JPH04369970A - Color image processor - Google Patents

Abstract

PURPOSE:To hold uniform gloss all over the screen by executing the undercolor processing with the substitution ratio to a black signal of a ratio lower than 100% and varying the substitution ratio according to the degree of the equivalent neutral density. CONSTITUTION:The read image signals are color-converted into yellow, magenta, and cyanogen density signals Y0 M0 C0 and inputted to a black generation circuit 101 of an image processing part 300. Thus, a black signal K0 is generated and the signals Y0 M0 C0 are inputted to an undercolor removal circuit 102. The YMCK signal inputted to the undercolor removal circuit 102 outputs a black signal K1 and three-color signals Y1 M1 C1 while referring to a lookup table 102A. In this case, the substitute ratio to the black signal is decided in advance corresponding to the degree of the equivalent neutral density so as to keep the difference of the degree of gloss between the achromatic area and color area in the output image within the allowable range. According to the substitution ratio, the variable setting of the density of the black signal to be generated and the color separation signal after the substitution can be performed.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image processing apparatus for processing, for example recording, color image signals.

0002

[Prior Art] Conventionally, color image output devices using electrostatic photography produce color images separated into four colors (yellow, magenta, cyan, and black) onto a photosensitive drum using a laser beam. Color images are formed through a latent image process in which exposure is drawn using a beam, a development process in which toner is adsorbed onto a photosensitive drum, a transfer process in which the toner is transferred to recording paper, and a fixing process in which the toner is fixed. For example, as an application example of such a color image output device, a color copying device using electrostatic photography separates a color image original into three colors of red (R), green (G), and blue (B) and reads them. Color conversion is performed into density signals for cyan (C), magenta (M), and yellow (Y), which are complementary colors of each RGB color, to obtain a subtractive color mixture system color signal using toner. Normally, black (
K) generate a signal, and based on this density signal, yellow,
Image formation is performed by determining the usage amounts of four color toners: magenta, cyan, and black. Furthermore, by adding black, undercolor removal is performed to replace the black obtained by mixing the CMY three colors. The effects of adding black toner and removing the undercolor are generally (1) improvement of density reproducibility in high-density areas of the image, (2) stabilization of color reproducibility in achromatic areas of the image, and (3) stabilization of color reproducibility in achromatic areas of the image. Improvement in sharpness (4) Reduction in running costs due to reduction in toner consumption is known.

On the other hand, in recent years, color copying machines copy color originals together with black and white originals in offices. For this reason, color copying machines are required to have the functionality and cost performance of conventional black-and-white copying machines, and in some cases, toner for conventional black-and-white copying machines is used as black toner, which is cheaper than color toner.

0004

[Problem to be Solved by the Invention] However, due to the use of toner from a black-and-white copying machine, the glossiness of the image surface differs between color areas and achromatic areas on one image, giving an impression of discomfort to the image viewer. was there.

[0005] That is, the toner used for conventional black and white copying machines as black toner has been designed to increase copying speed.
Durability is required due to the frequency of use. Therefore, the melting point of the toner is made high (180 to 190° C.) so that such use can be eliminated.

On the other hand, in order to improve the color reproducibility of yellow, magenta, and cyan color toners, the melting point of the toners is set to be low (140 to 150° C.) so that the toners can be well melted and mixed. For this reason, color toner is melted by heat in the fixing process of fixing the toner to recording paper, but black toner for black-and-white copying machines may not be sufficiently melted.

When viewed microscopically, the printing area surface of achromatic black toner has unevenness due to the magnetic powder component, and the light incident on the paper surface is diffusely reflected, resulting in less gloss on the image surface.On the other hand, color toner has uneven toner particles. The surface of the printing area is microscopically smooth, and the light incident on the paper surface has a large specular reflection component and is glossy.

For example, in a full-color human image as shown in FIG. 8, skin (Y+M), clothes (Y), and apples (M+Y
), Muscat (C+Y) are reproduced using color toners, and the surface of the image has a high level of gloss, but hair, eyes,
Black toner printed areas such as eyebrows have low glossiness, and the glossiness of the image surface differs between color areas and achromatic areas on one image, and evaluations for people are particularly severe, significantly degrading image quality.

SUMMARY OF THE INVENTION In view of the above-mentioned points, an object of the present invention is to provide a color image processing apparatus that can achieve a suitable balance between undercolor removal and gloss in image quality.

