Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
2000-11-16
2003-04-08
Nguyen, Lamson (Department: 2861)
Incremental printing of symbolic information
Ink jet
Controller
C347S043000, C358S534000
Reexamination Certificate
active
06543870
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image processing apparatus that carries out halftoning of multi-tone image data and a printing apparatus that creates dots according to halftone data obtained as a result of the halftoning process and thereby prints an image.
2. Description of the Related Art
Ink jet printers have widely been used as the output device of images processed by the computer. The ink jet printer creates dots on a printing medium with ink ejected from a plurality of nozzles formed on a print head, so as to record an image. The ink jet printer is generally capable of expressing only two tones, that is, the dot-on state and the dot-off state, with regard to each pixel. Image processing generally called the halftoning process is accordingly required, prior to printing an image. The halftoning process enables the multiple tones of original image data to be expressed by a distribution of dots. The ink jet printer is able to print images processed by the computer without the plate making process and thus has great facility.
In the ink jet printer, some effort is made to lower the visual conspicuousness of dots, in order to improve the granularity of the resulting image. The halftoning process adopted in the ink jet printer thus prevents local concentration of dots and attains the good dispersibility of dots. The dithering method using a discrete dither matrix like a Beiyer matrix is the known method to attain such halftoning process. The dithering method determines the dot on-off state, based on comparison between the tone values of the respective pixels and threshold values stored in a preset dither matrix.
The screen printing technique that uses a plate provided for each color is generally applied to print a large quantity of images. In the screen printing, the halftoning process is used for tone expression. The halftoning technique varies the dot percent to express each tone value.
FIG. 17
shows an example of tone expression by the halftoning technique. In this example, the tone level is varied in three stages. The upper-most figure corresponds to the lowest tone level, and the tone level is heightened towards the bottom. In the area of low tone, dots having a small dot percent are used for printing. With an increase in tone value and an increase in density to be expressed, dots having the greater dot percent are used for printing.
The screen printing is a suitable technique for mass printing but takes a high cost for plate making. With a view to reducing the cost required for plate making, the ink jet printer may be used for prepress. The prepress here means the trial printing before the actual plate making to allow the operator to check the printed image. The prepress using the ink jet printer advantageously reduces the cost required for plate making.
When the ink jet printer is used for prepress in the screen printing, it is desirable that the picture quality of the printed image by the ink jet printer is close to the picture quality in the screen printing. For this purpose, the halftoning process for the prepress adopts the dithering method using a halftone dot-simulating dither matrix (hereinafter referred to as the halftoning dither). As described previously, the halftoning process in the ink jet printer is generally carried out to ensure the sufficient dispersibility of dots. The dither matrix used in the dot distributed-type halftoning process is set to make the pixels with the high probability of dot creation, that is, the pixels having lower threshold values, appear in a discrete manner in the matrix. In the halftoning dither method, on the other hand, the dither matrix used for the halftoning process is set to make the pixels having lower threshold values locally concentrated and thereby create dots according to a halftone-dot simulating pattern.
The halftoning process using the halftoning dither may, however, extremely lower the picture quality of resulting images due to various factors discussed below. FIG.
18
A and
FIG. 18B
show the state of dots formed by an ink jet printer.
FIG. 18A
shows the state of dots created in the case of applying the halftoning dither, and
FIG. 18B
shows the state of dots created in the case of applying the discrete dither. In this example, a large matrix used corresponds to an area CE including a total of 100 pixels (10 pixels in length by 10 pixels in width). The tone level is varied in three stages, where the left part of the area CE expresses image data of high tones and the right part expresses image data of low tones. Each small matrix represents a pixel, and each closed circle represents a dot. The discrete dither gives dots in a distributed manner, whereas the halftoning dither gives local concentration of dots. I the case of the halftoning dither, the resolution is lowered by the local concentration of dots.
The reduced area CE and the shortened pitch of halftone dots can prevent the decrease in resolution due to the above reasons. This arrangement, however, narrows the expressible tone range. When the area CE includes 100 pixels as in the case of FIG.
18
A and
FIG. 18B
, the dot percent may be varied in 100 stages at most. The reduced area CE decreases the number of stages of varying the dot percent, thus narrowing the expressible tone range. The halftoning dither method thus can not attain printing of the appropriate picture quality as the prepress with the sufficient resolution and expressible tone range.
Another problem of the halftoning dither method is the possible deterioration of the picture quality due to the interference of the frequency of halftone dot creation with various frequencies intrinsic to the ink jet printer, for example, the frequency of main scan and the frequency of sub-scan. The ink jet printer generally has a large number of nozzles for ink ejection, and there is a variation in ink ejection characteristics among the respective nozzles. There may be a feeding error in the course of sub-scan. In the ink jet printer, the misalignment of dots due to the varying ink ejection characteristics and the feeding error in the course of sub-scan periodically varies in an image.
The halftoning dither technique gives local concentration of dots as shown in
FIGS. 17 and 18
. The local concentration of dots appear at the frequency corresponding to the magnitude of the halftone area CE. In some cases, the frequency of causing the local concentration of dots is close to the frequency of significant misalignment of dots. The closed frequencies emphasize the misalignment of dots and cause periodical unevenness of density, thus deteriorating the picture quality of the resulting image.
Each dot blots and expands over the pixel interval on the printing medium. In the distributed dot creation technique, the dot recording rate that is significantly lower than 100% causes the whole surface of the printing medium to be covered with ink in a substantially homogeneous manner. The halftoning dither technique, on the other hand, forms local concentration of dots. Regardless of some blotting, it is difficult to completely cover the whole surface of the printing medium with ink at the dot recording rate that is not very close to 100%. As in the example of the high tone area shown in
FIG. 17
, the halftoning dither technique may result in dropping out. The printing medium generally has a restriction in absorbable quantity of ink (hereinafter referred to as the duty restriction). Some printing media in specific print modes reach the duty restriction even at the dot recording rate that is significant lower than 100%. Especially in the case of recording a plurality of different color inks in an overlapping manner, there is a severe restriction in quantity of ink for each color. In such cases, the prior art halftoning dither method leaves some undesirable drop-outs to deteriorate the picture quality and moreover does not attain the sufficient density.
Because of the problems discussed above, the halftoning dither does not attain the picture quality in the screen p
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Seiko Epson Corporation
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