JP2012177779A - Image forming apparatus and method - Google Patents

Abstract

An object of the present invention is to perform banding correction with high accuracy while suppressing the occurrence of color unevenness due to a correction error.
A plurality of image forming units 100Y, 100M, 100C, and 100K, a secondary transfer device 111, and an intermediate transfer belt cleaning device 113. Further, a fixing device 112 is disposed on the downstream side of the secondary transfer device 111. For each of the image forming units 100Y, 100M, 100C, and 100K, corresponding color banding correction units 130Y, 130M, 130C, and 130K are installed. Further, a correction error storage unit 140 and an HT image storage unit (not shown) are installed.
[Selection] Figure 1

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile using an electrophotographic system or an electrostatic recording system.

  2. Description of the Related Art Conventionally, in an image forming apparatus such as an electrophotographic printer, generally, an electrostatic latent image corresponding to an image signal is formed by exposing an image carrier such as a photosensitive drum or a photosensitive belt with a laser beam or the like. An image is formed by developing on a sheet or the like after development. In addition, a color image forming apparatus that forms a plurality of toner images with different development colors and overlays them to form a full color image has been put into practical use. At that time, an electrostatic latent image is sequentially scanned in a one-dimensional direction, for example, from left to right, and the image carrier is rotated to perform a sub-scan in the direction orthogonal to the main scanning direction, for example, from top to bottom. Can be formed. Therefore, when the laser beam is scanned, a large number of electrostatic latent images in a straight line in the main scanning direction (hereinafter referred to as scanning lines) are parallel on the image carrier at constant intervals (hereinafter referred to as reference scanning line intervals) in the sub-scanning direction. Formed.

  In such an image forming apparatus, since scanning lines are used, horizontal stripes (hereinafter referred to as banding) due to the density of the image density are generated in the formed image due to various causes, thereby causing a problem that the quality of the image is remarkably impaired. For example, the above-mentioned image carrier may have uneven rotation speed, and non-uniformity occurs in the interval between scanning lines written on the image carrier, which appears as banding of the formed image. More specifically, when the rotation speed of the image carrier is high, the interval between the scanning lines is widened, so that the exposure amount per unit area is reduced and the output image is thinned. On the other hand, when the rotation speed is low, the exposure amount per unit area increases, so that the output image becomes dark. When the rotational speed of the image carrier is uneven, these irregularly occur and become banding.

  As means for solving the above problem, there has been proposed a method of detecting the speed error of the rotation speed of the image carrier and adjusting the exposure amount so as to cancel the banding caused by the speed error (Patent Document 1).

Japanese Patent Laid-Open No. 2-131956

  However, since the adjustment method of Patent Document 1 performs banding correction independently for each color, there is a problem that a correction error occurs in each banding correction and color unevenness occurs due to the color balance being lost. As an example, when performing banding correction of a certain color, if an error occurs and the exposure amount is excessive, the hue of the final image is different from the desired one, and the color of that color becomes strong. In addition, the color with an underexposed amount becomes weak in color and the color of other colors becomes strong.

  Accordingly, an object of the present invention is to perform accurate banding correction that suppresses the occurrence of color unevenness due to such correction errors.

  In order to achieve such an object, the image forming apparatus of the present invention has a plurality of latent images on the surface of the photoconductor by an exposure unit that exposes the photoconductor surface by scanning the photoconductor surface with an exposure amount for each pixel corresponding to input image data. An image forming apparatus that forms an image by overlapping images and is provided in an exposure unit, and a scanning position calculation unit that calculates a scanning position scanned for exposure, and a scanning position for scanning from the scanning position for exposure Targeted from the pixel interval prediction means for calculating the predicted value of the pixel interval, the pixel pattern acquisition means for acquiring the peripheral pixel pattern for each pixel to be printed from the input image data, and the predicted value and pixel pattern of the pixel interval. An exposure amount correction unit that corrects an exposure amount for each pixel according to input image data for the pixel, an exposure amount calculated by the scanning position calculated by the scanning position calculation unit, and an exposure amount compensation Characterized in that a correction error calculation means for calculating a correction error of the corrected exposure amount by means.

  The present invention has an effect of correcting banding with high accuracy.

1 is a configuration diagram of an image forming apparatus according to Embodiment 1 of the present invention. It is a schematic diagram showing the scanning position of the image forming apparatus of the present embodiment. It is a schematic diagram showing the fluctuation | variation of the scanning position of the image forming apparatus of a present Example. 6 is a flowchart illustrating an example of a banding correction process performed by the image forming apparatus. 1 is a block diagram illustrating a configuration of a processing system of an image forming apparatus according to an exemplary embodiment. It is a schematic diagram which shows an example of an optical position sensor. FIG. 4 is a diagram illustrating an example of a scanning position table held by the image forming apparatus. It is a schematic diagram showing the relationship between the scanning position of a present Example, and the predicted value of a scanning line space | interval. It is a conceptual diagram of density distribution calculation used for the correction factor calculation process of a present Example. FIG. 5 is a schematic diagram illustrating an example of an operation order of each process performed by the image forming apparatus. It is a schematic diagram showing the relationship between the scanning position and scanning line space | interval of a present Example. 6 is a diagram illustrating an example of a Y-color banding correction error table held by the image forming apparatus. FIG. 3 is a diagram illustrating an example of a UCR table held by an image forming apparatus. FIG. It is a schematic diagram of a banding image and an image appropriately corrected according to the present embodiment. It is a schematic diagram of a jitter shift caused by the accuracy of the polygon mirror of the present embodiment.

The best mode for carrying out the present invention will be described below with reference to the drawings. However, the relative arrangement, numerical formulas, numerical values, and the like of the components described in this embodiment are not intended to limit the scope of the present invention only to those unless otherwise specified.
[Example 1]
Hereinafter, the present invention will be described in detail according to preferred embodiments with reference to the accompanying drawings. FIG. 1 is a configuration diagram of the image forming apparatus shown in the present embodiment. The configuration of the image forming apparatus will be described below with reference to FIG.

  Along the intermediate transfer belt 110, which is an example of a conveyance body, modules described below are arranged in the order described below from the upstream in the rotation direction R1 of the intermediate transfer belt 110. Each module is a plurality of image forming units 100Y, 100M, 100C, 100K, a secondary transfer device 111, and an intermediate transfer belt cleaning device 113. Further, a fixing device 112 is disposed on the downstream side of the secondary transfer device 111. Here, a module that exists for each color is identified by attaching a character representing the color. That is, the image forming units 100Y, 100M, 100C, and 100K indicate image forming units for Y color, M color, C color, and K color, respectively.

  For each of the image forming units 100Y, 100M, 100C, and 100K, corresponding color banding correction units 130Y, 130M, 130C, and 130K are installed. Further, a correction error storage unit 140 and an HT image storage unit (not shown) are installed.

  The operation of the image forming apparatus will be described below. The image forming units 100Y, 100M, 100C, and 100K form toner images of the respective colors and primarily transfer them on the intermediate transfer belt 110. Each color toner image is formed at the same time by shifting the timing by a predetermined time in the order of Y color, M color, C color, and K color. The image forming units 100Y, 100M, 100C, and 100K are configured in the same manner except that toner colors used in the development processing described below are different from Y color, M color, C color, and K color, respectively. Therefore, details of the configuration and processing of the image forming units 100M, 100C, and 100K can be understood by appropriately replacing the description of the image forming unit 100Y below.

