JPH0330504B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an inkjet printing device that expresses the degree of shading by the adhesion density of ink droplets ejected per unit area.

When forming shading or intermediate tones using an inkjet printing device, this can be done by controlling the particle size of the ejected ink droplets, but this particle size control depends on temperature, humidity, ink used, etc. It is not necessarily constant due to fluctuations in external factors.

On the other hand, some inkjet printing devices, such as electric field control type devices, do not allow the particle size of ink droplets to be varied.

Conventionally, therefore, shading is expressed by controlling the adhesion density of ink droplets shot per unit area.

The applicant of the present application also filed a patent application in 1983 in view of such density control.
No. 50811 was filed for a color inkjet printing device.

FIG. 1 shows the main part of the color inkjet printing apparatus disclosed in Japanese Patent Application No. 50811/1982, and shows only the signal series of one color. In the figure, 1 is a comparator that compares the input image signal Vij and the threshold value Tij;
is a threshold value matrix in which the threshold values Tij to the comparator 1 are set in a matrix, and the threshold value pattern of this matrix 2 is, for example, assuming that the input image signal Vij includes gradation information of 17 gradations, the second As shown in the figure, the numbers are randomly scattered between 0 and 15. The pattern of this threshold matrix 2 is determined by the dither method. The above dither method compares the pixel level of the input image with a threshold value Tij and determines the level in a binary display device in which the display cell has only two brightness levels, bright and dark, and cannot produce shading by itself. This method converts the signal into a binary signal of 1 or 0, and displays the binary signal on a binary display device, so that it appears to the human eye as if shading is formed. The principle is that if cells that are turned on are clustered together, the area will appear white, and areas where cells that are turned on and turned off alternately will appear gray when viewed from a distance. It is. As mentioned above, the dither method is a type of algorithm that determines which cells should be turned on in order to obtain a binary image as close to the original image as possible in a binary display device.A more detailed explanation can be found at Nikkei Electronics. 1978, 5-1, pp. 50-65.

Returning to the explanation of FIG. 1 again, 3 is the comparator 1 mentioned above.
4 is a head drive circuit that converts the pixel signal read out from the pixel memory 3 into an ink droplet ejection signal, and 5 is a head drive circuit that ejects ink droplets 6, 6, . . . based on the above ejection signal. This is an ink head.

The transmitted image signal Vij is first introduced into a comparator 1, which converts it into a threshold matrix 2.
is compared with the threshold value Tij at the location (row i, column j) corresponding to that image signal. Input image signal of comparator 1
If the 4×4 pixel matrix by Vij is all gradation 4 as shown in FIG. 3A, the matrix of comparison output Rij output to the pixel memory 3 is
The result is as shown in Figure B. That is, the input image signal Vij,
Between the threshold value Tij and the comparison output Rij, the following relationship holds: Rij=0 when Vij≦Tij, and Rij=1 when Vij>Tij.

This comparison output Rij is stored in the pixel memory 3 until the comparison operation of the comparator 1 is performed for one row or one column in the ejection scanning direction of the ink head 5. The pixel "1" or "0" read from the pixel memory 3 is passed to the head drive circuit 4 to form an ejection signal if it is "1" and not to form an ejection signal if it is "0". An ejection signal of "1" is applied to the ink head 5 and ink droplets 6, 6, . . . are ejected. That is, in the matrix of the comparison output Rij shown in FIG. 3B, the ink droplets 6, 6, . . . adhere to the "1" locations and do not adhere to the "0" locations. As a result, the ink droplets 6, 6, . . . are scattered with regularity based on the threshold value matrix 2 by the dither method, and an image having halftones without deviation can be printed, which is extremely useful.

However, according to the conventional method, a total of 17 (0 to 16)
The number of ink droplets 6, 6, etc. proportional to each gradation of the input image signal Vij having gradation is applied to a unit area, and for the input image signal Vij of gradation 4, the ink droplets 6, 6, etc.
As shown in the figure, the actual situation is that four ink droplets 6, 6 are ejected, and for gradation 8, eight ink droplets 6, 6, . . . are ejected.

