JPH10200746A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To properly control image density so that reduced-line number processing is conducted only for the case that the reducedline number processing is to be conducted even when image density control is executed by referring to a density conversion table. SOLUTION: Density of 24 color patches in each color is measured and a present gradation is obtained (step S3), the present gradation is compared with prescribed object gradation and a density conversion table is generated based on the comparison result (step S5). The present gradation is compared with prescribed object gradation to generate a density conversion table based on the comparison result (step S5). Then the result of correcting a standard image density threshold level with the density conversion table is transferred as a new image density threshold level for the reduced-line number processing (step S6). The new image density threshold level is used to conduct the reduced-line number processing. Thus, the device is properly controlled to conduct the reduced-line number processing only for the case where the reduced-line number processing is to be processed and a disadvantage of image quality deterioration due to a difference from an image structure is avoided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus in which the number of lines of an image signal is reduced with respect to an image signal having an image density of not more than a predetermined reference density value. The present invention relates to an image forming apparatus that performs formation.

[0002]

2. Description of the Related Art Conventionally, in order to improve the reproducibility of dots in a low density portion, an image is formed with a low number of lines when the image density is low, and is formed with a high number of lines when the image density is medium and high. The invention has been proposed in Japanese Patent Application Laid-Open No. 7-254985. In one embodiment described in this publication, one of a plurality of pulse width modulation signals is selected by a waveform selection circuit in accordance with the density of an image density signal, and the number of lines is reduced at a low density. . Further, in another embodiment described in the publication, only when the image signal is less than 50%, the image signal switching unit converts the image signal into two sets.
By switching to transfer to the UT selection circuit, the number of lines is similarly reduced at low density. That is, the image signal density is compared with a predetermined image density threshold value, and the line frequency reduction process is performed on an image signal having a density lower than the image density threshold value. Note that the above technique is generally called a screen technique (or a HIEST technique).

FIGS. 14 and 15 are schematic diagrams of dot reproduction by the above-described technique. FIG. 14 shows a case where an image signal having a density lower than an image density threshold value is subjected to a low line ruling process.
Indicates a case where the line frequency reduction processing is not performed on an image signal having a density higher than the image density threshold.

On the other hand, a plurality of reference patches having different densities are formed on a non-image portion on the photosensitive member, and a density conversion table is created based on the densities of the formed plurality of reference patches. A technique relating to image density control for converting density characteristics of image data based on the image data has been proposed (see Japanese Patent Application Laid-Open No. 63-208368). 8 to 11 show the results of this image density control (the details will be described later).

Here, in a color image forming apparatus for four colors of black (K), yellow (Y), magenta (M), and cyan (C), image density control was executed using a density conversion table shown in FIG. Consider the case. In FIG. 16, since the gradation of K and Y is almost close to the target, the density conversion table is close to a 45-degree straight line, and the gradation of M is generally low in density. , And the density conversion table is in the direction of lowering the gray scale because the gray scale of C is high as a whole. Further, in this color image forming apparatus, if it is assumed that the image signal density is reduced to 30% or less and the number of lines is reduced by the technique described in Japanese Patent Application Laid-Open No. Hei 7-254985, the image density control table shown in FIG. 30% of the obtained image signal density (output image signal density) corresponds to an image signal density of 15% when the input image signal density before conversion is M (arrow A1), and corresponds to an image signal density of 55% when the input image signal density is C (arrow A1). Arrow A2). Here, in the image density control using the density conversion table, in order to correct the apparent macro tone density,
By the correction, the apparent macro density of each color matches the target density.

[0006]

However, since 15 to 30% before the conversion of M is corrected to 30% or more of the output image signal density by the above image density control, the line frequency reduction processing should be originally performed. Is not performed. Further, since the output image signal density is corrected to 30% or less by the above image density control before the conversion of C to 30% to 55%, the line frequency reduction processing is performed even though the line frequency reduction processing should not be originally performed. become. In these cases, the microscopic image structure is different.

[0007] This problem is remarkably exhibited, for example, in Y
This is the case of a smooth tone pattern of process black of three MC colors, in which the line frequency reduction processing is originally performed from the same image signal density for the three colors, but the image signal density for which the line frequency reduction processing is performed varies depending on the color. , And even though the macro density is the same, the granularity is roughened in the smooth gradation pattern due to the difference in the micro image structure.

FIGS. 17A and 17B are schematic diagrams for explaining this problem, and are examples of process black with an image signal density of 20% for each of the YMC colors. FIG. 17A shows a case where the gradation densities of the respective colors match each other, the density conversion table is close to a straight line of 45 degrees, and the image signal density hardly changes even when correction is performed using the density conversion table. Since the density threshold value is lower than 30%, the number of lines is reduced. In FIG. 17B, the aforementioned M concentration is low and C
This is the case where the density is high and the image density control is performed using the density conversion table shown in FIG. M20% is converted to 37% in the density conversion table, and the macro densities match, but since the image density threshold exceeds 30%, the line frequency reduction processing is not performed. Therefore, only M has a different image structure,
Granularity is generated in a smooth gradation pattern.

As another example in which this problem is conspicuous, when the gradation is reduced as shown in C in FIG. There is an example in which the dot structure becomes a low-resolution image in which the dot structure is conspicuous in the middle density portion.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem. Even when image density control is performed using a density conversion table, the number of lines is reduced only in the case where the number of lines should be reduced. It is an object of the present invention to provide an image forming apparatus that can appropriately control processing.

[0011]

According to another aspect of the present invention, there is provided an image forming apparatus, comprising: an image forming apparatus that forms an image based on an image signal; The density conversion is performed on the image signal based on the predetermined conversion information so that the image density is equal to or less than a predetermined reference density value among the density-converted image signals. An image forming apparatus for forming an image by reducing the number of lines of the image signal with respect to the image signal to be formed, wherein the predetermined reference density value is also subjected to density conversion based on the conversion information.

According to a second aspect of the present invention, there is provided an image forming apparatus for forming an image by reducing the number of lines of an image signal with respect to an image signal having an image density of not more than a predetermined reference density value. A reference patch forming means for forming a plurality of reference patches having different densities on the image carrier; a density measuring means for measuring the density of the reference patches formed by the reference patch forming means; Conversion information creation means for creating conversion information for converting density characteristics of an image signal based on a density target value of a reference patch and a density measurement value of the reference patch measured by the density measurement means; and Density conversion means for converting the density characteristics of the image signal and the predetermined reference density value based on the conversion information created by the creation means.

In the image forming apparatus according to the first aspect of the present invention, an image is formed based on predetermined conversion information such that an image density to be represented by an image signal is equal to an image density formed based on the image signal. Density conversion is performed on the signal.
Further, of the image signals subjected to the density conversion, an image signal having an image density formed based on the image signal and having an image density of a predetermined reference density value or less is formed by reducing the number of lines of the image signal. (Perform a so-called low screen ruling process).

At this time, the image forming apparatus according to the first aspect is characterized in that the density conversion is performed on the predetermined reference density value based on the conversion information, and then the line frequency reduction processing is performed. As described above, since the density conversion is also performed for the reference density value in the line frequency reduction processing, the line frequency reduction processing is performed on the image signal which should not be originally performed the line frequency reduction processing, Inappropriate control is performed so that the line frequency reduction processing is not performed on the image signal that should be performed, and that the line frequency reduction processing is performed only in the case where the frequency reduction processing is to be performed. be able to.

Next, in the image forming apparatus according to the present invention,
An image signal is formed by reducing the number of lines of the image signal for an image signal in which the image density of the formed image is equal to or less than a predetermined reference density value (so-called line number reduction processing is performed). In such an image forming apparatus, a plurality of reference patches having different densities are formed on the image carrier by the reference patch forming means, and the density of the formed reference patches is measured by the density measuring means.
The density characteristic of the image signal is converted based on the density target value of the predetermined reference patch and the density measurement value of the reference patch measured by the density measuring means so that the density measurement value becomes equal to the density target value. Conversion information (for example, a density conversion table) is created by the conversion information creating means. Furthermore, based on the created conversion information,
The density conversion means converts the density characteristics of the image signal and a predetermined reference density value.

As described above, in the image forming apparatus according to the second aspect, not only the density characteristic of the image signal but also the reference density value in the low frequency conversion processing is subjected to the density conversion based on the conversion information. Corrects inconveniences such as performing low frequency reduction processing on image signals that should not be subjected to digitization processing, and not performing low frequency reduction processing on image signals that should originally be subjected to low frequency reduction processing. Therefore, it is possible to appropriately control the line frequency reduction processing to be performed only in the case where the line frequency reduction processing is to be performed.

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

[Overall Configuration of Color Copier] FIG. 1 is an overall configuration diagram of a color copier to which the present invention is applied, and FIG. 2 is a block diagram of a color copier to which the present invention is applied.

As shown in FIGS. 1 and 2, the color copier 1
Numeral 0 denotes a reading unit 20 for reading a document, an image processing unit 30 for processing read image data, a ROS optical unit 40 for driving a laser according to the processed image data to irradiate a light beam to a photoconductor, and forming an image. And an image forming unit 60.

As shown in FIG. 2, in the reading unit 20, an original G placed on a predetermined position of the placing table 12 (see FIG. 1) is irradiated by an exposure lamp 22 and reflected light thereof is reflected by a CCD image sensor (hereinafter referred to as a CCD image sensor). ). The image signal read by the CCD 24 is amplified to an appropriate level by an amplifier 26, and the amplified image signal is A / D
The data is converted by the converter 28 into 8-bit digital image data. The shading correction and the gap correction are sequentially performed on the digital image data. The digital image data subjected to these corrections is converted into density data by the density converter 29 and sent to the image processing unit 30.

The image processing section 30 performs basic image processing as a color copying machine, that is, color signal conversion, black reproduction (UC
R), MTF processing and the like are performed, and the image data is converted into image data of four colors of yellow, magenta, cyan, and black. The converted image data of each color is subjected to gradation conversion in accordance with the gradation of the reading unit 20 and the image forming unit 60. Further, the image processing unit 30 receives image data (for example, image data (CG image data) created by computer graphics, image data stored in a CD-ROM, etc.) from an external image processing device or the like. External data input means 33 and an image density control means 31 for creating a density conversion table for converting the density characteristics of the image data. The image density control means 31 performs the gradation conversion. Image data or the external image data is subjected to density conversion based on a density conversion table. The external data input means 33 is, for example, a floppy disk reader,
It can be constituted by a CD-ROM reading device, a communication processing device for receiving data from an external image processing device or the like via a network, or the like.

The image data on which the density conversion has been performed is supplied to an image data switching means 102 or an LUT selection unit 10 which will be described later.
4 to the D / A converter 32 to be converted into analog image data. The analog image data is sent to the comparator 39 via the selector 34. In the comparator 39,
The sent analog image data and the triangular wave generator 38
The pulse width modulation is performed by comparing the signal with a predetermined period output from the analog signal, and the analog image data is converted into binary image data. In the pulse width modulation here, for example, as shown in FIG. 3, the input analog image data A is compared with the triangular wave B from the triangular wave generator 38, and the portion where the analog image data A is larger than the triangular wave B is " 0 "
(Laser off), and binary image data in which the portion of the analog image data A smaller than the triangular wave B is "1" (laser on) is generated, and sent from the comparator 39 to the ROS optical unit 40.

The color copier 10 compares the image density represented by the image data with a predetermined image density threshold value, and reduces the number of lines for image data having a density lower than the image density threshold value. Perform processing. As shown in FIGS. 2 and 13, the density-converted image data is input to the image data switching means 102 and compared with the image density threshold value transferred from the image density control means 31. Here, when the image density represented by the image data is larger than the image density threshold value, the image data is transferred to the D / A converter 32.

On the other hand, when the image density of the image data is equal to or less than the image density threshold value, the image data is transferred to LUTs 104B and 104C having different characteristics in the LUT selection unit 104 and is converted into digital data. 104A. LUT selection circuit 104A
Then, the digital data from the LUT 104B and the digital data from the LUT 104C are switched using the reference clock signal and transferred to the D / A converter 32. L above
Through the processing in the UT selection unit 104, a low frequency conversion process is performed on low-density image data equal to or less than the image density threshold value.

In the D / A converter 32, the image data directly input from the image data switching means 102 and the digital data input from the LUT selection circuit 104A are combined, and converted into an analog image density signal. Transfer to the selected selector 34.

In the process of reducing the number of lines in the color copying machine 10 of this embodiment, an image density threshold value subjected to density conversion based on a density conversion table by the image density control means 31 is used.

The image processing section 30 is provided with patch signal generating means 36 for generating image signals (hereinafter referred to as patch image signals) of a plurality of image density control patches (reference patches) having different densities. I have. The selector 34 selects the analog image data from the D / A converter 32 at the time of normal copying, and, when receiving an instruction to create a reference patch from the arithmetic unit 84 of the image forming unit 60 described later, generates a patch signal generating means. The patch image signal from 36 is selected, sent to the comparator 39, and binarized by the pulse width modulation as described above.

The ROS optical section 40 is controlled by a laser drive circuit 42 for controlling on / off of the laser 46 based on the binary image data sent from the comparator 39, and under control of an arithmetic unit 84 of the image forming section 60 described later. And a laser light amount varying device 44 for variably controlling the laser light amount. The laser light is deflected by the polygon mirror 48 and f
Image forming unit 60 via θ lens 50 and reflection mirror 52
To the photoreceptor 62.

As shown in FIGS. 1 and 2, the image forming section 6
0, a photoconductor 62 is provided.
Around the charging device 68, an electrometer 70 for measuring the potential on the photoconductor for controlling the potential of the photoconductor, a rotary developing device 72, and measuring the patch density on the photoconductor for controlling the toner dispensing. An optical sensor 74, a transfer device 80, a cleaner device 64, and a discharge lamp 66 are provided. Further, the image forming unit 60 includes a toner dispensing device 76 that supplies toner to the developing devices of the respective colors of the rotary developing device 72, a fixing device 88, and a sheet conveying device 9.
2 is also provided.

Further, the image forming section 60 includes an arithmetic unit 84 for controlling the entire image formation and controlling image forming conditions in accordance with the outputs of the electrometer 70 and the optical sensor 74, and an arithmetic unit 84.
And a developing bias variable device 78 that changes the developing bias under the control of the arithmetic device 84. The arithmetic unit 84 includes the patch signal generator 3
6 and the selector 34 are instructed to create a reference patch.

[Outline of Image Forming Process] Next, an outline of the image forming process executed by the image forming section 60 will be described. In the image forming section 60, the following image forming processing is executed according to a well-known xerographic process. That is, the photoconductor 62 rotating clockwise in FIG.
8, the first black latent image of the first color is formed on the photoconductor 62 by the laser light from the ROS optical unit 40. This latent image is developed with black toner by the black developing device of the rotary developing device 72. The developed black toner image is transferred by a transfer corotron 80 </ b> A to a sheet (not shown) that is transported from a paper tray 90 by a paper transport device 92 and wound around a transfer drum 80. The toner image remaining on the photoconductor 62 without being transferred is removed by the cleaner device 64, and the photoconductor 6 is removed.
2 is discharged by the discharge lamp 66.

Then, the photosensitive member 62 is uniformly negatively charged again by the charging device 68, and the second color yellow image is subsequently formed. In this manner, the toner images of a total of four colors from the third color magenta to the fourth color cyan are sequentially transferred to the paper wound around the transfer drum 80. The paper to which the four color toner images have been transferred is separated from the transfer drum 80 by the separation corotron 80B, and
The color toner image is fixed on the paper, and a color copy is formed. After the transfer of each color or the peeling of the paper, the extra charge on the paper and the transfer drum 80 is discharged by the charge removing corotron 80C.
The charge is removed.

[Overview of Photoconductor Potential Control] Next, the photoconductor 6
The control of the photoconductor potential by the charge amount varying device 82, the developing bias varying device 78, and the laser light amount varying device 44 by the electrometer 70 for measuring the potential on 2 will be briefly described.

In the present embodiment, the arithmetic unit 84 of the image forming unit 60 is provided immediately before the power of the color copier 10 is turned on and before the copy is started, and thereafter, after every 30 minutes before the start of the copy.
Thus, the photoconductor potential control is performed according to the flowchart of FIG. The memory of the arithmetic unit 84 stores in advance a target dark potential VHS, a target exposure partial potential VLS, and a fog prevention potential difference VC from the target dark potential VHS to the developing bias potential VB.

First, at step 152 in FIG.
8 is applied to the voltage VG by the charge amount varying device 82.
The dark potential VH1 when set to 1 and the dark potential VH2 when set to the voltage VG2 are detected by the electrometer 70. In the next step 154, the target dark potential VH is calculated using the following equation (1).
The grid voltage VGS for obtaining S is calculated.

VGS = VG1 + ((VG2−VG1) × (VHS−VH1) / (VH2−VH1)) (1) In the next step 156, the photosensitive member 62 is set in the above step 1.
It is charged with the grid voltage VGS obtained in 54. And
Laser light amount LD by laser light amount variable device 44
1. The laser drive circuit 42 is driven by two kinds of laser light amounts LD1 and LD2 so that two kinds of laser light amounts L
A reference patch is formed by D1 and LD2. Further, the exposure partial potential VL of each of the two formed reference patches
1. VL2 is measured by the electrometer 70. In the next step 158, a laser light amount LDS for obtaining the target exposure partial potential VLS is calculated using the following equation (2).

LDS = LD2 − ((LD2−LD1) × (VLS−VL2) / (VL1−VL2)) (2) In the next step 160, development is performed using the following equation (3). Calculate the bias potential VB. VC indicates a fog prevention potential difference.

VB = VHS-VC (3) In the next step 162, the grid voltage VGS obtained as described above is developed into the charging amount varying device 82, and the laser light amount LDS is developed into the laser light amount varying device 44. The bias potential VB is set in the developing bias variable device 78, and the process ends.

[Overview of Toner Dispensing Control]
Dispensing control of each color toner for the rotary developing device 72 will be described. The toner dispensing control is realized by measuring the patch density for toner dispensing control on the photoconductor 62 by the optical sensor 74 and controlling the operation of the toner dispensing device 76 by the arithmetic unit 84 based on the measured patch density. I do. The toner dispensing control patch is instructed to be created by the arithmetic unit 84.

When an instruction to create a patch is issued from the arithmetic unit 84, the selector 34 selects a patch image signal for each color for toner dispensing control with an image area ratio of 50% from the patch signal generating means 36 and sends it to the comparator 39. Then, a reference patch for each color having an image area ratio of 50% is formed on a non-image portion on the photoconductor 62 in the same procedure as the above-described image forming process of the color copying machine.

The optical sensor 74 for measuring the patch density on the photoconductor 62 irradiates the reference patch P on the photoconductor 62 with light from the LED 74A as shown in FIG. 4, and measures the reflected light with the photodiode 74B. The patch density is measured based on the measured reflected light amount.

If the patch density measured here is lower than the target value, the arithmetic unit 84 sets the toner dispensing unit 76
Is driven to increase the toner density so that the patch density approaches the target value. Conversely, when the measured patch density is higher than the target value, the arithmetic unit 84 stops the toner dispensing device 76 and brings the patch density closer to the target value.

[Image Density Control Processing in the Present Embodiment]
Next, an image density control unit that forms a plurality of reference patches having different densities, creates a density conversion table based on the density measurement results, and converts the density characteristics of the image data based on the created density conversion table, FIG. 6 will be described with reference to the flowchart of the density conversion table creation process of FIG. 6 and the schematic diagram of the image density control of FIG.

As shown in FIGS. 6 and 7, when creating the density conversion table, the arithmetic unit 84 sends a signal for creating a correction color patch to the patch signal generating means 36, and each color selector 34 sets the patch signal generating means 36. The correction color patch image signal is selected and sent to the comparator 39, and the correction color patch is printed and output on paper in the same image forming procedure as that of the above-described color copying machine (step S1).
Here, for example, as shown in FIG. 12, a correction color patch print composed of 24 gradation patches of four colors of yellow, magenta, cyan, and black and having different densities for each color is output. In FIG. 12, Y in the upper row indicates yellow, M indicates magenta, C indicates cyan, and K indicates black.

Next, the reading unit 20 of the color copying machine 10
Is used as a density measuring device for correction color patch prints, the operator sets the correction color patch prints on the mounting table (platen) 12 (step S).
2). The density measuring device for the color patch print for correction may use a densitometer other than the reading unit 20 of the color copying machine 10.

Next, the reading section 20 measures the density of 24 color patches of each color to determine the current gradation (step S3). The result of the density measurement is sent to the image density control means 31, and it is determined whether or not the measurement result has a problem (step S4). If there is no problem here, the image density controller 31 compares the current gradation with a predetermined target gradation, and creates a density conversion table based on the comparison result and stores it in the memory (step S5).

Further, in the present embodiment, the result (value) obtained by correcting the standard image density threshold value by the created density conversion table is used as a new value for the line number reduction processing by the image data switching means 102 in FIG. It is transferred to the image data switching means 102 as the image density threshold (step S6).

If there is a problem in the measurement result in step S4, a warning is displayed to the operator and the process is stopped because there is a possibility that the correction color patch print is not properly placed (step S7).

In step S5 of the above processing, a density conversion table is created. At the time of image output, the original image data sent from the reading unit 20 is color-coded by the image processing unit 30 as shown in FIG. After the conversion and the gradation conversion processing, the density of the image is converted based on the created density conversion table so that the gradation of the image matches the target gradation. Similarly, when printing image data transmitted from the outside, based on the created density conversion table, an external image data is output so that the gradation of the image matches the target gradation. Density conversion is performed on the image data.

The image data switching means 102 performs the line frequency reduction processing on the image data subjected to the density conversion as described above, using the new image density threshold value transferred in step S6.

For example, if the density conversion table shown in FIG. 16 is created in step S5, if the standard image density threshold is 30%, the corrected image density threshold is 30% for K and Y. 50% for M,
In C, it becomes 10%. Accordingly, in the image data switching means 102, the corrected image data of 30% or less for K and Y, the corrected image data of 50% or less for M, and the corrected image data of 10% or less for C are low. A line number conversion process is performed.

Thus, according to the present embodiment, the line frequency reduction processing should be performed on the image data which should not be subjected to the line frequency reduction processing, or the image data which should be subjected to the line frequency reduction processing should not be processed. Thus, it is possible to correct the inconvenience of not performing the line-reduction processing, and to appropriately control the line-reduction processing only in the case where the line-reduction processing is to be performed. That is, it is possible to avoid the disadvantage that the image quality is reduced due to the difference in the image structure.

[Example of Execution Result of Image Density Control] The execution result of the above-described image density control will be described with reference to FIGS. FIG. 8B shows the measurement results (curve K11) of 24 patches with different densities for black, the target value of 24 patches (curve K12), and the measurement results of the 24 patches, which are created to match the target values. 9 shows a density conversion table (curve K13). FIG. 8A shows the density gradation (curve K01), the target value of the density gradation (curve K02), and the density conversion table (curve K13 in FIG. 8B) for black before density control. ) Shows the density gradation (curve K03) after density control based on the above. Obviously, the curve K0
It can be seen that the curve K03 is closer to the curve K02 than to 1. That is, by performing the density control, the density gradation can be made closer to the target value.

Similarly, for yellow, FIG.
, Measurement results of 24 patches having different densities (curve Y11),
FIG. 9A shows a target value of 24 patches (curve Y12) and a density conversion table (curve Y13) for matching the measurement result of the 24 patches to the target value, and shows a density gradation before density control shown in FIG. (Curve Y01) is changed to a target value (curve Y0).
In order to approach 2), the density conversion table (FIG. 9)
Density control is performed based on the curve Y13) of (B), and a density gradation (curve Y03) after the density control can be obtained.

FIG. 10B also shows magenta.
, Measurement results of 24 patches having different densities (curve M11),
A density conversion table (curve M13) for matching the target value of 24 patches (curve M12) and the measurement result of 24 patches to the target value is shown, and the density gradation before density control shown in FIG. (Curve M01) is changed to a target value (curve M0).
In order to approach 2), the density conversion table (FIG. 10)
Density control is performed based on the curve M13) of (B), and a density gradation (curve M03) after the density control can be obtained.

FIG. 11B shows the measurement results of 24 patches having different densities (curve C11).
A density conversion table (curve C13) for matching the target value of four patches (curve C12) and the measurement result of 24 patches to the target value is shown, and the density gradation before density control shown in FIG. (Curve C01) is changed to the target value (curve C0).
In order to approach 2), the density conversion table (FIG. 11)
Density control is performed based on the curve C13) of (B), and a density gradation (curve C03) after the density control can be obtained.

In this embodiment, an example has been described in which a plurality of reference patches having different densities are formed on a sheet and a density conversion table is created based on the results of these density measurements. A plurality of reference patches having different densities may be formed on the belt body, these densities may be measured, and a density conversion table may be created from the measurement results. Similar effects can be obtained even when the reference patches are formed on the photoreceptor or the transfer belt.

[0058]

According to the image forming apparatus of the first or second aspect, not only the density characteristics of the image signal but also the reference density value in the low frequency conversion processing is used for density conversion. To perform a low frequency reduction process on an image signal that should not be subjected to a low frequency reduction process, or not to perform a low frequency reduction process on an image signal that should originally be subjected to a low frequency reduction process,
Such an inconvenience can be corrected, and appropriate control can be performed so that the line frequency reduction processing is performed only in the case where the frequency reduction processing is to be performed.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of a color copying machine according to an embodiment.

FIG. 2 is a block diagram of the color copying machine of FIG.

FIG. 3 is a diagram illustrating binarization of image data by pulse width modulation.

FIG. 4 is a diagram illustrating a configuration of an optical sensor.

FIG. 5 is a flowchart showing a processing routine of photoconductor potential control.

FIG. 6 is a flowchart showing a procedure for creating a density conversion table.

FIG. 7 is a diagram for explaining an outline of image density control.

8A is a graph showing the density gradation before black density control, the target value of the density gradation, and the density gradation after density control for black, and FIG. 8B is a graph showing the density of 24 patches for black. It is a graph which shows a measured value, a target value of 24 patches, and a density conversion table.

9A is a graph showing a density tone before density control, a target value of density tone, and a density tone after density control for yellow, and FIG. 9B is a graph showing density of 24 patches for yellow. It is a graph which shows a measured value, a target value of 24 patches, and a density conversion table.

10A is a graph showing density gradation before magenta density control, a target value of the density gradation, and density gradation after magenta density control, and FIG. 10B is a graph showing the density of 24 patches for magenta. It is a graph which shows a measured value, a target value of 24 patches, and a density conversion table.

11A is a graph showing density gradation before density control, a target value of density gradation, and density gradation after density control for cyan, and FIG. 11B is a graph showing density of 24 patches for cyan. It is a graph which shows a measured value, a target value of 24 patches, and a density conversion table.

FIG. 12 is a diagram showing an arrangement of 24 patches.

13 is a detailed configuration diagram of an image data switching unit, an LUT selection unit, and a D / A converter in FIG.

FIG. 14 is a schematic diagram of dot reproduction when an image signal having a density lower than an image density threshold is subjected to a low frequency ruling process.

FIG. 15 is a schematic diagram of dot reproduction in a case where an image signal having a density higher than an image density threshold is not subjected to a line ruling process.

FIG. 16 is a graph showing an example of a density conversion table in a color image forming apparatus for four colors of KYMC.

17A is a diagram showing a process black image structure using three colors of YMC having a dot area ratio of 20% when the gradation density matches the target density, and FIG. FIG. 11 is a diagram illustrating a process black image structure using three colors of YMC having a dot area ratio of 20% when an image signal is corrected by a density conversion table, unlike the case of FIG.

[Explanation of symbols]

 Reference Signs List 10 color copying machine (image forming apparatus) 20 reading unit 30 image processing unit 31 image density control unit 60 image forming unit 84 arithmetic unit 102 image data switching unit 104 LUT selection unit

Claims (2)

[Claims] And performing density conversion on the image signal based on predetermined conversion information such that an image density to be represented by an image signal is equal to an image density formed based on the image signal. An image forming apparatus that forms an image by reducing the number of lines of an image signal of which an image density formed based on the image signal is equal to or less than a predetermined reference density value among image signals subjected to density conversion. An image forming apparatus, wherein density conversion is also performed on the predetermined reference density value based on the conversion information. 2. An image forming apparatus for forming an image by reducing the number of lines of an image signal for an image signal in which an image formed image density is equal to or lower than a predetermined reference density value, wherein a plurality of standards having different densities Reference patch forming means for forming a patch on the image carrier; density measuring means for measuring the density of the reference patch formed by the reference patch forming means; density target value of the predetermined reference patch and the density measurement Conversion information creating means for creating conversion information for converting the density characteristic of the image signal based on the density measurement value of the reference patch measured by the means, and based on the conversion information created by the conversion information creating means. And a density conversion unit for converting the density characteristics of the image signal and the predetermined reference density value.