  The image forming unit 100Y includes a charging device 102Y, an exposure device 103Y, a developing device 104Y, a transfer device 105Y, a cleaning device 106Y, an encoder 121, and an optical position sensor 122 around the photosensitive drum 101Y. The photoreceptor drum 101Y has an organic photoconductor layer with a negative polarity on the outer peripheral surface thereof, and rotates in the direction of the arrow R3.

(Charging)
The charging device 102Y is applied with a negative polarity voltage and irradiates the surface of the photosensitive drum 101Y with charged particles, whereby the photosensitive drum 101Y charges the surface to a uniform negative polarity potential.

(exposure)
The exposure device 103Y drives a laser based on the Y color separation color image of the input image data, and scans the photosensitive drum 101Y with a rotating mirror. Thereby, an electrostatic image is written on the surface of the charged photosensitive drum 101Y.

(developing)
The developing device 104Y attaches the negatively charged toner to the electrostatic image on the photosensitive drum 101Y, and reversely develops the electrostatic image.

(Transcription)
The transfer device 105 </ b> Y is applied with a positive charge, is negatively charged, and primarily transfers the toner image carried on the photosensitive drum 101 </ b> Y onto the intermediate transfer belt 110.

(cleaning)
The cleaning device 106Y removes the residual toner image that has passed through the transfer device 105Y and remained on the surface of the photosensitive drum 101Y.

(Secondary transfer)
The secondary transfer device 111 collectively transfers the four color toner images carried on the intermediate transfer belt 110 onto the recording material P moving in the direction of the arrow R2.

(Fixing)
The fixing device 112 performs a process such as pressure heating on the recording material P onto which the four color toner images have been secondarily transferred at once, thereby fixing the full color image on the surface.

(Belt cleaning)
The intermediate transfer belt cleaning device 113 removes residual toner remaining on the intermediate transfer belt 110 after passing through the secondary transfer device 111.

(Banding correction)
The encoder 121Y calculates the rotation angle θ of the photosensitive drum 101Y and outputs it to the Y-color banding correction unit 130Y. The optical position sensor 122Y calculates the laser irradiation position by the exposure device 103Y and outputs it to the Y-color banding correction unit 130Y. Each of the color banding correction units 130Y, 130M, 130C, and 130K performs banding correction processing in the corresponding image forming unit. In addition, each banding correction error is input to and output from the correction error storage unit 140. The correction error storage unit 140 inputs / outputs each color banding correction error to / from each color banding correction unit 130Y, 130M, 130C, 130K, and holds the obtained color banding correction error.

  Each color image forming unit scans the scanning lines at equal intervals by controlling the rotation speed of the photosensitive drum to be constant and controlling the time interval of the writing timing of each scanning line by the laser light to be constant. To do. Details thereof will be described below with reference to FIG.

The photosensitive drum has a radius r and is controlled to rotate at a constant angular velocity ω. The time interval of write timing of each scan line is controlled to be constant at t _0.
First, the first scanning line is scanned. At this writing start timing, the time is set to 0, and the scanning position on the surface of the photoreceptor is set to 0. Next, the second scanning line is scanned. Since the time is t ₀ and the rotation angle of the drum is t ₀ ω at the writing timing at this time, the scanning position on the photosensitive member surface is rt ₀ ω from the relationship between the angle and the arc length. Further, the third scanning line is scanned. At this writing start time, the time is 2t ₀ and the drum is rotated by an angle 2t ₀ ω, so the scanning position on the surface of the photoconductor is 2rt ₀ ω.

Similarly, at the writing start timing when scanning the i-th scanning line, the time t (i) and the scanning position D ₀ (i) on the surface of the photosensitive member can be calculated by the following equations, respectively.

As described above, the scanning line interval is controlled to be constant at rt ₀ ω. However, in such an image forming apparatus, an error occurs in the scanning line interval due to various causes. The main causes are the positional error of laser irradiation due to the fluctuation of the rotational speed of the drum and the surface tilt of the polygon mirror. As shown in FIG. 3A, when the rotational speed of the drum changes, the actual scanning position D (i) on the surface of the photoconductor becomes different from D ₀ (i) obtained above. Further, when the reflection angle of the laser beam changes due to the surface tilt of the polygon mirror (angle variation of a plurality of reflection surfaces), an error occurs in the laser irradiation position as shown in FIG.

  As shown in the image a1 in FIG. 14, by operating the lasers with the same intensity and the same interval, an image having a uniform density as shown in the image a2 can be formed. However, in actuality, as described above, the scanning line interval is not an equal interval but may include an error, so that coarseness and density occur as shown in an image b1 in FIG. At this time, in the formed image, as shown in an image b2 in FIG. 14, a dark image is formed in a portion where the scanning line interval is narrow, and a thin image is formed in a portion where the scanning line interval is sparse. As described above, when the scanning line interval is not an equal interval but includes an error, the shading of the image changes and banding occurs.

  The image forming apparatus shown in the present embodiment corrects this banding by adjusting the laser light amount (exposure amount) by the banding correction process performed by each color banding correction unit 130Y, 130M, 130C, and 130K. That is, as shown in the image c1 ′ of FIG. 14, the portion where the scanning line interval is narrow is weak, and the portion where the scanning line interval is sparse is strongly irradiated with the laser, so that the scanning line interval is dense and dense. Cancels the shading of the image and corrects the banding.

  The banding correction process in this embodiment is performed in synchronization with the toner image forming process performed by the corresponding image forming unit in each of the color banding correction units 130Y, 130M, 130C, and 130K. For this reason, each color banding correction process is carried out simultaneously with the timing shifted by a certain time in the order of Y color, M color, C color, and K color in the same manner as the toner image forming process for each color. First, the configuration of the Y-color banding correction unit 130Y will be described with reference to the block diagram of FIG.

(HT image storage unit 501)
The HT image storage unit 501 holds Y, M, C, and K color HT images, and outputs the HT image to the exposure amount calculation unit 502Y and the peripheral pixel pattern acquisition unit 507Y. The Y color HT image is a Y color separation color image generated based on the input image, and is configured as a matrix of I rows in the sub-scanning direction and J columns in the main scanning direction. Each element of the Y color HT image is composed of “0” or “1”, and “0” corresponds to the corresponding pixel being OFF and “1” being ON. The same applies to the M, C, and K color HT images. In the following description, a pixel corresponding to the i-th row and the j-th column is described as a pixel (i, j).

(Exposure amount calculation unit 502Y)
The exposure amount calculation unit 502Y receives each color HT image output from the HT image storage unit 501, performs exposure amount calculation processing, and outputs the calculated exposure amount to the exposure amount correction unit 509Y.

(Scanning position calculation unit 503Y)
The scanning position calculation unit 503Y receives the pulse signal output from the encoder 121Y and the laser irradiation position output from the optical position sensor 122Y, performs a scanning position calculation process, and outputs the calculated scanning position to the scanning position storage unit 504Y. To do.

(Scanning position storage unit 504Y)
The scanning position storage unit 504Y receives the scanning position output from the scanning position calculation unit 503Y, holds the received scanning position, and outputs it to the scanning line interval prediction unit 505Y and the scanning line interval calculation unit 510Y.

(Scanning line interval prediction unit 505Y)
The scanning line interval prediction unit 505Y receives the scanning position output from the scanning position storage unit 504Y, performs a scanning interval prediction process, calculates a predicted value of the scanning line interval, and calculates the predicted value of the scanning line interval as a pixel interval prediction. Output to the unit 506Y.

(Pixel interval prediction unit 506Y)
The pixel interval prediction unit 506Y receives the predicted value of the scanning line interval output from the scanning line interval prediction unit 505Y, performs the pixel interval prediction process to calculate the predicted value of the scanning line interval, and calculates the calculated predicted value of the pixel interval. It outputs to the correction factor calculation unit 508Y.

(Peripheral pixel pattern acquisition unit 507Y)
The peripheral pixel pattern acquisition unit 507Y receives each color HT image output from the HT image storage unit 501, performs a peripheral pixel pattern acquisition process to acquire a peripheral pixel pattern, and uses the acquired peripheral pixel pattern as a correction factor calculation unit 508Y. Output to the correction error calculation unit 512Y.

(Correction rate calculation unit 508Y)
The correction rate calculation unit 508Y receives the predicted value of the pixel interval output from the pixel interval prediction unit 506Y and the peripheral pixel pattern output from the peripheral pixel pattern acquisition unit 507Y, and performs the correction rate calculation process to calculate the correction rate. . Also, the calculated correction rate is output to the exposure amount correction unit 509Y and the correction error calculation unit 512Y.

(Exposure amount correction unit 509Y)
The exposure amount correction unit 509Y receives the exposure amount output from the exposure amount calculation unit 502Y and the correction rate output from the correction rate calculation unit 508Y, performs exposure amount correction processing, and supplies the corrected exposure amount to the exposure apparatus 103Y. Output.

(Scanning line interval calculation unit 510Y)
The scanning line interval calculation unit 510Y receives the scanning position output from the scanning position storage unit 504Y, performs a scanning line interval calculation process to calculate the scanning line interval, and calculates the calculated scanning line interval to the pixel interval calculation unit 511Y. Output.

(Pixel interval calculation unit 511Y)
The pixel interval calculation unit 511Y receives the scanning line interval output from the scanning line interval calculation unit 510Y, performs pixel interval calculation processing to calculate the pixel interval, and outputs the calculated pixel interval to the correction error calculation unit 512Y. .

(Correction error calculation unit 512Y)
The correction error calculation unit 512Y receives the peripheral pixel pattern output from the peripheral pixel pattern acquisition unit 507Y, the correction rate output from the correction rate calculation unit 508Y, and the pixel interval output from the pixel interval calculation unit 511Y. Using these, a Y-banding correction error calculation process is performed to calculate a Y-banding correction error. Also, the calculated Y banding correction error is output to the correction error storage unit 140.

  Next, the Y color banding correction process performed in the Y color banding correction unit 130Y will be described with reference to the flowchart of FIG.

(Step S401Y)
Step S401Y is a process of setting 1 to the variable i. i is an index indicating the row number (scan line number) of the HT image. i increases by 1 each time the following steps S402Y to S417Y are performed, and is repeated until I (the number of pixels in the sub-scanning direction) is reached. This means that the following steps S402Y to S417Y are performed for each scanning line. Therefore, in the following description, general i is described without being limited to the case of i = 1.

(Step S402Y)
Step S402Y is a scanning line interval prediction process. The scanning line interval prediction unit 505Y predicts the scanning line interval between the i-th scanning line and the peripheral scanning line using the information in the scanning position table stored in the scanning position storage unit 504Y. Here, the peripheral scanning lines are the (i−1) th scanning line and the (i + 1) th scanning line.

  FIG. 7 is an example of a scanning position table stored in the scanning position storage unit 504Y. In this table, for each of the scanning lines from -2nd to i-th, the scanning position is stored in association with the scanning line number. The scanning position of each scanning line is a position in the sub-scanning direction on the surface of the photoconductor with respect to the first scanning line. This table is initialized as shown in FIG. 7B every time one image print process is completed. Further, the scanning position storage unit 504Y has a memory sufficient to store the scanning positions of all the scanning lines up to the I-th. In this embodiment, the scan positions are held on the assumption that there is no position error for the -2nd to 0th scan lines. However, the laser beam is also scanned in a region other than the image to obtain the scan position. The method of doing etc. can also be considered.

  Details of the scanning line interval prediction process will be described below. At this time, a predicted value of the scanning line interval between the i-th scanning line and the i′-th scanning line is written as G ^ (i, i ′). The actual scanning line interval between the i-th scanning line and the i′-th scanning line is written as G (i, i ′).

  FIG. 8 is a schematic diagram showing the relationship between the scanning position and the predicted value of the scanning line interval. Predicted value G ^ (i, i + 1) of the scanning line interval between the i-th scanning line and the i + 1th scanning line, and the predicted value G ^ of the scanning line interval between the (i-1) th scanning line and the i-th scanning line. (I-1, i) is calculated. At that time, the actual scanning line interval G (i, i + 1) between the i-th scanning line and the i + 1-th scanning line can be calculated from the difference between P (i + 1) and P (i). However, at the time of scanning the i-th scanning line, P (i + 1) is future information and cannot be acquired when processing is performed. Therefore, in this embodiment, the predicted values G ^ (i, i + 1) and G ^ (i-1, i) of the scanning line interval are calculated using the position information of the past scanning lines already detected. An example of the predicted value calculation formula for the scanning line interval is shown below.

  The prediction performed in this embodiment is known as a linear prediction method. The linear prediction method is a method for predicting a future value as a linear map of values observed in advance, and is generally described by the following equation.

Here, x ^ (i) is a predicted value, x (ik) is a value observed in advance, and _ak is a prediction coefficient. When this is rewritten according to the present embodiment, the following equation is obtained.

In general, the prediction coefficient a _k in the linear prediction method is selected such that the error | x (i) −x ^ (i) | In the present embodiment, this is set as a _k = 1 when k = 1, and a _k = 0 when k ≠ 1, and prediction is performed by applying Expression (6) as the following expression.

Therefore, G ^ (i-1, i) is calculated from the difference between P (i-1) and P (i-2). Similarly, G ^ (i, i + 1) is calculated from the difference between P (i) and P (i-1). As the prediction coefficient _{ak, an} appropriate value can be selected from the measurement values acquired in advance. Furthermore, in this embodiment, the linear prediction method is used as the prediction method, but other prediction methods such as a method using a Kalman filter or a simulator simulating the movement of the image forming apparatus can be used.

(Step S403Y)
Step S403Y is a pixel interval prediction process. The pixel interval prediction unit 506Y calculates a predicted value D ^ of the interval between each pixel (i, j) in the i-th row and surrounding pixels (i + k, j + l) by the following equation. At that time, the predicted value G ^ of the scanning line interval calculated in step S402 is used. The variation range of k and l is {-1, 0, 1}.

  Here, j is an index indicating the column number of the HT image, and originally takes all integer values from 1 to J (number of pixels in the main scanning direction). However, as is clear from Equation (8), the value of D ^ does not depend on j. Therefore, D ^ may be calculated once for each scanning line, and the value is commonly used for all the pixels in the i-th row. Further, Gx in Expression (8) is a distance when there is no position error between pixels adjacent in the main operation direction.

