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.