JPH0974490A - Image processing method and device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To generate a 2-color output image without a sense of incongruity from an original image. SOLUTION: A received RGB signal is converted into a signal L*a*b*, a lightness density conversion section 301 generates a density D, a weighting coefficient generating section 302 calculates a coefficient α=(cos(θ-θstart))/2+0.5 based on a hue angle θobtained by the signal a*b* and a hue angle θstart of a color having a saturation of an output color to use αD as the density. As to the remaining color, a value (D-αD) is used for the density. Thus, the density of the output image is preserved with respect to the density of the input image and the picture element of the color with the saturation gets higher density as close as to the color in the original image and the image without a sense of incongruity is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing method and apparatus for converting a color signal of three or more colors into two colors such as a copying machine or a printer which outputs an image of two colors.

[0002]

2. Description of the Related Art Conventionally, an image processing device capable of outputting two colors,
For example, in a copying machine, red and blue toners whose first color is black and whose second color is other than black are used, and most of the outputs of the second color are binary outputs.

[0003]

However, in the above-mentioned conventional apparatus, an image output for a manuscript having gradation information, such as a photograph or a printed matter, can be obtained by simply coloring the discriminator or a portion to be emphasized. The appearance of the original and the original match, resulting in a hard-to-see copy.

On the other hand, a copying machine having a gradation for the second output color has also begun to appear, but for the R, G, B signals obtained by reading the original, the CMY signal after log conversion is used. Only the black (black component) extraction and the second color component extraction are performed by using the black ink, and a phenomenon occurs in which the black component is not output depending on the hue.

The present invention has been made in view of the above conventional example. Even when an original image is output as a two-color image, a specific color is overemphasized or a specific color is not output. It is an object of the present invention to provide an image processing method and apparatus that can prevent natural image output and obtain an image that does not look strange to a document.

[0006]

In order to achieve the above object, the image processing method of the present invention has the following configuration.
That is,

An image processing method for inputting an image of at least three colors and outputting as an image of two colors, wherein the image density of the first color is the hue of the input image and the hue of the first color. And a second step of determining the image density of the second color such that the sum of the image density of the first color and the image density of the first color is the input image density. And a process.

Alternatively, there is provided an image processing method for inputting an image of at least three colors and outputting as an image of two colors,
A first step of determining the signal level of the first color based on the hue of the input color and the hue of the first color; and the signal level of the first color and the signal of the second color A characteristic indicating the relationship between the level and the density of the output image is stored, and the second signal level is based on the first signal level determined in the first step, the density level of the input image, and the characteristic. And a second step of determining. Further, the image processing apparatus of the present invention has the following configuration. That is, an image processing apparatus that inputs an image of at least three colors and outputs it as an image of two colors, wherein the image density of the first color is set to the hue of the input image and the hue of the first color. And a second means for determining the image density of the second color so that the sum of the image density of the first color and the image density of the first color is the input image density. With.

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment) FIG. 1 is a diagram relating to an image processing apparatus according to a first embodiment that best represents the features of the present invention. In the figure, 101 is an image signal input unit, 102 is a color conversion unit, and 103 is a two-color image conversion unit. The three color signals (RGB or CMY) input from the image signal input unit 101 are
The color conversion unit 102 converts the lightness or density, hue, and saturation signal. The two-color image conversion unit 103 obtains the lightness (or density) of the first color in accordance with the hue and saturation signals.
The lightness (or density) of the color is determined so as to preserve the lightness. The brightness (or density) of the first color is the same brightness (or density) as the brightness (or saturation) of the input image when the two-color output is obtained by using the output signal generating means of the second color for the output signal obtained in the first color. To be

For example, consider an output device in which the first color is black (Bk) and the second color is the output color red (R) with saturation. The document is input by the image signal input unit 101, and the R, G, B
To obtain the image signal of. The R, G, B image signals are converted by the color conversion unit 102 into a lightness (or density) signal, a hue signal, and a saturation signal. As an example, the uniform color space L * a * b *
Think using. Using L * as lightness information, the hue θ (deg) and the saturation S are obtained from the a * b * plane as shown in FIG. The two-color image conversion unit 103 is configured as shown in FIG. That is, the weighting coefficient generation unit 302 obtains the weight α from the hue θ and the saturation S. As the weighting function, for example, the following expression (1) is used.

