Multiple pass printing for achieving increased print resolution

Abstract: Raster data are depleted to provide for faster printing with a printed dot size that is larger than would otherwise be utilized with the undepleted raster data, on a pixel grid having the resolution of the undepleted raster data. Raster data at a particular resolution (e.g. 300 dpi) are scaled up in such a manner that the upscaled raster data include only printed pixels that correspond one-to-one with the printed pixels of the original data and wherein the relative locations of the printed pixels of the original data are maintained in the up scaled raster data. A pixel array having a particular resolution can be printed with a print element array having print elements disposed for printing at a lesser resolution.

Description: The subject invention is generally directed to swath type printers, and more particularly to apparatus and techniques for reducing pixel density to allow for higher resolution printing with print elements designed for lower resolution printing. A swath printer is a raster or matrix type printer that is capable of printing a plurality of rows of dots in a single scan of a movable print carriage across the print media. The possible locations for dots that can be printed by a raster printer can be represented by an array or grid of pixels or square areas arranged in a rectilinear array of rows and columns wherein the center to center distance or dot pitch between pixels is determined by the resolution of the printer. For example, if a printer is capable of printing 300 dots per inch (dpi), the dot pitch of the pixel array would be 1/300th of an inch. The print carriage of a swath printer typically includes a plurality of printing elements (e.g., ink jet nozzles) displaced relative to each other in the media motion direction which allows printing of a plurality of rows of dots. Depending upon application, the separation between the printing elements in the media scan direction can correspond to the dot pitch for the highest or finest resolution at which the printer can operate (e.g., 1/300th of an inch for 300 dot per inch (dpi) resolution). The printing elements of a swath printer are commonly implemented in a printhead such as a thermal ink jet printhead that is integral to a replaceable ink jet printhead cartridge. The quality of the printed images produced by a raster printer depends to large degree on the resolution of the printer. Higher or finer resolution wherein the printed dots are more closely spaced produces provides for higher quality images. A consideration with increasing the resolution of ink jet printers is that increased resolution requires more printed dots per unit area, with the number of dots increasing by the product of the increase factor along each dimension. For example, doubling print resolution from 300 dpi to 600 dpi results in four times as many dots per unit area. Since the number of dots per unit area increases with resolution, the size of each printed dot must decrease with increased resolution in order to avoid saturating the print media. For example, if a 300 dpi ink jet cartridge were utilized to print at 600 dpi, the amount of ink per unit area would increase by a factor of four, just as the number of dots increased. However, the design and implementation of higher resolution ink jet cartridges with reduced dot size may be impractical, and moreover the use of reduced dot size ink jet cartridges may also present difficulties in printing at reduced pixel densities that would be conveniently utilized for increased throughput in those situations where the reduced pixel densities would acceptable, such as drafts. It would therefore be an advantage to provide a thermal ink jet printer that processes raster data so that it can be printed with ink jet cartridges designed for a resolution that is lower than the resolution of the raster data to be printed. The foregoing and other advantages are provided by the invention in a printing procedure wherein an ink jet cartridge having a dot pitch at a particular resolution (e.g., 300 dpi) is utilized to provide printing on a grid having twice the resolution (i.e., 600 dpi for the particular example of a 300 dpi cartridge). U.S.4,401,991 discloses a method for printing the rows of the pixel array comprising the steps of: (A) advancing the print media to align a plurality of the nozzles of the nozzle array with a first plurality of alternating pixel rows; (B) moving the carriage in the direction of the carriage scan axis and printing the first plurality of alternating pixel rows; (C) advancing the print media in the direction of the media scan axis to align a plurality of the nozzles with a second plurality of alternating pixel rows including pixel rows interposed between the rows of the first plurality of rows; and (D) moving the carriage in the direction of the carriage scan axis and printing the second plurality of alternating pixel rows. whereby a plurality of adjacent pixel rows are printed in an interleaved manner. The method according the invention is set out in claim 1. The advantages and features of the disclosed invention will readily be appreciated by persons skilled in the art from the following detailed description when read in conjunction with the drawing wherein: FIG. 1 is a schematic perspective view of the major mechanical components of a thermal ink jet printer employing the disclosed print techniques. FIG. 2 is a schematic side elevational sectional view illustrating the relation between the downwardly facing ink jet nozzles and the print media of the printer of FIG. 1. FIG. 3 is a schematic plan view illustrating the general arrangement of the nozzle arrays of the printhead cartridges of the printer of FIG. 1. FIG. 4 is a detail view of a positionally adjustable printhead cartridge retaining shoe of the swath printer of FIG. 1. FIG. 5 is a detail view illustrating an example of a cam actuating mechanism for adjusting the position adjusting cam of the positionally adjustable printhead cartridge retaining shoe of FIG. 4. FIG. 6 is a simplified block diagram of a printer control system for controlling the swath printer of FIG. 1. FIG. 7 schematically illustrates a pixel array on which dots can be selectively printed by the printer to form a printed image, and further schematically illustrates printed dots of different sizes. FIG. 9 is a table that illustrates the sequence in which pixel rows are printed with a nozzle array having a nozzle pitch that is twice the pitch of the pixel rows. FIG. 10 sets forth a flow diagram of a printing procedure for printing a pixel array with a nozzle array having a nozzle pitch that is twice the row pitch of the pixel array. In the following detailed description and in the several figures of the drawing, like elements are identified with like reference numerals. Pursuant to raster data depletion, a higher print resolution is achieved with a printed dot size that is utilized for lower print resolution, and faster printing can be achieved while maintaining the print resolution for the depleted raster data. According to a technique of increased resolution printing a high resolution image raster is printed with a cartridge designed for printing at a lower resolution. Referring now to FIG. 1, set forth therein is a schematic frontal quarter perspective view depicting, by way of illustrative example, major mechanical components of a multiple printhead ink jet printer in which the techniques of the invention can be implemented. The printer includes a movable carriage 51 mounted on guide rails 53, 55 for translational movement along the carriage scan axis (commonly called the Y-axis in the printer art). The carriage 51 is driven along the guide rails 53, 55 by an endless belt 57 which can be driven in a conventional manner, and a linear encoder strip 59 is utilized to detect position of the carriage 51 along the carriage scan axis, for example in accordance with conventional techniques. The carriage 51 supports first and second cartridge retaining shoes 91, 92 located at the front of the carriage for retaining substantially identical removable first and second ink jet printhead cartridges C1, C2 (sometimes called "pens," "print cartridges," or "cartridges"). FIG. 1 shows the cartridge C2 in a removed condition, while in FIG. 5 shows the cartridge C2 in its installed position. As depicted in FIG. 2, the printhead cartridges C1, C2 include downwardly facing nozzles for ejecting ink generally downwardly to a print media 61 which is supported on a print roller 63 that is generally below the printhead cartridges. For reference, the print cartridges C1, C2 are considered to be on the front of the printer, as indicated by legends on FIG. 1, while left and right directions are as viewed while looking toward the print cartridges, as indicated by labelled arrows on FIG. 1. By way of example, the print media 61 is advanced while printing or positioning so as to pass from beneath the cartridge nozzles toward the front of the printer, as indicated on FIG. 2, and is rewound in the opposite direction. A media scan axis (commonly called the X-axis) as shown in FIG. 3 will be utilized as a reference for displacement of the media, as well as a reference for orientation of a line. The media scan axis can be considered as being generally tangential to the print media surface that is below the nozzles of the printhead cartridges and orthogonal to the carriage scan axis. In accordance with prior usage, the media scan axis is conveniently called the "vertical" axis, probably as a result of those printers having printing elements that printed on a portion of the print media that was vertical. Also in accordance with known usage, the carriage scan axis is conveniently called the "horizontal axis". From a practical viewpoint, if the printed output of the printer of FIG. 1 were placed vertically in front of an observer in the same orientation as it would hang down from the print roller 63, a line printed by with a single ink jet nozzle and media movement rather than carriage movement would be "vertical," while a line printed with a single ink jet nozzle and carriage movement rather than media movement. If the print media containing such lines were positioned horizontally in front of an observer, the line that extends away from the observer can be considered vertical by common convention; and the line that extends sideways as to the observer can be considered horizontal by common convention. FIG. 3 schematically depicts the arrangement of the nozzle plates 101, 102 of the first and second cartridges C1, C2 as viewed from above the nozzles of the cartridges (i.e., the print media would be below the plane of the figure). Each nozzle plate includes an even number of nozzles arranged in two columns wherein the nozzles of one column are staggered relative to the nozzles of the other column. By way of illustrative example, each nozzle plate is shown as including an array of 50 nozzles which are numbered as (a,1) through (a,50) starting at the lower end of the nozzle array with nozzles in the left column being the odd numbered nozzles and the nozzles in the right column being the even numbered nozzles, where "a" represents the printhead cartridge number. The distance along the media scan axis between diagonally adjacent nozzles, as indicated by the distance P in FIG. 3, is known as the nozzle pitch, and by way of example is equal to the resolution dot pitch of a dot resolution (e.g., 1/300 inch for 300 dpi) for which the cartridge has been designed. In use, the physical spacing between the columns of nozzles in a printhead is compensated by appropriate data shifts in the swath print data so that the two columns function as a single column of nozzles. The first and second cartridges C1, C2 are side by side along the carriage scan axis and are offset relative to each other along the media scan axis. The second retaining shoe 92 is fixedly secured to the carriage 51, while the first cartridge retaining shoe 91 is pivotally secured to the carriage 51 by a flexurally deformable, torsion bar like support member 93 located at the lower rear part of the retaining shoe 91 near the plane of the nozzle plate of the first printhead cartridge C1 and generally parallel to the carriage scan axis, as shown in FIGS. 4 and 4A. By way of illustrative example, the torsion bar like support member 93 is integrally formed with a backplate 95 of the first cartridge retaining shoe 91 and with a portion of the carriage frame, such that the first retaining shoe 91 is pivotable about a pivot axis PA that passes through the torsion bar like support member 93. The top of the first cartridge retaining shoe 91 includes a cam follower flange 97 that is structurally integral with the back plate 95 of the retaining shoe. As further shown in FIG. 5 the cam follower flange 97 is biased rearwardly against a position adjustment cam 111 by a pair of retaining springs 113 which are connected between the top of the carriage and the top of the first retaining shoe. The adjustment cam 111 is rotatably mounted on a pin 115 on the carriage 51 and is shaped so as to increase the distance between the cam pin 115 and the retaining shoe flange 97 with increased counterclockwise rotation of the cam, as viewed from above. The cam is rotated by a cam lever 117 that is integral with the cam, and is engageable with a right cam stop 119 which limits the clockwise rotation of the cam. Thus, as the cam lever 117 is rotated counterclockwise away from the cam stop 119, the nozzle plate 101 of the first cartridge C1 is rotated downward about the pivot axis PA, which aims the nozzle plate of the first cartridge so that its print area is closer to the print area of the second cartridge along the media scan axis. Rotation of the adjustment cam 111 in the counterclockwise direction as viewed from above effectively positions the first print cartridge C1 closer to the second print cartridge C2. The adjustment cam 111 is controllably moved pursuant to movement of the carriage 51 while the cam lever 117 is engaged against the downwardly extending tab 121a of a conveniently located pivoted adjustment lever arm 121 that can be pivoted so that the tab 121a is in or above the path of the cam lever 117 as the cam lever 117 moves with the carriage 51. As shown in FIG. 5, the cam actuator arm 121 can be in the proximity of one end of the carriage guide rails, and is actuated by an actuating lever 123 that is driven by a cam follower 125 which in turn is controlled by a cam 127 on the output of a stepper motor 129. A bias spring 131 ensures that the cam actuator arm 121 is fully raised when actuated to be in the raised position. It should be appreciated that the cam actuator arm 121 can be controlled by other mechanisms, and that the stepper motor 129 can be used of additional purposes. The use of an actuator arm 121 and carriage displacement relative to the actuator arm 121 for cam adjustment avoids the use a separate servo motor for cam adjustment. In use, the printhead cartridges C1, C2 are mechanically and electronically aligned so that the two printhead cartridges C1, C2 cooperatively function like a single printhead having a single column array of nozzles spaced apart along the media axis by the nozzle pitch P, as disclosed in commonly assigned U.S. application Serial No. 07/785,650, filed October 31, 1991 by Haselby and Nguyen, entitled "PRINT CARTRIDGE ALIGNMENT IN PAPER AXIS," incorporated herein by reference; and commonly assigned U.S. application Serial No. 07/786,326, filed October 31, 1991 by Chen, Corrigan and Haselby, entitled "FAST FLEXIBLE PRINTER/PLOTTER WITH ALIGNMENT CORRECTION," incorporated herein by reference. Referring now to FIG. 6, set forth therein is a simplified block diagram of a control system for controlling the thermal ink jet printer of FIG. 1 in which the techniques of the invention can be implemented. The control system includes an interface 32 which receives print data from a host computer, for example, and stores the print data in a buffer memory 34. A microprocessor controller 36 is configured to process the print data to produce raster data that is stored in a bit-map memory 42a contained in a random access memory (RAM) 42 provided for the use of the microprocessor controller. A read-only memory 44 is also provided as appropriate for the use of the microprocessor controller 36. Processes in accordance with the invention, as described further herein, can be performed by the microprocessor controller 36 in conjunction with look-up tables contained in the read-only memory 44. A print controller 31 transfers portions of the raster data from the bit-map memory 42a to a swath memory 41 and provides swath data to a printhead driver controller 38 which controls printhead drivers 45 that drive the ink firing elements of the ink jet cartridges C1, C2. The print controller 31 further controls a media axis driver motor 33 which moves the print roller 63 pursuant to media motion commands from the print controller 31. A media axis drive motor encoder 35 provides information for the feedback control of the media axis driver motor 33. Similarly, a carriage axis encoder 37 provides feedback information for the feedback control of a carriage scan axis drive motor 39 which positions the ink jet cartridge supporting carriage 51 pursuant to carriage motion commands from the print controller 31. In discussing the printing of dots onto a print medium, the area to be printed is commonly characterized as an array of pixels on which the printer can selectively print dots to form the desired image, and FIG. 7 schematically illustrates of a portion of a two dimensional grid or array of pixels P(1,1) through P(RMAX,CMAX) each of which defines a location on a two dimensional print medium where a dot can be printed. The pixels P are squares arranged in rows and columns in a rectilinear array, wherein each row is represented as extending horizontally and each column is represented as extending vertically, and each pixel P(R,C) has a corresponding location in the bit-map raster memory 42a contained in main RAM 42, such that there is a one-to-one correspondence between locations in the raster memory 42a and the pixel array to be printed. For the particular example of a binary printing, the data for each pixel comprises a binary bit that defines whether or not a dot is to be printed at that pixel. For ease of reference, in this disclosure a pixel bit contains a 1 if a dot is to be printed on the corresponding pixel, but it should be appreciated that a 0 can be utilized to indicate that a dot will be printed, depending upon implementation. By way of reference, the horizontal or row dimension of the pixel array corresponds to the carriage axis, while the vertical or column dimension of the pixel array corresponds to the media axis, and each of the pixels P(R,C) in the grid of FIG. 7 is uniquely identified by an ordered pair of numbers (R,C) wherein R is the row in which the pixel is located and C is the column in which the pixel is located. For convenience, the rows are number 1 through RMAX starting with the top row, and the columns are numbered 1 through CMAX starting with the leftmost column. Thus, the pixel at the top left corner is pixel P(1,1). The number of pixels per unit of horizontal distance is referred to as the horizontal resolution, and is commonly expressed in "dots per inch" or dpi, for example 300 dpi or 600 dpi. The vertical resolution can be similarly expressed. The bit-mapped raster data for the pixel array is logically organized to correspond to the rows and columns of the physical arrangement of the printed pixel array, although in accordance with conventional bit mapping techniques the bit-mapped raster data for an array of pixels does not necessarily have to be in the form of rows and columns. Thus, the schematic representation of the pixel array or grid also represents the bit-map raster data that defines the printed state of each of the pixels in the pixel array. For convenience each of the raster data bits for a pixel array will also be identified as P(R,C) since a pixel necessarily includes an associated one-bit data location in the bit-map memory, and it will be clear from the context as to whether the pixel or the pixel is being referenced. Dots printed at pixel locations generally comprise roughly circular dots generally centered on the pixel locations for which the dots are intended. In known raster printers, the size of the printed dot is selected to be slightly larger than the pixel size, as represented in FIG. 7 by circles 30 on a plurality of horizontally adjacent pixels. A draft mode of printing can be provided wherein raster data at a particular resolution (e.g., 300 dpi) is depleted along the horizontal axis and then printed on a grid that corresponds to that particular resolution with a dot size intended for that resolution. In this manner, for the same ink drop firing frequency as would be utilized with the non-depleted raster data, the carriage that supports the ink jet cartridges can be scanned at a higher rate, which provides for faster printing of the horizontally depleted raster data. Furthermore enhanced mode raster data depletion can be provided wherein the image raster data at a particular resolution (e.g., 600 dpi) is depleted so that the depleted image raster data can be printed on a raster grid having the particular resolution with dots that are larger than would otherwise be utilized with the particular resolution, as represented in FIG. 7 by circles 40 on a plurality of horizontal pixels that are respectively separated from each other by one pixel. By way of illustrative example, raster data for a 600 dpi grid is depleted so that it can be printed on a 600 dpi grid with a dot size intended for a 300 dpi grid. Since a 300 dpi dot has four times the area of a 600 dpi dot, printing a 600 dpi image with 300 dpi dots would result in about four times as much ink in comparison to printing the same image with 600 dpi dots, which would unacceptably saturate the printing medium. For the particular example of printing a 600 dpi image with 300 dpi dots, a technique in accordance with the invention depletes the 600 dpi raster data so that the depleted raster data can be printed on a 600 dpi grid with 300 dpi dots with the appropriate amount of ink. Raster data at a particular resolution (e.g., 300 dpi) can be scaled up to a higher resolution (e.g., 600 dpi) in such a manner that the expanded data does not require depletion for printing at the higher resolution with a dot size intended the lower resolution, and which locates the printed pixels on the higher resolution grid at locations such that the relation between the dots printed at the higher resolution is the same as the relation between the dots that would have been printed at the lower resolution. In other words, the expansion of the raster data to a higher resolution results in the printed pixels being located at the same locations relative to each other as if the unexpanded raster data were printed on the lower resolution grid that corresponds to the unexpanded raster data. This allows raster data at particular resolution to be combined with higher resolution raster data for printing in accordance with the enhanced mode of operation. In accordance with an aspect of the invention, a printing procedure is provided by which an ink jet cartridge having a dot pitch at a particular resolution (e.g., 300 dpi) is utilized to provide printing on a grid having twice the resolution (i.e., 600 dpi for the particular example of a 300 dpi cartridge). Increased Resolution Printing Along Media Axis In accordance with this aspect of the invention, an ink jet nozzle array having a nozzle pitch along the media axis that corresponds to a particular resolution (e.g., 1/300th of an inch for 300 dpi) is utilized to print on a grid having twice the resolution (i.e., 600 dpi which has resolution grid pitch of a 1/600th of an inch for the example of a nozzle pitch of 1/300th of an inch). Pursuant to increased resolution printing, the rows of a pixel array are printed in an interleaved manner wherein each row is printed on a different carriage scan than each of any adjacent rows. By way of simplified example, the table of FIG. 22 identifies the pixel rows that are printed pursuant to a series of carriage scans of a four nozzle array having a dot pitch that is twice the dot pitch of the printed pixel array. The vertical axis identifies the row in the pixel array, and the horizontal axis identifies the scan of the carriage. An X at a particular location indicates that the pixel row indicated by the vertical axis is printed on the carriage scan identified by the horizontal axis. From the table of FIG.9, it should be appreciated that the rows printed on each scan of the carriage are as follows: Scan 1: 1, 3 Scan 2: 2, 4, 6, 8 Scan 3 : 5 , 7 , 9 , 11 Scan 4: 10, 12, 14, 16 For the particular example illustrated by the table of FIG. 9, the nozzle array is initially positioned so that on the first pass one-half of the nozzles print alternating rows starting with the first row. The media is advanced by 5 dot pitches for the second pass, 3 dot pitches for the third pass, 5 dot pitches for the next, and so forth. More generally, for a nozzle array having N nozzles adjacent to each other along the media axis and spaced apart along the media axis by a nozzle pitch P, for the first carriage scan the media is advanced to align the leading M nozzles of the nozzle array with pixel row 1 and succeeding alternating pixels rows through the (2M-1)th pixel row, wherein pitch of the pixel rows along the media axis is P/2 and wherein the M leading nozzles are nozzles first encountered by the advance of the media and M is less than or equal to N. For the second and subsequent carriage scans, the media is advanced by 2(N-M)+1 pixels rows and M-1 pixel rows in an alternating manner. Referring now to FIG. 10, set forth therein by way of illustrative example of increased resolution printing in accordance with the invention. At 611, the print media is advanced to align the leading M nozzles with the first pixel row and succeeding alternating pixel rows up to the (2M+1) the pixel row. At 613, the carriage is scanned to print the pixel rows aligned with the leading M nozzles. At 615 the print media is advanced 2(N-M)+1 pixels rows, and at 617 the carriage is scanned to print pixel rows aligned with the nozzles of the nozzle array. At 619 a determination is made as to whether all of the raster data has been printed. If yes, the procedure ends. If the determination at 619 is no, all the raster data has not been printed, at 621 the print media is advanced by 2M-1 pixel rows, and at 623 the carriage is scanned to print pixel rows aligned with the nozzles of the nozzle array. At 625 a determination is made as to whether all of the raster data has been printed. If yes, the procedure ends. If the determination at 625 is no, all the raster data has not been printed, control transfers to 615.

Claims: A method for printing the rows of a rectilinear pixel array in a raster or matrix type printer capable of printing a plurality of rows of dots in a single scan and having (a) a carriage (51) that is movable along a carriage scan axis, and (b) a thermal ink jet nozzle array (C1,C2,101,102) supported by the movable carriage for printing onto a print media (61) that is selectively movable along a media scan axis, the nozzle array having at least N nozzles adjacent to each other along the media axis and spaced apart along the media scan axis by a predetermined nozzle pitch, the rows and columns of the pixels being respectively aligned with the carriage axis and the media axis, and the pixel array having a pixel row pitch along the media axis that is one-half the predetermined nozzle pitch, the method comprising the steps of: (A) advancing the print media to align the leading M nozzles of the array with the first and succeeding alternating pixel rows to be printed up to the (2M-1)th pixel row, wherein M is less than N and the M leading nozzles are those first encountered by advance of the print media; (B) scanning the carriage and printing the pixel rows aligned with the leading M nozzles of the array; (C) advancing the media by 2(N-M)+1 pixel rows; (D) scanning the carriage and printing the pixel rows aligned with the N nozzles of the array; (E) terminating the print process if all rows of the pixel array have been printed; (F) advancing the media by 2M-1 pixel rows; (G) scanning the carriage and printing the pixel rows aligned with the N nozzles of the array; (H) terminating the print process if all rows of the pixel array have been printed; and (I) repeating steps (C) through (H) until all rows of the pixel array have been printed. The method of Claim 1 wherein the number of nozzles N is an even number. The method of Claim 2 wherein M is equal to N/2.

Inventors: KADONAGA ANNE P, KADONAGA, ANNE P.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1993
Main classification: G06K15
IPC codes: G06K15, B41J2