JPH0414961A - Color picture forming device - Google Patents

Abstract

PURPOSE:To improve the reproducibility of line picture and granuality by providing 1st and 2nd gradation conversion means, deciding which of them is used in response to each arithmetic result of a picture signal and using the 1st means for at least one and using the 2nd means for the remaining processing. CONSTITUTION:A picture signal outputted from a masking UCR processing circuit 152 is inputted to either a dither processing circuit 156 or a mean error minimum processing circuit 157 via a data selector 154 or 155, and either an output of the dither processing circuit 156 or an output of the mean error minimum processing circuit 157 is selected by a data selector 158 and the result is fed to a laser driver 159. Then the processing method of gradation is selected for each color in response to the kind of the picture and at least one color is processed by the dither processing circuit 156 independently of the kind of the picture and at least one color is processed by the mean error minimum processing circuit 157. Thus, the reproducibility of a line picture is improved and the granuality of a recording picture is improved.

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 apparatus, the ratio between the number of recorded pixels and the number of non-recorded pixels in a plurality of pixels, that is, the size 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.

Therefore, for example, in Japanese Patent Application Laid-Open No. 63-288565, means for executing error diffusion processing and means for executing dither processing are both provided, and either processing is selected according to the result of identifying the type of image. automatically determines whether

[Problems to be Solved by the Invention] However, even when dither processing is selected to emphasize gradation, the image may include line drawings, so the image quality may deteriorate due to a decrease in resolution. There is. On the other hand, even if error diffusion processing is selected with emphasis on resolution because a large number of characters are included, it may be desirable to avoid deterioration of the graininess of the image. For example, when reproducing a color image, if the minimum average error method or error diffusion method is used, the level of the pixel of interest is determined independently for each color, so irregular patterns with different colors will appear in the recorded image, resulting in a grainy appearance. quality deteriorates significantly.

An object of the present invention is to improve both the reproducibility of line drawings such as characters and the apparent graininess when reproducing halftones of color images.

[Means for Solving Problem 11] In order to solve the above problems, the present invention 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 an input signal. In a color image forming apparatus that processes a plurality of image signals of mutually different color components to form one recorded image: the gradation processing circuit; a first gradation conversion means in which the influence of the tone is reflected; and a second gradation conversion means in which the gradation level of the conversion output is determined by comparison with one or more pre-fixed threshold values; determining whether the image signal of each color is to be processed by the first gradation conversion means or the second gradation conversion means according to the calculation result of each of the plurality of color image signals, and determining the calculation result of the calculation result. Regardless of the above, processing determining means is provided for processing at least one of the plurality of image signals by the first tone converting means and processing the remaining at least one by the second tone converting means.

[Operation] 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), 7M (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, an irregular pattern occurs, and for the color components processed by the second gradation conversion means, the continuity of the line drawing is good, but the color components processed by the second gradation conversion means are Although the pattern has good graininess in appearance, the continuity of the line drawing is lost. however,
When looking at the color image as a whole, the continuity of line drawings is ensured for at least one color component processed by the first gradation conversion means, making it easier to read characters, etc.
Also, there are color components that do not appear in irregular patterns that have been processed by the second gradation conversion means.

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. .

Furthermore, since the distribution of each color image between the processing by the first gradation conversion means and the processing by the second gradation conversion means is automatically changed depending on the type of image, the optimal color image processing is selected. .

[Example 1] Fig. 1 shows the structure of the mechanism 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 driven for scanning, and sequentially expose the entire area of the document to light, and form an image of the reflected light from the entire image on the reading unit 57. can do.

In this example, the lens unit 56 includes a spectral filter, and guides the separated R (let), 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 CCD 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 has a recording system using an electrostatic recording method similar to that of a general copying machine. A photoreceptor belt 1 is used as an image carrier for forming an image. 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 1-color correction to generate C, M, and Y image signals. Generate and output. These image signals are further processed by a masking/OCR 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/OCR processing circuit 15
The image signal output from 2 is subjected to primary 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 signal is input to either a dither processing circuit 156 or an average error minimum processing circuit 157 and is processed therein. 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 gradation 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, 16
threshold data is stored in advance, and one threshold value is selectively read out according to address designation and applied to the comparator 61. ROM62 address input terminal AX
and AY are applied with the outputs of counters 63 and 64, respectively. The counter 63 is.

Clock pulses that appear at the timing of each pixel in the main scanning direction are counted to generate a position address in the horizontal direction of the matrix shown in FIG. 3a. The counter 64 is.

The line synchronization signal L5ync is counted to generate the vertical position address of the matrix of FIG. 3a. Therefore, R
The OM 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 61. That is, the threshold data that is compared with the input gradation data in the comparator 61 changes periodically in synchronization with the progress of scanning.

As a result, the 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'. Threshold generation circuit 73 generates threshold T based on X' and applies it to one input terminal of comparator 72. The comparator 72 converts the input data X' to a threshold value T
The result is output 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 pixel of interest, and the other pixels indicate peripheral pixels. In the example of FIG. 4a, four peripheral pixels are each given a weight of "1".

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 surrounding pixels. FIG. 4b shows an output image obtained when an input image of uniform density with one 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 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 minimum average error 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 it is difficult to record The visual graininess of the image is poor.

Therefore, in this embodiment, in order to improve both resolution and apparent graininess when processing a color image, a gradation processing method is selected for each color depending on the type of image, and two types of gradation processing are used. It is used in combination with Wi style processing. Also, regardless of the type of image, at least one of the four colors C, M, Y, and BK is processed by the dither processing circuit 156, and at least one color is processed by the average error minimum processing circuit 157. It's in control. Therefore, C, M, Y, and BK data are always processed by combining both processes. This improves the reproducibility of line drawings and at the same time improves the graininess of recorded images.

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 irregular patterns for each color, so the graininess of the recorded image is significantly deteriorated. If only one color is used, the deterioration in graininess of the color image as a whole is not so noticeable.

In other words, by processing a color image by combining two types of gradation processing, it is possible to simultaneously satisfy both the resolution and graininess requirements for the color image as a whole.

The process control unit 153 performs switching of gradation processing. In reality, the original image is scanned (prescanned) prior to the copying operation, and the C, M,
Each color image data of Y and BK is processed, and it is determined in advance which method to perform gradation processing on the image data of each color. When the copy operation starts, the image reading, toner image formation, and transfer processes are sequentially repeated for the four colors C, M, Y, and BK, and a full-color image is formed in four processes. In the process, the process control unit 153 sets the signals S], S2, and S3 to perform gradation processing using a predetermined processing method.

Note that instead of the dither processing (156) in this 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.

The contents of the gradation processing determination routine of the process control unit 153 are shown in FIG. 5a, and the contents of the subroutine "density detection" therein are shown in FIG. 5b. First, "density detection" will be explained.

Generally speaking, in this example, the number of non-colored pixels (those with low density), that is, the density, is detected for an nXm pixel matrix area of n pixels in the main scanning direction and m pixels in the sub-scanning direction. In step 1, processing memory D(1,,1) to D
Input data is stored in (n, m), and the values of each register are initialized in steps B2 and B3.
4, the density of the pixel data D (N, M) at the target position is compared with a predetermined threshold THI to check whether the pixel is a non-colored pixel. If D(N, M) < TH1, it is regarded as a non-target pixel, and flag S is set to 1 in step B5; otherwise, S is cleared to O in step B6. This process is performed for all nXm pixels, and the integrated value of S obtained for each pixel is stored in Sun. Therefore, uncolored pixel density information is stored in register Su+m.

Reference is now made to Figure 5a. In the gradation processing determination routine,
First, in step A1, 0-color image data is input, processed by a density detection routine, and the result is stored in SumC. Furthermore, the value of Sui+ is compared with a predetermined threshold value TH2, and if Sun > TH2, that is, if the non-colored pixel density of the cyan component is large, the flag Fc is set.
Set to 1, otherwise clear Fc to 0. “0” in Fc indicates that the image data is processed by the dither processing circuit 156, and “1” indicates that the image data is processed by the dither processing circuit 156, and “1” indicates that the image data is processed by the dither processing circuit 156.
57 indicates processing. Similarly, flags Fm, Fy, and Fbk are set for magenta, yellow, and black, respectively, depending on the size of the non-colored pixel density of each color component.

Step A13 and subsequent steps are processing for exceptions.

In other words, in this embodiment, since both dither processing and average error minimum processing are always used to perform gradation processing on color images, if all of Fc, Fm, Fy, and Fbk are 1 or O, then need to be changed. Step A 1.
In step 3, the sum of Fc, Fm+F7+Fbk is stored in register Ft. If the value of Ft is 4, step A1
Proceed to step 5. In that case, among Fc, Fm, Fy, and Fbk, the color assigned to the color with the minimum Sum is changed to 0. In other words.

Find the smallest one among SumC, SumM, SumY, and SumBK, and select Fc, Fm+ corresponding to that color.
Change FY+ or Fbk from "1" to "0". Further, if the value of Ft is 0, the process proceeds to step A1.7. In that case, among Fc, Fm, Fy, and Fbk, the one assigned to the color with the largest Sum is changed to 1. In other words, S
uwrC, SuwrM, Su! llo, SumBH
Find the largest one among them and find the FCt corresponding to that color.
Change Fm, F'/ or Fbk from "0" to "1".

Therefore, even if the types of images vary,
C, M, Y, and BK are not all tonally processed in the same way; at least one color is processed by the dither processing circuit 156.
At least one other color is processed by the average error minimum processing circuit 157.

In general, images with a high density of uncolored pixels tend to have strong contrast and few midtones.

By determining the gradation processing method for each color based on the results of detecting the presence or absence of features in the image, more preferable results suitable for the type of input image can be obtained.

A modified example of the gradation process determination process will be described next.

In this example, the gradation process is determined by the same procedure as in FIG. 5a, but the gradation process is determined based on the difference between the maximum value and the minimum value of the image density. That is, the "contrast detection" routine shown in FIG. 6 is used instead of the "density detection" routine in FIG. 5a, and the register Sum is changed to R. The rest is the same as the previous embodiment. In general, the larger the difference between the highest density and the lowest density, the higher the contrast of the image and the fewer the intermediate tones, so gradation processing is determined based on this idea.

This will be explained with reference to FIG. In step 6I, input data is stored in processing memories D(1,,1) to D(n, m), and in steps 62 and 63, the values of each register are initialized. MIN serves as a threshold for the maximum detected concentration, and is normally zero.

MAX serves as a threshold for the minimum detected concentration, usually the theoretical maximum (255)
It is.

In step 64, the density D(a, b) of the pixel of interest is compared with the contents of P, which holds the maximum value up to that point.

If D(a,b) > P, store D(a,b) in P.

In step 66, the density D(a, b) of the pixel of interest is compared with the contents of Q, which holds the minimum value up to that point.

If D(a, b) < Q, store D(a, b) in Q.

This process is sequentially executed for all nXm pixels. When processing for all pixels is completed, the process proceeds to step 70. The contents of P and Q at this time are, respectively,
These are the maximum and minimum values of the density of nXm pixels. In step 70, the density difference between P and Q is stored in register R.

Therefore, as in the case of FIG. 5a, if the magnitude of the density difference is identified by comparing the contents of the register R with a predetermined threshold value, the preferable gradation processing method for each color can be determined. I can do it. However, it is not desirable for C, M, Y, and BK to all be processed in the same way, so when Ft becomes 4 and 0, as in the case of Fig. 5a, exception processing is performed and Fc, Fm, It is necessary to change one of Fy and Fbk.

[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. Furthermore, more preferable gradation processing is automatically selected depending on the type of input image.

[Brief explanation of the drawing]

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 the 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. 38. 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. . FIGS. 5a, 5b, and 6 are flowcharts showing processing related to tone processing determination in the embodiment. 1: Photoreceptor belt 4: Charging corona charger 5: Optical writing unit 7.9, 11.1311 Imager 14: Paper feed cassette 35: Image scanner 57: Reading unit 150: Image sensor] 52: Masking/UCR processing Circuit 153: Process control unit

Claims (2)

[Claims] (1) Contains 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 multiple image signals with different color components to form one recorded image. In a 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; Level is pre-fixed 1
a second gradation conversion means determined by comparison with one or more threshold values; At least one of the plurality of image signals is processed by the first tone converting means, and the remaining image signals are processed by the first tone converting means, regardless of the calculation result. A color image forming apparatus, comprising: processing determining means for processing at least one tone with a second tone converting means. (2) The processing determining means determines whether the image signal should be processed by the first tone converting means or the second tone converting means, depending on the result of calculating the non-colored pixel density of the image signal. 2. The color image forming apparatus according to claim 1, wherein the color image forming apparatus determines the color image forming apparatus.