JPH0369711B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an intermediate color representation method used in an inkjet printer that records color images.

Inkjet printers use at least cyan, magenta, and yellow ink liquids, and eject these ink liquids from nozzles to record color images with a large number of dots. In this inkjet printer, halftones are expressed by the amount of ink recorded within one pixel, and intermediate colors are expressed by mixing ink droplets of each color.

Various methods have already been proposed for expressing halftones. For example, in Japanese Patent Application Laid-Open No. 148428/1989, one pixel is constructed from an n x m matrix (n and m are integers), and this matrix cell is Gradation is expressed by the position and number of dots placed inside.

On the other hand, as a method of expressing intermediate colors, for example, as disclosed in Japanese Patent Application Laid-open No. 126416/1983, one ink head is used for each ink liquid used, and these ink heads are arranged on the same scanning line. ,
It is known to express intermediate colors by overlapping a plurality of color ink droplets ejected from different ink heads at the same point by delaying the signal by the number of dots corresponding to the distance between the ink heads. This method of JP-A-54-126416 uses subtractive color mixing because a plurality of color ink droplets are superimposed on the same point. This subtractive color mixing causes color turbidity, making it difficult to express vivid intermediate colors.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for expressing intermediate colors that can express vivid intermediate colors.

It is known that additive color mixing can express more colorful hues than subtractive color mixing when expressing neutral colors, and inkjet printers also use side-by-side color mixing to visually perform additive color mixing. It has been confirmed through experiments that it is possible to express more colorful colors when the color is warm.

Therefore, in order to perform this juxtaposed color mixing, the present invention has developed a dot matrix of cyan ink so that the centers of the n×m (n, m are integers of 3 or more) dot matrices of each color constituting the same pixel do not coincide. , the dot matrix of magenta ink is shifted vertically and horizontally by at least one dot, and the dot matrix of yellow ink is shifted vertically by at least one dot with respect to the dot matrix of cyan ink. When the density level is low, ink droplets of each color do not overlap, and juxtaposition color mixing is performed, and when the density level is high, juxtaposition color mixing and subtractive color mixing where ink droplets of each color overlap are mixed.

The juxtaposed color mixing method of the present invention causes noticeable color shift at high densities when drawing text or line drawings, but it is highly effective for recording images that do not require high clarity, such as landscapes, still lifes, and portraits. .

In order to record a gradation image, it is necessary to express gradation in at least 16 levels, and this requires
It is preferable to use a method of sequentially changing the number and arrangement of dots in an m.times.n dot matrix, as in No. 148428. However, with this method, the gradation expression is not necessarily smooth, and when the number of gradations is increased, there is a problem in that the number of dot matrix cells forming one pixel increases. Therefore, in the embodiment of the present invention, in addition to the number and arrangement of dots, the size of the dots is also changed to express halftones. This method makes it possible to express multiple levels of gradation with a small number of dot matrices, and
Convenient for expressing neutral colors through juxtaposed color mixing.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 schematically shows an inkjet printer embodying the present invention. Recording paper (blank paper) 2 is wrapped around the outer periphery of the rotating cylinder 1, and rotates together with the rotating cylinder 1 in the main scanning direction. The rotating cylinder 1 is driven by a main scanning pulse motor 5 that receives pulse signals sent from a central processing unit 4 via an interface 3 .
The rotational position of the rotating cylinder 1 is detected by a main scanning rotary encoder 6, and this rotary encoder 6 is reset every time the rotating cylinder 1 rotates once.

A head portion 7 is provided adjacent to the rotary cylinder 1, and this head portion 7 is fixed on a movable table 8. This moving table 8 is moved in the sub-scanning direction by a feed screw shaft 9 and a rail 10. The feed screw shaft 9 is rotated by a sub-scanning pulse motor 11, and the amount of rotation is detected by a sub-scanning rotary encoder 12. These rotary encoders 6 and 12 determine the position of the head section 7 with respect to the recording paper 2. The sub-scanning pulse motor 11 rotates one step for each rotation of the rotary cylinder 1 to send the head section 7 in the sub-scanning direction.