(Step S404Y)
Step S404Y is a process of setting 1 to the variable j. j is an index indicating the column number of the HT image. j is incremented by 1 each time the following steps S405Y to S407Y are performed, and is repeated until J (the number of pixels in the main scanning direction) is reached. This means that the following steps S405Y to S407Y are performed for each pixel. Therefore, in the following description, general j is described without being limited to the case of j = 1.

(Step S405Y)
Step S405Y is a peripheral pixel pattern acquisition process. The peripheral pixel pattern acquisition unit 507Y acquires the peripheral pixel pattern W from the Y color HT image stored in the HT image storage unit 501. That is, W (i + k, j + l) is defined by the following formula for k and l whose fluctuation range is {-1, 0, 1}.

(Step S406Y)
Step S406Y is a correction rate calculation process. The correction rate calculation unit 508Y calculates a modulation magnification R (correction rate) for modulating the exposure amount E calculated by the exposure amount calculation unit 502Y based on the Y color HT image stored in the HT image storage unit 501. . In this case, R is determined so as to reduce banding caused by an error in the scanning line interval. That is, R is determined so that the target density value and the predicted density value described below are equal. The target density value is a density value at an attention position of an ideal formed image in which no banding occurs. The predicted density value is a density value at the target position of the predicted image calculated from the corrected exposure amount R × E and the predicted value D ^ of the pixel interval calculated in step S403Y. The target density value and the predicted density value are calculated by the following formulas using the predicted pixel spacing value D ^ calculated in step S403Y and the peripheral pixel pattern W acquired in step S405Y.

  Here, Gy in equation (10) is the distance between adjacent pixels in the sub-operation direction when there is no position error, and sh in equations (10) and (11) is the original exposure amount. E is a function representing the density distribution when an isolated dot is printed. The variation range of k and l is {-1, 0, 1}.

  FIG. 9 is a schematic diagram showing the density distribution of dots corresponding to the pixel of interest (i, j) (hereinafter referred to as “dots of interest”) and dots corresponding to the peripheral pixels (hereinafter referred to as “peripheral dots”). is there. Equations (10) and (11) will be described below with reference to FIG. Note that FIG. 9 illustrates only the dots corresponding to the pixel (i + 1, j + 1) as peripheral dots in consideration of easy viewing. Since each dot has a certain width and is printed in an overlapping manner, the density value at a certain target position is calculated as the sum of density values at the target position of peripheral dots including the target dot.

The schematic diagram (a) of FIG. 9 is for explaining the formula (10). The target density value is obtained by calculating the sum of the value 901 and the value 902. A value 901 is a density value at the target position of the target dot. At this time, the target dot is exposed with the original exposure amount E without the exposure amount being modulated by the banding correction process. A value 902 is a density value at the target position of the peripheral dots. At this time, there is no change in the positions of the peripheral dots, and the distance from the target position to the peripheral dots is √ (Gx ² + Gy ² ). Therefore, the value 902 is calculated by sh (√ (Gx ² + Gy ² )).

  The schematic diagram (b) of FIG. 9 is for explaining the formula (11). The predicted density value is obtained by calculating the sum of the value 903 and the value 904. A value 903 is a density value at the target position of the target dot. At this time, the target dot is exposed with the corrected exposure amount R × E. A value 902 is a density value at the target position of the peripheral dots. At this time, fluctuations occur in the positions of the peripheral dots, and the distance from the target position to the peripheral dots is D ^ calculated in step S403Y. Therefore, the value 904 is sh (D ^), which is different from the value 902. The first term on the right side of Equation (11) represents the amount of increase or decrease in the density value when the light amount is corrected with the correction factor R.

  The density value at the target position of the peripheral dots needs to be calculated for all the pixels in the image, but the value is sufficiently small for a pixel far away to some extent and can be ignored. Therefore, in this embodiment, the sum is calculated only for pixels adjacent vertically, horizontally, and diagonally. The correction rate R is calculated by the following equation based on the condition for the target density value obtained by Equation (10) to be equal to the predicted concentration value obtained by Equation (11).

  Here, D ^ ((i, j), (j + k, j + l)) in the equation (12) is a predicted value of the pixel interval calculated in step S403Y, and W (i + k, j + l) is determined in step S405Y. This is a calculated peripheral pixel pattern.

(Step S407Y)
Step S407Y is an exposure correction process. The exposure amount correction unit 509Y corrects the exposure amount using the correction rate R calculated in step S406Y. That is, the calculated correction rate R is multiplied by the original exposure amount E, and the exposure amount E ′ to be written this time is calculated by the following equation.

（ステップＳ４０８Ｙ）
ステップＳ４０８Ｙは、ｊを１だけ増加させる処理である。現在の画素での処理を終え、次の画素（主走査方向下流側）での処理へと移行する。

(Step S408Y)
Step S408Y is a process for increasing j by one. After the processing at the current pixel is finished, the processing proceeds to the processing at the next pixel (downstream in the main scanning direction).

(Step S409Y)
Step S409Y is a process of determining the true / false of “j ≦ J”. If the determination result is true, the process returns to step S405Y, and the same processing is repeated for the next pixel. If the determination result is false, the repetition ends and the process proceeds to step S410Y. As described above up to step S409Y, for all the pixels in the i-th row, the exposure amount is modulated using the position information of the peripheral pixels and the pattern information of the peripheral pixels (ON / OFF information of each pixel), It corrects fluctuations in image density caused by the density of scanning line intervals. As a result, it is possible to accurately correct the banding due to the position variation of the scanning line. However, the peripheral pixel position information used for the correction is calculated based on the scanning line interval prediction process in step S402Y. For this reason, an error may occur in the calculated prediction position information, and banding may not be completely corrected.

  In the present embodiment, banding remaining in the Y-banding correction process (hereinafter referred to as Y-banding correction error) is canceled out in this way, and more accurate banding correction is realized. That is, the remaining Y-color banding correction error is detected, and control is performed to cancel the Y-color banding correction error in the M, C, and K-color banding correction processes that are subsequently performed.

  Steps S410Y and thereafter are processing for detecting a Y-color banding correction error. So far, the scanning of the i-th scanning line has been completed. Subsequently, immediately before the scan of the (i + 1) th scanning line is started, the scanning position is acquired, and the actual position information of the peripheral pixels is calculated. As a result, the actual print density is calculated and the remaining banding is detected.

(Step S410Y)
Step S410Y is an i + 1th scanning position calculation process. The scanning position calculation unit 503Y calculates the scanning position of the (i + 1) th scanning line using output information from the encoder 121Y and the optical position sensor 122Y. First, the scanning position calculation unit 503Y calculates the scanning position D (i + 1) when there is no error in the laser irradiation position. The encoder 121Y generates a pulse signal corresponding to the rotation of the photosensitive drum 101Y, and outputs the pulse signal to output the rotation angle θ of the photosensitive drum 101Y. The scanning position calculation unit 503Y calculates D (i + 1) using the rotation angle θ according to the following equation.

