Patent Publication Number: US-6663206-B2

Title: Systems and method for masking stitch errors

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to masking stitch errors between swaths during printing. 
     2. Description of Related Art 
     Fluid ejecting devices such as, for example, inkjet printers, fire drops of fluid from rows of nozzles of an ejection head. The nozzles are usually fired sequentially in groups beginning at one end of the head and continuing to the other end of the head. While the nozzles are being fired, the head moves at a rate designed to advance it by a resolution distance before the next firing sequence begins. If the nozzles are not fired simultaneously, the rows of nozzles can be tilted so that drops fired from all nozzles land in a substantially vertical column. 
     The ejection head can have one or more dies, each die having a plurality of nozzles. Some devices have ejection heads with only one die, and some devices have ejection heads with multiple dies. If an ejection head has multiple dies, the dies can be, for example, arranged vertically with respect to one another so that the head can eject more drops in a single swath of the head compared to a head having a single die. 
     The line at which the swaths ejected by adjacent dies meet, or at which the adjacent swaths meet, is called the stitch joint. Stitch joint errors occur when the swaths meeting at the stitch joint meet in such a way that the resulting arrangement of drops at one side of the stitch joint of a printed image are displaced from the drops on the other side of the stitch joint by a different distance than the displacement distance between drops within a swath. This creates a visible, undesirable print defect. Because of the spacing of the stitch joint errors, the stitch joint errors are very noticeable because the human eye is very sensitive to this spatial frequency region. 
     Stitch joint error can be, for example, the result of a gap between the drop of one die or swath adjacent the stitch joint and the drop of an adjoining swath or die adjacent the stitch joint. The gap is usually caused by difficulties in producing adjacent swaths close enough together to mask this apparent error. 
     SUMMARY OF THE INVENTION 
     It is desirable to cover up or mask the stitch joint error. Prior art techniques for masking the stitch error between swaths require alternating the firing of the nozzles of adjacent dies in a multi-die ejection head using different firing sequences. However, it is often difficult to precisely position adjacent dies so that the spacing between the lowermost nozzle of the upper swath and the uppermost nozzle of the lower swath is reduced enough so that the stitch joint error becomes less apparent. 
     This invention provides systems and methods for indexing the position of a sheet of recording medium conventionally and then measuring the position of the sheet of recording medium accurately by a sensor. 
     This invention separately provides systems and methods for shifting the data in the printhead so that the data is accurately aligned within a predetermined pixel accuracy to the known paper position. 
     This invention separately provides systems and methods for shifting the position of the printhead so that the data is accurately aligned within a predetermined pixel accuracy to the known paper position. 
     This invention separately provides systems and methods for covering up the resulting stitch joint error by modifying the pixels at the stitch joint interface to mask the apparent error. 
     In various exemplary embodiments of the systems and methods of this invention, a sheet of recording medium is indexed crudely. The resulting position is measured more accurately using a sensor. The sensor provides this information to a controller. In various exemplary embodiments, the systems and methods of this invention shift the data in the printhead so that the data is aligned within a predetermined pixel accuracy to the measured paper position. In various exemplary embodiments, the remaining sub-pixel stitch joint error is covered up by modifying the pixels at the stitch interface. 
     These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various exemplary embodiments of the invention will be described in relation to the following drawings, in which like reference numerals refer to like elements, and wherein: 
     FIG. 1 shows a stitch joint error between two swaths; 
     FIG. 2 is a perspective view of an exemplary image recording apparatus in which the systems and methods of the invention can be used; 
     FIG. 3 shows a first exemplary embodiment of pixel data fired from the next swath; 
     FIG. 4 shows another exemplary embodiment of pixel data fired from the next swath; 
     FIG. 5 shows a first exemplary embodiment for reducing a stitch joint error by showing the location of the fill pixels with respect to the position of the raster lines of the adjacent swaths; 
     FIG. 6 is another exemplary embodiment of a stitch joint error; 
     FIG. 7 shows a second exemplary embodiment for reducing a stitch joint error by showing the location of the fill pixels with respect to the position of the raster lines of the adjacent swaths; 
     FIG. 8 is a functional block diagram of an exemplary embodiment according to the invention; and 
     FIG. 9 is a flowchart outlining one exemplary embodiment of a method for reduced stitch error printing according to this invention. 
