Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
1999-06-24
2002-09-03
Hallacher, Craig A. (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
C347S015000
Reexamination Certificate
active
06443548
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a printing method and printing apparatus and, more particularly, to a printing method and printing apparatus for printing an image on a printing medium using a printhead according to an inkjet printing method.
Printing units incorporated in printers, copying machines, facsimile apparatuses, and the like print an image consisting of a dot pattern on a printing medium such as a paper sheet or thin plastic plate on the basis of image information. Such printing apparatuses are classified according to their printing methods into an inkjet type, wire dot type, thermal type, laser beam type, and the like.
Of these types, the inkjet type (inkjet printing apparatus) is designed to print an image by discharging ink (printing solution) droplets from the orifices of a printhead and adhering them to a printing medium.
In recent years, many printing apparatuses have been used, and high performance such as high-speed printing, high resolution, high quality, and low noise has been required for the printing apparatuses. As printing apparatuses that can meet such requirements, the above inkjet printing apparatuses have been used as a relatively compact printing apparatus and have become rapidly popular.
One of these inkjet printing apparatuses employs a printhead having an array of a plurality of orifices to realize high printing speed, and another one of these inkjet printing apparatuses employs a plurality of printheads in correspondence with the number of types of color inks in order to be capable of color printing.
In consideration of the demands for high resolution and high quality, half-tone processing methods such as a dither method and error diffusion method are used in these inkjet printing apparatuses as methods of faithfully reproducing the tonality of image information.
According to these tonality reproduction methods, when a printing apparatus has a high resolution (1,000 dots/inch or more), excellent multi-level printing can be performed. If, however, a printing apparatus has a low resolution (about 360 to 720 dots/inch), the printed dots of a highlight portion of a formed image become conspicuous, and the discontinuity of pixels tends to increase the graininess of a formed image.
In order to increase the number of tonality levels, a method of representing each print dot itself by a multi-valued dot is recently adopted.
For example, according to a known method, the voltage, the pulse width, or the like to be applied to a printhead is controlled to modulate the diameter of each print dot adhered to a printing medium, thereby attaining tonality. However, there is a limit to the minimum size of a printable dot according to this method. Although it is difficult to stably reproduce many tonality values, this method is put into practical use for representing several tonality levels.
A density modulation method is also available, in which the density of dots within a predetermined dot matrix (a predetermined area) is changed while the dot size remains the same. However, this method requires a considerably large area to increase the number of representable tonality levels, and hence the resolution decreases.
In consideration of the above methods for improving the tonality and obtaining high-density, multi-level images by using an inkjet printing apparatus, for example, the following methods have been proposed and put into practice: a so-called multiple droplet method of forming one dot by adhering a plurality of droplets to substantially the same portion on a printing medium, i.e., changing the number of droplets to be adhered onto the same portion, thereby expressing tonality; a printing method of reproducing tonality by forming at least two types of print dots having different densities in the same color by using a plurality of inks having different densities; and a combination of these methods.
As one of the pseudo tonality reproduction methods, an error diffusion method (R. FLOYD & L. STENBERG, “An Adaptive Algorithm for Spatial Grey Scale” SID' 75 DIGEST), pp. 36-37) is available.
As an image printing method using this error diffusion method, Katoh, Y. Arai, Y. Yasuda, “Multivalued Error Diffusion Method” (National Conference of Communication, Department in Showa 53 Year, Society of Electronic Communication in Japan (1973), p. 504 (in Japanese)) proposed an error diffusion method using a plurality of thresholds, unlike the conventional error diffusion method using one fixed threshold.
If, for example, the image data value per pixel ranges from 0 to 255, error diffusion is performed using “128” as a threshold to obtain binary data in the prior art. In contrast to this, in the “multi-valued error diffusion method” by Katoh, Y. Arai, Y. Yasuda, when a half-tone image is to be printed with two different ink densities, “85” and “175” are set as thresholds to obtain ternary print data with two different printing densities.
Furthermore, efforts have recently been made to realize a method of performing half-tone printing with three or more different ink densities to obtain multi-valued data with three or more different densities and represent a high quality image.
To recover the print quality from deterioration in an inkjet print apparatus, suction means and pressurizing means are used to remove undesirable substances and bubbles from liquid paths by forcibly drawing ink from the nozzles of the printhead. Alternatively, ink coagulations near orifices are removed by cleaning the ink discharge surfaces with a wiper.
In addition, since ink adhered to the orifice surfaces of the printhead upon ink discharge may cause a discharge failure, the orifice surfaces of the printhead are wiped at a predetermined timing.
Furthermore, to discharge ink whose viscosity has increased upon evaporation of the ink solvent from an unused nozzle during printing operation, preliminary discharge, which differs from ink discharge for printing, is performed at a predetermined timing. With this operation, fresh ink is always supplied into each nozzle to perform stable printing. In this way, recovery operation is done.
However, although fresh ink is supplied into a nozzle by the above-described recovery operation, the ink solvent gradually evaporates from the nozzle tip immediately after recovery operation, resulting in an increase in ink density in the nozzle tip with time.
In the multiple droplet method or the method of representing tonality by using inks having the same color and different densities, in particular, the tonality balance deteriorates with an increase in ink density, and the tonality continuity loses smoothness. As a result, pseudo outlines appears on a printed image, and the quality deteriorates in some case.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a printing method and printing apparatus capable of printing a high quality image showing excellent tonality even when a plurality of inks having the same color and different densities are adhered to substantially the same portion on a printing medium to form one print dot and represent tonality.
According to one aspect of the present invention, the foregoing object is attained by providing a printing method of performing printing on a printing medium by discharging ink from an inkjet printhead using a plurality of inks having different densities, comprising: an input step of inputting multi-valued image data; a calculation step of calculating a lapse time from previous ink discharge by the printhead; a prediction step of predicting a density change of ink according to the lapse time based on the lapse time calculated at the calculation step; a selection step of selecting a type of ink and ink discharge amount suitable for representing a density value indicated by the input multi-valued image data based on the density change of ink predicted at the prediction step; and the driving step of driving the printhead so as to cause ink discharge from the printhead based on the type of ink and ink discharge amount selected at the selection step.
The selectio
Matsumoto Kazumasa
Takayama Hidehito
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Hallacher Craig A.
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