JP2020199745A - Image formation apparatus, control method of the same and program - Google Patents

Abstract

To solve such a problem that interpolation processing causes image degradation when performing the interpolation processing using PWM in order to reduce steps and stripes that occur when performing pixel shift with geometric conversion of an image.SOLUTION: An image formation apparatus for forming an image with scanning of light from a plurality of light sources acquires an inclination amount in the scanning direction of each light, shifts formed image data by a pixel unit in a direction orthogonal to the scanning direction of the light in accordance with the acquired inclination amount, shifts the formed image data by a pixel unit in the scanning direction or the direction orthogonal to the scanning direction in accordance with the skew amount of a sheet, and performs interpolation processing on the image data shifted by the pixel unit in the direction orthogonal to the scanning direction of the light by a unit less than one pixel.SELECTED DRAWING: Figure 8

Description

The present invention relates to an image forming apparatus, a control method thereof, and a program.

The electrophotographic method is known as an image recording method used in a color image forming apparatus such as a color printer or a color copier. In this electrophotographic method, a latent image is formed on a photosensitive drum using a laser beam, and the latent image is developed with a charged coloring material (hereinafter referred to as toner). Then, the image by the developed toner is transferred to the transfer paper and fixed to form (print) the image.

In such a color image forming apparatus, it is common to use a tandem method in which the same number of developers and photosensitive drums as the number of toner colors are provided and images of different colors are sequentially transferred onto a transport belt or a recording medium. is there. In this tandem color image forming apparatus, it is known that there are a plurality of factors that cause registration deviation between colors, and various countermeasures have been proposed for each factor.

One of the factors is the non-uniformity of the lens of the deflection scanning device, the deviation of the mounting position, and the deviation of the assembly position of the polarization scanning device to the color image forming apparatus main body. Due to this misalignment, the scanning line is tilted or bent, and the degree of the bending (hereinafter, profile) differs for each color, resulting in a registration deviation. Further, this profile has different characteristics for each image forming apparatus, that is, for each printer engine, and further for each color.

On the other hand, in the image forming apparatus, the paper on which the image is formed may be obliquely conveyed. When the image is transferred from the transfer belt to the paper conveyed obliquely in this way, the image is formed tilted on the paper. Since this is due to the skew of the paper, unlike the registration deviation described above, each color has the same characteristics (the degree of skew).

As a countermeasure against registration deviation caused by the bending of the scanning line, the inclination and the magnitude of the bending are measured by using an optical sensor, and the bitmap image data is corrected so as to cancel them, and the correction is performed. There is a method of forming an image according to the obtained image data.

Further, as a technique for simultaneously correcting registration deviation and paper skew, for example, in Patent Document 1, both corrections are made by using a sensor for detecting inclination on a belt and a sensor for detecting the passing timing of paper. Is going.

When the image data is corrected to correct the above-mentioned deviation, a mechanical adjusting member and an adjustment step at the time of assembly become unnecessary. Therefore, the size of the color image forming apparatus can be reduced, and the registration deviation can be dealt with at low cost. The registration deviation correction based on such image data is divided into a correction for each pixel and a correction for less than one pixel.

The correction in units of one pixel shifts the pixels in the sub-scanning direction in units of one pixel according to the inclination of the scanning line and the correction amount of the bending. When this method is used, the bending or inclination of the scanning line is about several hundred to 500 μm, and in an image forming apparatus having a resolution of 1200 dpi, an image memory for several tens of lines is required to perform such correction. Hereinafter, the position to be shifted is referred to as a transfer point.

On the other hand, in the correction of less than one pixel, the gradation value of the image data is adjusted by the pixels before and after the sub-scanning direction. That is, when the image is bent upward due to the profile, the image data before correction is handled on the sub-scanning side in the direction opposite to the direction indicated by the profile. By correcting less than one pixel by such a method, it is possible to eliminate an unnatural step at the boundary of a transfer point caused by the correction in units of one pixel and to smooth the image.

When the above-mentioned smoothing process is performed on the image data that has been subjected to the screen processing immediately before printing, the smoothing is performed by using pulse width modulation (PWM) for the laser beam and the laser exposure time. Is performed by gradually switching to the sub-scanning direction. For example, in the case of correction in units of 0.5 pixels, which is less than one pixel, half the exposure is performed twice in the vertical direction of the sub-scanning direction.

Similarly, as a countermeasure when the image is tilted and formed on the paper, a measure is taken such as tilting the bitmap image data so as to offset them according to the amount of skew of the paper and forming the image on the paper. .. In this case, in addition to the offset in the sub-scanning direction performed at the time of the previous registration deviation, the image is pseudo-tilted by shifting the pixels in the main scanning direction as well. Although there is an error from the coordinate calculation by strict affine transformation, if it is skewed at a slight angle, it can be sufficiently approximated by the pixel shift processing.

JP-A-2010-140019

As described above, when the smoothing process is performed by modulating the laser exposure time using PWM, the edge portion is blurred, and the smoothing is performed using this blur. By performing this not only for correction of less than one pixel in the sub-scanning direction but also for the two axes in the main scanning direction, the amount of blurring is doubled, and the reproduction characteristics of small dots and fine lines are deteriorated. In particular, the reproducibility is not stable with small dots and thin lines that make up halftone dots, and the density changes at the transfer point, making it appear uneven. Therefore, in the interpolation processing by smoothing using PWM after shifting the two axes in the main scanning direction and the sub scanning direction, it is necessary to take measures to prevent unevenness of these fine lines and small dots.

An object of the present invention is to solve at least one of the problems of the prior art.

An object of the present invention is that when image defects due to tilting of each light in the scanning direction and skewing of paper are suppressed by correcting image data, deterioration of the formed image is prevented and occurs at the corrected portion. The purpose of the present invention is to provide a technique for making the step difference of the pixels inconspicuous.

In order to achieve the above object, the image forming apparatus according to one aspect of the present invention has the following configuration. That is,
An image forming apparatus that forms an image by scanning light from a plurality of light sources.
An acquisition means for acquiring the amount of inclination of each light in the scanning direction,
A first shift means that shifts the image data to be formed in pixel units in a direction orthogonal to the scanning direction of the light according to the amount of inclination.
A second shift means for shifting the image data to be formed in pixel units in the scanning direction and / or in a direction orthogonal to the scanning direction according to the amount of skew of the paper.
It is characterized by having an interpolation means that performs interpolation processing in units of less than one pixel on the image data shifted by the first shift means.

According to the present invention, when image defects due to tilting of each light in the scanning direction and skewing of paper are suppressed by correcting image data, deterioration of the formed image is prevented and occurs at the corrected portion. It is possible to make the step of the pixel to be inconspicuous.

Other features and advantages of the present invention will become apparent in the following description with reference to the accompanying drawings. In the attached drawings, the same or similar configurations are designated by the same reference numbers.

The accompanying drawings are included in the specification and are used to form a part thereof, show an embodiment of the present invention, and explain the principle of the present invention together with the description thereof.
FIG. 3 is a block diagram illustrating a configuration of each part related to the creation of an electrostatic latent image of the electrophotographic color image forming apparatus according to the first embodiment of the present invention. FIG. 3 is a schematic cross-sectional view illustrating an image forming portion of the color image forming apparatus according to the first embodiment. The figure which shows an example of the profile characteristic of the image forming apparatus which concerns on Embodiment 1. FIG. The figure which shows the correlation between the direction which should be corrected by an image processing part and the deviation direction of an image forming part based on a profile definition. The figure explaining an example of how to hold the data of a profile characteristic. The figure which shows the skew of the paper in an image forming apparatus and the measurement position thereof. The figure explaining the binarization method by the dither method. The flowchart explaining the process by the image shift part which concerns on Embodiment 1. The figure which shows an example of the image before and after the shift process schematically. The figure explaining the interpolation of less than 1 pixel in S804 of FIG. The figure which showed the white spot due to the shift processing schematically. A line parallel to the laser scanning in the second embodiment and a state of its shift are schematically shown. The figure which shows typically the line orthogonal to the laser scan and the state of the shift.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments do not limit the invention according to the claims. Although a plurality of features are described in the embodiment, not all of the plurality of features are essential to the invention, and the plurality of features may be arbitrarily combined. Further, in the attached drawings, the same or similar configurations are designated by the same reference numbers, and duplicate description is omitted.

[Embodiment 1]
FIG. 1 is a block diagram illustrating the configuration of each part related to the creation of an electrostatic latent image of the electrophotographic color image forming apparatus according to the first embodiment of the present invention.

This color image forming apparatus has an image forming unit 101 and an image processing unit 102, and the image processing unit 102 generates bitmap image information, and the image forming unit 101 is placed on a recording medium (sheet) based on the bitmap image information. Perform image formation. A description of each part of FIG. 1 will be described later.

FIG. 2 is a schematic cross-sectional view illustrating an image forming unit 101 of the color image forming apparatus according to the first embodiment. Here, a cross-sectional view of a color image forming apparatus using a tandem electrophotographic method of a plurality of colors using the intermediate transfer body 28, that is, four colors of cyan, magenta, yellow, and black (hereinafter, CMYK) is shown. There is.

The image forming unit 101 controls the exposure by the laser according to the exposure time based on the data processed by the image processing unit 102, forms an electrostatic latent image by the laser beam, and further develops the electrostatic latent image. To form a monochromatic toner image. Then, these monochromatic toner images are superposed to form a multicolor toner image, and the multicolor toner image is transferred to the recording medium 11 of FIG. 2 to fix the multicolor toner image on the recording medium.

The 23Y, 23M, 23C, and 23K in FIG. 2 are injection chargers, and are configured to include four to charge the photoconductors 22Y, 22M, 22C, and 22K for each color in the order of Y, M, C, and K. Here, Y, M, C, and K correspond to the above-mentioned four colors, respectively, and the portion for forming an image corresponding to each color is indicated by Y, M, C, and K added after the reference number. , When these parts are collectively described, they will be described with reference numbers not including Y, M, C, and K. Each injection charger includes sleeves 23YS, 23MS, 23CS, 23KS.

The photoconductors 22Y, 22M, 22C, and 22K rotate by transmitting the driving force of a drive motor (not shown), and the drive motor rotates the photoconductors 22Y, 22M, 22C, and 22K in a counterclockwise direction according to the image forming operation. Rotate to. The exposure unit irradiates the photoconductors 22Y, 22M, 22C, and 22K with laser light from the scanning unit (light source) 24Y, 24M, 24C, and 24K, and selectively exposes the surfaces of the photoconductors 22Y, 22M, 22C, and 22K. As a result, it is configured to form an electrostatic latent image.

