JP3012286B2 - Image forming device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser scanning type image forming apparatus provided with a self-test printing means and capable of switching pixel density.

[Conventional technology]

Personal computer, EWS (engineering work station), document creation device, digital copier,
There is an image forming apparatus as an external or built-in output device for OA equipment such as a high-speed facsimile.

As an image forming apparatus that responds to the improvement of the processing speed of the OA equipment, that is, the host machine, and the demand for high-quality images as described above, a laser scanning image forming apparatus having excellent printing speed and resolution, for example, laser printing (laser beam printer:
In the following, the use of a “printer” is increasing.

Such a laser printer not only has a large pixel density, but also can easily change the dot size, dot pitch or line pitch, that is, change the pixel density, as compared with other types of printers, and have high performance. Some models have a function of selecting a pixel density according to the purpose.

With respect to the technology relating to the variable pixel density, for example, the dot size and the image clock are controlled according to the dot pitch and the number of dots constituting an image unit (characters, etc.) as disclosed in Japanese Patent Application Laid-Open No. Sho 59-117372. Proposal, JP-A-60-12
Japanese Patent Application Laid-Open No. 60-93873 discloses a proposal for controlling the intensity and scanning speed of a laser beam (hereinafter, also simply referred to as a “beam”) according to the magnification of a created image as disclosed in Japanese Patent Application Laid-Open No. 60-93873. A proposal is made such that optical systems having different resolutions (pixel densities) from each other are detachably attached to the printer body and the dot size and scanning speed are controlled in accordance with the optical system mounted, or as disclosed in JP-A-62-162547. There have been many proposals, such as a proposal to divide the reference clock according to the pixel density as described above and control the rotation speed of the polygon motor.

Regarding the timing of switching the pixel density, as shown in, for example, Japanese Patent Application Laid-Open No. 1-285349, even if a pixel density switching command is input, it waits until printing of one page is completed, and then prints the next page. It has been proposed that the pixel density be switched before the start of.

Further, in the case of, for example, test printing, for the same image data, a plurality of registered resolutions (pixel densities) can be sequentially switched from a lower one or a higher one to develop and output a bitmap. The device is Japanese Patent Application No. Hei 2-
No. 65700 (JP-A-3-265922).

Such technology is used in the adjustment process in the assembly line in the factory or on-site service by service personnel.
Alternatively, on the user side, it is conceivable that the present invention is applied to a test print for frequently performed maintenance and inspection.

As a result, if the self-test print is performed by automatically changing the pixel density of the image data of the same pattern sequentially, it is possible to efficiently adjust the resolution and easily find an abnormality. Become.

[Problems to be solved by the invention]

In general, when performing self-test printing at a plurality of pixel densities, when comparing the print states of patterns due to differences in pixel densities, it is difficult to find subtle differences by juxtaposing and comparing print results at adjacent pixel densities. It is convenient.

However, in an image forming apparatus capable of performing double-sided printing, when performing self-test printing by switching a large number of pixel densities by performing double-sided printing that saves paper, it is necessary to change each pixel density in order from the lower one or the higher one. When printing, the first and second sides of the same paper
Since the printing result of the surface is a pattern in which the pixel densities are adjacent to each other, they cannot be compared side by side, and there is a disadvantage that it is difficult to see a subtle difference.

The present invention has been made in view of the above points,
When performing self-test printing with double-sided printing,
An object of the present invention is to make it easy to see a subtle difference in a printing result depending on each pixel density.

[Means for solving the problem]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a laser scanning type image forming apparatus having a self-test printing unit and a double-sided printing unit, which can switch the pixel density. In response to the test print command, the self-test print means controls the plurality of test prints having different pixel densities from each other continuously, and the control means controls the plurality of test prints continuously performed by the duplex print means. The pixel density on the first surface and the pixel density on the second surface of the same sheet are controlled so as to be different from each other. When performing test printing with three or more types of pixel densities, the pixel density on the first surface is reduced. The pixel density on the second surface of the same sheet is determined by three or more types of pixel densities that are adjacent pixel densities. It is provided with a control means for odd controlled.

(Operation)

In the image forming apparatus configured as described above, when performing self-test printing with three or more types of pixel densities in double-sided printing, the pixel densities of the first surface and the second surface of the same sheet are not adjacent to each other. Therefore, test print results based on adjacent pixel densities can be arranged side by side and compared, facilitating checking of subtle differences.

〔Example〕

Hereinafter, an embodiment of the present invention will be specifically described with reference to the drawings.

FIG. 2 is a schematic configuration diagram showing an internal mechanism of a laser printer having a double-sided printing device as a double-sided printing means according to one embodiment of the present invention.

Photoconductor drum 1 rotating counterclockwise as indicated by the arrow
A charging charger 2, an optical writing unit 3, a phenomenon unit 4, a transfer charging unit 5, and a cleaning unit 6 are arranged in that order in the rotation direction.

The photosensitive drum 11 is first uniformly charged on its surface by the charging charger 2 and then main-scanned by a spot where a beam modulated by image data from the optical writing unit 3 and deflected in the main scanning direction is combined. As a result, an electrostatic latent image is formed.

After the electrostatic latent image is developed by the developing unit 4 and converted into a visualized toner image, the toner image is transferred onto the paper conveyed by the transfer charger 5 along the route indicated by the dashed line. Transcribed.

The toner and the charge remaining on the photosensitive drum 1 are removed by the cleaning unit 6, and the removed toner is collected by the cleaning unit 6.

The paper is fed from a paper stack 9 on a paper feed cassette 8 by a paper feed roller 10, abuts against a pair of registration rollers 11, temporarily stops, and is then written on the photosensitive drum 1 by an optical writing unit 3. The toner image is transferred to the position of the transfer charger 5 by the registration roller pair 11 at a timing corresponding to the above image.

The sheet on which the toner image has been transferred is conveyed to the fixing unit 12, is pressed against the heated fixing roller 12a by the pressure roller 12b, is fixed by the heat and pressure, and then passes under the switching claw 13. The paper is then conveyed to the paper discharge rollers 14 and
From the paper tray 16.

In the case of the double-sided mode, the sheet after fixing on the first side passes over the switching claw 13 rotated clockwise from the illustrated position, and is conveyed to the reversing unit 20 through the conveyance path 19 by the conveyance roller group 18. You.

When the reversing sensor 21 detects that the sheet has been fed to the reversing unit 20, the switching claw 22 rotates counterclockwise from the position shown in the drawing, and the reversing roller 23 starts to rotate in reverse, so that an image is formed. The sheet having the first surface facing upward and the new second surface facing downward and hitting the reversing section 20 passes under the switching claw 22 and passes through the guide path 24.
, And abuts the registration roller pair 11 to temporarily stop.

Thereafter, as in the case of the one-sided mode, the toner image is transferred onto the downward second surface, fixed, and discharged onto the paper discharge tray 16.

FIG. 3 is a perspective view of an essential part showing an example of the optical writing unit 3.

The optical writing unit 3 includes an LD (laser diode) unit 30 as a modulation light source unit and a first cylinder lens 3.
1, 1st mirror 32, pre-object type imaging lens 33
A rotary deflector 36 comprising a polygon motor 34 and a polygon mirror 35 driven to rotate in the direction of arrow A by the polygon motor 34;
The scanning image forming optical system includes a second mirror 37 and a second cylinder lens 38, and a synchronous optical system including a third mirror 40, a light receiving lens 41, and a synchronous sensor 42.

The LD unit 30 integrally incorporates an LD, a collimator lens that converts a divergent beam emitted from the LD into a parallel light beam, and an aperture that regulates the thickness of the beam.

The parallel light beam emitted from the LD unit 30 passes through the first cylinder lens 31 and is reflected by the first mirror 32, then passes through the imaging lens 33 and enters the reflection surface 35a of the polygon mirror 35.

The imaging lens 33 acts so as to form an image of a parallel main scanning direction component of the incident beam on the photosensitive drum 10 described later, and the first cylinder lens 31 uses the combined power with the imaging lens 33. This acts so that the sub-scanning direction component of the parallel beam is once imaged on the reflection surface 35a of the polygon mirror 35.

The beam reflected by the polygon mirror 35 and repeatedly deflected in the main scanning direction is reflected by the second mirror 37, passes through the second cylinder lens 38, and reaches the photosensitive drum 10 rotating in the direction of arrow B.