0010

[Means for Solving the Problems] In order to achieve the above object, the present invention calculates an equivalent neutral density from color separation signals, generates a black signal corresponding to the equivalent neutral density, and generates a black signal corresponding to the equivalent neutral density. In a color image processing device that removes undercolor by replacing the equivalent neutral density portion of the color separation signal with the black signal, the difference in glossiness between the achromatic color area and the color area in the output image is The replacement ratio to the black signal is determined in advance in accordance with the magnitude of the equivalent neutral density so that the black signal to be generated and the color separation signal after replacement are determined to be within a permissible range. It is characterized by comprising an image processing circuit that variably sets the density.

[0011] Also, an image area separation circuit extracts a color separation signal of a black image portion from the color separation signal and replaces the color separation signal with a black signal at a ratio of 100%; The present invention is characterized in that it further includes switching means for switching and outputting a color signal to be image processed instead of the output signal of the image processing circuit.

0012

[Operation] In the present invention, the difference in gloss between the achromatic color area and the color area of the output image is tolerated by executing undercolor processing in the image processing circuit at a ratio of replacing the black signal with a ratio smaller than 100%. Stay within range. Further, by making the above-mentioned replacement ratio variable depending on the magnitude of the equivalent neutral density, uniformity of gloss can be maintained over the entire image area.

[0013] Furthermore, by performing 100% under color removal (UCR) on a black image, black characters and black lines become dull and easy to see.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a block diagram schematically showing an image processing section that performs black generation and undercolor removal processing according to the present invention, and an electrostatic image forming section.

Image signals read by an image input device (not shown) are color-converted into yellow, magenta, and cyan density signals Y0 M0 C0 and then sent to a black generation circuit 101.
is input. Here, by the method described later, the black signal K0 is
is generated, and the Y0 M0 C0 signal is then input to the undercolor removal circuit 102.

FIG. 2 schematically shows the level of each color of the Y0 M0 C0 signal input to the black generation circuit 101 at this time. The shaded area in the figure is the equivalent neutral concentration min (YMC
) and corresponds to the achromatic color component of the reproduced color.

FIG. 3 shows 100% UCR (under color removable) in which all equivalent neutral densities are replaced with black signals.

In this embodiment, at 100% UCR, all equivalent neutral densities are replaced with black signals as shown in the figure.
In addition, all equivalent neutral densities are subtracted from the YMC color signals. Therefore, the signal level of either YMC becomes zero.

Originally, 100% UCR is effective in stabilizing color reproduction in achromatic image areas, reducing costs due to toner consumption, and preventing color shift due to overlapping of three colors in thin lines, but in reality it Due to the discontinuity of the gradation when the color changes to an achromatic color, a part of the equivalent neutral density is empirically replaced with a black signal (skeleton black) as shown in FIG. In this embodiment, the amount of black signal replacement is determined by the glossiness of the printed image. now,
When reproducing an achromatic color of a certain density, color reproduction using a single black color has a low glossiness, and color reproduction using a mixture of three YMC colors has a glossiness that matches that of a chromatic color area. Therefore, in this embodiment, the black component resulting from the three-color mixture is gradually replaced with black toner at each density of achromatic colors, and the replacement rate of black toner at which the glossiness is within an acceptable level is detected in advance in correspondence with the neutral density. , the density of the black signal and three color signals determined from this replacement rate is expressed in the form of equivalent neutral density and printing rate of black toner and three color toner. Further, a lookup table 102A having this characteristic curve (skeleton curve) is created in advance.

FIGS. 5 and 6 show skeleton curves of the black signal and YMC signal amounts for the equivalent neutral density.
An example is shown.

YMC signals input to the black generation circuit 101,
And the YMCK signal input to the under color removal circuit 102 is
Refer to the lookup table to determine the black signal K after removing the undercolor.
1, 3 color signals Y1 M1 C1 are output.

As described above, the replaced Y1 M1 C
1 K1 signal is sent to D/D in sequence by the selector 103.
After being subjected to V/T conversion by an A converter, triangular wave generator, and comparator, it enters the laser driver. This three color signal Y
Based on the 1 M1 C1 signal, the laser light emitted for each color passes through an optical system such as a horigon mirror 110,
A latent image is formed on the photosensitive drum 112, and an output image is obtained through development, transfer, and fixing processes.

The toner used in this example is as follows.

Black toner: Number average molecular weight approximately 10.0
00 polyester main binder, 5 parts by weight of carbon black, and a charge control agent (hereinafter referred to as CA
It is written as agent. ) 4 parts by weight, and an external additive.

Yellow toner: Number average molecular weight approximately 3500
100 parts by weight of polyester main binder,
C. I. Pigment Yellow 17 5 parts by weight, CA agent 4
Toner consisting of parts by weight and external additives.

Magenta toner: Number average molecular weight approximately 3500
100 parts by weight of polyester main binder,
Content C. I. 4 parts by weight of Melvent Red 49, Dye C
．． I. A toner consisting of 0.7 parts by weight of Pigment Red 122, 4 parts by weight of a CA agent, and external additives.

Cyan toner: A toner consisting of 100 parts by weight of a polyester main binder having a number average molecular weight of about 3500, a phthalocyanine content of 5 parts by weight, a CA agent of 4 parts by weight, and an external additive.

Image forming experiments were conducted using the above four types of toners mixed with magnetic carrier particles and used as a developer.

In addition to this embodiment, the following examples are given.

1) As shown in FIG. 7, a well-known image area separation circuit 501 is provided in the image processing section 500, and the black character part and black line drawing part of the original image in the color image are separated. It is also possible to perform a 100% UCR like this. In this case, when a black image is detected by the image area separation circuit 501, a lookup table (not shown) is used to
Perform %UCR. As a result, the black (K) signal obtained is outputted via the selector 103 (switching means of the present invention). As a result, it is possible to copy a monochrome text document at a low copying cost without color shift, while obtaining the same effect as the first embodiment in the color image portion. In addition, by printing only the black and white character portion with a single color of black toner, there is an effect that the character becomes easy to read without gloss.

2) The black toner used in this example was:
All of these toners are non-magnetic toners, but a magnetic toner containing a magnetic material such as magnetite in the toner binder may be used instead. Since magnetite is an infusible substance, the melting temperature of magnetic toner containing it is generally high. Further, since the magnetite magnetic powder forms microscopic irregularities on the fixed image, incident light is likely to be diffusely reflected, and the glossiness of the image surface is significantly reduced. Therefore, when the present invention is applied to such a magnetic toner, even more effects can be exhibited effectively.

In this example, 60 parts by weight of magnetite, 100 parts by weight of a polyester main binder with a number average molecular weight of about 3500, and
An image forming experiment was conducted using a jumping development method as a magnetic one-component development using a toner consisting of 2 parts by weight of a CA agent and an external additive.

[0034]

[Effects of the Invention] As explained above, in the present invention, UC
The execution ratio of R is not 100%, but is executed at a ratio that provides good glossy image quality, and the ratio is set variably in accordance with the equivalent neutral density, so in the color image part, the entire image is High-quality color reproduction with uniform gloss is possible.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram schematically showing the configuration of main parts of an embodiment of the present invention.

FIG. 2 is an explanatory diagram for explaining image processing according to an embodiment of the present invention.

FIG. 3 is an explanatory diagram for explaining image processing according to an embodiment of the present invention.

FIG. 4 is an explanatory diagram for explaining image processing according to an embodiment of the present invention.

FIG. 5 is an explanatory diagram showing an example of a skeleton curve according to an embodiment of the present invention.

FIG. 6 is an explanatory diagram showing an example of a skeleton curve according to an embodiment of the present invention.

FIG. 7 is a configuration diagram showing the configuration of a second embodiment of the present invention.

FIG. 8 is an explanatory diagram for explaining conventional problems.

[Explanation of symbols]

101 Black generation circuit 102 Undercolor removal circuit 102A Lookup table 103 Selector 112 Photosensitive drum 113 Developing device 114 Transfer drum 300,500 Image processing section 501 Image area separation circuit

Claims (2)

[Claims] [Claim 1] Calculating an equivalent neutral density from the color separation signals and generating a black signal corresponding to the equivalent neutral density,
In a color image processing device that removes undercolor by replacing the equivalent neutral density part of the color separation signal with the black signal, the difference in glossiness between each area of the achromatic color area and the color area in the output image A replacement ratio to the black signal is predetermined in correspondence with the magnitude of the equivalent neutral density so that the ratio is within a permissible range, and according to the replacement ratio,
A color image processing device comprising an image processing circuit that variably sets the density of the black signal to be generated and the replaced color separation signal. 2. An image area separation circuit that extracts a color separation signal of a black image portion from the color separation signal and replaces the color separation signal with a black signal at a ratio of 100%; 2. The color image processing apparatus according to claim 1, further comprising switching means for switching and outputting a color signal to be image processed instead of an output signal of the image processing circuit.