The developing devices 26Y, 26M, 26C, and 26K of FIG. 2 are provided with four developing devices that develop each color of Y, M, C, and K in order to visualize the electrostatic latent image. Each developer is provided with sleeves 26YS, 26MS, 26CS, 26KS. Each developer 26 is removable.

The intermediate transfer member 28 in FIG. 2 rotates clockwise to receive a monochromatic toner image from the photoconductor 22. With the rotation of the photoconductors 22Y, 22M, 22C, 22K and the primary transfer rollers 27Y, 27M, 27C, 27K arranged to face each other, each photoconductor 22 corresponds to each color to the intermediate transfer body 28. The monochromatic toner image is transferred. At this time, by applying an appropriate bias voltage to the primary transfer roller 27 and making a difference between the rotation speed of the photoconductor 22 and the rotation speed of the intermediate transfer body 28, the monochromatic toner image is efficiently transferred onto the intermediate transfer body 28. Will be done. This is called primary transcription.

Further, the monochromatic toner images for each CMYK color are superimposed on the intermediate transfer body 28. The superimposed multicolor toner image is conveyed to the position of the secondary transfer roller 29 as the intermediate transfer body 28 rotates. At the same time, the recording medium 11 is conveyed from the paper feed tray 21 (21a, 21b) to the position of the secondary transfer roller 29, and the multicolor toner image on the intermediate transfer body 28 is transferred to the recording medium 11 such as paper. At this time, the toner image is electrostatically transferred by applying an appropriate bias voltage to the secondary transfer roller 29. This is called secondary transcription. The secondary transfer roller 29 comes into contact with the recording medium 11 at the position indicated by 29a while transferring the multicolor toner image onto the recording medium 11, and is separated from the position indicated by 29b after the transfer treatment. In the following description, the recording medium 11 is collectively referred to as paper.

When this paper is conveyed to the position of the secondary transfer roller 29, a slight skew occurs. Due to this skewing, the transferred image may be skewed on the paper. Normally, this skew is so small that it is hard to notice, but when cutting a double-sided print, it can be recognized as a misalignment of the image on the front and back.

The fixing device 31 is for pressing the fixing roller 32 that heats the recording medium 11 and the recording medium 11 against the fixing roller 32 in order to melt and fix the multicolor toner image transferred to the recording medium 11 on the recording medium 11. A pressure roller 33 is provided. The fixing roller 32 and the pressure roller 33 are formed in a hollow shape, and heaters 34 and 35 are built in, respectively. The fixing device 31 conveys the recording medium 11 holding the multicolor toner image by the fixing roller 32 and the pressure roller 33, and fixes the toner image on the recording medium 11 by applying heat and pressure.

After the toner image is fixed in this way, the recording medium 11 is then discharged to a paper discharge tray (not shown) by a discharge roller (not shown) to end the image forming operation. The cleaning unit 30 removes and cleans the toner remaining on the intermediate transfer body 28, and remains after transferring the four-color multicolor toner image formed on the intermediate transfer body 28 to the recording medium 11. Waste toner is stored in a cleaner container.

Next, with reference to FIGS. 3, 4, and 5, the profile characteristics of the scanning lines for each color of the color image forming apparatus according to the first embodiment will be described.

FIG. 3 is a diagram showing an example of profile characteristics of the image forming apparatus.

FIG. 3A is a diagram showing a region shifted upward with respect to the scanning direction of the laser, and FIG. 3B is a diagram showing a region shifted downward with respect to the scanning direction of the laser.

In FIG. 3, 301 shows an ideal scanning line and shows the characteristics when scanning is performed perpendicular to the rotation direction of the photoconductor 22.

The profile characteristics in the following description are performed on the premise of correction in the direction to be corrected by the image processing unit 102, that is, in the forward direction with respect to the profile, but the definition as the profile characteristics is not limited to this. Absent. That is, it may be defined as the deviation direction of the image forming unit 101, and the image processing unit 102 may be configured to perform correction in the opposite direction.

FIG. 4 is a diagram showing a correlation between the direction in which the image processing unit 102 should be corrected and the deviation direction of the image forming unit 101 based on the profile definition.

For example, in the case of the bending characteristic as shown in FIG. 4A, the profile characteristic of the image forming unit 101 is as shown in FIG. 4B in the opposite direction. On the contrary, in the case of the bending characteristic as shown in FIG. 4C, the profile characteristic of the image forming unit 101 is as shown in FIG. 4D in the opposite direction.

FIG. 5 is a diagram illustrating an example of how to retain profile characteristic data.

For example, as shown in FIG. 5, the pixel position in the main scanning direction of the transfer point and the direction of the change until the next transfer point are maintained. Specifically, taking FIG. 5 as an example, transfer points P1, P2, P3, ..., Pm are defined for the profile characteristic of (A). The definition of each transfer point is a point at which a pixel shift occurs in the sub-scanning direction, and the direction may change upward or downward until the next transfer point.

