as AREA X in  FIG. 9 .       

   More specifically,  FIG. 10  presents a number of processing alternatives to arrive at area  992  values, including:
         a. Selecting the final value in this common area randomly from one of the bands (areas  962  or  972 ), or   b. conducting a template matching approach, or   c. a combination of a and b, for example, if there is no template match, then randomly choosing the value, or   d. using the left band data until there is a match then use right band data so as to produce a “zig-zag” response that switches at the matches.       

   Specifically,  FIG. 10  illustrates a flowchart for completing the described operations. At S 1010 , S 1012 , the initial variables (matchno, currentPixel) are reset to zero. Next, at S 1020 , a test is conducted to determine if the variable matchno corresponding to the number of consecutive matches found is less than a pre-defined NOMATCHES value (for example, a value of 2). If matchno is not less, then it is reset at S 1024 . The, input of the error diffused image bands in AREA X is received in S 1030  and S 1032 , and test S 1040  is conducted to compare the corresponding pixel values in AREA X of each band. In the event the pixel values are a match (YES), processing continues at S 1044 , where the matchno variable is incremented to indicate a matching pixel and the output is selected from the second band. Subsequently, the currentpixel counter is incremented to assure the process continues a following iteration with a subsequent pixel (S 1046 ). 
   If no match is detected when the corresponding pixels in AREA X of the bands are compared at S 1040 , processing continues at S 1050 . In S 1050 , the values of the pixels are acknowledged as different and either a random value is generated as the output pixel ((a) above), or depending upon the magnitude of the matchno variable, the corresponding value from the AREA X of the first band or AREA X of the second band is output. Subsequently, the matchno variable is reset and processing continues at S 1046  with the incrementing of the curentPixel counter. Next, at S 1060 , the size of the currentPixel counter is checked against a predetermined value that represents the size of the common area (AREA X). If the entire area has not been processed, the method continues at S 1020 . Otherwise, as indicated by S 1070 , processing is complete and the resultant image is generated using the values produced as a result of the steps indicated above, and these values are output for the common area  992  ( FIG. 9 ). 
   In all the methods described herein, it is further contemplated that, as an option, the number of 1&#39;s and 0&#39;s is counted in both adjacent bands, and the ratio of 1&#39;s and 0&#39;s is maintained in the final output. It will be appreciated that in general this method of operation will also maintain a desired level of grayness in the final image. Referring also to  FIG. 11 , there is depicted a final image that is the result of processing in accordance with the method set forth in  FIG. 10 . Furthermore, the template matching of step (b), above, can be accomplished in a number of ways and is not simply limited to the manner in which described. 
   The various embodiments disclosed herein are not restricted to wide format images only. Processing of regular sized images in software can be improved by using the methods described above. It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many 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.