JPH0223352B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a color image forming method for forming a color image, and more particularly to a gradation method and a color image forming method with excellent color reproducibility.

Although the present invention is applicable to various printers such as inkjet printers, thermal transfer printers, and electrophotographic printers, the present invention will be described below using a color inkjet printer as an example.

<Prior Art> To form a color image, generally at least three types of ink, cyan, magenta, and yellow, are used, and the density and printing method of each color are controlled according to data for each pixel.

In order to obtain good image quality, it is necessary to have a wide gradation range for each of the colors cyan, magenta, and yellow, and for this purpose a method has been proposed in which inks of different densities are used for each color. According to this method, a wide range of gradations can be obtained as shown in FIG.

However, this method has a problem in that false contours occur at joints between dark and light inks because inks of different densities are switched and used.

Therefore, as shown in FIG. 2, if the difference in density between inks of each density is reduced, it is possible to prevent the occurrence of false contours, but this will narrow the range of gradations that can be reproduced.

To solve this problem, it is possible to obtain a wide range of gradations and prevent false contours by increasing the types of ink densities as shown in Figure 3, but in this case, the necessary head The number of head drivers required increases accordingly, and
Image processing also becomes complicated and the cost of the device increases. Furthermore, considering the reliability of the heads, in this type of method any trouble in any one head will be fatal to the image, so it is desirable to reduce the number of such troubles as much as possible.

In other words, in terms of image quality, it is desirable to use as many types of ink with different concentrations as possible, but when using a liquid coloring material such as ink for image formation, it is better to use ink with a lower concentration. The inventors have discovered that, as a result, the total amount of ink jetted onto paper for image formation increases, causing ink smearing or dripping, which in turn causes a problem in which resolution deteriorates.

Therefore, in the past, there was a problem in that it was not possible to maintain high reproducible gradation width and high resolution.

<Object of the Invention> In view of the above points, the object of the present invention is to provide a color image forming method which is sufficiently satisfactory in terms of image quality and which can be put into practical use considering the cost and reliability of the apparatus. .

<Background of the Invention> Furthermore, as shown in FIG. 4, in a region where the density gradually changes, dark dots (●) and light dots (○)
If a boundary exists, the reflected optical density (hereinafter referred to as
Although the dots are continuous in terms of OD value), since the light dots have a large area and the dark dots have a small area, the border looks visually unnatural and gives a false contour.
In addition, small, dark dots are highly irritating to the eyes, giving a sense of roughness.

On the other hand, as is well known, the resolution of the human eye varies depending on the color. The difference in dot diameter as mentioned above, or the feeling of roughness caused by small dots of dark ink, naturally depends on the resolution of the color, and the higher the resolution of a color, the more noticeable the difference in dot diameter and roughness will be. Cheap. By the way, the resolution for blue is lower than that for green and red, and since cyan, magenta, and yellow are inks that control red, green, and blue, respectively, the resolution for yellow is also lower than that for cyan and magenta. In other words, it can be said that the boundary between cyan and magenta is less conspicuous than the boundary between yellow.

In fact, we formed patterns as shown in FIG. 4 for each of the colors cyan, magenta, and yellow. At this time, of course, the density formed by each ink was adjusted so that the same OD value could be obtained for the same dot diameter.

As a result, it was confirmed that magenta and cyan feel more unnatural than the border between yellow and yellow. From this, it can be inferred that good image quality can be obtained even if the number of yellow ink density types is smaller than that of cyan and magenta.

<Description of Examples> In this example, inks with two different densities were used for yellow, and inks with three different densities were used for magenta and cyan. Yellow is dye concentration
Figure 5 shows the relationship between dot diameter and OD value using 0.5% ink YU and 4% ink YK.

Magenta is ink MU with dye concentration 0.2%, 0.7%
Ink MT, 3% ink MK, cyan is ink CU with dye concentration 0.3%, ink CT 0.7%, 2.0%
Figures 6 and 7 show the relationship between dot diameter and OD value using the respective inks CK. And from YU to YK
The switching point L1 is OD value 0.34, from MU to
For MT, L2 = 0.28, from MT to MK, L3 = 0.48, from CU to CT, L4 = 0.28, and from CT to CK, L5 = 0.58.

With this configuration, the difference in density between cyan and magenta at the ink switching point is reduced, and the occurrence of false contours is suppressed.
Furthermore, since small dark dots are not used, there is no roughness in the image. Furthermore, for yellow, even if there is a certain degree of density difference, it is not noticeable.

Next, a control block diagram of an inkjet recording apparatus for realizing this embodiment is shown. A video signal VS containing RGB color signals and a synchronization signal is sampled by a sample hold circuit 81 using a sample signal obtained from the synchronization signal for each color signal, and the A/D
It is output to converter 82. The A/D converter 82 converts the R, G, and B color signals into digital signals indicating gradations, and the obtained signals are stored for a predetermined number of lines.

The data in the line memory 83 is then converted into a digital signal for driving each inkjet head by an image processing circuit 84, which will be described in detail later. The digital signal is converted into an analog signal by a D/A converter 86, and is then driven to each inkjet head 92 via a driver 89.

In this embodiment, the head 92 includes yellow dark ink YK, yellow light ink YU, MU, MT, MK,
It is composed of individual heads for each of CU, CT, and CK inks, and since expressing black using Y, M, and C would cause the ink to become stale and pure black cannot be expressed, a head for black ink is also provided. As the ink jet head, a head using a piezoelectric element whose ejection amount changes depending on the applied voltage is used. For example, special public service 51-39495
A head such as that described in No. 1 is effective.

On the other hand, a system controller 85 that controls the sequence of the printer generates a head drive signal, a carriage motor drive signal, and a paper feed signal at a timing corresponding to the input video signal. The signal is supplied to a motor driver 88, and an inkjet head 92, a carriage motor and its mechanism 90, and a paper feed motor and its mechanism 91 are controlled at predetermined timing to print a reproduced image of the input video signal on a recording medium.

Next, the operation of the image processing section 84 shown in FIG. 8 will be explained using FIG. 9. Digital signals of each color from the line memory 83 are converted into YMC density data by a YMC converter 101, and further subjected to γ conversion by a γ converter 102. Then, a common component of YMC, that is, an achromatic color component K is obtained in the undercolor removal circuit 103, and the achromatic color component is subtracted from YMC. Next, the color correction circuit 104
A masking process is performed to remove the influence of cloudy components of the ink.

Then, the yellow digital value is input to a yellow reference table 105, and the table 105 determines whether to use light ink or dark ink according to the input digital value, and also indicates the voltage value to be applied to each ink head. Outputs digital signals YU′ and YK′. Similarly, the magenta reference table 106 is
MU′, MT′, MK′ in cyan lookup table 10
7 outputs CU', CT', and CK'. Further, the achromatic color component K' is output as is to the blackhead driver via the D/A converter 86. Similarly, other digital signals are also output to each head driver via the D/A converter 86.

As explained above, according to this embodiment, for magenta and cyan, which are highly sensitive to the human eye, inks of various densities are used, so it is possible to prevent the occurrence of false contours due to density differences, and As shown in FIGS. 5 to 7, the minimum dot diameter of ink is larger for magenta and cyan than for yellow. Therefore, the roughness that generally occurs when using small dots of dark ink is also improved.

In addition, as another embodiment, a kind of yellow,
A method using three types of magenta and cyan can also be considered.

In this embodiment, a type of printer that reproduces gradation and color by modulating the size of dots has been described as an example, but a printer that can control the colored area occupied within a predetermined area, i.e. The present invention can also be applied to printers that perform density modulation, such as a dither method or a halftone method.

<Description of Effects> As described above, the present invention uses three colors of yellow, magenta, and cyan, and when using a coloring material with multiple concentrations for at least one of the three colors, the concentration of each color of yellow, magenta, and cyan is The number of types is N _Y ,
When N _M and N _C are used, the respective magnitude relationships are N _M and N _C ＞
Since it is set to N _Y , it is possible to prevent false contours from occurring at the boundary between light and dark coloring materials in magenta and cyan, which have particularly high resolution for the human eye, and the type of coloring material for yellow is Since the number of inkjet heads is small, the type of coloring material is also limited.For example, if an inkjet head is used, the number of heads is also limited, and the entire device is simplified, improving reliability and reducing costs. It can be measured. Furthermore, in the present invention, by setting the type of coloring material for yellow to be smaller than that for cyan and magenta, it is possible to reduce not only the type of coloring material as a whole but also the total amount of coloring material, which reduces the resolution caused by ink bleeding etc. It is possible to prevent a decrease in color and reproduce a wide range of gradations. Furthermore, when the present invention is applied to a type of printer that reproduces gradation by modulating the size of dots, the size of the smallest dot of dark cyan and magenta colorants is also set larger than that of yellow. As a result, the roughness to the eyes, which generally occurs when using small dots of dark colorants, is also improved.

[Brief explanation of drawings]

Figures 1, 2, and 3 are diagrams showing the relationship between the ink dot diameter and OD value for each density, Figure 4 is a diagram showing the pattern at the boundary between dark dots and light dots,
Figures 5, 6, and 7 are diagrams showing the relationship between the dot diameter and OD value of the ink used in this example for yellow, magenta, and cyan, respectively, and Figure 8 is the relationship between the ink dot diameter and OD value of the ink used in this example. printer control circuit diagram,
FIG. 9 is a detailed block diagram of the image processing circuit 84 of FIG. 8. In the figure, 84 is an image processing circuit, 86 is a D/A
89 is a head driver, 92 is an inkjet head, and 105, 106, and 107 are reference tables for yellow, magenta, and cyan, respectively.

Claims (1)

[Claims] 1. In a color image forming method using at least three colors of yellow, magenta, and cyan, and using a plurality of types of liquid colorants having different intrinsic densities for at least one of the three colors, yellow, magenta,
The number of unique density types for each cyan color is N _Y , N _M , N _C
When, the size relationship of each is N _M ＞N _Y and N _C ＞
A color image forming method characterized by N _Y.