R = αD Bk = (1-α) D (1) α = (cos (θ−θstart)) / 2 + 0.5 D: Density θ: Hue θstart: Hue angle at which weighting is maximum Here, the weight α As the cos function used in the above, another functional system such as a Gaussian function may be used, or a parameter for adjusting the width in which R is generated may be added.

As can be seen from equation (1), R concentration and Bk
Since the output is performed by adding the densities to obtain the density of the original, the output image is output with the same density as the original. This is realized by the first color output unit 305 and the second color output unit 306 in FIG. In these, the density information obtained by reading the original is dispersed with the weight α so as to be the sum of the densities of the respective colors of the output image, and then output.

FIG. 4 is a graph in which the value of α when θstart = 30 (deg) is plotted on the vertical axis and θ on the horizontal axis. As can be seen from this figure, the value of the weight α continuously changes around θstart (30 deg). Therefore, the second color output is θ
It becomes the largest at start and becomes smaller as it goes away from θstart. Therefore, by setting the value of θstart to be the same as the hue angle of an output color having saturation (red, which is the second color output in the present embodiment), the weight α increases as the hue approaches red, and the red density increases. Go up. In this way, the two-color output image and the original look the same, the red color is overemphasized or not output at all, and no unnatural image is formed.

(Second Embodiment) In the present embodiment,
Expression (1) that gives the weighting coefficient in the first embodiment is changed to the following (2).

R = αD Bk = (1−α) D (2) α = ((cos (θ−θstart)) / 2 + 0.5) × β β = S / Smax D: Density θ: Hue θstart: Maximum weighting Hue angle S: Saturation Smax: Saturation maximum value or saturation of any size Saturation parameter β is introduced to the first embodiment to obtain a more natural two-color printing output. You will be able to get

That is, since β has a larger value as the saturation is larger, the weight α also has a larger value in proportion to the saturation. Therefore, the output of the second color also has a density corresponding to the hue angle and the saturation, and for example, even if the hue is close to red, the density decreases when the saturation decreases and the density of the second color is determined. Smax may be the maximum value of the saturation of the output value, or the maximum value of the saturation of the document or the read signal. Further, even if it is not the maximum value, the value may be arbitrarily changed so that the output of the second color depends on the saturation.

(Third Embodiment) The configuration of the two-color image conversion unit 103 of the first embodiment is changed as shown in FIG.
That is, of the input signals L *, a *, and b *, the weighting coefficient generation unit 602 calculates the coefficient α from a * and b *, and the lightness / density conversion unit 601 calculates the density of the document from L *. The output density of the second color is obtained by the multiplier 603 of α and D,
The second color output signal generation unit 605 generates a second color signal value. In the first embodiment, the signal value of the first color is (1
In contrast to the fact that −α) D is obtained independently of the second color, in the present embodiment, the signal value of the first color is output by the output signal generation unit 605 of the first color based on the result of the output density of the second color. Ask for.

FIG. 5A is a graph of the output device assumed in the first embodiment, in which the horizontal axis represents the signal value and the vertical axis represents the density. The lowest straight line plot is a graph of the signal value vs. density when the signal value of the first color is 0 and only the output of the second color is output. The plot in which the output signal of the first color is increased is in the higher order. The signal line of the first color is the 128th level in the plot of the straight line in the middle, and the first straight line is the output of the first color. It is a graph of the signal value versus density of the second color when the signal is at the maximum 255th level. FIG. 5 (a)
In the case where the output density is obtained by the linear calculation of the density obtained by the first color alone and the density obtained by the second color alone, the density can be stored without any problem with the device shown in the first embodiment. However, in many cases, it does not hold in an actual output device.

