JP2788479B2 - Color image processing equipment - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a color image processing device, and more particularly, to a color image processing device having excellent color reproducibility in both character images and color gradation images.

(Background of the Invention) A color image such as a character image or a photographic image is optically read by being divided into red R, green G, and blue B, and converted into recording colors such as yellow Y, magenta M, cyan C, and black K. (Color reproduction or color correction)
Then, there is a color image processing apparatus that records an image on a recording sheet using an electrophotographic color output device based on this.

FIG. 9 is an explanatory diagram showing a state when discriminating colors (determination of chromatic / achromatic colors) in the above-described color image processing apparatus. In the cube in the figure, the near side in the horizontal direction is the density of R. And the vertical direction is the density of B,
The depth direction is the density of G. Accordingly, the lower left part where the densities of R, G and B are all zero is white, and the upper right part where all the densities are maximum is black. Therefore, a region connecting white and black corresponds to an achromatic (gray) region, and the other region corresponds to a chromatic region.

By the way, the setting of the achromatic region has the following conflicting problems.

In order to reduce color ghosts (unnecessary colors that occur at the edges of black characters) due to color misregistration of the R, G, and B of the CCD sensor and chromatic aberration of the lens, achromatic regions should be reduced as much as possible. Need to be wider.

In the case of a color gradation image, in order to accurately reproduce low-saturation colors (for example, brown, dark blue, purple, etc.), it is necessary to make the achromatic region as narrow as possible.

(Problems to be Solved by the Invention) Due to the above conflicting demands, it has been a reality that a compromise is made by setting an achromatic region within a range that is not dissatisfied with both.

However, in practice, the low chroma color reproduction of the color gradation image in the photograph mode has not been satisfactory. That is, since the achromatic region has a certain width as described above, the low-saturation portion is reproduced as black.

On the other hand, even when a black character image is reproduced, color ghost occurs, and satisfactory results have not been obtained.

In addition, for color gradation images and black character images, gradation (γ) or MT
Although it is preferable to change the F correction amount, it is actually fixed and good reproduction cannot be performed.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems, and an object of the present invention is to improve low-saturation color reproduction in a color gradation image, improve gradation characteristics, and improve resolution in a character image. And a color image processing apparatus capable of reducing color ghosts.

(Means for Solving the Problems) The present invention for solving the above-mentioned problems includes a means for color-coding a color image signal into an achromatic region, a chromatic region, and an intermediate color region; Image determination processing means capable of determining whether the image is a black character image, and when the determination result of the image determination processing means is a color gradation image, the color image signal of the intermediate color area is converted to the color image signal of the chromatic color area. And correcting the color image signal of the intermediate color area to the color image signal of the achromatic color area if the result of the determination by the image determination processing means is a black character image. Image processing means for performing gradation processing or spatial frequency filter processing according to a color gradation image and a black character image on a color image signal after being divided into a chromatic region and a chromatic region Wherein the gradation processing in the image processing means is processing for lowering the gradation when the determination result of the image determination processing means is a color gradation image, and increasing the gradation when the determination result is a black character image. The spatial frequency filter processing in the image processing unit weakens the correction amount of the spatial frequency correction when the determination result of the image determination processing unit is a color gradation image, and reduces the spatial frequency correction amount in the case of a black character image. This is characterized in that the correction amount is increased.

(Operation) In the color image processing apparatus of the present invention, the color-coded area between the achromatic color and the chromatic color is changed according to the type of image, and different image processing is executed depending on the type of image.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a configuration diagram showing the configuration of one embodiment of the present invention. In FIG. 1, reference numeral 1 denotes a density conversion unit for converting 8-bit digital data of R, G, and B provided from outside into data of 6, 6, and 5 bits, respectively. Reference numeral 2 denotes a color code generation unit that receives the R, G, B data from the density conversion unit 1, discriminates white / black / chromatic / intermediate colors described below, and outputs a color code. Reference numeral 3 denotes a color reproduction unit that receives R, G, and B data and performs a color reproduction process for generating Y, M, C, and K data. 4
An image discriminating unit receives the 8-bit data of G and discriminates a black character image / color gradation image from the density gradient. 5
Reference numeral denotes a color code correction unit that receives a determination signal from the image determination unit 4 and sorts the intermediate color code into a chromatic or achromatic color code. Reference numeral 6 denotes a selector for selectively passing density data of Y, M, C, and K from the color reproducing section 3 in accordance with the corrected color code. Reference numeral 7 denotes a color ghost correction unit that performs color ghost correction, 8 denotes a filter processing unit that performs various types of filter processing, and 9 denotes a gradation correction unit that corrects gradation characteristics. In the processing in the filter processing unit 8 and the gradation correction unit 9, different processing is executed for each determination result according to the image determination signal.

