JP4569332B2 - Image forming apparatus and control method thereof - Google Patents

Description

  The present invention relates to an image forming apparatus such as a printer or a copying machine, and more particularly to an image forming apparatus that performs registration control.

  Today, apparatuses for forming color images are widely used in image forming apparatuses such as printers and copiers. As a general image forming apparatus of this type, for example, an image forming unit provided for each color of black (K), yellow (Y), magenta (M), and cyan (C) is a transfer target (an intermediate transfer member). There is a so-called tandem type image forming apparatus that is arranged facing a transfer belt, a recording material such as paper, or the like. In this tandem type image forming apparatus, images of different colors formed in the respective image forming units are sequentially transferred and multiplexed on a moving transfer object to form a color image.

  In this tandem type image forming apparatus, an image formed for each color is overlapped to form a color image. Therefore, an error in each mounting position of the image forming unit, a peripheral speed error in each image forming unit, and an exposure position with respect to the transfer target In some cases, color misregistration may occur in the formed image due to a difference in linear velocity or a change in the linear velocity of the transfer target. That is, the alignment of the image forming unit provided for each color, the mechanical error, and the like are the color shifts on the recording medium (such as paper) as they are. Therefore, in this type of image forming apparatus, it is indispensable to measure the color misregistration amount and perform color misregistration control (registration control) for suppressing the occurrence of color misregistration. As a general method of color misregistration control, a mark (pattern) of each color of yellow, magenta, cyan, and black is drawn on the transfer target, the position is read by a sensor, and the color misregistration is calculated from the read result and fed back. There is a method for controlling the image forming unit. In addition to the tandem type image forming apparatus, for example, in a cycle method in which a color image is formed by rotating a plurality of image carriers, or an image forming apparatus such as a so-called inkjet method, the same problem with respect to color misregistration or the like. There is.

  By the way, as one type of image shift to be registered, there is a shift caused by skew (hereinafter referred to as skew shift). That is, when the inclination of the scanning optical system or the photosensitive drum of a specific color is different from those of other colors, a deviation occurs between the image of the specific color and the image of the other color. Various conventional techniques for correcting this skew deviation include those that are corrected by a mechanism in the mechanical system and optical system, and those that are corrected by image processing that deforms the original image data in accordance with the deviation. There is. Since mechanical correction by a mechanism system or an optical system requires very high accuracy, correction by image processing can reduce cost and convenience for fine correction.

  As a conventional technique for correcting the skew of the image itself by correcting the image data, for example, two or more registration marks are formed at different positions in the main scanning direction on the transfer belt, and the reference color and other colors are changed. There is a technique in which a deviation amount is obtained, a correction approximation function is calculated from the deviation amount, and an image address is changed to correct a skew deviation (hereinafter, skew correction) (for example, refer to Patent Document 1). Further, there is disclosed a technique for correcting a skew deviation generated between printer heads in units of lines by changing an image writing address and outputting the image with a step (for example, Patent Document 2 and Patent Document 2). 3).

  In recent years, there has been proposed an image forming apparatus using a so-called multi-beam scanning optical system that collectively scans a plurality of laser beams from a plurality of light sources with a rotating polygon mirror. In addition, there is an image forming apparatus that performs so-called multiple exposure, in which the same image data is drawn on the surface of the photosensitive member a plurality of times by using this multi-beam scanning optical system (see, for example, Patent Document 4).

JP 2000-112206 A JP 2001-80124 A Japanese Patent Laid-Open No. 9-188000 JP 2004-109680 A

FIGS. 15A and 15B are diagrams illustrating an example of processing conventionally performed for skew correction in an image forming apparatus using a multi-beam scanning optical system (multi-beam ROS).
In FIGS. 15A and 15B, the main scanning direction (A to R) is shown horizontally, the sub-scanning direction (1 to 24) is shown vertically, and the case of drawing a line with a width of 4 dots is illustrated. Has been. Here, the main scanning direction is the scanning direction of the print source, and is the direction orthogonal to the moving direction of the image carrier (paper, intermediate transfer member, etc.). The sub-scanning direction is orthogonal to the main scanning direction and is the same as the moving direction of the image carrier.

  In the skew correction by image processing, as shown in FIG. 15B, the output image is changed by changing the position of the pixel to be drawn (address switching) stepwise in the main scanning direction of FIG. Data is shifted in the sub-scanning direction. The pixel position is changed by shifting image data used for multi-beam image formation. This shift in output image data makes it possible to correct skew distortion. For example, it is possible to perform good skew correction on a color drawn with an inclination with respect to the reference color.

  FIGS. 16A and 16B show the case where multiple exposure (double exposure in the illustrated example) is performed by an image forming apparatus using the same multi-beam scanning optical system as in FIGS. 15A and 15B. It is a figure which shows the mode of this skew correction. In multiple exposure, as disclosed in Patent Document 4 described above, scanning is performed a plurality of times per scanning line by scanning the multi-beams while shifting by several scanning lines (a number smaller than the total number of beams). . FIGS. 16A and 16B show an example in which scanning is performed with 16 beams shifted by 8 scanning lines. That is, in this image forming apparatus, exposure is performed twice for every eight scanning lines. FIG. 16A shows the exposure state in the n-th scanning and the exposure state in the n + 1-th scanning side by side, and FIG. 16B shows the two scannings in FIG. The state in which the exposures at are synthesized is shown.

In FIG. 16A, in the n-th scanning, four lasers corresponding to the sub-scanning directions 9 to 12 in the main scanning direction A to F are output. Further, the laser output position is shifted by one scanning line in the main scanning direction G to L, and four lasers corresponding to the sub scanning directions 10 to 13 are output. Further, the laser output position is further shifted by one scanning line in the main scanning direction M to R, and four lasers corresponding to the sub scanning directions 11 to 14 are output.
Similarly, in the (n + 1) th scanning, four lasers corresponding to the sub-scanning directions 1 to 4 in the main scanning direction A to F are output. The laser output position is shifted by one scanning line in the main scanning direction G to L, and four lasers corresponding to the sub scanning directions 2 to 5 are output. Further, the output position of the laser is further shifted by one scanning line in the main scanning direction M to R, and four lasers corresponding to the sub scanning directions 3 to 6 are output.

  In actual image formation, the n-th scan and the (n + 1) -th scan are superimposed while being shifted by eight scan lines, so the scan lines in the sub-scan direction in the two scans are as shown in FIG. The total number is 24. Then, the sub-scanning directions 9 to 16 overlap in the scanning width (for 24 scanning lines) in the two scans. The scanning lines in the sub-scanning directions 9 to 12 in the main scanning directions A to F, the scanning lines in the sub-scanning directions 10 to 13 in the main scanning directions G to L, and the main scanning directions M to R. Thus, each of the scanning lines in the sub-scanning directions 11 to 14 is exposed twice. For simplicity, only two scans have been described, but the scan lines in the sub-scan directions 9 to 16 in the (n + 1) th scan overlap the scan lines in the sub-scan directions 1 to 8 in the (n + 2) th scan. To do. In the same manner, each scanning is repeated by 8 scanning lines, and double exposure is sequentially performed.

  By the way, in a digital image forming apparatus, in general, pixels are often formed by binary dots. Therefore, area gradation is performed when a halftone is expressed. In area gradation, a screen having a fine pattern (dot pattern or line pattern) is drawn at a location where halftone is to be expressed in image data. By arbitrarily selecting a screen pattern, the density of a color visually recognized within a range where the screen is stretched is controlled, and a halftone is expressed.

Conventionally, when the above-described skew correction is performed in an image in which halftone expression is made by area gradation, depending on the shape of the screen applied to the image data and the angle of the screen, a streak is formed at the position where the skew correction is performed. An image quality defect (hereinafter referred to as a streak defect) may occur.
FIGS. 17A to 17C are diagrams schematically showing a streak defect generated by performing the skew correction.
FIGS. 17B and 17C show a state in which skew correction (shift) is performed on a monochrome original image with a screen stretched as shown in FIG. 17A. In FIG. 17B, a black streak defect appears, and in FIG. 17C, a white streak defect appears.

  The present invention has been made to solve the technical problems as described above, and an object thereof is, for example, an image forming apparatus using a multi-beam scanning optical system and an ink jet having a plurality of print sources. In an image forming apparatus adopting this method, it is intended to contribute to the improvement of image quality by suppressing streak defects that occur when correction is performed by dividing an image such as skew correction and shifting each region. .

  In order to achieve this object, the present invention is realized as an image forming apparatus that performs multiple exposure using a multi-beam scanning optical system. This apparatus has an input means for inputting image data, a plurality of printing sources, printing means for collectively scanning a beam from the printing sources to print an image carrier, and image data input by the input means. Control means for shifting the image stepwise in the sub-scanning direction which is the moving direction of the image carrier when the image is drawn by the printing means. The printing unit scans the beam from the printing source while shifting the scanning lines by several scanning lines, thereby scanning a plurality of times per scanning line, and the control unit performs scanning a plurality of times for one scanning line. Shift images at different positions. More specifically, the control means specifies a reference shift position so as to correct the image skew deviation, and sets the reference shift position as at least a part of a plurality of scans for one scanning line. On the other hand, the image is shifted at a position shifted by one or several pixels. More specifically, the control means shifts the image at a position shifted by one or a plurality of pixels in a predetermined direction with respect to the reference shift position in the (n + odd) scan, and the reference in the nth and n + even scans. The image is shifted at a shift position or a position shifted by one or a plurality of pixels in the opposite direction to the n + odd scan. More preferably, the control unit determines how many pixels the image is shifted from the reference shift position in accordance with the drawing content by the printing unit.

  The present invention has a plurality of printing sources, and scans the beam from the printing sources while shifting the scanning lines by several scanning lines so that scanning is performed a plurality of times per scanning line to form an image on the image carrier. It can also be grasped as a device control method. This method obtains a correction value for skew correction so as to shift an image at different positions in a step of performing screen processing on input image data and a plurality of scans for one scanning line. And a step of performing skew correction by shifting the image data subjected to the screen processing stepwise in the sub-scanning direction, which is the moving direction of the image carrier, based on the obtained correction value.

  The present invention is also realized as the following image forming apparatus. This apparatus has an input means for inputting image data, a plurality of print sources, a print means for printing the image carrier by collectively scanning a beam from the print source or the print source, and an input means. The image data is divided into a plurality of portions with respect to the main scanning direction orthogonal to the moving direction of the image carrier, and the divided portions are shifted stepwise in the sub-scanning direction which is the moving direction of the image carrier. And a control means for drawing. Then, the control means divides the image data so that the boundary line of the part has a shape other than the straight line in the sub-scanning direction. The scanning of the printing source can be considered including nozzle scanning by an ink jet method. In that case, the conveyed sheet (sheet) corresponds to the image carrier. In addition, “printing” is not limited to the formation of characters such as text. In the formation of various images other than characters, an image forming apparatus employing an electrophotographic method forms an image using an exposure function or an inkjet method. In the apparatus, the function of ejecting ink from a print head and the like are broadly meant. The same applies hereinafter. Here, the plurality of printing sources of the printing unit are a plurality of laser beam light sources, and the printing unit uses a multi-beam that collectively scans a plurality of laser beams from the plurality of laser beam light sources with a rotating polygon mirror. It can be characterized by being an exposure means.

  In this image forming apparatus, the control unit corrects the data of the empty pixels and the overlapping pixels generated when the image data portion is shifted based on the information of the pixels around the empty pixels or the overlapping pixels. More preferably, the control unit divides the image data in a boundary line shape that is not synchronized with the pattern drawn by the printing unit. Furthermore, the control means can also divide the image data at a boundary line having a shape along a part of the edge portion of the image drawn by the printing means.

