JP2012108388A - Image forming apparatus, image processing device, image processing method and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image processing technique capable of easily reducing the generation of ghosts without using a pre-exposing device in an image forming apparatus.SOLUTION: An image forming apparatus of the present invention reduces ghosts generated on images formed in a focus station located downstream in the circumferential surface movement direction of a photosensitive drum by correcting image data used for image formation at the focus station. Specifically, the correction amount applied for the image data is determined from image data used for each of the focus station and one or more stations further upstream than the focus station. The correction amount is determined at the focus station to be the amount reducing the difference between a reference potential, which is a surface potential of the photosensitive drum after exposure when a ghost is not generated, and a surface potential of the photosensitive drum after exposure when the ghost is generated.

Description

  The present invention relates to an electrophotographic image forming apparatus, an image processing apparatus that performs image processing on image data used in the image forming apparatus, an image processing method for the image data, and a program.

  In general, in an electrophotographic image forming apparatus, the surface of an image carrier, for example, a photoreceptor is uniformly charged by a charging device, and then a toner image is formed on the surface of the photoreceptor. The toner image formed on the surface of the photosensitive member is transferred to a recording material or an intermediate transfer member by a transfer device. In such an image forming apparatus, the surface potential of the photoreceptor after the transfer process may be non-uniform.

  When charging the surface of a photoconductor with a nonuniform surface potential with a charging device, if the degree of nonuniformity is large, the surface of the photoconductor cannot be uniformly charged and the surface potential is nonuniform. May remain. Such non-uniformity of the surface potential on the photoconductor can cause an image defect called ghost for an image that is subsequently formed on the photoconductor.

  In order to eliminate the non-uniformity of the surface potential of the photoreceptor as described above, for example, the following method has been proposed. Patent Document 1 proposes an image forming apparatus in which a charge eliminating device called a pre-exposure device is provided between a transfer device and a charging device along a rotation direction of a photosensitive member. In this image forming apparatus, before the surface of the photoconductor is charged by the charger, the surface of the photoconductor is neutralized by the exposure light from the pre-exposure device, and the surface potential is shifted to a uniform potential. Prevent the occurrence of defects. In Patent Document 2, in the inline type color image forming apparatus, as a pre-exposure device for neutralizing a photoconductor, a different light source is not provided for each photoconductor, but a common one is used for a plurality of photoconductors. A method of providing a light source has been proposed. In Patent Document 3, a pre-exposure device is not provided, and the charging bias is changed at an independent timing for each photoconductor so that the charging potential of the photoconductor continuously increases during image formation. A technique for reducing the occurrence of ghosts with a simple configuration has been proposed.

Japanese Patent Laid-Open No. 11-133824 JP 2007-219250 A JP 2007-171633 A

  In recent years, there has been a demand for an image forming apparatus that achieves cost reduction and size reduction. However, in order to realize cost reduction and miniaturization with the above-described conventional technology, there are the following problems. For example, in the method of Patent Document 1, an LED array is required as a pre-exposure device, so that the production cost increases and the size of the device is reduced due to the need to secure a space for installing the LED array. Increase.

  In the pre-exposure device of Patent Document 2, light from one light source is irradiated onto a plurality of photoconductors via a plurality of light guides. In this case, in order to obtain the required amount of light on the surfaces of the plurality of photoconductors in order to shift the surface potential of the plurality of photoconductors to a predetermined potential, the light amount of the light source is increased or the light guide is made transparent. It is necessary to improve, leading to an increase in production costs. In addition, the device size increases due to securing the installation space for the light guide.

  The method of Patent Document 3 does not require a pre-exposure device, so the device size does not increase. However, when the power source for charging is shared by a plurality of photoconductors for cost reduction, the charging bias for each photoconductor is changed at an independent timing for each photoconductor as in this method. It ’s difficult.

  SUMMARY An advantage of some aspects of the invention is that it provides an image processing technique that can easily reduce the occurrence of ghosts without using a pre-exposure device in an image forming apparatus.

  The present invention can be realized as an image forming apparatus, for example. The image forming apparatus includes an image carrier, an exposure unit that forms an electrostatic latent image on the surface of the image carrier by exposing the surface of the image carrier according to image data, and develops the electrostatic latent image with a developer. Development means for forming an image on the surface of the image carrier, each of which has a plurality of image forming means for forming images of different colors, and the image is transferred from the image carrier of the plurality of image forming means. Rotation that transports a recording material on which a multicolor image to be formed on the recording material is formed, or a multicolor image is formed by transferring an image from an image carrier of a plurality of image forming means A correction amount to be applied to the image data used in the first image forming means located downstream of the body and the moving direction of the peripheral surface of the rotating body among the plurality of image forming means, One or more second units positioned upstream of one image forming unit The difference between the surface potential of the image carrier after exposure according to the image data in the first image forming unit and the reference potential corresponding to the image data, which is caused by the image formed by the image forming unit, is reduced. A determining unit that determines a correction amount from image data used in the first and second image forming units, and a correcting unit that corrects image data used in the first image forming unit using the correction amount determined by the determining unit. It is characterized by providing.

  According to the present invention, it is possible to provide an image processing technique capable of easily reducing the occurrence of ghost without using a pre-exposure device in the image forming apparatus.

1 is a diagram illustrating a schematic configuration of an image forming apparatus according to a first embodiment. It is a figure which shows an example of the change of the surface potential of the said photosensitive drum at the time of forming a solid image on the surface of a photosensitive drum. FIG. 7 is a diagram illustrating an example of a change in surface potential of a photosensitive drum at a downstream station when a solid image formed on the intermediate transfer belt at the upstream station reaches a primary transfer nip portion of the downstream station. FIG. 4 is a diagram illustrating an example of a relationship between image data, a surface potential of a photosensitive drum after primary transfer, and a surface potential of a photosensitive drum after charging. It is a figure which shows an example of the relationship between image data and the surface potential of the photosensitive drum after exposure based on the said image data. It is a figure which shows an example of the image before ghost generation, the image after ghost generation, and the image after ghost suppression. It is a conceptual diagram which shows the mechanism in which a ghost generate | occur | produces. FIG. 2 is a block diagram illustrating a schematic configuration of an image processing unit in the image forming apparatus according to the first embodiment. 3 is a diagram illustrating an example of a reference table used for ghost suppression in the image forming apparatus according to the first embodiment. FIG. 5 is a flowchart illustrating a procedure of image forming processing including ghost suppression processing in the image forming apparatus according to the first embodiment. It is a conceptual diagram which shows the mechanism in which a ghost generate | occur | produces in the 2nd recording material when printing on a some recording material continuously. , 10 is a flowchart illustrating a procedure of image forming processing including ghost suppression processing in the image forming apparatus according to the second embodiment.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. The following embodiments do not limit the invention according to the claims, and all combinations of features described in the embodiments are not necessarily essential to the solution means of the invention.

[First Embodiment]
<Configuration of Image Forming Apparatus 100>
The configuration of the image forming apparatus 100 will be described with reference to FIG. The image forming apparatus 100 generally includes a color image forming engine unit 401 and an image processing unit 402. Hereinafter, the configuration and operation of the color image forming engine unit 401 will be described first.

  The color image forming engine unit 401 forms toner images (developer images) using yellow (Y), magenta (M), cyan (C), and black (K) toners (developers). Image forming units SY, SM, SC, and SK. Hereinafter, the image forming units SY, SM, SC, and SK are also referred to as a first station, a second station, a third station, and a fourth station, respectively. The image forming units SY, SM, SC, and SK are sequentially arranged along the peripheral surface of the intermediate transfer belt (rotating body) 14 from the upstream side to the downstream side with respect to the moving direction of the peripheral surface. The intermediate transfer belt 14 disposed to face the image forming units SY, SM, SC, and SK is stretched around three rollers: a drive roller 13a, a tension roller 13b, and a secondary transfer counter roller 13c. A constant tension is maintained on the intermediate transfer belt 14 by a tension roller 13b. The drive roller 13a drives the intermediate transfer belt 14 and conveys the intermediate transfer belt 14 in the direction of the arrow in FIG.

  Each of the image forming units SY, SM, SC, and SK includes an integral process cartridge, and each process cartridge includes drum units 10Y, 10M, 10C, and 10K, and developing units 8Y, 8M, 8C, and 8K. The drum units 10Y, 10M, 10C, and 10K include photosensitive drums (image carriers) 1Y, 1M, 1C, and 1K, cleaning devices 9Y, 9M, 9C, and 9K, and charging rollers 2Y, 2M, 2C, and 2K, respectively. . The developing units 8Y, 8M, 8C, and 8K include developing rollers 5Y, 5M, 5C, and 5K, toners 3Y, 3M, 3C, and 3K, toner application rollers 6Y, 6M, 6C, and 6K, and toner application blades 7Y and 7M. , 7C, and 7K. The toners 3Y, 3M, 3C, and 3K are non-magnetic one-component toners that are negatively charged.

  Each of the image forming units SY, SM, SC, and SK includes exposure devices 11Y, 11M, 11C, and 11K that include a scanner unit that scans laser light with a polygon mirror or an LED array. The exposure devices 11Y, 11M, 11C, and 11K respectively irradiate the surfaces of the photosensitive drums 1Y, 1M, 1C, and 1K with scanning beams 12Y, 12M, 12C, and 12K that can be scanned in accordance with image data, so that each photosensitive drum is exposed. An electrostatic latent image is formed on the surface. The image data can be expressed by, for example, 8-bit data for each color, that is, 256-level values from 00H to FFH. Here, “H” means hexadecimal display. Note that the value of the image data may be displayed not in hexadecimal but in decimal. When “dec” is added to the value of the image data instead of “H”, it means that the value is displayed in decimal.

  The exposure apparatuses 11Y, 11M, 11C, and 11K are input with signals that are modulated so that light is emitted in the light emission time corresponding to such image data. The image data may be expressed by percentage display with FFH as 100%. For example, an image referred to as “solid image” in this specification means an image formed using image data in which all pixel values are FFH (100%). The image density decreases as the pixel value of the image data decreases, and 00H (0%) corresponds to solid white. Further, in this specification, an image referred to as “solid white image” means an image formed using image data having all pixel values of 00H (0%), that is, in this case, the corresponding photosensitive image. This means that no image is formed on the surface of the drum.

  Inside the intermediate transfer belt 14, primary transfer rollers 4Y, 4M, 4C, and 4K are arranged at positions facing each other with the intermediate transfer belt 14 corresponding to the photosensitive drums 1Y, 1M, 1C, and 1K, respectively. . The primary transfer rollers 4Y, 4M, 4C, and 4K form primary transfer nip portions 23Y, 23M, 23C, and 23K with the corresponding photosensitive drums 1Y, 1M, 1C, and 1K via the intermediate transfer belt 14, respectively. . The primary transfer rollers 4Y, 4M, 4C, and 4K are applied with a predetermined primary transfer bias, so that the toner images of the respective colors on the photosensitive drums 1Y, 1M, 1C, and 1K are transferred to the primary transfer nip portions 23Y, 23M, and 23C. , 23K, the image is transferred onto the surface of the intermediate transfer belt 14 in a superimposed manner. As a result, a multicolor toner image is formed on the intermediate transfer belt 14. The secondary transfer counter roller 13 c forms a secondary transfer nip portion with the secondary transfer roller 20 via the intermediate transfer belt 14 and the recording material P. The secondary transfer roller 20 transfers a toner image formed on the intermediate transfer belt 14 onto the recording material P at the secondary transfer nip portion by applying a predetermined secondary transfer bias.

  The image forming operation by the color image forming engine unit 401 described above is executed as follows. When image formation is started in the color image forming engine unit 401, the photosensitive drums 1Y, 1M, 1C, 1K, the intermediate transfer belt 14, and the like start to rotate in a direction indicated by an arrow in FIG. 1 at a predetermined process speed. The surfaces of the photosensitive drums 1Y, 1M, 1C, and 1K are brought to a predetermined surface potential Vd (dark portion potential) by charging rollers 2Y, 2M, 2C, and 2K to which a predetermined negative charging bias is applied from a power source (not shown). Charged. After that, the exposure apparatuses 11Y, 11M, 11C, and 11K apply the scanning beams 12Y, 12M, 12C, and 12K to the corresponding photosensitive drums 1Y based on the signals that are input from the image processing unit 402 and modulated using the image data. Irradiate on 1M, 1C, 1K, respectively. As a result, electrostatic latent images of the respective colors are formed on the photosensitive drums 1Y, 1M, 1C, and 1K. Here, the electrostatic latent images of the respective colors are determined in advance so that a toner image developed later is superimposed and transferred on the intermediate transfer belt 14 to form a multicolor image on the intermediate transfer belt 14. Formed on each photosensitive drum at the same timing. The electrostatic latent images formed by using the scanning beams 12Y, 12M, 12C, and 12K are transferred to the developing rollers 5Y, 5M, 5C, and 5K and the photosensitive drums 1Y, It is conveyed to the nip portion between 1M, 1C, and 1K. In the nip portion, the electrostatic latent images on the photosensitive drums 1Y, 1M, 1C, and 1K are reversed by developing rollers 5Y, 5M, 5C, and 5K to which a predetermined negative developing bias is applied from a power source (not shown). Developed. Thus, Y, M, C, and K color toner images are formed on the photosensitive drums 1Y, 1M, 1C, and 1K, respectively.

  A Y-color toner image formed on the photosensitive drum 1Y in the image forming unit SY (first station) is conveyed to the primary transfer nip 23Y as the photosensitive drum 1Y rotates. This Y toner image is transferred from the photosensitive drum 1Y onto the intermediate transfer belt 14 at the primary transfer nip 23Y as described above. The Y toner image transferred onto the intermediate transfer belt 14 is conveyed as the peripheral surface of the intermediate transfer belt 14 moves. In synchronization with the movement of the Y-color toner image on the intermediate transfer belt 14, the M-color, C-color, and K-color toner images formed in the second to fourth stations are respectively transferred from the photosensitive drums 1M, 1C, and 1K. The toner image is transferred onto the Y color toner image. As a result, a multicolor toner image having four colors is formed on the surface of the intermediate transfer belt 14.

  On the other hand, the recording material P to which the toner image formed on the intermediate transfer belt 14 is to be transferred is fed and transported from the paper feed cassette 15. The recording material P is fed from the paper feed cassette 15 onto the transport path by a half-moon shaped paper feed roller 16. At that time, only one of the plurality of recording materials loaded in the paper feed cassette 15 is separated by the separation roller 17. The separated recording material P is transported on the transport path by the transport roller 18 to the position of the registration roller 19 and temporarily stops there. Thereafter, when the recording material P is supplied to the secondary transfer nip portion when the multi-color toner image formed on the intermediate transfer belt 14 reaches the secondary transfer nip portion, the recording material by the registration roller 19 is used. The conveyance of P is resumed. In the secondary transfer nip portion, the toner image on the intermediate transfer belt 14 is secondarily transferred onto the recording material P as described above. The transfer material P onto which the toner image has been transferred is separated from the intermediate transfer belt 14 and sent to the fixing device 21. The fixing device 21 includes a fixing roller 21a and a pressure roller 21b. In the fixing device 21, the recording material P is heated and pressed by the fixing roller 21a and the pressure roller 21b. As a result, the toner image is melted and fixed on the surface of the recording material P and fixed on the surface. As a result, a multicolor image is formed on the recording material P.

  Residual toner adhering to the photosensitive drums 1Y, 1M, 1C, and 1K after the primary transfer is removed by cleaning devices 9Y, 9M, 9C, and 9K including fur brushes, blades, and the like. Further, residual toner adhering to the intermediate transfer belt 14 after the secondary transfer is removed by an intermediate transfer belt cleaning device 22 including a blade, a fur brush, a web, and the like. The removed toner is collected in a waste toner container (not shown). Alternatively, the remaining toner after the secondary transfer may be electrostatically collected on the photosensitive drum by being charged with a polarity opposite to that of the normal toner. In the following, it is assumed that the surface potential of the photosensitive drum when the electrostatic latent image of a solid image is formed is VL (bright portion potential), and the surface potential changes from Vd (dark portion potential) to the VL side. It expresses.

<Ghost generation mechanism>
Next, a mechanism for generating a ghost in an image formed by the color image forming engine unit 401 that should be suppressed (reduced) in the present embodiment will be described. FIG. 2 shows, as an example, a case in which an electrostatic latent image 201 of a solid image is formed on the photosensitive drum 1C in the image forming unit SC (third station). Here, the entire surface of the photosensitive drum 1 </ b> C is charged in advance to a constant dark portion potential Vd (for example, about −500 V). In the image forming unit SC, as shown in 200a of FIG. 2, when the electrostatic latent image 201 of a solid image is formed in a predetermined small area on the photosensitive drum 1C, the surface potential at the dotted line unit 202 is as shown in 200b of FIG. To change. That is, the surface potential of the area where the electrostatic latent image 201 of the solid image exists on the photosensitive drum 1C changes from Vd to VL (for example, about −120 V), and the surface potential of the other areas remains Vd. .