  Here, r in the equation (14) is the radius of the photosensitive drum 101Y. Next, the scanning position calculation unit 503Y acquires the laser irradiation position fluctuation amount L (i + 1) from the optical position sensor 122Y.

  FIG. 6 is a schematic diagram illustrating an example of the optical position sensor 122Y. The light receiving element 601 in FIG. 6 outputs a pulse signal when detecting the laser beam. The triangular slit 602 covers the light receiving element 601. With this configuration, since the time during which the light receiving element 601 is irradiated with the laser changes according to the laser irradiation position, the laser irradiation position can be calculated from the width of the output pulse signal. The width of the pulse signal output from the optical position sensor 122Y when the laser beam irradiation reference position 603 is irradiated with the laser beam is set to w1. When the laser beam is irradiated to the position of the laser irradiation position 604 in FIG. 6, the time for the light receiving element 601 to receive the laser beam is longer than when the laser beam irradiation reference position 603 is irradiated with the laser beam. The pulse width w2 of the output signal is longer than w1. The optical position sensor 122Y can calculate the laser irradiation position fluctuation amount from the change in the pulse width. Specifically, the laser irradiation position fluctuation amount can be obtained by multiplying the difference between the pulse widths w2 and w1 by a proportional coefficient. This proportionality coefficient is a constant determined by the shape of the slit. As an example, if the shape of the slit is a right-angled isosceles triangle and one of the two orthogonal sides is parallel to the main scanning direction and the other is parallel to the sub-scanning direction, this proportionality factor is 1. That is, the difference in pulse width becomes the laser irradiation position fluctuation amount as it is. Thus, the laser irradiation position fluctuation amount L (i + 1) can be obtained from the pulse width of the signal output from the sensor. The scanning position calculation unit 503Y calculates the (i + 1) th scanning position P (i + 1) by using the following equation using D (i + 1) and L (i + 1) obtained above.

  As is apparent from the calculation method, the (i + 1) th scanning position P (i + 1) is the position of the i + 1th scanning line in the sub-scanning direction on the surface of the photosensitive drum 101Y. Next, the scanning position storage unit 504Y updates the scanning position table. The scanning position table is referred to in step S402Y. Immediately before updating the table, the scanning position table stores the scanning positions of the i-th scanning lines in association with the scanning line numbers. The scanning position of the (i + 1) th scanning line is stored in association with the scanning line number.

(Step S411Y)
Step S411Y is a scanning line interval calculation process. The scanning line interval calculation unit 510Y calculates the scanning line interval between the i-th scanning line and the peripheral scanning line using the information in the scanning position table updated immediately before. Here, the peripheral scanning lines are the (i−1) th scanning line and the (i + 1) th scanning line. FIG. 11 is a schematic diagram showing the relationship between the scanning position and the scanning line interval. A scanning line interval G (i, i + 1) between the i-th scanning line and the i + 1-th scanning line and a scanning line interval G (i-1, i) between the i-th scanning line and the i-th scanning line are expressed as follows. Calculate with the formula.

(Step S412Y)
Step S412Y is a pixel interval calculation process. The pixel interval calculation unit 511Y calculates an interval D between each pixel (i, j) in the i-th row and surrounding pixels (i + k, J + l) by the following formula. At that time, the scanning line interval G calculated in step S411 is used. The variation range of k and l is {-1, 0, 1}.

  Here, j is an index indicating the column number of the HT image, and originally takes all integer values from 1 to J (number of pixels in the main scanning direction). However, as is clear from Equation (18), the value of D ^ does not depend on j. Therefore, D ^ may be calculated once for each scanning line, and the value is commonly used for all pixels in the i-th row. Further, Gx in Expression (18) is a distance when there is no position error between pixels adjacent in the main operation direction.

(Step S413Y)
Step S413Y is a process of setting 1 to the variable j. j is an index indicating the column number of the HT image. j is incremented by 1 each time the following steps S414Y to S415Y are performed, and is repeated until J (the number of pixels in the main scanning direction) is reached. This means that the following steps S414Y to S415Y are performed for each pixel. Therefore, in the following description, general j is described without being limited to the case of j = 1.

(Step S414Y)
Step S414Y is a peripheral pixel pattern acquisition process. Since the process of step S414Y is the same as the process of step S405Y, a description thereof will be omitted.

(Step S415Y)
Step S415Y is a Y-banding correction error calculation process. The correction error calculation unit 512Y calculates a Y-color banding correction error that occurs due to an error in the scanning line interval prediction process.

  First, the correction error calculation unit 512Y calculates an actual print density value. The actual print density value is a density value at the target position of the actual print image calculated from the corrected exposure amount R × E and the pixel interval D calculated in step S412Y. The actual print density value is calculated by the following equation using the pixel interval D calculated in step S412Y, the peripheral pixel pattern W acquired in step S414Y, and the correction factor R calculated in step S406Y.

  Here, the variation range of k and l in the equation (19) is {−1, 0, 1}. Next, the correction error calculation unit 512Y calculates a Y-banding correction error. The Y color banding correction error ErrY (i, j) in the pixel (i, j) is calculated by the following equation using the actual print density value calculated immediately before and the target density value calculated in the same manner as in step S406Y. To do.

  Subsequently, the correction error storage unit 140 updates the Y color banding correction error table. FIG. 12 is an example of a Y-color banding correction error table stored in the correction error storage unit 140. In this table, for each pixel, the Y color banding correction error is stored in association with the row number (coordinate in the sub-scanning direction) and the column number (coordinate in the main scanning direction). Referring to FIG. 12, immediately before the table is updated, Y-banding correction errors are stored for the (i−1) th scanning line and the j−1th scanning line of the ith scanning line. The Y banding correction error is stored for pixel (i, j). The banding correction error table is updated by the above processing. Note that this table discards all stored information every time printing of one image is completed. In addition, the correction error storage unit 140 has a memory sufficient to store Y banding correction errors of all the pixels in I rows and J columns at the maximum.

  The Y-banding correction error ErrY (i, j) calculated and stored in step S415Y is referred to in the subsequent M-color, C-color, and K-color banding correction processes. In the M-color, C-color, and K-color banding correction processes, the Y-color banding correction error ErrY (i, j) is canceled out, and the color unevenness caused by the Y-color banding correction error ErrY (i, j) is suppressed. To control.

(Step S416Y)
Step S416Y is a process of increasing j by 1. After the processing at the current pixel is finished, the processing proceeds to the processing at the next pixel (downstream in the main scanning direction).

(Step S417Y)
Step S417Y is a process for determining the authenticity of “j ≦ J”. If the determination result is true, the process returns to step S414Y, and the same processing is repeated for the next pixel. If the determination result is false, the repetition ends and the process proceeds to step S418Y.

(Step S418Y)
Step S418Y is a process of increasing i by 1. After the processing on the current scanning line is finished, the process proceeds to processing on the next scanning line (downstream in the sub-scanning direction).

(Step S419Y)
Step S419Y is a process for determining the authenticity of “i ≦ I”. If the determination result is true, the process returns to step S402Y, and the same processing is repeated for the next scanning line. If the determination result is false, the repetition is finished and the Y-banding correction process is finished.

  The Y-color banding correction process is executed in synchronization with the Y-color exposure process executed by the image forming unit 100Y (FIG. 1). The operation sequence of the Y color exposure process and the Y color banding correction process will be described with reference to FIG.

(Y color exposure process)
In the i-th scan of the Y-color exposure process, first, a laser irradiation position detection signal 1001a is issued for a certain period of time. Next, after the issuance of the laser irradiation position detection signal 1001a, a predetermined time is left and the image signal 1002a is issued for a certain time. While the laser irradiation position detection signal 1001a and the image signal 1002a are issued, the exposure apparatus 103Y (FIG. 1) performs laser irradiation. Similarly, in the i + 1th scan, a laser irradiation position detection signal 1001b and an image signal 1002b are issued.

(Y color banding correction process)
The exposure amount correction signal 1005a is issued after the issue of the laser irradiation position detection signal 1001a is finished and before the issue of the image signal 1002a is started. At the timing of the exposure amount correction signal 1005a, the processing for correcting the exposure amount from step S402Y to step 409Y in FIG. 4 in the Y color banding correction processing for the i-th scan is executed. The laser irradiation position detection signal 1003b and the drum position detection signal 1004b are issued after the issue of the laser irradiation position detection signal 1001b is completed and before the issue of the image signal 1002b is started. At the timing of the laser irradiation position detection signal 1003b, the optical position sensor 122Y (FIG. 1) detects the irradiation position of the laser beam and outputs the detected irradiation position to the Y-banding correction unit 130Y (FIG. 1). At the timing of the drum position detection signal 1004b, the encoder 121Y (FIG. 1) detects the rotation angle of the photosensitive drum 101Y (FIG. 1), and outputs the detected rotation angle to the Y-banding correction unit 130Y (FIG. 1).

  After the laser irradiation position detection signal 1003b and the drum position detection signal 1004b, a correction error calculation signal 1006b is issued. At the timing of the correction error calculation signal 1006b, a process of detecting a correction error from step S410Y to step 419Y in FIG. 4 in the banding correction process for the i-th scan is executed. The above process is repeated for each scan.

  Next, the configuration of the M color banding correction unit 130M will be described with reference to the block diagram of FIG. The M color banding correction unit 130M is configured in the same manner as the Y color banding correction unit 130Y. Therefore, Y at the end of the symbol is rewritten as M, and detailed description is omitted. However, only the correction rate calculation unit 508M is different from the correction rate calculation unit 508Y. More specifically, the correction rate calculation unit 508M is different in that it receives the M-color banding correction error output from the correction error storage unit 140 in addition to the processing performed by the correction rate calculation unit 508Y. Further, unlike 508Y, the correction rate calculation unit 508M holds a lower color removal (hereinafter referred to as UCR) table (not shown) for lower color removal conversion. FIG. 13 is an example of the UCR table stored in the correction rate calculation unit 508M. In this table, density values when an achromatic image is formed by mixing Y, M, and C colors are associated with density values when an achromatic image is formed using only the K color. Stored. That is, an image formed by superimposing Y, M, and C color toner images having density values y (n), m (n), and c (n), respectively, has a density value k (n ) Achromatic color.

  Next, the M color banding correction process performed in the M color banding correction unit 130M will be described with reference to the flowchart of FIG. The M banding correction process is different from the Y color banding correction process only in target density value calculation in the correction factor calculation process (step S406M). Of the M-banding correction process, the same part as the Y-color banding correction process is illustrated by simply rewriting Y at the end of the symbol to M and omitting the description, and only for calculating the target density value which is the difference. Details will be described below.

  The target density value calculation in the M color banding correction process will be described below. When a banding correction error occurs in the Y-color banding correction process, color unevenness occurs in an image formed by superimposing the toner images of the respective colors. Therefore, in this embodiment, when a banding correction error occurs in the Y color, control is performed so as to suppress the occurrence of color unevenness by changing the target density value of the M color. That is, the target density value of M color is increased or decreased so that the hue of the image formed by superimposing the colors approaches the desired hue.

  A method for determining the M color target density value increase / decrease amount Sm at that time will be described below. First, the peripheral pixel pattern acquisition unit 507M acquires the peripheral pixel patterns Wm, Wc, and Wk for each color from the M, C, and K color HT images stored in the HT image storage unit 501. The details of the process are the same as those in step S405Y of the Y-color banding correction process, and a description thereof will be omitted.

Next, the Y banding correction error ErrY (i, j) is acquired from the correction error storage unit 140, and the value y (ny) closest to the value of ErrY (i, j) is acquired from the Y color string of the URC table. To do. Further, the value m (ny) stored in the same row as y (ny) in the M color string is acquired. Then, the following equation (21), equation (22), in a range that satisfies both the equation (23), | m (ny ) -m (n 0) | a seek _{n 0} minimized. However, it is assumed that the density values obtained by modulating the exposure amount of the exposure apparatus have minimum values for minM, minC, and minK and maximum values for maxM, maxC, and maxK for each of the M, C, and K colors. minM, minC, and minK are density values when exposure is performed with the minimum laser power. MaxM, maxC, and maxK are density values when exposure is performed with the maximum laser power.

Obtaining n ₀ within a range satisfying both of the expressions (23), (24), and (25) increases or decreases the M color target density value only in a range where both the M, C, and K colors can be modulated. It means that That is, as an example, the K color exposure amount cannot be reduced in an area that is not originally exposed in K color, and therefore, in such an area, the M color target density based on the subsequent decrease in the K color target density value is assumed. Do not increase the value.

Finding n ₀ so as to minimize | m (ny) −m (n ₀ ) | is to set the target density value of M color so that the hue of the image formed by superimposing the colors approaches the desired hue. Means to increase or decrease. Finally, using the obtained n ₀ , the M color target density value increase / decrease amount Sm is calculated by the following equation.

  The target density value is increased or decreased by the M color target density value increase / decrease amount Sm calculated as described above. That is, the target density value in the M banding correction process is calculated by the following equation.

  By this process, the M color is controlled to be printed with a density value obtained by adding Sm to the original density value. This cancels the banding correction error in the Y-color banding correction process and suppresses the occurrence of color unevenness.

  Next, the configuration of the C-color banding correction unit 130C will be described with reference to the block diagram of FIG. The C banding correction unit 130C is configured in the same manner as the M color banding correction unit 130M. For this reason, M at the end of the symbol is rewritten as C, and detailed description thereof is omitted.

  Next, the C color banding correction process performed in the C color banding correction unit 530C will be described with reference to the flowchart of FIG. The C-banding correction process is different from the Y-color banding correction process only in target density value calculation in the correction factor calculation process (step S406C). Of the C-banding correction process, the same part as the Y-color banding correction process is illustrated by simply rewriting Y at the end of the symbol to C and omitting the description, and only for calculating the target density value, which is the difference, Details will be described below.

  The target density value calculation in the C color banding correction process will be described below. When a banding correction error occurs in the Y color and M color banding correction processing, color unevenness occurs in an image formed by superimposing the toner images of the respective colors. Therefore, in the present embodiment, when a banding correction error occurs in the Y color and M color banding correction processing, control is performed so as to suppress the occurrence of color unevenness by changing the target density value of the C color. That is, the target density value for C color is increased or decreased so that the hue of the image formed by superimposing the colors approaches the desired hue. A method for determining the C color target density value increase / decrease amount Sc at that time will be described below.