  Furthermore, the present invention is grasped as a control method of an image forming apparatus that has a plurality of print sources and prints an image carrier by collectively scanning the print sources or beams from the print sources. In this method, screen processing is performed on input image data, the main scanning direction in which the image data is orthogonal to the moving direction of the image carrier, and the boundary line is the moving direction of the image carrier. The method includes a step of dividing the plurality of portions into a shape other than a straight line in the sub-scanning direction, and a step of performing skew correction by shifting the plurality of divided portions stepwise in the sub-scanning direction.

  According to the present invention configured as described above, it is possible to suppress streak defects that occur when correction is performed by dividing an image into regions and shifting each region, starting with skew correction on the original image. As a result, the image quality can be improved.

The best mode for carrying out the present invention (hereinafter referred to as an embodiment) will be described below in detail with reference to the accompanying drawings.
FIG. 1 is a diagram illustrating an image forming apparatus to which the exemplary embodiment is applied. This image forming apparatus is a so-called tandem type digital color machine employing an electrophotographic system. As shown in FIG. 1, the image forming apparatus includes an image forming unit 10 that forms an image and an exposure that forms an electrostatic latent image on a photosensitive drum 11 of the image forming unit 10 as a printing function (printing function). The apparatus 13 includes a transfer belt 21 as an intermediate transfer member that superposes and carries a toner image carried on the photosensitive drum 11. The image forming unit 10 is provided corresponding to each color of yellow (Y), magenta (M), cyan (C), and black (K). Hereinafter, when it is necessary to distinguish between them, they are expressed as image forming units 10Y, 10M, 10C, and 10K. When they are not necessary to be distinguished, they are simply expressed as image forming units 10. A primary transfer roll 23 for carrying an image on the transfer belt 21 is provided inside the transfer belt 21 at a position facing the photosensitive drum 11 of each image forming unit 10. Further, a secondary transfer roll 24 and a counter roll 25 provided inside the transfer belt 21 are arranged at a so-called secondary transfer position where the toner image carried on the transfer belt 21 is transferred to a sheet. Furthermore, a paper feed cassette 27 for storing paper as a recording medium and a fixing device 28 for fixing the transferred paper are provided. The image forming apparatus also includes a control unit 31 that controls image shift for skew correction, and a color misregistration sensor 32 that reads a color misregistration control pattern formed in a predetermined area of the transfer belt 21.

  The control unit 31 generates an image signal such as a digital image signal of an image obtained from image data input means such as an image reading apparatus (IIT: Image Input Terminal) or a pattern image for color misregistration control, and an exposure apparatus. 13, writing on the transfer belt 21 is performed. The control unit 31 acquires the detection result of the color misregistration control pattern from the color misregistration sensor 32, analyzes the color misregistration amount based on the acquired information, and performs necessary correction. These functions in the control unit 31 are realized by, for example, a program-controlled CPU (Central Processing Unit) or the like. The control unit 31 includes a nonvolatile ROM (Read Only Memory) and a readable / writable RAM (Random Access Memory) as memories. The ROM stores a software program for controlling an image forming operation and color misregistration detection and correction operation executed by the controller, image information of a color misregistration control pattern, and the like. The RAM stores various types of information acquired along with the operation of the image forming apparatus, such as various counter values, the number of job executions, and execution information (time information, etc.) of the previous color misregistration detection process.

  For each color exposure device 13, a digital image signal obtained from, for example, an image reading device (IIT) or an external personal computer device (PC) and converted by an image processing device (not shown) is supplied to the control unit 31. Is supplied via. The color misregistration sensor 32 forms an image of a color misregistration control pattern (ladder-like toner patch, chevron patch) formed on the transfer belt 21 on a detector composed of a PD (PhotoDiode) sensor, and the center of gravity of the patch. This is a reflective sensor that outputs a pulse when the line matches the center line of the detector. This color misregistration sensor 32 detects, for example, the downstream side of the most downstream image forming unit 10K in FIG. 1 in order to detect the relative color misregistration of the color misregistration control pattern due to the patches formed in each image forming unit 10. And two are arranged along the main scanning direction. For example, two infrared LEDs (wavelength 880 nm) are used for the light emitting part of the color misregistration sensor 32, and the light emission quantity of the two LEDs can be adjusted (for example, in two stages) in order to ensure stable pulse output. It is configured.

  In each of the four color image forming units 10Y, 10M, 10C, and 10K, various units for image formation are similarly formed around the photosensitive drum 11 that is an image carrier. That is, a charging device for charging the photosensitive drum 11, a developing device for developing a toner image on the photosensitive drum 11 exposed by the exposure device 13, and residual toner remaining on the photosensitive drum 11 after the transfer of the toner image to the transfer belt 21. Various units, such as a cleaner, are provided. The configuration of the image forming unit 10 is not used for normal color image formation in addition to so-called regular colors such as yellow (Y), magenta (M), cyan (C), and black (K). It is also possible to provide a specific color image forming unit corresponding to a special drawing material such as a color. In addition to the four colors Y, M, C, and K described above, five or more colors including dark yellow can be used as regular colors. In the present embodiment, the axial direction of the photosensitive drum 11 that is an image carrier is the main scanning direction, and the moving direction by the rotation of the photosensitive drum 11 is the sub-scanning direction.

  Here, in the exposure device 13 that exposes the photosensitive drums 11 of the four color image forming units 10Y, 10M, 10C, and 10K, a multi-beam ROS (Raster Output Scanner) is used, each of which includes a plurality of laser diodes. It has a plurality of light sources composed of (LD). The laser beams emitted from the plurality of light sources are collimated by a collimating lens, then scanned by a deflecting / reflecting surface of a rotary polygon mirror (polygon mirror), and the photosensitive drum 11 is scanned by a laser spot narrowed down by an imaging lens (mainly (Scanning) exposure. The photosensitive drum 11 is rotationally driven by a driving unit, and is exposed in a direction (sub-scanning direction) orthogonal to laser scanning (main scanning) by the exposure device 13, thereby realizing two-dimensional exposure recording.

