Image forming apparatus

Abstract

To prevent the occurrence of a void in a contour part of an image in a scan direction center part even in a case of correcting the density of an image subjected to processing of adding an addition pixel to the contour part of the image in an image formation apparatus using an optical system that does not have the fθ characteristic.SOLUTION: An image formation apparatus comprises: scan means which scans a laser beam to a photoreceptor; an optical system in which the scan speeds at which the laser beams move on the surface of the photoreceptor in the main-scanning direction do not become the constant speed; light amount correction means which corrects a light amount of a pixel according to the position in the main-scanning direction of the pixel constituting the image; image processing means which performs processing of adding an addition pixel immediately before or immediately after the pixel of the contour part of the image before correcting by the light amount correction means; and smoothing processing means which performs processing of adjusting the density value of the addition pixel being the object pixel in the direction to bring the density value closer to the density value of the front pixel or rear pixel according to the density value of the front pixel immediately before and the rear pixel immediately after the object pixel in the main-scanning direction before the light amount correction means corrects the image added with the addition pixel by the image processing means.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to an electrophotographic image forming apparatus such as a copying machine or a printer having an optical scanning device for exposing a photoreceptor to form an electrostatic latent image.

Background Art

[0002] An electrophotographic image forming apparatus has an optical scanning device for exposing a photoreceptor to form an electrostatic latent image. The optical scanning device uses a scanning lens having an fθ characteristic that forms a laser beam on the surface of the photoreceptor so that the laser beam moves on the surface of the photoreceptor at a constant speed when a rotating polygon mirror rotates at a constant angular velocity.

[0003] However, a scanning lens having an fθ characteristic is larger in size and higher in cost than a scanning lens not having an fθ characteristic. Therefore, in order to reduce the size and cost of the image forming apparatus, an image forming apparatus is proposed that is provided with an optical scanning device that does not use a scanning lens or uses a scanning lens not having an fθ characteristic to scan a laser beam at a non-constant scanning speed.

[0004] In Patent Document 1, an image forming apparatus with less uneven image density is proposed even when using an optical scanning device in which the scanning speed of the laser beam on the surface of the photoreceptor is not constant. Specifically, the exposure time of the latent image corresponding to each pixel is corrected (partial magnification correction) so that the width in the scanning direction (moving direction) of the laser beam is constant. Further, in order to compensate for the shortage of the exposure amount per latent image due to the correction of the exposure time of the latent image corresponding to each pixel, the luminance of the laser beam for each latent image is corrected (partial light amount correction).

[0005] In Patent Document 2, in both the image exposure method (IAE) and the background exposure method (BAE), an image processing (thickening process) method is proposed that keeps the width of the white line constant regardless of the process conditions for each color in the image forming apparatus.

[0006] In Patent Document 3, in an optical scanning device including an imaging optical system composed of only one imaging optical element, parameters of the imaging optical system are proposed to achieve good imaging performance of the imaging optical element while realizing cost reduction and miniaturization.

[0007] In Patent Document 4, an image forming apparatus has been proposed that can correct the exposure amount per unit area on a photoreceptor to be substantially uniform by correcting image data based on correction data corresponding to the position in the scanning direction of laser light.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0009] In Patent Document 2, in both the image exposure method (IAE) and the background exposure method (BAE), an image processing (thickening process) technology is provided to keep the thickness of the white line constant by thickening thin lines such as white characters with additional pixels.

[0010] Here, consider the case where an image subjected to the thickening process as in Patent Document 2 is subjected to partial magnification correction processing and partial light amount correction processing as in Patent Document 1 or Patent Document 4 in an image forming apparatus using an optical system that does not have fθ characteristics.

[0011] In this case, the laser light imaged on the photoreceptor through a scanning lens without fθ characteristics has a large light amount at the center in the scanning direction and a small light amount at the ends in the scanning direction due to the difference in optical path length and the difference in transmittance due to the incident angle and exit angle. Therefore, when correcting the density (light amount) of the image subjected to the thickening process according to the position in the scanning direction (partial light amount correction), the light amount correction value for the image at the center in the scanning direction becomes smaller than the light amount correction value for the image at the ends in the scanning direction. Therefore, when correcting the density (light amount) of the image subjected to the thickening process according to the position in the scanning direction (partial light amount correction), white gaps may occur in the contour portion (thin line boundary portion) of the thin lines at the center in the scanning direction.

[0012] Therefore, an object of the present invention is to prevent white gaps from occurring in the contour portion of the image at the center in the scanning direction even when correcting the density of an image obtained by performing a process of adding additional pixels to the contour portion of the image in an image forming apparatus using an optical system without fθ characteristics.

Means for Solving the Problems

[0013] The present invention Typical configuration includes a photoreceptor, a light source that outputs laser light based on an input image, scanning means that scans the laser light output from the light source with respect to the photoreceptor, and an optical system provided between the scanning means and the photoreceptor in the optical path of the laser light, the laser light passing through the optical system, and the scanning speed at which the laser light moves on the surface of the photoreceptor in the main scanning direction along the axis of the photoreceptor is not constant; light amount correction means for correcting the light amount of the pixels according to the position of the pixels constituting the image in the main scanning direction; and before being corrected by the light amount correction means, As the contour portion of the image, a contour portion of an image in which black pixels equal to or less than a predetermined number are continuous in the main scanning direction is detected, and an additional pixel is added immediately before or after the pixel of the contour portion of the image to thicken the image in the main scanning direction, and a thickening process is performed image processing means, and before the image with the additional pixels added by the image processing means is corrected by the light amount correction means, the density value of the additional pixel to be the target pixel is adjusted in a direction closer to the density value of the previous pixel immediately before the target pixel or the next pixel immediately after the target pixel according to the density values of the previous pixel and the next pixel in the main scanning direction. Among them, the pixels of the thin line contour portion before the thickening process smoothing processing means for performing a process of adjusting in a direction closer to the density value of the previous pixel or the next pixel, and is characterized by comprising the same.

