Printer and recording medium

Details Printer and recording medium Printer and recording medium

1999-06-28

2001-09-04

Le, N. (Department: 2861)

Incremental printing of symbolic information

Ink jet

Controller

C347S040000, C347S043000

Reexamination Certificate

active

06283571

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a printer that prints an image with a head having nozzles that enable creation of dots having different quantities of ink.
2. Description of the Related Art
Ink jet printers that create dots with a plurality of color inks ejected from a plurality of nozzles formed on a head to record an image have been proposed as an output apparatus of a computer. The ink jet printers are widely used to print images processed by the computer in a multi-color, multi-tone manner. In such printers, each pixel is generally expressed by two tones, that is, the dot-on state and the dot-off state. The image is accordingly printed after the halftone processing, which is the image processing to enable the tones of original image data to be expressed by dispersibility of dots.
Multi-value printers, which are ink jet printers that enable expression of two or more tones, have recently been proposed to enrich the tone expression. Such printers include a printer that enables expression of three or more different densities by changing the quantity of ink or the density of ink and a printer that enables multi-tone expression by creating a plurality of dots in an overlapping manner in each pixel. The halftone processing is still required in such printers, since the tone of the original image data is not sufficiently expressible in each pixel.
In the multi-value printer, it is required to determine the recording ratio of each type of dot according to the tone value of the original image data in the course of the halftone processing. The conventional technique sets the recording ratio of each type of dot to appropriately express a variation in tone value and ensure the favorable granularity of the resulting printed image. Especially from the viewpoint of the improved granularity, there is a tendency of creating a large number of dots having a less quantity of ink.
In the multi-value printer that changes the quantity of ink, however, when a large number of a specific type of dot, which has a substantially identical size with a recording pitch of dots, banding often appears because of the reason discussed below.
FIG. 23
shows a state of recording only the specific type of dots in a predetermined image area. The rectangle shown on the left side of
FIG. 23
represents a head with five nozzles. The open circles shown on the right side represent the specific type of dots. The hatched square denotes one pixel. In order to enable the whole image area to be filled with dots, the size of the specific type of dot is set to be substantially identical with or more precisely only a little greater than each side of the pixel, that is, the recording pitch of dots. In the example of
FIG. 23
, dots are created at the most ideal positions in the respective pixels. In this case, the predetermined image area can be filled uniformly with the dots.
In the ink jet printer, the respective nozzles generally have different ink ejecting characteristics, which cause a deviation of the dot recording positions.
FIG. 24
shows a state of recording the specific type of dots with a deviation of the dot recording positions. In the illustrated example, ink is ejected in oblique directions from the first nozzle and the second nozzle, so that the positions of the dots created by the first nozzle and the second nozzle are deviated from the expected positions. The deviation of the dot recording positions causes unevenness of density or banding in the resulting printed image as clearly shown in FIG.
24
. In an extreme case, there is a dropout between adjoining rows of dots.
FIG. 25
shows a state of recording another type of dots, which has a greater area than that of the specific type of dot, with a deviation of the dot recording positions. The symbols in
FIG. 25
have the same meanings as those explained in
FIGS. 23 and 24
. Since there are significant overlaps of dots in the example of
FIG. 25
, the dots are expressed by the solid line and the dotted line alternately, for the clarity of illustration. There is no practical difference between the dots by the solid line and the dots by the dotted line. As clearly understood from the comparison with the example of
FIG. 23
, the dots shown in
FIG. 25
have the greater size than each side of the pixel or the recording pitch of dots. This increases the overlapped area of the adjoining dots and thereby makes the unevenness of density, which is due to the deviation of the dot recording positions, relatively inconspicuous in the example of
FIG. 25
, compared with the example of FIG.
24
. In the case of the specific type of dot having the substantially identical size with the recording pitch, even a little deviation of the dot recording positions makes the banding significantly conspicuous. The multi-value printer has been developed to enrich the tone expression and enable the high quality printing. The occurrence of the banding, which results in lowering the picture quality, is thus not negligible.
SUMMARY OF THE INVENTION
The object of the present invention is thus to provide a technique that ensures high-quality printing while reducing the occurrence of banding due to recording of a specific type of dot in a multi-value printer.
At least part of the above and the other related objects is attained by a printer with a head having a plurality of nozzles that enable creation of at least two different types of dots having different sizes. The printer determines which of the at least two different types of dots is to be created in each pixel according to a printing condition and a tone value of image data and creates dots based on a result of the determination with the head, thereby printing an image on a printing medium. The printer includes: a memory unit that stores relations between the recording ratio of each type of dot and the tone value with regard to printing conditions; a printing condition input unit that inputs a specified printing condition; and a decision unit that determines whether or not each type of dots is to be created in each pixel, based on the recording ratio corresponding to the specified printing condition stored in the memory unit. Different values are set to a limit recording ratio of a specific type of dot, which is selected among the at least two different types of dots and enables independent expression of a certain tone value, corresponding to the printing conditions. The limit recording ratio is specified against a limit tone value, at which a recording ratio of another type of dot having a greater size than the specific type of dot practically starts recording to have a significant value as a recording ratio thereof.
It is preferable that the specific type of dot has a size that is substantially identical with a dot pitch in printing.
It is also preferable that the limit recording ratio is set based on a possibility of occurrence of banding.
The following describes the relation between the recording ratio of the specific type of dot and the banding, prior to description of the functions and effects of the printer of the present invention. As described above with
FIG. 24
, the banding often appears in the case of recording the specific type of dots. The possibility of the occurrence of banding depends upon the recording ratio of the specific type of dot as discussed below.
FIG. 15
shows a state of recording the specific type of dots. The open circles in
FIG. 15
represent the specific type of dots. The example of
FIG. 15
regards a relatively low recording ratio and there are a large number of pixels in which no dot is created. Like the example of
FIG. 24
, there is a deviation of the dot recording positions in the example of FIG.
15
. The presence of a gap B
2
, which is ascribed to pixels where no dot is created, makes a gap B
1
, which is due to the deviation of the dot recording positions, relatively inconspicuous. This means that the banding is relatively inconspicuous when the specific type of dot has a low recording ratio.
FIG. 16
shows a state of recording the specif

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