In order to facilitate understanding of the present invention, the relationship between the structure of the head section 7 and the position of the dot matrix will first be explained with reference to FIGS. 2 to 4. Second
As shown in the figure, cyan heads 13 to 15 are arranged side by side in the main scanning direction. Although the heads 13 to 15 are shown separated in the drawings, they are actually arranged as close as possible. Similarly, in order to record different scanning lines, magenta heads 16 to 18 and yellow heads 19 to 21 are provided side by side in the main scanning direction, three for each color, nine in total. Record a dot with the head. A nozzle is provided at the center of each of these heads 13-21, and ink droplets are ejected from this nozzle.

In an m×n dot matrix, m=4
In this case, it is sufficient to arrange four heads for each color in the main scanning direction. Note that since n is determined by the number of ink droplets ejected from the same head, it can be specified in terms of the circuit regardless of the number of heads.

As shown in FIG. 3, cyan heads 13-1
The directions of the ink droplets 5 are changed so that the ink droplets are ejected at positions shifted by one dot in the sub-scanning direction. Since the recording paper 2 is moving in a direction perpendicular to the paper surface of the drawing, at any given time it extends in the direction perpendicular to the paper surface, and a 3×3 image is formed on the scanning line l having a width of 3 dots. A dot matrix is recorded by heads 13-15. That is, 1
When expressing a pixel with a 3×3 dot matrix, line Y1 in the main scanning direction shown in FIG.
The matrix cells above (Y1X1, Y1X2,
Y1X3) is recorded in head 13.

The head 14 records the matrix cells on the Y2 line, and the head 15 records the matrix cells on the Y3 line. Note that the grid is an imaginary line to indicate the dot position.
Dotted lines indicate the positional relationships of dot matrices of other colors, which will be described later.

Because the nozzles of the heads 13 to 15 are spaced apart in the main scanning direction due to space constraints of the heads,
The reference head 13 is moved by the number of dots corresponding to this distance.
The signal is delayed relative to that, and the alignment of inks of the same color is performed.

The macenter heads 16 to 18 record dot matrices on scanning lines m located at recording positions at arbitrary times. This magenta head 16-1
8 is cyan head 1 for juxtaposition color mixing.
The dot matrix is shifted one dot to the left by changing the orientation of the heads relative to numbers 3 to 15.
Further, as shown in FIG. 4B, this dot matrix is shifted downward by one dot with respect to the cyan heads 13-15.

The yellow heads 19-21 record the dot matrix of the image on scanning line n at any given time. This head 19-2
The dot matrix according to No. 1 is shifted downward by one dot with respect to the cyan dot matrix, as shown in FIG. 4C.

Since the head section 7 moves to the left in FIG. 3 by the width of the scanning line (3 dots) each time the rotating cylinder 1 rotates once, the magenta head 16
-18 is at the position of the cyan heads 13-15, magenta ink is injected into the matrix cells of YiXj (i, j=2-4) in FIG. 4A. In this case, since the Y4 line and the X4 line are part of adjacent pixels for the cyan ink liquid, cyan dots and magenta dots of different pixels overlap in a part on these lines.

Furthermore, when the yellow heads 19 to 21 come to the positions of the cyan heads 13 to 15, YiXj (i=1 to 15)
3. Inject yellow ink into the matrix cells of j=2 to 4). Therefore, since they are dot matrices of the same pixel, the ones that should originally overlap are intentionally shifted. Juxtaposed color mixing is performed by shifting the center of the dot matrix of each color vertically and horizontally.

In the embodiment described above, parts of different pixels overlap for different colors, but the pixels may be made larger by the amount of shift in the dot matrix of each color. For example, in Fig. 4, one pixel is a 4 x 4 dot matrix including dotted lines, no dots are recorded on the X line indicated by the dotted line, and the width of the scanning line is set to 4 x 4 from Y1 to Y4 lines. This can be easily carried out by moving the head portion 7 by four dots each time the rotating cylinder 1 rotates once. In this case, 1
As the pixels become larger, the overlap of colors decreases, and the effect of juxtaposed color mixing becomes greater.

Also, since the heads of the same color lined up in the main scanning direction are tilted so that the dot positions are shifted by one dot in the sub-scanning direction, it is easier to make adjustments compared to a method in which each head is set parallel to the paper surface and shifted by one dot. It is simple and the dot position can be determined accurately.
Of course, the present invention can also be applied to this remote arrangement.

Returning to FIG. 1 again, the configuration of the inkjet printer will be explained. The storage device 23 stores density signals of three colors (blue, green, red) for each pixel, which are read out according to the sub-scanning position signal from the rotary encoder 12 and subjected to masking processing by the central processing unit 4. is applied and converted into cyan, magenta, and yellow density signals. Here, since the heads for each color are installed so as to record three different scanning lines at the same time, the density signals stored in the storage device 23 are sequentially read out for each scanning line in an extremely short period of time.

For each pixel of scanning line l to be stored by the cyan heads 13 to 15, only the cyan density signal is extracted and temporarily stored in the cyan line buffer memory 24. The magenta line buffer memory 25 temporarily stores the magenta density signal of the pixel to be recorded on the scanning line m, and the yellow line buffer memory 26 stores the yellow density signal of the pixel to be recorded on the scanning line n. Memorize the signal.

The density signals for one scanning line stored in each of the line buffer memories 24 to 26 are sent to dot pattern converters 27 to 29, respectively, and the positions, numbers, and sizes of dots to be injected into the dot matrix are converted into density signals. Specified according to level. The dot signals output from the dot pattern converters 27 to 29 are sent to head drive units 30 to 32, respectively, which simultaneously drive the heads 13 to 21 to convert each pixel on the scanning lines l, m, and n. Ink droplets are simultaneously fired into the dot matrix.

FIG. 5 shows a circuit for recording a cyan dot matrix. A scanning line temporarily stored in the line buffer memory 24, for example,
The cyan density signal of each pixel to be recorded on the
It is decoded by the decoder 34 and specifies a table in the table memory 35. This table memory 35 stores the relationship between a dot pattern, that is, each cell of a dot matrix, and a dot signal (level of applied voltage for driving the head) according to the density level. The Y line shown in FIG. 4 is designated by the XY address counter 36, and then the address of the X line is counted up and a dot signal is read out for each Y line. The dot signals read out for each Y line are sent to the XY address counter 36.
The data is written to a predetermined address in the buffer memories 37 to 39 selected by . Thereafter, dot signals are read out for each pixel in the same manner and written into buffer memories 37 to 39 provided for each Y line.

Here, the leading addresses of the buffer memories 38 and 39 are shifted by a dot corresponding to the distance of each head 14 and 15 from the reference head 13, so even if the dot signals are on the same X line, they are at different addresses. It is written.

Buffer memories 37-39 are read out according to position signals from main scanning rotary encoder 6, and sent to head drive circuits 40-42. Here, it is converted into a voltage applied to the heads, further superimposed with a bias signal, and sent to the heads 13 to 15, respectively. The heads 13-15 simultaneously record dots on the Y1-Y3 lines.

FIG. 6 shows an example of a dot matrix table for each color stored in the table memory. The numbers displayed within each dot matrix indicate the level of voltage applied to the head.
In this embodiment, there are seven stages, 1 to 7. Level "7" means a high voltage, the size of the ejected ink droplet becomes large, and the diameter of the dot on the recording paper also becomes large. Conversely, level "1" indicates a small voltage and the recorded dot diameter becomes small. Note that this voltage level does not necessarily have a linear relationship with the applied voltage or the dot diameter. Moreover, a blank column indicates a state in which ink droplets are not ejected.

The grid shown in the drawing is a virtual line indicating the dot position in the dot matrix, and in this embodiment, one pixel is 500 μm square, so one side of the matrix cell is approximately 167 μm. When drawing, each color is drawn in units of 500 μm square pixels shown by the solid line, but in the case of multicolor drawing, juxtaposed colors are mixed within the 667 μm square pixels shown by the dotted line, so intermediate color pixels have an apparent area of becomes larger. That is, since the same pixel is recorded in a 4×4 dot matrix, it appears larger by 7 dots.