Effects of the Invention

[0014] According to the present invention, even when correcting the density of an image obtained by performing a process of adding additional pixels to a contour portion of an image in an image forming apparatus using an optical system that does not have fθ characteristics, it is possible to prevent white gaps from occurring in the contour portion of the image at the center in the scanning direction.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Modes for Carrying Out the Invention

[0016] Hereinafter, with reference to the drawings, preferred embodiments of the present invention will be exemplarily described in detail. However, the dimensions, materials, shapes, relative arrangements, etc. of the components described in the following embodiments should be appropriately changed according to the configuration of the apparatus to which the present invention is applied and various conditions, and are not intended to limit the scope of the present invention only thereto.

[0017] 〔Example 1〕 The image forming apparatus 100 of this embodiment will be described with reference to FIG. 1. FIG. 1 is a schematic cross-sectional view of the image forming apparatus. Here, as an example of the image forming apparatus, a digital monochrome printer that forms an image using black toner will be exemplified and described.

[0018] (Image forming apparatus) The image forming apparatus 100 is provided with an image forming unit 101 for forming an image.

[0019] The image forming unit 101 is provided with a photosensitive drum 102 which is a photoreceptor. Around the photosensitive drum 102, process means acting on the photoreceptor are provided. Here, as the process means, a charging device 103 which is a charging means, an optical scanning device 104 which is an exposure means, and a developing device 105 which is a developing means are provided. Further, a cleaning device 106 which is a cleaning means is provided as the process means.

[0020] Also, below the photosensitive drum 102, an endless belt-shaped intermediate transfer belt 107 which is an intermediate transfer member is disposed. The intermediate transfer belt 107 is stretched over a driving roller 108 and driven rollers 109 and 110, and rotates in the direction of arrow B in the figure during image formation.

[0021] Also, at a position facing the photosensitive drum 102 via the intermediate transfer belt 107, a primary transfer device 111 that abuts on the inner peripheral surface of the intermediate transfer belt 107 is provided. And a primary transfer portion is formed as a nip portion between the intermediate transfer belt 107 with each primary transfer device 111 abutted thereon and each photosensitive drum 102. Further, the image forming apparatus 100 of this embodiment is provided with a fixing device 113 for fixing the toner image on the recording medium S.

[0022] Here, the image forming process from the charging process to the developing process of the image forming apparatus 100 will be described.

[0023] First, the surface of the photosensitive drum 102 is charged by the charging device 103 of the image forming unit 101. The charged photosensitive drum 102 is exposed by the laser light emitted from the optical scanning device 104. As a result, an electrostatic latent image is formed on the surface of the photosensitive drum 102 that is rotationally driven. Thereafter, the electrostatic latent image is developed as a black toner image by the developing device 105.

[0024] Next, the image forming process after the transfer step will be described. The primary transfer device 111 applies a transfer bias to the intermediate transfer belt 107. As a result, the black toner image formed on the photosensitive drum 102 of the image forming unit is transferred (primary transfer) to the intermediate transfer belt 107 in the primary transfer section. Note that the toner remaining on the photosensitive drum 102 after transfer is removed by the cleaning device 106.

[0025] The toner image transferred onto the intermediate transfer belt 107 is transferred (secondary transfer) to the recording medium S conveyed to the secondary transfer section from the manual feed cassette 114 or the feed cassette 115 by the secondary transfer device 112. Then, the recording medium S onto which the toner image has been transferred is heat-fixed by the fixing device 113 and discharged by the discharge section 116. In this way, a monochrome image is obtained on the recording medium S.

[0026] (Photosensitive Drum and Optical Scanning Device) FIG. 2 shows the configuration of the photosensitive drum 102, the optical scanning device 104, and the control unit of the optical scanning device 104.

[0027] The optical scanning device 104 includes a light source 201, a collimator lens 202, a cylindrical lens 203, a deflector 204, and an imaging lens 205.

[0028] The light source 201 is a light source that generates laser light (optical beam) with a beam number of 1 based on the input image data. In this embodiment, the light source 201 is a light source in which laser diodes (light emitting elements) with a beam number of 1 are arranged, but the number of light emitting elements is not limited to this, and it may be a multi-beam light source with a beam number of 2 or more. The collimator lens 202 shapes the laser light emitted from the light source 201 into parallel light. The cylindrical lens 203 condenses the laser light that has passed through the collimator lens 202 in the sub-scanning direction (the direction corresponding to the rotation direction of the photosensitive drum). The deflector 204 is a scanning means that scans the laser light output from the light source 201 in the main scanning direction (the direction along the axis z of the photosensitive drum 102) with respect to the photosensitive drum 102. The deflector 204 is a polygon mirror (rotating multi-faceted mirror) that is attached to the motor shaft 206 of a motor that is a drive source and is driven by the motor to rotate. Here, the polygon mirror is exemplified by a mirror with 4 reflecting surfaces, but it is not limited to this, and other numbers of surfaces may be used.

[0029] Furthermore, the optical scanning device 104 includes a Beam Detector (hereinafter referred to as BD) 207, which is a signal generation means that detects the laser light deflected by the deflector 204 and outputs a horizontal synchronization signal (hereinafter referred to as BD signal) in response to detecting the laser light. The control unit 208 reads the input image data 212 in the memory 209 and drives the light source 201 via the laser drive unit 210 with a PWM waveform corresponding to the read input image data 212. The laser light output from the light source 201 driven by the laser drive unit 210 is deflected by the deflector 204 via a lens and scanned in the main scanning direction on the photosensitive drum 102 for exposure. An imaging lens 205 is provided between the deflector 204 and the photosensitive drum 102 in the optical path of the laser light. The imaging lens 205 is an optical system through which the laser light passes and is a lens that does not have so-called fθ characteristics. That is, the imaging lens 205 is an optical system (hereinafter also referred to as a non-uniform scanning optical system) in which the scanning speed at which the laser light moves along the main scanning direction along the axis z of the photosensitive drum 102 on the surface of the photosensitive drum 102 is not constant. The optical scanning device 104 is positioned so as to scan the photosensitive drum 102 with the laser light parallel to the axis z of the photosensitive drum 102. Each time the reflecting surface of the polygon mirror of the deflector 204 scans once in the main scanning direction on the photosensitive drum, scanning lines corresponding to the number of light-emitting elements of the light source 201 are formed simultaneously.