For example, the transfer point P2 is a point at which the transfer should be performed upward until the next transfer point P3. Therefore, the transfer direction in P2 is upward (↑) as shown in FIG. 5 (B). Similarly, in P3, the direction is upward (↑) until the next transfer point P4. The transfer direction at the transfer point P4 is downward (↓) unlike the previous directions. As a method of holding the data in this direction, for example, if the data indicating the upward direction is "1" and the data indicating the downward direction is "0", the data is as shown in FIG. 5 (C). In this case, the number of data to be held is only the same as the number of transfer points, and if the number of transfer points is m, the number of bits to be held is also m bits.

Reference numeral 302 in FIG. 3A is an inclination due to the positional accuracy and diameter deviation of the photoconductor 22 and the positional accuracy of the optical system in the scanning unit 24 (24C, 24M, 24Y, 24K) of each color shown in FIG. And the actual scanning line where the bend occurred. The image forming apparatus has different profile characteristics for each recording device (printer engine), and further, in the case of a color image forming apparatus, the characteristics differ for each color.

Next, with reference to FIG. 3A, a transfer point in a region where the laser scanning direction is shifted upward will be described.

The transfer point in the first embodiment means a point at which a deviation of one pixel is accumulated in the sub-scanning direction and the bitmap image data needs to be shifted (shifted) by one pixel. That is, in FIG. 3A, P1, P2, and P3, which are points shifted by one pixel in the sub-scanning direction on the upward bending characteristic 302, correspond to transfer points. In addition, in FIG. 3A, it is described assuming that P0 is used as a reference. As can be seen from FIG. 3 (A), the distances (L1, L2) between the transfer points become shorter in the region where the bending characteristic 302 changes rapidly, and become longer in the region where the bending characteristic 302 changes slowly. ..

Next, with reference to FIG. 3B, a transfer point in a region shifted downward in the laser scanning direction will be described. Even in the region showing the characteristic of being shifted downward, the definition of the transfer point indicates the point shifted by one pixel in the sub-scanning direction. That is, in FIG. 3B, Pn and Pn + 1, which are points shifted by one pixel in the sub-scanning direction on the downward bending characteristic 302, correspond to transfer points. Also in FIG. 3 (B), as in FIG. 3 (A), the distance (Ln, Ln + 1) between the transfer points becomes shorter and gradually changes in the region where the bending characteristic 302 is abruptly changed. It becomes longer in the area where it is.

As described above, the transfer point is closely related to the degree of change in the bending characteristic 302 of the image forming apparatus. Therefore, in an image forming apparatus having a sharp bending characteristic, the number of transfer points is large, and conversely, in an image forming apparatus having a gentle bending characteristic, the number of transfer points is small.

As already explained, the bending characteristics of the image forming apparatus are different for each color, so that the number and positions of transfer points are different for each. This difference between the colors appears as a registration shift in the image in which the toner images of all colors are transferred onto the intermediate transfer body 28.

Next, with reference to FIG. 6, the correction of the paper skew of the image forming apparatus according to the first embodiment will be described.

FIG. 6 is a diagram showing skewing of paper and its measurement position in the image forming apparatus.

During the secondary transfer, if the paper is conveyed in a skewed state, the toner image is not transferred to the correct position on the paper. Therefore, the image processing unit 102 corrects the skew by correcting the image data so that the toner image is tilted and formed so as to match the skew amount of the paper. In the first embodiment, it is assumed that the user measures the amount of skew of the paper and instructs the image forming apparatus. Here, "matching" is a state in which the amount of skew of the paper to be secondarily transferred and the amount of skew of the toner image transferred to the intermediate transfer body 28 match. In this way, the toner image transferred to the paper with the skew amounts matched has the skew amount suppressed with respect to the paper as compared with the case where the skew amounts do not match (no correction). Become.

FIG. 6A is a diagram in which straight lines L3 and L4 extending in the main scanning direction (direction orthogonal to the conveying direction) and straight lines L1 and L2 extending in the sub-scanning direction are formed with respect to the non-oblique paper. FIG. 6B is a diagram in which straight lines L3 and L4 extending in the main scanning direction and straight lines L1 and L2 extending in the sub-scanning direction (conveying direction) are formed with respect to the skewed paper. Here, the straight lines L1 to L4 are test patterns for measuring the skew amount of the paper.

In FIG. 6B, since the straight lines L1 to L4 are formed obliquely with respect to the sides of the paper, the user can confirm that the paper is skewed. The user measures the distances i and j between the straight line L2 and the sides of the paper, and the values are input to the image processing unit 102. The image processing unit 102 stores the input values of the distances i and j as data related to the skew amount of the paper, and uses it for the skew correction of the paper. A sensor may be provided near the paper ejection port of the image forming apparatus, and the distances i and j may be measured by this sensor.

As described above, the amount of paper skew of the image forming apparatus is the amount of paper skew with respect to the intermediate transfer body 28, so that the toner images of all colors are the same, unlike the registration deviation described above. Will be skewed to.

Next, the processing of the image processing unit 102 in the color image forming apparatus according to the first embodiment will be described with reference to FIG.

The image generation unit 104 generates raster image data capable of print processing from print data received from a computer device (not shown) or the like, and outputs it as RGB data for each pixel. The image generation unit 104 may have a reading unit (scanner) inside the color image forming device instead of the image data received from a computer device or the like, and may be configured to handle the image data from the reading unit. The reading unit referred to here includes a CCD (Chaerged Couple Device) or a CIS (Contact Image sencor), and is configured to also have a processing unit that performs predetermined image processing on the read image data. It may have been done. Further, the data may be received from the reader via an interface (not shown) instead of inside the color image forming apparatus. This image data is generated at a resolution corresponding to the exposure resolution of the exposed portion. For example, if the scanning line spacing is 1200 dpi, it is generated as image data having a resolution of 1200 dpi.

The color conversion unit 105 converts RGB data into CMYK data according to the toner color of the image forming unit 102, and stores the CMYK data in a storage unit 106 having a bitmap memory. The storage unit 106 is a first storage unit included in the image processing unit 102, and temporarily stores raster image data to be printed. The storage unit 106 may be configured as a page memory for storing image data for one page, or may be configured as a band memory for storing data for a plurality of lines.

The HT processing units 107C, 107M, 107Y, and 107K execute half-toning processing on the data of each color (for example, 8-bit / 256-gradation image data) output from the storage unit 106. In this way, the input multi-gradation image data is converted into a pseudo halftone (for example, two-gradation) expression represented by the dither method. Here, too, C, M, Y, and K indicate an HT processing unit that performs half-toning processing on the image data corresponding to each color.

FIG. 7 is a diagram illustrating a binarization method by the dither method.

The input continuous gradation image data 700 (for example, 8-bit / 256 gradation image) is divided into N × M (8 × 8 in the figure) blocks. After that, the gradation value of the pixels in the block is compared with the threshold value in the dither matrix 701 in which N × M threshold values of the same size are arranged for each pixel. Then, for example, if the pixel value is larger than the threshold value, "1" is output, and if it is equal to or less than the threshold value, "0" is output. By performing this for all pixels for each size of the matrix, the entire image can be binarized. An example of this binarization is shown by 702.

In an electrophotographic color image forming apparatus, a dither matrix in which a plurality of pixels are concentrated is periodically used in order to realize stable dot reproducibility on paper. On the contrary, if the dots are diffused or if there are many isolated dots that do not have dots around them, stable dot reproducibility cannot be obtained. A screen in which each dot size is small, the dot spacing is narrow, and the number of dots for expressing the same density is large is called a high-line number screen, which is used to enhance the sense of resolution. On the contrary, in the case of a low line number screen, each dot size is large, the dot spacing is wide, the number of dots for expressing the same density is small, and smooth gradation expression is possible.

Then, the process returns to FIG. The image shift units 108C, 108M, 108Y, and 108K perform shift processing (offset) on the image binarized by the above-mentioned half toning process at the coordinate position obtained by the above-mentioned profile characteristics and the amount of skew. Perform geometric correction of the image. The flow of processing in the image shift unit 108 will be described with reference to FIG.

FIG. 8 is a flowchart illustrating processing by the image shift unit 108 according to the first embodiment.

First, in S801, the image shift unit 108 acquires the coordinates at which the pixels should be shifted and the shift direction (direction) thereof. Since the shift obtained by the skew of the paper needs to be corrected in both the main scanning direction and the sub-scanning direction, the coordinates in the sub-scanning direction and the coordinates in the main scanning direction are required. On the other hand, the correction caused by the bending of the laser scanning needs to be corrected only in the sub-scanning direction, and the coordinates are stored in the profile as the coordinates in the main scanning direction and the shift direction thereof. In this way, the correction coordinates and the correction direction due to the bending of the laser scan are obtained from the profile. Further, the skew of the paper is detected by the skew detection described with reference to FIG. 6, and the shift coordinates obtained by the detection are the same for all four CMYK colors. On the other hand, the shift coordinates obtained by the bending of the laser scan are independent coordinates for all four CMYK colors. Therefore, the coordinate information to be shifted needs to be calculated independently for each of the four CMYK colors.

Next, proceeding to S802, the image shift unit 108 performs shift processing in the main scanning direction based on the coordinates in the sub-scanning direction obtained in S801. Similarly, in S803, the image shift unit 108 performs the shift process in the sub-scanning direction based on the coordinates in the main scanning direction. This situation is shown in FIG.

FIG. 9 is a diagram schematically showing an example of an image before and after the shift process.

FIG. 9A shows an image before the shift process. FIG. 9B shows the result of shifting in the main scanning direction in S802 of FIG. 8, and FIG. 9C shows the result of shifting in the sub-scanning direction in S803 of FIG. FIG. 9 shows an example in which shifts are performed three times in each of the main scanning direction and the sub scanning direction. Normally, the step in the shift coordinates (shift position) generated by such shift processing is a step of about 20 μm at the most if the image data has a high resolution such as 1200 dpi, and it is difficult to visually recognize it, which causes a problem. Hateful.

Next, proceeding to S804, the image shift unit 108 performs interpolation processing of less than one pixel on the image data shifted in the sub-scanning direction.

FIG. 10 is a diagram illustrating interpolation of less than one pixel in S804 of FIG.

FIG. 10A shows the amount of deviation of the inclination due to the bending of the laser scanning in the main scanning direction, and here, the deviation occurs in the upward direction by one pixel for every five pixels in the main scanning direction. FIG. 10B shows a bitmap image corrected by this, and in order to correct the above-mentioned upward deviation, the image data is offset every 5 pixels in the main scanning direction so as to cancel the upward deviation. , It is shifted by one pixel in the opposite direction (downward direction). And FIG. 10C shows the corrected bitmap image. Further, in FIG. 10D, by adjusting the gradation value of the image data with the pixels before and after the sub-scanning direction, the correction of less than one pixel with respect to the pixels before and after the main scanning direction of the shift position is performed. Is going. An image of the light emission time obtained as a result is shown in FIG. 10 (D). The image exposed by this is as shown in FIG. 10 (E).

By performing correction of less than one pixel by such a method, it is possible to eliminate an unnatural step at a transfer point boundary caused by a shift in units of one pixel and to smooth the image. This smoothing requires an intermediate level (a pixel representing a density between white and black) for a pixel represented by a binary value. This intermediate level is generated by adjusting the light emission time by PWM, which will be described later.

In this interpolation processing, since the interpolation processing is performed only for the shift caused by the bending of the laser scanning, the correction processing is performed only in the sub-scanning direction. Shifting at different coordinates for each color due to the bending of the laser scanning may cause a problem because even a step of about 20 μm may be visually recognized.

FIG. 11 is a diagram schematically showing white spots caused by shift processing.

For example, as shown in FIG. 11 (A), a case where only one of cyan and black is shifted in a place where two different colors are adjacent to each other is shown in FIG. 11 (B). Here, only black is shown when it is shifted. In such a case, even if printing can be performed at an ideal position, a gap of 10 μm, which is at least half of 20 μm, is formed on the paper. If such a gap occurs at a certain coordinate as a boundary, it may be visible even if the width is 10 μm.

Therefore, in this shift process, the interpolation process is performed so that the gap does not appear only for the shift at the coordinates different for each color. FIG. 11C shows the result of performing such interpolation processing. Further, FIG. 11 (D) shows a state when two adjacent colors overlap.

In this way, by blurring the image by the interpolation processing by smoothing, it is possible to prevent problems such as white spots caused by shifting at different coordinates for each color.

The storage unit 109 is a storage unit included inside the image forming apparatus, and stores the image data to which the shift process has been performed.

The pulse width modulation units 113C, 113M, 113Y, 113K convert the image data for each color read from the storage unit 109 into the exposure time of the scanning units 24Y, 24M, 24C, 24K. Then, the converted image data is image-formed by the photoconductors 22Y, 22M, 22C, 22K of the image forming unit 101. This pulse width modulation makes it possible to control the exposure time of one pixel and generate the intermediate signal described above. That is, for example, by allocating half the exposure time of one pixel to the exposure time, it is possible to form a pixel having a density of 50%.

The scanning unit 24Y, 24M, 24C, 24K emits a pulse width-modulated exposure time signal as a laser to expose the image photoconductors 22Y, 22M, 22C, 22K.

The profile characteristic data described above with reference to FIG. 5 is held inside the image forming unit 101 as a characteristic of the image forming apparatus, and the image processing unit 102 holds the profile held by the image forming unit 101. Processes for image data are executed according to the characteristics (profiles 116C, 116M, 116Y, 116K).

In the above-described first embodiment, it has been described that when the shift process is performed only in the sub-scanning direction, the interpolation process by smoothing is performed by using PWM. The smoothing process using this PWM has an effect of preventing white spots, but at the same time, it may impair the reproducibility of dots. This causes the dots to become unstable due to an intermediate signal of less than 100% generated by PWM, and if this intermediate signal is used frequently, there arises a problem that reproducibility is impaired. This may become apparent as unevenness in a particularly fine region such as a thin line or a small dot region, which makes it difficult to control the intermediate density using PWM. This means that there is no linearity between the minute exposure time and the density, and it is desirable to keep the smoothing using PWM, which can impair the reproducibility of dots, to the minimum necessary.

Further, in the first embodiment, in order to minimize the generation of the intermediate density due to PWM, the interpolation processing using PWM is performed only for the direction in which the shift at the coordinates different for each color occurs. As a result, deterioration in image quality due to shift processing can be minimized.

In the first embodiment, an example in which the shift processing and the interpolation processing are performed only in the sub-scanning direction has been described, but the shift processing and the interpolation processing are not limited to the sub-scanning direction in the sense that only the direction requiring an independent shift for each color is performed. ..

Further, although the exposure by laser scanning is shown as an example, the same effect can be expected for an exposed portion such as an LED that is not subjected to laser scanning. In this exposure, the LEDs are arranged in a row in contact with the photoconductor for the number of pixels on one side of the image, and the rotating photoconductor is exposed. In this case, it is not the bending of the laser scanning, but the correction of the deviation for each color that occurs due to the assembly accuracy of the LED rows and the like.

Further, although an image having the same resolution in the main scanning direction and the sub scanning direction has been described as an example, the present invention is not necessarily limited to this. For example, when the resolution in the sub-scanning direction is higher than that in the main scanning direction, the interpolation processing may not be performed in the sub-scanning direction, and the resolution may be switched to the correction processing. On the contrary, the interpolation process may be performed in the direction of low resolution. In this case as well, it is possible to minimize the smoothing of the image using PWM.

Further, in the first embodiment, the shift process that approximates the skew of the paper and the bending of the laser has been described, but when the coordinates are converted into different directions for each color and the same direction for each color, interpolation is performed according to each direction. It is not limited to this as long as it switches the presence or absence. For example, in pixel interpolation from affine transformation including enlargement, reduction, rotation, etc., interpolation is not performed for interpolation in the same direction for each color (nearest neighbor interpolation), and interpolation for different directions for each color. Interpolation may be performed (bilinear, etc.).

[Embodiment 2]
In the above-described first embodiment, an embodiment in which the interpolation processing is performed or not performed is described as an example, focusing on the different directions for each color and the shift to the same direction for each color. This is to achieve both whiteout and dot reproducibility by stopping blurring on one side and minimizing the number of intermediate signals.

On the other hand, in the second embodiment, a case where the dot reproducibility is different between the direction parallel to the scanning direction of the laser and the direction orthogonal to the scanning direction will be described as an example. The linearity of the emission time and density of PWM selects an advantageous direction.

In the second embodiment, the description regarding the configuration of the image forming apparatus and the description of the overlapping parts, which are the same as those in the first embodiment, will be omitted, and the relationship between the scanning direction and the dot reproducibility, which are the points, will be described. In the second embodiment, unlike the first embodiment described above, it is assumed that the laser is mechanically adjusted so as not to have an inclination or a bend for each color. That is, the case where only the skew correction of the paper that needs to be shifted at the same position for each color is corrected will be described as an example.

First, the details of the shift processing by the image shift units 108C, M, Y, and K will be described. However, since there is no difference from the contents described in the above-described first embodiment with respect to the other parts, the description thereof will be omitted. Further, since the processing flow is the same as that of FIG. 8 of the first embodiment, the description thereof will be omitted.

As described in the first embodiment, the interpolation process is not always necessary because the colors are shifted at the same position for each color, but a step due to the shift may be noticeable depending on the type of the input image. For example, when drawing a high-density repeating pattern, the shift position may be visible as a streak rather than a step. Therefore, it may be better to perform interpolation processing even if the shift is performed at the same position for each color. However, since interpolation using an intermediate signal using PWM may cause a problem of deterioration of dot reproducibility as described above, it is preferable to set the interpolation direction to one side in order to minimize the deterioration of image quality. .. Therefore, the optimum direction is selected as the direction of interpolation, and interpolation is performed only in that direction.

FIG. 12 is a diagram schematically showing a line parallel to the laser scanning in the second embodiment and a state of the shift thereof. FIG. 13 is a diagram schematically showing a line orthogonal to the laser scan and a state of its shift.

FIG. 12A shows an example of a line having a width of 2 pixels parallel to the laser scanning and an image shifted in the sub-scanning direction at the center thereof. FIG. 13A shows an example of a two-pixel line perpendicular to the laser scan and an image shifted to the main scan at the center thereof.

12 (B) and 13 (B) show the results of interpolation processing using PWM with respect to FIGS. 12 (A) and 13 (A), respectively. An intermediate signal is generated for each. For example, in line 1202 of FIG. 12B, a signal whose emission time is narrowed down to 75% with respect to one pixel is generated, and in line 1203, a signal is generated which is narrowed down to 25%. ing. Further, the line 1204 has a light emission time of 75% like the line 1202, and the line 1205 has a light emission time of 25% like the line 1203. Together with a 100% light emitting line such as line 1201, the step is suppressed while preserving the width of two pixels of the line shown in FIG. 12 (A).

Similarly, in FIG. 13B, these intermediate signals are generated, but they are oriented vertically.

12 (C), (D), and (E) of FIG. 12 are diagrams showing the light emission time of each line with the horizontal axis as the PWM pulse light emission time with respect to the laser scanning direction. 12 (C) shows the light emission time of line 1201 (100%), FIG. 12 (D) shows the light emission time of line 1202 (75%), and FIG. 12 (E) shows the light emission time of line 1203 (25%). Shown. The width shown by 1206 in FIG. 12C corresponds to the light emission time of one pixel. In FIG. 12D, the light emission time of one pixel is 75%, and in FIG. 12E, the light emission time of one pixel is 25%. The line shown in FIG. 12 (B) has a relationship in which the lines formed by the light emission times of FIGS. 12 (D) and 12 (E) are vertically interchanged with the line 1201 as the boundary with the shift position 1210 as a boundary. The total line width can be saved and smooth interpolation processing can be performed.

Similarly, in FIGS. 13 (C), (D), (E), and (F), the horizontal axis is the PWM emission time with respect to the laser scanning direction, and the sections 1301, 1302, 1303 of FIGS. 13 (A) and 13 (B) are used. The emission time corresponding to 1304 is shown. That is, the section 1301 of FIG. 13 (A) is shown in FIG. 13 (C), the section 1302 of FIG. 13 (B) is shown in FIG. 13 (D), and the section 1303 of FIG. 13 (B) is shown in FIG. 13 (E). And the section 1304 of FIG. 13 (B) corresponds to FIG. 13 (F), respectively.

In FIG. 13, unlike the above-mentioned FIG. 12, the pulse width changes in pixel units during the main scan. Therefore, similarly, even if the light emission is 100% in the center, there is a case of FIG. 13 (D) in which there is a 25% pulse before scanning and a case of FIG. 13 (E) in which there is a 75% pulse in front of scanning. , Occurs on the same line.

