JPH043658A - Color image forming device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an apparatus for forming a color image by superimposing a plurality of colors, and particularly relates to reproduction of halftones and improvement of recorded image quality.

[Prior Art] For example, in a recording device that uses an electrostatic recording method, such as a digital copying machine, it is not possible to adjust the recording density of each pixel that makes up a recorded image in multiple stages, and generally black A binary recording of (recording) and white (non-recording) is adopted. Therefore, when expressing halftones with this type of recording device, the ratio of the number of recorded pixels to the number of non-recorded pixels among multiple pixels.

In other words, the magnitude of the average density of a plurality of pixel areas is adjusted. Broadly speaking, two types of methods are known as specific gradation processing methods of this type. One is a method that determines the level of a pixel of interest by comparing it with a pre-fixed threshold value or thresholds, such as the dither method or density pattern method, and the other is a method that uses the minimum average error method or This method, like the error diffusion method, reflects the density of surrounding pixels in determining the level of the pixel of interest. All of these have advantages and disadvantages.

In other words, in the case of the dither method and the density pattern method, since they have a threshold matrix that is unrelated to the original image data, the continuity of lines, etc. that exist in the original image is lost, and the reproducibility of characters, etc. is extremely poor. However, on the other hand, regularity appears in the image after halftone processing regardless of the content of the original image, so the appearance of graininess is good. On the other hand, in the case of the minimum average error method or the error diffusion method, the density level of surrounding pixels is reflected in determining the level of the pixel of interest, so if there is a line in the original image, the continuity of the line is is preserved in the image after processing, and the reproducibility of characters etc. is good, but irregular patterns appear in the image after halftone processing, and the graininess in appearance deteriorates. In particular, when reproducing color images, the minimum average error method and error diffusion method determine the level of the pixel of interest for each color independently.
Irregular patterns with different colors appear in the recorded image, further worsening the graininess in appearance.

[Problems to be Solved by the Invention] An object of the present invention is to improve both the reproducibility of a line drawing such as a single character and the apparent graininess when reproducing halftones of a color image.

[Means for Solving Problem 11] In order to solve the above problems, the present invention has been developed.
It includes a gradation processing circuit that converts the number of gradations of each pixel included in an image signal to be smaller than that of the input signal, and processes a plurality of image signals of mutually different color components to produce one recorded image. In the color image forming apparatus, the gradation processing circuit includes: a first gradation conversion means that reflects the influence of the gradation of surrounding pixels of the pixel of interest in determining the gradation level of the conversion output; and a gradation of the conversion output. One-way [
a second gradation conversion means determined by comparison with a plurality of threshold values; at least one of the plurality of color image signals is processed by the first gradation conversion means; At least one other of the signals is processed by the second gradation conversion means.

[Function] As the first gradation conversion means in the present invention, a circuit that converts the gradation of an image signal by, for example, the minimum average error method or the error diffusion method can be used, and as the second gradation conversion means, For example, a circuit that converts the gradation of an image signal using a dither method or a density pattern method can be used.

According to the present invention, Y (yellow), 9M (magenta), and C (cyan) constitute a color image signal.

One or more of BK (black) etc. are processed by the first gradation conversion means, and the remaining one or more are processed by the second gradation conversion means. Therefore, for example, Y, M, C,
In the recorded image formed by combining BK, both the processing characteristics of the first gradation conversion means and the processing characteristics of the second gradation conversion means appear.

In other words, for the color components processed by the first gradation conversion means, while the continuity of the line drawing is good, irregular patterns occur, and for the color components processed by the second gradation conversion means, Since it is a regular pattern, the graininess in appearance is good, but the continuity of the line drawing is lost. However, when looking at the color image as a whole, the continuity of the line drawing is ensured for at least one color component processed by the first gradation conversion means, making it easier to read characters, etc. Since there are also color components that do not appear in the irregular pattern processed by the gradation conversion means, the deterioration in graininess is relatively small. Therefore, compared to cases in which color image processing is performed by the first gradation conversion means alone or color image processing is performed by the second gradation conversion means alone, the overall image quality is significantly improved by the present invention. .

By the way, when color processing is performed in a copying machine or the like, colors such as Y, M, C, and BK are sequentially superimposed to reproduce a color image. In such a case, the color component recorded last has a large influence on the entire image. Therefore, in a preferred embodiment of the present invention, the first gradation conversion means is
Used to process the color components recorded last. This further improves the legibility of characters, etc. Also, since letters and line drawings are often recorded in achromatic colors, Y, M, and C are used.

Regarding the order in which colors such as BK are superimposed, the BK (black) component is placed last, and the BK acid component is processed by the first gradation conversion means.

[Embodiment] FIG. 1 shows the structure of a mechanical section of a - type copying machine. 1st
The copying machine shown in the figure is a digital color copying machine, and is composed of an image scanner 35 located above and a color laser printer located below.

The image scanner 35 irradiates a document image placed on a contact glass 51 with light from an exposure lamp 52 and sends the reflected light to a first mirror 53 . Second mirror 54. Reading unit 57 via third mirror 55 and lens unit 56
image is formed. Exposure lamp 52. First mirror 53. The second mirror 54 and the third mirror 55 are configured to be mechanically scanned and driven, and sequentially expose the entire area of the document and form an image on the reading unit 57 by reflecting light from the entire image. can do.

In this example, the lens unit 56 includes a spectral filter, and guides the separated R (red), G (green), and B (blue) lights to the reading unit 57. The reading unit 57 includes three independent one-dimensional CCD image sensors, each of which receives R, G, and B light, respectively. Each COD image sensor has one
Line images can be read pixel by pixel. Two-dimensional image data can be obtained by performing reading with the reading unit 57 while performing mechanical sub-scanning of the exposure lamp and mirror.

The printer uses a photoreceptor belt 1 as an image carrier for forming one image, which constitutes a recording system using an electrostatic recording method similar to that of a general copying machine. Photoreceptor belt 1
The surface of is exposed by the optical writing unit 5. By modulating the laser light emitted from the semiconductor laser provided in the optical writing unit 5 according to the recorded image information, a potential distribution corresponding to the density distribution of the image, that is, an electrostatic latent image, is created on the surface of the photoreceptor belt 1. Form. The surface of the photoreceptor belt 1 is
Before the exposure process, it is uniformly charged to a predetermined high potential by a charging corona charger 4. In order to develop the electrostatic latent image on the photoreceptor belt 1, four developing units 7,
9, 11 and 13 are provided. Developing device 7, 9, 1
1 and 13 contain toners of Y (yellow), 9M (magenta), C (cyan), and BK (black), respectively, and develop the respective color components. Y,M.

The image on the photoreceptor belt visualized with the C or BK color toner, that is, the toner image, is transferred to a sheet of transfer paper 33 fed from the paper feed cassette 14 .

When performing full-color recording, image formation and transfer processes for Y, M, C, and BK colors are repeatedly performed, and one color image is transferred to transfer paper in four processes. In order to overlay multiple color images on the transfer paper, in this example, when the transfer of one color image is completed, the transfer paper is moved toward the right and returned to the previous position), and then the image of the next color is transferred. Prepare for. The transfer paper on which the transfer has been completed passes through a fixing device 31, where the toner image is fixed, and is discharged onto a paper discharge tray 32.

FIG. 2 shows the configuration of an electric circuit for processing signals related to images of the copying machine shown in FIG. The electric circuit will be explained below with reference to FIG.

R and G output from the image sensor 150.

The image signal (analog signal) of B is sent to the image processing circuit 151.
is applied to The image processing circuit 151 converts the image signal of each color into an 8-bit digital signal by A/D conversion, and then performs image processing such as γ correction and complementary color generation two-color correction to generate C, M, and Y image signals. Generate and output. These image signals are further processed by a masking/UCR processing circuit 152. This circuit executes a predetermined masking process, extracts the black (BK) component included in the color image, removes the black component from each color component of C, M, and Y,
Outputs four signals: C, M, Y, and BK. Each signal is output as 4-bit gradation data.

In this example, the modulation of the laser beam is controlled by an on/off binary signal, so the masking/UCR processing circuit 15
The image signal output from 2 is then subjected to gradation processing,
converted to a binary signal. However, since halftones cannot be reproduced on the output image by simple binarization, processing is performed to reproduce halftones in a pseudo manner on the output image. Specifically, it is possible to reproduce halftones using two methods: the dither method and the minimum average error method.

The image signals C, M, Y, BK output from the masking/UCR processing circuit 152 are sent to the data selector 154 or 1.
55, the dither processing circuit 156 and the minimum average error processing circuit 157 are input to one side and processed. Further, either the output of the dither processing circuit 156 or the output of the average error minimum processing circuit 157 is sent to the data selector 158.
is selected and applied to the laser driver 159, and used to modulate the energization state of the semiconductor laser LD. The image signal of each color is processed by a dither processing circuit 156 and an average error minimum processing circuit 1.
The process control unit 1 determines which of the 57 is used for processing.
53 is decided.

The dither processing circuit 156 will be explained. The 4-bit rank data output from the data selector 154 is transferred to a ROM (read-only memory) 62 by a comparator 61.
The signal is compared with the threshold data output from the signal generator and converted into a binary signal. Inside the ROM 62, as shown in FIG. 3a, 1
Six threshold data are stored in advance, and one threshold is selectively read out according to address designation and applied to the comparator 61. ROM62 address input terminal A
The outputs of counters 63 and 64 are applied to X and AY, respectively. The counter 63 counts clock pulses that appear at timings for each pixel in the main scanning direction, and
Generate the horizontal position address of the diagram matrix. The counter 64 counts the line synchronization signal L5ync,
Generate the vertical position address of the matrix of FIG. 3a. Therefore, the ROM 62 selects one threshold value from the position on the matrix corresponding to the scanning position of the pixel at that time, and applies it to the comparator 6I. In other words.

The threshold data that is compared with the input gradation data in the comparator 61 changes periodically in synchronization with the progress of scanning.

This creates a recording area (4x4
The distribution of the number of black pixels and the number of white pixels within a pixel (pixel) is adjusted according to the input gradation, and halftones are reproduced. Figure 3b shows
This shows an output image obtained by processing an input image of uniform density with a gradation value of 10. Note that the pixels with hatching are black pixels (recording pixels), and the pixels without hatching are white pixels (non-recording pixels). In dither processing, the threshold value is fixed, so as shown in Figure 3b,
Periodicity (regularity) appears in the distribution of black pixels and white pixels on the recorded image.

The average error minimum processing circuit 157 will be explained.

The 4-bit gradation data X output from the data selector 155 is input to the adder 71. The adder 71 adds the outputs E and X of the error calculation circuit 74 and outputs the result X'. Apply to one input terminal. The comparator 72 converts the input data X' to a threshold value T
and outputs the result as a binary signal Y. The subtracter 76 subtracts the output Y from the input data X', resulting in F,
That is, the error between the input and the output is detected and written into the RAM (read/write memory) 75. The error calculation circuit 74 reads error data F of peripheral pixels located around the pixel of interest at the scanning position from the RAM 75, calculates the weighting, and outputs the normalized value E to the adder 71.

In this example, error calculation circuit 74 uses the weighting pattern shown in FIG. 4a. In FIG. 4a, the hatched pixels indicate the pixels of interest.

Other pixels indicate peripheral pixels. In the example of Figure 4a, 4
A weight of "1" is attached to each of the two surrounding pixels.

In the average error minimum processing circuit 157, the value X' compared with the threshold value T is corrected by the difference E that reflects the error amount F of the surrounding pixels. The upper threshold value is not constant but changes dynamically depending on the influence of one surrounding pixel. FIG. 4b shows an output image obtained when an input image of uniform density with a gradation value of 10 is processed. Note that the pixels with hatching are black pixels (recording pixels), and the pixels without hatching are white pixels (non-recording pixels). In the average error minimum processing, since the threshold value is not constant, black pixels and white pixels appear irregularly on the recorded image, as shown in FIG. 4b.

The reason why two types of gradation processing are performed in this embodiment is to improve the quality of recorded images in color images. That is,
When using a processing method with a fixed threshold such as dither processing, regularity appears in the dot distribution and the apparent graininess of the recorded image improves, but the resolution deteriorates significantly, especially for characters etc. A line drawing cannot be reproduced unless the line is sufficiently small compared to the size of the matrix. Furthermore, when using a processing method such as average error minimum processing in which the threshold value changes depending on surrounding pixels, line drawings can be reproduced because the degradation in resolution is small, but the dot distribution is not regular, so the recorded image The graininess in appearance is poor.

Therefore, in this embodiment, when processing a color image, in order to improve both resolution and apparent graininess, the gradation processing method is switched according to changes in color components.
Two types of gradation processing are used in combination. Specifically, when recording the C, M, and Y color components, the laser light is modulated according to the results processed by the dither processing circuit 156, and the B
When recording the K color component, the average error minimum processing circuit 1
The laser beam is modulated according to the result of processing in step 57.

Therefore, line drawings can be reproduced for BK colors, C, M,
For each color component of Y, preferable graininess is obtained.

In full-color recording, toner images of C, M, Y, and BK colors are superimposed on one sheet of transfer paper to form one copy image, so even if the BK color component contains all the line drawing components. If so, even if line drawings cannot be reproduced in other colors such as C, M, and Y, line drawings such as characters can be recognized using only BK color images. Furthermore, when all color components are processed by the average error minimum processing circuit 157, dots are recorded in a scattered irregular pattern for every nine colors, so the graininess of the recorded image is significantly deteriorated. If only one color is used, the deterioration of graininess in the color image as a whole is not so noticeable.

In other words, by processing a color image by combining two types of WJiIJ processing, it is possible to satisfy both the resolution and graininess requirements for the entire color image at the same time.

The process control unit 153 performs switching of gradation processing. That is, in this example, image reading.

C, M, Y toner image formation and transfer process.

The four colors of BK are sequentially and repeatedly processed to form a full-color image in four processes. In each process, the process control unit 153 controls the signals Sl and S.
2. The processing is switched by setting S3 as shown in Table 1 below.

Table 1 In this example, the toner images of each color are transferred to the transfer paper in the order of C, M, Y, and BK, so the BK toner image is finally transferred to the top of the transfer paper.

In this example, the BK color is used to increase the resolution of the entire color image, and gradation processing is performed using the minimum average error process, so by transferring the BK color to the top of the transfer paper, other colors can be Components do not overlap on top of each other, and resolution deterioration can be avoided. In other words, the resolution can be increased by transferring the color components processed by the minimum average error process so that they are placed in an upper layer of the image than the color components processed by the dither process.

Therefore, for example, in the case of a device that superimposes four-color toner images on a drum and then transfers them to transfer paper, conversely, image formation with high-resolution toner components is more important than with other color components. By doing this first, a toner component with a high resolution can be placed on the top of the transfer paper, and the resolution can be increased.

Note that instead of the dither processing (156) in the above embodiment, a processing circuit that executes, for example, a density pattern method may be employed, and instead of the average error minimum processing (157), a processing that executes, for example, an error diffusion method may be employed. A circuit may also be used.

In the example, the case where the color that emphasizes resolution is set as BK color is shown, but depending on the type of image, characters and lines may be printed in chromatic colors, so for example, keys on the operation panel The color for which resolution is important may be arbitrarily switched to another color such as C, M, or Y by an operation. In that case, for BK color, it is better to switch to processing with a fixed threshold value, such as a dither method.

[Effects] As described above, according to the present invention, when recording a halftone color image, the legibility of characters etc. on the recorded image is improved,
Moreover, the graininess of the image appearance also improves.

[Brief explanation of drawings]

FIG. 1 is a front view showing the structure of a mechanical section of a color copying machine of one type that embodies the present invention. FIG. 2 is a block diagram showing the main parts of the electrical equipment of the copying machine shown in FIG. 1. FIG. 3a is a plan view showing an arrangement of threshold matrices used in dither processing, and FIG. 3b is a plan view showing an image binarized by dither processing using the threshold values shown in FIG. 3a. Fig. 4a is a plan view showing the arrangement of weight patterns used in the average error minimum processing, and Fig. 4b is a plan view showing an image binarized by the average error minimum processing using the weight patterns in Fig. 4a. . 1: Photosensitive belt 4: Charging corona charger 5: Optical writing unit 7.9, 11, 13: Developing device 14: Paper feed cassette 35: Image scanner 57: Reading unit 150: Image sensor 152: Masking/UCR processing circuit 153: Process control unit

Claims (1)

[Claims] It includes a gradation processing circuit that converts the number of gradations of each pixel included in an image signal to be smaller than that of the input signal, and processes a plurality of image signals with different color components to form one record. In a color image forming apparatus that forms an image: in the gradation processing circuit; a first gradation conversion means that reflects the influence of the gradation of surrounding pixels of the pixel of interest in determining the gradation level of the conversion output; and a conversion output. 1 whose gradation level is fixed in advance
at least one of the plural color image signals is processed by the first tone converting means, and the plural color image signals are processed by the first tone converting means; A color image forming apparatus characterized in that at least one other of the image signals is processed by a second gradation conversion means.