 Hereinafter, the operation of the present embodiment will be described.

The original image is read by an image reading unit (not shown), and R, G,
It is converted into 8-bit digital data for each B. Then, the digital data of each of R, G, and B is supplied to the density converter 1. In the density converter 1, 8-bit data is converted into 6, 6, and 5 bit data in accordance with human visual characteristics. The output data of the R, G, B density converter 1 is applied to the color code generator 2 and the color reproducer 3. The color code processing unit 2 outputs a color code indicating whether each pixel belongs to white / black / achromatic / intermediate colors described below, based on the level of each data of R, G, B.

FIG. 2 is an explanatory diagram showing how the color code generation unit 2 generates a color code. In the cube in the figure, the near side in the horizontal direction is the density of R. The vertical direction is the density of B, and the depth direction is the density of G. Therefore, R,
The lower left part (and its periphery) where the densities of G and B are all zero is white (color code: 00), and the upper right part (and its periphery) where all densities are maximum is black. Here, an achromatic color (white is not included. An achromatic color is recorded with black toner, and is hereinafter referred to as black) connecting white and black (color code: 11).
And a relatively wide intermediate color area (color code; 01) is set around this black area. Then, the other area is set as a chromatic area (color code; 10). In other words, in both cases of a black character image and a color gradation image, only an achromatic (black) area is set as a black area. Then, in the case of a color gradation image, the region is a low-saturation region, and in the case of a black character image, a region where a color ghost is possible is set as an intermediate color region. FIG. 3A shows this state in the CIE L ^* a ^* b ^* uniform color space.

Therefore, the color code generation unit 2 generates a 2-bit color code (white; 00, black; 11, intermediate color; 01, chromatic color; 1
0) is output.

This color code is supplied to the color code correction unit 5. Then, the color code correcting unit 5 determines whether the intermediate color code (01) is black (11) based on the image discrimination signal (signal for discriminating a color gradation image / character image) generated by the image discrimination unit 4. Correct the color code to chromatic (10).
That is, if the image being processed is a color gradation image, the intermediate color code is corrected to a chromatic color code (FIG. 3C) to improve the reproducibility of low chroma colors. If the image being processed is a black character image, the intermediate color code is corrected to a black color code (FIG. 3B) to suppress the occurrence of color ghosts.

FIG. 4 is an explanatory diagram for explaining the state of image discrimination. In the figure, X is a pixel of interest for which image determination is to be performed, V is a pixel one line before, W is a pixel one pixel before,
Y is a pixel one pixel later, and Z is a pixel one line later.
Here, a density gradient is obtained using the density data (8 bits) of each pixel. That is, the density gradient S of the target pixel X
Is obtained by the following equation.

S = | V−X | + | W−Y | In this manner, the S-parameter of the density gradient is determined for the peripheral pixels.

In addition to the S parameter, a parameter of S ′ = | V−X | + | W−X |... S ″ = | V−Z | + | W−Y |. 2
Since only pixels are used, the discrimination ability is not sufficient, and S "has a disadvantage that it requires three pixels in the sub-scanning direction and thus requires a large number of image memories. I will use.

FIG. 5 is a configuration diagram showing details of the image discrimination unit 4. In the figure, reference numeral 12 denotes a register for holding data of the pixel Y, and 13
Is a register for holding the data of the pixel X, 14 is a register for holding the data of the pixel W, 15 is a subtraction absolute value generating circuit that subtracts W and Y to generate an absolute value (| W−Y |), 16 , 17 are registers for holding the data of the pixel V, 18 is a subtraction absolute value conversion circuit for subtracting V and X to generate an absolute value (| V−X |), and 19 is a subtraction absolute value conversion circuit 15, 18. Output (| W−Y |
+ | V−X |), 20 is a threshold value generating circuit for generating a threshold value of the density gradient, and 21 is an image discriminating signal by comparing the added output of the adding circuit 19 with the threshold value. This is a comparison circuit.

Next, this image discrimination will be described in terms of color.
On the chromaticity plane a ^* b ^* of the CIE L ^* a ^* b ^* uniform color space, each color code of black, intermediate color, and chromatic color is set in a color area as shown in FIG. 3A.

Note that the area of this color code is represented by Q and R, G, B in the following equation.
(8 bits; 0 to 255).

Here, W ₂ = (R + 2G + B) / 5, W ₀ = 255.

(1) When Q ≦ 15 and G> 180 Color code: 00 (white) (2) When Q ≦ 7.5 and G ≦ 180 Color code: 11 (black) (3) 7.5 ≦ Q ≦ 15 and G ≦ 180 Color code: 01 (intermediate color) (4) Q> 15 Color code: 10 (chromatic color) Then, the image discriminating unit 4 obtains the parameter S as described above, and compares this S with the threshold value T. When S> T, it is determined that the character is a black character, and the intermediate color area is set as a black area (FIG. 3B). When S ≦ T, it is determined that the image is a color gradation image, and the intermediate color region is set as a chromatic color region (FIG. 3C).

FIG. 6 shows the cumulative distribution obtained from the histogram of the density gradient parameter S of the intermediate color. In the figure,
The color gradation image indicates the ratio of the chromatic color code, and the black characters indicate the ratio of the black code. The intersection between the two has the same discrimination rate. In this case, the threshold T = 51 and the discrimination rate 82.4
%become. Accordingly, the threshold value T is set as 51, and the color gradation image and the black character image are determined.

After the color code has been corrected in this manner, based on the corrected color code and scan code (a code indicating the color recorded by the printer),
The data of Y, M, C, K selectively passes through the selector 6. That is, when the corrected color code is chromatic (10), Y, M, and C from the color reproduction section 3 pass through the selector 6.
When the corrected color code is black, K from the color reproduction unit 3 passes through the selector 6.

Then, color ghost correction, filter processing, and gradation correction are performed. In this filter processing, MTF correction, smoothing processing, and the like are performed.

The MTF correction is a process for correcting a resolution reduced due to various causes. Therefore, it is desirable to change the correction amount of the MTF correction between the character mode and the photograph mode. For example, the correction amount is increased in the character image mode, and the correction amount is reduced in the photograph mode. To do this, N × N
A convolution filter using image data of pixels (N = 3, 5, 7) may be employed, and the filter coefficient may be changed according to the mode. That is, the convolution filter is a high-pass filter for a character image, and a low-pass filter for a color gradation image.

Further, when processing a gradation image, a smoothing process is performed to reduce beat interference (moire) caused by halftone dots.

Examples of the filter coefficient include the following, for example.

The gradation correcting section 9 lowers the gradation (γ) when processing a gradation image as shown in FIG. 7 so as to obtain a smooth gradation characteristic. Also, when processing a character image, γ is set to be high so that a sharp image can be obtained.

Then, the image signal for which the image processing has been completed is supplied to an external printer unit or the like, and an image is formed on a recording sheet.

As described above, in the present embodiment, the color code is divided into chromatic color / black / white / intermediate color, the intermediate color is assigned to black or chromatic color according to the processing mode, and image processing (MTF correction, γ correction) is performed according to the mode. Was changed. For this reason, in a black character image, the intermediate color region becomes a black region, and color ghost does not occur, and the resolution is improved and a sharp image can be obtained. Further, in the color gradation image, the intermediate color region becomes a chromatic color region, and a chromatic color with low saturation can be reproduced well, and an image with excellent gradation reproducibility can be obtained.

Next, the configuration and operation of each section of a color copying machine to which the color image processing apparatus of the present invention is applied will be described with reference to FIG. The color copier is developed using a color dry developing system. In this example, two-component non-contact development and reversal development are employed. That is, the transfer drum used in the conventional color image formation is not used, and the superposition is performed on the electrophotographic photosensitive drum on which the image is formed. In the following example, an OPC for image formation is
A four-color image of yellow, magenta, cyan, and black is developed on a photoreceptor (drum) by four rotations of a drum, and transfer is performed once after development, so that the image is transferred to recording paper such as plain paper.

The document reading unit A is driven by turning on the copy button of the color copier. And the platen 12
The eight originals 101 are optically scanned by the optical system.

This optical system includes a carriage 132 provided with light sources 129 and 130 such as halogen lamps and a reflection mirror 131, and a V mirror 133.
And 133 'are provided.

The carriage 132 and the movable mirror unit 134 are caused to travel on a slide rail 136 at predetermined speeds and directions by a stepping motor 135, respectively.

Optical information (image information) obtained by irradiating the original 101 with the light sources 129 and 130 is guided to the optical information conversion unit 137 via the reflection mirror 131 and the mirrors 133 and 133 '.

A standard white plate is provided on the back side of the left end of the document table 128. This is because the image signal is standardized into a white signal by optically scanning the standard white plate.

The optical information conversion unit 137 has a lens 139 and a prism 14.
0, two dichroic mirrors 102 and 103, a CCD 104 for capturing a red color separation image, and a green color separation image
It comprises a CCD 105 and a CCD 106 for capturing a blue color separation image.

Optical signals obtained by the optical system are collected by the lens 139, and color-separated into blue optical information and yellow optical information by the dichroic mirror 102 provided in the prism 140 described above. Further, the dichroic mirror 103 separates the yellow optical information into red optical information and green optical information. Thus, the color optical image is decomposed by the prism 140 into three-color optical information of red R, green G, and blue B.

Each color separation image is formed on the light receiving surface of each CCD, so that an image signal converted into an electric signal is obtained.
After the image signal is processed by the signal processing system, each color signal is output to the writing unit B.

The signal processing system includes an A / D converter and the like in addition to the various signal processing circuits of the density conversion unit 1 to the gradation correction unit 9 shown in FIG.

The writing section B has a deflector 141. This deflector 1
As 41, a deflector made of an optical deflector using quartz or the like may be used in addition to a galvanometer mirror, a rotating polygon mirror, or the like. The laser beam modulated by the color signal
Deflection scanning is performed by 141.

When deflection scanning is started, beam scanning is detected by a laser beam index sensor (not shown),
Beam modulation by the first color signal (for example, a yellow signal) is started. The modulated beam is charged uniformly by the charger 154 to the image forming body (photosensitive drum)
It is made to scan over 142.

Here, the main scanning by the laser beam and the image forming body 142 are performed.
Due to the sub-scanning due to the rotation of
An electrostatic latent image corresponding to the color signal is formed.

This electrostatic latent image is developed by a developing device 14 containing yellow toner.
3 to form a yellow toner image.
Incidentally, a predetermined developing bias voltage from a high voltage source is applied to this developing device.

The toner supply of the developing device 143 is C for system control.
By controlling the toner replenishing means (not shown) based on a command signal from a PU (not shown), toner is replenished when necessary. The above-described yellow toner image is rotated in a state where the pressure of the cleaning blade 147a is released, and an electrostatic latent image is formed based on a second color signal (for example, a magenta signal) in the same manner as in the case of the first color signal. You. Then, by using the developing device 144 that stores the magenta toner, the developing device 144 is developed to form a magenta toner image.

It goes without saying that a predetermined developing bias voltage is applied to the developing device 144 from a high-voltage power supply.

Similarly, an electrostatic latent image is formed based on the third color signal (cyan signal), and a developing device 145 containing cyan toner is formed.
As a result, a cyan toner image is formed. Further, an electrostatic latent image is formed based on the fourth color signal (black signal), and is developed by the developing device 146 filled with black toner in the same manner as the previous time.

Therefore, a multicolor toner image is formed on the image forming body 142 in an overlapping manner.

Here, the formation of a multicolor toner image of four colors has been described, but it goes without saying that a two-color or single-color toner image can be formed.

As described above, in the state where the AC and DC bias voltages are applied from the high-voltage power supply,
An example of so-called two-component non-contact development in which each toner is caused to fly toward the image forming body 142 for development has been described.

The toner is supplied to the developing units 144, 145, and 146 in the same manner as described above, based on a command signal from the CPU.

On the other hand, the recording paper P fed from the paper feeding device 148 via the feed roll 149 and the timing roll 150 is the image forming body 1
The image forming body is synchronized with the rotation of 42.
Conveyed over 142 surfaces. Then, the multicolor toner image is transferred onto the recording paper P by the transfer pole 151 to which the high voltage is applied from the high power supply, and is separated by the separation pole 152.

The separated recording paper P is conveyed to the fixing device 153, where a fixing process is performed to obtain a color image.

The image forming body 142 after the transfer is cleaned by the cleaning device 147, and is prepared for the next image forming process.

In the cleaning device 147, a predetermined DC voltage is applied to the metal roll 147b in order to easily collect the toner cleaned by the cleaning blade 147a.
The metal roll 147b is arranged on the surface of the image forming body 142 in a non-contact state. After the cleaning is completed, the cleaning blade 147a is released from the pressure contact.However, at the time of release, an auxiliary roller 147c is further provided to release unnecessary toner that is left, and the auxiliary roller 147c is rotated in a direction opposite to the image forming body 142, By pressing, unnecessary toner is sufficiently cleaned,
Removed.

In the above description, an example in which the color image processing apparatus of the present embodiment is applied to a color copying machine has been described. However, it goes without saying that the color image processing apparatus of the present invention can be used for various other devices.

(Effects of the Invention) As described above in detail, in the present invention, an intermediate color region is set for color separation between a chromatic color and an achromatic color, and the intermediate color region is allocated to a chromatic color or an achromatic color according to the type of image. I did it. Further, the content of the image processing is deflected according to the type of the image. For this reason, a color image processing apparatus that can improve color reproduction of low chroma in a color gradation image, improve gradation characteristics, improve resolution in a character image, and reduce color ghosts is realized. can do.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a configuration of an embodiment of the present invention, FIG. 2 is an explanatory diagram for explaining a state of color code generation, FIG. 3 is an explanatory diagram for showing a state of color coding, and FIG. FIG. 5 is a configuration diagram showing a configuration example of an image discrimination unit, FIG. 6 is a characteristic diagram showing a relationship between a density gradient and image discrimination, and FIG. 7 is a character image and gradation. FIG. 8 is a diagram showing the overall configuration of a color copying machine, and FIG. 9 is an explanatory diagram showing how a conventional color code is generated. DESCRIPTION OF SYMBOLS 1 ... Density conversion part, 2 ... Color code generation part 3 ... Color reproduction part 4 ... Image discrimination part 5 ... Color code correction part 6 ... Selector 7 ... Color ghost correction part 8 ... Filter processing part , 9 ... gradation correction unit

Claims (1)

(57) [Claims] A means for classifying a color image signal into an achromatic color area, a chromatic color area and an intermediate color area; an image discrimination processing means capable of discriminating whether the image is a color gradation image or a black character image from the color image signal. When the determination result of the image determination processing means is a color gradation image, the color image signal of the intermediate color area is corrected to the color image signal of the chromatic color area, and the determination result of the image determination processing means is a black character image. In the case of, the correction means for correcting the color image signal of the intermediate color area to the color image signal of the achromatic color area, and the color after being divided into an achromatic color area and a chromatic color area by the correction by the correction means. Image processing means for performing gradation processing or spatial frequency filter processing according to each of a color gradation image and a black character image on the image signal, wherein the gradation processing in the image processing means is When the determination result of the image determination processing means is a color gradation image, the gradation is lowered, and when the result is a black character image, the gradation is increased. The spatial frequency filtering processing by the image processing means A color image processing apparatus for reducing the correction amount of the spatial frequency correction when the determination result of the determination processing unit is a color gradation image, and increasing the correction amount of the spatial frequency correction when the determination result is a black character image; .