Printing system

Abstract: A swath printing system (210) such as multi-colour inkjet printing which uses lower resolution printheads (120-126) 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) 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 colour 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.

Description: 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. Patent No. 5541625 issued 30 July 1996 for METHOD FOR INCREASED PRINT RESOLUTION IN THE CARRIAGE SCAN AXIS OF AN INKJET PRINTER; U.S. Patent No. 5469198 issued 21 November 1995 entitled MULTIPLE PASS PRINTING FOR ACHIEVING INCREASED PRINT RESOLUTION; and U.S. Patent No. 5535307 issued 9 July 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 labelled for wide format printers/plotters. An increased colour resolution printer from Epson was introduced which provided some colour depletion after increasing addressable resolution to 720 x 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. US-A-5,649,071 and its earlier-published equivalent JP 08116464 A disclose a printing system and method employing a depletion mask which is capable of eliminating excessive ink .drops in area fills. Continuous tone separation signals representing a plurality of separation colour images are used to derive an estimate of marking material coverage. This estimate is then used in association with a random number generator to turn off pixels during printing. EP-A-0687565 relates to. a printing system and method which provides ink jet recording with high resolution. Embodiments of the invention involve the thinning of recording data in non-edge portions of the image. When multiple colour printing is undertaken, the treatment depends on whether an edge portion is an edge portion in monochrome or an edge portion as the recording area. The present invention seeks to provide an improved printing system. According to a first aspect of the present invention, there is provided a swath printing system as specified in claim 1. According to a second aspect of the present invention, there is provided a method as specified in claim 6. The preferred embodiment can provide a faster increased resolution higher resolution print technique which includes improved dot depletion to assure better print quality for a wider selection of media. The preferred embodiment provides a swath printing system such as multi-colour 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) axes. Increased resolution is achieved in the X axis in a first embodiment by printing on a first carriage pass a first set of colour 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 this embodiment 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 an 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 in 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 preferred feature 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. An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: Fig. 1A is a perspective view of a large format inkjet printer/plotter capable of incorporating the features of the preferred embodiments; Fig. 1B is a close-up view of the carriage mechanism of the printer/plotter of Fig. 1A; Fig. 2 shows a presently preferred embodiment of a print cartridge arrangement in a scanning carriage wherein four separate print cartridges for yellow, cyan, magenta and black ink can be utilized to generate higher resolution X/Y axis addressability from lower resolution printheads; Fig. 3 shows another embodiment of a print cartridge arrangement in a scanning carriage where one separate print cartridge for black ink along with a tri-compartment print cartridge for cyan, magenta, and yellow ink can be utilized to generate the higher resolution addressability; Fig. 4 shows an additional embodiment of a print cartridge arrangement in a scanning carriage where a first tri-compartment print cartridge for yellow, light magenta, and dark magenta ink along with a second tri-compartment print cartridge for black, light and dark cyan ink can be utilized to generate the higher resolution addressability; Fig. 5 shows an additional embodiment of a print cartridge arrangement in a scanning carriage where two separate print cartridges for each of yellow, cyan, magenta and black inks can be utilized to generate the higher resolution addressability; Fig. 6 shows a further embodiment of printhead arrangements in a scanning carriage wherein four separate cartridges for different ink colors which are staggered to provide non-overlapping swaths for each color can be utilized to generate the higher resolution addressability; Fig. 7 shows yet another embodiment of a tri-compartment printhead wherein three separate nozzle arrays for different ink colors such as yellow, cyan, and magenta which are staggered to provide non-overlapping swaths for each color can be utilized to generate the higher resolution addressability; Fig. 8 shows still a further embodiment of a tri-compartment printhead wherein three separate nozzle arrays for different ink colors such as black, light cyan, and dark cyan which are aligned to provide overlapping swaths for each color can be utilized to generate the higher resolution addressability; Fig. 9 shows a different embodiment of two separate printheads of a given nozzle resolution such as 300 dpi both having the same color of ink, and being relatively positioned on a scanning carriage to have a small nozzle overlap with no direct nozzle alignment in the direction of the Y axis; Fig. 10 shows the printheads of Fig. 9 being relatively positioned on a scanning carriage to have approximately 50% nozzle overlap; Fig. 11 shows the printheads of Fig. 9 being relatively positioned on a scanning carriage to have almost 100% nozzle overlap; Fig. 12 is a schematic drawing showing a comparison between the theoretical drop size and the actual drop size for typical 300 dpi and 600 dpi ink drops; Fig. 13 shows an embodiment of area fill depletion mask which can be used when oversized color ink drops are applied to an increased resolution addressable grid; Fig. 14 is a schematic drawing showing four virtually identical 300 dpi nozzle arrays as used in the print cartridge arrangement of Fig. 2; Fig. 15 shows a presently preferred embodiment of a multipass print mask as used with the area fill depletion mask of Fig. 13; Fig. 16 is a schematic drawing showing the amount of media advance for a preferred four-pass increased resolution addressable print mode; Fig. 17 is an enlarged schematic representation showing and area fill of magenta ink made with the print masks of Figs. 13 and 15 for 300 dpi drops on a 600 x 600 addressable grid, as compared to an area fill of black ink made with undepleted 600 dpi drops on the same 600 x 600 addressable grid; Figs. 18-19 are self explanatory flow charts showing the preferred depletion techniques; Figs 20 and 21 show the depletion masks for horizontal and vertical edges, respectively; Fig. 22 is a comparison of lines made with 600 dpi size non-depleted drops and 300 dpi size depleted drops on a 600 x 600 dpi grid; Fig. 23 shows the 300 dpi dot size on a 600 x 600 dpi grid; Figs. 24-29 are self-explanatory schematic representations of the various steps of the preferred depletion techniques; Fig. 30 is a self-explanatory flow chart for the representations of Figs. 24-29, and Figs. 31 and 32 show a thickened line on a 600 x 600 dpi grid before and after depletion, respectively. 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 230 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 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 preferred embodiment 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 198 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 facilitate 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 x 300 dpi grid, as well as for a 600 x 600 dpi grid. The problem of too much ink is especially magnified when printing 300 dpi size ink drops on a 600 x 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 preferred embodiment 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 modes 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 x 600 addressable print modes, 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 x 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 Fig.s 31 and 32 show how the depletion techniques are applied to a thickened line. The subject matter of this application is related to the applicant's copending European applications, numbered 0837421, 0837423, 0837424 and 0837425.

Claims: A swath printing system (210) where a scanning carriage (102) carries printheads along a carriage scan axis over media (230) which is periodically advanced through a print zone in a media advance direction (201), comprising: a plurality of printheads (120-126), providing different ink colours for application of different colour ink drops to the media, each printhead including an active nozzle swath width and a given nozzle resolution based on the distance separating adjacent nozzles as measured along the media advance direction; media advance means for moving the media in the media advance direction through the print zone a predetermined distance after completion of a printing swath by the carriage in the direction of the scan axis; and ink application control means for controlling the amount of each different colour ink applied to the media and employing depletion masks to eliminate excessive ink drops in area fills, characterised in that ; said ink application control means employs a set of different depletion masks independently but according to the same depletion rules to each different colour ink, so that each colour ink is depleted without having to change said different depletion mask. The printing system of claim 1 wherein said depletion mask includes a first masking pattern applicable to area fills of the figure and a second masking pattern applicable to edges of the figure. The printing system of claim 2 wherein said second masking pattern includes a horizontal edge masking pattern and a vertical edge masking pattern. The printing system of claim 2 or 3 wherein said second masking pattern includes a corner edge mask. The printing system of any of claims 2 to 4 wherein said second masking pattern includes a slanting edge mask. A method of controlling distribution of different colour drops applied by an inkjet printer (210) to an addressable pixel grid, comprising the steps of: separating a figure to be printed into a plurality of different colour planes corresponding to the different colour ink drops; and applying a set of different depletion masks independently but according to the same depletion rules to each different colour plane, so that each colour plane is depleted without having to change said different depletion masks, said different depletion masks including an area fill mask, a horizontal edge mask, and a vertical edge mask. The method of claim 6 wherein said different depletion masks further include a corner edge mask. The method of claim 6 or 7 wherein said different depletion masks further include a slant edge mask.

Inventors: ALFARO VICTOR, ALFARO, VICTOR
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1997
Main classification: G06K15
IPC codes: B41J2, G06K15