Abstract:
A method and system for a printing device is disclosed. The method and system comprise printing a test pattern on a print medium and generating a digital image of the printed test pattern by an imaging device. The method and system include analyzing an interference pattern to measure for distortion of the print medium and calibrating the printing device based upon the measured distortion. In a preferred embodiment, the present invention utilizes the reticle patterns, which are printed in the margins of the paper, which are measured real-time during printing. The interference or Moiré patterns created by superimposed reticles may be used to measure image distortion, process direction misalignment, and misregistration caused by web distortion. The advantage of this invention is that image distortion compensation, RIP (Raster Image Processor) parameters, timing, or other printer characteristics may be adjusted on-the-fly in a closed feedback system, for high-speed textile or paper color printing, utilizing on-the-fly distortion or stretch measurement for accurate color and/or duplex images registration. In a duplex printer, automatic images alignment front-to-back is obtained by combining optically or logically the two images for the evaluation of interference patterns and amount of distortion in the process and scan direction.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to high-speed printing systems and more particularly to a system and method for controlling distortion in a high-speed printing system.  
       BACKGROUND OF THE INVENTION  
       [0002]     In high-speed inkjet systems with high-tension webs, the substrate may experience significant stretching and distortion as a result of the absorption of the ink while the web is under tension. For example, when the web is paper, the distortion and stretching causes noticeable image distortion errors between the color planes of a multi-component system. With some inkjet systems, the resulting image distortion has caused significant customer satisfaction problems, and (along with other significant factors) has led some customers to reserve the printer for one-component printing. Furthermore, drying of the ink during processing causes the paper to shrink, and subsequent component printing causes the paper to stretch again. Stretching may be different in the “scan” direction (i.e., perpendicular to the direction of travel of the web) than in the “process” direction (i.e., the direction of travel of the web) because of the tension in the web. Since the ink content of the components can differ greatly, the degree of stretching or distortion may differ between printing stations.  
         [0003]     Conventional inkjet systems have had significant problems with web distortion, which have been addressed mechanically with custom unwinders. The custom unwinder is costly, but its primary shortcoming is that it is not part of a closed-loop system. Specifically, the unwinder does not measure local stretching of the web and adjust its work appropriately.  
         [0004]     Furthermore, the unwinder works at only the entry point of the system, so that non-uniform distortion along the process direction cannot be addressed.  
         [0005]     Accordingly, what is needed is a system and method for overcoming the above-identified problems. The present invention addresses such a need.  
       SUMMARY OF THE INVENTION  
       [0006]     A method and system for a printing device is disclosed. The method and system comprise printing a test pattern on a print medium and generating a digital image of the printed test pattern by an imaging device. The method and system include analyzing an interference pattern to measure for distortion of the print medium and calibrating the printing device based upon the measured distortion.  
         [0007]     In a preferred embodiment, the present invention utilizes the reticle patterns, which are printed in the margins of the paper, which are measured real-time during printing. The interference or Moiré patterns created by superimposed reticles may be used to measure image distortion, process direction misalignment, and misregistration caused by web distortion. The advantage of this invention is that image distortion compensation, RIP (Raster Image Processor) parameters, timing, or other printer characteristics may be adjusted on-the-fly in a closed feedback system, for high-speed textile or paper color printing, utilizing on-the-fly distortion or stretch measurement for accurate color and/or duplex images registration. In a duplex printer, automatic image alignment front-to-back is obtained by combining optically or logically the two images for the evaluation of interference patterns and amount of distortion in the process and scan direction. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  illustrates a block diagram of a printing environment in which certain described aspects of the invention are implemented;  
         [0009]      FIG. 2  illustrates a block diagram of software elements, hardware elements, and data structures in which certain described aspects of the invention are implemented;  
         [0010]      FIG. 3  illustrates logic implemented in an application to configure a print system in accordance with certain described implementations of the invention;  
         [0011]      FIG. 4  illustrates logic implemented in an application for color image distortion compensation of a printer in accordance with certain described implementations of the invention; and  
         [0012]      FIG. 5  illustrates logic implemented in an application to indicate how color image distortion compensation of a printer is performed while printing a print job in accordance with certain described implementations of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]     The present invention relates generally to high-speed printing systems and more particularly to a system and method for controlling distortion in a high-speed printing system. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein.  
         [0014]      FIG. 1  illustrates a block diagram of a printing environment in which certain described aspects of the invention are implemented. A printer  100  includes one or more printing stations  102 . The printing stations  102  may include a cyan printing station  102   a , a magenta printing station  102   b , a yellow printing station  102   c , and a black printing station  102   d , capable of printing with cyan, magenta, yellow, and black inks or toners respectively.  
         [0015]     The printer  100  may be any multi-component printer known in the art including an electrostatic printer, an inkjet printer, a laser printer, a plotter, a network printer, a stand-alone printer etc. Alternative implements may use other devices that function in a manner analogous to printers such as digital duplicating machines, photocopiers, fax machines etc. While the current implementation describes a four-component printer, in alternative implementations printer  100  could be a two- or three-component printer.  
         [0016]     Printer  100  could also be a single component printer, if each of at least two single component printers prints one color component. Also, printer  100  could be a single component printer where the reticle-based method is used for ink jet alignment within the print head.  
         [0017]     While  FIG. 1  shows four printing stations  102   a ,  102   b ,  102   c , and  102   d , there may be fewer or more printing stations in alternative implementations. In some implementations, the black printing station  102   d  may be omitted. The printing stations  102   a ,  102   b ,  102   c ,  102   d  may also print with inks or toners different from cyan, magenta, yellow and black. While the printing stations  102   a ,  102   b ,  102   c ,  102   d  are indicated within separate blocks in  FIG. 1  the printing stations  102   a ,  102   b ,  102   c ,  102   d  may be constructed as a single hardware unit or as multiple hardware units. If the printing stations are constructed as a single hardware unit, the single hardware unit may at different times print with a different colored ink or toner.  
         [0018]     Printer  100  may also include a controller  104  coupled to a computational unit  106 . The computational unit  106  may be any computational unit known in the art, including a processor  106   a  and memory  106   b . The computational unit  106  may be inside or outside the printer  100 . The memory  106   b  may include volatile memory  107   a  such as RAM or non-volatile memory  107   b  such as disk storage. The controller  104  may be implemented in several ways including software, hardware or a combination of software and hardware. The controller  104  may lie within or outside the computational unit  106 . In one implementation the controller  104  works cooperatively with the computational unit  106 . In some implementations, software or hardware present with or within the printer  100  may absorb the functions of the controller  104 .  
         [0019]     The controller  104  may be able to calibrate the printing stations  102 , a print media supply  108  and a print media cutter  110 , and other components of the printer  100  not shown in  FIG. 1 . The controller  104  may adjust the timing of the firing of the printing stations  102 , to compensate for distortion in a printed color plane. The controller  104  may also perform pixel shifts as part of rasterization, i.e. the controller  104  may shift a color plane an integral and/or fractional number of pixels in memory before printing the color plane.  
         [0020]     The print media supply  108  may include a collection of any type of print medium  108   a  known in the art on which the printer  100  is capable of printing, including paper, transparencies, fabric, glass, plastic, labels, metal, cardboard, etc. The print medium  108   a  may also be a container made up of a variety of material, including plastic, cardboard, metal etc. In one implementation the print medium  108   a  is a roll of paper. The print medium  108   a  passes through the cyan, magenta, yellow, and black printing stations  102   a ,  102   b ,  102   c ,  102   d . Subsequently, the print media cutter  110  may crop parts of the print medium  108   a.    
         [0021]     A scanning device  112  is coupled to the printing stations  102  and the computational unit  106 . The scanning device  112  may include any scanning device known in the art, including a charge coupled device (CCD) camera, a scanner, or any other imaging device capable of digitizing images printed on the print medium  108   a . The scanning device  112  can image the print medium  108   a  as the print medium  108   a  moves through the printing stations  102 . While  FIG. 1  shows only one scanning device, in alternative implementations multiple scanning devices may be positioned to scan the outputs of the cyan, magenta, yellow, and black printing stations  102   a ,  102   b ,  102   c ,  102   d . In the current implementation, the scanning device  112  scans the print medium  108   a  after the four printing stations  102   a ,  102   b ,  102   c ,  102   d  have printed on the print medium, i.e. a page is scanned after the printer  100  has overlaid all color planes on the page.  
         [0022]     An application  114  coupled to the printer  100  may implement aspects of the invention. While the application  114  has been shown in a separate block outside the printer  100 , part or all of the functions of the application  114  may be integrated into the computational unit  106 , into the controller  104  or into any other unit not illustrated in  FIG. 1  such as a printer driver resident on a computational device outside the printer  100 .  
         [0023]      FIG. 2  illustrates a block diagram of software elements, hardware elements, and data structures in which certain described aspects of the invention are implemented. Referring to  FIGS. 1 and 2  together, a reticle pattern  200  is a predetermined marking pattern that is capable of being printed at an appropriate location on the print medium  108   a  by the printing stations  102 . Further details of reticle patterns are described in the publication “Reticles in Electro-Optical Devices” (copyright  1966  by Lucien M. Biberman), which publication is herein incorporated by reference.  
         [0024]     The scanning device  112  is capable of digitizing the reticle pattern  200  printed on the print medium  108   a  and can produce a digital image of the reticle pattern  202 . When the printer  100  prints the reticle pattern  200  onto the print medium  108   a , if there is color image distortion or reticle image distortion on the printer  100 , the printed reticle pattern  200  may have interference patterns, such as Moiré patterns. The test patterns are patterns of light and dark lines, and the interference patterns appear when two repetitive patterns of lines, circles, or arrays of dots overlap with imperfect alignment. Interference patterns magnify differences between two repetitive patterns. If two patterns are exactly lined up, then no interference pattern appears. The misalignment of two patterns will create an easily visible interference pattern. As the misalignment increases, the lines of the interference pattern appear thinner and closer together. Interference patterns are well known in the art and some applications of interference patterns in imaging are described in the doctoral dissertation “Analysis and reduction of Moiré patterns in scanned halftone pictures” (May 1996, Virginia Polytechnic Institute and State University). In the implementation, interference patterns may arise because the printer  100  prints the same reticle pattern  200  by overlaying ink or toner from at least two of the cyan, magenta, yellow, and black printing stations  102   a ,  102   b ,  102   c , and  102   d  respectively. Interference patterns may appear prominently when reticle patterns have comparable intensity values in the different color planes.  
         [0025]      FIG. 2  also illustrates a digital image analyzer unit  204 , where the digital image analyzer unit  204  is capable of processing the digital image of the reticle pattern  202  and extracting a digital image of interference pattern  206  corresponding to the digital image of the reticle pattern  202 . The digital image analyzer unit  204  may include an edge detector  204   a  that determines edges by applying prior art edge detectors such as the Sobel operator, Canney edge operator or other image gradient-based operators to the digital image of the reticle pattern  202 . The digital image analyzer unit  204  and the edge detector  204   a  may be implemented in hardware or software, or via a combination of hardware and software.  
         [0026]     A distortion error analyzer  208  is capable of processing the digital image of interference pattern  206  and producing distortion adjustment control instructions  210 . Analysis of patterns obtained from reticle patterns is well known in the art and described in the publication “Reticles in Electro-Optical Devices” (copyright  1966  by Lucien M. Biberman). The distortion adjustment control instructions  210  are instructions for adjusting the components of the printer  100 , such as the printing stations  102  and the print media supply  108 , that reduces the distortion.  
         [0027]     The controller  104  may be capable of processing the distortion adjustment control instruction  210 , and may produce printing station adjustment instructions  214  to adjust the printing stations  102 . The newly adjusted printing stations  102  may print the reticle pattern  200  on the print medium  108   a.    
         [0028]      FIG. 3  illustrates logic, implemented in an application  114  of  FIG. 1 , coupled to the printer  100  to configure the printer  100  in accordance with an implementation of the invention. As stated earlier, the application  114  may reside within the printer  100  or may reside in an external computational device outside of the printer  100  and from the external computational device control the printer  100 . Referring to  FIGS. 1, 2 , and  3  together, at block  302 , the application  114  enables an entity (such as an operator, a programmer, a computer program, a predetermined data file etc.) to enter predetermined reticle patterns  200 , where each of the reticle patterns  200  may optionally be associated with one or more printing stations  102 . The application  114  stores (at block  304 ) the reticle patterns  200  in the non-volatile memory  107   b . The application  114  may then enable the entity to enter (at block  306 ) a predetermined periodicity of printing of each reticle pattern  200 . The periodicity of printing of each reticle pattern  200  may depend on how frequently printer  100  has to adjust for distortion. At block  308 , the application  114  stores the periodicity of printing of the reticle patterns  200  in the non-volatile memory  107   b.    
         [0029]     The application  114  may then enable the entity to enter (at block  310 ) the predetermined positions on print medium  108   a  for printing each reticle pattern  200 . Control proceeds to block  312 , where the printer  100  stores the positions in non-volatile memory  107   b . Control proceeds to block  314  where the print system configuration ends.  
         [0030]     In alternative implementations, the entire logic of  FIG. 3  may be preprogrammed such that no entity has to provide any input or predetermine any values. The entire system may come pre-programmed with default reticle patterns, values for periodicity of printing, and positions on print medium for printing each reticle pattern.  
         [0031]      FIG. 4  illustrates logic implemented in the application  114  of  FIG. 1  for minimizing image distortion from the printer  100  in accordance with implementations of the invention, referring to  FIG. 1-4  together. The application  114  starts at block  400 , and the application  114  prints (at block  402 ) a reticle pattern  200  on one part of the print medium  108   a  via the printing stations  102 . The application  114  may print user requested data on the other parts of the print medium  108   a . The scanning device  112  scans the digital image and generates (at block  404 ) a digital image of the reticle pattern  202 . At the conclusion of block  404 , control passes in parallel to blocks  408  and  406 . At block  408 , the printer  100  ejects the page. The reticle pattern may be removed by post-processing equipment such as the print media cutter  110 . The post processing equipment may process a job much later than the original printing. For example, the printed medium may be re-rolled after printing, stored somewhere, and postprocessed later. In alternate implementations, the reticle pattern may also be removed from the print medium  108   a  without using the print media cutter  110 , such as for example by overprinting the reticle pattern with the same color on the print medium, or in any other manner known in the art.  
         [0032]     Parallel to the execution of block  408 , control proceeds to block  406  from block  404 . At block  406 , the digital image analyzer unit  204  processes the digital image of the reticle pattern  202  and isolates a digital image of an interference pattern  206 . Control proceeds to block  410 , where the distortion error analyzer  208  compares the digital image of the interference pattern  206  with the reticle pattern  200 . The distortion error analyzer  208  determines (at block  412 ) if the printer  100  needs to make adjustments to minimize distortion. If no distortion adjustments are needed, control proceeds to block  414  and the process comes to a stop.  
         [0033]     If at block  412 , the distortion error analyzer  208  determines that distortion adjustments are needed, control proceeds to block  416  where the distortion error analyzer  208  generates distortion adjustment control instructions  210 .  
         [0034]     Control proceeds to block  418 , where the application  114  adjusts the printing stations  102 . While the printing stations  102  may be adjusted in several ways, in one implementation the distortion error analyzer  208  sends the distortion adjustment control instructions to the controller  104  and the controller  104  adjusts the printing stations  102  by generating printing station adjustment instructions  214 .  
         [0035]     Control proceeds to block  402 , and a control loop formed by blocks  404 ,  406   b ,  410 ,  412 ,  416 ,  418  may be repeated. Within the control loop the application  114  repeatedly adjusts the printer  100  until no further distortion adjustments are needed. The application  114  may periodically execute the logic of  FIG. 4  depending on how often distortion adjustment is required for the printer  100 .  