  As the transfer belt 21, for example, a flexible synthetic resin film such as polyimide is formed in a belt shape, and both ends thereof are connected by means such as welding to form an endless belt. The transfer belt 21 is stretched by a driving roll and a backup roll in a loop shape that is at least partially straightened. Then, the four-color image forming units 10Y, 10M, 10C, and 10K and the opposing primary transfer rolls 23 are arranged at substantially regular intervals with respect to the substantially linear portion of the transfer belt 21. Yes. In the example shown in FIG. 1, a yellow image forming unit 10Y, a magenta image forming unit 10M, and a cyan image forming unit 10C are sequentially arranged from the upstream side to the downstream direction with respect to the moving direction of the transfer belt 21 during the transfer operation. , Black image forming portions 10K are arranged. A color toner image is formed on the transfer belt 21 by sequentially superimposing the images of the respective colors formed by the image forming unit 10 on the belt according to the movement of the transfer belt 21. Then, the movement of the transfer belt 21 and the timing of sheet conveyance are matched, and the color toner image formed on the transfer belt 21 is transferred to the sheet at a position including the secondary transfer roll 24 and the opposing roll 25. Thereafter, the sheet on which the color toner image has been transferred is conveyed to the fixing device 28 where the color toner image is fixed on the sheet and discharged to a discharge tray provided outside the casing of the image forming apparatus. The

  FIG. 2 is a view showing an example of a laser device used in the exposure apparatus 13. In the present embodiment, the exposure apparatus 13 is provided with a surface emitting laser device 40 as shown in FIG. This surface emitting laser device 40 is provided with a total of 16 laser diodes (LD) 41 of LD1 to LD16 in a 4 × 4 array (arrangement shape) as a printing source in one device. The 16 laser diodes 41 can simultaneously scan 16 lines with 16 beams. Since the 16 multi-beams are emitted from the surface emitting laser device 40 that is one device, the relative positional relationship of the 16 multi-beams is maintained with a certain degree of accuracy. However, when a relatively different inclination occurs between the surface emitting laser devices 40 in the exposure apparatuses 13 of the respective colors due to device attachment failure, temperature change, or the like, the scanning positions of the respective colors are shifted to cause a skew shift.

FIG. 3 is a diagram illustrating a state of skew correction by image processing.
It is possible to correct the skew deviation described above by performing image processing on the image data of each color so as to cancel the skew. As shown in FIG. 3, the image data of each color is divided into a plurality of strips in the main scanning direction, and each divided part is shifted stepwise in the sub-scanning direction. The number of divisions and the division position (that is, the theoretical shift position) in the main scanning direction are adjusted according to the amount of skew deviation (correction amount). Thereby, the skew deviation at the time of drawing is canceled.

  On the other hand, when such image processing is performed, a streak defect occurs depending on the drawn image (pattern). Typically, when the halftone of an image is expressed by area gradation, a screen with a fine pattern is drawn at a portion where the halftone is expressed, and an image for skew correction is drawn at a portion where the screen is drawn. When the shift is applied, a streak defect occurs. The root cause of the occurrence of the streak defect has not been clarified. For example, the following can be considered. That is, at the boundary portion where the screen pattern is shifted by the skew correction, the density change occurs because the ratio of binary dots changes compared to other locations. Therefore, when the number of black dots increases (for example, in the case of FIG. 17B), the black streak is recognized macroscopically, and on the contrary, the number of white dots increases (for example, the case of FIG. 17C). The idea is that macroscopically white stripes are recognized. However, regardless of what is the root cause of the streak defect, the pixels shifted by the skew correction are arranged in a line on a straight line. It is thought that the image quality defect will be recognized.

  Therefore, in the present embodiment, the generation of streak defects is suppressed by preventing pixels shifted for skew correction from being aligned in a line on a straight line. As specific methods for that purpose, in the present embodiment, a method using multiple exposure (first method), a method of making the boundary line when dividing image data non-linear (second method), and Indicates. Hereinafter, each method will be described in detail.

<First method>
In multiple exposure, scanning is performed a plurality of times per scanning line by scanning the multi-beams while shifting by several scanning lines. Therefore, in the first method, the pixel shift position is changed for skew correction in each scan of the same element of the image data. In the following, for the sake of simplicity, an example will be described in which scanning is performed twice per scanning line (double exposure) while shifting 16 scanning beams by 8 scanning lines.

  4A and 4B are diagrams illustrating the first method. FIG. 4A shows the exposure state in the n-th scanning and the exposure state in the n + 1-th scanning side by side, and FIG. 4B shows the two scannings in FIG. 4A. The state in which the exposures at are synthesized is shown. In the example shown in FIG. 4, the shift positions (image data division positions) of the pixels set for skew correction in the sub-scanning direction are the main scanning directions G and M.

  Referring to FIG. 4A, in the n-th scan, the laser output position is shifted in the sub-scanning direction by one scanning line at the positions in the main scanning directions G and M. On the other hand, in the (n + 1) th scanning, the laser output position is shifted in the main scanning directions H and N, and the shift position moves by one pixel in the forward direction (right direction in the figure) in the main scanning direction. ing. That is, the pixels shifted for skew correction are arranged in two rows with a width of one pixel although it is a straight line by two scans.

  Since these two scans are overlapped, at the position in the main scanning direction G, the laser corresponding to the sub-scanning directions 9 to 12 is output and exposed in the first scanning, and the sub-scanning direction 10 in the second scanning. Lasers corresponding to ˜13 are output and exposed. Similarly, at the position in the main scanning direction M, the positions in the sub-scanning directions 10 to 13 are exposed in the first scanning, and the positions in the sub-scanning directions 11 to 14 are exposed in the second scanning. Therefore, as shown in FIG. 4B, five pixels are exposed at the positions in the main scanning directions G and M, respectively. Of these, the pixels at both ends, ie, the pixels in the sub-scanning directions 9 and 13 at the position in the main scanning direction G, and the pixels in the sub-scanning directions 10 and 14 at the position in the main scanning direction M are each exposed only once. The density in the image is lower than in other parts.

  As a result, in FIG. 4B, the boundary between the area where the scanning lines in the sub-scanning directions 9 to 12 are exposed and the area where the scanning lines in the sub-scanning directions 10 to 13 are exposed becomes ambiguous (smoothing). Similarly, the boundary between the area where the scanning lines in the sub-scanning directions 10 to 13 are exposed and the area where the scanning lines in the sub-scanning directions 11 to 14 are exposed becomes ambiguous. As described above, since the boundary of the pixel shift position due to the skew correction becomes ambiguous, the visibility of the streak defect appearing along the boundary line of the pixel shift position is suppressed (becomes difficult to recognize). .

  Here, consider n + 2 and subsequent scans. In the above operation, in the (n + 1) th scanning with respect to the nth scanning, the pixel shift position by the skew correction is moved by one pixel in the forward direction of the main scanning direction. If this is applied to the n + 2 and subsequent scans as it is, the pixel shift position gradually moves along the main scanning direction, so that it deviates greatly from the shift position set according to the amount of skew deviation. In order to avoid this, in the (n + 2) th scan, the pixel is shifted at the original shift position by the skew correction as in the nth scan. In other words, in general, the pixels are shifted at the original shift position in the n-th and n + even-th scans, and moved one pixel in the forward direction of the main scanning direction from the original shift position in the n + odd-th scans. The pixel is shifted by the position.

  As an extension of the first method shown in FIG. 4, it is conceivable to move the pixel shift position in the nth and n + even scans. FIGS. 5A and 5B are diagrams showing the operation of the first method in this case. As in the case of FIG. 4, FIG. 5A shows the exposure state in the n-th scanning and the exposure state in the n + 1-th scanning side by side, and FIG. A state in which the exposures in the two scans of (a) are combined is shown. In the example of FIG. 5, as in the example of FIG. 4, the shift positions (image data division positions) of the pixels set for skew correction are the main scanning directions G and M.

  Referring to FIG. 5A, in the n-th scan, the laser output position is shifted by one scanning line in the sub-scanning direction at positions F and L in the main scanning direction. That is, the actual shift position is shifted by one pixel in the reverse direction of the main scanning direction (left direction in the figure) with respect to the original shift position set for skew correction. On the other hand, in the (n + 1) th scanning, the laser output position is shifted at the positions of the main scanning directions H and N, and the shift position is the forward direction in the main scanning direction (see FIG. 4A). Is moved by one pixel to the right). Therefore, in the example shown in FIG. 5, the pixels shifted for skew correction are arranged in two rows with a width of two pixels by two scans.

  Since these two scans are superimposed, at the positions of the main scanning directions F and G, the laser corresponding to the sub-scanning directions 9 to 12 is output and exposed in the first scanning, and the sub-scanning is performed in the second scanning. A laser corresponding to directions 10 to 13 is output and exposed. Similarly, at positions in the main scanning directions L and M, positions in the sub-scanning directions 10 to 13 are exposed in the first scanning, and positions in the sub-scanning directions 11 to 14 are exposed in the second scanning. Therefore, as shown in FIG. 5B, five pixels are exposed at positions in the main scanning directions F, G, L, and M, respectively. As in the case of FIG. 4, the pixels at both ends, that is, the pixels in the sub-scanning directions 9 and 13 at the positions in the main scanning directions F and G, and the sub-scanning directions 10 and 14 at the positions in the main scanning directions L and M, respectively. Since each pixel is exposed only once, the image has a lower density in the image than the other parts.

  Accordingly, in FIG. 5B, the boundary of the pixel shift position by the skew correction becomes ambiguous, and the streak defect appearing on the boundary line of the pixel shift position is suppressed. Note that the n + 2 and subsequent scans can be considered in the same manner as in FIG. That is, in general, in the n-th and n + even-th scans, the pixel is shifted at a position shifted by one pixel in the reverse direction of the main scanning direction from the original shift position set for skew correction, and n + odd In the second scanning, the pixel is shifted at a position moved by one pixel in the forward direction of the main scanning direction from the original shift position.

<Second method>
As described above, the stripe defect is considered to be an image quality defect that can be visually recognized because the pixels shifted by the skew correction are aligned in a line. Therefore, in the second method, when the image data is subjected to deformation for skew correction, image processing is performed so that the boundaries of the pixel shift positions are not aligned in a straight line. In other words, when the image data is divided into a plurality of strips in the main scanning direction, the image data is shaped so that the boundary line of each strip is not a straight line in the sub-scanning direction.

  FIGS. 6A and 6B are diagrams for explaining the second method. FIG. 6A shows a state where the image data is divided into strips, and FIG. 6B shows a state where the divided image data is shifted step by step for each strip. In the example shown in FIGS. 6A and 6B, the boundary line of the image data (denoted by a broken line in the figure) is bent in the main scanning direction every certain length to form a rectangular wave boundary line. Thus, by making the boundary line into a complicated shape when dividing the image data, it is possible to avoid the shifted pixels from being arranged in a line on a straight line. As a result, the density change at the pixel shift position due to the skew correction is dispersed in the main scanning direction, so that the visibility of the streak defect appearing along the boundary line of the pixel shift position is suppressed.

  By the way, when a strip having a rectangular wave boundary line is shifted in the sub-scanning direction, a gap (empty pixel) is formed at a position where the boundary line is bent in the main scanning direction, as shown in FIG. There may be a part (overlapping pixel) where two adjacent strips overlap. Specifically, as shown in FIG. 7, focusing on two adjacent strips A and B, considering the portion where strip A protrudes toward strip B, strip B is shifted with reference to strip A. Empty pixels are generated on the downstream side in the direction, and overlapping pixels are generated on the upstream side.

These empty pixels and overlapping pixels are greatly different from other surrounding pixels as they are. Therefore, for these empty pixels and overlapping pixels, the surrounding pixels are copied and replaced, or the pixel data is corrected based on the information of the surrounding pixels, so that the difference from other surrounding pixels can be avoided. Suppress. As a specific processing method,
1. One pixel of the two areas arranged in the main scanning direction is copied.
2. An adjacent pixel is specified based on the coordinates in the main scanning direction before the shift, and the pixel is copied.
3. The average value of the pixel data in the peripheral pixels of the sky pixel and the overlapping pixel after the shift is calculated, and the obtained average value is set as the pixel data of the sky pixel and the overlapping pixel.
Such a method is conceivable.

  As shown in FIG. 6, in the case where the strip boundary line obtained by dividing the image data is formed in a rectangular wave shape, if the length of the rectangular wave boundary line bent in the main scanning direction is increased, the boundary line in the main scanning direction is formed. Another defect may occur. Therefore, it is desirable to make the length in the sub-scanning direction larger than the length in the main scanning direction in the rectangular wave shape of the boundary line. Further, as shown in FIGS. 8A and 8B, the defect in the main scanning direction can be suppressed by forming irregularities also in the portion where the shape of the boundary line extends in the main scanning direction.

  The second technique is to suppress the visibility of the streak defect by making the border line of the strip obtained by dividing the image data in the image processing into a complicated shape. Therefore, the shape of the boundary line need not be a simple straight line, and is not limited to the rectangular wave shape described above. 9A to 9C are diagrams illustrating examples of complicated boundary lines. FIG. 9A shows a boundary line that is bent a plurality of times in the main scanning direction and repeats a stepped pattern. FIG. 9B shows a boundary line that forms an enclave-like shape by arranging regions that are shifted together with other adjacent strips within the strip region. FIG. 9C shows a boundary line formed so as to have a curved shape as a whole without a portion extending linearly in the sub-scanning direction or the main scanning direction. In this manner, the strip boundary line obtained by dividing the image data can be arbitrarily formed.

In the second method, the strip boundary line obtained by dividing the image data as described above is formed in a complicated shape. However, if the boundary line shape is determined arbitrarily arbitrarily, the boundary line shape and the screen pattern may be synchronized depending on the screen drawn when the halftone is expressed by the area gradation.
FIGS. 10A and 10B are diagrams showing an example of synchronization between the shape of the boundary line and the screen pattern. In the example of FIG. 10A, the period of the rectangular wave boundary line (denoted by a broken line) bent in the main scanning direction is synchronized with the period of the screen pattern. In the example of FIG. 10B, a part of the shape of the boundary line and the angle of the screen pattern are synchronized.

  When the shape of the border line of the strip is synchronized with the screen pattern, the occurrence of density change due to the shift of the strip is linearly arranged in a specific direction according to the synchronization of the shape of the border line and the screen pattern. A streak defect may appear. Therefore, it is necessary to select the shape of the boundary line in consideration of the shape having a period different from the period of the screen pattern and the shape having no portion having the same angle as the angle of the screen pattern. Become. Specifically, if a screen pattern for area gradation is determined in advance, a boundary line shape that does not synchronize with all screen patterns is set, or several types of boundary line shapes are prepared and image data is prepared. It may be selectively applied according to the screen pattern used in the above. On the other hand, when a screen pattern for area gradation is created and added by the user, it is necessary to perform image analysis and create a boundary line shape that is not synchronized with the added screen pattern.

  Further, since the streak defect is generated by a combination of a shift of image data for skew correction and a screen pattern for area gradation, it is considered that the streak defect is likely to occur in a place where some image exists. Therefore, when an edge portion of a predetermined image exists in the vicinity of the boundary line that divides the image data, the shape of the boundary line (indicated by a bold line in the drawing) is changed to a shape along the edge portion of the image as shown in FIG. By setting, it is possible to prevent the boundary line from crossing the image and to make it difficult to generate a streak defect.

As described above, as a technique for suppressing the streak defect in the skew correction according to the present embodiment, the technique using the multiple exposure (first technique) and the technique of making the boundary line when dividing the image data non-linear ( Two types of methods have been described.
Next, a configuration for realizing the above-described streak defect suppressing method will be described.
FIG. 12 is a diagram illustrating a functional configuration of the control unit 31 that performs the skew correction by applying the above-described streak defect suppressing method.

  Referring to FIG. 12, the control unit 31 includes an image data generation unit 51 that converts an input image into image data, a screen processing unit 52 that performs screen processing on the image output from the image data generation unit 51, and skew correction. And a skew correction processing unit 53 to be executed. Further, the control unit 31 uses the ROS (Y ROS, M ROS, and C for ROS) of the exposure apparatus 13 corresponding to the image forming units 10Y, 10M, 10C, and 10K that form images of Y, M, C, and K colors. The image forming instruction unit 54 outputs image information to the ROS and the ROS for K). Furthermore, the control unit 31 includes a pattern storage unit 55 that stores pattern information suitable for each color of Y, M, C, and K and for each object. Furthermore, the control unit 31 uses, for example, the color misregistration sensor 32 to detect the skew of each color with reference to black (K), and outputs the image by changing the image writing address in the skew correction. A correction value calculation processing unit 57 for performing correction value calculation processing when the correction value is calculated.

  The image data generation unit 51 converts image data output from a PC or IIT, such as page description language or bitmap data, into image data unique to the image forming apparatus. The image data generated by the image data generation unit 51 is expressed as, for example, 600 dpi (8 bits) + Tag (4 bits) as multi-value data, or as 600 dpi (1 bit) or 1200 dpi as binary data. (1 bit) or the like. The image data generation unit 51 may be configured by an external controller provided in a separate housing from the image forming apparatus, in addition to being provided in a controller inside the image forming apparatus.

  The screen processing unit 52 performs preferable screen processing for each specific color and specific object (for example, a photograph, a character, or the like), and outputs, for example, image data of 2400 dpi (1 bit). In the screen processing unit 52, text / image separation (T / I separation) processing is executed, and a preferred pattern for performing screen processing is read from the pattern storage unit 55 and used.

  The skew detection processing unit 56 determines whether or not skew correction is necessary for the image data subjected to the screen processing. Whether or not skew correction is necessary can be determined based on the detection result of the color misregistration control pattern acquired from the color misregistration sensor 32, for example. In addition, skew correction may be performed at the time of color registration alignment. In this case, more specifically, for example, one color such as black (K) is fixed as a reference, and the color registration is adjusted by performing skew correction for the other colors.

  The correction value calculation processing unit 57 performs calculation processing for implementing the above-described streak defect suppression method. Specifically, for example, in the case of the first method described above, the pixel shift position in each scan is calculated. Further, in the case of the second method, the pixel shift position for dividing the image data into strips at a boundary line other than a straight line along the sub-operation direction is calculated, and empty pixels and overlapping pixels are corrected. For the operation.

  The skew correction processing unit 53 uses the correction value calculated by the correction value calculation processing unit 57 to perform the above-described streak defect suppression method, that is, change of pixel shift position and division of image data. Deform the boundary line.

Next, processing executed by the control unit 31 will be described.
FIG. 13 is a flowchart for explaining processing by the control unit 31 shown in FIG.
As shown in FIG. 13, when the control unit 31 receives an image output request (step 131), the screen processing unit 52 performs, for example, character / image recognition on the image data output from the image data generation unit 51. The object judgment is executed (step 132).
Further, the screen processing unit 52 reads a predetermined pattern from the pattern storage unit 55 based on the color of the image data and the object determination (step 133), and executes screen processing (step 134).

Next, the skew detection processing unit 56 determines whether or not skew correction is necessary for the image data (step 135). If it is determined that skew correction is not necessary, the process proceeds to step 138. On the other hand, when it is determined that skew correction is necessary, the correction value calculation processing unit 57 calculates a correction value based on the above-described streak defect suppression method (step 136). Then, the skew correction processing unit 53 executes skew correction based on the correction value calculated by the correction value calculation processing unit 57 (step 137).
Thereafter, image data is output from the image formation instruction unit 54 to an IOT (Image Output Terminal) (step 138). This image data is the image data that has undergone skew correction in step 137, or the screen processing that has been subjected to screen processing in step 134 if it is determined that skew correction is not necessary in step 135 (the state before skew correction). ) Image data.

  By the way, a streak defect is likely to occur when an image shift for skew correction is applied to a place where a line type screen is drawn, and a place where another type (for example, a dot type) screen is drawn. It is difficult to occur. In addition, even in a line type screen, when the line angle is 45 degrees with respect to the image shift direction, the streak defect is easily recognized, and the line defect is recognized as the line angle moves away from 45 degrees. It has been empirically found that it is less likely to be recognized (becomes difficult to recognize). Therefore, it is necessary to determine whether or not the above-described method for suppressing the streak defect according to the present embodiment is performed according to the screen pattern for area gradation.

For example, in the case of the first method, on a screen other than the line type (dot type screen or the like), there is a case where the image quality level is not lowered when the processing (shift position shift) for suppressing the streak defect is not performed.
FIGS. 14A to 14C are diagrams showing examples of line-type screens having various angles. Since the image shift for skew correction is performed in the sub-scanning direction, the angle of the line is expressed here as an angle with respect to the main scanning direction orthogonal to the image shift direction. 14A shows a screen with a line angle of 45.0 degrees, FIG. 14B shows a line angle of 69.4 degrees, and FIG. 14C shows a screen with a line angle of 25.0 degrees (both 40 % Gradation). Among these screens, the screen of the 45.0 degree line shown in FIG. 14A has the highest defect level (easy to be recognized), and the other screens shown in FIGS. 14B and 14C show the defect level. Is low (not easily recognized).

  This can be dealt with by applying the first method as follows. A line type screen having a high defect level as shown in FIG. 14A (that is, an angle close to 45.0 degrees (eg, ± several degrees)) is drawn at the image shift position in the skew correction. In this case, the shift position of the pixel in the sub-scanning direction in a plurality of scans is greatly shifted (see FIG. 5). On the other hand, a line type screen with a low defect level as shown in FIGS. 14B and 14C (that is, an angle greatly different from 45.0 degrees) is drawn at the image shift position in the skew correction. If there is, the shift of the shift position of the pixel in the sub-scanning direction in a plurality of scans is reduced (see FIG. 4), or the shift position is not shifted (the first method is not applied).

  For which type of screen, how much the shift position is shifted by a plurality of scans can be arbitrarily set. However, if the shift position shift amount is increased, the effect of smoothing is increased, but other image quality factors such as the sharpness of the edge of the line may be reduced. Therefore, the shift amount of the shift position should be as small as possible. In consideration of individual differences in the IOT (Image Output Terminal) of the image forming apparatus and the subjectivity of the user, the screen may be actually drawn and set individually. In addition, when generating a screen by IOT, the control part 31 can grasp | ascertain screen information, such as a pattern type and a line angle.

  The above explanation is about the case of drawing a line type screen.However, even if it is another type of screen (for example, a dot type screen), when the gradation becomes high, it becomes a line shape pattern. Since a streak defect may occur, it is effective to apply the suppression method according to the present embodiment.

As described above in detail, according to this embodiment, it is possible to suppress streak defects that occur when skew correction is performed on an original image, thereby contributing to improvement in image quality.
Although the present embodiment has been described using an electrophotographic image forming apparatus, the image quality defect correction method according to the present embodiment can also be applied to an inkjet or thermal image forming apparatus. For example, in the case of an inkjet method, when a plurality of nozzles that are printing sources are provided instead of the above-described multi-beam scanning, the plurality of nozzles are collectively scanned and printed on an image carrier (paper or the like). An image shift process is performed on the printing unit with respect to the sub-scanning direction, which is the moving direction of the image carrier (such as paper). As a result, the same effect as the electrophotographic method can be obtained. Further, in this embodiment, the suppression of streak defects that occur in skew correction has been described. However, a similar method is used to divide an image into regions, such as misalignment by a bow, and shift each region to change between colors. If the displacement can be reduced (for example, meandering of the belt), the image quality can be improved by applying the method of the present embodiment.

  Furthermore, in the present embodiment, the skew correction performed to prevent color misregistration in an image forming apparatus such as a color printer or a color copying machine has been described. However, a similar method is applied to a monochrome output image in a black monochrome printer or copying machine. Needless to say, it can be applied to the shape correction (skew correction for paper).

1 is a diagram illustrating an image forming apparatus to which the exemplary embodiment is applied. It is the figure which showed an example of the laser device used for exposure apparatus. It is a figure which shows the mode of the skew correction by image processing. It is a figure explaining the 1st method of suppressing the stripe defect in this embodiment. It is a figure explaining the other Example of the 1st method of suppressing the stripe defect in this embodiment. It is a figure explaining the 2nd method of suppressing the stripe defect in this embodiment. It is a figure explaining the empty pixel and overlapping pixel which arise with the 2nd method of suppressing the stripe defect in this embodiment. It is a figure explaining the other Example of the 2nd method of suppressing the stripe defect in this embodiment. It is a figure explaining the further another Example of the 2nd method of suppressing the stripe defect in this embodiment. It is a figure which shows the example of the synchronization in the shape of the boundary line which divides | segments an image, and the pattern of a screen. It is a figure explaining the further another Example of the 2nd method of suppressing the stripe defect in this embodiment. It is a figure which shows the function structure of the control part of this embodiment. It is a flowchart explaining the process by the control part of this embodiment. It is a figure which shows the example of the screen of a line type of various angles. It is a figure which shows the example of the process currently performed about the skew correction in the image forming apparatus using a multi-beam scanning optical system. FIG. 16 is a diagram showing a state of skew correction when multiple exposure is performed by an image forming apparatus using a multi-beam scanning optical system similar to FIG. 15. It is the figure which represented typically the streak defect which arises by performing skew correction.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Image formation part, 11 ... Photosensitive drum, 13 ... Exposure apparatus, 21 ... Transfer belt, 31 ... Control part, 32 ... Color shift sensor, 40 ... Surface emitting laser device, 41 ... Laser diode (LD), 51 ... Image data generation unit 52 ... Screen processing unit 53 ... Skew correction processing unit 54 ... Image formation instruction unit 55 ... Pattern storage unit 56 ... Skew detection processing unit 57 ... Correction value calculation processing unit

Claims (12)

Input means for inputting image data;
One scanning line having a plurality of printing sources and collectively scanning a plurality of beams from the plurality of printing sources while shifting the scanning beams by several scanning lines so that some of the plurality of beams overlap. A printing means for printing the image carrier by scanning a plurality of times per ,
Control means for shifting the image stepwise in the sub-scanning direction which is the moving direction of the image carrier when the image data subjected to screen processing is drawn by the printing means;
The control means specifies a reference shift position in the image so as to correct an image skew shift, and serves as the reference shift position in at least a part of a plurality of scans for one scanning line. An image forming apparatus that shifts an image at a position shifted by one or several pixels with respect to the image forming apparatus. The control means shifts the image at a position shifted by one or a plurality of pixels in a predetermined direction with respect to the reference shift position in the (n + odd) scan, and in the nth and n + even scans, the reference the image forming apparatus according to claim 1, characterized in that shifting the image at a position shifted one or a plurality of pixels in the direction opposite to the shift position or the n + odd-numbered scan made. The control means determines how many pixels the image is shifted from the reference shift position according to an angle of a line of a screen pattern applied by the screen processing. The image forming apparatus according to claim 1 . Input means for inputting image data;
A printing unit having a plurality of printing sources and printing the image carrier by collectively scanning the printing source or a beam from the printing source;
The screen-processed image data is divided into a plurality of parts with respect to a main scanning direction orthogonal to the moving direction of the image carrier, and the divided plurality of parts are sub-directions that are the moving direction of the image carrier. Control means for shifting in a scanning direction stepwise and causing the printing means to draw,
The image forming apparatus, wherein the control unit divides the image data so that a boundary line of the portion has a shape other than a straight line in the sub-scanning direction. The control means corrects the data of a sky pixel and an overlapping pixel that are generated when the portion of the image data is shifted based on information of pixels around the sky pixel or the overlapping pixel. Item 5. The image forming apparatus according to Item 4 . The image forming apparatus according to claim 4 , wherein the control unit divides the image data in a boundary line shape that is not synchronized with a pattern drawn by the printing unit. The image forming apparatus according to claim 4 , wherein the control unit divides the image data along a boundary line having a shape along a part of an edge portion of an image drawn by the printing unit. The plurality of printing sources of the printing unit are a plurality of laser beam light sources, and the printing unit uses a multi-beam that collectively scans a plurality of laser beams from the plurality of laser beam light sources with a rotary polygon mirror. The image forming apparatus according to claim 4 , wherein the image forming apparatus is an exposure unit. One scanning line having a plurality of printing sources and collectively scanning a plurality of beams from the plurality of printing sources while shifting the scanning beams by several scanning lines so that some of the plurality of beams overlap. A method for controlling an image forming apparatus that prints an image carrier by scanning a plurality of times
Applying screen processing to input image data;
A reference shift position in the image is specified so as to correct skew of the image, and at least a part of a plurality of scans with respect to one scanning line is 1 or 2 with respect to the reference shift position. Obtaining a correction value so as to shift the image at a position shifted by several pixels ;
A step of performing skew correction by shifting the image data subjected to the screen processing stepwise in a sub-scanning direction, which is a moving direction of the image carrier, based on the obtained correction value. And a control method of the image forming apparatus. Claim 9, wherein the determining in accordance with whether to shift the image in many pixels the position shifted from the shift position serving as the reference, the angle of the line pattern of the screen is performed by the screen processing Image forming apparatus control method. A control method for an image forming apparatus that has a plurality of print sources and prints an image carrier by collectively scanning the print sources or beams from the print sources,
Applying screen processing to input image data;
A plurality of portions such that the image data has a shape other than a straight line in the main scanning direction orthogonal to the moving direction of the image carrier and a boundary line in the sub-scanning direction that is the moving direction of the image carrier. Dividing into steps,
And a step of performing skew correction by gradually shifting an image of the plurality of divided portions in the sub-scanning direction. The method further comprises a step of correcting data of sky pixels and overlapping pixels generated when the portion of the image data is shifted based on information of pixels around the sky pixels or the overlapping pixels. 11. A method for controlling an image forming apparatus according to 11 .

Images