JP5402654B2 - Image processing apparatus, image processing method, program, and recording medium - Google Patents

Description

  The present invention relates to an image processing apparatus used in an image forming apparatus using an electrophotographic process such as a laser printer, a digital copying machine, and a fax machine.

  In recent years, image forming apparatuses such as laser printers and digital copying machines using an electrophotographic process have been increased in speed and density, and an optical scanning apparatus using a laser beam has been adopted as an apparatus for performing an exposure process. Responding to the demand for high-speed and high-density image formation by increasing the speed of the polygon scanner on the optical scanning device side increases noise such as wind noise from the polygon mirror and drive noise from the drive motor. There are also problems such as increase in durability and deterioration in durability. On the other hand, the method of using a multi-beam optical scanning device is effective as a method for realizing high-speed and high-density image formation without causing the environmental problems described above.

  In a method of converting an optical scanning device into a multi-beam, a method of performing interlaced scanning using an LD array in which a plurality of light emitting points are arranged at equal intervals, and further making scanning line intervals non-uniform is proposed (for example, (See Patent Document 1). When such interlaced scanning is used, even if the lateral magnification in the sub-scanning direction between the light source and the surface to be scanned varies depending on the deflection angle, it is possible to reduce variations in scanning line intervals and density unevenness depending on the main scanning position, A high-resolution image can be formed.

  By the way, the above-described image forming apparatus has been used in a field that has been conventionally called light printing, but double-sided printing technology is important because of recent demands for resource saving. In electrophotographic double-sided printing, the first side of the paper is recorded, heat-fixed, and then the second side is recorded. For example, 0.2% for the first side and the second side. A magnification difference of ~ 0.4% occurs. Thus, pixel thinning or pixel insertion techniques have been proposed for magnification adjustment, bending correction, and the like (see, for example, Patent Documents 2 and 3).

  If the resolution of the exposure system is, for example, 4800 dpi, the sub-scanning pitch of the beam formed by the interlaced scanning is about 5.2 μm. If the scanning width of the laser beam is A3 size, a pitch of 5.2 μm must be maintained over a scanning width of about 420 mm. This is difficult to adjust, and the man-hour (cost) for adjustment increases.

  Further, since the pitch changes due to a temperature change in the apparatus, it is difficult to maintain 5.2 μm. Therefore, as the apparatus is operated and the temperature changes, there is a problem that density unevenness occurs in the image and the image quality is not stable.

  In addition, regarding the occurrence of density unevenness due to uneven scanning pitch in a reciprocating scanning environment using a galvanometer mirror, a method in which cells having an odd number of 3 or more pixels in the sub-scanning direction are arranged adjacent to each other in the main scanning direction is also proposed. (For example, see Patent Document 4). Since the density of the scanning lines by the galvanometer mirror is regular, the density difference is eliminated even if the cells are odd in the sub-scanning direction and the formed halftone dots are even in the sub-scanning direction.

  However, in this method, the interlaced scanning optical system using polygons does not necessarily eliminate the density difference, and the cell size selection is restricted by making the number of cells odd, thereby obtaining the desired number of screen lines. There is a problem that it cannot be done. Furthermore, when performing pixel thinning or pixel insertion as described above, this method has a problem that density unevenness cannot be eliminated.

In addition to this, a method using anisotropic dither has been proposed in order to prevent light amount unevenness and pitch unevenness (see, for example, Patent Document 5).
It is disclosed.

  However, anisotropic dither generally gives a rough impression and tends to select the performance of the output device, and is not necessarily applicable to an image forming apparatus for an electrophotographic process.

The present invention has been made in view of the above problems,
The object of the present invention is optimal for a polygon optical system based on multi-beam interlaced scanning, in which there is no roughness and the image quality does not deteriorate due to temperature changes inside the apparatus, and pixels are inserted and deleted for magnification adjustment. An object of the present invention is to provide an image processing apparatus optimal for an image forming apparatus.

The present invention scans a plurality of beams in the main scanning direction on the photosensitive member by a photosensitive member driven in the sub-scanning direction and a polygon rotating in a fixed direction, and the plurality of beams are scanned by a scanning scanning optical system. In an image forming apparatus that exposes according to an image signal and forms an image by an electrophotographic process , a density conversion unit that converts an input image into an even number of times and a density conversion unit that aligns the density-converted image in the sub-scanning direction Among the pixels, the number of on-pixels having a high density and the number of off-pixels having a low density are counted, and a value obtained by correcting the number of on-pixels by a first coefficient, and the number of off-pixels. Is compared with a value corrected by a second coefficient larger than the first coefficient to determine a density level of pixels arranged in the sub-scanning direction of the image, and based on the density level A 1 pixel expansion means for one pixel expansion in the sub-scanning direction by selecting the buried leaving portions of the halftone dots or halftone dots, and most important feature and this with a.

1. Since the halftone dot or the unfilled portion of the halftone dot is composed of an odd number of continuous pixels in the sub-scanning direction, density unevenness is suppressed even when there is pitch unevenness in interlaced scanning.
2. Since the dither mask cell size is not limited to an odd number, there is no restriction on the degree of freedom of the screen angle and the number of screen lines. In addition, since the dither mask may be isotropic, a stable and high-quality printed matter can be output regardless of the electrophotographic image forming apparatus.
3. Since the original input image is subjected to density conversion to an even multiple, and the result of density conversion is expanded by one pixel in the sub-scanning direction, this can be realized with a simple circuit configuration.
4). Since the density level around the target pixel is determined, and based on the determination result, the halftone dot or the unfilled portion of the halftone dot is selected to expand one pixel in the sub-scanning direction. Density unevenness can be appropriately suppressed in the low density part and the extremely high density part.
5. The determination of the density level around the target pixel can be detected with a simple configuration because it determines the density level from the arrangement of pixels in the sub-scanning direction.

1 shows a configuration of an image forming apparatus of the present invention. The structure of a surface emitting semiconductor laser is shown. 1 shows a configuration in which an optical device exposes a photosensitive drum. It is a figure explaining the scanning position of interlaced scanning. The structure of a control unit is shown. The detailed structure of GAVD is shown. 1 illustrates a configuration example of an image processing unit according to a first embodiment. It is a figure explaining operation | movement of a sub-scanning scaling part. It is a figure explaining operation | movement of a sub-scanning scaling part. It is a figure explaining generation | occurrence | production of density unevenness. 3 shows a configuration example of an image processing unit according to a second embodiment. The structure of a level judgment part is shown. It is a figure at the time of expanding the pixel of the halftone dot or the unfilled part of a halftone dot by 1 pixel below. An example in which one pixel is converted to four times the density is shown.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Description of image forming apparatus)
FIG. 1 shows a configuration of an image forming apparatus of the present invention. The image forming apparatus 100 includes an optical device 102 including optical elements such as a polygon mirror 102a, an image forming unit 112 including a photosensitive drum, a charging device, and a developing device, and a transfer unit 122 including an intermediate transfer belt. The The optical device 102 includes a VCSEL 200 (see FIGS. 2 and 3) (not shown) as a semiconductor laser. In the example shown in FIG. 1, the light beam emitted from the VCSEL 200 is once condensed by a first cylindrical lens (not shown) and deflected by the polygon mirror 102a to the reflection mirror 102b.

  The VCSEL (Vertical Cavity Surface Emitting Laser) 200 is a surface emitting semiconductor laser in which a plurality of light sources (semiconductor lasers) are arranged in a lattice pattern on the same chip. There are various techniques as an image forming apparatus using the VCSEL 200, and the VCSEL 200 is incorporated in the optical device 102 of the image forming apparatus 100 of the present invention with the same configuration as those known techniques.

  FIG. 2 shows a configuration of the VCSEL 200 incorporated in the optical device 102. As shown in FIG. 2, the VCSEL 200 forms a semiconductor laser array in which a plurality of light sources ch1 to ch40 (a plurality of light emitting points) are arranged in an oblique lattice shape.

  The image forming apparatus 100 constitutes a post-object type optical device 102 that does not use an fθ lens. In the illustrated embodiment, light beams L composed of ch1 to ch40 are generated in a number corresponding to each color of cyan (C), magenta (M), yellow (Y), and black (K), and the polygon mirror 102a. The beam L deflected by the laser beam is reflected by the reflection mirror 102b and condensed again by the second cylindrical lens 102c to expose the photosensitive drums 104a, 106a, 108a, and 110a.

  Since the irradiation of the light beam L is performed using a plurality of optical elements as described above, the timings in the main scanning direction and the sub-scanning direction are synchronized. Hereinafter, the main scanning direction is defined as the scanning direction of the light beam, and the sub-scanning direction is defined as a direction orthogonal to the main scanning direction.

  Each of the photosensitive drums 104a, 106a, 108a, and 110a includes a photoconductive layer including at least a charge generation layer and a charge transport layer on a conductive drum such as aluminum. The photoconductive layers are disposed corresponding to the photosensitive drums 104a, 106a, 108a, and 110a, respectively, and surface charges are imparted by the chargers 104b, 106b, 108b, and 110b including a corotron, a scorotron, or a charging roller. Is done.

  The electrostatic charges imparted on the photosensitive drums 104a, 106a, 108a, 110a by the respective chargers 104b, 106b, 108b, 110b are imagewise exposed by the light beam L to form an electrostatic latent image. The electrostatic latent images formed on the photoconductive drums 104a, 106a, 108a, and 110a are developed by the developing devices 104c, 106c, 108c, and 110c including a developing sleeve, a developer supplying roller, a regulating blade, and the like. Is formed.

  The developer carried on the photosensitive drums 104a, 106a, 108a, and 110a is transferred onto the intermediate transfer belt 114 that moves in the direction of arrow A by the conveying rollers 114a, 114b, and 114c. The intermediate transfer belt 114 is conveyed to the secondary transfer unit while carrying C, M, Y, and K developers. The secondary transfer unit includes a secondary transfer belt 118 and conveying rollers 118a and 118b. The secondary transfer belt 118 is conveyed in the direction of arrow B by the conveyance rollers 118a and 118b. An image receiving material 124 such as high-quality paper or a plastic sheet is supplied to the secondary transfer portion from an image receiving material storage portion 128 such as a paper feed cassette by a conveying roller 126.

  The secondary transfer unit applies a secondary transfer bias to transfer the multicolor developer image carried on the intermediate transfer belt 114 onto the image receiving material 124 held by suction on the secondary transfer belt 118. The image receiving material 124 is supplied to the fixing device 120 along with the conveyance of the secondary transfer belt 118. The fixing device 120 includes a fixing member 130 such as a fixing roller including silicone rubber, fluorine rubber, and the like, pressurizes and heats the image receiving material 124 and the multicolor developer image, and outputs the printed material 132 to the outside of the image forming apparatus 100. Output. After the multicolor developer image is transferred, the transfer belt 114 is supplied to the next image forming process after the transfer residual developer is removed by the cleaning unit 116 including a cleaning blade.

(Description of polygon optical system)
FIG. 3 shows a configuration in which the optical device 102 including the VCSEL 200 exposes the photosensitive drum 104a. The light beam L emitted from the VCSEL 200 is collected by the first cylindrical lens 202 used for shaping the light beam bundle, passes through the reflection mirror 204 and the imaging lens 206, and is then deflected by the polygon mirror 102a. . The polygon mirror 102a is rotationally driven by a spindle motor that rotates several thousand to several tens of thousands. The light beam L reflected by the polygon mirror 102a is reflected by the reflection mirror 102b and then reshaped by the second cylindrical lens 102c to expose the surface of the photosensitive drum 104a.

  In order to synchronize the scanning start timing of the light beam L in the sub-scanning direction, a reflection mirror 208 is disposed. The reflection mirror 208 reflects the light beam L to the synchronization detection device 210 including a photodiode or the like before starting scanning in the sub-scanning direction. When detecting the light beam, the synchronization detection device 210 generates a synchronization signal to start sub-scanning, and synchronizes processing such as generation of a drive control signal to the VCSEL 200.

  The VCSEL 200 is driven by a pulse signal sent from a GAVD 310, which will be described later. As will be described later, the light beam L is exposed at a position corresponding to a predetermined image bit of the image data, and an electrostatic latent image is formed on the photosensitive drum 104a. Form.

(Explanation of interlace scanning method)
FIG. 4 is a diagram for explaining the scanning position at which interlaced scanning is performed on the photosensitive member by the light beam L composed of 40 beams ch1 to ch40.

  The exposure resolution on the photoconductor is 4800 dpi, and therefore the interval (pixels) between the pixels in FIG. 4 is about 5.2 μm. A scan S1 at a certain time point and a scan S2 by the next polygon mirror surface will be described. First, the polygon mirror surface scans 40 beams at a time. The beam is 40 channels, and the 20th channel (ch20) and 21st channel (ch21) in the center are close to each other. Between the other channels, a positional relationship is formed such that a gap of one scanning line is formed and scanning is performed in the main scanning direction.

  The position of the intermediate point a between the channel 20 and the channel 21 is the center of the lens group in the sub-scanning direction and is symmetric in the sub-scanning direction. When horizontal scanning is performed in such a positional relationship on a certain polygonal surface, the line indicated by S2 in the figure is scanned on the next surface, and the 21 channel (ch21) and 22 channel (ch22) of the S1 line are scanned. The gap is now scanned by one channel (ch1). The same applies to Y, C, and M. In this way, a writing unit with 4800 dpi resolution is configured.

  Such an interlaced scanning optical system consisting of 40 beams as a whole, and can be interpolated by a multiple of 8 because the central 20-21ch is close. The ability to process at a multiple of 8 makes it easier to handle even when configuring a circuit for image processing. In addition, when a VCSEL (surface emitting laser diode) is used as the light source, there is an advantage that the degree of freedom of light source arrangement is high. In addition, the sub-scanning beam interval between 1ch and 40ch is narrower than normal interlaced scanning, and the outermost beam is not so far away from the optical axis. Therefore, there are variations in sub-scanning beam pitch and beam spot diameter. Can be reduced. Further, there is an advantage that the effective range of the scanning lens can be reduced and the optical element can be miniaturized.

(Description of image processing apparatus)
FIG. 5 shows the configuration of the control unit 300 of the image forming apparatus 100. The control unit 300 is configured as a scanner unit 302, a printer unit 308, and a main control unit 330. The scanner unit 302 functions as a means for reading an image, and a VPU 304 that performs A / D conversion on a signal read by the scanner to perform black offset correction, shading correction, and pixel position correction, and an acquired image as an RGB table. The IPU 306 performs image processing for digital conversion from the color system to image data in the CMYK color system. The read image acquired by the scanner unit 302 is sent to the printer unit 308 as digital data.

  The printer unit 308 includes a GAVD (Gate Array Video Device: video processing gate array) 310 that functions as a control unit that performs drive control of the VCSEL 200, and a current for driving the semiconductor laser element using a drive control signal generated by the GAVD 310. The LD driver 312 is supplied to the semiconductor laser element, and the VCSEL 200 is mounted with a two-dimensionally arranged semiconductor laser element. The GAVD 310 of the present embodiment divides the image data sent from the scanner unit 302 so as to correspond to the spatial size of the semiconductor laser element emitted by the VCSEL 200, and executes the high resolution processing.

  The scanner unit 302 and the printer unit 308 are connected to the main control unit 330 via the system bus 316, and image reading and image formation are controlled by commands from the main control unit 330. The main control unit 330 includes a central processing unit (hereinafter referred to as CPU) 320 and a RAM 322. The CPU 320 receives a command from the user via the interface 328, calls a program module that executes processing corresponding to the command, and executes processing such as copying, facsimile, scanner, and image storage. Further, the main control unit 330 includes a ROM 324 and stores initial setting data, control data, programs, and the like of the CPU 320. The image storage 326 is configured as a fixed or detachable memory device such as a hard disk device or a memory card, stores image data acquired by the image forming apparatus 100, and can be used for various processes by the user.

  When the printer unit 308 is driven for the image data acquired by the scanner unit 302 to output an image as an electrostatic latent image on the photosensitive drum 104a or the like, the CPU 320 performs main scanning and sub-scanning of an image receiving material such as fine paper or plastic film. Control in the scanning direction is executed. The CPU 320 outputs a start signal to the GAVD 310 when starting scanning in the sub-scanning direction. When the GAVD 310 receives the start signal, the IPU 306 starts the scanning process. Thereafter, the GAVD 310 receives the image data stored in the buffer memory or the like, then processes the received image data, and outputs the processed image data to the LD driver 312. When the LD driver 312 receives image data from the GAVD 310, the LD driver 312 generates a drive control signal for the VCSEL 200. Thereafter, the LD driver 312 sends the drive control signal to the VCSEL 200, thereby turning on the VCSEL 200. The LD driver 312 drives the semiconductor laser element using PWM control or the like. The VCSEL 200 of this embodiment includes 40 channels of semiconductor laser elements, but the number of channels of the VCSEL 200 is not limited to this.

(Function block diagram of GAVD)
FIG. 6 shows a detailed configuration of the GAVD 310. The GAVD 310 includes a memory 340 such as a FIFO buffer that receives the synchronization signal and stores and stores the image data output from the IPU 306, and performs image processing on the image data transmitted from the IPU 306 in a first-in first-out manner. Part 342. The image processing unit 342 reads the image data from the memory 340, performs resolution conversion of the image data, assignment of the semiconductor laser element channel, and addition / deletion of image bits (that is, correction pixels for scaling the image data). Processing and correction processing of the image data in the sub-scanning direction are executed.

  In the image data, a position where the photosensitive drum 104a is exposed is defined by a main scanning line address value defined in the main scanning direction and a sub scanning line address value defined in the sub scanning direction. Hereinafter, in this embodiment, the address coordinate is an address value that gives a specific image bit when image data is designated by a main scanning line address value (R address value) and a sub scanning line address value (F address value) Define as a set of These address values are determined by the address generation unit 354 (FIG. 7). Further, these address coordinates are determined for each pixel (that is, a pixel column) arranged in a line in the main scanning direction and the sub-scanning direction. Then, the sub-scanning scaling unit 352 (FIG. 7) performs a scaling process such as inserting a pixel bit into the pixel located at the coordinate address designated by the R address value and the F address value. The scaled image data is corrected in the sub-scanning direction by the sub-scan correction unit 360 (FIG. 7).

  The output data control unit 344 converts the output data serving as a write signal corresponding to the image data generated by the image processing unit 342 from the F address value and the sub-scanning speed into a time-series drive pulse, and further, the synchronization detection device 210. Is generated by adding a synchronization control signal for providing a synchronization signal. The generated drive control signal is transmitted to the LD driver 312 to drive the VCSEL. The output data control unit 344 receives a synchronization signal from the synchronization detection device 210 and synchronizes transmission of the drive control signal to the LD driver 312. The processing of the memory 340, the image processing unit 342, and the output data control unit 344 is synchronized with the operation clock by the PLL 346.

  FIG. 7 shows a configuration (Example 1) of the image processing unit 342 of FIG. The resolution conversion unit 350 performs density processing of four times in length and width so that a unit pixel of 1200 dpi image data acquired from the memory 340 is assigned to a laser element channel that performs irradiation of the pixel corresponding to the exposure resolution by the VCSEL 200. To implement. As a result, as shown in FIG. 14, one pixel (a) having a data resolution of 1200 dpi is converted to 4 × 4 pixels (b) having a writing resolution of 4800 dpi, which is a multiple of 2 in the sub-scanning direction.

  The sub-scanning scaling unit 352 includes an image path selector 358 and a shift holding memory 356. The sub-scanning scaling unit 352 receives the F address and R address used to form an image from the address generation unit 354, and adds (inserts) or deletes image bits in the address value being processed. It is determined whether or not an address value is included. The sub-scanning scaling unit 352 generates, for example, a scaling command signal such as an addition flag or a deletion flag for the address at which the image bit is added / deleted, and passes it to the image path selector 358 and the shift holding memory 356. The shift holding memory 356 stores a shift amount for shifting an image bit, and counts and holds a scaling command signal. The image path selector 358 sets the data of the image bit to white data and shifts the subsequent image data by 1 bit at a time when the image is enlarged and an additional scaling command signal is set. When the scaling command signal is not set, the input data from the resolution conversion unit 350 is selected and output based on the shift amount from the shift holding memory 356.

  In this embodiment, when the 40ch VCSEL 200 is used as a semiconductor laser, signals indicating the position to be added / deleted and signals indicating the shift amount are allocated for 40ch and used for driving the VCSEL 200. The calculation of addition / deletion of image bits can be configured as a dedicated module of the image processing unit 342 and can be configured as a part of other modules. The reason for counting the magnification command signal is to specify the position where the image bit is added at the beginning of the second scan after the image bit is added at the first scan, for example, when the image bit is shifted.

The operation of the sub-scan scaling unit will be described with reference to FIGS.
As shown in FIG. 8, the sub-scanning zooming unit performs a process of shifting the image as a whole in the sub-scanning direction by inserting pixels (white pixels) at positions of black marks regularly arranged.

  FIG. 9A shows an original image at 4800 dpi. Pixels a0, a1, a2,. . . The next row has pixels b0, b1, b2,. . . And expanded in memory. Further, the addresses in the main scanning direction are set to R0, R1, R2,. . . , And addresses in the sub-scanning direction are F0, F1, F2,. . . It will be expressed as

  On the other hand, a process of shifting (stretching) the image in the sub-scanning direction is performed. When shifting, a pixel is inserted (conversely, when reducing, the pixel is deleted).

  FIG. 9B shows the insertion position of the pixel (white pixel) in association with the image by * (magnification command signal). Since the insertion positions are regularly arranged, whether or not the insertion positions are satisfied is checked by applying main scanning and sub-scanning by the logic circuit without actually developing them in the memory.

A reading process for zooming will be described.
For F0, first read a0 of R0. Since this corresponds to the insertion position as shown in FIG. 9B, white is output as pixel data. Then, as shown in FIG. 9D, “1” is written in the shift holding register R0. Next, a1 of R1 is read. Since this does not correspond to the insertion position, a1 is output as it is. In this way, the first line is processed. The row F0 in FIG. 9C shows the processing result.

  Processing moves to the next F1 line. For R0, the first pixel originally reads b0, but for R0, since the shift holding register is “1”, the data of F0 that is one level higher is read. Therefore, the output is a0.

  In R1, since the shift holding register is “0”, the data is read as it is, and the output is b1. The line F1 in FIG. 9C shows the processing result.

  Processing is performed in the same manner, and F14 will be described. Since the shift holding register of R0 is “1”, the data one level higher is read and the output is i0. Since R1 corresponds to the insertion position, white pixels are output. Then, as shown in FIG. 9E, “1” is set to R1 of the shift holding register. Since R2 is “0” in the shift holding register, j2 is output as it is.

  By repeating the above operation, the output image is gradually shifted in the sub-scanning direction as compared with the original image (a) as shown in FIG. 9C. Thereby, a shift operation is achieved.

  At this time, since the beam used for the exposure does not change, the pixels of h0 to k0 in the sub-scanning direction are four beams of F12 to F in FIG. 9A, but F13 to F in FIG. 9C. When the formation of halftone dots is considered, the beam to be used is shifted within the paper.

  The reason why the above scaling process is executed will be described. As described in the prior art, when the first side (front side) is recorded and fixed by double-sided printing, the paper may shrink in the sub-scanning direction. When the second side (back side) is recorded, the front and back sides are printed. Deviation occurs. Therefore, in order to eliminate shrinkage in the sub-scanning direction at the time of fixing, the printing misalignment between the front and back sides is corrected by shifting the image in the sub-scanning direction when printing the second surface (back surface).

  Returning to FIG. 7, the sub-scan correction unit 360 in this embodiment is a circuit block for expanding the lower edge side of the image data after sub-scan scaling by one pixel. The image data scaled in the sub-scanning direction is sent to the 1-line delay unit 361, and the original data and the data delayed by 1 line are OR-combined in the combining unit 362. As a result, the lower edge portion of the image data is expanded by one pixel.

  FIG. 13A shows a halftone dot of 4 × 8 pixels. FIG. 13B is a diagram when the halftone dot of FIG. 13A is expanded downward (in the sub-scanning direction) by one pixel. As described above, the image data input to the sub-scanning correction unit 360 is configured to be even pixels (8 pixels in FIG. 13A) in the sub-scanning direction, and the sub-scanning correction unit 360 is further arranged in the sub-scanning direction. By expanding by one pixel, the number of continuous pixels in the sub-scanning direction is necessarily an odd pixel (9 pixels in FIG. 13B).

(Explanation of the effect of +1 pixel expansion)
The reason why the image density unevenness due to the interlaced pitch unevenness in the sub-scanning is eliminated by setting the odd number of pixels in the sub-scanning direction in this way will be described.

  In the case of interlaced scanning, for example, a scanning line is configured as shown in FIG. However, the scanning beam pitch shifts due to adjustment and temperature change.

  FIG. 10 shows dot formation when the scanning line is not shifted and when a beam pitch shift occurs due to the shift. A circle indicates the beam scanning position. A halftone dot where an ellipse is formed is shown.

  When the halftone image is formed by four even number of combined pixels in the sub-scanning direction when the beam pitch is not shifted in the sub-scanning direction, the combination in the sub-scanning direction as shown by the halftone dot 210 in FIG. It becomes. However, the beam pitch shifts due to temperature changes. Then, ch2, 3, 4, 5, 6 and ch23, 24, 25, 26, and 27 in FIG. 10 are scanning by different polygon planes, and the sub-scanning positions as shown in FIG. The dots formed in the halftone dots 210 in (a) are like the halftone dots 212 in (b). If the area (a) and the area (b) exist in the apparatus, the same exposure energy as the halftone dot 210 and the halftone dot 212 is generated, but the sub-scanning size is different, that is, the density unevenness. Will occur.

  Further, consider a case where the image position is shifted due to pixel insertion by sub-scanning scaling. For example, if the halftone dot 212 in (b) is shifted by one pixel in the sub-scanning direction (the lower side in the figure), the combined state of four pixels becomes the halftone dot 214 in (c). Comparing (b) and (c), it can be seen that both are the same in exposure with four beams, but the lengths in the sub-scanning direction are different. For this reason, the toner image is thinner in (c) than in (b). That is, further density unevenness occurs due to sub-scanning scaling and beam pitch deviation.

  Here, in the present invention, since one pixel is expanded in the sub-scanning direction, the combination of four pixels in the sub-scanning direction is converted into a combination of five pixels, and exposure is performed. In this case, in the case of the exposure shown in FIG. 10A, the halftone dot 216 shown in FIG. Similarly, FIG. 10B is exposed to a halftone dot 218 in FIG. 10E, and FIG. 10C is exposed to a halftone dot 220 in FIG.

  Comparing the halftone dots 216, 218, and 220 in FIG. 10D to FIG. 10F, the exposure energy is the same in the exposure with five beams. It can also be seen that the length in the sub-scanning direction is almost the same. For this reason, since the toner images have substantially the same density, density unevenness can be prevented even if there is a beam pitch shift.

  In the configuration of FIG. 7, a mode is also provided in which expansion in the sub-scanning direction is turned off by a synthesis control signal. That is, when the sub-scanning direction is expanded, the density is slightly increased. Some users do not want to increase the darkness when the internal adjustment of the device is sufficient and sub-scanning magnification is not used. In such a case, the expansion function is turned off by turning off the expansion in the sub-scanning direction. You can also

  FIG. 11 illustrates a configuration example of the image processing unit 342 according to the second embodiment. The 1200 dpi input image is converted by the resolution conversion unit 350 into a 4800 dpi image that is four times in length and width. The sub-scanning scaling unit 352 performs predetermined pixel insertion or pixel deletion on the converted image.

  The scaled image is input to the sub-scanning correction unit 360. The sub-scanning correction unit 360 temporarily stores the image in the image buffer 363. The value of this buffer is determined by the level determination unit 364, and for each pixel in the main scanning direction, a portion where the gradation level of the image is dark (predetermined threshold value) from the ON / OFF arrangement of the pixel of interest in the sub scanning direction. Or more) or a light portion (less than a predetermined threshold).

  When it is determined that the light portion is a light portion, the OR synthesis circuit 366 is caused to function so as to enlarge the black pixel (ON side) as in the first embodiment. Thereby, the lower edge of the ON pixel is enlarged.

  If it is determined that the area is dark, the AND composition circuit 367 is caused to function to enlarge the pixel on the OFF side (white side) of the pixel. Thereby, the lower edge of the OFF pixel is enlarged. The selector 368 selects the output of the OR combining circuit 366 when the level determining unit 364 determines that it is a light portion, and selects the output of the AND combining circuit 367 when determining that it is a dark portion.

  FIG. 12 shows a configuration example of the level determination unit 364. The level determination unit 364 determines the gradation level by examining the arrangement of pixels in the sub-scanning direction for each main scanning address. A buffer 381 for 40 pixels before and after the target pixel is provided in the sub-scanning direction. At this time, if the target pixel is white (OFF), the number of consecutive OFF pixels is counted by the target level arrangement number unit 382. Further, the non-attention level arrangement number section 383 counts the number of consecutive pixels (in this case, ON pixels) opposite to the next pixel of interest in the sub-scanning direction.

  Then, the count values of the arrangement number units 382 and 383 are corrected by the correction coefficient units 384 and 385. The coefficient correction unit differs depending on whether the arrangement number units 382 and 383 count white or black, and in this embodiment, the coefficient 1 is black (ON) for the number of white (OFF) arrangements. The coefficient 3/4 is applied to the number of lines. The reason why the number of black lines is 3/4 is that the threshold value for determining the light part is shifted to the dark side so that it is determined that the part is light even when the number of white and black lines is the same. In the case of FIG. 12, the level determining unit 364 determines that the portion is dark. Accordingly, the AND synthesis circuit 367 functions so that one white pixel is added below the target pixel.

  FIG. 13 shows an example in which halftone dots (a) or unfilled portions (c) of halftone dots are enlarged by one pixel in the sub-scanning direction. The light dot halftone dot (a) is an array of 8 pixels in the sub-scanning direction, and by the process of the present invention, as shown in (b), an array of 9 odd pixels is expanded in the sub-scanning direction. . In the dark halftone dot unfilled portion (c), which is an array of 4 pixels in the sub-scanning direction, an odd pixel of 5 pixels in the sub-scanning direction as shown in FIG. It is inflated.

  According to the present invention, a storage medium in which a program code of software for realizing the functions of the above-described embodiments is recorded is supplied to a system or apparatus, and a computer (CPU or MPU) of the system or apparatus is stored in the storage medium. This is also achieved by reading and executing the code. In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments. As a storage medium for supplying the program code, for example, a hard disk, an optical disk, a magneto-optical disk, a nonvolatile memory card, a ROM, or the like can be used. Further, by executing the program code read by the computer, not only the functions of the above-described embodiments are realized, but also an OS (operating system) operating on the computer based on an instruction of the program code. A case where part or all of the actual processing is performed and the functions of the above-described embodiments are realized by the processing is also included. Further, after the program code read from the storage medium is written into a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer, the function expansion is performed based on the instruction of the program code. This includes the case where the CPU or the like provided in the board or function expansion unit performs part or all of the actual processing, and the functions of the above-described embodiments are realized by the processing. Further, the program for realizing the functions and the like of the embodiments of the present invention may be provided from a server by communication via a network.

350 Resolution Conversion Unit 352 Sub-Scanning Scaling Unit 354 Address Generation Unit 356 Shift Holding Memory 358 Image Path Selector 360 Sub-Scanning Correction Unit 361 1 Line Delay 362 OR Combining Unit

JP 2006-215270 A JP 2009-83472 A Japanese Patent No. 4253843 JP 2007-62099 A Japanese Patent No. 4220654

Claims (5)

A plurality of beams are scanned in the main scanning direction on the photosensitive member by a photosensitive member driven in the sub-scanning direction and a polygon rotating in a fixed direction, and the plurality of beams are writing scanning optical systems by interlaced scanning. In an image forming apparatus that exposes according to a signal and forms an image by an electrophotographic process,
Density conversion means for converting the density of the input image into even multiples;
Of the pixels arranged in the sub-scanning direction of the image subjected to the density conversion, the number of on-pixels having a high density and the number of off-pixels having a low density are counted, and the number of on-pixels is set to the first number. Judgment for judging the density level of the pixels arranged in the sub-scanning direction of the image by comparing the value corrected by the coefficient with the value corrected by the second coefficient larger than the first coefficient for the number of pixels of the off-pixel Means,
A one-pixel expansion means for selecting a halftone dot or an unfilled portion of a halftone dot based on the density level and expanding one pixel in the sub-scanning direction;
The image processing apparatus according to claim and this with a. The image processing apparatus according to claim 1 , further comprising a scaling unit that inserts and deletes pixels in the image. A plurality of beams are scanned in the main scanning direction on the photosensitive member by a photosensitive member driven in the sub-scanning direction and a polygon rotating in a fixed direction, and the plurality of beams are writing scanning optical systems by interlaced scanning. In an image forming method of exposing according to a signal and forming an image by an electrophotographic process,
A density conversion step of converting the input image to an even number of times, and
Of the pixels arranged in the sub-scanning direction of the image subjected to the density conversion, the number of on-pixels having a high density and the number of off-pixels having a low density are counted, and the number of on-pixels is set to the first number. Judgment for judging the density level of the pixels arranged in the sub-scanning direction of the image by comparing the value corrected by the coefficient with the value corrected by the second coefficient larger than the first coefficient for the number of pixels of the off-pixel Process,
A one-pixel expansion step of selecting a halftone dot or an unfilled portion of a halftone dot based on the density level and expanding one pixel in the sub-scanning direction;
An image processing method including : A program for causing a computer to realize the image processing method according to claim 3 . A computer-readable recording medium on which a program for causing a computer to realize the image processing method according to claim 4 is recorded.

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