         [0036]     The printer does not have to stop printing during distortion adjustments. For example, with reference to  FIG. 4 , while the printing station  102  is being adjusted at block  418 , the reticle patterns  200  may be ejected (at block  408 ) from the printer  100 . Alternatively, the reticle patterns  200  may be printed in area of the media that may not be visible, may be cropped later or may be part of the desired print area. Additionally, printed media may be rejected until distortion is minimized.  
         [0037]      FIG. 5  illustrates logic implemented in an application to indicate how distortion adjustment of a printer is performed while printing a print job in accordance with certain implementations of the invention, referring to  FIGS. 1 and 5  together. At block  500 , the application  114  starts processing a print job. After the application  114  processes (at block  502 ) part of the print job, the application  114  performs (at block  504   a ) distortion adjustment of the printer and optionally concurrently processes (at block  504   b ) part of the print job. Control proceeds to block  506 , at the conclusion of either of blocks  504   a  or  504   b , where the application  114  determines if the print job is complete. If so, the application  114  stops (at block  508 ) the processing of the print job. If at block  506 , the application  114  determines that the print job is incomplete, control passes to block  502 , and the logic of blocks  502 ,  504   a ,  504   b , and  506  are repeated.  
         [0038]     The method, system, and article of manufacture can perform distortion adjustment on a printer on-the-fly. In this way, the printer is adjusted while printing the print job, such that the distortion measured on a printed page is used to adjust the printer when printing subsequent pages of the print job. Additionally, the periodicity of printing of reticle patterns may be adjusted depending on how frequently printing stations need to be adjusted for distortion. By performing periodic adjustments of the printing station while printing, a printer may print very long print jobs continuously without the intervention of a human operator. The interference patterns provide enough details to adjust the printer to minimize distortion.  
         [0000]     Additional Implementation Details  
         [0039]     The described techniques for distortion adjustment may be implemented as a method, apparatus or article of manufacture using standard programming and/or engineering techniques to produce software, firmware, hardware, or any combination thereof. The term “article of manufacture” as used herein refers to code or logic implemented in hardware logic (e.g., an integrated circuit chip, Programmable Gate Array (PGA), Application Specific Integrated Circuit (ASIC), etc.) or a computer readable medium (e.g., magnetic storage medium, such as hard disk drives, floppy disks, tape), optical storage (e.g., CD-ROMs, optical disks, etc.), volatile and non-volatile memory devices (e.g., EEPROMs, ROMs, PROMs, RAMs, DRAMs, SRAMs, firmware, programmable logic, etc.). Code in the computer readable medium is accessed and executed by a processor. The code in which implementations are made may further be accessible through a transmission media or from a file server over a network. In such cases, the article of manufacture in which the code is implemented may comprise a transmission media, such as a network transmission line, wireless transmission media, signals propagating through space, radio waves, infrared signals, etc. Of course, those skilled in the art will recognize that many modifications may be made to this configuration without departing from the scope of the implementations, and that the article of manufacture may comprise any information bearing medium known in the art.  
         [0040]     While the implementations have been described with respect to analysis of interference patterns, such as Moiré patterns, analysis of other patterns similar to interference patterns, or patterns caused via phenomenon or principles similar to interference may also be used. Furthermore, the implementations analyze the interference patterns after all the printing stations have laid down the color planes. In alternative implementations, the scanning device may scan the printed reticle patterns in between printing stations, and secure additional clues for minimizing distortion of the printer. The reticle pattern may also be printed on media to be used for distortion adjustment at a later time and even at a different location.  
         [0041]     The implementations of  FIGS. 3 and 4  describe specific operations occurring in a particular order. Further, the steps may be performed in parallel as well as sequentially. In alternative embodiments, certain of the logic operations may be performed in a different order, modified or removed and still implement preferred embodiments of the present invention. Morever, steps may be added to the above described logic and still conform to the preferred embodiments.  
         [0042]     Variations of the implementations may be constructed for various types of printing devices. For example, in an ink-jet printer the implementation may include reticle patterns that generate interference patterns only if the ink spots printed by an ink-jet printer are small enough not to bleed into each other. In such a case the implementation would attempt to secure interference patterns rather than eliminate interference patterns in the digital image of the reticle pattern. Manual or automatic adjustments may be made to the ink-jet printer, if the spots are judged to be bleeding too much.  
         [0043]     Alternatively, the presence of the interference patterns may be used as a security feature on printed materials such as legal documents or currency, where the presence of a correct interference pattern is used to validate the legitimacy of the printed matter. Because only the superimposed reticles, with resulting interference pattern, will be present on the final printed matter, additional security is maintained, since counterfeiters will not have easy access to the original reticle patterns used to create the interference patterns.  
         [0044]     In variations of the implementation the calibration may be performed at a later time or at a location different from the printing device. In some printers, a color head on a printing station may comprise of a multiple head array, where each head of the multiple head array may have alignment errors. In one implementation, reticle patterns that cover most of a page may be used to provide diagnostics on each head of the multiple head array. The scanning device may be movable such that the scanning device can be moved over the reticle patterns to return diagnostics as to which heads in the multiple head array are providing the distortion, and to suggest a direction for correction.  
         [0045]     The implementation can have a test pattern of interference patterns that cover most of the page to give diagnostics on each of the head arrays. The implementation can have the CCD or scanner that reads the interference patterns be moveable.  
         [0046]     The implementation could also include a test pattern of interference patterns, either whole page or across the scan width, so that scan direction distortion of the paper can be measured and adjusted for on a component-by-component basis. The whole pages are used for calibration, where the single-line or-column interference patterns are used for on-the-fly adjustment. Furthermore, rather than a whole “scan line” of interference patterns, one interference pattern can be used at each side (and potentially between pages for n-up configurations) to do coarse measurement of the scan direction distortion, based on the assumption that the distortion is uniform. Since scan direction distortion is going to be less than process direction distortion (because the web is under higher tension in the process direction), the assumption of uniformity is probably sufficient for measurement of scan direction.  
         [0047]     A whole scan line of interference patterns can be used to measure and compensate for local changes in distortion; i.e., where distortion is not uniform across the entire scan width, but varies within a print job.  
         [0048]     The implementation could allow ink jet printers to have an interference pattern for the test pattern that can indicate if a single jet is out. Interference patterns can be printed in areas where they do not need to be removed, e.g., where they will be hidden by binding or other processing.  
         [0049]     In another embodiment, the interference patterns could be used to build a model to assist with on-the-fly or preRIP adjustment. Measured information could be used to develop a model for a closed-loop feedback system for predicting the stretch for this particular paper based on the component coverage (e.g., by pel counting). This can be used to reduce the amount of on-the-fly calculation required.  
         [0050]     This model can also be used in preRIP if the paper is known to be the same as the paper used in the model-building run, and if the job coverage/content is known to be comparable to that of the model-building run. This is particularly useful where a job does not need careful image distortion compensation, and where the run performance of the printer is more critical. If content/coverage/paper/environment may have changed “somewhat” from the measurement run, this information in preRIP can be used to bring the print “closer to feedback loop lock” for the on-the-fly adjustment. Model information can be part of the forms definition, for example.  
         [0051]     Interference patterns can be used in calibration pages to precalibrate for the paper. Then one may use the prebuilt model to preRIP the data. These interference patterns can be laid out or chosen in such a way to emulate the range of coverage of jobs; e.g., light-to-heavy coverage. They can also be chosen and placed to emulate the actual layout of the non-variable parts of the actual job.  
         [0052]     A checksums on overlay projects could be stored, tied to distortion models and form definitions. When the checksum recurs, the distortion model can be pulled up. These stored checksums can be expired out of the database over time if not referenced again, or not stored at all unless the overlay occurs some threshold number of times. For paper with preprinted marks or pinholes, the measured information can be combined with this information to produce a more accurate model. This is also applicable to other printing technology that has not dealt with distortion of the paper, e.g., due to fusing of wet papers on EP technologies.  
         [0053]     The present invention could be utilized for applications such as statements, books, or digital newspaper where the image must be registered, but the image distortion of the (usually single-component) text is not important. Thus, only the image is adjusted on-the-fly or pre-adjusted in preRIP, based on the measured or model information.  
         [0054]     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.