In order to accurately switch the pulse width on a pixel-by-pixel basis, it is necessary for the laser emission response to follow immediately, and in reality, it is not possible to emit light with an ideal width as shown in FIG. Can not. That is, the light emission of the pulse shown in FIG. 13 (D) that emitted light in the order of 25%, 100%, and 75% and the pulse emission of FIG. 13 (E) that emitted light in the order of 25%, 100%, and 75% It is not possible to emit light for the same time in total, and the line density is different. As a result, the image becomes such that the line density repeats shading with the shift coordinates as the boundary, and the image quality is deteriorated by the interpolation processing.

On the other hand, in FIGS. 12 (D) and 12 (E), even if the response cannot be immediately followed, the conditions are the same for the upper and lower lines, so that the difference in line density with respect to the shift position 1210 is Hard to come out. Even if the line 1202 does not follow and is slightly thin, the line 1202 is also thin, so that the line density is the same.

In such a situation where the laser emission response is not immediately followed, it is not preferable to perform interpolation processing in that direction. In the image shift processing unit 108, in the shift in the direction orthogonal to the laser scan, interpolation processing using PWM is performed in addition to the shift, and in the shift in the direction parallel to the laser scan, only the shift is performed. Interpolation using PWM is not performed. That is, since the scanning direction of the laser is horizontal, the shift in the vertical direction performs interpolation, and the shift in the horizontal direction does not perform interpolation.

According to the second embodiment, in order to minimize the generation of the intermediate concentration due to the PWM, the PWM is used only in the direction orthogonal to the laser scan when the laser emission response is not immediately followed. Perform interpolation processing. By doing so, it is possible to minimize the deterioration of image quality due to the shift process.

In the second embodiment, an example in which the shift process and the interpolation process are performed only in the sub-scanning direction has been described, but when time-division light emission (exposure) using PWM is performed in the scanning direction of the light emitter, the direction is not necessarily the same. It is not limited to the horizontal direction. Further, in the second embodiment, the exposure by laser scanning is shown as an example, but the vertical-horizontal relationship is reversed for the exposed portion that is not scanned such as LED exposure. In that case, the same effect can be expected.

(Other embodiments)
The present invention supplies a program that realizes one or more functions of the above-described embodiment to a system or device via a network or storage medium, and one or more processors in the computer of the system or device reads and executes the program. It can also be realized by the processing to be performed. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

The present invention is not limited to the above embodiments, and various modifications and modifications can be made without departing from the spirit and scope of the present invention. Therefore, in order to make the scope of the present invention public, the following claims are attached.

22Y to 22K ... Photoreceptor, 24Y to 24K ... Scanning unit, 101 ... Image forming unit, 102 ... Image processing unit, 108Y to 108K ... Image shifting unit, 109 ... Storage unit, 113Y to 113K ... Pulse width modulation unit, 116Y to 116K ... Profile

Claims (11)

An image forming apparatus that forms an image by scanning light from a plurality of light sources.
An acquisition means for acquiring the amount of inclination of each light in the scanning direction,
A first shift means that shifts the image data to be formed in pixel units in a direction orthogonal to the scanning direction of the light according to the amount of inclination.
A second shift means for shifting the image data to be formed in pixel units in the scanning direction and / or in a direction orthogonal to the scanning direction according to the amount of skew of the paper.
An interpolation means that performs interpolation processing on the image data shifted by the first shift means in units of less than one pixel, and
An image forming apparatus characterized by having. Further, it has a storage means for storing the amount of inclination of each light in the scanning direction.
The image forming apparatus according to claim 1, wherein the acquisition means acquires the amount of inclination of each light in the scanning direction from the storage means. The interpolation means is characterized in that the interpolation processing is executed in units of less than one pixel by driving the plurality of light sources with a signal PWM (pulse width modulation) based on the image data. Or the image forming apparatus according to 2. The image forming apparatus according to any one of claims 1 to 3, further comprising a means for obtaining the skew amount of the paper. The plurality of light sources correspond to a plurality of colors of the image data.
The image forming apparatus according to any one of claims 1 to 4, wherein the shift by the first shift means is different for each of the image data of the plurality of colors. The image forming apparatus according to claim 5, wherein the shift by the second shift means is the same for the image data of the plurality of colors. The interpolating means according to any one of claims 1 to 6, wherein the interpolation means corrects less than one pixel with respect to the pixels before and after the position shifted by the first shifting means in the main scanning direction. The image forming apparatus described. Further having a binarization means for binarizing multi-gradation image data,
The first shift means and the second shift means according to any one of claims 1 to 7, wherein the shift means performs the shift on the image data binarized by the binarization means. Image forming device. The image forming apparatus according to any one of claims 1 to 8, wherein the light from the plurality of light sources is laser light. A control method for controlling an image forming apparatus that forms an image by scanning light from a plurality of light sources.
The acquisition process for acquiring the amount of inclination of each light in the scanning direction,
A first shift step of shifting the image data to be formed in pixel units in a direction orthogonal to the scanning direction of the light according to the amount of inclination.
A second shift step of shifting the image data to be formed in pixel units in the scanning direction or in a direction orthogonal to the scanning direction according to the amount of skew of the paper.
An interpolation step in which the image data shifted by the first shift step is interpolated in units of less than one pixel, and
A control method characterized by having. A program for causing a computer to function as each means of the image forming apparatus according to any one of claims 1 to 9.

Images