The second cylinder lens 38 re-images the component in the sub-scanning direction of the beam once formed on the reflection surface 35a on the photosensitive drum 10, so that the beam is mainly formed by the image forming lens 33. The photosensitive drum forms an image on the spot with the scanning direction component.
The main scanning line 10a is main-scanned in the direction of arrow C.

Immediately before the beam deflected by the rotary deflector 36 enters the effective image area on the main scanning line 10a, the beam is reflected by the third mirror 40, and is imaged on the light receiving surface by the light receiving lens 41. A synchronization sensor 42 that outputs a synchronization signal for main scanning is also provided.

FIG. 4 is a block diagram showing an example of the control unit of the laser printer.

The controller 50 is composed of a main micro computer (hereinafter referred to as “MPU”) 51 and a ROM 5 storing programs, constant data, character fonts and the like required by the MPU 51.
2 and R to store temporary data, dot patterns, etc.
AM53 and I / O54 to control input and output of commands and data
And an operation panel 55 connected to the MPU 51 via the I / O 54
And an internal interface (I / F) 56, and are mutually connected by a data bus, an address bus, and the like.

A host machine 58 that outputs a print command, character data, and image data is also connected to the MPU 51 via the I / O 54.

The engine driver 60 includes a sub-microcomputer (hereinafter referred to as a “CPU”) 61, a ROM 62 storing programs and constant data required by the CPU 61, a RAM 63 storing temporary data, and a data input / output. I control
/ O64 and are connected to each other by a data bus, an address bus and the like.

The I / O 64 is connected to the internal interface 56 of the controller 50, and receives a video signal and an operation panel from the controller 50.
Input the status of various switches on the controller 55 and send status signals such as image clock and paper end to the controller 50.
Output to

The I / O 64 includes an optical writing unit 3 and other sequence equipment groups 66 constituting a printer engine 65 as printing means, and various sensors 67 including the synchronous sensor 42.
Is also connected.

The controller 50 receives a print command, a character code, and image data from the host machine 58, edits those codes and data, and if the character code is a character code, the character font stored in the ROM 52 is used to write an image. After converting them to dot patterns, the dot patterns of those characters and images (hereinafter collectively referred to as "images") are stored in RAM 53.
It is stored in the VRAM (video RAM) area in the memory.

When the image clock is input together with the ready signal from the engine driver 60, the controller 50 outputs the dot pattern stored in the VRAM area to the engine driver 60 serially or in parallel as a video signal synchronized with the image clock.

The engine driver 60 controls the optical writing unit 3 of the printer engine 65 and the sequence device group 66 based on data from the controller 50, and inputs a video signal required for image writing from the controller 50 to the optical writing unit 3. Output to the synchronous sensor 42 and other sensors
A signal indicating the state of each part of the engine is input from 67 and processed, and necessary information and an error signal are output to the controller 50.

FIG. 1 is a circuit diagram showing an example of a control system of a printer engine 65 and an engine driver 60 mainly for a part related to optical writing in the control unit shown in FIG. 4, and is the same as FIG. Portions are given the same reference numerals and description thereof is omitted.

The printer engine 65 includes an MM (main motor) 70, an MM driver 71 that controls the MM 70, and a PM (polygon motor) 34.
And the PM driver 72 and paper feed clutch (C
L) 73, a resist clutch (CL) 75, a sequence device group 66 including CL (clutch) drivers 74, 76 and others for operating them, an LD (laser diode) 77 of the optical writing unit 3 and its driving. It comprises an LD driver 78, a synchronous sensor 42 composed of a PD (photodiode), a synchronous signal detection circuit 43, a sensor 67 composed of a resist sensor 79 and others.

Each part constituting the printer engine 65 is connected to the engine driver 60 directly or via a driver, or to the controller 50 via a control circuit.

The MM driver 71 receives the on / off signal MMC and performs constant-speed rotation control of the MM 70, which is a common power source of each unit of the printer.

The PM driver 72 inputs the on / off signal PMST and the signal PMDI for instructing the pixel density, performs synchronous control so as to maintain the PM at a constant speed according to the pixel density, and is locked when the synchronization is maintained. Signal PMLK indicating that the engine driver 60
Output to

When the feed command signal FC is turned on, the CL driver 74 turns on the feed clutch 73, which is a one-way clutch that connects the feed roller 10 and the MM 70, and feeds paper from the feed cassette 8 by the feed roller 10. Paper is a registration roller pair 11
When the registration sensor 79 detects this, the engine driver 60 turns off the signal FC, and the CL driver 74 turns off the paper feed clutch 73 to stop the paper feed roller 10.