  First, the peripheral pixel pattern acquisition unit 507C acquires the peripheral pixel patterns Wc and Wk of each color from the C color and K color HT images stored in the HT image storage unit 501. The details of the process are the same as those in step S405Y of the Y-color banding correction process, and a description thereof will be omitted.

  Next, the Y color banding correction error ErrY (i, j) is acquired from the correction error storage unit 140, and the value y () closest to the value of ErrY (i, j) is obtained from the Y color string of the URC table shown in FIG. ny). Further, the value c (ny) stored in the same row as y (ny) in the C color string is acquired. Further, the M color banding correction error ErrM (i, j) is acquired from the correction error storage unit 140, and the value m (nm) closest to the value of ErrM (i, j) is acquired from the M color string of the URC table. . Further, a value c (nm) stored in the same row as m (nm) in the C color column is acquired.

Next, equation (24), in a range that satisfies both the equation (25), | (c ( ny) + c (nm)) / 2-c (n 0) | a seek _{n 0} minimized. Finally, with n ₀ obtained, to calculate the C color target density value decrease amount Sc by the following equation.

  The target density value is increased or decreased by the C color target density value increase / decrease amount Sc calculated as described above. That is, the target density value in the C color banding correction process is calculated by the following equation.

  With this process, the C color is controlled to be printed with a density value obtained by adding Sc to the original density value. Thereby, the banding correction error in the Y-color and M-color banding correction processing is canceled, and the occurrence of color unevenness is suppressed.

  Next, the configuration of the K-color banding correction unit 130K will be described with reference to the block diagram of FIG. The K-color banding correction unit 130K does not have some modules in the banding correction units for other colors. That is, for example, the M-banding correction unit 130M to the scanning line interval calculation unit 510M, the pixel interval calculation unit 511M, and the correction error calculation unit 512M are unnecessary. Therefore, such unnecessary processing units are deleted, M at the end of the symbol is replaced with K, and detailed description is omitted.

  Next, the K-color banding correction process performed in the K-color banding correction unit 130K will be described with reference to the flowchart of FIG. In the K banding correction process, steps S411 to S418 are not performed in the steps of the Y color banding correction process. That is, the banding correction process for other colors is not performed after the banding correction process for K color, and the K banding correction error calculation process and the related process are not performed because the K banding correction error is not referred to. . The target density value calculation in the correction factor calculation process (step S406K) is different from that in the Y color banding correction process. Of the K-color banding correction process, the same part as the Y-color banding correction process is illustrated by simply rewriting Y at the end of the symbol to K and omitting the description, and only for calculating the target density value which is the difference. Details will be described below.

  The target density value calculation in the K color banding correction process will be described below. In the M color and C color banding correction processing, control was performed so as to suppress a change in hue due to each color banding correction error. However, these processes do not consider changes in density. Therefore, in the K banding correction process, the target density value of K color is increased or decreased so that the density of the image formed by superimposing the colors approaches the desired density. A method for determining the K color target density value increase / decrease amount Sk at that time will be described below.

  First, the peripheral pixel pattern acquisition unit 507C acquires a K-color peripheral pixel pattern Wk from the K-color HT image stored in the HT image storage unit 501. The details of the process are the same as those in step S405Y of the Y-color banding correction process, and a description thereof will be omitted. Next, the Y color banding correction error ErrY (i, j) is acquired from the correction error storage unit 140, and the value y () closest to the value of ErrY (i, j) is obtained from the Y color string of the URC table shown in FIG. ny). Further, the value k (ny) stored in the same row as y (ny) in the K color string is acquired.

  Further, the M color banding correction error ErrM (i, j) is acquired from the correction error storage unit 140, and the value m (nm) closest to the value of ErrM (i, j) is obtained from the M color string of the URC table shown in FIG. ) To get. Further, the value k (nm) stored in the same row as m (nm) in the K color column is acquired. Subsequently, the C banding correction error ErrC (i, j) is acquired from the correction error storage unit 140, and the value c (nc) closest to the value of ErrC (i, j) is acquired from the C color string of the URC table. To do. Further, the value k (nc) stored in the same row as c (nc) in the K color string is acquired.

Then, in a range satisfying the equation (25), | (k ( ny) + k (nm) + k (nc)) / 3-k (n 0) | a seek _{n 0} minimized. Finally, with n ₀ obtained, to calculate the K color target density value decrease amount Sk by the following equation.

  The target density value is increased or decreased by the K color target density value increase / decrease amount Sk calculated as described above. That is, the target density value in the K-banding correction process is calculated by the following formula.

  By this process, the K color is controlled to be printed with a density value obtained by adding Sk to the original density value. As a result, the banding correction error in the Y color, M color, and C color banding correction processing is canceled, and the occurrence of color unevenness is suppressed.

  As described above, the image forming apparatus according to the present exemplary embodiment cancels the banding correction error in the banding correction process before the current process in the M color, C color, and K color banding correction processes performed after the second time. Suppresses the occurrence of uneven color. That is, in the M color and C color banding correction processing, control is performed so that the hue of an image obtained by superimposing the colors is the same as the desired hue. In the K banding correction process, the density of an image in which each color is superimposed is controlled to be the same as a desired density. As a result, it is possible to obtain a good image with less color unevenness caused by each color banding correction error. Furthermore, in the present embodiment, the processing at the time of forming a color image with four colors has been shown, but the present invention is not limited to this, and can be used when an image having an arbitrary number of colors is formed.

  In this embodiment, the position in the sub-scanning direction on the surface of each color photosensitive drum (101Y, 101M, 101C, 101K) of each scanning line is stored in the scanning position table. However, the present invention is not limited to this. Absent. As another example, the distance in the sub-scanning direction with the immediately preceding scanning line on the surface of each color photosensitive drum (101Y, 101M, 101C, 101K) may be stored.

  In this embodiment, the scanning positions of all scanned scanning lines are stored in the scanning position table. However, the present invention is not limited to this, and scanning positions are stored only for some of the scanning lines. May be. In this embodiment, the banding correction error of all processed pixels is stored in the banding correction error table. However, the present invention is not limited to this, and the banding correction error is stored only for some of the pixels. May be. In this embodiment, in the scanning line interval prediction process (steps S402Y, S402M, S402C, and S402K), the entire scanning line interval is calculated by prediction, but the present invention is not limited to this. As another example, a part of the scanning line interval may be calculated by prediction, and the remaining interval may be actually detected.

  Further, the density distribution sh of the isolated dots has been described as being isotropic. However, sh is anisotropic, for example, when the distribution differs between the main scanning direction and the sub-scanning direction, sh can be a function representing a two-dimensional distribution. At that time, it is possible to cope with the pixel interval prediction value D ^ calculated in the pixel interval prediction process (steps S403Y, S403M, S403C, and S403K) as a two-dimensional vector of the main scanning direction distance and the sub-scanning direction distance. is there. In the present embodiment, in the correction factor calculation processing (steps S406Y, S406M, S406C, and S406K), only the neighboring dots that are adjacent to the target dot are used. However, the present invention is not limited to this. You may use a dot.

In the correction rate calculation processing (steps S406Y, S406M, S406C, and S406K), the dot density calculation is proportional to the correction rates Ry, Rm, Rc, and Rk for each color, but this may not be true. In that case, equivalent processing can be performed by using a look-up table (LUT) or a function indicating the relationship between the exposure amount and the dot density.
Further, the predicted density at the target position is obtained from the sum of the isolated dots, but when such a relationship does not hold, the predicted density and the correction rate may be calculated using a previously created LUT.
In addition, in equation (11) in the correction factor calculation process (steps S406Y, S406M, S406C, and S406K), the predicted density value is calculated assuming that the exposure amount of the peripheral pixels is not corrected. However, it is also possible to reflect the calculated correction factors in subsequent calculation of predicted density values by sequentially storing the correction factors.

  The HT image held by the HT image storage unit 501 is assumed to be a binary value of “0” or “1”. Even in a configuration in which the HT image is multi-valued and multi-value exposure processing is performed using pulse width modulation, exposure intensity modulation, or the like, the same processing as in this embodiment is possible. In this case, for example, it is possible to cope with this by using a function sh representing a plurality of density distributions corresponding to the number of levels of image data. Furthermore, when the center of gravity of the dot moves due to pulse width modulation, the amount of center of gravity movement can be reflected in equation (11). In addition, in each color banding correction error calculation process (steps S415Y, S415M, S415C, and S415K), an actual print density value may be acquired by a sensor. In other words, this is a method of acquiring, with a sensor, an electrostatic latent image or a toner image formed on each color photosensitive drum (101Y, 101M, 101C, 101K) or a toner image transferred to the intermediate transfer belt 110 or the recording material P. .

  Further, the method for determining the M color target density value increase / decrease amount Sm in the M color banding correction process is not limited to the exemplified one. That is, the value of Sm may be determined by other methods as long as the target density value of M color is changed so that the hue of the image formed by superimposing the colors approaches the desired hue. Further, the method for determining the C color target density value increase / decrease amount Sc in the C color banding correction process is not limited to the exemplified one. That is, the value of Sc may be determined by other methods as long as the target density value of C color is changed so that the hue of the image formed by superimposing the colors approaches the desired hue.

Further, the method for determining the K color target density value increase / decrease amount Sk in the K color banding correction process is not limited to the exemplified one. That is, as long as the target density value of K color is changed so that the density of the image formed by superimposing the colors approaches the desired density, the value of Sk may be determined by another method.
[Example 2]
In the first embodiment, the configuration in which the exposure amount is modulated according to the scanning position shift in the sub-scanning direction has been described. In this embodiment, a description will be given of a configuration that modulates the exposure amount in response to a scanning position shift in the main scanning direction (hereinafter referred to as jitter shift) in addition to a scanning position shift in the sub-scanning direction. In general, in an image forming apparatus, jitter deviation may occur due to the accuracy of each surface of a polygon mirror.

  FIG. 15 is a schematic diagram showing the relationship between the polygon mirror accuracy and the scanning position in the main scanning direction. FIG. 15A is a schematic diagram of a polygon mirror and a scanning position when there is no error in polygon mirror accuracy. The polygon mirror shown in FIG. 15B has an inclination in the surface 1 and the surface 4 among the six surfaces. At this time, the variation in Ex occurs in the main scanning direction for the scanning lines scanned by the surface 1 and the surface 4.

  Since the basic configuration and processing of the image forming apparatus shown in the present embodiment are the same as those in the first embodiment, only differences will be described. Compared with the first embodiment, the image forming apparatus according to the present embodiment has two pixel interval prediction processes (steps S403Y, S403M, S403C, and S403K) and a pixel interval calculation process (steps S412Y, S412M, S412C, and S412K). It is different in point. The scanning position storage unit 504Y previously acquires and holds the position fluctuation amount Ex in the main scanning direction for each scanning line.

  In the pixel interval prediction process (steps S403Y, S403M, S403C, and S403K), the predicted value D ^ of the interval between the pixel (i, j) and the surrounding pixel (i + k, j + l) is calculated by the following equation.

  In the pixel interval calculation process (steps S412Y, S412M, S412C, and S412K), the interval D between the pixel (i, j) and the surrounding pixels (i + k, j + l) is calculated by the following equation.

  In Expression (30) and Expression (31), it is possible to calculate a more accurate pixel interval (predicted value thereof) by considering the jitter deviation of the current scanning line and the jitter deviation of the surrounding scanning lines. Accurate banding correction can be performed.

[Other Embodiments]
The present invention can also be realized by executing the following processing. That is, software (program) for realizing 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, etc.) of the system or apparatus reads the program. It is a process to be executed. The present invention can also be realized by a plurality of processors cooperating to perform processing.

Claims (6)

An image forming apparatus that forms an image by forming a plurality of latent images on a surface of a photoconductor by an exposure unit that exposes the photoconductor by scanning the surface of the photoconductor with an exposure amount for each pixel according to input image data,
A scanning position calculating means provided in the exposure means for calculating a scanning position scanned for the exposure;
Pixel interval prediction means for calculating a predicted value of the pixel interval when scanning for the exposure from the scanning position;
Pixel pattern acquisition means for acquiring a peripheral pixel pattern for each pixel to be printed from the input image data;
Exposure amount correction means for correcting an exposure amount for each pixel corresponding to the input image data for a target pixel from the predicted value of the pixel interval and the pixel pattern;
And a correction error calculation unit that calculates a correction error between the exposure amount calculated by the scanning position calculated by the scanning position calculation unit and the exposure amount corrected by the exposure amount correction unit. Image forming apparatus.   The correction of the exposure amount of a part or all of the exposure means processed after the second by the exposure means is executed so as to reduce the correction error of the exposure means processed before the second and subsequent processes. The image forming apparatus according to claim 1.   The correction of the exposure amount of a part or all of the exposure means processed after the second by the exposure means is performed by changing the hue of the formed image caused by the correction error of the exposure means processed before the second and subsequent processes. The image forming apparatus according to claim 1, wherein the image forming apparatus is executed to suppress the image forming apparatus.   Suppression of the hue change of the formed image is executed by canceling the hue change due to the correction error by correcting the exposure amount of a part or all of the exposure means that processes the second and subsequent processes by undercolor removal conversion. The image forming apparatus according to claim 3.   The correction of the exposure amount of a part or all of the exposure unit processed after the second by the exposure unit is performed by changing the density of the formed image caused by the correction error of the exposure unit processed before the second and subsequent processing. The image forming apparatus according to claim 1, wherein the image forming apparatus is executed to suppress the image forming apparatus. An image forming method for forming an image by superposing a plurality of latent images on a surface of a photoconductor by an exposure step of performing exposure by scanning the surface of the photoconductor with an exposure amount for each pixel according to input image data,
A scanning position calculating step for calculating a scanning position scanned for exposure in the exposure step;
A pixel interval prediction step of calculating a predicted value of the pixel interval when scanning for the exposure from the scanning position;
A pixel pattern acquisition step of acquiring a peripheral pixel pattern for each pixel to be printed from the input image data;
An exposure amount correction step of correcting an exposure amount for each pixel corresponding to the input image data for a target pixel from the predicted value of the pixel interval and the pixel pattern;
A correction error calculating step for calculating a correction error between the exposure amount calculated by the scanning position calculated in the scanning position calculating step and the exposure amount corrected in the exposure amount correcting step. Image forming method.

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