Edge enhancement depletion technique for over-sized ink drops to achieve high resolution X/Y axes addressability in inkjet printing

A swath printing system such as multi-color inkjet printing which uses slower resolution printheads of at least 300 dpi nozzle spacing to achieve high resolution output of an least 600 dpi addressability in both the X (media advance) and Y (carriage scan) axes. Synchronized depletion masks for area fills and edge enhancement is provided which is plot independent and prevents drop overlap. It is especially suited for multi-pass print modes since each row has a balanced number of pixels, and there are no rows which have no "on" pixels. The depletion includes separate depletion masks and rules for edge enhancement as compared to area fills. Each color plant is depleted separately, but subject to the same depletion rules and masks. A separate depletion step provides for narrowing the vertical and horizontal dimensions of the figure to be printed.

BACKGROUND OF THE INVENTION
 This invention relates generally to swath printing, and more particularly
 to inkjet printers using separate nozzle arrays for each different color
 ink.
 While much research and development has been directed toward increasing the
 nozzle resolution on inkjet printheads as the best way to improve print
 quality, some benefits can also be obtained through print mode techniques.
 Higher resolution addressability has already been obtained to some degree
 with monochrome print mode techniques. Examples of this are found in U.S.
 Pat. No. 5,541,625 issued Jul. 30, 1996 for METHOD FOR INCREASED PRINT
 RESOLUTION IN THE CARRIAGE SCAN AXIS OF AN INKJET PRINTER; U.S. Pat. No.
 5,469,198 issued Nov. 21, 1995 entitles MULTIPLE PASS PRINTING FOR
 ACHIEVING INCREASED PRINT RESOLUTION; and U.S. Pat. No. 5,535,307 issued
 Jul. 9, 1996 entitled PRINTING OF VARIABLE DOT SIZES DEPENDENT UPON IMAGE
 DENSITY FOR IMPROVED GRAPHICS.
 The monochrome techniques for increased addressable resolution, and the
 related depletion techniques, are not easy to apply to color printing,
 particularly where the goal is to have increased resolution in both the X
 and Y axes (media advance and carriage scan axes, respectively) as
 generally labeled for wide format printers/plotters.
 An increased color resolution printer from Epson was introduced which
 provided some color depletion after increasing addressable resolution to
 720.times.720. This product was unfortunately based on a very low nozzle
 resolution printhead of 90 dpi, which required a slow tedious eight pass
 print mode as well as special media to decrease ink dot gain.
 So there still remains a need for a faster increased resolution higher
 resolution print technique which includes improved dot depletion to assure
 better print quality for a wider selection of media.
 BRIEF SUMMARY OF THE INVENTION
 The invention provides a swath printing system such as multi-color inkjet
 printing which uses lower resolution printheads of at least 300 dpi nozzle
 spacing to achieve high resolution output of at least 600 dpi
 addressability in both the X (media advance) and Y (carriage scan) aces.
 Increased resolution is achieved in the X axis in a first embodiment by
 printing on a first carriage pass a first set of color ink drops onto a
 first group of pixel rows spaced apart at the lower resolution distance
 (e.g., 1/300 inch) and extending longitudinally in the Y axis direction,
 advancing the media to re-position the nozzles, and then printing on a
 second carriage pass a second set of ink drops onto a second group of
 different pixel rows also spaced apart at the lower resolution distance
 and extending longitudinally in the Y axis direction, with the second
 group of pixel rows interlaced between the first group of pixel rows. A
 related feature of the invention is to advance the media in a preferred
 four pass print mode, with even and odd numbered pixel rows being printed
 on alternate swaths based on advancing the media different distances
 between each swath.
 A second embodiment provides increased resolution in the X axis by the
 offset positioning of two printheads of the same color ink with their
 respective nozzles mis-aligned in the Y axis direction so that both the
 aforesaid first and second group of interlaced pixel rows can be printed
 on the same carriage pass, the first group by nozzles of one printhead and
 the second group by nozzles of the other.
 Increased resolution is provided in the Y axis in a first embodiment having
 a single printhead for each color by any of the following:
 1) maintaining the same carriage speed and the same firing frequency for
 the printhead, and printing on a first carriage pass a first set of color
 drops onto a first group of pixel columns spaced apart at the lower
 resolution distance (e.g., 1/300 inch) and extending in the X axis
 direction, and then printing on a second carriage pass a second set of ink
 drops onto a second group of different pixel columns also spaced apart at
 the lower resolution distance and extending to the X axis direction, with
 the second group of pixel columns interlaced between the first group of
 pixel columns;
 2) maintaining the same carriage speed but doubling the firing frequency
 for the printhead, so that both the aforesaid first and second group of
 interlaced pixel columns can be printed on the same carriage pass; or
 3) moving the carriage at half the usual speed, but maintaining the same
 firing frequency for the printhead, so that both the aforesaid first and
 second group of interlaced pixel columns can be printed on the same
 carriage pass.
 Increased resolution is provided in the Y axis in a second embodiment by
 having two printheads for each color offset in the X axis direction so
 that both the aforesaid first and second group of interlaced pixel rows
 can be printed on the same carriage swath.
 A further feature of the invention is the use of a plot-independent
 depletion mask for area fills which improves print quality.
 Synchronized depletion masks for area fills and edge enhancement are
 provided which are plot independent and prevent drop overlap. It is
 especially suited for multi-pass print modes since each row has a balanced
 number of pixels, and there are no rows which have no "on" pixels. The
 depletion includes separate depletion masks and rules for edge enhancement
 as compared to area fills. Each color plane is depleted separately, but
 subject to the same depletion rules and masks. A separate depletion step
 provides for narrowing the vertical and horizontal dimensions of the
 figure to be printed.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION
 A typical embodiment of the invention is exemplified in a large format
 color inkjet printer/plotter as shown in FIGS. 1A-1B. More specifically,
 FIG. 1A is a perspective view of an inkjet printer/plotter 210 having a
 housing 212 mounted on a stand 214. The housing has left and right drive
 mechanism enclosures 216, 218. A control panel 220 is mounted on the right
 enclosure 218. A carriage assembly 300, illustrated in phantom under a
 cover 222, is adapted for reciprocal motion along a carriage bar 224, also
 shown in phantom. The position of the carriage assembly 300 in a
 horizontal or carriage scan axis is determined by a carriage positioning
 mechanism 310 with respect to an encoder strip 320 (see FIG. 1B). A print
 medium 330 such as paper is positioned along a vertical or media axis by a
 media axis drive mechanism (not shown). As used herein the media axis is
 called the X axis denoted as 201, and the carriage scan axis is called the
 Y axis denoted as 301.
 FIG. 1B is a perspective view of the carriage assembly 300, the carriage
 positioning mechanism 310 anmd the encoder strip 320. The carriage
 positioning mechanism 310 includes a carriage position motor 312 which has
 a shaft 314 which drives a belt 324 which is secured by idler 326 and
 which is attached to the carriage 300.
 The position of the carriage assembly in the scan axis is determined
 precisely by the encoder strip 320. The encoder strip 320 is secured by a
 first stanchion 328 on one end and a second stanchion 329 on the other
 end. An optical read 366 is disposed on the carriage assembly and provides
 carriage position signals which are utilized by the invention to achieve
 optimal image resistration in the manner described below.
 Referring to FIG. 2, a carriage 102 is slidably mounted on support bar 172
 through a bearing sleeve 171, and includes four slots 121, 123, 125, 127
 for removably receiving four inkjet print cartridges. From right to left
 in the carriage slots are respectively mounted a black ink cartridge 120,
 a magenta ink cartridge 122, a cyan ink cartridge 124 and a yellow ink
 cartridge 126. Although the invention has been successfully demonstrated
 with four 300 dpi print cartridges of the type shown in FIG. 2 (see also
 FIG. 14), in a currently preferred embodiment the black ink cartridge has
 a 600 dpi nozzle resolution and therefore prints 600 dpi sized drops which
 require no depletion (see the area fill comparison in FIG. 17).
 Referring to FIG. 3, a modified carriage 102a carries a removably mounted
 black ink cartridge 130, and a tri-compartment ink cartridge 132 which has
 separate ink reservoirs 133, 134, 136 for cyan, magenta and yellow ink,
 respectively.
 Referring to FIG. 4, a further modified carriage 102b carries a first
 tri-compartment ink cartridge 140 which has separate ink reservoirs 142,
 144, 146 for yellow, light magenta, and dark magenta ink, respectively. A
 second adjacent tri-compartment ink cartridge 150 has separate ink
 reservoirs 152, 154, 156 for black, light cyan, and dark cyan ink,
 respectively.
 Referring to FIG. 5, another modified carriage 102c has a plurality of
 individual mounting slots 158 for carrying two adjacent yellow ink
 cartridges 160, 161, two adjacent cyan ink cartridges 162, 163, two
 adjacent magenta ink cartridges 164, 165, and two adjacent black ink
 cartridges 166, 167.
 Referring to FIG. 6, a different arrangement of staggered color ink
 printheads C1, C2, C3 and C4 are shown for cyan, magenta, yellow and black
 ink, respectively. The cartridge C4 can be aligned with either the magenta
 or cyan cartridge, or can be separately positioned at position 168 to be
 non-overlapping with the other print cartridges. The nozzle resolution P1
 can be the same as P2 as shown in the drawing (e.g. 300 dpi), or can be
 600 dpi (not shown in this drawing).
 The tri-compartment nozzle plate 175 in FIG. 7 shows three nozzle arrays
 176, 177, 178 for yellow, cyan, and magenta ink, respectively, which are
 staggered in order to print separate swaths during a single pass. The
 tri-compartment nozzle plate 180 in FIG. 8 shows three nozzle arrays 181,
 182, 183 for black, light cyan, and dark cyan ink, respectively.
 FIGS. 9 and 10 show two identical 300 dpi printheads 185, 186 for the same
 color ink being positioned in different overlapping nozzle positions, but
 with no direct alignment in the Y axis direction between any nozzles in
 either printhead. This enables the possibility of printing on pixel rows
 which are 1/600th inch apart on a single carriage pass. The schematic of
 FIG. 11 shows the relationship between a nozzle pitch 188 of the 300 dpi
 for adjacent nozzles on the same printhead, as compared to a nozzle pitch
 189 of 600 dpi for adjacent nozzles on different printheads.
 The non-aligned nozzles of FIGS. 9, 10 and 11 are contrasted with the
 aligned nozzles of the four identical 300 dpi printheads 190, 191, 192,
 193 of FIG. 14. By printing with only 48 of the 50 nozzles, the remaining
 nozzles at one end or the other of the printhead can be turned "off" in
 order to facilitat the best possible alignment (see arrows 195, 196). The
 printhead arrangement of FIG. 14 has been implemented in combination with
 the area fill depletion mask 197 in FIG. 13 in order to print the depleted
 area fill of magenta drops schematically shown in FIG. 17.
 The schematic drawing of FIG. 12 shows the size difference between
 theoretical and actual ink drops for a 300.times.300 dpi grid, as well as
 for a 600.times.600 dpi grid. The problem of too much ink is especially
 magnified when printing 300 dpi size ink drops on a 600.times.600 dpi
 grid. For this reason, it is very desirable to develop an improved
 depletion mask. Prior monochrome depletion techniques often provided
 inconsistent results because they were based in part on the external shape
 of a figure to be printed. In contrast, the present invention provides a
 plot independent mask as shown in FIG. 13. This is the basic grid which is
 preferably used for area fills, and it extends over the entire plot area
 to provide unique synchronized depletion. There is no drop overlap and
 this depletion mask has pixels "on" in every row. Both even and odd rows
 have 25% of the original amount of ink prior to depletion, so that
 interleaving print mods will work in a perfectly balanced way. Moreover,
 this depletion mask follows the periodic "on/off" firing mask 197 which
 enables the printheads to maintain the same firing frequency
 (approximately 5.5 KHertz) while at the same time maintaining the same
 scanning speed for the carriage (approximately 18.33 inches/sec.).
 In that regard, the four-pass cluster mode shown in FIG. 15 increases print
 quality and provides improved optical density. It is related to the media
 advance shown in FIG. 16 wherein the printheads alternate between printing
 on even and odd numbered rows (49 rows, 47 rows, etc.).
 It will be understood that the various arrangements and embodiments of
 color printheads shown in the drawings can all be used to achieve the
 benefits of the increased 600.times.600 addressable print modes of the
 present invention, all without the need for an excessive number of nozzle
 passes over the same printing area on the media.
 It will further be understood that the synchronized depletion mask assures
 balanced and predictable ink depletion, independent of the shape or color
 configuration of the figure(s) to be printed, thereby improving color
 print quality.
 FIGS. 18-19 show the separation of figures into separate color planes
 before depletion, and also show the separate depletion techniques applied
 to area fills as compared to edge enhancement.
 The horizontal edge depletion mask of FIG. 20 shows the locations 206 for
 the area fill "on" pixels, and the vertical edge depletion mask of FIG. 21
 also shows such locations 207.
 The non-depleted pixel separations in FIG. 22 are shown at 208, while a
 depleted pixel separation is shown at 209.
 Dot next to dot secondary colors 213, 215 as well as dot on dot secondary
 colors 217 are shown in FIG. 23, on a 600.times.600 dpi grid, both present
 problems of excessive ink.
 The illustrative drawings of FIGS. 24-29 show what happens when all the
 depletion rules and masks are applied to a figure, and the flow chart of
 FIG. 30 is a self-explanatory sequential step by step visual explanation
 thereof.
 Finally, the drawings of FIGS. 31 and 32 show how the depletion techniques
 are applied to a thickened line.
 While exemplary and preferred embodiments of the invention have been shown
 and described, it will be appreciated by those skilled in the art that
 various modification and revisions can be made without departing from the
 spirit and scope of the invention as set forth in the following claims.