  Reference numeral 200c in FIG. 2 shows a state in which the dotted line portion 202 on the surface of the photosensitive drum 1C has reached the primary transfer nip portion 23C after the electrostatic latent image 201 is developed. In this state, a primary transfer bias is applied from the primary transfer roller 4C side to the region 211 of the dotted line 202 in the primary transfer nip 23C only through the intermediate transfer belt 14. As a result, in the region 211, the primary transfer current flows directly from the primary transfer roller 4C to the surface of the photosensitive drum 1C via the intermediate transfer belt 14, so that the surface potential of the photosensitive drum 1C becomes about −200 V on the positive polarity side. Shift to On the other hand, a primary transfer bias is applied to the region 212 of the dotted line portion 202 from the primary transfer roller 4 </ b> C side via the toner image 201 and the intermediate transfer belt 14. Here, in the region 212, the surface potential has already shifted from Vd to VL, and the toner image becomes an impedance component. Therefore, in the area 212, the amount of primary transfer current flowing from the primary transfer roller 4C to the surface of the photosensitive drum 1C is smaller than in the area 211. Accordingly, the degree of shift of the surface potential to the + polarity side in the region 212 is smaller than that in the region 211, and the surface potential of the region 212 is about −70V. That is, the surface potential at the dotted line 202 after passing through the primary transfer nip portion 23C changes to different surface potentials in the region 211 and the region 212 as indicated by 200d in FIG.

  Next, on the photosensitive drums 1Y and 1M, respectively, at the first and second stations (image forming units SY and SM), which are upstream of the image forming unit SC with respect to the moving direction of the peripheral surface of the intermediate transfer belt 14. Assume that a solid toner image is formed. In this case, the toner images on the photosensitive drums 1Y and 1M are primarily transferred and superimposed on the intermediate transfer belt 14 in the primary transfer nip portions 23Y and 23M, and a secondary color toner image is formed on the intermediate transfer belt 14. The The secondary color toner image reaches the primary transfer nip 23C of the third station from the upstream primary transfer nips 23Y and 23M by the movement of the peripheral surface of the intermediate transfer belt 14.

  FIG. 3 is a sectional view (300a) in the main scanning direction and a surface in the main scanning direction when no image is formed on the photosensitive drum 1C of the third station (that is, when a solid white image is formed). The state of potential change (300b to d) is shown. 3A, the secondary color toner image 301 including Y and M solid image toner images 301Y and 302M formed on the intermediate transfer belt 14 at the first and second stations, respectively, is shown. A state in which the primary transfer nip portion 23C of 3 stations has been reached is shown. In the primary transfer nip 23C, a region 311 on the surface of the photosensitive drum 1C is applied with a primary transfer bias from the primary transfer roller 4C side only through the intermediate transfer belt 14. On the other hand, the primary transfer bias is applied to the region 312 on the surface of the photosensitive drum 1C from the primary transfer roller 4C side via the secondary color toner image 301 and the intermediate transfer belt 14.

  300b and 300c in FIG. 3 are surface potentials of portions where the toner image 301 contacts on the photosensitive drum 1C when the secondary color toner image 301 formed on the intermediate transfer belt 14 passes through the primary transfer nip 23C. Is shown. Reference numerals 300b and 300c denote surface potentials of the photosensitive drum 1C before and after the toner image 301 passes through the primary transfer nip portion 23C, respectively.

  Before the toner image 301 passes through the primary transfer nip 23C, the surface of the photosensitive drum 1C is uniformly charged, so that the surface potential is uniform as shown in 300b. When the toner image 301 passes through the primary transfer nip 23C, a primary transfer bias is applied from the primary transfer roller 4C side through the intermediate transfer belt 14 only in the region 311. As a result, the surface potential in the region 311 is shifted to about −200V. On the other hand, since the toner image 301 becomes an impedance component in the region 312, the amount of primary transfer current flowing from the primary transfer roller 4 C side to the photosensitive drum 1 C is smaller than that in the region 311. Further, since the toner image 301 is composed of Y-color and M-color toner images 301Y and 301M, the amount of applied toner in the region 312 is larger than that in the case of a single-color toner image, and the impedance of the toner image is increased. Also gets higher. As a result, since the amount of primary transfer current flowing into the photosensitive drum 1C is smaller than when a single color toner image is formed on the intermediate transfer belt 14, the surface potential in the region 312 is as shown by 300c. -400V.

  Reference numeral 400a in FIG. 4 denotes image data corresponding to the toner image formed at the upstream station, and the surface potential of the photosensitive drum at the downstream station after the toner image has passed through the primary transfer nip portion of the downstream station. Represents the relationship. In the downstream station, the surface potential of the photosensitive drum is charged to Vd (−500 V) in advance, and a predetermined primary transfer bias is applied. Here, “upstream station” and “downstream station” mean image forming stations disposed on the upstream side and the downstream side relative to the moving direction of the peripheral surface of the intermediate transfer belt 14, respectively. Further, when the value of the horizontal axis at 400a exceeds 100%, it corresponds to a case where a toner image composed of two or more colors is formed on the intermediate transfer belt 14 by a plurality of stations.

  As indicated by 400a in FIG. 4, the absolute value of the surface potential of the photosensitive drum gradually increases when the image data is in the range from 0% to 200%, while the surface potential is almost equal in the range exceeding 200%. It becomes saturated. A toner image formed with 0 to 200% image data in the upstream station becomes an impedance component in the downstream station. As a result, the primary transfer current flows into the surface of the photosensitive drum, whereby the surface potential of the photosensitive drum is lowered. On the other hand, a toner image formed with image data exceeding 200% at the upstream station has a high impedance component exceeding a predetermined level at the downstream station. As a result, a state in which the primary transfer current hardly flows to the surface of the photosensitive drum is generated, and the surface potential of the photosensitive drum is saturated.

  Reference numeral 400b in FIG. 4 shows an example of the relationship between the surface potential of the photosensitive drum after the primary transfer of the toner image to the intermediate transfer belt 14 and the surface potential when the photosensitive drum is charged again by the charging roller. As indicated by 400b in FIG. 4, when the surface potential of the photosensitive drum after the primary transfer is Vth (300V) or less in absolute value, the surface potential of the photosensitive drum becomes Vd due to charging by the charging roller. On the other hand, when the surface potential of the photosensitive drum after primary transfer exceeds Vth in absolute value, the surface potential of the photosensitive drum is higher in absolute value than Vd (for example, about −520 V) due to charging by the charging roller. )

  In this case, for example, as shown by 200d in FIG. 2, when the surface potential of the photosensitive drum after the primary transfer is Vth or less in absolute value in both the regions 211 and 212, the surface potential in both the regions 211 and 212 is Is charged by the charging roller so as to have a uniform potential. On the other hand, as shown by 300c in FIG. 3, in the region 312 (−400V) where the surface potential of the photosensitive drum after the primary transfer exceeds the absolute value Vth, the surface potential of the photosensitive drum after being charged by the charging roller. Changes to a value (−520 V) exceeding Vd in absolute value. 300d in FIG. 3 indicates the surface potential of the photosensitive drum after being charged by the charging roller. Thus, depending on the surface potential of the photosensitive drum after the primary transfer, a surface potential difference of about 20 V is generated between the surface potential after charging in the region 312 and the surface potential after charging in the region 311.

  500a in FIG. 5 is a graph of the surface potential of the photosensitive drum after exposure when the photosensitive drum charged with surface potentials of −520 V and −500 V is exposed according to image data of 0 to 255 dec. 501 and 502 are shown. Note that 500b and 500c in FIG. 5 are enlarged portions of 500a. As shown by 500a in FIG. 5, for example, when halftone exposure is performed using 60H (96 dec) image data, the surface potential after exposure is also 10V due to the surface potential difference of 20V after charging. It can be seen that a certain potential difference remains. That is, the surface potential after exposure in the region 312 where the surface potential before exposure is −520 V is about 10 V, compared to the surface potential after exposure in the region 311 where the surface potential before exposure is −500 V, on the Vd side. The value shifted to. As a result, the density of the image formed in the region 312 by development becomes lighter than the density of the image formed in the region 311.

  Next, an image formed in such a case will be described with reference to FIG. FIG. 6 shows an image 600a to be formed on the recording material and an image 600b in which a ghost is generated when the image 600a is formed on the recording material. The image 600a includes a secondary color (red) solid image 601 formed in the first and second stations on the upstream side in the sub-scanning direction of the photosensitive drum, and is formed in the third station on the downstream side. This is an image including a C halftone image 602. For example, when the color image forming engine unit 401 forms the image 600a on a recording material, the surface potential after exposure in the region where the solid image 601 is formed on the photosensitive drum becomes a value indicated by 501 in FIG. An image 600b can be formed on the recording material. That is, a C having the same shape as that of the solid image 601 at a position separated from the position where the solid image 601 is formed, such as the image 600b, by the length of one circumference of the photosensitive drum in the sub-scanning direction. A light-colored image 603 may appear as a ghost. This ghost is a phenomenon having a property that is caused in the target station due to an image formed in one or more stations located on the upstream side with respect to each downstream target station (second to fourth stations). It is. Therefore, it can be said that this phenomenon is unique to the color image forming apparatus.

  As described above, after the image (toner image) formed on the intermediate transfer belt 14 at the upstream station passes through the primary transfer nip portion of the downstream station, the absolute value of the surface potential of the photosensitive drum of the downstream station. May not fall below the threshold value Vth. In such a case, a ghost may occur in the formed image. However, for example, when an image is formed also in the third station so as to overlap the secondary color solid image 601 formed in the first and second stations, the photosensitive drum 1C of the third station is used for image formation. To be exposed. As a result, the absolute value of the surface potential after exposure on the photosensitive drum 1C may be lower than the threshold value Vth. In that case, since the surface potential of the photosensitive drum 1C is charged to Vd in the subsequent charging process, the occurrence of ghost is avoided. For example, referring to 500a in FIG. 5, when the surface of the photosensitive drum 1C is exposed with image data of 88 dec or more in the third station, the surface potential of the photosensitive drum 1C after the exposure is an absolute value of a threshold value Vth (300V) or less. Therefore, no ghost is generated.

<Ghost suppression method>
Considering the ghost generation mechanism described above, whether or not a ghost is generated in an image formed in the color image forming engine unit 401 can be predicted based on the following three conditions. Here, FIG. 7 shows the state in which the image A formed on the photosensitive drum of the upstream station is transferred onto the intermediate transfer belt 14 and conveyed in the direction of the arrow 701 in the order of 700a, 700b, and 700c. Shown in the series.
(Condition 1)
The sum of image data (corresponding to the amount of applied toner) for the image (image A in FIG. 7) formed at the upstream station.
(Condition 2)
When the image A formed at the upstream station reaches the primary transfer nip portion of the downstream target station, the image is formed at a position on the surface of the photosensitive drum where the image A is in contact with the primary transfer nip portion (see FIG. 7 image data of image B).
(Condition 3)
Image data of an image (image C in FIG. 7) formed in the next round of the photosensitive drum with respect to a position on the surface of the photosensitive drum where the image A is in contact with the primary transfer nip portion of the downstream target station.

  In the present embodiment, with reference to the bitmap image data generated by the image processing unit 402, the color, position, and generation level of a ghost generated in an image to be formed based on the above conditions 1 to 3. Predict. Further, based on the prediction result, the bitmap image data is corrected in advance so that the ghost is reduced, and an image is formed on the recording material in the color image forming engine unit 401 using the corrected bitmap image data. It is characterized by that.

  Next, a method for suppressing ghosts in the present embodiment will be described with reference to the configuration of the image processing unit 402 shown in FIG. In FIG. 8, an image generation unit 403 generates bitmap image data that can be printed based on print data received from an external device such as a host computer. The print data is data including drawing commands for data such as characters, graphics, and images. The print data is generally created in a printer description language for creating bitmap image data called PDL (Page Description Language). The image generation unit 403 generates bitmap image data of each color of R, G, and B by analyzing the received print data and performing rasterization processing. Note that the image generation unit 403 may handle image data read from a document by a reading unit such as a scanner provided in the image forming apparatus 100 instead of print data received from an external device such as a host computer.

  The color conversion unit 404 converts the bitmap image data of R, G, B to the bitmap image data of Y, M, C, K in accordance with the toner color of the color image formation engine unit 401, and after the conversion Each color data is temporarily stored in the image buffer 405. The image buffer 405 is a page memory that stores bitmap image data for one page. The image buffer 405 may be a band memory that temporarily stores image data for a plurality of lines necessary for a ghost suppression process described later.

  The ghost suppression unit 406 executes a ghost suppression process according to the present embodiment. As described above, the stations where the ghost can occur in the formed image are the second to fourth stations on the downstream side of the first station. For this reason, the ghost suppression unit 406 does not perform ghost suppression processing on the Y color image data corresponding to the first station, and the M, C, and K color image data corresponding to the second to fourth stations. The ghost suppression process is performed only on the.

  The generation level of the ghost can be predicted from the above three conditions. For example, in FIG. 7, an image A is an image corresponding to 200% image data formed by FFH image data for Y color and FFH image data for M color, and image B is 00H (solid). A cyan image formed with white image data. The image C is a cyan image formed with 30H (48 dec) image data. In this case, a ghost that can occur in the third station (C color) can be predicted as follows.

  First, referring to 400a in FIG. 4, after the primary transfer in the third station, that is, after the image A of 200% of the image data has passed through the primary transfer nip portion 23C of the third station, the photosensitive drum of the third station. The surface potential of 1C is −400V. Referring to 400b in FIG. 4, when the surface potential of the photosensitive drum 1C after the primary transfer is −400V, the surface potential of the photosensitive drum 1C becomes −520V due to charging by the charging roller 2C. Here, 500b in FIG. 5 is an enlarged view of a part of 500a. Referring to 500b, when an area on the surface of the photosensitive drum charged to −520 V is exposed at 48 dec which is image data of the image C, the surface potential of the area after the exposure is that of the area charged to Vd. The absolute value is 16V higher than the surface potential (reference potential). In order to reduce the surface potential difference of 16V on the photosensitive drum, it is necessary to expose an area charged to −520V with image data of 54 dec as shown in 500b. Therefore, it is understood that the image data of the image C may be corrected to 54 dec obtained by increasing the image data 48 dec of the original image by 6 dec as a correction amount.

  Next, for example, in FIG. 7, an image A is an image corresponding to 200% of image data formed by FFH image data for Y color and FFH image data for M color, and image B is: A cyan image is formed with 28H (40 dec) image data. The image C is a cyan image formed with 40H (64 dec) image data. In this case, a ghost that can occur in the third station (C color) can be predicted as follows.

  First, referring to 400a in FIG. 4, when the surface potential of the photosensitive drum of the third station is Vd (−500 V), the image A of 200% image data passes through the primary transfer nip portion 23C of the third station. Then, the surface potential of the photosensitive drum 1C becomes −400V. That is, when the image A is formed with 200% image data, the absolute value of the surface potential of the photosensitive drum 1C after the primary transfer is reduced to 80%. On the other hand, referring to 500a in FIG. 5, the surface potential of the photosensitive drum 1C is −400 V after the exposure of the image B of 16% (40 dec). Therefore, when the image A on the intermediate transfer belt 14 passes through the primary transfer nip portion 23C of the third station and the image B is primarily transferred from the photosensitive drum 1C onto the image A, the surface of the photosensitive drum 1C. The potential can be expected to change to -320V, which is 80% of -400V.

  As described above, in the third station, when the surface potential of the photosensitive drum 1C after the primary transfer is −320 V, referring to 400b in FIG. 4, the area where the image B is formed on the photosensitive drum 1C is as follows. It is charged to a surface potential of −510 V by the next charging. Here, FIG. 5 shows a graph 503 plotted by proportional calculation from graphs 501 and 502 shown in 500a at 500c in which 500a is enlarged. According to this graph 503, when the area where the image B on the photosensitive drum 1C, which is charged to −510 V, is exposed at 64 dec which is the image data of the image C, the surface potential of the area after the exposure is exposed. Is 7V higher in absolute value than normal Vd (graph 502). Further, according to 500c in FIG. 5, in order to reduce the surface potential difference by 7V, it is necessary to expose the region charged to −510V with 67 dec image data. Therefore, it is understood that the image data of the image C may be corrected to 67 dec obtained by increasing the image data 64 dec of the original image by 3 dec as a correction amount.

  Further, as described above, when the image A is formed with 200% image data and the image B is formed with 28H (40 dec) image data, the surface potential of the photosensitive drum 1C after the primary transfer in the third station is −320V. Can be considered as follows. Here, referring to 400a in FIG. 4, the image data corresponding to the surface potential of −320 V after the primary transfer is 120% (305 dec). That is, forming image A with 200% image data and image B with 28H image data is equivalent to forming image A with 120% image data and image B with 00H image data. . Therefore, even when the image data of the image B is other than 00H, the surface potential of the photosensitive drum after the primary transfer is obtained, and the image data of the image A is converted into the image B using the relationship of 400a in FIG. Can be converted into data corresponding to the image data of 00H. Further, the image data after the conversion can be used to correct the image data in order to suppress the ghost.

  As described above, it is possible to uniquely determine the correction amount of the image data necessary for suppressing the occurrence of the ghost from the image data of the images A, B, and C shown in FIG. In this embodiment, the correction amount to be applied to the image data used in each of the second to fourth stations on the downstream side is the image data used in the downstream target station and one or more positioned upstream from the image data. It is determined from the image data used in the station. That is, the correction amount, that is, the surface potential of the photosensitive drum after exposure according to the image data at the station of interest, which is caused by the image formed by the upstream station, and the reference potential corresponding to the image data The amount of correction to reduce the difference is determined. Here, the reference potential corresponds to the surface potential of the photosensitive drum when the corresponding photosensitive drum is exposed using the image data when no ghost occurs at the station of interest.

  Specifically, in this embodiment, a reference table in which three image data of images A, B, and C are used as indexes and the correction amount is associated with the image data is prepared in advance. This reference table is a table in which image data (images B and C) used in the station of interest, image data (image A) used in the upstream station, and the correction amount are associated with each other. The necessary correction amount can be determined with reference to this reference table. However, in this embodiment, instead of using a three-dimensional reference table using the image data of the images A, B, and C as an index, the required data can be obtained by using two two-dimensional reference tables. The capacity is reduced.

  FIG. 9 shows reference tables 901 and 902 for determining the correction amount of the image data. The reference table 901 is a table for determining the correction amount of the image data when the image data of the image B is 00H. The reference table 902 is a table for converting the image data of the image A into the image data of the image A when the image data of the image B is 00H when the image data of the image B is other than 00H. is there.

  In the reference table 901, the vertical axis index represents the sum of the image data of the image A, that is, the image formed at the station upstream of the downstream target station. The index on the horizontal axis represents the image data of the image C, that is, the image formed in the next round of the photosensitive drum after the image A has passed through the primary transfer nip portion in the downstream target station. In the present embodiment, the parameter referred to from the index on the vertical axis and the horizontal axis is determined as a correction amount to be applied for suppressing ghost with respect to image data used for image formation at the downstream target station. In the reference table 901, the maximum value of the index on the vertical axis is 765 dec which is the sum of image data when a solid image for three colors is formed, and the maximum value of the index on the horizontal axis is a solid image for one color. Is 255 dec corresponding to the image data when forming the image.

  For example, as described above, when the image A is formed of 200% image data, the image B is formed of 00H image data, and the image C is formed of 30H (48 dec) image data, the sum of the image data of the image A is 510 dec. (= FFH + FFH). In this case, when the reference table 901 is used, 6 dec is determined as the image data correction amount as a value corresponding to 510 dec for the vertical axis and 48 dec for the horizontal axis.

  In the reference table 902, the index on the vertical axis represents the sum total of image data of the image A, that is, the image formed at the station upstream of the downstream target station. The index on the horizontal axis represents the image data of the image B, that is, the image formed on the downstream target station and transferred onto the image A in an overlapping manner. In the present embodiment, the parameters referred to from the indexes on the vertical axis and the horizontal axis are determined as the image data of the converted image A corresponding to the case where the image data of the image B is 00H. In the reference table 902, the maximum values of the vertical and horizontal axes are the same as those in the reference table 901.

  For example, as described above, when the image A is formed of 200% image data, the image B is formed of 28H image data, and the image C is formed of 40H image data, first, the reference table 902 is used to The image data is converted to 305 dec. Next, using the reference table 901, 3dec is determined as the correction amount of the image data as a value corresponding to 305dec as the image data of the image A and the image data 40H of the image C.

  In the above ghost, after the image that has been transferred to the intermediate transfer belt 14 at the upstream station and causes the ghost is passed through the primary transfer nip portion of the downstream station, the photosensitive drum of the downstream station rotates once. At this stage, it appears on the intermediate transfer belt 14. For this reason, the correction of the image data described above is performed within the range of the peripheral length d of the photosensitive drum from the leading edge of the image in the sub-scanning direction among the M, C, and K color image data of the second to fourth stations. There is no need to perform this process on the image data of the pixel to which it belongs. That is, the determination of the correction amount and the correction of the image data using the correction amount correspond to the peripheral length d of the photosensitive drum from the leading edge of the image to be formed with respect to the moving direction of the peripheral surface of the intermediate transfer belt 14. What is necessary is just to perform about the pixel further back than the back pixel by the number of pixels. Thereby, it is possible to reduce the processing load required for determining the correction amount and correcting the image data using the correction amount.

  The reference tables 901 and 902 can be expressed as a two-dimensional array. Hereinafter, the correction amount obtained from the reference table 901 is represented as G (X1, Y1), and the value obtained from the reference table 902 is represented as H (X2, Y2). The correction amount corresponds to a correction amount to be applied to image data used in a downstream station (first image forming unit). However, X1 and X2 represent the horizontal axis index in the reference tables 901 and 902, and Y1 and Y2 represent the vertical axis index in the reference tables 901 and 902, respectively. Each pixel value included in the bitmap image data can also be expressed as a two-dimensional array. In the following, the pixel at the upper left end of the image (the direction opposite to the main scanning direction and the sub scanning direction) is defined as the origin (0, 0). Also, the image data of each pixel included in the bitmap image data of each color of Y color, M color, C color, and K color is converted into Y (x, y), M (x, y), C (x, y), respectively. ), K (x, y). However, 0 ≦ x ≦ W and 0 ≦ y ≦ L, W represents the number of pixels in the sub-scanning direction (horizontal direction) of the image, and L represents the number of pixels in the main scanning direction (vertical direction) of the image.

The ghost suppression unit 406 converts image data represented by image data Y (x, y), M (x, y), C (x, y), and K (x, y) for the pixel (x, y). The image data Y ′ (x, y), M ′ (x, y), C ′ (x, y), and K ′ (x, y) represented by the following expressions are corrected. Here, d is the number of pixels corresponding to the length (circumferential length) of one circumference of the photosensitive drum. In this case, as described above, in the sub-scanning direction, no ghost is generated for pixels included in the range of the circumference of the photosensitive drum from the leading edge of the image (0 ≦ y <d). For this reason, the ghost suppression unit 406 does not correct the image data for the pixels included in the range as shown in the following equation.
Y '(x, y) = Y (x, y)
M '(x, y) = M (x, y)
C '(x, y) = C (x, y)
K ′ (x, y) = K (x, y) (0 ≦ x ≦ W, 0 ≦ y <d) (1)

On the other hand, in the sub-scanning direction, a ghost may occur in pixels included in a range (d ≦ y ≦ L) exceeding the circumference of the photosensitive drum from the leading edge of the image. Therefore, the ghost suppression unit 406 corrects the image data for the pixels included in the range as in the following equation. However, since no ghost occurs in the pixel corresponding to the first station (Y color) on the most upstream side with respect to the moving direction of the peripheral surface of the intermediate transfer belt 14, no correction is performed.
Y '(x, y) = Y (x, y)
M '(x, y) = M (x, y) + G (M (x, y), H _M (x, yd))
C '(x, y) = C (x, y) + G (C (x, y), H _C (x, yd))
K ′ (x, y) = K (x, y) + G (K (x, y), H _K (x, yd)) (0 ≦ x ≦ W, d ≦ y ≦ L) (2)
here,
H _M (a, b) = H (M (a, b), Y (a, b))
H _C (a, b) = H (C (a, b), Y (a, b) + M (a, b))
H _K (a, b) = H (K (a, b), Y (a, b) + M (a, b) + C (a, b)) (3)
It is.

  In the expressions (2) and (3), the pixel (x, y) corresponds to the first pixel, and the pixel (x, y-d) corresponds to the second pixel. The correction amount G (X1, Y1) is the image data of the pixel (x, y) used at the target station and the pixel (x, y) downstream from the pixel (x, y) used at all stations upstream from the target station. x, yd) image data. Further, the image data of the station of interest is corrected with the determined correction amount. Here, the target station corresponds to an example of the first image forming unit, and all stations upstream from the target station correspond to an example of the second image forming unit. Since the circumference of the photosensitive drum is normally the same regardless of the station (color), d is the same for all the colors here, but d may be a different value for each color. In that case, different d may be used for each color in the equations (1) and (2).

<Image formation processing procedure>
Next, with reference to FIG. 10, an image forming process procedure including the ghost suppressing process according to the present embodiment executed by the ghost suppressing unit 406 of the image forming apparatus 100 will be described. When the image forming apparatus 100 receives print data from a host computer or the like, image processing using the print data is started in the image processing unit 402. In step S101, the image generation unit 403 generates R, G, and B bitmap image data based on the received print data. In step S102, the color conversion unit 404 converts the generated bitmap image data of R, G, and B into Y, M, C, and K bitmap image data Y (x, y), M (x, y). ), C (x, y), and K (x, y). In step S103, the converted data is stored in the image buffer 405. The ghost suppression unit 406 performs the above-described ghost suppression process on the bitmap image data stored in the image buffer 405 in order from the pixel at the coordinate (0, 0) to the pixel at the coordinate (W, L). First, in S104, the ghost suppression unit 406 initializes coordinates (variables) x and y indicating pixels to which the ghost suppression process is applied as x = 0 and y = 0. Here, x and y represent the positions of the pixel of interest in the main scanning direction and the sub scanning direction.

  Next, in S105, the ghost suppression unit 406 refers to the variable y and determines whether or not 0 ≦ y <d is satisfied, whereby the target pixel satisfies 0 ≦ y <d with respect to the sub-scanning direction. It is determined whether or not the pixel is within the range. If 0 ≦ y <d is satisfied, the process proceeds to S106. Since no ghost is generated for pixels in the range, the ghost suppression unit 406 uses equation (1) in S106 and does not correct the image data. That is, the ghost suppression unit 406 converts Y (x, y), M (x, y), C (x, y), and K (x, y) into Y ′ (x, y), M ′ (x, y), C ′ (x, y), and K ′ (x, y). On the other hand, in S105, the ghost suppressing unit 406 advances the process to S107 when 0 ≦ y <d is not satisfied. Since a ghost may occur for the pixels within the range, the ghost suppression unit 406 uses the equation (2) in S107 to correct the bitmap image data Y ′ (x, y), M ′ (x , y), C ′ (x, y), and K ′ (x, y). Next, in S108, the ghost suppression unit 406 determines the bitmap image data Y ′ (x, y), M ′ (x, y), C ′ (x, y), K ′ () obtained in S106 or S107. x, y) are sent to the halftone processing units 407Y, 407M, 407C, and 407K, respectively.

  Next, in S109, the halftone processing units 407Y, 407M, 407C, and 407K respectively input bitmap image data Y ′ (x, y), M ′ (x, y), and C ′ (x, y ), K ′ (x, y) is subjected to halftone processing such as multi-value dither. In S110, the halftone processing units 407Y, 407M, 407C, and 407K store the processed data in the transfer buffers 408Y, 408M, 408C, and 408K, respectively.

  The processes of S105 to S110 are sequentially executed for pixels on one main scanning line in the main scanning direction (x direction) of the bitmap image data. In step S111, the image processing unit 402 determines whether or not the variable x = W is satisfied, and determines whether the processing in steps S105 to S110 has been completed for all the pixels on one main scanning line. Here, if x = W is not satisfied, the image processing unit 402 increments x by +1 in S112, and moves the process to S105. On the other hand, when x = W is satisfied, x is initialized to 0 in step S113 (x = 0) in order to shift to processing for the first pixel of the next main scanning line. In step S114, the image processing unit 402 determines whether y = L is satisfied, and determines whether processing for all main scanning lines in the image to be printed has been completed. Here, when the image processing unit 402 determines that y = L is not satisfied, the processing for all the main scanning lines is not completed. In this case, in order to execute the process for the next main scanning line, the image processing unit 402 increments y by +1 and returns the process to S105. On the other hand, if it is determined that y = L is satisfied, the processing of S105 to S110 has been completed for all the pixels included in the image to be printed, so the processing proceeds to S116.

  In step S116, the color image formation engine unit 401 starts an initial operation for image formation. In S117, the PWM processing units 409Y, 409M, 409C, and 409K convert the bitmap image data stored in the transfer buffers 408Y, 408M, 408C, and 408K to the light emission times of the exposure apparatuses 11Y, 11M, 11C, and 11K. Convert. Further, the PWM processing units 409Y, 409M, 409C, and 409K synchronize with the image formation timing in the color image formation engine unit 401, and convert the converted data into exposure devices 11Y, 11M, and 11C in the color image formation engine unit 401. , 11K. The color image formation engine unit 401 performs image formation on the recording material based on the received data. With the above processing, the color image forming process for the recording material is completed.

  As described above, in this embodiment, the ghost generated in the image formed at the target station located downstream with respect to the moving direction of the peripheral surface of the photosensitive drum is corrected for the image data used for image formation at the target station. To reduce it. Specifically, the correction amount to be applied to the image data is determined from the image data used in each of the target station and one or more stations upstream from the target station. The correction amount is obtained by calculating the difference between the surface potential of the photosensitive drum after exposure when a ghost occurs and the reference potential when the ghost is generated with the surface potential of the photosensitive drum after exposure when no ghost is generated as the reference potential. The amount of correction to be reduced is determined. Further, the correction amount is determined by comparing the image data of the pixel of interest used in the station of interest and the number of pixels corresponding to the circumference of the photosensitive drum with respect to the pixel of interest in the upstream station. Use image data. By performing image formation with the image data corrected with the correction amount determined in this way, in the present embodiment, it is possible to easily reduce the occurrence of ghosts without using a pre-exposure device.

[Second Embodiment]
In the first embodiment, the ghost suppression process has been described in the case where a ghost occurs in another pixel in the same page due to any pixel in the image of one page. However, when executing a print job that continuously prints images of multiple pages on a recording material, a ghost may occur in pixels in the next page due to any pixel in the image of one page. There is sex. Therefore, in the second embodiment, a ghost suppression process that can occur when images of a plurality of pages are continuously printed on a recording material will be described. Note that description of parts common to the first embodiment is omitted or simplified.

  FIG. 11 shows a state in which a ghost is generated in the image 1102 of the second page due to the image 1101 of the first page when two pages of images are continuously printed on two recording materials. ing. Reference numeral 1121 denotes a red solid image, 1123 denotes a C halftone image, and 1122 denotes a ghost of an image 1121 appearing in the halftone image 1123. FIG. 11 shows a state in which two pages of images (toner images) 11011 and 1102 are continuously formed on the intermediate transfer belt 14, and the rear end of the image 1101 and the front end of the image 1102 in the sub-scanning direction. H (distance between images) and h <d. That is, the inter-image distance h is shorter than the length of each photosensitive drum. In this case, as shown in FIG. 11, an image 1121 formed near the rear end of the image 1101 on the first page may appear as a ghost 1122 in an area near the front end of the image 1102 on the second page. This can occur when the image of the next page is formed in the next round in the area where the image 1101 that may cause ghosts is formed on the surface of the photosensitive drum when the distance between successive images is short. . In this embodiment, when a plurality of images are continuously formed as described above, a ghost that may occur in the second and subsequent images is suppressed.

  In the present embodiment, the configuration of the image forming apparatus 100 and the configuration of the image processing unit 402 are the same as those in FIGS. 1 and 8 in the first embodiment. In the present embodiment, bitmap image data for a plurality of pages is stored in the image buffer 405 and the transfer buffers 408Y, 408M, 408C, and 408K. In this case, the ghost suppression unit 406 determines that the distance between the image and the image of the immediately preceding page is the photosensitive drum in the image forming process for the second and subsequent pages when a plurality of images are continuously formed. The following processing is executed when the circumference is shorter than the circumference d.

In a print job that prints images of multiple pages, the pixel values included in the image data of the first image are Y1 (x1, y1), M1 (x1, y1), C1 (x1, y1), K1 (x1 , y1). The pixel values included in the image data of the second image are Y2 (x2, y2), M2 (x2, y2), C2 (x2, y2), and K2 (x2, y2). However, 0 ≦ x1 ≦ W, 0 ≦ y1 ≦ L, 0 ≦ x2 ≦ W, 0 ≦ y2 ≦ L, and W is the number of pixels in the sub-scanning direction (horizontal direction) of the image as in the first embodiment. , L represents the number of pixels in the main scanning direction (vertical direction) of the image. In this case, the ghost suppression unit 406 converts the image data of all pixels represented by the image data Y (x, y), M (x, y), C (x, y), K (x, y), Image data Y ′ (x, y), M ′ (x, y), C ′ (x, y), and K ′ (x, y) are corrected. Image data Y ′ (x, y), M ′ (x, y), C ′ (x, y), and K ′ (x, y) are represented by the following expressions. The ghost suppression unit 406 performs the same processing as that of the first embodiment on the first bitmap image data as shown in the following equation. That is, the corrected image data is
Y1 '(x1, y1) = Y1 (x1, y1)
M1 '(x1, y1) = M1 (x1, y1)
C1 '(x1, y1) = C1 (x1, y1)
K1 '(x1, y1) = K1 (x1, y1) (0≤x1≤W, 0≤y1 <d) (4)
When,
Y1 '(x1, y1) = Y1 (x1, y1)
M1 '(x1, y1) = M1 (x1, y1) + G (M1 (x1, y1), H _M (x1, y1-d))
C1 '(x1, y1) = C1 (x1, y1) + G (C1 (x1, y1), H _C (x1, y1-d))
K1 '(x1, y1) = K1 (x1, y1) + G (K1 (x1, y1), H K (x1, y1-d)) (0 ≦ x1 ≦ W, d ≦ y1 ≦ L) (5)
And is represented by here,
H _M (a, b) = H (M1 (a, b), Y1 (a, b))
H _C (a, b) = H (C1 (a, b), Y1 (a, b) + M1 (a, b))
H _K (a, b) = H (K1 (a, b), Y1 (a, b) + M1 (a, b) + C1 (a, b)) (6)
It is.

On the other hand, the corrected data Y2 ′ (x2, y2) for the second bitmap image data Y2 (x2, y2), M2 (x2, y2), C2 (x2, y2), K2 (x2, y2) ), M2 ′ (x2, y2), C2 ′ (x2, y2), and K2 ′ (x2, y2) are obtained by the following equations. That is, the corrected image data is
Y2 '(x2, y2) = Y2 (x2, y2)
M2 '(x2, y2) = M2 (x2, y2) + G (M2 (x2, y2), I _M1 (x2, y2))
C2 '(x2, y2) = C2 (x2, y2) + G (C2 (x2, y2), I _C1 (x2, y2))
K2 '(x2, y2) = K2 (x2, y2) + G (K2 (x2, y2), I _K1 (x2, y2)) (0 ≦ x2 ≦ W, 0 ≦ y2 ≦ dh) (7)
When,
Y2 '(x1, y1) = Y2 (x1, y1)
M2 '(x2, y2) = M2 (x2, y2)
C2 '(x2, y2) = C2 (x2, y2)
K2 '(x2, y2) = K2 (x2, y2) (0 ≦ x2 ≦ W, dh <y2 <d) (8)
When,
Y2 '(x1, y1) = Y2 (x2, y2)
M2 '(x1, y1) = M2 (x2, y2) + G (M2 (x2, y2), I _M2 (x2, y2-d))
C2 '(x2, y2) = C2 (x2, y2) + G (C2 (x2, y2), I _C2 (x2, y2-d))
K2 '(x2, y2) = K2 (x2, y2) + G (K2 (x2, y2), I _K2 (x2, y2-d)) (0≤x2≤W, d≤y2≤L) (9)
And is represented by here,
I _M1 (a, b) = H (M1 (a, L- {d- (h + b)}), Y1 (a, L- {d- (h + b)})
I _C1 (a, b) = H (C1 (a, L- {d- (h + b)}), Y1 (a, L- {d- (h + b)}) + M1 (a, L- {d- (h + b)}))
I _K1 (a, b) = H (K1 (a, b), Y1 (a, L- {d- (h + b)}) + M1 (a, L- {d- (h + b)}) + C1 (a, L- {d- (h + b)}))
I _M2 (a, b) = H (M2 (a, b), Y2 (a, b))
I _C2 (a, b) = H (C2 (a, b), Y2 (a, b) + M2 (a, b))
I _K2 (a, b) = H (K2 (a, b), Y2 (a, b) + M2 (a, b) + C2 (a, b)) (10)
It is. Here, h is the number of pixels corresponding to the distance between the first image and the second image.

  As described above, in the second image, a region where a ghost may occur due to the first image is a range (d−h) from the front end in the sub-scanning direction of the second image. (0 ≦ y2 ≦ d−h). Therefore, in this embodiment, the ghost suppression process for the image data of the pixels belonging to the range is executed based on the bitmap image of the first image. In the first image, an area including a pixel that can generate a ghost in the second image is a distance (d−h) from the rear end in the sub-scanning direction of the first image. (L− (d−h) ≦ y1 ≦ L).

  On the other hand, in the second image, in the range of (d−h <y2 <d) in the sub-scanning direction, there is no possibility of ghosting, so the image data is not corrected. Also, in the range of (d ≦ y2 ≦ L) in the second image, a ghost attributed to other pixels on the same page can occur as in the first image. For this reason, the bitmap image data is corrected as in the case of the first image (first embodiment). For the third and subsequent images, the same processing as the ghost suppression processing performed for the second image may be executed using the bitmap image data of the image of the immediately preceding page.

  For example, by filling the space between the first image and the second image with 00H pixels, the first image and the second image are considered as one image, Ghosting can also be suppressed by performing processing similar to that of the first embodiment on the image.

<Image formation processing procedure>
Next, with reference to FIG. 12A and FIG. 12B, an image forming process procedure including the ghost suppressing process according to the present embodiment executed by the ghost suppressing unit 406 of the image forming apparatus 100 will be described. In the figure, the same reference numerals as those in FIG. 10 are assigned to steps for executing the same processing as in the first embodiment.

  When the image forming apparatus 100 receives print data from a host computer or the like, image processing using the print data is started in the image processing unit 402. In this embodiment, since the print data includes data for printing a plurality of pages of images on a recording material, a variable n indicating the page number of the bitmap image data being processed is introduced. In step S201, the image processing unit 402 initializes a variable n to 1 representing the first page. In S101 to S103, the same processing as that of the first embodiment is executed for the nth page image. Thus, Y, M, C, and K bitmap image data Yn (x, y), Mn (x, y), Cn (x, y), and Kn (x, y) are generated and stored in the image buffer 405. Stored.

  In step S <b> 202, the ghost suppressing unit 406 refers to the data stored in the image buffer 405 and determines whether the image to be formed is the first page (n = 1) or the image to be formed. It is determined whether the inter-image distance h between the image and the image formed immediately before is equal to or greater than d. Here, the determination regarding the inter-image distance h can be realized by confirming what stage of the image forming operation the color image forming engine unit 401 is performing at the time point. If the ghost suppression unit 406 determines in step S202 that n = 1 or the inter-image distance h is greater than or equal to d, the process proceeds to step S104. In S104 to S114, processing similar to that in the first embodiment is performed on bitmap image data of all pixels included in the image of the nth page to be printed.

  Thereafter, in S203, the image processing unit 402 determines whether or not the color image forming engine unit 401 is operating. If it is determined that the color image forming engine unit 401 is operating, the process proceeds to S117, but is determined not to be operating. If so, the process proceeds to S116. In step S116, the color image formation engine unit 401 starts an initial operation for image formation. Thereafter, in S117, as in the first embodiment, the color image formation engine unit 401 performs image formation on the recording material based on the bitmap image data stored in the transfer buffers 408Y, 408M, 408C, and 408K. The

  In step S <b> 204, the image processing unit 402 performs image forming processing by checking whether or not data for an image to be printed remains in print data received from, for example, a host computer. It is determined whether or not to end. Here, if image data of the page to be printed remains, the image processing unit 402 determines that the image forming process is not ended in order to form an image of the remaining page, and the page is determined to be S219. The number is incremented by +1, and the process returns to S101. On the other hand, if it is determined that there are no remaining pages to be printed, it is determined that the image forming process is to be terminated, and the process is terminated.

(Process when n ≠ 1 and h <d)
If the ghost suppression unit 406 determines in step S202 that n ≠ 1 and the inter-image distance h <d, the process proceeds to step S205. In S205, as in S104, variables x and y are initialized to 0. In step S <b> 206, the ghost suppressing unit 406 determines whether 0 ≦ y ≦ d−h is satisfied with reference to the variable y indicating the position of the target pixel in the sub-scanning direction. Thereby, it is determined whether or not the target pixel is a pixel within a range of 0 ≦ y ≦ d−h with respect to the sub-scanning direction. If it is determined that 0 ≦ y ≦ d−h is satisfied, the process proceeds to S207. For the pixels within the range, a ghost may occur due to the image data of the pixels included in the image of the immediately previous page. For this reason, in S207, the ghost suppressing unit 406 uses the equation (7) to calculate the corrected bitmap image data Yn ′ (x, y), Mn ′ (x, y), Cn ′ (x, y) and Kn ′ (x, y) are obtained. Thereafter, the process proceeds to S211.

  If the ghost suppression unit 406 determines in step S206 that 0 ≦ y ≦ d−h is not satisfied for the variable y, the process proceeds to step S208. In S208, the ghost suppressing unit 406 determines whether or not dh <y <d is satisfied for the variable y, so that the pixel of interest is within the range of dh <y <d in the sub-scanning direction. It is determined whether or not it is a pixel. If it is determined that dh <y <d is satisfied, the process proceeds to S209. A ghost does not occur for pixels within the range. For this reason, the image data Yn (x, y), Mn (x, y), Cn (x, y), and Kn (x, y) before correction are not corrected, and are directly changed according to the equation (8). Output as Yn ′ (x, y), Mn ′ (x, y), Cn ′ (x, y), Kn ′ (x, y). Thereafter, the process proceeds to S211.

  On the other hand, if the ghost suppression unit 406 determines in step S208 that dh <y <d is not satisfied, the process proceeds to step S210. When the target pixel is a pixel in the range of d ≦ y ≦ L in the sub-scanning direction, a ghost may occur in the target pixel due to image data of other pixels in the same page. For this reason, in S210, the ghost suppression unit 406 uses the equation (9) to calculate the corrected bitmap image data Yn ′ (x, y), Mn ′ (x, y), Cn ′ (x, y) and Kn ′ (x, y) are obtained. Thereafter, the process proceeds to S211.

  In S211, the ghost suppression unit 406 determines the bitmap image data Yn ′ (x, y), Mn ′ (x, y), Cn ′ (x, y), Kn ′ (x) obtained in S207, S209, or S210. , y) are sent to the halftone processing units 407Y, 407M, 407C, and 407K, respectively. In S212 and S213, as in S109 and S110, halftones for bitmap image data Y ′ (x, y), M ′ (x, y), C ′ (x, y), K ′ (x, y) Processing is performed. Further, the processed data is stored in the transfer buffers 408Y, 408M, 408C, and 408K, respectively.

  The subsequent processing of S214 to S218 is the same as S111 to S115. With this processing, the processing of S206 to S213 is sequentially executed for pixels on one main scanning line in the main scanning direction (x direction) of the bitmap image data, and all the main scanning lines in the image of one page are The processes of S206 to S213 are sequentially executed. In S217, the image processing unit 402 determines that y = L is satisfied, and if it is determined that the processing for all main scanning lines in the image to be printed has been completed, the process proceeds to S117. The processing after S117 is as described above. If the image processing unit 402 determines in S204 that there is no page to be printed, the image processing unit 402 determines to end the image forming process and ends the process.

  As described above, according to the present embodiment, even when images of a plurality of pages are continuously formed, ghosts that can occur in an image formed at a downstream target station can be reduced with a simple method. Is possible.

[Other Embodiments]
The present invention is not limited to the above-described embodiment, and various modifications can be made, and any of the modifications exemplified below can obtain the same effect as the above-described embodiment. For example, as the reference tables 901 and 902, a plurality of reference tables are prepared in advance according to environmental information such as the absolute water content, and appropriate tables are prepared according to the detection result of the environment in which the color image forming engine unit 401 is arranged. It may be used by switching. Further, as in the above-described embodiment, common reference tables 901 and 902 may be used in the second to fourth stations, or different tables may be prepared for each station.

  In the above-described embodiment, the reference tables 901 and 902 are created based on the data shown in FIGS. However, for example, based on the output result of the test pattern image, a correction amount that can suppress the ghost may be determined, and the reference table may be created. Further, the correction amount is not obtained from the reference table, but is derived for each pixel at the time of image formation using a predetermined conversion formula or data corresponding to the graphs shown in FIGS. Also good. At that time, as data corresponding to the graph prepared in advance, a plurality of data may be prepared for each station, each durable sheet, or each environment.

  The reference tables 901 and 902 are not tables that include all image data that can be considered as shown in FIG. 9. For example, data corresponding to an index that does not require correction, such as image data of 100% or less, is not used. , May be omitted. In addition, for image data on the high density side, data that is indexed by such image data may be omitted so that no ghost can occur and correction of the image data is unnecessary. In this way, the size of the reference tables 901 and 902 can be reduced.

  Referring to the reference table 901 shown in FIG. 9, it can be seen that even if the ghost suppression processing in the above-described embodiment is applied to the image data for the second station, the image data is actually hardly corrected. Therefore, it is not necessary to apply the ghost suppression process to the image data for the second station, and it is only necessary to apply the image data for the stations after the third station. Thereby, the processing amount required for the ghost suppression process can be reduced.

  In general, in a color image forming apparatus, from the viewpoint of color reproduction range and fixability, the maximum amount of applied toner when all the toners of Y color, M color, C color, and K color are used, and each applied toner amount The combinations are determined in advance. Therefore, for example, when the image data of the image A shown in FIG. 6 is data that can generate a ghost, the image data of the image B may always be 00H. In such a case, the correction process using the reference table 902 is not always necessary.

  The image processing related to determination of the correction amount of image data and correction using the correction amount in the present invention may be executed not on the image forming apparatus 100 but on the host computer side, for example. In this case, the host computer corresponds to the image processing apparatus of the present invention. The image forming apparatus 100 is not limited to the inline type color image forming apparatus as in the above-described embodiment. For example, the intermediate transfer body type color image forming apparatus including a plurality of developing devices for one photosensitive drum It may be a device. Alternatively, the image forming apparatus 100 forms a toner image of a plurality of colors on a recording material adsorbed on a recording material conveying member (rotary member) such as an electrostatic transfer belt that electrostatically adsorbs and conveys the recording material. A multiple transfer type image forming apparatus that forms an image by sequentially transferring images may be used.

  The present invention can also be realized by executing the following processing. That is, software (program) that realizes the functions of the above-described embodiments is supplied to a system or apparatus via a network or various storage media, and a computer (or CPU, MPU, or the like) of the system or apparatus reads the program. It is a process to be executed.

Claims (9)

An image forming apparatus,
An image carrier, exposure means for forming an electrostatic latent image on the surface of the image carrier by exposing the surface of the image carrier according to image data, and developing the electrostatic latent image with a developer. A plurality of image forming means for forming images of different colors, each of which includes a developing means for forming an image on the surface of the image carrier;
A multicolor image to be formed on a recording material is formed by transferring images from the image carriers of the plurality of image forming units, or images are transferred from the image carriers of the plurality of image forming units. A rotating body for conveying a recording material on which a multicolor image is formed,
Among the plurality of image forming means, a correction amount to be applied to image data used in the first image forming means positioned downstream with respect to the moving direction of the circumferential surface of the rotating body, The post-exposure after exposure according to the image data in the first image forming means, which is caused by an image formed by one or more second image forming means located upstream from the one image forming means Determining means for determining the correction amount for reducing the difference between the surface potential of the image carrier and the reference potential corresponding to the image data from the image data used in the first and second image forming means;
An image forming apparatus comprising: a correcting unit that corrects image data used in the first image forming unit using the correction amount determined by the determining unit. The determining means includes
The correction amount for correcting the image data of the first pixel used in the first image forming unit is set to the image data of the first pixel used in the first image forming unit and the second image formation. Among the image data used in the means, the second formed on the downstream side by the number of pixels corresponding to the circumferential length of the image carrier relative to the first pixel with respect to the moving direction of the peripheral surface of the rotating body. The image forming apparatus according to claim 1, wherein the image forming apparatus is determined from image data of the pixels. The first pixel image data used by the first image forming unit, the second pixel image data used by the second image forming unit, and the first image forming unit used by the first image forming unit. Storage means for storing in advance a table in which correction amounts for correcting image data of the pixels are associated;
The determining means includes
Referring to the table stored in the storage unit, the image data of the first pixel used in the first image forming unit and the image data of the second pixel used in the second image forming unit The image forming apparatus according to claim 2, wherein the correction amount corresponding to each of the two is a determined correction amount. The determining means and the correcting means are:
With respect to the moving direction of the circumferential surface of the rotating body, the correction amount is determined for pixels that are further rearward than the rear pixels by the number of pixels corresponding to the circumferential length of the image carrier from the leading edge of the image to be formed. The image forming apparatus according to claim 1, wherein the image data is corrected using the correction amount. The determining means includes
In the case where the image forming unit continuously forms images of a plurality of pages, when the distance between images on the peripheral surface of the rotating body is shorter than the peripheral length of the image carrier,
The pixels used in the first image forming unit for pixels included in a range corresponding to the circumference of the image carrier from the rear end of the image of the page formed immediately before the page with respect to the moving direction. 4. The image according to claim 1, wherein a correction amount is determined from image data for a page and image data for the immediately preceding page used by the second image forming unit. 5. Forming equipment. The first image forming unit is any one of the third and subsequent image forming units from the upstream side with respect to the moving direction of the peripheral surface of the rotating body among the plurality of image forming units. The image forming apparatus according to any one of claims 1 to 5. An image carrier, exposure means for forming an electrostatic latent image on the surface of the image carrier by exposing the surface of the image carrier according to image data, and developing the electrostatic latent image with a developer. A plurality of image forming means for forming images of different colors, each of which includes a developing means for forming an image on the surface of the image carrier;
A multicolor image to be formed on a recording material is formed by transferring images from the image carriers of the plurality of image forming units, or images are transferred from the image carriers of the plurality of image forming units. An image processing apparatus that performs image processing on image data used in an image forming apparatus including a rotating body that conveys a recording material on which a multicolor image is formed,
Among the plurality of image forming means, a correction amount to be applied to image data used in the first image forming means positioned downstream with respect to the moving direction of the circumferential surface of the rotating body, The post-exposure after exposure according to the image data in the first image forming means, which is caused by an image formed by one or more second image forming means located upstream from the one image forming means Determining means for determining the correction amount for reducing the difference between the surface potential of the image carrier and the reference potential corresponding to the image data from the image data used in the first and second image forming means;
An image processing apparatus comprising: a correction unit that corrects image data used in the first image forming unit using the correction amount determined by the determination unit. An image carrier, exposure means for forming an electrostatic latent image on the surface of the image carrier by exposing the surface of the image carrier according to image data, and developing the electrostatic latent image with a developer. A plurality of image forming means for forming images of different colors, each of which includes a developing means for forming an image on the surface of the image carrier;
A multicolor image to be formed on a recording material is formed by transferring images from the image carriers of the plurality of image forming units, or images are transferred from the image carriers of the plurality of image forming units. An image processing method for image data used in an image forming apparatus including a rotating body that conveys a recording material on which a multicolor image is formed,
Among the plurality of image forming means, a correction amount to be applied to image data used in the first image forming means positioned downstream with respect to the moving direction of the circumferential surface of the rotating body, The post-exposure after exposure according to the image data in the first image forming means, which is caused by an image formed by one or more second image forming means located upstream from the one image forming means A determination step of determining the correction amount for reducing a difference between a surface potential of the image carrier and a reference potential corresponding to the image data from image data used in the first and second image forming units;
An image processing method comprising: a correction step of correcting image data used in the first image forming unit using the correction amount determined in the determination step.   A program for causing a computer to execute the image processing method according to claim 8.

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