Also, when the width of the scanning line is 4 dots,
A voltage level at which no ink droplets are ejected to the matrix cells of the X4 line is written in the table memory 35. and XY address counter 36
accesses up to X4. In this case, since one pixel is 4×4 as shown by the dotted line, the area of one pixel becomes substantially large.

In order to minimize the overlap of each color, cyan is formed into a dot pattern that starts from the matrix cell in the upper left corner and expands the recording area toward the lower right corner as the density increases. Magenta has a dot pattern that expands from the lower right corner to the upper left corner.
The yellow has a dot pattern that expands from the bottom left corner to the top right corner.

At density level "1", no ink is ejected for each color, and at density level "2", only one small dot of, for example, about 100 μm is struck in one matrix cell, so the optical density is low. At the density level "32", for example, nine dots of 180 μm are lined up within a 500 μm square, so the optical density is high.

In FIG. 7A, the left side of the equal sign shows a cyan dot matrix table with a density level of "21," and the right side of the equal sign shows the size of the recorded dot in a circle. B indicates the magenta density level "17", and the size of the dot is indicated by a square. C indicates the yellow density level "10" and the size of the dot is indicated by a triangle. Note that this shape is for distinguishing colors, and in reality, all shapes are close to circles.

FIG. 8 depicts a deep sky blue using ink having the density level shown in FIG. 7. The dot matrix of each color has its center shifted, so
A juxtaposed color mixture is created in which the dot positions of each color are different, and an intermediate color similar to an additive color mixture is expressed.

In the above embodiment, an on-demand type ink head is used, but the present invention can also be applied to other types of ink heads.
Also, the dot matrix of each color is m=3, n=3
Although the description has been made for a 3×3 dot matrix, the present invention can also be applied to a 4×4 dot matrix. Furthermore, when using a special color such as skin color, recording may be started from the empty corner matrix cell.

As explained in detail above, the present invention is capable of forming dots of cyan ink so that the centers of n×m (n, m are integers of 3 or more) dot matrices of each color ink constituting the same pixel do not coincide. The dot matrix of magenta ink is shifted vertically and horizontally by at least one dot with respect to the matrix, and the dot matrix of yellow ink is shifted vertically by at least one dot with respect to the dot matrix of cyan ink. Since the ink droplets of each color are shifted side by side when the density level is low, they do not overlap, so it is possible to express a bright intermediate color without turbidity. Furthermore, even when the density level is high, some of the matrix cells undergo side-by-side color mixing, so that color turbidity can be reduced.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram showing an inkjet printer embodying the present invention, Fig. 2 is a front view showing an embodiment of the head portion, Fig. 3 is a plan view thereof, and Fig. 4 is a diagram showing the arrangement of the head and dot matrix. An explanatory diagram showing the relationship,
FIG. 5 is a block diagram showing a circuit for driving three cyan heads, FIG. 6 is an explanatory diagram showing an example of a dot matrix table, and FIG. 7 is an explanatory diagram showing the relationship between the dot matrix table and dots. ,
FIG. 8 is an explanatory diagram showing an example of juxtaposed color mixing. 1...Rotating cylinder, 2...Recording paper, 4...Central processing unit, 7...Head section, 13-15...Head for cyan, 16-18...Head for magenta, 19-21...Yellow Head for use.

Claims (1)

[Claims] 1. At least cyan ink, magenta ink,
Using yellow ink, n x m that constitutes one pixel
In an inkjet printer that records a color image by ejecting ink droplets into a dot matrix (where n and m are integers of 3 or more), the centers of the dot matrices of each color ink constituting the same pixel do not coincide. First, the dot matrix of magenta ink is shifted vertically and horizontally by at least one dot with respect to the dot matrix of cyan ink, and the dot matrix of yellow ink is shifted by at least one dot with respect to the dot matrix of cyan ink. By shifting one dot vertically, when the density level is low, juxtaposition color mixing is performed where ink droplets of each color do not overlap, and when the density level is high, juxtaposition color mixing and subtractive color mixing where ink droplets of each color overlap are used. An intermediate color expression method characterized by: