Abstract:
In a method of operating an inkjet printer, an intermediate transfer member is movable in an advance direction. A carrier supports a printhead, and is movable relative to the intermediate transfer member in a direction generally perpendicular to the advance direction. The printhead defines a plurality of raster lines extending over the intermediate transfer member at a non-perpendicular, fixed angle vector relative to the advanced direction. A bitmap image is defined which corresponds to an image to be formed on the intermediate transfer member. The bitmap image includes a plurality of rows and columns of pixels, with at least one image data corresponding to each pixel. The bitmap image is skewed such that the image data for at least one column within the bitmap image is shifted a predetermined number of pixel locations, dependent upon the fixed angle vector.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to ink jet printers, and, more particularly, to a method of rasterizing image data printed on an address space associated with an intermediate transfer member. 
     2. Description of the Related Art 
     Ink jet printers typically use one or more monochrome or color printheads to produce a printed document. In typical inkjet printers, the carrier moves horizontally across the print medium and the print medium is indexed in an, advance direction independently between scans of the carrier. This motion allows typical inkjet printers to print using an orthogonal, rectilinear address space. That is, all addressable pixels are located on a rectangular grid with an orthogonal axis. Inkjet printers print on the print medium using a desired ink dot density, such as a 600×600 dots per inch (dpi) grid. This process produces a printed document of high quality; however, often the time associated with printing is undesirably long. Non-printing time occurs during which the printheads are mechanically moved without jetting ink. This waste of mechanical energy in turn leads to unnecessary delays before an image is placed on the print medium and delivered to the user. 
     In order to minimize the non-printing time in an ink jet printer, it is known to use an intermediate transfer member (ITM), wherein an image is printed onto a repeating surface, such as a cylinder, and transferred to a print medium in a subsequent operation. The use an ITM to print upon minimizes the time and mechanical energy wasted during non-printing operations and provides a known, controlled and repeatable surface upon which to form the image. During the printing operation, the ITM rotates at a fixed speed. At the same time, the carrier moves from one end of the ITM to the other end at a constant linear speed as to follow a helical path on the ITM. The entire imaging operation is performed with no stops or starts with either the carrier system or the ITM system, thus minimizing energy waste. Therefore, it is no longer possible to produce or use an orthogonal, square, rectangular address space with the printheads during printing. 
     A continuous ITM as described above is also used with laser printers. A laser beam typically is reflected from a rotating polygon mirror and traversed across a photoconductive ITM as the polygon rotates. This occurs very fast and thus the helical effect associated with each line of pixels is negligible. 
     Ink jetting printers may also include an ink jet cartridge having a printhead with multiple or redundant major columns of ink jetting orifices. Each major column typically consists of multiple, staggered columns of ink jetting orifices, with the major columns being spaced apart from and parallel to each other. By providing redundant major columns of ink jetting orifices, each including multiple staggered columns of ink jetting orifices, print artifacts caused by clogged nozzles, faulty circuitry or the like may be avoided. 
     What is needed in the art is a method of printing with an inkjet printer using a continuous ITM, wherein the helical effect associated with printing on the ITM is minimized. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of operating an ink jet printer wherein the image data is skewed to offset the helical effect caused by printing on an intermediate transfer member. 
     The invention comprises, in one form thereof, a method of operating an inkjet printer. An intermediate transfer member is movable in an advance direction. A carrier supports a printhead, and is movable relative to the intermediate transfer member in a direction generally perpendicular to the advance direction. The printhead defines a plurality of raster lines extending over the intermediate transfer member at a non-perpendicular, fixed angle vector relative to the advance direction. A bitmap image is defined which corresponds to an image to be formed on the intermediate transfer member. A bitmap image includes a plurality of rows and columns of pixels, with at least one image data corresponding to each pixel. The bitmap image is skewed such that the image data for at least one column within the bitmap image is shifted a predetermined number of pixel locations, dependent upon the fixed angle vector. 
     The invention comprises, in another form thereof, a method of operating an ink jet printer. An intermediate transfer member is movable in an advance direction. A carrier supports a printhead, and is movable relative to the intermediate transfer member in a direction generally perpendicular to the advance direction. The printhead defines a plurality of raster lines extending over the intermediate transfer member at a non-perpendicular, fixed angle vector relative to the advance direction. A bitmap image is defined which includes an array of pixels. The bitmap image includes image data corresponding to each pixel. The bitmap image is skewed such that the image data for a selected pixel location is shifted to a different pixel location, dependent upon the fixed angle vector. 
     An advantage of the present invention is that the helical affect associated with printing on the continuous ITM is minimized. 
     Another advantage is that skewing of the image data to offset the helical affect may be selectively carried out depending on the quantitative value of the fixed angle vector. 
     Yet another advantage is that the spacing of the address space on the ITM and print medium may be varied by proportionally adjusting the linear carrier speed and rotational ITM speed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a graphical illustration of a conventional method of printing with an inkjet printer; 
     FIG. 2 is a graphical illustration of a method of printing with an inkjet printer using an intermediate transfer member; 
     FIG. 3 is a graphical illustration of the geometric relationship associated with the helical effect of printing on an intermediate transfer member; 
     FIG. 4 illustrates a modified address space associated with an intermediate transfer member; 
     FIG. 5 illustrates another modified address space associated with an intermediate transfer member; 
     FIG. 6 illustrates yet another modified address space associated with an intermediate transfer member; 
     FIG. 7 illustrates rasterized image data after skewing to offset the helical effect associated with an intermediate transfer member; 
     FIG. 8 illustrates a printed image in an address space on an intermediate transfer member after being skewed as shown in FIG. 7; 
     FIG. 9 is a flow chart of an embodiment of a method of operating an inkjet printer of the present invention. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and more particularly to FIG. 1, there is shown a graphical illustration of a typical address space using an ink jet printer. A carrier  20  typically carries an ink jet cartridge (not specifically shown for simplicity sake) with one or more inks therein to be jetted onto print medium  22 , such as paper. Each color ink carried buy carrier  20  is associated with a respective print head (not shown) with a plurality of ink jetting orifices form therein. Carrier  20  is movable in a perpendicular direction  24  across print medium  22 , with respect to an advance direction  26  of print medium  22 . Each ink dot is placed at an addressable location (i.e., pixel) on print medium  22 , with the addressable locations defining an address space for placing the ink dots on print medium  22 . A portion of the address space on print medium  22  is designated with the reference number  28 . The address space  28  using a typical ink jet printer is an orthogonal, rectilinear address space. That is, all addressable pixels are located on a rectangular grid with an orthogonal axis. Carrier  20  moves across print medium  22  in scan direction  24  to place ink dots at selected pixel locations within address space  28 . Print medium  22  is then advanced in advance direction  26  and carrier  20  is again scanned across print medium  22  to place ink dots at selected pixel locations. 
     Although it will be appreciated that the address space consists of circles corresponding to the ink dots placed on print medium  22 , it should also be understood that a bitmap image is formed in the ink jet printer using suitable electrical processing circuitry, such as a microprocessor, memory, etc. The bitmap image consists of an array of square or rectangular cells arranged in an orthogonal; rectilinear manner. Image data for one or more colors of ink is associated with each cell or pixel in the bitmap image corresponding to ink drops which are to be placed on the address space overlying print medium  22 . 
     In the embodiment shown in FIG. 1, address space  28  is an orthogonal, rectilinear address space with a dot spacing of 600×600 dpi. Printhead  20  is assumed to traverse across the width of print medium  22  at a traveling speed of 26.66 inches per second. Each ink jetting orifice or nozzle of printhead  20  (not shown) is assumed to be capable of firing a single drop of ink every 125 microseconds (8,000 hertz rate), providing the orthogonal, square, rectilinear 600×600 dpi address space shown in FIG.  1 . Each drop, represented by an open circle, is placed on a rectangular grid location at a spacing of about 42.33 micrometers (600×600 dpi) from the nearest drop in all directions. This is quite convenient since the majority of digital image formats are an orthogonal, square, rectilinear arrangement of pixels. Since the digital image defined by the bitmap image in the electronic circuitry of the printer matches pixels for spots, this makes for easy rasterization of images for printing. 
     Referring now to FIG. 2, there is shown an embodiment of a method of printing using a carrier  20  which is positioned in association with an ITM  30 . In the embodiment shown, ITM  30  is assumed to be a cylinder which rotates during printing; however, ITM  30  may also be in the form of a belt, etc. ITM  30  transfers an image formed thereon to print medium  22 . 
     Carrier  20  typically carries an inkjet cartridge having a printhead with a plurality of ink jetting orifices, and is positioned in association with ITM  30  such that ink dots may be placed on ITM  30  at selected pixel locations. Carrier  20  moves in a longitudinal direction across ITM  30  as indicated by arrow  32 , and ITM  30  is assumed to rotate at a selected rotational speed as indicated by arrow  34 . As carrier  20  translates and ITM  30  rotates, ink dots are placed on ITM  30  at selected pixel locations within a band  36  which helical around ITM  30 . For illustration purposes, a selected portion of an address space  38  within band  36  will be discussed in greater detail. 
     To the right of the cylindrical representation of ITM  30 , if the periphery of the cylinder was “unrolled”, is a two-dimensional representation consisting of a plurality of adjacent bands  36  positioned relative to each other at the same angular orientation at which carrier  20  helical around cylindrical ITM  30 . The ink dots placed within address space  38  therefore are not orthogonal and rectilinear with respect to each other. Carrier  20  (and the printhead carried thereby) no longer traverses along an orthogonal direction with respect to the media at a rate of 26.66 inches per second, but rather traverses along a fixed 6° angle vector at a rate of 26.66 inches per second. The possible ink dot placement locations within the illustrated portion of address space  38  therefore likewise extend at a 6° angle vector relative to each other as may be readily observed in the enlarged portion of address space  38  on the right of FIG.  2 . 
     Also for explanation purposes, an approximate triangle  40  is shown overlying ITM  30 . Triangle  40  has a hypotenuse positioned generally parallel to an edge of band  36 , with the other two legs of the right triangle extending parallel and perpendicular to the axis of rotation of ITM  30 , respectively. Triangle  40  is shown in more detail in FIG.  3 . The angle θ, corresponding to the helical or fixed angle vector of carrier  20  on ITM  30 , is assumed to be 6° as indicated above. Moreover, the hypotenuse represented by the letter C is assumed to be 26.66 inches per second as indicated above. Therefore, the rotational surface speed of ITM  30  is 26.52 inches per second and carrier  20  travels in a scan direction across ITM  30  at a speed of 2.5 inches per second. Expressed as a ratio, this is approximately a 1:10 ratio, expressed as rise over run. By varying the rotational speed of ITM  30  and the translational speed of carrier  20 , the actual scan speed of carrier  20  along band  36  may be selected with a different value. 
     As evident from the portion of address space  38  shown on the right of FIG. 2, each drop of ink from the printhead carried by carrier  20  is represented by an approximate circle. In the embodiment shown, each circle is assumed to be represented by an open 65 μm diameter circle, with the resulting address space being neither orthogonal nor rectilinear. There are spaces on ITM  30  (and in turn print medium  22 ) that are not addressable with this address space. There is also 100 percent dot overlap with this address space such that there exists wasted ink at locations where an ink drop is placed where a printed ink drop has already been placed. 
     Utilizing the ability to adjust timing between adjacent columns of ink jetting orifices in the printhead, the address space  38  described above with reference to FIG. 2 can be modified. By delaying one side of the printhead timing by 62.5 microseconds (42.33 μm at 26.66 inches per second), the overlapping drops can be moved, resulting in the address space shown in FIG.  4 . This address space is uniform and rectilinear, but not orthogonal. The address space shown in FIG. 4 provides 100% addressability of all points on ITM  30  and print medium  22 . The address space shown in FIG. 4 is described herein as a skewed, rectilinear address space. 
     The printhead carried by carrier  20  has certain physical characteristics, such as a maximum firing frequency, thermal response times, etc. These physical characteristics and limitations may be associated with the electronics, heaters, ink flow channel geometries, ink, etc. The present invention optionally selectively adjusts the trajectory speed of carrier  20 , without adjusting the firing rate (e.g., 8,000 hertz) or the nozzle firing order, to provide a different address space without increasing the stress level, either thermally or electrically, upon the printhead electronics or power supply. When adjusting the trajectory speed of carrier  20 , the drive system for ITM  30  is adjusted by a relative amount such that the carrier  20  traverses over ITM  30  at the same fixed angle vector as before (e.g., 6°). 
     Referring to FIG. 5, the trajectory speed of carrier  20  is increased to 29.75 inches per second (in the direction of hypotenuse C in FIG.  3 ). This is a 12% increase in the trajectory speed, as compared to the previous trajectory speed of 26.66 inches per second. The resulting address space shown in FIG. 5 is uniform, but not rectilinear or orthogonal. The address space does provide 100% coverage; therefore, all points on print medium  22  can be addressed using this address space. The address space shown in FIG. 5 is very similar to the original skewed, rectilinear address space shown in FIG. 4, yet provides for a 12% increase in printing speed with only a small loss in perceived image quality or image information. 