However, assuming that the maximum reflection density (the darkest part) obtained with an inkjet printing device is 1.0,
The relationship between this reflection density and the number of ink droplets ejected per 4×4 unit area is as shown in FIG. For example, the input image signal Vij of gradation 4 as mentioned above
In response to the input of 4 ink droplets 6, 6 as shown in FIG.
If you enter ..., the reflection density obtained will be 0.38, which does not match the reflection density of gradation 4, which is 0.25. Similarly, gradation 8
Also, the reflection density obtained is 0.68, making it impossible to obtain the desired reflection density of 0.5. That is,
For input image signal Vij of actual gradation 4, gradation 6 is applied.
For gradation 8, a reflection density of 0.68 corresponding to gradation 11 was printed, and the input image signal
The gradation of Vij was not faithfully expressed. As a result, the density conversion characteristic expressed by the print density with respect to the gradation of the input image signal Vij follows a curve as shown in FIG. 5, and the resulting print has a high contrast.

The present invention provides a plurality of threshold values based on different density conversion characteristics, such as equal density conversion characteristics that faithfully print the gradation of the input image signal Vij and soft density conversion characteristics, in addition to such high-tone density conversion characteristics. Equipped with a matrix,
The purpose is to form an image print having desired characteristics by selectively using the plurality of threshold value matrices in consideration of the density conversion characteristics to be obtained.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 6 is a schematic block diagram of an inkjet printing apparatus according to the present invention, which differs from the conventional example shown in FIG. 1 in that four threshold matrices 2a, 2b, 2c, and 2d are provided based on different density conversion characteristics. The four threshold matrices 2a, 2b, 2c, and 2d are artificially selected and the selected one is connected to the input terminal of the comparator 1. For example, the different density conversion characteristics include, as mentioned above, soft tone conversion characteristics such as high tone, and characteristics where halftones are emphasized, and FIG. 7A is an equal density conversion characteristic, and B is a characteristic where halftones are emphasized. In the density conversion characteristics, C corresponds to a high-tone density conversion characteristic, and D corresponds to a soft-tone density conversion characteristic. FIGS. 8A to 8D are pattern diagrams of threshold matrices determined based on each of the density conversion characteristics described above. The pattern of threshold matrix 2, that is, each threshold Tij is the seventh
The theoretical density, which is the printing density for each gradation of the input image signal Vij, is calculated based on Figures A to D, and this theoretical density is compared with the reflection density in Figure 4 to obtain a reflection density that is approximately equal to the theoretical density. It is determined from the number of ink droplets that can be produced.

Incidentally, FIGS. 8A to 8D show the threshold value matrix 2 when an input image signal Vij containing gradation information of 0 to 64 is printed with up to 16 ink droplets 6, 6, . . . per unit area. In other words, although the number of gradations (excluding 0) of the input image signal Vij is 64, the number of ink droplets is 16, so if you simply calculate, there are 4 gradation widths for one ink droplet. This is more preferable than the case where the number of ink droplets and the gradation width are 1:1, since it is possible to realize accurately selected density conversion characteristics.

Next, the specific procedure for determining the pattern of threshold matrix 2 will be explained.
A case will be explained in which 17 pieces of gradation information varying in the range of 16 to 16 are included. This equal density conversion characteristic means that the maximum level of gradation in the input image signal Vij is the maximum value of printing density (1.0 in the inkjet printing apparatus of this embodiment, but the ink that can be used is It varies depending on the printing medium, etc.) and is divided equally into each gradation, and by using this equal density conversion characteristic, each gradation of the input image signal Vij can be faithfully converted into printing density. I can do it.

Each threshold value Tij of the threshold value matrix 2 in this example is determined by calculating the gradation level/number of gradation levels=theoretical density, comparing the theoretical density with FIG. This can be easily determined from the number of ink droplets produced, as shown in FIG. More specifically, as mentioned above, the number of gradations is 16.
The theoretical density of gradation 8 is 0.5, and this 0.5
The optimal number of ink droplets 5 to achieve this is determined from FIG. 4, and the threshold Tij
The above 8 is placed at the location of =5.

The threshold value matrix 2 formed in this manner has a uniform density conversion characteristic, and by selecting this matrix 2, it is possible to obtain an image printing faithful to the input image signal Vij as intended.

Now, the explanation will be returned to the embodiment of the present invention shown in FIGS. 6 to 8.

Then, a pattern selection signal PS is applied to select the threshold matrices 2a, 2b, 2c, and 2d based on the density conversion characteristics to be obtained. As a result, comparator 1 selects one threshold value matrix 2
The threshold values Tij of a, 2b, 2c, and 2d are compared with the input image signal Vij, and the ink head 5 is driven by the comparison output Rij. Figures 10A to 10D are shown in Figure 8A as threshold matrices 2a, 2b, 2c, and 2d.
This figure schematically shows the adhesion state of ink droplets 6, 6, . In the figure, the row direction of A to D corresponds to each density conversion characteristic, and the column direction of I to I is a group for each gradation of the input image signal Vij, and the Arabic numerals represent the groups for each gradation of the input image signal Vij. It represents
The equal density conversion characteristic of A is 1 for input of gradation 8.
Ink droplets 6, 6, . . . are attached at 0 locations in the density conversion characteristic B where halftones are emphasized and in the soft tone density conversion characteristic D, and in 2 locations in the hard tone density conversion characteristic C.

Although the above explanation has been made using a monochromatic image as an example, if it is applied to the color inkjet printing device disclosed in Japanese Patent Application No. 55-50811, it is possible to obtain color images with a variety of different tastes. . The threshold matrix 2 at this time is shown in FIG. 11 when the input image signal Vij has 64 gradations. In Figure 11, A
The ~D rows correspond to equal density conversion characteristics or soft density conversion characteristics, and the A column corresponds to magenta, the R column corresponds to cyan, and the C column corresponds to black.
Since it is the same as in Figures A to D, it will be omitted.

As is clear from the above description, the inkjet printing apparatus of the present invention uses each threshold of one matrix selected from a plurality of randomly different threshold matrices based on density conversion characteristics, and an input image signal containing gradation information. , and the ink head is driven by the comparative output, so that the ink droplets ejected from the ink head are scattered in a unit area based on the selected density conversion characteristic, making it easy to print multiple images with different styles. can be obtained.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an outline of a conventional inkjet printing device, Fig. 2 is a pattern diagram of a conventional matrix, and Figs. 3 A and B are patterns of input image signals and comparison outputs to explain the conventional example. figure,
Fig. 4 is a reflection density characteristic diagram showing the relationship between the number of ink droplets per unit area and reflection density, Fig. 5 is a conventional density conversion characteristic diagram, and Fig. 6 is a block diagram schematically showing the inkjet printing apparatus of the present invention. Figures and Figure 7 are A to D.
are various concentration conversion characteristic diagrams for obtaining the present invention, No. 8
Figures A to D are pattern diagrams of threshold value matrices corresponding to A to D in Figure 7, Figure 9 is a pattern diagram of threshold value matrices in other embodiments of the present invention, and Figures A to D are pattern diagrams of threshold value matrices corresponding to A to D in Figure 7. A schematic diagram illustrating the state of ink droplets ejected into a unit area based on the threshold matrices of Figures A to D, according to the gradations of the input image signal.
FIG. 1 shows a pattern diagram of a threshold matrix in still another embodiment of the present invention. 1... Comparator, 2... Threshold matrix, 5...
Ink head.

Claims (1)

[Scope of Claims] 1. In an inkjet printing device that expresses the degree of shading by the adhesion density of ink droplets ejected per unit area, the value corresponds to the unit area and varies randomly and The patterns include a plurality of threshold matrix groups whose patterns differ based on predetermined density conversion characteristics, and each threshold and gradation information of a group of threshold matrices selected from the plurality of matrix groups according to the density conversion characteristics. The ink head is provided with a comparator that compares the input image signals with the respective input image signals, and an ink head that is driven by the comparison output of the comparator, and the ink head adjusts the amount of ink droplets deposited per unit area based on the pattern of the selected threshold matrix. An inkjet printing device characterized by scattering. 2. The ink jet printing apparatus according to claim 1, wherein the pattern of the threshold value matrix is determined by a dither method.