According to the signal of the registration sensor 79, the engine driver 60
Starts printing, the LD driver 78 turns on and off the LD 77 while monitoring the light amount output according to the signal VIDEO, and outputs a modulated laser beam.

A laser beam is received for each scanning line by a synchronous sensor (PD) 42, and the engine driver 60 detects the signal and uses it as a reference for the timing of starting scanning line writing.

Further, the engine driver 60 turns on the transport command signal RC at a certain timing from the start of writing to the photosensitive drum 1.

The CL driver 76 controls the registration roller pair 11 according to the signal RC.
Turn on the resist clutch 75 that couples the
The registration roller pair 11 is rotated to convey the sheet until the signal RC is turned off according to the length of the sheet.

 Hereinafter, the optical writing will be described in detail.

The engine driver 60 is connected to an optical writing control circuit 45, a test print switch (STPSW) 46, which is a start-up unit for instructing an operator to execute a self test print, and an LD driver 78.

The optical writing control circuit 45 is further connected to a controller 50, a synchronization signal detection circuit 43, and an LD driver 78.

When the engine driver 60 inputs a print start command from the host machine 58 via the controller 50,
At the same time as outputting a standby or start command to each sequence device group 66 at the same time,
A signal LDP instructing the optimum power of the laser beam according to the ambient temperature or the like is output to the LD driver 78, respectively.

Next, when the registration sensor 79 detects a sheet, it outputs an optical writing start signal FSYNC to the optical writing control circuit 45.

When detecting the signal FSYNC, the optical writing control circuit 45 outputs a signal FGATE held at “H” to the controller 50 until printing of one page is completed according to the length of the sheet, and outputs the signal FGATE to the controller 50 in the main scanning direction. To detect the synchronization signal DETP of
The signal VIDEO output to the LD driver 78 is set to “H” to cause the LD 77 to emit light.

The synchronization signal detection circuit 43 detects the laser beam reception of the synchronization sensor 42 and outputs a synchronization signal DETP, and the optical writing control circuit
45, when the signal DETP is input, returns the signal VIDEO to "L", and sets the signal LSYNC indicating the start of line printing and the signal LSYNC indicating the start of line printing in accordance with the width of the paper, with the timing of the left margin (alignment on the left side). And outputs a signal LGATE held at “H” to the controller 50 until the printing of is completed.

When the controller 50 detects the signal LSYNC, it outputs an image data signal WDATA for one line synchronized with the image clock, and the optical writing control circuit 45 outputs the input signal WDATA to the signal VIDEO.
And outputs it to the LD driver 78.

The LD driver 78 outputs to the LD 77 a drive current signal that is turned on / off in response to the signal VIDEO, and controls the power to be the power specified by the signal LDP from the engine driver 60 while monitoring the power.

As described above, one signal is kept until the signal FGATE returns to “L”.
Image writing for each line is repeated, and an electrostatic latent image for one page is formed on the photoconductor surface of the photoconductor drum 10.

As described above, the laser printer generally receives data from the host machine 58 and sends the data from the RAM 50 to the controller 50.
The bit map is developed as a bit map on the VRAM area 3 and the bit map is decomposed for each line, and the input signal WDATA is converted into a signal VIDEO by the optical writing control circuit 45, and an image is formed by the optical writing unit 3. You.

However, when an abnormal image is generated, whether the signal WDATA is abnormal due to the controller 50 is determined by the optical writing control circuit 45 and the LD driver 7 after the engine driver 60.
In many cases, it is not possible to judge whether an abnormal image has occurred due to 8.

Therefore, regardless of the controller 50, a self-test print function for generating a predetermined pattern independently after the engine driver 60 and checking the print is provided.

That is, in the circuit diagram shown in FIG. 1, when the operator presses the test print SW 46, the engine driver 60
Detects this, performs a series of printing operations, activates a test pattern generator (not shown) in the optical writing control circuit 45, and outputs the image of the test pattern to the LD driver 78 as a signal VIDEO. .