     FIG. 6 illustrates an address space wherein the trajectory speed of carriage  20  is increased to 34.0 inches per second, a 28% increase over the original trajectory speed of 26.66 inches per second. The address space shown in FIG. 6 is uniform, but not rectilinear or orthogonal. The resulting address space provides near 100% coverage; therefore, all points on the print media  22  can be addressed. This address space provides near 100% coverage since the printed spots take on an approximate hexagonal packing structure. The address space shown in FIG. 6 can be described as a rotated, pseudo-hexilinear address space. 
     The trajectory speed of carrier  20  can be further increased to alter the address space on ITM  30  and print medium  22 . For example, the trajectory speed of carrier  20  can be increased to 39.0 inches per second (a 46% increase) or 53.33 inches per second (a 100% increase). At this latter trajectory speed of carrier  20 , the resulting address space no longer provides 100% addressability or coverage on ITM  30  or print medium  22 . This address space can be used in a draft printing mode, and is referred to herein as a draft skewed rectilinear address space. 
     As is apparent from each of the address spaces illustrated and described above in FIGS.  2  and  4 - 6 , the ink dots are placed at a fixed angle vector relative to each other, regardless of the selected trajectory speed of carrier  20 . This in turn means that the image formed on print medium  22  is slanted at the same fixed angle vector. In other words, the resulting address space does not match pixel for pixel with typical computer raster formats. The present invention also skews the input bitmap image to reduce the effect of the slanted fixed angle vector caused by printing with ITM  30 . 
     More particularly, referring to FIG. 7, an input bitmap image formed in the electrical processing circuitry of the ink jet printer is illustrated. The input bitmap image is skewed by an angle equivalent to the helical or fixed angle vector associated angle vector may also be expressed as a ratio of 1:10. Using this ratio of the fixed angle vector, the input bitmap image is “pre-skewed” before it is mapped to the final address space. In the example of FIG. 8, the original image is shifted up one pixel for every 10 pixels (or at another appropriate rate depending upon the fixed angle vector). In other words, one or more of the columns are skewed 1 pixel for every 10 pixels in the cross direction. This working image is still fundamentally a square rectilinear grid. Therefore, it still lends itself to typical computer stored structures and manipulations. 
     With respect to the cylindrical ITM  30  shown in FIG. 2, the skewing of the bitmap image as described above with reference to FIG. 7 occurs in a direction which is opposite to the direction of rotation  34  of ITM  30 . By skewing the bitmap image in a direction opposite to the direction of rotation of ITM  30 , the net effect is that the fixed angle vector associated with the address space is compensated to provide an improved print quality. 
     FIG. 8 illustrates placement of ink dots on the address space of ITM  30  and print medium  22  after the skewing operation is carried out as described above with reference to FIG.  7 . In the embodiment shown, the letter “P” is printed on the address space. Normally, as show in the left hand block, the top of the letter “P” would continue in a downwardly angled direction corresponding to the fixed angle vector associated with printing the address space on the rotating ITM  30 . However, as is apparent in the right hand block, the printed pixel locations within each column in the array of pixel locations are shifted upwardly at a proportionate ratio to the fixed angle vector. In this example, the pixel locations within each column are shifted upwardly a distance of 1 pixel for every 10 pixels in a lateral direction. Keeping in mind that the actual ink dot is approximately {fraction (1/600 )} inch, this provides an improved print quality to the printed image. 
     FIG. 9 illustrates a flow chart for the inventive method of operating an inkjet printer as described above. At block  42 , the image data is mapped to a bitmap image within electrical processing circuitry in the ink jet printer. This generally consists of an array of square or rectangular pixels, with one or more image data associated with each pixel. For example, the printer may be a monochrome or a color printer, in which case one or more inks would typically be used to print the image, respectively. At block  44 , the bitmap image is skewed corresponding to the fixed angle vector associated with placing the image on rotating ITM  30 . This is carried out by skewing the pixel locations at a ratio corresponding to the fixed angle vector in a direction opposite to the direction of rotation of ITM  30 , as described above with reference to FIG.  7 . The image may be optionally resized by proportionally adjusting the scan speed of carrier  20  and the rotational speed of ITM  30 , as described above with reference to FIGS. 4-6 (block  46 ). At block  48 , the digital image may be optionally manipulated using a halftoning technique, as is known. For reference to halftoning techniques in general, reference is hereby made to U.S. Pat. Nos. 6,363,172; 6,356,363; and 6,307,647, which are owned by the assignee of the present invention and incorporated herein by reference. At block  50 , the image data is mapped to the data stream used to fire the ink jetting heaters within the printhead carried by carrier  20 . 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.