Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
2002-04-30
2004-07-20
Nguyen, Lamson D (Department: 2861)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
C347S041000, C347S015000
Reexamination Certificate
active
06764162
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to printing equipment and is particularly directed to ink jet printers of the type which uses multi-pass printing, called shingling, to form bitmap images of full intended coverage. The invention is specifically disclosed as a shingle mask that is derived from a shingle mask density distribution which exhibits a substantially trapezoidal shape, and thereby reduces banding effects by effectively increasing a number of printed-density bands which are correspondingly decreased in size, while at the same time not increasing the number of printhead passes over a given area on the print media, and thus not negatively impacting printed throughput.
BACKGROUND OF THE INVENTION
Banding is currently the primary defect in ink jet printing. Without banding, existing ink jet printing technology can easily achieve quality comparable to conventional photography. Typically, ink jet printers approach “photo quality” by using multi-pass printing. As the name implies, such printing makes multiple passes of the printhead, rather than the ordinary single pass printing. Each printing pass sub-samples the image by using a special “shingle mask” or “print mask.” The sub-sampling, or “shingle mask,” distributes the location errors of the individual ink drops caused by nozzle or nozzle firing abnormalities or other system errors. Such misplaced drops are blended with other normal ink drops, making the misplaced drops more difficult to detect. Multi-pass printing also increases the banding frequency, which makes the banding less visible and less objectionable to human visual systems. Therefore, the larger number of passes made using the multi-pass process, the better the print quality can be. However, increasing the number of passes involves a substantial penalty in throughput.
Various methods for designing a shingle mask to “average out” printing defects and to suppress banding are disclosed in existing patent documents. For example, Hewlett-Packard owns a number of patents involving using some type of print mask to reduce print artifacts, including banding-type artifacts. One of these patents is U.S. Pat. No. 5,992,962 (by Yen), which discloses a print mask used for ink jet printers that is designed to reduce print artifacts, both to reduce banding and print ink migration. The Yen invention reduces the banding by using multi-pass printing (also known as “shingling”), and states that the earlier prior art print masks had provided checkerboard patterns. In Yen, the print mask provides triangular clusters that are complimentary from the first pass to the second pass of printing. The primary example of the triangular clustered patterns used in Yen is illustrated on FIG. 6 (of Yen), in which the top row of one print pass is all dots, while that same top row in the second pass would be all non-dots. In the first pass, the top row is divided up into 4×4 tiles, and the dots in the top row represent the base of the triangle (per tile). Yen describes the complimentary print masks as being asymmetric, and also provides the benefits of turning off one of the top or bottom nozzles in each of the passes, which further helps to reduce banding artifacts. In addition to the above reduction of banding artifacts, the Yen patent describes “muffling” one or more nozzles of a first mask matrix in situations where a defective nozzle is determined, and that nozzle is then disabled in the first print pass. Then a complimentary nozzle is enabled in the second print pass to print all of the dots that would have been printed in the first pass by the defective nozzle.
U.S. Pat. No. 6,213,586 (by Chen, also owned by Hewlett-Packard) discloses an ink jet printer that produces temporally or spatially shingled images for a multicolor printhead. The example in the Chen patent is for a six-color ink jet printer, in which there are two different shades each of cyan and magenta. Each color has a “deposition mask” that comprises a matrix of threshold values, and each color has a set of “shingle control values.” The deposition masks allow for both temporal and spatial shingling to occur during successive scans of the printhead so as to avoid or reduce image artifacts. The deposition mask is tiled on the bitmap, and the shingle control value set for each color determines whether or not a particular colored dot will be placed on a particular scan (or pass) of the printhead. The only clear example as to how this spatial or temporal shingling is supposed to reduce image artifacts is described on column 4 starting at line 46, where it states that the threshold values in each deposition mask are arranged to assure that the color intensities and amounts of ink deposited at swath extremities “closely match” between succeeding swaths.
U.S. Pat. No. 4,999,646 (by Trask, also owned by Hewlett-Packard) discloses a method for enhancing the uniformity and consistency of ink jet dot formation. This patent uses multiple pass complimentary dot patterns to minimize many undesirable characteristics, including coalescing, beading, and color bleed. Trask uses a partial overlap between multiple passes that use complimentary dot patterns. The dot spacing in coincident dot rows within the overlapped portions is alternated between dots of the first and second patterns. In addition to the above, Trask uses an “improved dot-next-to-dot” super pixeling to further optimize ink drop drying conditions to produce optimized uniformity and consistency of dot formation. The partial dot overlap process alleviates print quality problems in three ways: (1) a 50% checkerboard or other overlap pattern is chosen to minimize interactions between individual drops while they are drying; (2) the 50% dot pattern overlap of two swaths breaks up the horizontal drying patterns and minimizes banding; and (3) the use of alternating nozzles in the overlap dot rows minimizes the impact of nozzle variations.
Another U.S. patent in this area is owned by Colorspan Corporation, U.S. Pat. No. 5,790,150 (by Lidke), which discloses a method for controlling an ink jet printer in multipass printing. The pixel locations for each pass are controlled so that no pixel on a particular pass is orthogonally or diagonally adjacent to any other pixel location that is to be printed in the same pass. At least four passes are made on the print media before the printhead is advanced to a new swath. This also means that no pixel (dot) location is immediately adjacent to any other pixel (dot) location being printed on that pass.
Many conventional ink jet printers use a swath-by-swath approach and this approach typically causes various defects to appear periodically across the page, and is commonly known as the “banding defect.” There are typically two types of density variations that comprise banding defects: high-frequency variations and low-frequency variations. Hi-frequency errors are generally caused by location errors in the placement of individual ink drops, probably originated from the printhead. Low-frequency density errors have a variety of causes, including halftone moiré patterns, alignment errors, and color difference caused by ink printing order changes. High-frequency density variations are the subject of conventional shingle mask designs. However, low-frequency density variations contribute substantially to the overall banding defects present in high-throughput printing methods that reduce the number of passes. The present inventors have conducted a study from which it was found that the human visual system is more sensitive to square wave variations than to other types of smooth variations of the same density contrast. It would therefore be advantageous to produce a smooth “banding profile,” that will tend to reduce the visibility of banding defects.
Multi-pass printing on carriage-based printing technologies (e.g., used for ink jet printers) helps to relieve print defects created by mechanical tolerances such as banding and pel location error. Currently a bitmap is divided into swathes of information. Each swath contains a portion of the
Biddle Mary Ellen
Cooper Brian Edward
Cui Chengwu
Goble Buford Rodney
McFarland Ross William
Gribbell, LLC Frederick H.
Nguyen Lamson D
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