[0030] (Image Processing) FIG. 3 is a diagram showing a case where laser light is emitted one pixel at a time from the light source 201 at the end and the center in the scanning direction, deflected by the deflector (polygon mirror) 204, scanned on the photosensitive drum 102, and exposed.

[0031] In FIG. 3, the PWM control waveforms W1, W2, the main scanning LSF (Line Spread Function) profiles P1, P2 on the photosensitive drum 102, and the spot diameters R1, R2 at the end and the center in the scanning direction are shown. Here, for simplicity of explanation, the collimator lens 202 and the cylindrical lens 203 in the optical path are omitted.

[0032] In FIG. 3, the main scanning LSF profiles P1 and P2 indicate the total exposure amount (integrated light amount) of the spot profile formed on the photosensitive drum 102 while moving the irradiation position in the main scanning direction while emitting light from the light source 201 at a predetermined luminance and time.

[0033] In FIG. 3, it is assumed that the control unit 208 has a partial magnification correction function and a partial light amount correction function, which will be described later.

[0034] In FIG. 3, the latent image exposed by the laser light for one pixel at the end in the scanning direction and the center in the scanning direction uses the same density in the input image data 212, and it is assumed that partial magnification correction and partial light amount correction are performed so that the same density is obtained when this latent image is developed and fixed. For example, it is assumed that the input image data 212 at the end in the scanning direction and the center in the scanning direction are both 600 dpi and 1-bit gradation per pixel, and the density value is 1.

[0035] Here, the control of the PWM control waveforms W1 and W2 by the partial magnification correction function and the partial light amount correction function of the control unit 208 will be described.

[0036] First, the modulation of the scanning time at the end in the scanning direction (here, the left end on the scanning start side) and the center in the scanning direction by the partial magnification correction function will be described. Let the scanning time for one pixel at the end in the scanning direction be t0 [ns], and the scanning time for one pixel at the center in the scanning direction be t1 [ns]. The scanning time per pixel at each position in the main scanning direction is measured and determined as the partial magnification characteristic of a non-uniform scanning optical system, which is an optical system without fθ characteristics. However, detailed description is omitted here.

[0037] In this embodiment, due to the characteristics of the non-uniform scanning optical system, the scanning speed at the end of the scanning direction is faster than that at the center of the scanning direction. Therefore, a partial magnification correction process is performed to shorten the scanning time per pixel. Also, at the center of the scanning direction, the scanning speed is slower than at the end of the scanning direction. Therefore, a partial magnification correction process is performed to lengthen the scanning time per pixel. For example, at the end of the scanning direction, the scanning time t0 = 38.4 [ns] for one pixel, and at the center of the scanning direction, the scanning speed t1 = 50 [ns].

[0038] Next, the light amount correction at the end of the scanning direction (here, the left end which is the scanning start side) and at the center of the scanning direction by the partial light amount correction function will be described. Let the light amount correction value for one pixel at the end of the scanning direction be N0 [%], and the light amount correction value for one pixel at the center of the scanning direction be N1 [%].

[0039] In this embodiment, due to the characteristics of the scanning optical system, at the end of the scanning direction, the laser light is incident obliquely on the imaging lens 205, resulting in a low transmittance and a long optical path length. Therefore, the exposure amount on the photosensitive drum 102 is lower compared to the center of the scanning direction. This is measured as a difference in peak light amount between the end of the scanning direction and the center of the scanning direction, like the LSF profiles P1 and P2 shown in FIG. 3. Therefore, as the partial light amount correction, the light amount correction value at the end of the scanning direction is set high, and the light amount correction value at the center of the scanning direction is set low. The light amount correction values at the positions in each main scanning direction are measured and determined as the partial light amount characteristics of a non-uniform scanning optical system that does not have an fθ characteristic. However, detailed explanations are omitted here.

[0040] In this embodiment, for example, the light amount correction value N0 = 100 [%] at the end of the scanning direction, and the light amount correction value N1 = 70 [%] at the center of the scanning direction.

[0041] Furthermore, the PWM control waveforms W1 and W2 at the scanning direction end and the scanning direction center will be described. As described above, at the scanning direction end, the scanning time for one pixel is t0 [ns] due to the partial magnification correction function, and the light amount correction value for one pixel is N0 [%]. Therefore, the PWM control waveform outputs one pixel with a pulse width of t0 × N0 [ns]. Since the density value 15 in 4-bit gradation is the pulse width for maximum lighting of one pixel, the PWM control waveform W1 at the scanning direction end outputs with a pulse width of t0 × N0 [ns]. Similarly, the PWM control waveform W2 at the scanning direction center outputs with a pulse width of t1 × N1 [ns].

[0042] Therefore, in this embodiment, the PWM control waveform W1 at the scanning direction end is output with a pulse width of t0 = 38.4 [ns] and N0 = 100%, that is, a pulse width of t0 × N0 = 38.4 [ns]. Similarly, the PWM control waveform W2 at the scanning direction center is output with a pulse width of t1 = 50 [ns] and N1 = 70%, that is, a pulse width of t1 × N1 = 35.0 [ns].

[0043] FIG. 4 is a diagram showing enlarged portions of each pixel at the scanning direction end and the scanning direction center when a part of the input image is thickened. Here, a part of a vertical thin line, which is a thin line in the vertical direction as a part of the input image, is exemplified. The vertical direction is the sub-scanning direction orthogonal to the main scanning direction and is also the rotation direction of the photosensitive drum. Also, the scanning direction end is the left end on the scanning start side in the main scanning direction. Further, FIG. 4 shows the main scanning pixel position n of each pixel, the input pixel density value D nin and the interpolated pixel density value D n , the PWM control waveform W, the LSF profile P, and the main scanning LSF diameter R.

[0044] The thickening process used here is a known process that detects the contour part (thin line boundary part) of a thin line that is a part of the input image, and adds additional pixels immediately before or after the pixels of the contour part of the thin line to thicken the thin line in the main scanning direction. In Patent Document 2, as an example, a process of thickening a horizontal or vertical line of 2 pixels by 10 μm to 40 μm with additional pixels at 1200 dpi and 1 bit is given. In this embodiment, for example, it is assumed that the input image data is 600 dpi, 1 bit, and the additional pixels used in the thickening process are additional pixels with a pulse width of the PWM control waveform W of less than 1 pixel, and the output image data is 600 dpi and 4 bits. In this case, the input pixel density value D nin of the input image data is the density value 0 (white pixel), and the interpolated pixel density value D n of the image data after the thickening process is the density value 0 (white pixel). When the input pixel density value D nin of the input image data is the density value 1 (black pixel), the interpolated pixel density value D n of the image data after the thickening process is the density value 15 (black pixel). In FIG. 4, as a thin line in the vertical direction, a thin line in which three pixels are continuous in the main scanning direction is illustrated. In the contour part of this thin line, a thickening process of adding additional pixels of 10 μm (1 / 4 pixel at 600 dpi) to 40 μm (1 pixel at 600 dpi) to the thin line is performed. In that case, the interpolated pixel density value D n of the additional pixels is a density value of 8 (1 / 2 pixel) or a density value of 4 (1 / 4 pixel) less than 1 pixel.

[0045] Here, as an image in which a predetermined number or less of pixels that are a part of the input image are continuous, a thin line in which three pixels are continuous in the main scanning direction is illustrated, but the number of continuous pixels is not limited to this, and should be appropriately set for each target (image) for which the thickening process is performed.

[0046] Here, attention is paid to each LSF profile P of the central part in the scanning direction (main scanning pixel position n = 3595 to 3601) and the end part in the scanning direction (main scanning pixel position n = 100 to 106) after the thickening process.

[0047] First, at the center of the scanning direction, the LSF profile of the pixels in the contour portion on one side of the thin line after the thickening process (main scanning pixel position 3596) is barely connected to the LSF profile of the adjacent pixel (the pixel at the main scanning pixel position 3597) (the portion shown in P21). However, the LSF profile of the pixels in the contour portion on the other side of the thin line after the thickening process (main scanning pixel position 3600) is not connected to the LSF profile of the adjacent pixel (the pixel at the main scanning pixel position 3599) (the portion shown in P22). Here, the pixels in the contour portion on one side of the thin line after the thickening process (main scanning pixel position 3596) and the pixels in the contour portion on the other side of the thin line (main scanning pixel position 3600) are additional pixels added by the thickening process. The pixel immediately after, which is the adjacent pixel to the additional pixel at the main scanning pixel position 3596 (the pixel at the main scanning pixel position 3597), is a pixel in the contour portion on one side of the thin line detected before the thickening process. The pixel immediately before, which is the adjacent pixel to the additional pixel at the main scanning pixel position 3600 (the pixel at the main scanning pixel position 3599), is a pixel in the contour portion on the other side of the thin line detected before the thickening process.

[0048] On one hand, at the end in the scanning direction, the LSF profile of the pixels of one side of the thin line after the thickening process (main scanning pixel position 101) is connected to the LSF profile of the adjacent pixels (pixels at the main scanning pixel position 102) (the part shown by P11). The LSF profile of the pixels of the other side of the thin line after the thickening process (main scanning pixel position 105) is connected to the LSF profile of the adjacent pixels (pixels at the main scanning pixel position 104) (the part shown by P12). That is, at the end in the scanning direction, the LSF profiles of the pixels of both the one side and the other side of the thin line after the thickening process are connected to the LSF profiles of the adjacent pixels (main scanning pixel positions 102 and 104). Here, the pixels of one side of the thin line after the thickening process (main scanning pixel position 101) and the pixels of the other side of the thin line (main scanning pixel position 105) are additional pixels added by the thickening process. The pixel immediately after, which is the adjacent pixel of the additional pixel at the main scanning pixel position 101 (the pixel at the main scanning pixel position 102), is the pixel of one side of the thin line detected before the thickening process. The pixel immediately before, which is the adjacent pixel of the additional pixel at the main scanning pixel position 105 (the pixel at the main scanning pixel position 104), is the pixel of the other side of the thin line detected before the thickening process.

[0049] When the LSF profile of the additional pixel of the thin line contour is connected to the adjacent pixel of the additional pixel, the following occurs. That is, by synthesizing the LSF profiles of the additional pixel of the thin line contour and its adjacent pixel (adjacent pixel), the thin line contour moves to the center of gravity position where the thin line contour is synthesized, and the possibility that toner is deposited without the thin line contour less than 1 pixel being white is increased. However, when the LSF profile of the additional pixel of the thin line contour is not connected to the adjacent pixel of the additional pixel, the LFS profiles are not synthesized, so the thin line contour does not move, and toner may not be deposited on this unconnected part.

[0050] Furthermore, in the thin line contour portion at the center in the scanning direction, since the light quantity peak of the pixel is higher than that at the end in the scanning direction, in addition to toner not being deposited on the unconnected portion, toner is likely to be deposited on the light quantity peak portion of the pixel. Therefore, in the thin line contour portion at the center in the scanning direction, toner is likely to adhere as isolated dots without being connected to adjacent pixels.

[0051] As described above, no white space occurs between the additional pixel of the thin line contour portion less than one pixel and the adjacent pixel at the end in the scanning direction, and white space where toner is not deposited occurs between the pixel of the thin line contour portion less than one pixel and the adjacent pixel at the center in the scanning direction.

[0052] In order not to cause this white space, the density value of the pixel in the thin line contour portion at the center in the scanning direction may be controlled to be darkened to such an extent that the LSF profile of the pixel is connected to the adjacent pixel.

[0053] Therefore, in this embodiment, a smoothing filter 501 (see FIG. 5) that can change the density value of each pixel constituting the image after the thickening process according to the position of the target pixel in the main scanning direction for density value adjustment is further provided. The smoothing filter 501, which is a smoothing processing means, performs smoothing processing on the image in which additional pixels are added to the thin line contour portion by the thickening processing unit (thickening processing means) 401, which is an image processing means, before being corrected by the light quantity correction rate calculation unit 303, which is a light quantity correction means. The smoothing processing by the smoothing filter 501 is a process of adjusting the density value of the additional pixel to be the target pixel in a direction closer to the density of the previous pixel or the subsequent pixel according to the densities of the previous pixel immediately before the additional pixel and the subsequent pixel in the main scanning direction. More specifically, in this embodiment, before the image in which additional pixels are added to the thin line contour portion by the thickening processing unit 401 is corrected by the light quantity correction rate calculation unit 303, the filter coefficient is switched according to the position of the pixel in the main scanning direction to perform smoothing processing. Note that the filter coefficient will be described later, and it is a coefficient used in the calculation formula for adjusting the density value of the additional pixel to be the target pixel.

[0054] FIG. 5 is a configuration diagram inside control unit 208 provided with the smoothing filter of this embodiment. In FIG. 5, the one-dot chain line portions 301 to 307 are modules that perform known partial magnification correction and partial light amount correction, 401 is a thickening processing module, and 501 to 502 are smoothing filter modules. That is, control unit 208 of this embodiment includes modules 301 to 307 that perform partial magnification correction and partial light amount correction, thickening processing module 401, and smoothing filter modules 501 to 502. Hereinafter, each part included in control unit 208 will be described.

[0055] The main scanning position detection unit 300 is notified of the main scanning start timing from BD207 and detects the main scanning position. The pixel magnification calculation unit 301, which is a magnification correction means, calculates the magnification (partial magnification) of the pixels according to the main scanning position information notified from the main scanning position detection unit 300. The storage unit 302 stores a partial magnification profile and a partial light amount profile corresponding to the position in the main scanning direction. The light amount correction rate calculation unit 303 calculates the correction rate (light amount correction rate) of the partial light amount of the pixels according to the main scanning position information notified from the main scanning position detection unit 300. The pixel size calculation unit 304 calculates the pixel size (number of pixel pieces) of the partial magnification from the pixel magnification calculated by the pixel magnification calculation unit 301. The correction value level conversion unit 305 performs correction value level conversion from the light amount correction rate calculated by the light amount correction rate calculation unit 303. The PWM conversion unit 306 converts the image data (density value) smoothed by the smoothing filter 501 with the LUT stored in the PWM table 307. The thickening processing unit (thickening processing means) 401, which is an image processing means, detects the contour portion (thin line boundary portion) of the input image and performs a known process of adding additional pixels immediately before or after the pixels of the detected contour portion of the image. The output of the thickening processing unit 401 is the interpolated pixel density value D n (see FIG. 4). The filter coefficient table 502 has filter coefficients corresponding to the main scanning position information notified from the main scanning position detection unit 300. The smoothing filter 501 can change the filter coefficients that the filter coefficient table 502 has according to the position of the pixels in the main scanning direction. Note that the smoothing filter 501 of this embodiment is a known 3×1 smoothing filter.

[0056] Also, the input pixel density value D of the smoothing filter 501 nin is the 4-bit interpolated pixel density value D of the output of the thickening processing unit 401 n as well. Therefore, in the following description, the input pixel density value D of the smoothing filter 501 nin will be uniformly referred to as the interpolated pixel density value D n .

[0057] This smoothing filter 501 calculates the output pixel density value D n-1 D n D n+1 for three pixels in the main scanning direction, with the filter coefficients being A n-1 B n C n+1 respectively, and outputs it according to the following formula 1 nout .

[0058] D nout ={A n-1 D n-1 +B n D n +C n+1 D n+1} / {A n-1 +B n +C n+1} ··· Formula 1

[0059] Here, the three pixels in the main scanning direction refer to the target pixel at position n in the main scanning direction, the previous pixel at position n - 1 immediately before the target pixel in the main scanning direction, and the subsequent pixel at position n + 1 immediately after the target pixel. These three pixels are three consecutive pixels in the main scanning direction in the order of position n - 1, position n, and position n + 1. That is, they are three consecutive pixels in the order of the previous pixel, the target pixel, and the subsequent pixel in the main scanning direction. Here, the additional pixel added by the above-described thickening process becomes the target pixel. The additional pixel is a pixel in the contour portion of the thin line after the thickening process, and one of the pixels immediately before or after this target pixel is a pixel in the contour portion of the thin line before the thickening process

[0060] Note that the smoothing filter 501 outputs the pixel density value D of the target pixel (additional pixel)nout When calculating using Equation 1, the target pixel (additional pixel) may be a pixel at one or the other end in the main scanning direction, and there may be no pixel immediately before or after it. In that case, the density value of the pixel that does not exist immediately before or after is set to 0, and the output pixel density value of the target pixel (additional pixel) is calculated using the filter coefficient on the end side in the scanning direction.

[0061] Also, the filter coefficient table 502 has filter coefficients A n , B n , C n corresponding to the main scanning position (position in the main scanning direction) n. In Equation 1 above, the filter coefficients B n , A n-1 , C n+1 are the filter coefficients used for each of the three aforementioned pixels.

[0062] The interpolated pixel density values D n , D n-1 , D n+1 are the interpolated pixel density values (input pixel density values) of the pixels at positions n, n - 1, and n + 1 that have undergone the thickening process and are input for each of the three aforementioned pixels.

[0063] FIG. 6 is a diagram showing the interpolated pixel density value D n and the output pixel density value D nout , the PWM control waveform W, the LSF profile P, and the main scanning LSF diameter R at the end in the scanning direction (here, the left end which is the scanning start side) and the center in the scanning direction when the present invention is applied. FIG. 6 shows the same input image data and main scanning pixel positions as in FIG. 4. Using FIG. 6, the process of smoothing (here, changing the filter coefficient) according to the main scanning position will be described by taking the end in the scanning direction and the center in the scanning direction as examples.

[0064] At the contour part on the right side of the thin line at the end in the scanning direction in FIG. 6, the interpolated pixel density value D 104 = 15 at the main scanning pixel position n - 1 = 104, the interpolated pixel density value D 105 = 4 at the main scanning pixel position n = 105, and the interpolated pixel density value D 106It is equal to 0. Here, the pixel at position n is the target pixel, an additional pixel added by the thickening process, and a pixel on the contour of the thin line after the thickening process. The pixel at position n - 1 is the previous pixel immediately before the target pixel, and a pixel on the right contour of the thin line before the thickening process (before the additional pixel is added). The pixel at position n + 1 is the subsequent pixel immediately after the target pixel.

[0065] At the main scanning pixel position n = 105, the filter coefficients of the three pixels in the main scanning direction are each A 104 = 0, A 105 = 0, A 106 = 0, B 104 = 64, B 105 = 64, B 106 = 64, C 104 = 0, C 105 = 0, C 106 Let it be 0. Then, the output pixel density value D of the pixel at position n = 105, which becomes the target pixel 105out From Equation 1, it becomes as follows, and the interpolated pixel density value D 105 and the output pixel density value D 105out become the same value.

[0066] That is, at the end of the scanning direction, even if smoothing processing is performed on the right contour of the thickened thin line before performing light amount correction of the pixel, the output pixel density value of the target pixel becomes the same value as the interpolated pixel density value.

[0067] D 105out ={A 104 D 104 +B 105 D 105 +C 106 D 106} / {A 104 +B 105 +C 106}={0×15 + 64×4 + 0×0} / {0 + 64 + 0}=4 (however, decimal places are rounded off)

[0068] On the other hand, at the right contour of the thin line in the center of the scanning direction in FIG. 6, the interpolated pixel density value D at the main scanning pixel position n - 1 = 3599 3599 is 15, and the interpolated pixel density value D at the main scanning pixel position n = 36003600 =4. Interpolation pixel density value D at the main scanning pixel position n+1 = 3601 3601 =0. Here, the pixel at position n is the target pixel, an additional pixel added by the thickening process, and a pixel on the contour of the thin line after the thickening process. The pixel at position n-1 is the previous pixel immediately before the target pixel, and a pixel on the right contour of the thin line before the thickening process (before the additional pixel is added). The pixel at position n+1 is the subsequent pixel immediately after the target pixel.

[0069] At the main scanning pixel position n = 3600, the filter coefficients for 3 pixels in the main scanning direction are A 3599 =12, A 3600 =12, A 3601 =12, B 3599 =56, B 3600 =56, B 3601 =56, C 3599 =12, C 3600 =12, C 3601 =12. Then, the output pixel density value D of the pixel at position n = 3600, which becomes the target pixel 3600out is, from Equation 1, as follows, and the interpolation pixel density value D 3600 =4, and the output pixel density value D 3600out =5 are different values.

[0070] That is, at the center of the scanning direction, when smoothing processing is performed on the right contour of the thickened thin line before performing pixel light amount correction, the output pixel density value of the target pixel becomes a value different from the interpolation pixel density value. Specifically, the output pixel density value of the target pixel at position n is adjusted in a direction closer to the density value of the previous pixel immediately before the target pixel. The density value of the additional pixel (target pixel) added by the thickening process is adjusted in a direction closer to the density value of the pixel on the contour of the thin line before the thickening process (the previous pixel immediately before the target pixel).

[0071] D 3600out ={A 3599 D 3599 +B 3600 D 3600 +C 3601 D 3601} / {A 3599 +B3600 +C 3601} = {12 × 15 + 56 × 4 + 12 × 15} / 64 = 5 (where the decimal part is rounded off)

[0072] The filter coefficients used when obtaining the output pixel density value of the target pixel in this way are those of the filter coefficient table 502 (see FIG. 5). And the filter coefficients of the target pixel at the end of the scanning direction and the pixels before and after it, and the filter coefficients of the target pixel in the center of the scanning direction and the pixels before and after it, which the filter coefficient table 502 has, have the following relationship.

[0073] Here, let the filter coefficient of the target pixel at the end of the scanning direction be X n and the filter coefficient of the pixel immediately before the target pixel be X n-1 and the filter coefficient of the pixel immediately after the target pixel be X n+1 . Also, let the filter coefficient of the target pixel in the center of the scanning direction be Y n and the filter coefficient of the pixel immediately before the target pixel be Y n-1 and the filter coefficient of the pixel immediately after the target pixel be Y n+1 . Then, the filter coefficients X n 、X n-1 、X n+1 of the target pixel at the end of the scanning direction and the pixels before and after it, and the filter coefficients Y n 、Y n-1 、Y n+1 of the target pixel in the center of the scanning direction and the pixels before and after it have the following relationship.

[0074] The filter coefficient Y n of the target pixel in the center of the scanning direction is smaller than the filter coefficient X n of the target pixel at the end of the scanning direction. The filter coefficients Y n-1 、Y n+1 of the pixels before and after the target pixel in the center of the scanning direction are larger than the filter coefficients X n-1 、X n+1 of the pixels before and after the target pixel at the end of the scanning direction. The filter coefficient Y n of the target pixel in the center of the scanning direction is such that, compared to the end of the scanning direction, the filter coefficients Y n-1 、Y n+1The ratio is small with respect to this. The filter coefficients of the pixels at the center in the scanning direction and at the ends in the scanning direction are in such a relationship.

[0075] And the smoothing filter 501 switches the filter coefficients in the above relationship according to the position of the target pixel in the main scanning direction, and performs a process of adjusting the density value of the additional pixel, which is the target pixel, in a direction closer to the density values of the pixels of the thickening-processed thin line contour portion before thickening, which are the pixels before and after it.

[0076] In this way, the smoothing filter 501 of the control unit 208 switches the filter coefficient used in the calculation for adjusting the density value of the target pixel according to the position of the target pixel in the main scanning direction. Specifically, as described above, the density value of the target pixel at the center in the scanning direction is adjusted in a direction closer to the density values of the pixels immediately before or after the target pixel, as compared with the case of adjusting the density value of the pixel at the end in the scanning direction.

[0077] In this way, by changing the filter coefficient according to the main scanning position, it is possible to control to change the output pixel density value according to the main scanning position for the same input image data (= interpolated pixel density value). That is, the output pixel density value does not change at the ends in the scanning direction, and the output pixel density value of the thin line contour portion changes at the center in the scanning direction, and the thin line contour portion can be darkened.

[0078] Therefore, the thin line contour portion at the center in the scanning direction becomes darker, the LSF profile is more likely to connect to the adjacent pixels, and toner is likely to be loaded without white spots.

[0079] As described above, in an image forming apparatus including a non-uniform scanning optical system, after performing a thickening process of adding additional pixels to the contour portion of the image, a smoothing process is performed in which the filter coefficient is switched according to the main scanning position to adjust the density value of the additional pixels. Thereby, when partial magnification correction processing and partial light amount correction processing are performed thereafter, it is possible to prevent white spots in the contour portion of the image at the center in the scanning direction.

[0080] In addition, due to its characteristics, the smoothing filter of this embodiment is likely to cause effects such as image blurring at the thin line contour part, which is a sharp change point. However, at a gentle change point, there is almost no effect. Therefore, it is difficult to cause adverse effects outside the thin line contour part, and it can be said to have suitable characteristics.

[0081] In this embodiment, for the purpose of explaining the principle, the smoothing filter is taken as an example for explanation. However, it goes without saying that another known filter (for example, the Sobel method or the Laplacian filter) that extracts the edge of the thin line contour part and applies a filter coefficient to darken only the pixels of the edge part at the center of the scanning direction may also be used.

[0082] In addition, in this embodiment, for the purpose of explaining the principle, it is configured to have a filter coefficient table for each main scanning pixel position. However, there is no need to have a filter coefficient table with the accuracy of each pixel, and the filter coefficient may be set for each region composed of a plurality of pixels (for example, 10 pixels or 100 pixels), and the filter coefficient may be switched for each region.

[0083] In addition, in this embodiment, a monochrome image forming apparatus with one color is taken as an example for explanation. However, it goes without saying that a color image forming apparatus having multiple colors can also be applied to the color image forming apparatus if the same configuration is adopted for each color.

[0084] [Embodiment 2] Next, the image forming apparatus according to Embodiment 2 will be described. In Embodiment 2, only the differences from Embodiment 1 will be described. The difference from Embodiment 1 is that the control is performed not by changing the filter coefficient of the smoothing filter according to the main scanning position, but by another parameter related to the main scanning position.

[0085] FIG. 7 is a diagram showing the configuration of the control unit of Embodiment 2. In FIG. 7, the point that the filter coefficient table 502 refers to the pixel size output of the pixel size calculation unit 304 is different from the point that the filter coefficient table 502 refers to the main scanning position in FIG. 5.

[0086] In the partial magnification correction function for correcting the magnification of pixels, this utilizes the correlation between the main scanning position and the pixel size (pixel magnification). The control for switching the filter coefficient according to the main scanning position in the first embodiment can be replaced with the control for switching the filter coefficient according to the pixel size.

[0087] Also, the same applies to the partial light amount correction function for calculating the correction value for correcting the light amount of pixels. Since there is a correlation between the main scanning position and the correction value level conversion result, the filter coefficient may be switched according to the conversion result (light amount correction value) of the correction value level conversion unit 305 in FIG. 7.

[0088] In this way, even if the control for switching the filter coefficient of the smoothing filter with another parameter (pixel size) related to the main scanning position is performed, it is possible to prevent the whiteout of the contour portion of the image at the center in the scanning direction, similar to the above-described embodiment.

[0089] [Embodiment 3] Next, an image forming apparatus according to Embodiment 3 will be described. In Embodiment 3, only the differences from Embodiment 1 will be described. The difference from Embodiment 1 is that the control unit 208 further includes a count unit which is a cumulative means for accumulating the input image density values. And when the count value which is the accumulated image density value exceeds a predetermined threshold value, the filter coefficient provided from the filter coefficient table 502 is switched.

[0090] FIG. 8 is a diagram showing the configuration of the control unit of Embodiment 3. In FIG. 8, the input image density count unit 600 counts the density values of the image data input from the input image data 212 for each pixel, and switches the filter coefficient of the filter coefficient table 502 when the count value which is the accumulated image density value exceeds the threshold value.

[0091] Generally, the light amount of a laser diode gradually decreases according to the number of irradiations. That is, the light amount decreases due to changes over time. When the light amount decreases due to changes over time, the density becomes thinner.

[0092] When the amount of light decreases over time, the possibility that the LSF profile at the thin line contour part in the center of the scanning direction in Example 1 fails to connect with adjacent pixels becomes even higher.

[0093] Therefore, when the amount of light decreases and the density becomes lower due to changes over time, white gaps in the contour parts of the image may become prominent.

[0094] Thus, in this embodiment, such changes over time are grasped by the input image density counting unit 600, and when the count value of the input image density value becomes equal to or greater than a predetermined threshold value, control is performed to switch the filter coefficient so as to further darken the density of the contour part of the image.

[0095] By doing so, in addition to the same effects as in Example 1, it is possible to prevent white gaps in the contour part of the image in the center of the scanning direction even when the amount of light decreases due to changes over time.

[0096] In addition, in the above-described Examples 1 to 3, an image forming apparatus having one image forming unit is exemplified, but the number of image forming units used is not limited and may be appropriately set as needed.

[0097] Also, in the above-described Examples 1 to 3, a printer is exemplified as the image forming apparatus, but the present invention is not limited thereto. For example, other image forming apparatuses such as a copying machine, a facsimile apparatus, or a multifunction machine combining these functions may be used. Further, an image forming apparatus that uses an intermediate transfer body, sequentially overlays toner images of each color on the intermediate transfer body and transfers them, and collectively transfers the toner image carried on the intermediate transfer body to a recording medium is exemplified, but the present invention is not limited thereto. An image forming apparatus that uses a recording medium carrier and sequentially overlays toner images of each color on the recording medium carried on the recording medium carrier and transfers them may also be used. By applying the present invention to these image forming apparatuses, similar effects can be obtained.

Explanation of Reference Numerals

[0098] 100…Image forming apparatus 101…Image forming unit 102…Photosensitive drum 104…Optical scanning device 107…Intermediate transfer belt 201…Light source 202…Collimator lens 203…Cylindrical lens 204…Deflector (polygon mirror) 208…Control unit 209…Memory 210…Laser drive unit 300…Main scanning position detection unit 301…Pixel magnification calculation unit 302…Storage unit 303…Light quantity correction rate calculation unit 304…Pixel size calculation unit 305…Correction value level conversion unit 306…PWM conversion unit 307…PWM table 401…Thickening processing unit 501…Smoothing filter 502…Filter coefficient table 600…Counting unit

Claims

1. A photoreceptor, A light source that outputs laser light based on the input image, Scanning means for scanning the laser light output from the light source onto the photoreceptor, An optical system provided between the scanning means and the photoreceptor in the optical path of the laser light, through which the laser light passes, and the scanning speed at which the laser light moves along the main scanning direction along the axis of the photoreceptor on the surface of the photoreceptor is not constant, Light amount correction means for correcting the light amount of the pixel according to the position of the pixel in the main scanning direction constituting the image, Before being corrected by the light amount correction means, as the contour portion of the image, a contour portion of the image in which a predetermined number or less of black pixels are continuous in the main scanning direction is detected, and an additional pixel is added immediately before or after the pixel of the contour portion of the image to thicken the image in the main scanning direction. Image processing means for performing a thickening process, Before the image with the additional pixel added by the image processing means is corrected by the light amount correction means, the density value of the additional pixel to be the target pixel is adjusted in a direction closer to the density value of the pixel of the thin line contour portion before the thickening process among the previous pixel and the subsequent pixel according to the density values of the previous pixel immediately before and the subsequent pixel immediately after the target pixel in the main scanning direction. Smoothing processing means for performing processing, An image forming apparatus, characterized by comprising the above.

2. The smoothing processing means performs processing to adjust the density value of the target pixel in the central portion in the scanning direction closer to the density value of the pixel immediately before or after the target pixel compared to the case of adjusting the density value of the pixel at the end portion in the scanning direction. The image forming apparatus according to claim 1, characterized by this.

3. The smoothing processing means switches the filter coefficient used in the calculation for adjusting the density value of the target pixel according to the position of the target pixel in the main scanning direction. The image forming apparatus according to claim 1 or 2, characterized by this.

4. The smoothing processing means calculates the input pixel density values D of the pixels at positions n, n-1, and n+1 input through the image processing means for the target pixel at position n in the main scanning direction, the previous pixel at position n-1 immediately before the target pixel, and the subsequent pixel at position n+1 immediately after the target pixel in the main scanning direction. ｎ , D ｎ－１ , D ｎ＋１ , filter coefficients B ｎ , A ｎ－１ , C ｎ＋１ and sets the output pixel density value D of the target pixel ｎｏｕｔ to be D ｎｏｕｔ = {A ｎ－１ D ｎ－１ + B ｎ D ｎ + C ｎ＋１ D ｎ＋１} / {A ｎ－１ + B ｎ + C ｎ＋１} The image forming apparatus according to claim 3, wherein the calculation and output are performed in this manner.

5. having a filter coefficient table having filter coefficients corresponding to the position n of each pixel in the main scanning direction, wherein the filter coefficients of the filter coefficient table are the filter coefficient of the target pixel at the end of the scanning direction is X ｎ , the filter coefficient of the previous pixel immediately before the target pixel is X ｎ－１ , the filter coefficient of the subsequent pixel immediately after the target pixel is X ｎ＋１ , and the filter coefficient of the target pixel at the center of the scanning direction is Y ｎ , the filter coefficient of the previous pixel immediately before the target pixel is Y ｎ－１ , the filter coefficient of the subsequent pixel immediately after the target pixel is Y ｎ＋１ , then the relationship between the filter coefficients X ｎ , X ｎ－１ , X ｎ＋１ of the target pixel at the end of the scanning direction and the pixels before and after it, and the filter coefficients Y ｎ , Y ｎ－１ , Y ｎ＋１ of the target pixel at the center of the scanning direction and the pixels before and after it is The filter coefficient Y of the target pixel at the center in the scanning direction ｎ is smaller than the filter coefficient X of the target pixel at the end in the scanning direction ｎ , The filter coefficients Y, Y ｎ－１ of the pixels before and after the target pixel at the center in the scanning direction ｎ＋１ are larger than the filter coefficients X, X ｎ－１ of the pixels before and after the target pixel at the end in the scanning direction ｎ＋１ , The filter coefficient Y of the target pixel at the center in the scanning direction ｎ has a smaller ratio to the filter coefficients Y, Y ｎ－１ of the pixels before and after it than at the end in the scanning direction ｎ＋１ , The image forming apparatus according to claim 3 or 4, wherein the smoothing processing means performs processing of switching the filter coefficients in the relationship according to the position of the target pixel in the main scanning direction and adjusting the density value of the target pixel in a direction closer to the density values of the pixels before and after it.

6. The image forming apparatus according to claim 5, wherein the smoothing processing means switches the filter coefficients in the filter coefficient table according to the scanning position on the photoreceptor.

7. having magnification correction means for correcting the magnification of the pixel according to the position of the pixel in the main scanning direction that constitutes the image, The image forming apparatus according to claim 5, wherein the smoothing processing means switches the filter coefficients in the filter coefficient table according to the magnification of the pixel.

8. The image forming apparatus according to claim 5, wherein the smoothing processing means switches the filter coefficients in the filter coefficient table according to the light amount correction value for correcting the light amount of the pixel.

9. further comprising accumulation means for accumulating the input pixel density values, The image forming apparatus according to any one of claims 5 to 8, wherein the smoothing processing means switches the filter coefficient of the filter coefficient table when the accumulated pixel density value exceeds a predetermined threshold value.

Images