FIG. 5B is a graph in which the electrophotographic recording portion is plotted in the same manner as in FIG. 5A. In this case, if there is a signal value for the first color, the output density will not be a simple sum with the signal value for the second color. In the third embodiment, the output density is the first color,
It is used in consideration of a system that cannot be obtained from a simple sum of densities obtained from the signal value of the second color.

Actually, from the second color signal value generated by the second color output signal generation unit 604 and the density value of the original document,
The first color output signal generation unit 605 selects the output signal value of the first color by the table (LUT) containing the plots of FIG. 5B so that the density of the document is stored. Where L
The UT may be replaced by an operation using a function that approximates the plot of FIG.

By doing so, an output image that is more comfortable with the original image can be obtained.

The present invention may be applied to a system composed of a plurality of devices or an apparatus composed of a single device. Needless to say, the present invention can be applied to a case where the present invention is achieved by supplying a program to a system or an apparatus. In this case, the storage medium storing the program according to the present invention constitutes the present invention. Then, by reading the program from the storage medium to the system or device, the system or device operates in a predetermined manner.

As described above, according to the image processing method and apparatus of the present invention, the density of the original is saved, and the density of the original and the density of the two-color image obtained by reading the original are stored. Can be matched.

Further, regarding the output color having saturation, the output density is calculated by considering not only the hue but also the saturation, so that the appearance of the original and the output image can be matched.

Further, by having a table or a function of the signal value vs. density when two colors are simultaneously output, it becomes possible to more accurately match the density of the two-color output image with the density of the original.

[0024]

[Brief description of drawings]

FIG. 1 is a block diagram best representing the features of the present invention.

FIG. 2 is a diagram illustrating a hue θ and a saturation S.

FIG. 3 is a block diagram illustrating a two-color conversion unit in the first and second embodiments of the present invention.

FIG. 4 is a diagram illustrating a weighting function α.

FIG. 5 is a diagram illustrating density versus signal value when two colors are simultaneously output.

FIG. 6 is a block diagram illustrating a two-color image conversion unit according to the third embodiment of the present invention.

[Explanation of symbols]

 Reference numeral 101 is an image signal input unit, 102 is a color conversion unit, 103 is a 2-color image conversion unit, and 104 is a 2-color image output unit.

Claims (5)

[Claims] 1. An image processing method for inputting an image of at least three colors and outputting as an image of two colors, wherein the image density of the first color is the hue of the input image and that of the first color. A first step of determining the image density of the second color based on the hue, and a second step of determining the image density of the second color such that the sum of the image density and the image density of the first color is the input image density. 2. The image processing method, comprising: 2. The first step adds the image density of the first color to the hue of the input color and the hue of the first color as well as the saturation of the first color. The image processing method according to claim 1, wherein the image processing method is determined based on the above. 3. The first step is maximized at the hue angle of the first color, and becomes smaller as the difference between the hue angle of the input image and the hue angle of the first color becomes larger. The image processing method according to claim 1, wherein a weighting coefficient is generated, and the density of the first color is obtained by multiplying the coefficient by the input image density. 4. An image processing method for inputting an image of at least three colors and outputting as an image of two colors, wherein the signal level of the first color is the hue of the input color and the first color. A first step of determining based on the hue, storing a characteristic indicating the relationship between the signal level of the first color, the signal level of the second color, and the density of the output image, and storing the second signal level An image processing method comprising: a first signal level determined in the first step; a density level of an input image; and a second step of determining based on the characteristic. 5. An image processing apparatus for inputting an image of at least three colors and outputting as an image of two colors, the image density of the first color being the hue of the input image and that of the first color. A first means for determining the image density of the second color, and a first means for determining the image density of the second color such that the sum of the image density and the image density of the first color is the input image density. 2. An image processing device comprising: