JP2001071560A - Imaging apparatus and imaging method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high quality halftoning image by minimizing the effect of uneven pitch on the halftoning of print mechanism when an image is formed by a multibeam scanning electrophotographic system. SOLUTION: An image pattern is formed while taking account of a position subjected to disturbance of scanning by matching the growing point of mesh point with an optical system being scanned continuously. The numeric values indicate the order of growth and do not indicate the threshold value. Two growing points are set in a dither. Boundary of scanning of four continuous light source groups is shown by a dashed line. Since the growing point and a converging point exist on a dashed line, the boundary of scanning intersects the interface of writing area and non-writing area perpendicularly at all times. Consequently, the interface of writing area and non-writing area unstable on the electrophotographic process is prevented from being positioned on the boundary of scanning as much as possible.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gradation expression in an image forming apparatus such as a laser printer, a copying machine or the like using an electrophotographic system by multi-beam scanning. The present invention relates to an intended image forming apparatus and image forming method.

[0002]

2. Description of the Related Art Generally, an image forming apparatus such as an electrophotographic laser printer or a digital copying machine has a built-in computer. For example, an image that communicates with an external device such as a host computer by the operation of a built-in computer, receives commands and print data from the host computer side, performs drawing development on a bitmap memory, and prints out. A forming apparatus is provided. Alternatively, there is also provided an image forming apparatus that performs image processing of image information read from an image reading device such as a scanner by an operation of a built-in computer and outputs the processed image information.

[0003] Further, in order to obtain higher print quality, image forming apparatuses are required to have improved print resolution and color, and are being mounted and implemented. For example, the size of the image to be handled is A4 size monochrome 2
4 as a 600 dpi (dot / inch) image of the value
M bytes are required, which is twice as large in A3 size, and four times as large in color, and a computer with high performance in processing this is required.

[0004] Similarly, the print mechanism is required to have the same speed and resolution. In order to realize these with a scanning type electrophotographic apparatus, a method of increasing a drawing clock of a printing mechanism is fundamental.

[0005] For example, in the case of a laser beam printer, in order to demand a higher quality print output, a clock of a pixel signal for driving a laser beam is further improved, and optical scanning at a higher density is performed. There is a need. In addition, for a demand for high-quality print output, the clock of a pixel signal for driving a laser beam is further improved,
It is necessary to perform higher density optical scanning. Further, the laser beam printer is basically binary, and the unit area of the gradation expression needs to be reduced as much as possible in the electrophotographic printer unit which performs the gradation expression by increasing the area of the paper covering with the toner. In order to achieve higher quality print quality in a small area, it is necessary to make the minimum drawing pixel smaller, and one pixel may be further subdivided during a printing operation for gradation expression. Further, in order to improve the number of prints per unit time, it is required to increase the paper transport speed. All of these result in demands for higher frequency image clocks.

In a laser beam printer, if the printing resolution is doubled, a clock four times in the horizontal and vertical directions is required, and if the paper transport speed is increased, the clock frequency is proportionally increased. Due to these various demands for high speed, the frequency of the image clock in the printing mechanism stage required has reached an area where the drive transmission by the conventional logic design is difficult.

As a countermeasure, there is a method of using a high-speed circuit other than the conventional logic design. However, especially in a medium-to-high-speed machine, the stage of transmitting the image generated by the image forming unit to the light emitting element is also a problem.

As another solution, there is a method realized by a conventional design by parallelizing circuits. In this case, in order to divert the conventional design, prepare multiple laser beams,
It is designed to keep the pixel drive frequency of the image low. Features of such a printing mechanism include:
The point is that scanning light of a plurality of laser beams is scanned on the same mirror surface. When scanning is performed with a single beam, the data write timing is constant. In addition, the timings of writing the individual column data do not overlap. However, when a plurality of beams are scanned on the same mirror surface, it is necessary to transmit a plurality of column data almost at the same time, and each column data is overlapped. After the data for a plurality of columns has been written out, blanks in the column data transfer request are vacant until the next scan of the mirror surface. The column images required almost simultaneously are drawn by scanning the same mirror. The feature is that the interval between the drawing pixels between the column data is more constant than the laser beam scanning interval of the same light emitting source scanned by different mirror surfaces.

This is because the mechanism is hardly affected by instability such as mechanical vibration in the operation of the mechanism because scanning is performed at the same mirror surface at the same or almost the same timing.
To be precise, the operation of the mechanical system is similarly unstable, so that the relative scanning interval is stable. Therefore, the stability of the pixel interval is high.

[0010]

In the conventional laser beam printer as described above, the laser beam scanning operates at the same position, but the scanning position on the paper is sequentially moved in combination with the paper conveyance. More precisely, the photosensitive drum rotates, and the scanning position of the light beam changes. The paper is conveyed in synchronization with the rotation of the photosensitive drum, the toner is transferred, and the toner is fixed on the paper, whereby a corresponding image is drawn on the paper. Optical scanning is generated by the rotational movement of the polygon mirror. Stability control of the rotational motion is performed with relatively high accuracy.

However, the mechanical drive mechanism of the transport system uses paper, which does not have stable conditions, so that the movement distance varies due to various factors, the scanning interval varies, and pitch unevenness and the like occur. Cause. In gradation expression in a printing mechanism using an electrophotographic method, pitch unevenness of the printing mechanism due to positional displacement such as mechanical vibration is a great enemy. This is because the disturbance of the scanning interval is reflected in the print output as the disturbance of the density, and causes a reduction in image quality (print quality). In particular,
Human vision is sensitive to changes in color tone, and when a color image is printed out, it appears as a change in color in a single density portion, resulting in a very unsightly print output.

In order to eliminate such a phenomenon, not only is the mechanical instability of the printing mechanism reduced, but also this type of mechanical instability occurs in the pattern of a printed image that is simultaneously drawn and generated. It is important to select a pattern that is not easily affected by the print quality in order to obtain a print output with high-quality gradation expression.

An object of the present invention is to provide an image forming apparatus and an image forming method capable of stably outputting a gradation image expression.

[0014]

According to a first aspect of the present invention, there is provided an electrophotographic printer for generating an image by sequentially scanning a plurality of light sources.
In an image forming apparatus for generating a halftone image by periodic halftone processing, the halftone processing cycle is set to an integral multiple of the number of light sources, and pixel coordinates having a remarkable tone threshold of tone variation due to disturbance such as mechanical vibration. Are arranged in optical scanning by the same mirror surface, and the net processing is performed by using a pattern in which pixel coordinates having a gradation threshold value stable against gradation fluctuation due to disturbance are arranged in optical scanning coordinates by a different mirror surface. It is characterized by having processing means.

Here, the halftone processing means matches the growing point of the halftone dot with an optical system which is continuously scanned, and forms an image pattern in consideration of a position where scanning is likely to be disturbed. Can be.

Further, a stable output of a gradation image expression is measured by placing a portion of the halftone dot growth pattern particularly sensitive to pitch unevenness at a scanning position between a plurality of beams with a stable scanning interval. Can be characterized.

Further, the position and the growth direction of the halftone dot may be individually set in accordance with the characteristics of the electrophotographic process of each image forming apparatus.

Further, since the growth point and the convergence point exist on the scanning boundary of the plurality of light source groups, the interface between the drawing area and the non-drawing area always intersects perpendicularly with the scanning boundary.
The feature is that the interface between the unstable drawing portion and the non-drawing portion in the electrophotographic process is minimized from being located at the scanning boundary.

The growth point may start growing from a boundary position of scanning of the plurality of light source groups.

Further, the growth point may be located at the center of the scanning of the plurality of light source groups.

In order to achieve the above object, the invention according to claim 8 has an electrophotographic printer unit for generating an image by sequential scanning by a plurality of light sources, and generates a halftone image by periodic halftone processing. In the image forming method of the image forming apparatus, the period of the halftone processing is set to an integral multiple of the number of light sources, and pixel coordinates having a remarkable gradation threshold of gradation fluctuation due to disturbance such as mechanical vibration are converted to light scanning by the same mirror surface. Arranging the pixel coordinates having a gradation threshold value that is stable with respect to gradation fluctuation due to disturbance, using a pattern arranging the light scanning coordinates by a different mirror surface, and performing the halftone processing step. I do.

(Function) In the present invention, in an image output apparatus having an electrophotographic printing mechanism for performing multi-beam scanning, of a halftone dot growth pattern, a portion which is particularly sensitive to pitch unevenness is determined by controlling the scanning interval. By providing the scanning position between the plurality of beams, a stable output of the gradation image expression is measured. For this purpose, in the present invention, the period of the halftone dot is set to an integral multiple of the number of multi-beams. For example, if the number of beams is three, the dither cycle is set to 3 × n, 6 × m, or the like. That is, the dither cycle in the sub-scanning direction in which the beams are connected is an integral multiple of the number of beam light sources. On the other hand, since the processing unit in the main scanning direction is irrelevant to the number of multi-beams, it is arbitrary, and the setting of the optimum halftone dot period corresponding to the electrophotographic process is adjusted by the dither period in the main scanning direction. .
Further, since the density region particularly sensitive to pitch unevenness differs depending on the characteristics of the electrophotographic process, the position and the growth direction of the halftone dot growth point are individually set in accordance with the characteristics of the electrophotographic process.

[0023]

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

(First Embodiment) FIG. 1 shows an example of a hardware configuration of an image forming apparatus according to an embodiment of the present invention. In this apparatus, the scanning unit 100 has a scanning density of 120 dpi.
Scanning is performed by four laser diodes 101 to 104.

Here, reference numeral 200 denotes an image forming unit. 20
1 is a CPU. Reference numeral 202 denotes a ROM, and the CPU 20
1 and various data for image generation. Reference numeral 203 denotes a RAM, which is used as a work area for temporarily storing data before processing sent from an external device, receiving and storing a program from an external device, and storing data from a scanner. Is done. Reference numeral 204 denotes an interface with an external device. The laser printer receives an external command and data via the interface 204, generates an image, and prints out the image. Reference numeral 205 denotes a scanner, which may not be mounted depending on the configuration like a laser printer. 300 is a bus line.

Reference numeral 100 denotes an outline of an optical system of a printing mechanism in which a paper transport section is omitted. 101 to 104 are laser light sources. Usually, a timing shift occurs in the scanning of the semiconductor laser light source, and the scanning is performed at a slightly shifted timing for each light source. This is because the light sources are different elements, and the mounting positions are slightly different physically.
Alternatively, a plurality of light emitting elements may be prepared on the same semiconductor. However, it is not preferable to concentrate the device positions of the light emitting points on one point due to a problem of light amount controllability due to heat generation. When the laser light sources 101 to 104 cannot be scanned exactly the same, it is necessary to individually drive the light emitting elements in synchronization with the image clock. However, if the scanning timings are not exactly the same due to the sensor described later, it is more convenient to separate them a little at a certain interval to separate the synchronization.

Since a plurality of elements having the same optical axis aligned on the same element have not been developed yet, this embodiment shows an embodiment in which each light source is constituted by another element. The laser light source 101 is scanned first, and the print position is also at the front end. Hereinafter, the laser light sources 102, 103, and 104 are assumed to follow. The laser beam emitted from the laser light sources 101 to 104 is scanned by the polygon mirror 105, and passes through the optical systems 106 to 108.
09 is converted into a constant speed scan. Optical systems 106 to 108
Is constituted by an f-θ lens or the like, and performs constant speed scan conversion and the like. 110 is a synchronous sensor. This synchronous sensor 110
Denotes a light detection sensor, which serves as a synchronization signal for transmitting image data in response to optical scanning.

The interval on the photosensitive member 109 in each scanning when drawing at 1200 dpi is about 20 μm. An optical system that individually prepares an optical sensor for detecting the scanning timing for each light source, adjusts the threshold by suppressing mutual optical interference, and independently synchronizes the light source and the synchronization sensor in a one-to-one correspondence, In practice, implementation is difficult. Because, in order to realize this, a large amount of adjustment and precision adjustment are required for each printing mechanism, and furthermore, a high-precision optical system is required, which makes the product expensive and not suitable for mass production. is there.

For this reason, in this embodiment, a method is adopted in which light from all light sources is guided to a single synchronous sensor 110 in consideration of productivity. As the synchronization signal of each light source, the output pulse of the synchronization sensor 110 is determined as a synchronization signal of a different light source depending on the order. For example, the k-th pulse is used as the synchronization signal of the k-th light source.

The corresponding pulse is individually synchronized with the light source, or the first pulse is used as a synchronization signal, and the second and subsequent timings are determined by the pulse interval. Usually, the pulse interval of the synchronous sensor 110 is different depending on the manufacturing individual. However, since the same individual has a constant value as long as the optical system is not affected by thermal expansion or the like, the pulse interval is stored once for each individual. In some cases, the value is used as a second or subsequent synchronization timing.

As an example of a circuit for separating the sensor output of the synchronous sensor 110, a timer circuit that issues a reset output when there is no input for a certain period of time, for example, about half of the mirror scanning period, a counter that counts the number of pulses, By providing an encoder that encodes the counter output, it is possible to separate and generate the synchronization signal of each light source.

In this embodiment, the synchronous sensor 110
Is counted by the counter 206 and each pulse is separated by the decoder 209. The counter 206 is 20
7 is initialized by a timer. The timer 207 issues a reset output if there is no output from the synchronous sensor 110 for a fixed time. When all the optical scanning of the same mirror surface is completed, a scanning gap occurs. Therefore, the timer 207 having adjusted the time constant issues a reset output, resets the counter 206, and prepares for the encoded output of the next mirror surface. This time constant is shorter than the optical scanning time and is much larger than the interval between pulses generated by different light sources. For example, when the optical scanning cycle is several milliseconds and the pulse interval between different light sources is tens of μsec, the time constant τ is tens to hundreds of μs.
ec is set.

Reference numeral 208 denotes a transmitter which outputs an image clock. The image clock output of the oscillator 208 needs to be in phase with the synchronization signal of each optical scan separated by the decoder 209. Reference numerals 211 to 214 denote phase synchronization circuits for this purpose.
By 4, clock synchronization is performed on the synchronization signal, and the scanning timing is adjusted. The line buffers 221 to 224 include light emitting elements (laser light sources) 101 to 104, respectively.
Phase synchronization circuits 211 to 21 synchronized with the scanning of
4 driven by the four clock outputs.

The light emitting elements 101 to 104 are driven by the output data of the line buffers 221 to 224, and drawing is performed on the photosensitive member 105. The CPU 201 converts and expands the multivalued image data into a binary bitmap by the dither shown in FIG. 2 and outputs the binary bitmap to the line buffers 221 to 224 according to the print timing. The generation part of the output image is equivalent to that of the conventional image processing apparatus.

In the present embodiment, a square dither matrix process is performed as an example of the image processing. FIG.
The following shows an example of a dither matrix growth pattern. FIG.
Indicate the order of growth, not the threshold. The size of the gradation processing unit dither is 8 × 8. Two growth points are set in the dither, and the dot density is 1200 線 8 ÷ 2 ≒ 212.

The numerical values of the dither matrix in FIG. 2 are the order of growth, and the two growth points start growing from positions corresponding to the boundaries of the quadruple light sources (light emitting elements 101 to 104). The scanning boundary of the quadruple light source group is indicated by a broken line. By having a growth point and a convergence point on the broken line,
The interface between the drawing area and the non-drawing area always intersects vertically with the scanning boundary. As described above, the vertical intersection minimizes the interface between the drawing portion and the non-drawing portion which are unstable in the electrophotographic process at the scanning boundary.

FIG. 3 shows an example of the output waveform of the synchronous sensor 110 of FIG. Since four light sources of the laser light emitting elements 101 to 104 are present, four pulses A to D continue during normal printing, a blank period exists for a while, and four pulses A 'to D' continue. Is detected by the synchronization sensor 110. The four consecutive pulses are the inputs of the four light sources scanned by the same mirror (the same mirror of the polygon mirror 105), and the four pulses starting again at intervals are the inputs scanned by the next mirror.

Since the interval between the four consecutive pulses depends on the accuracy of the mechanical mounting of the four light sources 101 to 104, the respective pulses are actually not at equal intervals but slightly different. This interval also differs depending on the individual device of each printer unit.

The pulse width is determined by the synchronous sensor 10.
9 is determined by the time when the laser beam scans the light receiving surface (photosensitive surface). If the pulses overlap, it becomes impossible to separate and synchronize the waveforms of the individual beams, so that the width of the light receiving surface for beam scanning of the synchronous sensor 110 is narrower than the deviation of the scanning start position of each beam. There is a need.

Although the rising edge of the output pulse of the synchronous sensor 110 is steep due to the characteristics of the optical sensor and peripheral circuits, the output value of the synchronous sensor 110 quickly returns after the scanning of the light receiving surface of the synchronous sensor 110 is completed. May not be. In this case, the edge of the falling waveform is not clear,
Synchronization must be taken on the rising edge.

In addition, it is necessary to mount the four light emitting elements 101 to 104 so that a sufficient shift occurs in the scan start timing of the light emitting elements in anticipation of the fall time in the optical scanning time of the surface of the synchronous sensor 110. .

When performing a printer operation, the CPU 20
1 generates a bitmap image on an image memory according to a command and data from an external device (not shown) and transfers the bitmap image to the line buffers 221 to 224. At the time when the gradation information for developing into the bitmap image is developed into the binary bitmap, the gradation processing is performed using the dither shown in FIG.

In this case, since the phase of the dither and the phase of the actual scanning need to always coincide with each other, in this embodiment, the synchronization between the paper conveyance and the main scanning is always performed on a polygon surface basis.
That is, the design is such that the scanning is always performed in units of four optical scans, and the scanning is not started from the element at the individual optimum position. Although the print resolution is improved for the purpose of enhancing the quality of the gradation expression, the mechanical accuracy of the paper transport position is not so high, so that the print start accuracy of 300 dpi scanning is sufficient.

(Second Embodiment) The first embodiment of the present invention described above.
In the embodiment, as shown in FIG. 2, a pattern that is robust against pitch unevenness caused by the transport mechanism in the entire gradation pattern is selected. However, depending on the process characteristics of the printing mechanism, there is a case where it is desired to reduce pitch unevenness in a specific density region. In such a case, it may be more effective to place the gradation pattern corresponding to the density region in a portion where the distance between drawing pixels is stable.

In the second embodiment of the present invention, an example in which a specific gradation area is arranged in an area with a stable scanning interval from such a viewpoint will be described. The print mechanism according to the second embodiment has a configuration of a quadruple multi-beam, similarly to the first embodiment shown in FIG.

The second embodiment shows a dither growth pattern in a region where the gradation is bright, that is, when the dither growth start portion is sensitive to pitch unevenness. FIG. 4 shows a dither configuration example according to the second embodiment.

As shown in FIG. 4, the growth point is located at the center of the scanning of the four beams of the light sources 1 to 4. At each growth point, up to the first 16 pixels fall within a frame that is scanned in quadruple and almost simultaneously on the same mirror. This is 50% of the number of hit points. In the electrophotographic small area printing process, 50% of the drawing and 50% of the density do not correspond to each other. Will be possible.

(Other Embodiments) Even if the present invention is applied to a system including a plurality of devices (for example, a host computer, an interface device, a reader, a printer, etc.), an apparatus (one device) For example, the present invention may be applied to a copying machine, a facsimile machine, and the like.

Another object of the present invention is to provide a recording medium (storage medium) recording software program codes for realizing the functions of the above-described embodiments to a system or an apparatus, and to provide a computer for the system or the apparatus. Needless to say, the present invention is also achieved when the CPU (or the CPU or the MPU) reads out and executes the program code stored in the recording medium.

In this case, the program code itself read from the recording medium implements the functions of the above-described embodiment, and the recording medium on which the program code is recorded constitutes the present invention.

As a recording medium for recording the program code and variable data such as a table, for example, a floppy disk (FD), hard disk, optical disk, magneto-optical disk, CD-ROM, CD-R, magnetic tape, A nonvolatile memory card (IC memory card), a ROM, or the like can be used.

When the computer executes the readout program code, not only the functions of the above-described embodiment are realized, but also the OS (Operating System) running on the computer based on the instruction of the program code. It goes without saying that an operating system) may perform some or all of the actual processing, and the processing may realize the functions of the above-described embodiments.

[0053]

As described above, according to the present invention,
In electrophotographic image formation by multi-beam scanning, the growth points of halftone dots are matched with the optical system that is continuously scanned,
Since the image pattern is formed in consideration of the position where scanning disturbance is likely to occur, the effect of pitch unevenness that affects the gradation of the printing mechanism is minimized, and the output of a higher quality gradation processed image is achieved. Can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an example of a hardware configuration according to first and second embodiments of the present invention.

FIG. 2 is a conceptual diagram showing an example of a dither matrix growth pattern according to the first embodiment of the present invention.

FIG. 3 is a waveform chart showing an example of an output waveform of the synchronous sensor 110 according to the first embodiment of the present invention.

FIG. 4 is a conceptual diagram illustrating an example of a dither matrix growth pattern according to a second embodiment of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 Optical system of printing mechanism omitting paper transport unit 101 to 104 Laser light source (laser light emitting element) 105 Polygon mirror 106 to 108 Optical system 109 Photoconductor 110 Synchronous sensor 200 Image forming unit 201 CPU 202 ROM 203 RAM 204 External devices Interface 205 Scanner 206 Counter 207 Timer 208 Transmitter 209 Decoder 211-214 Phase Synchronous Circuit 221-224 Line Buffer 300 Bus Line

Claims (14)

[Claims] 1. An image forming apparatus having an electrophotographic printer unit for generating an image by sequential scanning by a plurality of light sources and generating a halftone image by periodic halftone processing, wherein the halftone processing cycle is determined by the light source. Pixel coordinates having a tone threshold of remarkable gradation fluctuation due to disturbance such as mechanical vibration are placed in an integer multiple of the number, and arranged in optical scanning by the same mirror surface, and a gradation threshold stable against gradation fluctuation due to disturbance. An image forming apparatus comprising: a halftone processing unit that performs the halftone processing using a pattern in which pixel coordinates having the following are arranged at optical scanning coordinates by a different mirror surface. 2. The method according to claim 1, wherein the halftone processing unit matches a growth point of the halftone dot with an optical system that is continuously scanned, and forms an image pattern in consideration of a position where scanning disturbance is likely to occur. 2. The image forming apparatus according to 1. 3. A method of measuring a stable output of gradation image expression by placing a portion of a halftone dot growth pattern particularly sensitive to pitch unevenness at a scanning position between a plurality of beams with a stable scanning interval. The image forming apparatus according to claim 1, wherein: 4. The image forming apparatus according to claim 3, wherein a position and a growth direction of the halftone dot are individually set in accordance with characteristics of an electrophotographic process of each image forming apparatus. 5. An electrophotographic process in which a growth point and a convergence point are present on a scanning boundary of a plurality of light source groups, so that an interface between a drawing area and a non-drawing area always intersects vertically with a scanning boundary. 5. The image forming apparatus according to claim 1, wherein the interface between the unstable drawing portion and the non-drawing portion is positioned at a scanning boundary to a minimum. 6. The image forming apparatus according to claim 5, wherein the growth point starts growing from a boundary position of scanning of the plurality of light source groups. 7. The image forming apparatus according to claim 5, wherein the growth point is located at a center of scanning of the plurality of light source groups. 8. An image forming method for an image forming apparatus having an electrophotographic printer unit for generating an image by sequential scanning by a plurality of light sources and generating a halftone image by periodic halftone processing. The cycle is set to an integral multiple of the number of light sources, and pixel coordinates with a remarkable gradation threshold of gradation fluctuation due to disturbance such as mechanical vibration are arranged in optical scanning by the same mirror surface, and stable against gradation fluctuation due to disturbance. An image forming method, comprising: a halftone processing step of performing the halftone processing using a pattern in which pixel coordinates having an appropriate gradation threshold are arranged at optical scanning coordinates by different mirror surfaces. 9. In the halftone processing step, a growth point of a halftone dot is made coincident with an optical system which is continuously scanned, and an image pattern is formed in consideration of a position where scan irregularity is likely to occur. 9. The image forming method according to 8. 10. A stable output of gradation image expression is obtained by placing a portion of a halftone dot growth pattern that is particularly sensitive to pitch unevenness at a scanning position between a plurality of beams with a stable scanning interval. The image forming method according to claim 8, wherein: 11. The image forming method according to claim 10, wherein a position and a growth direction of the halftone dot are individually set in accordance with characteristics of an electrophotographic process of each image forming apparatus. 12. An electrophotographic process in which a growth point and a convergence point are present on a scanning boundary of a plurality of light source groups, so that an interface between a drawing area and a non-drawing area always intersects vertically with a scanning boundary. 12. The image forming method according to claim 8, wherein an interface between the unstable drawing portion and the non-drawing portion is positioned at a scanning boundary to a minimum. 13. The image forming method according to claim 12, wherein the growth point starts growing from a scanning boundary position of the plurality of light source groups. 14. The image forming method according to claim 12, wherein the growth point is located at a center of scanning of the plurality of light source groups.