Abstract:
The velocity of a printhead scanning across a sheet of paper or other print medium is varied on the basis of whether masking patterns for selectively activating nozzle of the printhead include activations in consecutive pixel locations of the masking pattern. The pixel locations are “firing opportunities” as defined by the maximum firing frequency assigned to the nozzles when the printhead is scanned at a particular velocity. The printhead is moved at that velocity when a masking pattern includes activation in consecutive pixel locations. On the other hand, a higher scanning velocity is available when a constraint is enforced to prevent occasions of firings in consecutive pixel locations. In one embodiment, the constraint is not enforced, but masking patterns are analyzed on a pattern-by-pattern basis to detect those patterns which satisfy the constraint, so that the scanning velocity can be increased during the suitable masking patterns.

Description:
BACKGROUND ART 
     There is a wide variety of known printing systems. A printing system may be self-contained or may be one that requires cooperation of two or more units, such as a computer printer that is controlled using drive software installed on a personal computer. Print material, such as ink, may be deposited upon a sheet of paper or other print medium by sequential movements of the depositing structure relative to the sheet. As one well known example, an inkjet printhead may be repeatedly scanned across a sheet of paper to apply ink in a series of swaths, until the composite image is formed. 
     Referring to  FIG. 1 , an example of a printer  10  is shown. The printer includes a body  12  and a hinged cover  14 . An inkjet printhead  16  is attached to a carriage  18  that moves bidirectionally along a carriage transport rail  20 . A flexible cable  22  connects the components of the print carriage to a print engine, not shown. The flexible cable includes electrical power lines, clocking lines, control lines, and data lines. Nozzles of the inkjet printhead are individually activated to project droplets of ink onto a print medium delivered from a supply  24 , such as a tray of paper. During each print operation, the print medium is stepped in one direction, while the inkjet printhead is moved along the transport rail in the perpendicular direction. 
     In the design of a printing system, a number of factors are considered to be significant. These factors include cost, speed, and print quality. A concern is that there is a tradeoff among these factors, particularly when a printer is designed to provide photo-quality printing. The inks of an inkjet printer  10  are water-based and are delivered to the medium as droplets. The quality of an image is dependent upon the consistency of droplet development at the printhead, the accuracy of delivery, and the droplet cooperation at the print medium. 
     Inkjet printing may be considered to be a droplet-on-demand (DOD) technology. Techniques for forming the droplets include thermal activation and piezoelectric pumping. Regardless, sufficient time between two activations of a single nozzle must be provided, if a sufficient volume of ink is to be accumulated for consistency in firing. Thus, a maximum “firing frequency” is enforced. For any particular nozzle of an inkjet printhead, this firing frequency limits the firing opportunities of the nozzle for a given period of time. Merely for the purpose of example, the firing frequency may be set at 12,400 activations per second. 
     At the print medium, there are concerns with “bleeding” and other phenomena. Bleeding of one color into another color is most detectable along edges of sharp color contrast within an image. Printers use a multi-pass concept to reduce the likelihood of bleeding and to provide compensation for other phenomena that affect image quality. Using the multi-pass concept, less than all of the droplets are deposited on a single pass over a particular area of the print medium. Each area of the print medium is scanned multiple times in order to deposit all of the droplets. The portion of the droplets which are deposited on a particular pass is controlled by a predefined masking pattern. As defined herein, a “masking pattern” is associated with a single pass, although multiple passes may be necessary in order to complete the printing. This use of the term is consistent with U.S. Pat. No. 6,310,640 to Askeland. In a multi-pass process, there is a “composite masking pattern” to which the print data is applied in defining droplet deposition. The composite masking pattern provides the basis for the individual masking patterns. Typically, the composite masking pattern is determined at the design stage for a particular printer. In printing two photographs, the composite masking pattern is applied in the same manner to the image data of the two photographs, so that it is the difference in the image data (from photograph to photograph) that causes differences between the two series of masking patterns. 
     The determination of which droplets are to be deposited on a particular pass includes a degree of randomness. In general, artifacts are more apparent when masks are more regular and uniform. While the generation of printing masks involves significant randomization, it is known to apply restrictions to such pattern generation. U.S. Pat. No. 6,250,739 to Serra describes some possible restrictions. A checkerboard pattern may be imposed on each masking pattern. Each image region is divided into distinct complementary patches. Bleeding among droplets placed in adjacent pixels of a composite image is less likely to occur, since horizontally neighboring pixels do not receive droplets in the same pass. However, bleeding may occur between diagonal pixels. Serra states that this can be overcome using the “Hickman system” in which printing in intervening lines and in intervening columns is presented in a single pass. The patent states that a concern with this system is that it forfeits the ability to print in the intervening lines and columns even with respect to printmodes in which bleeding and similar problems are absent, such as in a single-pass mode for printing black-and-white text. Serra describes bidirectional scanning printheads which discharge color-ink droplets at ultra high resolution, with each swath of printing on the paper being completed in either eight passes with four paper advances, or four passes with two paper advances, or two passes in a single paper advance. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, the speed at which a printhead is scanned across a medium, such as a sheet of paper, has a direct relationship with the selection or deselection of a mode of operation in which each nozzle of the printhead is restricted from being activated for successive firing opportunities of that nozzle. The “firing opportunities” are defined by the maximum firing frequency assigned to the nozzles of the printhead moving at a particular scanning velocity. With respect to the sheet of paper, these firing opportunities may be referred to as pixel locations on the sheet. The speed at which the printhead is moved over the medium is significantly increased when the restriction against activation of a nozzle in successive firing opportunities is imposed. 
     A printmask controller is configured to generate masking patterns which establish sequencing of activations of the nozzles during passes. The printmask controller is further configured to impose the limitation on the firing frequency of activations for each nozzle and to impose the restriction as to the activation for successive firing opportunities, if a speed-enhancement mode is selected. The printmask controller includes a standard mode of operation in which masking patterns may include activations of the individual nozzles in successive firing opportunities, but with a reduction in the speed of the printhead across the medium. 
     The selection between the standard mode of operation and the speed-enhancement mode of operation may be manual. For example, a user may be presented the option of selecting between a “Best Photo” option or a “Fast Best Photo” each time that a photograph is to be printed. The printing system may include a graphical user interface (GUI) program that enables the user selection. If “Best Photo” is selected, masking hardware employs a higher quality composite masking mask and the printhead is moved at the speed required to ensure that the optimal nozzle firing frequency is not violated. On the other hand, if the “Fast Best Photo” option is selected, the masking hardware uses the restricted-location composite masking mask and the printhead is moved at the higher speed. 
     As another possibility, the selection may be transparent to the user. While the processing requirements would be significant, the selected mode of operation may be varied during execution of the single print task. For example, the printmask controller may include a comparator that is activated when the standard mode of operation is selected. The comparator may be computer programming which determines whether individual masking patterns (i.e., pass-by-pass masking patterns) are without nozzle activations in successive firing opportunities, despite the absence of the restriction. The multi-speed drive is placed in its accelerated state when the comparator identifies a masking pattern in which the condition is satisfied. Thus, the standard mode may include printhead passes at two different scan velocities when the printmask controller is in the standard mode. 
     In the use of the printing system, a method of forming an image on a sheet of paper or other medium includes enabling at least two modes of operation for printing the image. Specifically, the standard mode and the speed-enhancement mode are enabled for generating masking patterns as subsets of the print data. The standard mode allows utilization of the maximum firing frequency. On the other hand, the speed-enhancement mode may be utilized when the masking patterns are based on a set of constraints that include restricting the masking patterns from including use of an individual nozzle in consecutive firing opportunities permitted on the basis of the maximum firing frequency. The method further includes controlling the speed of printing the image on the basis of the current mode of operation, with a higher relative speed between the nozzles and the medium being used with the speed-enhancement mode. For applications in which the nozzles are components of an inkjet printhead, it is the scanning velocity of the printhead that is controlled. 
     The method may include enabling a user to select between the two modes at the initiation of a print task. Alternatively or additionally, the method may include providing a pattern-analysis of the masking patterns when the printing is in the standard mode. The analysis detects specific masking patterns that satisfy the constraint regarding consecutive firing opportunities. If the constraint is satisfied, the scanning velocity may be increased for that masking pattern. In such an application, the scanning velocity is determined on a scan-by-scan basis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a printer that may be used in the implementation of the invention. 
         FIG. 2  is a block diagram of components for use in the printer of  FIG. 1 . 
         FIG. 3  is an example of a composite masking pattern and a pair of individual masking patterns resulting therefrom. 
         FIG. 4  is a conceptual view of the use of masking patterns to form an image. 
         FIG. 5  is an example of a process flow of steps for implementing the invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 2 , a printing system  26  in accordance with one embodiment of the invention is shown as including components that may be self-contained within the printer  10  of  FIG. 1  or may be distributed between two or more components. For example, the program memory  28  having a graphical user interface (GUI)  30  may be an element of a computer that is supported by the printer  10 . The program memory may contain printer drive software. 
     The printing system is shown as having a printhead  16  that includes only four nozzles  32 ,  34 ,  36  and  38 . As is well known in the art, printheads typically include a much greater number of nozzles. For example, an inkjet printhead may include 100 nozzles having a pitch of approximately 0.085 millimeters. The nozzles are arranged in a column that is perpendicular to the scanning direction of the printhead. In addition to having a greater number of nozzles within the column, many printheads include multiple columns. For example, a printhead that is used for forming color images will often include a number of parallel columns. As another possible modification to the embodiment of  FIG. 2 , there may be multiple printheads, such as a system that includes a color cartridge and an adjacent cartridge which is used to deposit only black ink. While the invention will be described primarily with reference to inkjet printheads, the invention may be applied to other technologies in which ink or other print materials are deposited from nozzles. 
     If the program memory  28  and the GUI  30  are components of a personal computer, the printing system  26  includes an input  40 . In response to a print command, an image to be printed is transferred to an image buffer  42  via the input. The image buffer may be internal memory of the printer  10  shown in  FIG. 1 . The input  40  may include a cable port or may be a wireless transceiver. 
     Connected to the image buffer  42  is a printmask controller  44 . The printmask controller is configured to generate masking patterns which establish sequencing for activations of the nozzles  32 ,  34 ,  36  and  38  during passes of the printhead  16  over a sheet of paper or other print medium. In accordance with the invention, the printmask controller has at least two modes of operation and a user selects a preferred mode for a particular print task. Automated mode selection is also a desired feature of the invention. When the controller is in a standard mode, the composite masking pattern is defined on the basis of a first set of constraints. Among the constraints is a limitation regarding the maximum firing frequency of each nozzle  32 ,  34 ,  36  and  38 . The maximum firing frequency is selected for the purpose of ensuring that each nozzle is allotted time to be sufficiently replenished with ink following an activation. The purpose of the comparator  46  of the printmask controller will be described below, when referring to  FIG. 5 . 
     The printmask controller  44  is shown as providing inputs to a multi-speed drive  48  and a paper-advance system  50 . With reference to  FIG. 1 , the paper-advance system may be any known approach to progressing paper from the paper supply  24  past the printhead  16 . The paper advances in a direction perpendicular to the scanning direction of the printhead, as is well known in the art. The multi-speed drive  48  provides movement of the printhead in the scanning direction. In  FIG. 2 , there is a standard mode drive  52  and a higher speed drive  54 . 
     As is well known in the art, an individual masking pattern from the printmask controller  44  defines the partial image to be formed for a particular pass of the printhead  16  over the sheet of paper. A masking pattern may be considered to be a grid of pixels representing locations on the sheet of paper. For example, each pixel location in the grid may be represented by a data “0” or a data “1.” Pixel locations having a “1” will trigger activation of the nozzle at the corresponding location of the sheet of paper. On the other hand, a “0” will result in the corresponding location on the sheet of paper  66  being passed without deposition of a droplet. 
     Referring now to  FIG. 3 , a composite masking pattern  56  includes the letter “X” for each location on a sheet of paper that is to receive a droplet. For example, pixel location  58  will receive a droplet, while pixel location  60  will not. However, not all of the pixel locations will receive a droplet in a single pass of a printhead. Rather, the composite masking pattern is divided into two masking patterns  62  and  64  for use in separate passes of the printhead across the sheet of paper  66 . These two masking patterns  62  and  64  satisfy the condition that adjacent pixel locations in the scanning direction will not receive a droplet in the same pass. This is a condition established for the speed-enhancement mode of operation for the printmask controller  44  of  FIG. 2 . With the condition being satisfied for the two masking patterns, the speed at which the printhead is moved over the sheet of paper  66  can be significantly increased. Theoretically, the scanning speed can be doubled while maintaining the same firing frequency. However, there may be considerations which reduce this theoretical speed increase. For example, if the accuracy of droplet placement is reduced to an unacceptable level when the scanning speed is doubled, a smaller increase may be provided. On the other hand, if the speed can be doubled without imposing adverse effects, the theoretical relationship between increasing scanning speed and deposition intermittency can be extended. For example, if the imposed condition is that there be two deposition-free pixel locations between every droplet deposition of a masking pattern, it is theoretically possible to triple the scanning speed relative to the scanning speed at the “standard mode” of operation. 
       FIG. 4  is a conceptual view of a two-pass print operation in which after the first two rows of pixel locations, each location is to receive a droplet. However, the printhead  16  may be operated at a scan velocity that is significantly higher than the scan velocity of the “standard mode,” since the individual masking patterns for each pass do not include depositing droplets in adjacent pixel locations. In “Pass  1 ,” droplets are not deposited in either the horizontally adjacent or the vertically adjacent pixel locations. Then, in “Pass  2 ,” the printhead  16  has been stepped downwardly the equivalent of two rows of pixels and the full coverage of the third and fourth rows is completed. While not shown in  FIG. 4 , a third pass will completely fill in the pixel locations of the fifth and sixth rows. 
     A process flow of steps for implementing the invention is shown in  FIG. 5 . At step  68 , at least two modes of printer operation are enabled. In a standard mode, masking patterns are generated under a first set of constraints and the printhead is driven at a standard speed. In a speed-enhancement mode, masking patterns are generated based upon a second set of constraints, which include disablement of nozzle firings for consecutive pixel locations in the scanning direction of the masking patterns. Optionally, there may be a third mode of printer operation, such as a text mode in which quality considerations are less significant to a particular print task. For text-only print tasks, a one-pass process may be sufficient. 
     At step  70 , a particular print task is received. For example, a user of a personal computer may request printing of a document. The decision step  72  identifies the requested print quality. Many conventional printer drivers allow a user to select “draft” quality, photo quality, or a level of quality between these two. The user&#39;s selection may be used at decision step  72 . When a draft quality or text-only printing is detected, printing may be executed in a text mode at step  74 . 
     If an affirmative response is received at decision step  72 , the process advances to decision step  76 , wherein it is determined whether the print task is to be processed using the standard mode of printer operation or the speed-enhancement mode of printer operation. The decision step  76  may be executed manually. For example, the printing of a photograph-quality image may enable the user to select either a “Best Photo” or “Fast Best Photo” mode via the GUI. If “Best Photo” is selected, the process may follow the conventional approach of utilizing a composite masking pattern associated with the standard mode (step  82 ) and progressing the printhead at the same velocity for each scan across the print medium. A more complex approach in which the print speed is varied when the standard mode is selected will be described below when referring to steps  84 ,  86 ,  88  and  90 . The more complex approach will have advantages in some applications. 
     If the “Fast Best Photo” (i.e., speed-enhancement) mode is selected at step  76 , the process advances to step  78 . At this step, a different composite masking pattern is used, so that the individual masking patterns are in effect generated according to the second set of constraints, which include disabling nozzle firings within consecutive pixel locations. Masking patterns  62  and  64  of  FIG. 3  satisfy this constraint. The document or documents may then be printed at the higher speed, as indicated at step  80 . 
     Optionally, the masking patterns that are provided at step  82  (following a negative response at step  76 ) progress immediately to the processing necessary for directing the masking patterns to the printhead. As a more process-intensive option, the comparator  46  of  FIG. 2  is used to analyze each single-pass masking pattern to determine whether the pattern is one in which there are no nozzle firings in consecutive pixel locations. This may occur for some single-pass patterns despite the absence of a constraint that ensures such occurrence. Thus, the masking patterns are reviewed on a pattern-by-pattern basis at step  84  to identify such occurrences. 
     The review of masking patterns at step  84  is followed by the decision step  86 . If it is determined that a particular masking pattern does not require nozzle firings for consecutive pixel locations, the masking pattern is operatively associated with printing at the higher scanning velocity associated with the speed-enhancement mode, as indicated at step  88 . On the other hand, a masking pattern that includes the requirement of a nozzle firing for consecutive pixel locations will be printed at the standard scanning velocity for the printhead, as indicated as step  90 . For either step  88  or  90 , the process will loop back to step  84  until the final masking pattern of the print task has been forwarded to the printhead.