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     The following detailed description of various exemplary embodiments of the fluid ejection systems according to this invention are directed to one specific type of fluid ejection system, an ink jet printer, for sake of clarity and familiarity. However, it should be appreciated that the principles of this invention, as outlined and/or discussed below, can be equally applied to any known or later developed fluid ejection systems, beyond the ink jet printer specifically discussed herein. 
     Fluid ejector systems, such as drop-on-demand liquid ink printers, such as piezoelectric, acoustic, phase-change wax-based or thermal type printers, have at least one fluid ejector from which droplets of fluid are ejected towards a receiving sheet. Within the fluid ejector, the fluid is contained in a plurality of channels. Power pulses cause the droplets of fluid to be expelled as required from orifices or nozzles at the end of the channels. 
     When the fluid ejector is an ink jet printhead, the fluid ejector may be incorporated into, for example, a carriage-type printer, a partial-width array-type printer, or a page-width-type printer. The carriage-type printer typically has a relatively small printhead containing the ink channels and nozzles. The printhead can be sealingly attached to a disposable ink supply cartridge. The combined printhead and cartridge assembly is attached to a carriage that is reciprocated to print one swath of information at a time, on a stationary receiving medium, such as paper or a transparency, where each swath of information is equal to the length of a column of nozzles. 
     After the swath is printed, the receiving medium is stepped a distance at most equal to the height of the printed swath so that the next printed swath is contiguous or overlaps with the previously printed swath. This procedure is repeated until the entire image is printed. 
     In contrast, the page-width printer includes a stationary printhead having a length sufficient to print across the width or length of the sheet of receiving medium. The receiving medium is continually moved past the page-width printhead in a direction substantially normal to the printhead length and at a constant or varying speed during the printing process. A page width fluid ejector printer is described, for instance, in U.S. Pat. No. 5,192,959, incorporated herein by reference in its entirety. 
     Fluid ejection systems typically eject fluid drops based on information received from an information output device, such as a personal computer. Typically, this received information is in the form of a raster, such as, for example a full page bitmap or in the form of an image written in a page description language. The raster includes a series of scan lines comprising bits representing individual information elements. Each scan line contains information sufficient to eject a single line of fluid droplets across the receiving medium in a linear fashion. For example, fluid ejection printers can print bitmap information as received or can print an image written in the page description language once it is converted to a bitmap of pixel information. 
     FIG. 1 shows the systematic stitch joint error between a pixel discharged or fired from a last nozzle of a first swath and a pixel discharged or fired from the first nozzle of an adjacent or subsequent swath. The gap between the printed pixels  60  of a first swath  1  and the black pixels  65  of a second swath  2  is the stitch joint error  85 . The stitch joint error  85  is caused, for example, by mispositioning of the recording media or the ejection head between swaths. 
     This mispositioning of the last pixel in the first swath  1  and the first pixel of the second swath  2  usually arises due to errors resulting from manufacturing tolerances and limitations. As the swath width of a fluid ejection system becomes larger, the difficulty in having the proper position of the fluid receiving substrate for adjacent swaths increases. Thus, it is difficult to position the fluid receiving substrate accurately to within an acceptable margin, which is usually about 10 μm but which can be even smaller. 
     FIG. 2 shows a portion of a fluid ejecting apparatus that incorporates the systems and methods of the invention. As shown in FIG. 2, a fluid ejection head  10  moves in a first direction A along a guide rod  15 . It should be appreciated that the fluid ejection head  10  is movable along the guide rod  15  in a first direction and a second direction opposite the first direction. 
     A receiving substrate  30  is supported by a platen  25 . As the fluid ejection head  10  moves back and forth along the guide rod  15 , an image is created on the receiving substrate  30 . The receiving substrate  30  is typically in a flat position when it receives the created image as the fluid ejection head  10  moves back and forth along the guide rod  15 . However, it should be appreciated that the receiving substrate  30  can be in any position suitable to adequately receive the created image from the fluid ejection head  10 . The fluid ejection apparatus shown in FIG. 2 includes a sensor  35  connected to a controller  20 . The sensor  35  detects marks  33  located on the platen  25 . In particular, the sensor  35  detects the marks  33  to detect an amount of rotation of the platen  25 . 
     The information detected by the sensor  35 , concerning the amount of rotation of the platen  25 , is output to the controller  20 . The controller  20  uses the information provided by the sensor  35  to determine the amount of movement of the fluid receiving substrate  30  relative to the fluid ejection head  10 . Accordingly, the position of the fluid receiving substrate is determined by the controller  20 . 
     In various exemplary embodiments, the sensor  35  can be implemented using any one of a number of sensors that accurately sense the position of a moving surface having a primary movement direction, where the moving surface is marked with a plurality of detectable marks. 
     For ease of understanding and clarity, the following description of the systems and methods of this invention are directed to a specific type of sensor, a bi-directional linear incremental position sensor, or BLIP sensor, that is usable to accurately measure the position of the fluid receiving substrate  30  relative to the fluid ejection head  10 . However, it should be appreciated that the systems and methods of this invention can use any type of sensor that is usable to accurately measure the position of the fluid receiving substrate  30  relative to the fluid ejection head. 
     As indicated above, while any suitable type of sensor can be used with the systems and methods of this invention, the following description will focus on a bi-directional linear incremental position sensor. In general, the bi-directional linear incremental position sensor has sharp edge detection quality. Conventionally, an optical sensor in an ink jet printer sequentially detects a linear array of transverse belt timing marks, such as the marks  33  discussed above. Accurately sensing the position of the sheet being printed by an inkjet printer can provide improved quality printing and reduce the stitch joint error. 
     It should be appreciated that the individual detected lines of a mark may be much thicker than the pixel spacing of the linear array detector marks sensor. For example, a typical detectable mark line could be 200 or more pixels wide and the inter pixel spacing of a 2000 pixel array could be only 10 microns or less. While the mark thickness is not critical with the bi-directional linear incremental position sensor  35 , a sharp edge detection quality of the marks is desirable. 
     In particular, the bi-directional linear incremental position sensor  35  is used to detect the marks  33  spaced together around the circumference of the platen  25 . The marks  33  are spaced incrementally around the platen  25 . It should be appreciated that the marks  33  can be spaced in any manner as long as they are detectable by the sensor  35 . 
     When the platen  25  rotates, the bi-directional linear incremental position sensor  35  detects the marks  33  as they pass by the bi-directional linear incremental position sensor  35  and the corresponding position of each individual timing mark  33  on the platen  25 . 
     The bi-directional linear incremental position sensor  35  detects the movement of each individual mark  33  relative to the last mark  33  that was detected. By detecting the motion of the marks  33 , the bi-directional linear incremental position sensor  35  detects the positional change of the fluid receiving substrate  30 . That is, the marks  33  provide movement information to the bi-directional linear incremental position sensor  35 . The bi-directional linear incremental position sensor  35  converts this position information into a signal that is output to the controller  20 . Thus, the bi-directional linear incremental position sensor  35  provides highly accurate information of the position of the fluid recording medium relative to the fluid ejection head  10 . 
     The last nozzle of the uppermost swath and the first nozzle of the lowermost adjacent swath are desirable precisely aligned such that the lowermost nozzle of the first swath and the uppermost nozzle of the second swath are spaced correctly to produce an image without any resulting stitch joint error. However, as discussed above, when nozzles of the first and second swath are not spaced correctly, a stitch joint error results. 
     Thus, the systems and methods of this invention reduce stitch joint error by shifting the data in the printhead  10  to reduce stitch joint error. Shifting data in the printhead  10  allows a nozzle, which was not necessarily originally designated to fire the pixel data prior to the shift of data, to fire pixel data. Shifting the data in the printhead  10  allows the resulting swaths on the fluid recording medium to be aligned such that an apparent stitch joint error is reduced or eliminated. 
     Shifting of data in the printhead  10  occurs after the controller  20  receives the positional information of the fluid recording medium detected by the sensor  35 . In response, the controller  20  controls which nozzles in the printhead  10  receive which raster line of data for the next swath. In this manner, the controller  20  controls the printing of the image by the printhead  10 . 
     According to various exemplary embodiments of the systems and methods of this invention, the position of the nozzles of the second swath which are to be fired is determined from the marks  33  and the sensor  35  as described above. In various exemplary embodiments of the systems and methods of this invention, when the controller  20  determines that a stitch joint error will occur based on the current relative location between the printhead  10  and the image receiving medium  30  and the location of the previous swath on the image receiving medium  30 , the location of the second swath, and corresponding nozzles which fire the pixel data of the second swath, are adjusted relative to the position of the first swath. 
     Thus, the image data is shifted in the printhead  10 , resulting in the lines of pixel data being fired from nozzles to which the lines of pixel data would not have been originally designated. It should be appreciated that the data for any given raster line of the second swath can be shifted to fire from any nozzle in the array. Shifting the data in the printhead  10  for the second swath moves the fired lines of pixel data relatively closer to the position on the image receiving medium of the first swath. As such, the stitch joint error will be reduced. 
     Accordingly, the controller  20  utilizes the information about the relative position of the fluid recording medium and the printhead provided by the sensor  35  to determine which nozzle of the second swath  2  will be most accurately positioned adjacent the last fired nozzle of the first swath  1  so that the stitch joint error is reduced to at most 0.5 pixel. Once the controller  35  determines which nozzle of the second swath  2  should be fired first, the data in the printhead is shifted accordingly. 
     According to another exemplary embodiment of the invention, after the controller  20  receives information about the relative position of the fluid recording medium  30 , the printhead can be shifted relative the fluid recording medium so that a nozzle of the second swath  2  will be most accurately positioned adjacent the last fired nozzle of the first swath  1  so that the stitch joint error is reduced to at most 0.5 pixels. 
     FIG. 3 shows the extent of the printed information from a first swath  52 . The bottom of the first swath  52  corresponding to the position of the last raster line and last pixels fired from the nozzles of the first swath. FIG. 3 also shows the relative position of the printhead and corresponding nozzles of the printhead of the second swath after the sheet of fluid recording medium  30  has been advanced. According to the various exemplary embodiments of this invention, the controller  35 , shown in FIG. 8, determines the location of the bottom of the first swath  52  and where that location is positioned relative the nozzles of the second swath. With this determination, the controller  35  shifts the data in the printhead with regard to the second swath so that the appropriate nozzles of the second swath are fired which, as discussed in more detail below, reduces minimizes or prevents stitch joint error. 
     In shifting the data in the printhead with regard to the second swath, the position of the image data is shifted relative to the ejection nozzles in the printhead so that a nozzle, other than the nozzle originally designated to fire the corresponding line of pixel data of the second swath, will be fired. That is, for example, if the first nozzle of the second swath was originally designated to fire a corresponding first line of pixel data, according to exemplary embodiments of this invention, a nozzle other than the first nozzle of the second swath is used to fire the first line of pixels in the second swath. Thus, a nozzle firing the corresponding line of pixels other than that first nozzle of the second swath is selected by the controller  20  as the uppermost firing nozzle of the second swath. In other words, the uppermost one or more nozzles of the second swath may not be used to print image data. 
     It should be understood that at least one nozzle of the second swath should overlap the pixels fired from the last raster line of the previous swath, wherein the overlapping at least one nozzle does not print image data. Such overlapping avoids the requirement for costly precision assembly that would normally be required to prevent stitch joint error, because misalignment between the two swaths can be limited to at most approximately one-half of the center-to-center nozzle spacing by selecting the appropriate uppermost firing nozzle for printing the second swath. If there is no overlapping of nozzles, there cannot be a shifting of the data in the printhead to reduce or eliminate the stitch joint error. This situation results in an unmaskable stitch joint error. 
     For example, FIG. 3 shows an exemplary situation in which the number of nozzles which will fire the pixel data is a nominal set of nozzles  22  and where every nozzle has been designated to fire pixel data. The nozzles of the second swath that overlap with the printed image data of the first swath do not print image data. Because of the overlap, the second swath will only fire pixel data from the actual number of nozzles  26  used in the second swath. Thus, the amount of pixel data printed by the second swath will be less than originally planned. However, according to the exemplary embodiments of FIG. 3, the pixel data corresponding to the number of nozzles overlapped  24 , will be shifted to the next swath and printed in that next swath. 
     In FIG. 3, if the controller  35  determines that the sixth nozzle  27  should fire the first line of pixels of the second swath to be printed, the pixel data is shifted so that the sixth nozzle  27  fires the pixel data originally set to be fired by the first nozzle. The data remaining to be fired in subsequent nozzles, i.e. nozzles  26  other than the sixth nozzle  27 , is shifted and fired accordingly. As a result of firing the first pixel from the sixth nozzle  27 , the number of lines printed in the swath will be less than the number of nozzles. The number of lines printed will be reduced by the number of nozzles  24  overlapped at the top of the second swath. In FIG. 3, the first  5  nozzles  24  will not fire pixel data, and thus the number of lines of printed pixel data of the second swath will be less than originally planned. 
     According to the exemplary embodiment of FIG. 3, the bottom of the first swath is located in a position between the fifth and sixth nozzle of the second swath to be printed. If the bottom of the previous swath is located exactly between the nozzles, then the remaining stitch joint error will be in the range of ±0.5 pixels. It should be appreciated that the bottom of the previous swath might not be located exactly between the nozzles. As such, the resulting remaining stitch error can be a value different from ±0.5 pixels. 
     According to another exemplary embodiment of this invention shown in Fig. the nominal set of nozzles in the printhead used to print the second swath can be more than the number of nozzles designated to fire pixel data in the second swath. In this situation, the data in the printhead can be shifted in either an up or down direction to reduce, minimize or prevent stitch joint error. 
     The exemplary embodiment of FIG. 4 includes nozzles which are not originally designated to fire pixel data located on either side of the nozzles  44  which are designated to fire pixel data. If the data in the printhead is shifted up to mask the stitch error, then there can be more nozzles at the bottom of the printhead which will not fire pixel data. Alternatively, if the data in the printhead is shifted down to mask the stitch error, there can be more nozzles at the top of the printhead which will not fire pixel data. Of course, it should be understood that the nozzles which are originally designated to fire pixel data, according to the exemplary embodiment of FIG. 4, can be any designated set of nozzles which allows for movement of the data in an up or down direction in the printhead. 
     In FIG. 4, the nominal set of nozzles  42  used to print the second swath is the number of nozzles of the printhead which are available to fire pixel data. However, according to the embodiment of FIG. 4, not all of the nominal set of nozzles  42  are originally designated to fire pixel data. Accordingly, there is a set of nozzles  44  which are designated to fire pixel data. This set of nozzles  44  contains the pixel data to be fired in the second swath and is located between the first and last nozzle of the nominal set of nozzles  42 . According to the exemplary embodiment of FIG. 4, having the designated set of nozzles  44  located in between the nominal set of nozzles  42 , allows the controller  35  to shift the data in the printhead in either direction, when a stitch joint error is detected, allowing for the stitch joint error to be reduced, minimized or prevented. 
     As discussed above, when the controller  35  determines that a stitch joint error will occur, the data is shifted in the printhead to mask the stitch joint error. However, according to the exemplary embodiment shown in FIG. 4, the data is fired from a different set of nozzles  46  from the set of nozzles originally designated to fire the pixels  44 . As shown in FIG. 4, for example, the controller  35  determines that in order to reduce the stitch joint error, the pixel data should be shifted five nozzles. That is, the nozzles originally designated to fire pixel data  44  will be shifted in the upward direction. Originally, nozzle  11  was designated to be the uppermost nozzle firing pixel image data. However, after the pixel data has been shifted in the printhead, the sixth nozzle will be the uppermost nozzle to fire the pixel data. Correspondingly, the last five nozzles of the nozzles  44  originally designated to fire pixel data, will not fire pixel data because the data has been shifted upward five nozzles. Additionally, nozzles  6 - 10 , which were not nozzles  44  originally designated to fire pixel data, will now be used and a part of the nozzles  46  used to fire pixel image data. 
     As discussed previously, it should be understood that the pixel data can also be shifted downward to reduce the stitch joint error because the nozzles  44  originally designated to fire pixel data are located in between the nominal set of nozzles  42 . It should also be appreciated that the amount of shifting of the image data within the printhead can be any number of nozzles. 
     FIG. 5 shows the situation where there is a stitch error that is in the range of ½ pixel. Thus, the first nozzle fired in the second swath  2  is mispositioned from the last fired nozzle of the first swath  1  by a pixel margin  15  that is at most 0.5 pixel on either side of the bottom edge  52  of the first swath  1 . If printed even after shifting the relative position of the image data to the nozzles of the printhead to compensate for including advancing the sheet of recording medium, a stitch error would still be formed in the printed image. 