FIG. 5 is a partially enlarged view showing an example of a test pattern printed on paper by self-test printing,
FIG. 1A shows an example of a lattice pattern, and FIG. 1B shows an example of a checkered pattern.

The type and size of this test pattern are CKX2, SEL
The signal is designated by a 4-bit signal consisting of 0 to 2. If the signal SEL2 indicating the type is 0, the lattice pattern is output. If the signal SEL2 is 1, the checkered pattern is output.

Table 1 shows an example of the relationship between the size of the other three-bit signals CKX2, SEL0, and SEL1, and the pitch a and the line width or interval b of the pattern shown in FIG. Have been.

When performing self-test printing at a pixel density of 240 dpi, 300 dpi, 400 dpi, and 480 dpi, if the combination with signals as shown in Table 2, for example, all pitches are reduced to 1/2.
A pattern aligned to the inch is printed.

FIGS. 6 and 7 are routines related to self-test printing in the main routine of the engine driver 60, which is self-test printing means and control means for controlling the optical writing control circuit 45 having a built-in test pattern generator.
FIG. 7 is a flowchart showing an example of each of an STP and a subroutine TP used in the routine.

When the routine STP shown in FIG. 6 is entered from another routine of the main routine, it is first determined whether or not the printing is possible, and then it is determined whether or not the test print SW 46 is pressed.

If any one of them is not satisfied, immediately the next routine
Move to ELSE, and if both are acceptable, set the pixel density to 240 dpi and jump to the sub-rating TP.

When the subroutine TP shown in FIG. 7 is entered, the rotation speed of the PM (polygon mirror) 34 is switched according to the set pixel density, and the signals CKX2 and CKX2 are sent to the test pattern generator in the optical writing control circuit 45. SEL0 to SEL2 are output to instruct the type and size of the pattern, and to instruct paper feeding and other preparations.

Next, whether PM34 has reached the predetermined number of revolutions,
It is determined whether or not the signal PMLK from the PM driver 72 has become “H”. If not, the process waits until the signal PMLK becomes “H”.

When the PM 34 has reached a predetermined number of revolutions, it is determined whether or not preparations for sheet feeding and the like have been completed.

When the preparation is completed, the pattern is outputted and written one line at a time from the optical writing control circuit 45, and the writing is repeated until the printing of one page is completed, that is, until the signal FGATE becomes "L".

When the test print for one page is completed, the routine returns.

After returning to the main routine STP, the pixel density is set to 300 pdi, and the process jumps to the subroutine TP again.

Similarly, switch the pixel density to 400pdi and 480dpi,
When the test prints for all the switchable pixel densities are executed, the self-test print is completed, and the routine proceeds to the next routine ELSE.

As described above, just by pressing the test print SW 46 once, the engine driver 60 as the control means successively,
Since the test print is performed by changing the pixel density (so as not to overlap), the operation of the self-test print is extremely easy, and there is no waste in performing the test print with the duplicated pixel density by mistake or skipping a certain pixel density. In addition, the required time is reduced as compared with the conventional method of manually switching the pixel density.

Further, even if the pixel density is printed at the beginning or end of each page, if the pixel density is switched from the lower one or the higher one, the check of the printed pattern becomes easy. .

In particular, as shown in FIG. 6, when the pixel density is switched from a low density to a high density, generally, the time during which the PM 34 stabilizes at a predetermined speed is much longer when the rotation speed is higher than when the rotation speed is lower. Since there is a tendency to be short, there is an effect that the total time required for the self-test print is further reduced.

Since the embodiment shown in FIG. 2 is a laser printer capable of double-sided printing, the self-test printing is also performed in the double-sided mode, so that paper is saved and the storage space is reduced.

When the self-test printing is performed in the double-sided mode and there are three or more types of switchable pixel densities, the pixel density on the first side and the pixel density on the second side of the same sheet are mutually different. Control is performed so that the pixel density does not become adjacent. This is a feature of the present invention.

FIG. 8 is a flowchart showing an example of the main routine STPW in that case, and this routine STPW also uses the subroutine TP shown in FIG.

As is clear from FIG. 8, this routine STPW
The difference from the routine STP shown in the figure is that the order of pixel density is 2
40dpi, 400dpi, 300dpi, and 480dpi, and the other parts are the same, so that the description of the flow is omitted.

FIG. 9 shows a print example of the result of performing the self-test print in the duplex mode according to these routines, and FIGS. 9A and 9B are based on the routines STP and STPW, respectively.

In FIG. 9, the symbols (1-1), (1
-2), (2-1), and (2-2) show the first and second surfaces of the first sheet, respectively, and the first and second surfaces of the second sheet. This is illustrated by a broken-line frame.

That is, the printing in the duplex mode by the routine STP shown in FIG.
i, (1-2) 300dpi, (2-1) 400dpi, (2-2)
Has a 480dpi pattern printed on it.

On the other hand, printing in the duplex mode by the routine STPW shown in FIG.
-2) 400dpi, (2-1) 300dpi, (2-2) 480d
The pi pattern is printed.

In a case where patterns of 240 dpi, 300 dpi, 400 dpi, and 480 dpi (hereinafter referred to as P24, P30, P40, and P48, respectively) are arranged and compared in the order of pixel density, in the example shown in FIG. Since two adjacent combinations of “P24, P30” and “P40, P48” are printed on both sides of the same sheet, they cannot be compared side by side.

However, in the example shown in FIG. 9B by the routine STPW, the combinations of “P24, P40” and “P30, P48” are not arranged, but “P24, P30” and “ P3
All combinations of "0, P40" and "P40, P48" can be compared side by side.

In general, when comparing the printing state of patterns due to differences in pixel density, it is more convenient to see subtle differences if patterns with adjacent pixel densities can be arranged than to arrange patterns with greatly separated pixel densities. It is clear that is better. Therefore, as described above, the pixel density of the first surface and the pixel density of the second surface of the same sheet are the same.
By controlling the pixel density so as not to be adjacent to each other, even in the case of double-sided printing, it is possible to arrange and compare the patterns of the printing results based on the adjacent pixel densities. It is easier to check the difference.

The laser printer that performs a series of self-test printing by pressing the test print SW (STPSW) 46 once has been described. Of course, this self-test printing means may be configured to be activated by a command from the host machine 58.
The printer itself is an independent startup means, for example, test print SW
It is more convenient to have 46.

In some cases, such as in the adjustment process, it is desired to perform a self-test print only for the pixel density being adjusted.
The present invention does not preclude such a conventional method of self-test printing.

Although the embodiment in which the present invention is applied to the laser printer has been described above, the present invention is not limited to the laser printer, and is not limited to the laser printer.
It goes without saying that the present invention can be applied to a laser scanning type image forming apparatus built in a device.

〔The invention's effect〕

As described above, in the image forming apparatus according to the present invention, when performing self-test printing with three types of abnormal pixel densities in double-sided printing, the pixel densities of the first surface and the second surface of the same sheet are adjacent to each other. Because it does not become density,
It becomes possible to arrange and compare the patterns of the print result by the adjacent pixel densities.

Therefore, even in the self-test printing using the two-sided printing, it is easy to check a delicate difference due to a difference in pixel density.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of a control system relating to optical writing of a printer according to the present invention, FIG. 2 is a schematic configuration diagram of an internal mechanism of a laser printer showing one embodiment of the present invention, FIG. FIG. 4 is a perspective view of an essential part of the writing unit, FIG. 4 is a block diagram showing an example of the control unit, FIG. 5 is a partially enlarged view showing an example of the test pattern, and FIGS. FIG. 9 is a flowchart showing an example of the processing routine, and FIG. 9 is an explanatory view showing an example of the test pattern printed on both sides. 3. Optical writing unit, 30 LD unit 45 Optical writing control circuit (self-test printing means) 46 Test print switch (starting means) 60 Engine driver (control means) 77 LD (Laser diode)

Claims (1)

(57) [Claims] 1. A laser scanning type image forming apparatus comprising a self-test printing means and a two-sided printing means capable of switching pixel density, comprising: a starting means for the self-test printing means; and a test print command issued by the starting means. Control means for controlling the self-test printing means to continuously perform a plurality of test prints having different pixel densities from each other; and a first surface of the plurality of test prints continuously performed by the double-sided printing means. And the pixel density on the second surface of the same sheet are controlled so as to be different from each other. When performing test printing with three or more types of pixel densities, the pixel density on the first surface is the same as that on the second surface. Control means for controlling the pixel density on the second surface of the paper so as not to be adjacent to each other. An image forming apparatus characterized in that: