JPH1178070A - Ink density fixing method and image output apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve visual recognizability and graininess for ink dots in highlights while maintaining balance in graininess for the entire image and to devise to keep balance in the amount of ink consumption on fixing up density for ink on outputting digital images with inks in the same color in a plurality of different densities. SOLUTION: The range of variation for inputted image data is divided approximately in equal parts, and one kind or one or more kinds of ink dots are generated for the inputted image data at a certain point of equally divided parts, and ink density is fixed up so that optical density of the outputted images obtained by generating the maximum amount of ink dots per unit area until reaching the maximum ink capacity of a recording medium to be used can be made to correspond to the optical density at the required point of equally divided parts.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to selection of an ink density used in an image output device such as an ink jet printer, and more particularly, to an ink density when outputting a digital image using the same color ink having a plurality of different densities. It is about optimization.

[0002]

2. Description of the Related Art In general, many image output devices such as an ink jet printer realize output of an image by controlling whether or not ink is ejected onto a recording medium. In order to drive these image output devices,
A binary image drive signal corresponding to the output image is required.

[0003] In order to obtain the binary image drive signal, binarization is performed on an input image having continuous gradation, and the continuous gradation image data is converted into binary image data. As a method of binarization, it is an error diffusion method that can obtain a binary image of relatively high image quality. The error diffusion method weights an error (ie, a difference between continuous tone image data and binary image data) generated when converting continuous tone image data into binary image data to a plurality of adjacent pixels. It "spreads" by adding a coefficient. By this operation, the error generated between the input image having the continuous tone and the binarized binary image can be minimized on average,
A binary image having excellent image quality can be obtained. However,
The error diffusion method requires some calculations, including multiplication, to diffuse the error. This is a significant drawback when high-speed processing and large-capacity image processing are required.

On the other hand, there are several methods for reducing the amount of calculation and converting input image data having continuous tone into binary image data at high speed. Among them, the most frequently used is the dither method. In the dither method, as shown in FIG. 5, the input continuous tone image data f (x, y) 5
00 and the threshold value t (x, y) of the dither matrix are compared on a pixel-by-pixel basis, and if f (x, y)> t (x, y), the binary image data is 1, f In the case of (x, y) ≦ t (x, y), the input continuous tone image data is converted into binary image data so that the binary image data becomes 0, and a binary image 502 is obtained. . Here, one pixel of the input continuous tone image corresponds to one dot of the binary image.

Using the binary image data generated by such a binarization method, an image output device is driven to realize image output. At the dot position of the output image, there is a binary state as to whether or not there is a dot. In order to express the gradation of an image, it is general to control and realize the number of ink dots in a unit pixel (consisting of a fixed number of dots) on a recording medium (referred to as area modulation). Figure
6 is a unit pixel composed of a 2 × 2 dot matrix, 1
It shows the number of gradations that can be expressed when using seed density ink. At this time, the number of gradations that can be expressed is 5.

In actuality, when an image is output, it may not be possible to eject ink dots at all dot positions within a unit pixel, depending on the type of recording medium used, the weight of ink dots, the output resolution, and the like. . This is because the maximum amount of ink that can be ejected within a unit area is already determined according to the type of recording medium, and if the maximum amount of ink used is exceeded, bleeding or the like occurs in the recording medium, resulting in poor image quality. Generally, the relationship required between the input image data and the optical density (OD) of the output image is not linear. Accordingly, the relationship between the input image data and the number of ink dots in the unit pixel is not linear.

In this area modulation method, there is a problem that it is difficult to achieve both the number of gradations that can be expressed in an output image and the resolution. That is, as the number of dots constituting a unit pixel increases, the number of gradations that can be expressed increases, but the resolution of the image decreases. Conversely, if the number of dots is small, the resolution of the image is high, but the number of gradations that can be expressed is small.

Therefore, several prior arts have been proposed in order to enhance the gradation expression of an image of an image output device such as an ink jet printer and to achieve both gradation expression and resolution. Among them, there is an application called “ink jet recording apparatus” (Japanese Patent Application Laid-Open No. 60-19538). In this application, a plurality of inks having different densities are used, and the gradation of an image is expressed by a combination of ink dots in a unit pixel (for example, composed of a 2 × 2 dot matrix), and a relatively high image gradation is used. At the same time, the image resolution does not decrease because the number of dots constituting the unit pixel does not increase. FIG. 7 shows the number of gradations that can be expressed when the unit pixel is also composed of a 2 × 2 dot matrix and the ink type is 2. in this case,
The number of gradations that can be expressed is nine. Assuming that there are K types of ink densities and the maximum optical density of the required output image is Dmax for the densities of a plurality of different inks, the optical density of the image output using only this ink is n · D
Determined to be max / K. Here, n = 1,2, .. K.
That is, a technique has been devised in which the density of each ink is determined based on the reflection optical density value obtained by substantially equally dividing the optical density range of the output image.

[0009]

However, the above-described method of determining the density of each ink based on the reflection optical density values obtained by substantially equally dividing the reflection optical density range of the output image has a significant disadvantage. I have. FIG. 8 shows an example of a typical gradation characteristic curve (relation between input image data and optical density OD of an output image). Here, assuming that the input image data changes between 0 and 255 and that the unit pixel is composed of an 8 × 8 dot matrix using four density inks, the nth (n = 1, 2, 3) , 4) Optical density ODn of the image output only with the ink (however, this optical density ODn is the optical density of the output image when the n-th ink dot is generated at all dot positions of the 8 × 8 dot matrix. 7) is determined by equally dividing the entire density range as shown in FIG. Therefore, Ink 1 (the thinnest ink) is used for about two-fifths (between about 155 and 255) of the input data range including the highlight part, and conversely, Ink 3 and Ink 4 (relatively Dark ink)
Is narrower. This degrades the graininess of the image low density portion including the highlight portion. That is, in Ink 1, a considerably high ink density is required to reach the required optical density of the output image (OD1 in FIG. 7). As a result, the visibility of the ink dots in the highlight portion becomes poor, and the granularity of the image also becomes poor. On the contrary, the gradation expression power of relatively dark ink is not used up much. Further, in such a method of determining the ink density, the usage amounts of the respective inks are very uneven, which causes an inconvenient situation for replenishing the ink.

In actuality, when outputting a digital image, the density of the ink used in the highlight portion is made as low as possible, and a large number of ink dots are ejected in order to reach a certain optical density. It is desirable to improve the graininess of the image so that the usage of each ink is as balanced as possible.

SUMMARY OF THE INVENTION An object of the present invention is to improve the visibility and granularity of ink dots in a highlight portion in determining an ink density when outputting a digital image using the same color inks having different densities. The purpose of the present invention is to balance the overall granularity and balance the usage of each ink.

[0012]

SUMMARY OF THE INVENTION In an exemplary embodiment of the present invention, the above-mentioned problem is solved and the above-mentioned object is achieved, but the range of change of the input image data is substantially reduced. One or more ink dots are generated for the input image data at a certain dividing point, and the maximum amount of ink within a unit area is reached until the maximum ink receiving capacity of the recording medium to be used is reached. It is characterized in that the ink density is determined so that the optical density of the output image obtained by generating the dots matches the optical density required at these equal points.

[0013]

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

FIG. 1 shows an image output device used in the present embodiment. In this image output device, the same color ink (Ink1, Ink2, Ink3, Ink4) 105 with four different densities
Where Ink 1 is the lightest ink and Ink 4 is the darkest ink.

The range of the image data input to the image input unit 101 is 0 to 255, and the multi-value halftone processing unit 102 processes the input image data. Multi-level halftone processing unit 10
The output image signal from 2 is sent to the ink head control unit 103
, The Ink 1, Ink 2, Ink 3, and Ink 4 heads are respectively controlled, each ink is ejected, and an image is recorded.

In this embodiment, it is assumed that the required gradation characteristic curve is the curve shown in FIG. Here, the input image data range 0 to 255 is equally divided, and the optical densities corresponding to each equally divided point are indicated by OD1, OD2, OD3, and OD4.

Here, the multi-value halftone method used in this embodiment will be described.

As described above, in the dither method shown in FIG. 5, the output image data is in a binary state of only 0 and 1 as a result of the binarization. This is applicable when only one type of ink is used. However, when using inks of a plurality of densities, the output image data for one dot must be multi-valued in accordance with the type of ink to be used, and a so-called multi-valued dither method is used. For example, when four types of inks are used, the output image data for one dot takes one of 4 + 1 = 5 values (0, 1, 2, 3, 4). These 1,2,3,4 are Ink 1, Ink 2, Ink 3, In
Generates k 4 dots, 0 means do not generate all dots. Also, up to two types of dots are generated for a uniform area having a certain value in the input image.

FIG. 3 shows the relationship between the input image data used in this embodiment and the amount of each ink generated within a unit area. Here, the change range of the input image data 0 to 255 is equally divided, and the input data is the thinnest ink I between 192 and 255.
Using only nk1, gradations are represented by combinations of Ink1 and Ink2, Ink2 and Ink3, and Ink3 and Ink4 between 0 to 64, 64 to 128 and 128 to 192, respectively. In this embodiment, a fixed amount of dots corresponding to the amount of each ink is generated by the multi-value dither method described below. FIG. 4 shows this halftone method. First, the input image data of 0 to 255 is input to the LUT (Look Up Tabl
Output the generated amount of each ink by referring to e). Where L
The UT represents the relationship between the input image data and the amount of each ink generated.In fact, the UT describes the relationship between the input image data and the amount of each ink generated shown in FIG. 3 for each level. is there. Through this LUT, a certain amount of generated ink is output for certain input image data, and among them, there are at most two types of ink whose ink amount is not 0.