     That is, in various exemplary embodiments, in addition to shifting the data and firing the information set to be printed, the controller  20  will also fire a line of pixels from the nozzle prior to and immediately adjacent to the first-fired nozzle. In the example illustrated in FIG. 3, the controller  20  will fire a pixel from the fourth or fifth nozzle which is immediately adjacent and prior to the fifth or sixth nozzle used to fire the first line of pixels of the next swath. The line of pixels fired from the nozzle prior to and immediately adjacent the first nozzle to be fired, are called fill pixels  70 . 
     The purpose of a fill pixel  70  is to bridge the gap between a printed pixel the last fired nozzle of swath  1  and a corresponding adjacent printer pixel that will be formed when the first line of pixels is formed by the nozzle that will be used for the first line of pixels for the second swath  2 . As shown in the exemplary embodiment illustrated in FIG. 5, a fill pixel  70  is fired from the fourth or fifth nozzle resulting in the masking of the ½ pixel gap between the first and second swaths, thus further reducing the perception of the stitch joint error. 
     According to this exemplary embodiment, to reduce the effects of the stitch spacing, the fill pixels  70  are produced in a space between the first swath  1  and the second swath  2 . The fill pixels  70  bridge the gap between adjacent pixels of the first swath  1  and the adjacent pixels to be fired in the second swath  2  as determined by the controller  20 . The fill pixels  70  create a printed image having more uniform continuity and density. 
     In various exemplary embodiments, the fill pixels  70  are not produced for all of the pixels located in the last raster line  30  of the first swath  1 . Instead, the fill pixels  70  are produced when a printed pixel  60  is located in the same position in both the first swath  1  and the second swath  2 . Accordingly, as shown in FIG. 5, a fill pixel  70  is located between the printed pixel  60  of the first swath  1  and the printed pixel  60  of the second swath  2  which is located in the same position. A fill pixel  70  is not located below the printed pixel  65  because there is no corresponding printed pixel printed in the corresponding location in the second swath  2 . However, it should be appreciated that a fill pixel  70  can be generated for any number of printed pixels of the first swath even if there will not be an adjacent printed pixel in the second swath. 
     It should be appreciated that the fill pixels  70  do not have to be directly in the center of the fill pixel raster line  40 , nor do the fill pixels  70  have to be directly between the adjacent printed pixels  60  of the first swath  1  and the second swath  2 . However, the fill pixels  70  should be located in the fill pixel raster line  40  within the region between the two printed pixels. The situation of FIG. 5 thus illustrates one desirable position for the fill pixels  70 . 
     In various other exemplary embodiments, for a pixel error of 0.5 pixel, the fill pixels  70  are of a ½ smaller size or are ½ less dense than the corresponding printed pixels  60 . Having the fill pixels  70  at a reduced size or density lessens the effect of overlapping of the fill pixel  70  and the printed pixels  60 , which could create a darker image upon printing and/or could overload the fluid receiving substrate with too much fluid. It should be appreciated that for any size pixel error, the fill pixels  70  can be of any arbitrary size. Accordingly, the fill pixel  70  can be larger, the same size as, or smaller than the printed pixels  60 . 
     In the exemplary embodiment illustrated in FIG. 5, the second swath  2  is mispositioned by 0.5 pixel from the first swath  1 . Accordingly, fill pixels  70  are located in the fill pixel raster line  40  to reduced the stitch joint error. In addition to reducing the appearance of the stitch joint error, because a 0.5-sized pixel is added to the image, the image is lengthened by 0.5 pixel. It should be appreciated that the image is lengthened or shortened by approximately the size of the stitch joint error, and thus the size of the fill pixels  70  located in the fill pixel raster line  40  between the printed pixels  60  of the last raster line  30  of the first swath  1  and the first raster line  50  of the second swath  2 . 
     According to another exemplary embodiment of the invention, the pixels created in the region between the last raster line of the first swath and the first raster line of the next swath, can be a duplicate line of either the last raster line of the first swath or the first raster line of the next swath. The duplicate line is a reprinted line of the same pixels in either the last raster line of the first swath or first raster line of the next swath. For example, if duplicating the last raster line of the first swath, the pixels printed in the region between the last raster line of the first swath and the first raster line of the next swath will be the same pixels printed in the last raster line of the first swath. It should be appreciated that the size and/or density of the duplicated line can be changed similar to changing the size and/or density of the fill pixels  70  discussed above. 
