Abstract:
A method for controlling the printing of overlapping swathes, the swathes extending perpendicular to a succession of print lines; for each print line a transition is defined between one swathe and the next, with the location of the transition in the print line varying between print lines, for example varying pseudo-randomly or as a curve extending in the swathe direction.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of printing and particularly raster printing. In particular, examples of the invention are concerned with the difficulties faced in printing a ‘page-wide’ image with a printhead that is not ‘page-wide’. 
     2. Description of Related Art 
     This problem is commonly addressed either by using a single printhead to perform several passes over the same page, in which case the different passes may occur in different scanning directions, or with several printheads attached to a rigid frame known as a printhead bar. In each case, the area addressed by each printhead during printing is known as a swathe. 
     In order to form a page-wide image the swathes addressed by the printheads must form a mosaic covering the whole of the printed image area. However, the printed image resulting from a mosaic of print swathes is often found to have visible artefacts. Causative factors include “end effects” at the ends of the print heads and also mechanical errors and random errors in the both the heads and the system mechanics. The discontinuities can be in image density, image features or both. 
     The above effects are especially noticeable in printing situations where the substrate is printed in a single pass (material passes under the print heads only once) and especially in coating and deposition systems where film uniformity is critical. An array of static print heads (or actuators) printing onto a web moving continuously in one direction is a common example. 
     Similarly, a head (or array of heads) capable of printing a fully-formed image in a single pass can be used in a scanning mode—where the substrate is moved in a feed direction perpendicular to the print direction after each pass—thus, the second pass addresses a swathe parallel to the first printed swathe, but spaced in the substrate feed direction. 
     Typically it is desirable to have some overlap between the swathes so that errors in alignment of neighbouring swathes do not lead to elongate regions of unprinted substrate, which are highly visible to the human eye. In these regions of overlap, the substrate is therefore addressed twice (or possibly more)—either by a single pass of multiple overlapping printheads or by a single printhead passing multiple times or a combination of the two. Since the overlap region must not be printed with double weight it is necessary to manage the deposition of print-pixels in the overlap regions. 
     “Stitching” in this disclosure is defined as the management and printing of the print-pixels deposited at the interface between two swathes as stated above. Stitching is concerned with the edges of a specific swathe and that of another swathe that is adjacent to it. Interleaving of swathes may be employed to print alternate drops in a print area and then filling in the rest during a later print of a different swathe. This interleaving technique is independent of stitching and may be used in conjunction with it. 
     One common factor in the implementation of these techniques is that the effective width of the printhead is reduced as the swathes are required to overlap. 
     Raster printing involves the deposition of print-pixels in a grid, in the overlapping regions it is therefore possible to select on a pixel-by-pixel basis to which swathe a pixel in a print image is assigned. Inkjet printing is a typical raster printing method where the printed image is formed of a grid of ink dots on the substrate. 
     An example of the process of stitching for two swathes is illustrated in  FIG. 1 , which shows two swathes—A and B—and their region of overlap (diagonally shaded area), which is m pixels wide. In this and the following figures, no presumption is made as to the order, the direction or the sequencing of swathe ‘A’ and swathe ‘B’. They need not be printed consecutively or from the same printhead. For example, swathe A may be formed by a first pass of a single printhead in direction  1  and swathe B formed by the second pass in direction  2 , or equally both may be formed by a single pass in direction  1  or  2  of two overlapping printheads. Printheads may produce drops of a single consistent size (binary) or variable size (greyscale). All print-pixels in  FIG. 1  are of identical size for the purposes of clarity. 
     Two swathes, swathe ‘A’ and swathe ‘B’, are printed in such a way that there is an overlap region (diagonally shaded area) where a number of pixel locations can be printed as part of either swathe ‘A’ or swathe ‘B’. 
     If a specific pixel in the stitching area is required to be printed then it is either printed during the printing of swathe ‘A’ or swathe ‘B’. It is required to determine on a pixel by pixel basis for each pixel in the stitching area (diagonally shaded area) whether it will be printed as part of swathe ‘A’ or swathe ‘B’. 
     In an example stitching method illustrated in  FIG. 2 , referred to herein as ‘soft-stitch’, the stitching area (diagonally shaded area) of the swathes is ‘m’ pixels wide.  FIG. 2  shows this as 3 pixels but it may be any number provided that it is less than the width of the printhead. Typically it would be between 1 line and 10% of the head width. 
     The probability of deciding whether to print a given pixel in swathe ‘A’ is a mathematical function of ‘m’ and ‘n’ as defined above.
         p(A)∝(n/(m+1)) i  where ‘i’ is any suitable power.       

     For example:
         p(A)=(n/(m+1)) i *RND where RND produces a uniform probability.   if p(A)&gt;0.5 then print using swathe A, else print using swathe B       

     The value of (n/(m+1)) i  may also be used as an index into a “dither table”, suitably scaled by the dimension of the table. This table may be any form of dither table and would include; “ordered dither”, “random dither” or “blue noise dither”. 
     SUMMARY OF THE INVENTION 
     However, the applicant has recognised that such a stitching method will nonetheless result in visible artefacts in the printed image. Through identification and analysis of the causes of stitching artefacts with such methods the applicant has arrived at the solution offered by the present invention.  FIGS. 3-6  illustrate common causes of stitching artefacts. 
       FIG. 3  shows an ideal pattern printed by overlapping swathes A and B and the overlapping area of width m where ‘soft-stitching’ has been performed. The print-pixels of the two swathes (A and B) are illustrated with different shadings for clarity, so that the assignment of pixels in the overlap region to each swathe is visible. Here, the pixels in the two swathes (A and B) are of identical size and are aligned with imperceptible error. 
       FIG. 4  shows stitching between swathes A and B that are offset by a distance d perpendicular to the swathe direction ( 1  or  2 ), where d is a non-integral number of pixel-spacings. Such alignment errors are common both with printhead bars and with printheads working in a scanning mode. Here, the stitching process has resulted in the creation of overlapping pixels ( 11 ), thus leaving several gaps ( 10 ) in the printed pattern. The bulk effect of such gaps is to create a band of lighter tone in the finished image. This band is aligned with the print direction, thus being elongate and as such highly visible to the human eye, which is adept at identifying straight lines. 
       FIG. 5  shows stitching between swathes A and B that are offset by a distance d parallel to the swathe direction ( 1  or  2 ), where d is a non-integral number of pixel-spacings. Again, this results in a highly visible band of lighter tone of width m in the finished image. 
       FIG. 6  shows stitching between swathes A and B that have print-pixels of different sizes. With large enough errors in optical density this may also result in a visible band of width m in the printed image. 
     Accordingly, the present invention provides a method for controlling the printing of overlapping swathes, resulting in improved print quality. 
     According to a first aspect of the invention there is provided a method for controlling the printing of overlapping swathes extending in a swathe direction with a succession of print lines extending perpendicular to the swathe direction, comprising the step of defining for each print line a transition between one swathe and the next, with the location of the transition in the print line varying between print lines. 
     Preferably, the location of the transition in the print line varies as a function of displacement in the swathe direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described with reference to the Figures, wherein: 
         FIG. 1  shows two swathes of print and the region of overlap between. 
         FIG. 2  displays a known stitching system referred to herein as ‘soft-stitch’ applied to the swathes of print shown in  FIG. 1 . 
         FIG. 3  illustrates the result of a prior art stitching system carried out for two perfectly aligned print swathes. 
         FIG. 4  illustrates the result of the prior art stitching system of  FIG. 3  when applied to swathes misaligned perpendicular to the swathe direction. 
         FIG. 5  illustrates the result of the prior art stitching system of  FIG. 3  when applied to swathes misaligned parallel to the swathe direction. 
         FIG. 6  illustrates the result of the prior art stitching system of  FIG. 3  when applied to swathes having mismatched print-pixel sizes. 
         FIG. 7  illustrates an embodiment of the invention referred to herein as ‘profile stitching’. 
         FIG. 8  illustrates an embodiment of the invention referred to herein as ‘diagonal stitching’. 
         FIG. 9  illustrates an embodiment of the invention referred to herein as ‘pin stitching’. 
         FIG. 10  illustrates how an embodiment of the invention alleviates visual artefacts. 
         FIG. 11  illustrates a further embodiment of the invention using a combination of ‘profile stitching’ and ‘soft stitching’. 
         FIG. 12  illustrates a further embodiment of the invention using a combination of ‘diagonal stitching’ and ‘soft stitching’. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The path traced by the interface of the two swathes is called the stitch line. This line is traditionally a straight line running parallel to the relative motion of the printhead and the substrate. In the example illustrated in  FIG. 2 , referred to as a “soft-stitch”, the nominal centreline of the stitch runs parallel to the direction of travel of the swathes ( 1  or  2 ). 
       FIG. 7  illustrates a method in accordance with a first embodiment of the invention, referred to herein as “profile-stitching”. The figure shows two swathes (A and B) that combine to form an image of continuous tone. The two swathes are illustrated as regions of shaded pixels in the figure and therefore are spaced apart perpendicular to the possible swathe directions ( 1  and  2 ) for clarity. As before, no presumption is made as to the order of printing of the two swathes (A and B) and the swathes may be printed in the same or opposite directions (possible swathe directions are shown as  1  and  2 ). In this technique, the centreline of the stitch no longer runs parallel to the direction of travel of the printhead(s). The path of this line may be of any non-straight line whether mathematically calculated or randomly derived. In the figure the centreline is illustrated as a waved line. In the printed image the visibility of the centreline to the human eye will be minimised as it is no longer a linear feature. 
     Using this method the usable width of the swath is reduced and an overlap area is used to achieve a continuous print. The size of the overlap also dictates the magnitude of the profile of stitch. 
       FIG. 8  illustrates in a similar manner a method in accordance with a second embodiment of the invention, referred to herein as ‘diagonal stitching’. This is very similar to the ‘profile-stitch’ except that the centreline of the stitch is made up of diagonal lines at any set or varying angles so as to prevent a stitch centreline running parallel to any printed swath at any time. 
     Using this method the usable width of the swath is reduced and an overlap area is used to achieve a continuous print. Again, the size of the overlap also dictates the magnitude of the profile of stitch. 
       FIG. 9  illustrates in the same manner a method in accordance with a third embodiment of the invention, referred to herein as ‘pin-stitching’. This is again similar to the ‘profile stitch’ but in this case the stitch centreline is set randomly for each line of pixels that is laid down in the print direction ( 1  or  2 ) and results in a jagged edge to region printed by each swathe (A and B). The position of the centreline of the stitch is not influenced in any way by the position of the stitch centreline in either the preceding or the subsequent lines of pixels. Again, size of the overlap also limits the magnitude of the profile of stitch. 
     The ‘pin-stitch’ method is particularly advantageous in overcoming image artefacts due to alignment errors perpendicular to the swathe direction. As illustrated in  FIG. 10  the resultant print image will have only one gap ( 10 ) per print line, these gaps being randomly distributed throughout the stitching region. Such gaps are far too few in number to cause a perceptible difference in the optical density of the stitching region. 
     The stitching methods described above may optionally be combined with the ‘Soft Stitching’ technique. In this case the ‘soft stitching’ can be used to soften the transition between regions printed in different swathes. 
       FIG. 11  displays two print swathes (A and B) generated using a combination of a ‘profile-stitch’ and ‘soft-stitch’. Again, the two regions have been separated for the purposes of illustration.  FIG. 12  displays an example of the combination of ‘diagonal stitch’ and ‘soft stitch’ 
     Care however must be taken when using ‘soft stitch’ to soften the transition in order that the limits of the variation of the stitch line added to the width of the soft stitch do not exceed the limits of the overlap. To this end the probability function used to determine which swath is to print a particular pixel in the soft stitch algorithm may be modified to include positional information about the centreline of the stitch within the overlap area. Indeed, with greyscale printing, it is possible to form an overlapping pixel in part during one print swathe, in part during another print swathe. 
     It will be apparent to those skilled in the art that the aforementioned techniques for stitching may be applied to any raster image format printing apparatus, including both greyscale and binary inkjet printing.