JPH1184961A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a stable half tone image whose density is not influenced by temperature and humidity by previously preparing a temperature and humidity detecting means and plural threshold matrices, properly selecting the threshold matrix in accordance with the detected value of the temperature and the humidity and controlling how to assign the tone expression number of a threshold per pixel in the threshold matrix. SOLUTION: A temperature and humidity sensor 515 detects ambient temperature and ambient humidity on the inside of an image forming device and transmits a signal (x) to a control part 520. A tone expression number changing means 521 provided in the control part 520 generates a selection signal A for changing how to assign the tone expression number of the threshold per pixel in the threshold matrix in accordance with the detected value from the sensor 515. The generated selection signal A is transmitted to a threshold memory 512. How to assign the tone expression number of the threshold per pixel is changed by selecting the threshold matrices 512a and 512b based on the selection signal A.

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 for performing halftone processing, and more particularly, to detecting a change in the environment of a main body of a device and, based on the detected value, a threshold level per pixel of a threshold matrix. The present invention relates to an image forming apparatus that adjusts image density by changing a method of assigning the number of tonal expressions.

[0002]

2. Description of the Related Art FIG. 5 shows a schematic configuration of a laser beam printer which is a conventional image forming apparatus. This laser beam printer is a terminal device connected to a computer via a predetermined interface cable, and outputs, for example, a result of information processed by the computer on recording paper.

In the laser printer shown in FIG.
Is a signal processing unit to which an image signal is input. This image signal is input via an image input unit (not shown). As the image signal, a color image signal including R (red), G (green), and B (blue) components is input, but is not limited to this color image signal.

A pseudo-halftone processing unit 202 adjusts the image density level of a color image signal. 203 is a laser driver, and 204 is an amplifier. This amplifier 204
Is output to the semiconductor laser 205. 2
06 is a polygon mirror, 207 is an fθ lens, 208 is a reflection mirror, and 209 is a photosensitive drum. The subsequent configuration and processing of the photosensitive drum 209 will be described with reference to FIGS. 8 and 9 described later.

Here, the image forming process will be described.
First, a color image signal is input to the signal processing unit 201 as R (red), G (green), and B (blue) components. The image signal input in the signal processing unit 201 is subjected to predetermined signal processing, and Y (yellow), M
(Magenta), C (cyan), and K (black).

[0006] The Y,
The M, C, and K color image signals are supplied to the pseudo halftone processing unit 2.
At 02, image processing is performed.

FIG. 6 shows image processing in the pseudo halftone processing section 202. When the original image 220 of FIG. 6A is input, AD conversion is performed, and a digitized original image 221 as shown in FIG. 6B is obtained. Here, it is assumed that the image density level of each pixel in the digitized original image 221 is at seven levels.
Then, the original image 221 at the predetermined image density level
Are the thresholds (slice levels) of the threshold matrix 222 in which 4 × 4 pixels constitute one block as shown in FIG.
Compare with

FIG. 7 shows a comparator 230 for performing the comparison processing. An original image 221 having a predetermined image density level is converted into a threshold matrix 22 having a predetermined threshold.
2, pixels having an image density level higher than the slice level are black, and pixels having a lower image density level are white, and a pseudo halftone image is formed. As a result, a pseudo halftone image 223 having an image density distribution as shown in FIG. 6D is output.

The pseudo halftone image 223 formed in the pseudo halftone processing unit 202 in this manner is sent to a laser driver 203 as shown in FIG. Then, the laser driver 203 modulates the input pseudo halftone image 223, that is, Y, M, C, and K color image information, and outputs the modulated signal to the amplifier 204 as a modulation signal. The amplifier 204 drives the semiconductor laser 205 according to the modulation signal output from the laser driver 203. The laser light output from the semiconductor laser 205 scans the photosensitive drum 209 via the polygon mirror 206, the fθ lens 207, and the reflection mirror 208.

Hereinafter, the printing process shown in FIG. 8 will be described. First, in the charging step 301, the photosensitive drum 2
In the surface layer 09, a photoconductor is laminated on a conductive substrate, and a constant current control is performed by a primary charger (not shown) so that the surface potential of the photosensitive drum 209 is constant regardless of environmental conditions and history of the photosensitive drum. Accordingly, the surface of the photosensitive drum 209 is uniformly charged.

Next, in an exposure step 302, as shown in FIG. 5, the photosensitive drum 209 is irradiated with a laser beam corresponding to the image information.
By scanning above, an electrostatic latent image is formed on the photosensitive drum 209 by an electrostatic force corresponding to the laser beam.

Next, in a developing step 303, an exposure step 30
In order to visualize the electrostatic latent image formed in step 2, each color is developed. Details of the developing step 303 will be described with reference to the configuration diagram of the developing roller 400 shown in FIG. The developing roller 400 includes a fixed magnet 403 and a magnetic cylinder (sleeve) 404 rotating on the outer periphery thereof. The toner 402 is charged by friction with the sleeve 404. A distance of several hundred μm is maintained between the photosensitive drum 208 and the sleeve 404, and the toner 402 is caused to fly on the photosensitive drum 209 according to the electrostatic latent image formed in the exposure process 302 while flying the toner 402 in the alternating magnetic field. By attaching
Visualize the electrostatic latent image. Thus, in the developing process 303, the visualization by the toner 402 according to the electrostatic latent image is performed.

Next, in a transfer step 304 shown in FIG. 8, the print paper supplied from the supply step 309 is brought into contact with the photosensitive drum 209 in synchronization with the transfer paper, and a transfer charger (not shown) is applied to the surface of the print paper opposite to the toner. By applying a polarity charge, the toner on the photosensitive drum 209 is copied onto a print sheet. In the subsequent drum cleaning step 306, the residual toner on the photosensitive drum 209 of the photosensitive drum 209 on which the visible image has been transferred is removed by a cleaning blade (not shown).

The exposure step 302, the development step 303, the transfer step 304, and the drum cleaning step 3
06 are performed according to the number of colors used.
K is repeated four times. When the transfer is completed for all colors, in the next separation step 305, the above-described transfer step 3
In 04, the print paper on which the toner image has been transferred is separated from the photosensitive drum 209.

Finally, in a fixing step 307, the toner transferred onto the print sheet is heat-fixed by a fixing roller (not shown), and the toner image is fixed on the print sheet. Thereafter, the print paper on which the toner image has been fixed in the paper discharge step 308 is discharged.

The color printing is completed according to the steps described above.

[0017]

However, in the above-described conventional example, since the threshold matrix 222 used for the halftone processing is fixed, the toner may be depleted due to environmental conditions of the image forming apparatus, for example, temperature and humidity. Photosensitive drum 20
9 causes a difference in the flying ratio, and as a result, there is a problem that the image density after transfer to the recording paper changes.

Therefore, an object of the present invention is to store one or more values in a threshold matrix according to a change in temperature and humidity around the apparatus main body.
An image forming apparatus capable of obtaining a stable image, particularly a color image, which is not affected by changes in temperature and humidity, by adjusting the image density level by changing the method of allocating the number of gradation representations of the threshold value per pixel. To provide.

Another object of the present invention is to provide a method of allocating the number of gradations of a threshold value per pixel in a threshold value matrix in accordance with the ratio of toner flying on a photosensitive drum according to an electrostatic latent image. An object of the present invention is to provide an image forming apparatus capable of obtaining a stable image which is not affected by environmental changes, particularly a color image, by changing the image density level.

[0020]

According to the present invention, image information subjected to predetermined signal processing is input to an image processing unit to form a halftone image having a converted image density level, and the halftone image is converted into a halftone image. An image forming apparatus that forms an image by forming an electrostatic latent image on a photoreceptor by modulating and driving a light source, wherein an environmental state detecting unit that detects an environmental state of the apparatus main body; Storage means for storing a threshold matrix having a threshold corresponding to each pixel of the image information; and a threshold value per pixel in the threshold matrix according to a detection value of the environmental state detection means. Means for changing the number of tonal expressions, and comparing means for comparing each pixel of the image information with a threshold value of the threshold matrix and outputting a halftone image in which the image density level is converted With the By driving the light source on the basis of the halftone image, by controlling the ratio of the electrostatic latent image formed on the photosensitive member, constituting an image forming apparatus.

Here, the gradation expression number changing means selects a predetermined threshold matrix from a plurality of threshold matrices stored in the storage means, thereby obtaining the threshold gradation expression number per pixel. Change the allocation method.

Further, according to the present invention, image information subjected to predetermined signal processing is input to an image processing section to form a halftone image having a converted image density level, and the halftone image is modulated to form a light source. An image forming method for forming an electrostatic latent image on a photosensitive member to form an image by driving the image forming apparatus, comprising: an environmental state detecting unit that detects an environmental state of the apparatus main body; and each pixel of the image information. Using a threshold matrix having a threshold value corresponding to the threshold value, and changing a method of assigning the number of gradation expressions of the threshold value per pixel in the threshold matrix according to the detection value of the environmental state detecting means. And a comparing step of comparing each pixel of the image information with a threshold value of the threshold value matrix and outputting a halftone image in which the image density level is converted, based on the output halftone image. Drive the light source It makes by controlling the ratio of the electrostatic latent image formed on the photosensitive member, an image forming method.

Here, in the step of changing the number of gradation representations, a method of allocating the number of gradation representations per pixel is changed by selecting a predetermined threshold matrix from the plurality of threshold matrices.

The environmental condition detecting means may comprise a temperature and humidity sensor for detecting a temperature and a humidity around the apparatus main body.

The environmental condition detecting means may comprise a toner detecting sensor for detecting a rate at which the toner according to the electrostatic latent image flies to the photosensitive member.

The light source may be constituted by a laser light source using a semiconductor laser or an LED light source.

The present invention also provides a computer
A recording medium on which a control program for controlling an image density level of image information subjected to predetermined signal processing to form a halftone image is recorded, wherein a threshold having a threshold corresponding to each pixel of the image information is provided. Using a matrix, the control program causes a computer to detect the temperature and humidity around the apparatus main body using a temperature and humidity sensor, and according to the detected value of the temperature and humidity sensor, a threshold level per pixel in the threshold matrix. The method of allocating the number of tonal expressions is changed, and by comparing each pixel of the image information with the changed threshold value of the threshold matrix, the halftone image whose image density level is converted is output. Driving the light source based on the halftone image, thereby controlling the ratio of the electrostatic latent image formed on the photoreceptor, To provide a recording medium which records a growth control program.

Further, the present invention provides a computer
A recording medium on which a control program for controlling an image density level of image information subjected to predetermined signal processing to form a halftone image is recorded, wherein a threshold having a threshold corresponding to each pixel of the image information is provided. Using a matrix, the control program causes the computer to detect the rate at which the toner according to the electrostatic latent image flies to the photoreceptor using a toner detection sensor, and according to the detection value of the toner detection sensor, The halftone image whose image density level has been converted by changing the way of assigning the number of gradation representations of the threshold value per pixel and comparing each pixel of the image information with the changed threshold value of the threshold matrix. And controlling the ratio of the electrostatic latent image formed on the photoreceptor by driving the light source based on the output halftone image. By providing a recording medium which records a halftone image formation control program.

Here, by selecting a predetermined threshold matrix from the plurality of threshold matrices, it is possible to change the way of assigning the number of gradation representations of thresholds per pixel in the threshold matrix.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings.

First, the first embodiment of the present invention will be described with reference to FIG.
A description will be given with reference to FIG.

In this embodiment, a laser beam printer used in a conventional example will be described as an example of an image forming apparatus. The overall flow of the image forming process is the same as that of the conventional example (see FIGS. 5 to 9), and thus the description thereof is omitted, and the same parts are denoted by the same reference numerals.

FIG. 1 shows a schematic configuration of the signal processing unit 201 and the pseudo halftone processing unit 202. Signal processing unit 20
Reference numeral 1 describes in detail the internal configuration of the conventional signal processing unit 201 shown in FIG. The pseudo halftone processing unit 202 shows a configuration according to the present invention.

First, the signal processing unit 201 will be described. The color image signal is A / D converted and converted into 8-bit R, G, B signals in the ROM 5 in the signal processing unit 201.
01 is input to the input terminal.

The ROM 501 performs a process of converting the R, G, and B signals as luminance signals into Y, M, and C signals as density signals. In order to perform this conversion processing, the input signal 0 is calculated using the coefficients represented by the following equations (1) to (3).
Data corresponding to 0 (H) to FF (H) is output from an LUT (look-up table) not shown.

Y = −log (aB) (1) M = −log (aG) (2) C = −log (aR) (3) where a is a coefficient.

Then, Y,
The 8-bit data of each of the M and C signals is input to a UCR (under color control) circuit 503 via a selector 502.
The UCR circuit 503 performs a masking process by a known undercolor removal method using a Min (minimum value) detection circuit 504 and a conversion table 505, and each of the Y, M, C, and K signals is / VDO7 to / VDO0. Are sequentially output as 8 bits. The Y, M, C, and K signals output in this manner are input to the pseudo halftone processing unit 202.

Next, the pseudo halftone processing section 20 according to the present invention
2 will be described.

In FIG. 1, reference numeral 510 denotes a γ table (hereinafter referred to as a first γ table) for correcting the nonlinearity of the output device. Reference numeral 511 denotes a comparison processing unit that compares the output of the first γ table 510 with a threshold and outputs the comparison result. 512 is a threshold matrix 512a, 51
This is a threshold memory for storing the threshold defined in 2b. 5
Reference numeral 13 denotes a γ table (hereinafter, referred to as a second γ table) for obtaining an actual pixel modulation intensity from the comparison result obtained for each pixel.

The temperature / humidity sensor 515 is a sensor provided inside the image forming apparatus, detects the ambient temperature and the ambient humidity inside the image forming apparatus, and sends the detected signal x to the control section 520. The control unit 520 is provided with a gradation expression number changing unit 521. The gradation expression number changing means 521 generates a selection signal A for changing the way of assigning the gradation expression number of the threshold value per pixel in the threshold value matrix according to the detection value of the temperature / humidity sensor 515. By transmitting the generated selection signal A to the threshold memory 512, the way of assigning the number of gradations of the threshold value per pixel in the threshold matrix is changed.

In this example, a plurality of (two) threshold matrices 512a and 512b (see FIG. 2 to be described later) having different ways of assigning the number of threshold gradations per pixel are prepared. , 512b based on the selection signal A, it is possible to change the way of assigning the number of gradation representations of the threshold value per pixel. The method of changing the method of assigning the number of gradation representations will be described in detail later.

514 is a horizontal number H of processed pixels.
And a number V in the vertical direction. The count value (H, V) of the counter 514 is stored in the threshold memory 512.
, A threshold in the threshold matrices 512a and 512b corresponding to each pixel is selected.

Here, the operation of the pseudo halftone processing section 202 will be described.

Now, in the initial state, the value of the counter 514 is initialized to (H, V) = (0, 0), and the size of the threshold matrices 512a, 512b is set to M × N (M, N
Is the size in the horizontal direction and the vertical direction, respectively), and if the number of gradation representations of the threshold per pixel is P, the threshold memory 5
From 12, P types of thresholds corresponding to the address (0, 0) are loaded into the comparison processing unit 511. This comparison processing unit 5
In P1, P comparators 230 (see FIG. 7) are arranged in parallel, and different P thresholds are input as their reference values.

FIG. 2 shows that the size of the threshold matrix is 4 × 4.
(M × N), a representative fattening type (centralized type) threshold matrix 512a in which the number of gradation representations of thresholds per pixel in each threshold matrix is expressed as 16 levels (P), and a Bayer type ( (Distributed) threshold matrix 512b. Each threshold matrix 512a, 512
The numerical value in b indicates the threshold density level corresponding to one pixel. The input / output characteristics of the image density change depending on the way of assigning the number of gradation representations of one pixel in the threshold matrices 512a and 512b. FIG. 3 shows an example of the input / output characteristics of the image density. It can be seen that the output level of the image density differs between the characteristic A and the characteristic B depending on the position of the input value.

In general, since the image forming apparatus causes toner to fly on the photosensitive drum 209 by an electrostatic recording method, it is easily affected by temperature and humidity, and particularly in the case of color image formation, the degree of the influence is remarkable.

FIG. 4 shows a state in which the value of the toner density with respect to the input level of the image signal in the image forming apparatus changes according to the environmental conditions of temperature and humidity. As can be seen from FIG. 4, in general, the concentration level increases in a low-temperature and low-humidity environment, and conversely, in a high-temperature and low-humidity environment.
In humid environments, the concentration level tends to decrease.

Therefore, in this embodiment, a temperature / humidity sensor 515 for detecting temperature and humidity is provided in the image forming apparatus to detect temperature / humidity, and one pixel of the threshold matrices 512a and 512b described above according to the detected information. Control is performed so as to change the method of assigning the number of gradations of the hit threshold. Such control is performed by the gradation expression number changing unit 521 in the control unit 520.
(This is a control program stored in a storage means such as a ROM).

That is, when the signal x detected by the temperature / humidity sensor 515 is input to the control section 520, the environmental condition of the image forming apparatus is grasped by the gradation expression number changing means 521. When the environmental condition of the image forming apparatus is low temperature and low humidity, since the toner density is high from FIG. 4, it is necessary to adjust so that the toner density is set low. Therefore, the control unit 520 generates a selection signal A that selects the Bayer type (distributed type) threshold matrix 512b in FIG.

On the other hand, when the environmental conditions of the image forming apparatus are high temperature and high humidity, since the toner density is low from FIG. 4, it is necessary to make an adjustment so that the toner density is set high. Accordingly, the control unit 520 generates a selection signal A that selects the fatning type (centralized type) threshold matrix 512a in FIG.

The selection signal A thus generated in the control section 520 is input to the threshold memory 512, whereby the threshold matrix 512a or the threshold matrix 512b is selected. Then, for example, when the threshold matrix 512a is selected, (H, V)
Is sent to the comparison processing unit 511. At this time, assuming that the number of threshold gradation representations per pixel in the threshold matrix 512a is 16 levels,
The six thresholds are respectively input to the sixteen comparators 230 in the comparison processing unit 511. On the other hand, the first γ table 51
0 is converted to a table by the comparison processing unit 51.
It is input to 16 comparators 230 in 1. Thereby, in the comparison processing unit 511, the comparison processing between the input image signal and the threshold value is performed in parallel by the 16 comparators 230.

If the comparator 230 is set so as to output "True" when the input image signal is larger than the threshold value, the value of the output "True" becomes the second γ Input to the table 513. The pseudo halftone image 223 representing n-bit pixels output from the second γ table 513 is input to the laser driver 203. The pseudo halftone image 223 thus obtained is an image signal that is not affected by environmental changes, that is, temperature and humidity.

The laser driver 203 modulates the pseudo halftone image 223, that is, the Y, M, C, and K color image information, and outputs the modulated signal to the amplifier 204 as a modulation signal. In the amplifier 204, according to the modulation signal output from the laser driver 203, the semiconductor laser 2 shown in FIG.
05 is driven. Thereafter, an exposure step 302 shown in FIG.
Then, by scanning the photosensitive drum 209 with a laser beam corresponding to the modulation signal of the pseudo halftone image 223, an electrostatic latent image is formed on the photosensitive drum 209 by an electrostatic force corresponding to the laser beam. Further, in a development step 303, development of each color is performed, and the electrostatic latent image formed in the exposure step 302 is visualized. By the above processing, the visualized image is a stable image in which the image density level is not influenced by the temperature and humidity.

As described above, a pseudo halftone image which is not affected by temperature and humidity is created by comparing the plurality of types of the gradation expression number of the threshold value per pixel to be compared in the comparison processing unit 511. Thus, the image density level can be controlled.

Next, a second embodiment of the present invention will be described with reference to FIG.
It will be described based on. The description of the same parts as in the first embodiment is omitted.

In the above-described first embodiment, the temperature / humidity sensor 515 is used as the environmental state detecting means, and the method of allocating the number of gradation representations of the threshold per pixel in the threshold matrix according to the ambient temperature in the image forming apparatus. Is changed to output a stable image density, but in this example, another configuration example is shown as the environmental state detecting means.

For example, there is a case where one of the color toners Y, M, C, and K for printing has been consumed, the toner replacement is warned, and a stable color image cannot be output. In such a case, a toner detection sensor 530 is prepared as an environmental state detecting means, and the way of allocating the number of gradations of the threshold value per pixel in the threshold value matrix is changed according to the signal y for warning of toner replacement. To control. In other words, a plurality of threshold matrices are prepared in advance, and the threshold matrices are switched according to the rate at which toner according to the electrostatic latent image flies onto the photosensitive drum 209. By changing the method of assigning the number of tonal expressions and performing halftone processing, a stable image is provided. The method of controlling the image density is the same as in the first embodiment described above.
The description here is omitted.

In the first and second embodiments described above, as a method for changing the allocation of the number of gradation representations of the threshold value per pixel in the threshold value matrix,
Although a plurality of threshold matrices are prepared and selected in advance in the threshold memory 512, the present invention is not limited to such a method. For example, the gray scale of the threshold per pixel in the threshold matrix according to the detection signal A similar effect can be obtained by calculating the value for changing the number of expressions by the CPU and rewriting the threshold value in the memory.

The image forming apparatus of the present invention is exemplified by a laser beam printer. However, the present invention is not limited to this. An image forming apparatus using another electrostatic latent image system, such as a copying machine or a facsimile, is used. Also applicable to The light source is not limited to a semiconductor laser, but may be an LED light source.

The temperature / humidity sensor 515 and the toner detection sensor 530 are used as the environmental state detecting means of the present invention. However, the present invention is not limited to this. For example, the state of transfer to print paper after the transfer step is detected. It is also possible to adopt a configuration in which the image density level in the halftone processing is controlled according to this detection value.

The present invention may be applied to a system composed of a plurality of devices or an apparatus composed of one device. Needless to say, the present invention can be applied to a case where the present invention is achieved by supplying a program to a system or an apparatus. In this case, by reading a storage medium storing a program represented by software for achieving the present invention into the system or the apparatus, the system or the apparatus can enjoy the effects of the present invention.

[0062]

As described above, according to the present invention,
A method of allocating the number of gradation representations of thresholds per pixel in the threshold matrix by preparing temperature and humidity detecting means and a plurality of threshold matrices in advance and appropriately selecting the threshold matrix according to the detected temperature and humidity values. And an image forming apparatus capable of forming a stable halftone image whose image density is not affected by temperature and humidity can be obtained.

Further, according to the present invention, means for detecting the rate at which the toner according to the electrostatic latent image flies to the photoreceptor and a plurality of threshold matrices are prepared in advance, and the ratio at which the toner flies to the photoreceptor is determined. By appropriately selecting a threshold matrix in accordance with the above, it is possible to control a method of allocating the number of gradations of the threshold value per pixel in the threshold matrix, and to create a stable halftone image in which the image density is not affected by environmental changes. An image forming apparatus capable of performing the above operation can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram mainly illustrating a halftone processing circuit according to first and second embodiments of the present invention;

FIG. 2 is an explanatory diagram schematically showing a configuration example of a threshold matrix.

FIG. 3 is a characteristic diagram showing an example of input / output characteristics of image density.

FIG. 4 shows the relationship between the toner density and the input level of an image signal.
FIG. 9 is a characteristic diagram showing a state of changing according to environmental conditions of temperature and humidity.

FIG. 5 is an explanatory diagram showing an overall flow of an image forming process.

FIG. 6 is an explanatory diagram showing a process of creating a pseudo halftone image.

FIG. 7 is a configuration diagram of a comparator.

FIG. 8 is an explanatory diagram showing a printing process.

FIG. 9 is a cross-sectional view illustrating a configuration of a developing unit.

[Explanation of symbols]

 202 Image processing unit 205 Semiconductor laser 209 Photosensitive drum 511 Comparison means 512 Storage means 512a, 512b Threshold matrix 515 Temperature / humidity sensor (environment sensor) 521 Gradation expression number changing means 530 Toner detection sensor (environment sensor)

Claims (15)

[Claims] 1. An image processing apparatus according to claim 1, wherein the image information subjected to predetermined signal processing is input to an image processing unit to form a halftone image having a converted image density level, and the halftone image is modulated to drive a light source. An image forming apparatus for forming an electrostatic latent image on a photoreceptor to form an image, wherein the image information is provided in the image processing unit; Storage means for storing a threshold matrix having a threshold value corresponding to each pixel, and a method of allocating the number of gradation representations of thresholds per pixel in the threshold matrix according to the detection value of the environmental state detection means. Means for changing the number of gradation representations to be performed, and comparing means for comparing each pixel of the image information with a threshold value of the threshold value matrix and outputting a halftone image in which the image density level has been converted. Based on halftone image There by driving the light source, an image forming apparatus characterized by controlling the ratio of the electrostatic latent image formed on the photosensitive member. 2. The method according to claim 1, wherein the tone expression number changing means selects a predetermined threshold matrix from a plurality of threshold matrices stored in the storage means, thereby assigning a threshold tone number per pixel. The image forming apparatus according to claim 1, wherein the direction is changed. 3. An image forming apparatus according to claim 1, wherein said environmental condition detecting means comprises a temperature / humidity sensor for detecting a temperature and a humidity around said apparatus main body. 4. An image according to claim 1, wherein said environmental condition detecting means is a toner detecting sensor for detecting a rate at which toner according to said electrostatic latent image flies to said photosensitive member. Forming equipment. 5. The image forming apparatus according to claim 1, wherein the light source is a laser light source using a semiconductor laser. 6. The image forming apparatus according to claim 1, wherein the light source is an LED light source. 7. An image processing apparatus according to claim 1, wherein the image information on which predetermined signal processing has been performed is input to an image processing unit to form a halftone image having a converted image density level, and the halftone image is modulated to drive a light source. An image forming method for forming an electrostatic latent image on a photoreceptor to form an image, comprising: an environmental state detecting means for detecting an environmental state of an apparatus main body; and a threshold value corresponding to each pixel of the image information. A gradation expression number changing step of changing a method of assigning a gradation expression number of a threshold value per pixel in the threshold value matrix according to a detection value of the environmental state detection means, Comparing each pixel of information with a threshold value of the threshold value matrix to output a halftone image having the converted image density level, and driving the light source based on the output halftone image. By doing An image forming method, comprising: controlling a ratio of an electrostatic latent image formed on the photoconductor. 8. The method of changing the number of gradation representations, comprising: selecting a predetermined threshold matrix from the plurality of threshold matrices to change a method of allocating the number of gradation representations per pixel. The image forming method according to claim 7, wherein: 9. The image forming method according to claim 7, wherein said environmental condition detecting means comprises a temperature / humidity sensor for detecting a temperature and a humidity around said apparatus main body. 10. An image according to claim 7, wherein said environmental condition detecting means is a toner detecting sensor for detecting a rate at which toner according to said electrostatic latent image flies to said photoreceptor. Forming method. 11. The image forming method according to claim 7, wherein the light source is a laser light source using a semiconductor laser. 12. The image forming method according to claim 7, wherein the light source is an LED light source. 13. A recording medium recording a control program for forming a halftone image by controlling an image density level of image information on which predetermined signal processing has been performed by a computer, wherein: Using a threshold matrix having threshold values corresponding to pixels, the control program causes a computer to detect the temperature and humidity around the apparatus main body by using a temperature and humidity sensor. The method of allocating the number of gradation representations of the threshold value per pixel is changed, and each pixel of the image information is compared with the changed threshold value of the threshold value matrix, so that the image density level is converted into the halftone. An image is output, and the light source is driven based on the output halftone image, whereby an electrostatic latent image formed on the photoconductor is formed. A recording medium storing a halftone image formation control program, characterized in that the ratio is controlled. 14. A recording medium storing a control program for forming a halftone image by controlling the image density level of image information on which predetermined signal processing has been performed by a computer, wherein: Using a threshold matrix having threshold values corresponding to the pixels, the control program causes the computer to detect the rate at which the toner according to the electrostatic latent image flies to the photoconductor by the toner detection sensor, and calculates the detection value of the toner detection sensor. Accordingly, the method of allocating the number of gradations of the threshold value per pixel in the threshold value matrix is changed, and each pixel of the image information is compared with the changed threshold value of the threshold value matrix, thereby obtaining the image density. By outputting a halftone image whose level has been converted, and driving the light source based on the output halftone image, A recording medium storing a halftone image formation control program, wherein a ratio of an electrostatic latent image formed on the medium is controlled. 15. A method of selecting a predetermined threshold matrix from among the plurality of threshold matrices, thereby changing a method of allocating the number of threshold gradations per pixel in the threshold matrix. 15. A recording medium recording the halftone image formation control program according to 13 or 14.