     FIG. 6 illustrates another exemplary embodiment of the invention. In FIG. 6, the size of the stitch joint error between the last raster line  30  of the first swath  1  and the first raster line  50  of the second swath  2  is a 0.25 of a pixel. 
     It should be appreciated that, in various exemplary embodiments, when the size of the stitch joint error is ±0.25 pixel or less, the last raster line  30  of the first swath  1  and the first raster line  50  of the second swath  2  are considered to be located in close proximity. Accordingly, in such exemplary embodiments, it might not be desirable for the user to produce a fill pixel  70  in the fill pixel raster line  40 , to avoid an undesirably darker image in the area of the printed pixels  60 . Accordingly, in this exemplary embodiment, the controller  20  can be designed to determine a pixel error below which using the fill pixels  70  in the fill pixel raster line  40  will not be required. 
     In various exemplary embodiments, if a fill pixel  70  is desired, a fill pixel  70  of ¼ size or ¼ density is produced in the fill pixel raster line  40  between adjacent printed pixels  60  of the last raster line  30  and the first raster line  50 . Of course, for the reasons outlined above, the image will be elongated by a 0.25 of a pixel length. 
     FIG. 7 illustrates another exemplary embodiment of the residual or remaining stitch joint error according to this invention. In FIG. 7, there is a negative 0.5 pixel error between raster  30  of swath  1  and raster  50  of swath  2 . That is, in contrast to the situations illustrated in FIGS. 3 and 4, where the first raster line  50  of the second swath was spaced away from the last raster line  30  to obtain a maximum stitch joint error of ±0.5 pixel, in this case, to limit the stitch joint error to ±0.5 pixel, the second swath  2  overlaps the first swath  1 . In particular, as shown in FIG. 7, there is an overlap of the last raster line  30  of the first swath  1  and the first raster line  50  of the second swath  2  of at most 0.5 pixel. This situation results in the printed image having areas of dark densities corresponding to the overlapping pixels and areas of light densities where there is no overlap. This situation produces inconsistent image quality and density, and is undesirable. 
     In various exemplary embodiments, the printed pixels of the first raster line  50  of the second swath  2 , which overlap with printed pixels of the last raster line  30  of the first swath  1 , are reduced in density. The reduced density pixels  70  lessen the effect of dark banding caused by an overlap of standard size and standard density pixels. 
     Thus, as shown in FIG. 7, the reduced density pixels  70  of the first raster line  50  of the second swath  2  are reduced in density inversely proportional to the amount of overlap. However, it should be appreciated that the reduced density pixels  70  can be reduced by any amount which will reduce the effects of dark banding. Of course, for the reasons outlined above, the printed image with an overlap between the first and second swaths  1  and  2 , a negative stitch error, such as a 0.5 pixel error, will be foreshortened by approximately 0.5 of the pixel length. 
     FIG. 8 is a functional block diagram of one exemplary embodiment of a printing device  300  incorporating the systems and methods of the invention. The printing device  300  has an input/output device  310  that connects the printing device  300  to an input device  320 . 
     In general, the image data source  330  can be any one of a number of different sources, such as a scanner, a digital copier, a facsimile device that is suitable for generating electronic image data, or a device suitable for storing and/or transmitting electronic image data, such as a client or server of a network, or the Internet, and especially the World Wide Web. For example, the image data source  330  may be a scanner, or a data carrier such as a magnetic storage disk, CD-ROM or the like, or a host computer, that contains image data. Thus, the image data source  330  can be any known or later developed source that is capable of providing image data to the printing device  300  of this invention. 
     When the image data source  330  is a personal computer, the data line connecting the image data source  330  to the printing device  300  can be a direct link between the personal computer and the printing device  300 . The data line can also be a local area network, a wide area network, the Internet, an intranet, or any other distributed processing and storage network. Moreover, the data line can also be a wireless link to the image data source  330 . Accordingly, it should be appreciated that the image data source  330  can be connected using any known or later developed system that is capable of transmitting data from the image data source  330  to the printing device  300 . 
     The printing device also includes, in addition to the input/output device  310 , a sensor  345 , a memory  340 , an overlap determining circuit  350 , a state determining circuit  360 , and a controller  380 , each communicating over a data/control bus. The overlap determining circuit  350  determines a degree of overlap of the next swath in order to select the most appropriate uppermost fired nozzle for the print head when printing the next swath. The state determining circuit  360  determines which state is most appropriate to produce the minimum stitch joint error (i.e., positive or negative stitch error). The printing apparatus  370  can include, for example, the print head. 
     In operation, according to one exemplary embodiment of FIG. 8, the input device  320  provides data to be printed and subsequently, a first swath of data is printed. A relative position between the recording medium and the printhead is crudely advanced to position the printhead relative to the recording medium for printing a next swath of the image data. The position of the fluid receiving medium is detected by the sensor  345  and the relative position of the recording medium and the printhead is determined based on the detected position of the recording medium. 
     After the relative position is determined, the controller  380  determines whether a stitch joint error will occur between the first and next swaths. If a stitch joint error will occur, the relative position of the raster lines of the next swath is shifted within the printhead to reduce the stitch joint error within a predetermined or dynamically determined maximum positive or negative value for the remaining or residual stitch joint error. In various exemplary embodiments, this predetermined or dynamically determined maximum error is ±0.5 pixel, but any useful values can be used as the predetermined maximum positive and negative values, such as 0 pixel and 1 pixel as the negative and positive values. 
     After the raster lines are shifted in the printhead, the overlapping determining circuit  350  determines, based on the value of the remaining or residual stitch joint error, whether the first swath and the second swath overlap. If an overlap is determined, the first raster line of the second swath is altered to reduce those printed pixels in that raster line that overlap printed pixels in the last raster line of the first swath. Then the next swath is printed. 
     However, if the overlap determining circuit determines there will be no overlap, a fill pixel line is generated to print pixels between the adjacent printed pixels in the last raster line of the first swath and the first raster line of the next swath. Then, the next swath is printed. 
     It should be understood that each of the circuits shown in FIG. 8 can be implemented as portions of a suitably programmed general purpose computer. Alternatively, each of the circuits shown in FIG. 8 can be implemented as physically distinct hardware circuits within an ASIC, or using a FPGA, a PDL, a PLA or a PAL, or using discrete logic elements or discrete circuit elements. The particular form each of the circuits shown in FIG. 8 will take is a design choice and will be obvious and predicable to those skilled in the art. 
     FIG. 9 is a flowchart outlining one exemplary embodiment of a method of masking a stitch joint error according to this invention. Beginning in step S 100 , operation continues to step S 200 , where a first swath is printed. Then, in step S 300 , the relative position between the recording medium and the printhead is crudely advanced to position the printhead relative to the recording medium for printing a next swath of the image data. Next, in step S 400 , the position of the fluid receiving medium is detected by a sensor. Operation then continues to step S 500 . 
     In step S 500 , the relative position of the recording medium and the printhead is determined based on the detected position of the recording medium. Next, in step S 600 , a determination is made whether a stitch joint error will occur between the first and next swaths. If so, operation proceeds to step S 700 . Otherwise, if no stitch joint error will occur, operation jumps directly to step S 1100 . In step S 700 , the relative position of the raster lines of the next swath is shifted within the printhead to reduce the stitch joint error with a predetermined or dynamically determined maximum positive or negative value for the remaining or residual stitch joint error. In various exemplary embodiments, this predetermined or dynamically determined maximum error is ±0.5 pixel, but any useful values can be used as the predetermined maximum positive and negative values, such as 0 pixel and 1 pixel as the negative and positive values. 
     Then, in step S 800 , a determination is made, based on the value of the remaining or residual stitch joint error, whether the first swath and the second swath overlap. If so, operation continues to step S 900 . Otherwise, operation jumps to step S 1000 . In step S 900 , the first raster line of the second swath is altered, such as changing the size or density of the pixel image data, to change those printed pixels in that raster line that overlap pixels in the last raster line of the first swath. Operation then jumps to step S 1100 . In contrast, in step S 1000 , a fill pixel line is generated to print pixels between the adjacent pixels in the last raster line of the first swath and the first raster line of the next swath. Operation then continues to step S 1100 . 
     In step S 1100 , the next swath is printed. Operation then continues to step S 1200 , where the method ends. 
     While this invention has been described in conjunction with the exemplary embodiment outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the exemplary embodiment of the invention, as set forth above, is intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention.