Typewriting machines – Including selection of type-face by...
Reexamination Certificate
2001-08-28
2004-05-25
Hirshfeld, Andrew H. (Department: 2854)
Typewriting machines
Including selection of type-face by...
C400S061000, C400S062000, C347S015000
Reexamination Certificate
active
06739772
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a printing method and apparatus and, more particularly, to a printing method and apparatus which print multilevel grayscale images.
Among printers based on various printing schemes, some printers are designed to form texts and images on printing media by making printing materials adhere to the printing media. Of the printers based on such printing schemes, an ink-jet printing apparatus is a typical one. Recently, with advances in the performance of ink-jet printing apparatuses, images have been printed as well as texts.
A typical ink-jet printing apparatus uses an array of a plurality of orifices (nozzles) capable of discharging inks having the same color and density. Such arrays of nozzles are generally arranged for inks having the same color and different densities or inks having different colors, respectively. Some printing apparatuses can discharge inks having the same color and density while changing the discharge amount in several steps.
While a head having these nozzles is moved relative to a printing medium, ink is discharged from the nozzles, thereby printing an image.
As methods of moving a head relative to a printing medium, the following are practiced:
(i) A so-called swath printing scheme, in which nozzles are arranged substantially parallel in the X direction. While a printing medium is at rest, the printhead is moved in a direction (Y direction) perpendicular to the X direction, and printing is performed during this period. Thereafter, the printing medium is intermittently moved by a predetermined distance in the X direction. The printhead is then moved again in the Y direction. Subsequently, this operation is repeated to print.
(ii) A so-called full multi-printing method, in which nozzles are fixed to cover the entire width of a printing medium in the Y direction. Printing is performed while the printing medium is moved at a constant speed in the X direction.
When images are printed by these methods, a pixel is defined as a unit of an image. A pixel is not necessarily formed by one dot (a portion formed on a printing medium by discharging ink from one nozzle once) and may be formed by a plurality of dots. When each pixel is to be formed by a plurality of dots, dots may be overlaid and printed on substantially the same point or may be printed on adjacent points. In either case, overlaying operation is determined in accordance with a predetermined rule. Image data to be printed is subjected to enlargement interpolation, reduction, or the like by an image processing means to have an image size conforming to a printing apparatus. The color to be printed and its density are determined for each pixel in accordance with predetermined rules. Printing is then executed in accordance with this determination. As described above, since one pixel may be constituted by a plurality of dots, dots do not necessarily have the same density, and inks having different densities can be selected. If a head capable of changing the discharge amount is used, the discharge amount, i.e., the ink amount of a dot, may be changed as needed. Alternatively, these methods may be combined.
When an image is to be printed, halftoning such as dithering or error diffusion is used as a method of faithfully reproducing the gradation of image data. In dithering or error diffusion, by increasing the number of gray levels of one pixel, a larger number of gray levels can be expressed. Such a printing method is disclosed in detail, for example, in Japanese Patent Laid-Open No. 10-324002.
More specifically, nozzles capable of discharging inks having different densities are prepared for one color, and printing is selectively performed a plurality of number of times (to be referred to as overlaying hereinafter) for one pixel by using these nozzles within a predetermined limit, thereby increasing the number of gray levels or densities (printing OD values) that can be expressed on this pixel. Assume that nozzles capable of discharging inks having six different densities are prepared, and overlaying is to be performed four times or less with respect to one pixel based on 600 dpi. In this case, 50 gray levels or more can be expressed. If one pixel is constituted by 2×2 adjacent points and is to be formed by a total of 16 times or less of overlaying/printing, 200 gray levels or more can be expressed. Gradation may be expressed by changing the amount of ink discharged from each ink and changing the ink amount of each dot instead of preparing nozzles capable of discharging inks having different densities. Alternatively, gradation may be expressed by combining these methods.
In these cases, a rule that makes the density (desired OD value) of a pixel to be expressed correspond to an ink overlaying/printing method is determined in advance, and actual printing, i.e., which nozzles are used and when inks are discharged, is determined in accordance with this rule. Printing is then actually performed by a printing control means in accordance with this determination.
For example, the printing OD value of each pixel printed by using the respective inks is measured in advance, and a printing OD value obtained by overlaying is determined by this measurement value, thereby preparing a table in which the printing OD values of pixels corresponding to the respective overlay patterns are written. An overlay pattern corresponding to a printing OD value near a desired OD value of a pixel to be printed is selected. In error diffusion processing, the difference between the desired OD value of the pixel to be printed and the corresponding printing OD value in the table is obtained and is distributed as errors to adjacent pixels.
There are various kinds of images, and hence various characteristics are required for printers depending on applications and purposes. In designing various printers in accordance with the application purposes, it is preferable that printing characteristics be freely designed.
As an example of an image for which special printing characteristics are required, a medical image will be described below.
In some fields, e.g., the field of medical images, many monochrome images printed in monochrome are still used for the following reason. A monochrome image exhibits high human eye density resolution. Therefore, in a field in which high density resolution is required, the amount of information that can be recognized by a human is higher in a monochrome image than in a color image. It is known that a transmission type printing medium increases the human eye density resolution as compared with a reflection type printing medium. In general, the human eye density resolution with respect to a color image is about 8 bits, whereas that with respect to a monochrome transmission image is 10 to 11 bits. A medical X-ray photograph or CT/MRI image printed on a transmission medium is actually read up to the human eye resolution limit to provide information for diagnosis. As a printer for printing such a high-quality monochrome image, a laser imager is available, which irradiates a silver halide film with a laser beam modulated in accordance with an image signal, and developing the film, thereby obtaining an image on the film. In such a laser imager, an image is often printed with a density resolution of 12 bits in consideration of a certain margin. However, such a laser image is expensive. In addition, wet type developing processing is required, and hence problems arise in terms of waste liquid disposal, cumbersome maintenance, and the like. Although a dry silver halide type laser imager which develops by heating is available, the image quality is inferior to that in the wet type.
An apparatus based on the ink-jet scheme capable of expressing 50 density gray levels or more in 600 dpi is disclosed in Japanese Patent Laid-Open No. 10-324002, which can print a 256-level grayscale image by further performing error diffusion processing. This reference has exemplified 256-level grayscale printing. If input image data is 4096-level grayscale data inst
Matsumoto Kazumasa
Suwa Hidetomo
Suzuki Ken'ichi
Chau Minh
Hirshfeld Andrew H.
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