Next, dither processing is performed to generate ink dots. If the amount of ink generation is not 0, 1
In the case of a seed, the occurrence of a dot is determined in step 403 using the dither method for this ink. If there are two types of inks in which the amount of generated ink is not 0, the dots of the two types of ink are mixed. In this case, first, the dither method is applied to the relatively dark ink in step 405, and it is determined whether or not dots of this ink are generated. If the ink dot has not been generated, it is determined in step 407 whether a relatively thin ink dot has been generated. In this way, a fixed amount of ink dots corresponding to the amount of each ink can be generated accurately.

In order to determine the relationship between the input image data shown in FIG.
Ink 1 between 5 and γ correction is performed so that the optical density of the output image obtained by the combination of each ink between 0 and 192 matches the gradation characteristic required in FIG. It is necessary to adjust the amount generated. Actually, since the relationship between the amount of ink ejected in a unit area and the optical density of the output image obtained is affected by factors such as the recording medium used, the generation of each ink with respect to the input image data according to the output conditions. The amount needs to be changed. As shown in FIG. 3, only one type of ink dot exists where the input image data is 0, 64, 128, or 192. At this time, the generation amounts of Ink 1, Ink 2, Ink 3, and Ink 4 are shown in FIG.
c, d are indicated by four points. At these four points, the maximum amount of each ink is generated, and the optical density of each output image is obtained from these ink amounts. The greater the maximum amount of ink generated, the more lightly concentrated ink can be used to obtain a certain optical density. The use of this thin ink can improve the visibility and granularity of dots, especially in highlights. However, this maximum amount of ink generation is limited to the maximum amount of ink received by the recording medium used. If the amount of the ink used is too large, bleeding or the like occurs on the recording medium, and the image quality deteriorates. Therefore, it is ideal to determine the maximum ink generation amount so as to reach the maximum ink reception amount of the recording medium to be used. In the present embodiment, after determining the maximum amount of ink generation, the optical density of the output image obtained with this maximum amount of ink generation matches the OD1, OD2, OD3, OD4 in the gradation characteristic curve shown in FIG. Ink 1, Ink 2, Ink 3, and Ink 4 are adjusted so as to perform the above operations. The ink concentrations are OD1, OD2, OD
3, It is not necessary to completely match with OD4, it is only necessary to roughly match.

As described above, the densities of the four different inks are determined. As a result, the density of Ink1 can be considerably reduced, and by using a large number of dots of only Ink1 between 192 and 255, the visibility of the dots in the highlight portion and the graininess of the image can be improved. In the determination of the ink density when outputting a transmission type image using the same color ink of different densities, such an ink density determination method is used,
The visibility of dots in the highlight portion of the output image and the granularity of the image can be improved.

[0023]

According to the present invention, in determining the ink density when outputting a digital image using the same color ink of a plurality of different densities, the visibility and granularity of the ink dots in the highlight portion are improved. At the same time, the balance of the granularity of the entire ink can be maintained, and the usage of each ink can be balanced.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an image output device used in the present embodiment.

FIG. 2 is a diagram illustrating a relationship between an optical density of each ink and a gradation characteristic curve in the present embodiment.

FIG. 3 is a diagram illustrating a relationship between input image data and an amount of generation of each ink in the present embodiment.

FIG. 4 is a diagram illustrating a process of a halftone process in the embodiment.

FIG. 5 is a diagram showing an image of an image binarization process.

FIG. 6 is an example of gradation expression using only one type of ink.

FIG. 7 is an example of gradation expression using two types of ink.

FIG. 8 is a diagram illustrating a relationship between the optical density of each ink and a gradation characteristic curve according to a conventional method.

[Explanation of symbols]

 101 Image data input unit 102 Multi-value halftone processing unit 103 Ink head control unit 104 Ink head 105 Ink cartridge

Claims (4)

[Claims] In determining an ink density when a reflection type image is output using the same color ink of a plurality of different densities,
The change range of the input image data is substantially equally divided, and one or more types of ink dots are generated for the input image data at a certain dividing point until the maximum ink receiving capacity of the recording medium to be used is reached. An ink characterized in that the ink density is determined such that the reflection optical density of an output image obtained by generating the maximum amount of ink dots within a unit area matches the required reflection optical density at this equidistant point. How to determine the concentration. 2. A method for determining an ink density when a transmission type image is output using the same color ink of a plurality of different densities,
The change range of the input image data is substantially equally divided, and one or more types of ink dots are generated for the input image data at a certain dividing point until the maximum ink receiving capacity of the recording medium to be used is reached. An ink characterized in that the ink density is determined so that the transmission optical density of an output image obtained by generating the maximum amount of ink dots in a unit area matches the required transmission optical density at these equal points. How to determine the concentration. 3. The method according to claim 1, wherein in determining the ink density, the same amount of ink is generated at all equally divided points. 4. A method for determining an ink density when outputting a reflective image using the same color ink having a plurality of different densities,
The change range of the input image data is substantially equally divided, and one or more types of ink dots are generated for the input image data at a certain dividing point until the maximum ink receiving capacity of the recording medium to be used is reached. An image characterized in that the ink density is determined so that the reflection optical density of the output image obtained by generating the maximum amount of ink dots within a unit area matches the required reflection optical density at this equidistant point. Output device.