Method of multi-dot interlace printing

Details Method of multi-dot interlace printing Method of multi-dot interlace printing

2000-02-07

2001-05-29

Nguyen, Thinh (Department: 2861)

Incremental printing of symbolic information

Ink jet

Ejector mechanism

C347S014000

Reexamination Certificate

active

06238037

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ink jet printers, and, more particularly, to a method of printing multiple drops of ink at any pixel location with an ink jet printer.
2. Description of the Related Art
With printers which use a columnar array of print elements or nozzles, a typical mode of operation requires that the column of nozzles be swept horizontally across the paper while the nozzles are selectively fired at points in the bitmap which represent printed pixels. At the end of such a swath, the paper is indexed vertically by the height of the printhead and the printhead is again swiped across the paper. With this process, there are inherent print defects introduced by such things as paper feed inaccuracies and nozzle-to-nozzle variations in drop size or placement. Two swaths of a solid-fill pattern produced by a printhead with twelve nozzles or print elements are shown in
FIG. 1A. A
½-pel positive index overshoot results in a print artifact in the form of a visible light band in the print.
It is known to interlace the swaths as shown in
FIG. 1B
in order to minimize the visible effects of the overshoot. In the first swath of the printhead, a mask is applied to the printed data allowing ½ of the possible pels to be printed. In this case the mask applied is a checkerboard. After printing the first swath, the paper is indexed ½ of the head height and a complement of the previous mask is applied. This second swath completes the application of the pels for the bottom half of the first swath and applies a new set of 50%-fill pels for the bottom half of the second swath. This process is repeated for the rest of the page with half of the pels for any swath masked and indexes reduced to ½ of the head height. This is an example of two-pass binary interlacing or shingling.
As is apparent, the visible effect of the index error caused by the ½ pel index overshoot between every ½ head-height index is significantly reduced.
A deviant dot shape in a single nozzle can also cause a print artifact in the form of a band in the print, as can be seen in a comparison of
FIGS. 2A and 2B
. The printed results with this type of head fault when two-pass shingling and four-pass shingling are applied are shown in
FIGS. 2C and 2D
, respectively. As is apparent with higher shingling ratios, the deviant nozzle's effect is averaged throughout the printed pattern, producing a less noticeable artifact.
The use of binary shingling to produce a desired image is shown in
FIGS. 3A through 3D
. A shingling mask (
FIG. 3A
) of checkerboard 1's and 0's is laid over a bitmap of desired dots to be printed, and a twelve nozzle-high printhead (not shown) is passed over the bitmap. On the first swath (FIG.
3
B), the bottom half of the head (the bottom six nozzles) is passed over the bitmap and every desired pixel that matches a 1 in the shingle mask is printed. As the paper is indexed by ½ of the head height (six nozzles), the shingle mask is complemented and, again, the matching bits are printed (FIG.
3
C). The process is repeated (
FIG. 3D
) until the page is complete. This is termed “two-pass shingling” since two indexes each spanning six nozzles are required to move the paper by the equivalent of the head height of twelve nozzles.
In binary shingling, there are several variations on the algorithms. For example, the number of passes can be increased while decreasing the density of 1's in the mask. Another variation is to keep the mask for the entire head constant but divide the head into sections each with different masks. Primarily, the concept involves comparing a mask against the desired print pattern and altering the masks on each swath such that all portions of the page receive enough masks to allow all pels to be printed. For example, if 16-pass shingling is desired, the masks would have a density of {fraction (1/16)} 1's and there would be 16 masks. This allows all locations to see a mask value of “1” a single time.
The known methods of shingling are used with printers which place, at most, one dot in every pixel location. On these uni-layer printers, the dot size must be large enough to allow full saturation fill if a single dot is placed on every grid location. At 600 dpi, the required dot size to meet this requirement is large enough to be distinctly visible. However, it is desirable to produce images and fill patterns that do not have visually discernable dots. A printer may be designed such that the individual dot-size is reduced to produce a less-noticeable single dot. With such a small drop-size, full saturation fills are not possible with a single layer of drops. Thus, the printer must place multiple drops in at least some of the pixel locations. Further, the printer must use a bitmap of pels that specifies the drop-count at every pixel location, not simply the existence of a drop.
If, for example, a maximum of two ink drops were required at each pixel location, it would not be appropriate, in two-pass mode, to fire all drops of the first layer on the first pass and all drops of the second layer on the second pass. In areas with slightly greater than 50% fill, there may be only a few drops on the second layer while the first layer is fully utilized. This would create an imbalance of printed dots per swath, which would result in undesirable print artifacts, as discussed above. Thus, with multidot bitmaps too, it would be advantageous to use masking or shingling to reduce the conspicuousness of print artifacts.
With the advent of multidot bitmaps, however, it is no longer possible to simply apply a binary mask over the print bitmap. The print bitmap now includes encoded values for each pixel location. These values represent the desired number of drops to be printed at each location.
What is needed in the art is a method of interleaving or shingling which may be used with a multidot bitmap.
SUMMARY OF THE INVENTION
The present invention provides a method of interlace printing which accommodates bitmaps that require multiple ink drops to be placed at at least one pixel location.
The invention comprises, in one form thereof, a method of performing interlace printing on a print medium with a printhead of an ink jet printer. An array of pixel locations is provided on the print medium. Each pixel location has a common maximum number M of ink drops that the pixel location may receive. Each pixel location is assigned a respective desired number D of ink drops which are to be placed at the pixel location. Each desired number D is less than or equal to M. Sets of threshold values are created. A number N of the sets of threshold values is equal to or greater than M. Each of the pixel locations corresponds to one of the threshold values of each set of threshold values. A respective, single ink drop is selectively jetted at each pixel location. Whether the single ink drop is jetted onto a respective pixel location is dependent upon the desired number D assigned to the respective pixel location and the corresponding threshold value of one of the sets of threshold values. The selectively jetting step is repeated N−1 additional times. Each selectively jetting step uses a different one of the sets of threshold values.
An advantage of the present invention is that shingling can be performed with any number of ink drops being placed at any pixel location.
Another advantage is that shingling can be performed with various numbers of passes of the printhead over a given pixel location. Yet another advantage is that the method is simple enough to be implemented in code or hardware.
A further advantage is that the drops are evenly distributed across all of the swaths that make up a printed area For example, in four-pass shingling with a single layer of drops required over an area, the masks applied allow 25% of the pixel locations to be fired on each pass; if two layers of drops are required across an area in four-pass mode, half of the pixels on each layer are allowed to fire on each p

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