JP5188097B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus for obtaining a color image of two or more colors.

  Space saving of digital composite copying machines tends to be more emphasized especially in low-end products. Conventionally, an oblique incidence system is known as an optical system for realizing space saving.

  FIG. 9 is a side view showing an example of the oblique incidence method. According to FIG. 9, a light emitting element 901 that outputs a beam for forming a latent image, a photosensitive drum 903 that holds the latent image, and a polygon mirror 902 for scanning the beam are located at different positions in the height direction. Has been placed. The beam output from the light emitting element 901 is obliquely incident on a mirror surface (scanning plane) provided on the side surface of the polygon mirror 902. Reference numeral 902 ′ denotes the upper surface of the polygon mirror 902.

  According to the oblique incidence method, the distance from the center position (rotation axis) of the polygon mirror 902 is different between the center and end of the mirror surface (distances a and b). This difference in distance causes the position of the beam applied to the photosensitive drum 903 to fluctuate in the height direction.

  FIG. 10 is a diagram for explaining a difference between an actual scanning line formed on the photosensitive drum 903 in the oblique incidence method and an ideal scanning line. A curve indicated by a solid line 1001 is an actual scanning line. A straight line indicated by a broken line 1002 is an ideal scanning line. In the oblique incidence method, the scanning line is curved in this way.

  In order to alleviate this bending, a correction lens may be disposed on the beam path. The correction lens is shaped so that the refractive index in the height direction differs depending on the scanning position. This correction lens alleviates the curvature to some extent and achieves high image quality.

  However, in order to effectively alleviate the bending, it is necessary to ensure the processing accuracy of the correction lens and adjust the optical system to a desired state. This is not suitable for low-end products because it increases production time and increases production costs.

  Incidentally, even in the vertical incidence method in which the beam is incident perpendicularly to the rotation axis of the polygon mirror 902, the same fluctuation as in the oblique incidence method can occur. In an ideal vertical system, a beam trajectory exists in the rotation plane of the polygon mirror 902.

  FIG. 11 is a diagram illustrating an example of an ideal optical system having no significant shift in the rotation axis and an actual optical system having a significant shift in the rotation axis. FIG. 11A shows an ideal optical system. Since the rotation axis is not deviated, the height of the beam irradiation position on the photosensitive drum 903 is kept constant even if the distance from the rotation axis to the mirror reflection position is changed by the rotation of the polygon mirror 902. On the other hand, FIG. 11B shows a state in which the rotation axis is displaced due to an attachment error. For this reason, the distance from the rotation axis to the reflection position of the mirror surface is changed by the rotation of the polygon mirror 902, and the height of the irradiation position is also shifted in conjunction therewith. Such a shift is not preferable because image quality is deteriorated. In particular, in high-end models, such a mounting error of the rotating shaft is a problem that cannot be ignored.

  FIG. 12 is a diagram illustrating an example of a method for correcting curvature by changing lines. Here, a conventionally proposed correction method (Patent Documents 1 to 4) will be described with reference to FIG. In FIG. 12, the broken line indicates an ideal scanning locus L0. Each solid line represents actual scanning trajectories L1, L2, and L3 that pass within ± 0.5 lines with respect to an ideal scanning trajectory.

When attention is paid to both ends of the scanning trajectory, among the three scanning trajectories L1, L2, and L3, L1 is closest to the ideal scanning trajectory L0. In the scanning region located at the center of the scanning locus, L3 is closest to the ideal scanning locus L0. In the region located between the end and the center, L2 is closest to the ideal scanning locus L0. Therefore, in order to realize an ideal straight line, it is almost ideal if the line is changed to a different line in the sub-scanning direction in order of L1 => L2 => L3 => L2 => L1 according to the scanning region. A straight line can be realized. In this way, dividing one scanning cycle into a plurality of regions and selecting an actual scanning line for each region to form a desired image is referred to as line transfer. Further, the partition between regions is called a transfer point.
Japanese Patent Laid-Open No. 02-050176 JP 2003-182146 A JP 2003-276235 A JP 2005-304011 A

  However, the correction method described above is a highly difficult technique in terms of the characteristics of electrophotography. In particular, in a low-end product, the uniformity of the thickness of a thin line is likely to be impaired due to the poor reproducibility of fine dots. Therefore, it is desirable to apply the following related technologies.

  FIG. 13 is a diagram illustrating an example of a problem associated with line transfer. Each square in the figure indicates one dot formed by PWM (pulse width modulation) of the beam. As is well known, the density of an image to be formed is changed by changing the beam width (spot size) by PWM.

  (A) shows an ideal thin line of 1 dot width. (B) shows an example in which the line is changed, but the interpolation processing is not applied at the transfer point. In this case, as a detrimental effect due to the line transfer, a thin line with noticeable jaggy is formed. (C) is an example in which line transfer is performed and interpolation for mitigating jaggies is applied in the vicinity of the transfer point. In this case, jaggies are alleviated, but it becomes difficult to uniformly reproduce the thickness of the thin line in the interpolation portion. For this reason, as shown in (d), by minimizing the range of pixels to which the interpolation is applied, the uniformity of the fine lines can be maintained while mitigating jaggies.

  By the way, when forming a color image, it is common to superimpose images of a plurality of colors such as cyan, magenta, yellow and black. Since the optical system is different for each color or the photosensitive drum is different, the actual scanning locus of each color does not always match. That is, the preferred transfer point differs for each color. As described above, if the transfer points of the image data are different among a plurality of colors, color misregistration and color unevenness may occur near the transfer points. This is not preferable because it leads to a decrease in image quality.

  FIG. 14 is a diagram for explaining the principle that color misregistration and color unevenness occur in the vicinity of a transfer point. A vertical broken line indicates the transfer point described above. (A) shows a correction target curve to be corrected for the first toner color (Color A) and a correction function for correcting the correction target curve in units of lines. (B) shows the difference (error component) between the correction function and the correction target curve.

  (C) shows an interpolation function for pixel interpolation applied in the vicinity of the transfer point. By this pixel interpolation, the non-uniformity of the thin line thickness is alleviated. As shown in (d), even when the correction process and the interpolation process are applied, an error (a mountain or a valley in the figure) may occur at a position away from the transfer point. Such an error is not a problem when an image is formed in a single color, but can be a problem when a color image is formed.

  For example, when an image is formed with only one of cyan, magenta, yellow, and black, which are basic colors of the color space in the electrophotographic system, the problem is less likely to become obvious. However, when a plurality of toner colors such as R (Red), G (Green), and B (Blue) need to be superimposed, color misregistration may occur.

  Assume that the correction target curve for the second toner color (Color B) is different from the correction target curve for Color A, as shown in FIG. In this case, the error when correcting in line units and applying pixel interpolation can be a value as shown in (f). The error of Color A indicated by (d) and the error of Color B indicated by (f) are different in size and direction. Therefore, as shown in (g), it can be seen that the amount of color misregistration between two colors is twice as large as the error of each color.

  In FIG. 14, the transfer point of Color A and the transfer point of Color B are assumed to be the same, but in actuality, a suitable transfer point is different for each color. This difference in transfer points further complicates the problem.

  Therefore, an object of the present invention is to solve at least one of such problems and other problems. For example, an object of the present invention is to reduce the occurrence of color misregistration and color unevenness between a plurality of colors due to line switching. Other issues can be understood throughout the specification.

The present invention is, for example,
An image forming apparatus that forms a multicolor image by superimposing a plurality of colors,
Storage means for storing data of pixels included in each line for each color in a plurality of lines in the main scanning direction;
An image carrier provided for each color;
Scanning means provided for each color for scanning the corresponding image carrier using a beam modulated by pixel data read from the storage means for each color;
Correction means for correcting the curvature of the scanning line in the main scanning direction of the image carrier by changing lines read from the storage means according to a predetermined order when scanning one line by the scanning means; ,
Interpolating means for interpolating data of one or more pixels located before and after a transfer point when changing the read line;
Have
The interpolation section corresponding to the color of the pixel of interest is from the transfer point corresponding to the color of the pixel of interest to the transfer point of another color closest to the transfer point .

  According to the present invention, when performing line transfer for each color and interpolation in the vicinity of the transfer point, the line transfer is performed by setting an interpolation interval from the transfer point of the own color to the transfer point of another color. As a result, the occurrence of color misregistration and color unevenness between a plurality of colors is reduced.

  An embodiment of the present invention is shown below. Of course, the individual embodiments described below will be helpful in understanding various concepts such as the superordinate concept, intermediate concept and subordinate concept of the present invention. Further, the technical scope of the present invention is determined by the scope of the claims, and is not limited by the following individual embodiments.

[Example 1]
FIG. 1 is a schematic cross-sectional view of an image forming apparatus that forms a multicolor image by superimposing a plurality of colors according to the embodiment. This image forming apparatus is a four-drum color copying machine in which four photoconductors are arranged in tandem. Note that the image forming apparatus may be realized as, for example, a printing apparatus, a printer, a multifunction peripheral, or a facsimile. The number of colors may be two or more. Therefore, it is sufficient that the number of photoconductors is two or more. In the following, an outline of a color image reading apparatus (hereinafter referred to as “color scanner”) 1 and a color image recording apparatus (hereinafter referred to as “color printer”) 2 constituting the color copying machine 100 will be described.

  The color scanner 1 forms an image of the document 13 on the color sensor 17 through the illumination lamp 14, the mirror groups 15 </ b> A, B, C, and the lens 16. Further, the color scanner 1 reads the color image information of the document for each color separation light of, for example, blue (hereinafter referred to as B), green (hereinafter referred to as G), and red (hereinafter referred to as R), and converts it into an electrical image signal. To do.

  The color scanner 1 performs color conversion processing based on the intensity levels of the B, G, and R image signals. As a result, color image data of black (K), cyan (C), magenta (M), and yellow (Y) is obtained.

  Next, the color printer 2 will be described. One writing optical unit 28M (for magenta), 28Y (for yellow), 28C (for cyan), 28K (for black) is provided for each color toner. Note that the suffix MYCK attached to the reference number indicates the color of the toner. These writing optical units are an example of a scanning unit for each color that scans the corresponding image carrier using a beam modulated by pixel data read from the storage unit for each color. The writing optical unit is sometimes called an exposure device or a scanner device.

  The writing optical unit converts the color image data from the color scanner 1 into an optical signal and performs optical writing. Thereby, electrostatic latent images are formed on the photoreceptors 21M, 21Y, 21C, and 21K provided for the respective colors. These photoreceptors are examples of an image carrier.

  These photoconductors 21M, 21Y, 21C, and 21K rotate counterclockwise as indicated by arrows. Chargers 27M, 27Y, 27C, and 27K for uniformly charging the photoconductor are provided around the photoconductor. Further, an M developing unit 213M, a C developing unit 213C, a Y developing unit 213Y, and a Bk developing unit 213K for developing an electrostatic latent image using a developer (eg, toner) are also provided. Further, the intermediate transfer belt 22 as an intermediate transfer member is stretched around a driving roller 220 and driven rollers 219 and 237. In addition, first transfer bias blades 217M, 217Y, 217C, and 217K, which are first transfer units, are also provided so as to face the respective photoconductors.

  The second transfer bias roller 221 is disposed at a position facing the driven roller 219. The second transfer bias roller 221 is separated from or in contact with the intermediate transfer belt 22 by a separation / contact mechanism (not shown).

  In the color printer 2, image formation is first started from magenta. Thereafter, cyan image formation is started at a timing delayed with respect to the rotational speed of the intermediate transfer belt 22 corresponding to the separation distance between the photoconductor 21M and the photoconductor 21C. Next, yellow image formation is started at a timing delayed with respect to the rotational speed of the intermediate transfer belt 22 corresponding to the separation distance between the photosensitive member 21C and the photosensitive member 21CY. Finally, black image formation is started at a timing delayed with respect to the rotational speed of the intermediate transfer belt 22 corresponding to the distance between the photosensitive member 21Y and the photosensitive member 21K. Thus, on the intermediate transfer belt 22, the developer images of the respective colors are superimposed to form a multicolor image. This multicolor image is transferred to the recording material conveyed by the conveying rollers 228, 227, 226, and 225 at the secondary transfer position formed by the driven roller 219 and the second transfer bias roller 221. Thereafter, a color image is fixed on the surface of the recording material in the fixing device 25. Note that the recording material may be called, for example, a recording medium, paper, sheet, transfer material, or transfer paper.

  FIG. 2A is a schematic cross-sectional view of the writing optical unit according to the embodiment. FIG. 2B is a schematic plan view of the writing optical unit according to the embodiment. The light emitting element array 281 includes four light emitting elements, and can irradiate four beams (laser lights) at the same time. Each beam is applied to the mirror surface of the rotating polygon mirror 283 via the lens 282. The polygon mirror 283 is rotationally driven by a polygon motor. When the polygon mirror 283 rotates once, the photosensitive member can be scanned six times. This is because the polygon mirror 283 has six mirror surfaces. The beam deflected by the polygon mirror 283 is first detected by a beam detection (hereinafter referred to as BD) element 286. The BD signal output from the BD detection element 286 serves as a trigger for starting exposure for each main scan. That is, pixel data included in the corresponding four lines are sequentially read out and applied to the four light emitting elements, respectively. The scanning speed is corrected by the fθ lens 284 so that the scanning speed of the beam on the photosensitive member is constant. Thereafter, the beam is deflected by a plane mirror 285 to expose and scan the photoreceptor.

  FIG. 3 is an exemplary block diagram of a control system and an image processing system according to the embodiment. The reading system image processing unit 301 applies image processing such as shading correction to the image signal output from the color scanner 1 and passes the image signal to the central image processing unit 305. The central image processing unit 305 stores the image signal in the image memory 304, and passes the image signal to the output system image processing units 306 to 309 at an appropriate timing reflecting the distance between the photoconductors. The output system image processing unit 306 is an image processing unit for yellow (Y). The output system image processing unit 307 is an image processing unit for magenta (M). The output system image processing unit 308 is an image processing unit for cyan (C). The output system image processing unit 309 is an image processing unit for black (K). The output system image processing units 306 to 309 execute correction processing and interpolation processing according to each color of Y, M, C, and K, respectively.

  The central image processing unit 305 transmits and receives data via an external interface 303 connected to a telephone line, a network, and the like. If the received data is data described in PDL (page description language), the PDL processing unit 302 develops the image information. The CPU 310 is a control device that performs overall processing of each unit.

  FIG. 4 is an exemplary block diagram of an output system image processing unit according to the embodiment. The line buffer 401 is an example of an accumulation unit that accumulates data of pixels included in a line for each color in a plurality of lines in the main scanning direction. The number of lines that can be stored by the line buffer 401 is, for example, the sum of the number of lines corresponding to the maximum line shift width and the number of lines used for the subsequent interpolation processing calculation. For example, if the maximum deviation width is 20 lines and the number of lines used for the interpolation calculation is 1, the size of the line buffer 401 is 21 lines. The line buffer 401 accumulates data of the next line stored in the image memory 304 when the data of all the pixels constituting one line is read by the line selector 402.

  The line selector 402 receives all the data stored in the line buffer 401 and outputs the data designated by the correction calculation unit 404 to the interpolation calculation unit 403. The correction calculation unit 404 reads the information of the line transfer point from the curve characteristic memory 406. When the current main scanning position x counted by the main scanning counter 405 has a value corresponding to the line changing point, the correction calculation unit 404 instructs the line selector 402 to change the read target line. In this way, the correction calculation unit 404 changes the scanning line curve in the main scanning direction of the image carrier by changing the lines read from the storage unit according to a predetermined order when one line is scanned by the scanning unit. It is an example of the correction | amendment means to correct | amend.

  In the bending characteristic memory 406, information on transfer points for correcting the curvature of the scanning line in the main scanning direction of the photosensitive member is acquired and stored at the time of factory shipment or the like. In other words, the bending characteristic memory 406 is an example of a holding unit that holds the number of the pixel that is a transfer point from the first pixel to the last pixel constituting one line for each color.

  The interpolation calculation unit 403 applies the interpolation calculation to the data output from the line selector 402 and outputs the data. The interpolation calculation unit 403 is an example of an interpolation unit that interpolates data of one or more pixels positioned before and after a transfer point when a line read from the storage unit is changed. For example, the interpolation calculation unit 403 acquires interpolation section information used for the interpolation calculation, weight coefficient information applied to each interpolation section, and the like from the correction calculation unit 404 and executes the interpolation calculation. The weight coefficient may also be stored in the bending characteristic memory 406.

  The correction calculation unit 404 is an example of an interpolation interval determination unit, and determines an interpolation interval to be interpolated by the interpolation calculation unit 403 for the nth color among a plurality of colors. For example, the correction calculation unit 404 determines an interpolation interval from the nth color change point to the mth color change point closest to the change point. Note that the information about the m-th color transfer point is acquired from the CPU 310. The CPU 310 acquires transfer point information from the bending characteristic memory 406 of each output system image processing unit 306 to 309.

[Method for determining interpolation interval]
FIG. 5 is a diagram for explaining an interpolation interval determination method according to the embodiment. The left end in the figure corresponds to the main scanning start position, and the right end corresponds to the main scanning end position. That is, the beam scans on the photoreceptor from left to right.

  Here, the interpolation interval applied to the nth color is determined from the transfer point of the nth color and the transfer point of the mth color. In FIG. 5, for convenience, the nth color line and the mth color line are shown as being shifted in the sub-scanning direction (direction orthogonal to the main scanning direction). However, in practice, there is no shift in the sub-scanning direction (however, there may be a local shift causing color shift or color unevenness). That is, both lines are superimposed as one line on the intermediate transfer belt 22.

  According to FIG. 5, the transfer points for the nth color are n1 to n5. On the other hand, the transfer points for the mth color are m1 to m4. Here, the interpolation section of the transfer point n1 is a front section v1a and a rear section v1b. The interpolation section of the transfer point n2 is a front section v2a and a rear section v2b. The transfer points n3 to n4 are also shown.

  Here, among the interpolation sections of the transfer point n1, the rear section v1b is a section from the transfer point n1 to the m-th color transfer point m1. Similarly, in the interpolation section of the transfer point n2, the forward section v2a is a section from the m-color transfer point m1 to the n-color transfer point n2. The rear section v2b is a section from the n-color transfer point n2 to the m-color transfer point m2.

  Thus, the interpolation interval for the nth color is from the nth color transfer point to the mth color transfer point closest to the transfer point. The interpolation section can be separated into, for example, a front section (example: v2b) and a rear section (example: v2b). The forward section (eg, v2a) is from the nth color change point (eg, n2) to the mth color change point (eg, m1) closer to the scan start position than this change point. On the other hand, the rear section (e.g., v2b) starts from the n-th color change point (e.g., n2) to the m-th color change point (e.g., m2) that is closer to the scanning end position than this transfer point. Become.

  The m-th interpolation interval is as follows. The forward section (e.g., w1a) is a section from the m-th color switching point (e.g., m1) to the n-th color switching point (e.g., n1) closer to the scanning start position than this switching point. Become. On the other hand, the rear section (e.g., w2b) is from the m-th color change point (e.g., m1) to the n-th color change point (e.g., n2) closer to the scan end position than this transfer point. It becomes a section.

  Note that the first forward section v1a for the nth color is a section from the start position of the line to the first transfer point n1. This is because there is no m-th color transfer point that is closer to the start position than the n1-th transfer point. Similarly, the last rear section v5b is a section from the transfer point n5 to the end position of the line. This is because there is no m-th color transfer point closer to the end position of the line than the transfer point n5.

  Here, if the transfer point n1 or m1 is expressed by coordinates in the main scanning direction (the number of pixels included up to the transfer point), the interpolation interval for the nth color shown in FIG. It becomes as follows.

v1a = | n1-0 |
v1b = | m1-n1 |
v2a = | n2-m1 |
v2b = | m2-n2 |
v3a = | n3-m2 |
v3b = | m3-n3 |
v4a = | n4-m3 |
v4b = | m4-n4 |
v5a = | n5-m4 |
v5b = | pixel (max) −n5 |
Here, pixel (max) is the total number of pixels included in one line. Also, the interpolation interval for the mth color shown in FIG. 5 is as follows.

w1a = | m1-n1 |
w1b = | n2-m1 |
w2a = | m2-m2 |
w2b = | n3-m2 |
w3a = | m3-n3 |
w3b = | n4-m3 |
w4a = | m4-m4 |
w4b = | n5-n4 |
As described above, when four colors of MCYK are used, the interpolation section of the second color is determined based on the transfer point between M, which is the first color, and C, which is the second color. That's fine. Similarly, an interpolation interval for the third color may be determined based on a transfer point between C, which is the second color, and Y, which is the third color. Based on the transfer point between Y as the third color and K as the fourth color, the interpolation interval of the fourth color may be determined. Note that the interpolation section of the first color may be determined based on the transfer point between K, which is the fourth color, and M, which is the first color. These are merely examples. For example, the interpolation interval of the second color may be determined based on the transfer point between the fourth color and the second color. From the viewpoint of alleviating color misregistration and color unevenness, it may be desirable to determine a combination of colors for determining the interpolation interval.

  6A, 6B, and 6C are diagrams for explaining an example of pixel interpolation processing according to the embodiment. In particular, FIG. 6A shows three lines before the line transfer is performed and the pixels included in them. Here, data B, C, and D of three pixels with different lines are shown in front of the transfer point. Behind the transfer point, data b, c, and d of three pixels with different lines are shown. As can be seen from the figure, for example, pixel data B and pixel data b belong to the same i-th line. Similarly, it is assumed that pixel data C and pixel data c belong to the same i + 1-th line. It is assumed that the pixel data D and the pixel data d belong to the same i + 2th line.

  FIG. 6B shows the positional relationship of each pixel after line transfer is executed. In this example, the data of the rear pixel moves to the upper line at the transfer point. That is, the i-1th line including the pixel data A in the drawing is read in order, and after passing through the transfer point, the ith line including the pixel data b is sequentially read out.

  FIG. 6C shows an example of pixel interpolation. Pixel data B 'indicates pixel data after interpolation. For example, the pixel data B ′ is defined by the sum of the product of the pixel data A and the first weighting factor α and the product of the pixel data B and the second weighting factor β.

B '= α ・ A + β ・ B
Similarly, the pixel data d ′ is defined by the sum of the product of the i + 3th pixel data e and the first weighting factor α and the product of the pixel data d and the second weighting factor β.

d ′ = α · e + β · d
Here, it is desirable that the first weighting coefficient α and the second weighting coefficient β change according to the distance from the transfer point. For example, since the first weighting coefficient α is a coefficient that is multiplied by the pixel data included in the original line, the value increases as the distance from the transfer point increases. On the other hand, since the second weighting coefficient β is a coefficient that is multiplied by the pixel data included in the lower or upper line, the value decreases as the distance from the transfer point increases. This alleviates jaggies that occur near the transfer point.

  6D and 6E are diagrams for explaining an example of pixel interpolation processing according to the embodiment. FIG. 6D shows the positional relationship of each pixel after line transfer is executed. In FIG. 6B, the line located behind the transfer point has moved upward. In FIG. 6D, the line located behind the transfer point has moved downward. The direction of movement is determined by the characteristics of the curvature.

  FIG. 6E shows an example of pixel interpolation. Pixel data D 'and b' indicate pixel data after interpolation. For example, the pixel data D ′ is defined by the sum of the product of the pixel data E and the first weighting factor α and the product of the pixel data E and the second weighting factor β.

D ′ = α · E + β · D
Similarly, the pixel data b ′ is defined by the sum of the product of the pixel data a and the first weighting factor α and the product of the pixel data b and the second weighting factor β.

b ′ = α · a + β · b
FIG. 7 is a flowchart illustrating a method for determining an interpolation interval according to the embodiment. Here, it is assumed that the interpolation interval is dynamically calculated. However, at the time of factory shipment or the like, the interpolation section may be determined in advance from the test result, and the information on the interpolation section may be stored in the bending characteristic memory 406.

  In step S <b> 701, the correction calculation unit 404 provided in the output system image processing unit in charge of the nth color reads information on the transfer point from the bending characteristic memory 406.

  In step S <b> 702, the correction calculation unit 404 reads information on the transfer point for the mth color and information on the transfer point from the bending characteristic memory 406 provided in the output image processing unit in charge of the nth color. For example, the correction calculation unit 404 requests the CPU 310 to transmit information on the transfer point for the mth color. The CPU 310 requests the correction calculation unit 404 provided in the output system image processing unit in charge of the mth color to transmit information on the transfer point for the mth color. The correction calculation unit 404 reads information on the transfer point for the mth color from the bending characteristic memory and transmits the information to the CPU 310. The CPU 310 transmits the received transfer point information for the mth color to the correction calculation unit 404 provided in the output image processing unit in charge of the nth color. Note that the information on the transfer point includes, for example, information indicating the position of the transfer point (coordinates (pixel numbers) in the main scanning direction).

  In step S703, the correction calculation unit 404 provided in the output system image processing unit in charge of the nth color determines an interpolation section for each transfer point according to a predetermined interpolation section determination rule. An example of a predetermined rule for determining an interpolation interval is the rule described with reference to FIG. For example, from the position of the i (i is 1 to the total number of pixels) -th transfer point ni, the transfer point of the m-th color located before and after that is extracted. Normally, two transfer points of the mth color are extracted, but only one may be extracted for the first transfer point n1 and the last transfer point. The correction calculation unit 404 determines an interval between the noticed ni-th transfer point and the extracted transfer point as an interpolation section (front section, rear section). Information on the determined interpolation interval is sent to the interpolation calculation unit 403.

  FIG. 8 is a flowchart illustrating the pixel interpolation method according to the embodiment. In step S <b> 801, the CPU 310 determines whether a BD signal is output from the BD detection element 286. When the BD signal is output, the CPU 310 transmits an instruction to restart the main scanning counter to the correction calculation unit 404. Thereafter, the process proceeds to step S802.

  In step S802, the correction calculation unit 404 restarts the main scanning counter 405. In step S <b> 803, the line selector 402 reads out data of pixels belonging to the line designated by the correction calculation unit 404 from the line buffer 401 and outputs the data to the interpolation calculation unit 403. The main scanning counter 405 increments the count value by one.

  In step S804, the correction calculation unit 404 determines whether the count value of the main scanning counter 405 is a value corresponding to a transfer point. It is assumed that the position information of the transfer point is read from the bending characteristic memory 406. If it is not a transfer point, the process proceeds to step S807. On the other hand, if it is a transfer point, the process proceeds to step S805.

  In step S805, the correction calculation unit 404 instructs the line selector 402 to switch the readout line. Note that information on which line to switch to is stored in the bending characteristic memory 406 together with the position information of the transfer point. The correction calculation unit 404 reads information on which line to switch from the bending characteristic memory 406 and notifies the line selector 402 of the information. The line selector 402 selects the line designated by the correction calculation unit 404 as a reading target.

  In step S806, the correction calculation unit 404 acquires information on the interpolation section and information on the weighting factor. The correction calculation unit 404 may read the interpolation section information and the weight coefficient information from the curve characteristic memory 406, or may dynamically calculate the information from the change point position information. The correction calculation unit 404 may also dynamically calculate the weighting factor applied to each pixel located in the interpolation section from the weighting determination function determined in advance for the weighting factors α and β. The weight determination function may be, for example, a linear function or a non-linear function. In any case, the weight determination function is determined from the viewpoint of mitigating jaggies, color misregistration, and / or color unevenness. It is desirable.

  In step S807, the correction calculation unit 404 determines whether the count value of the main scanning counter 405 is a value corresponding to the interpolation interval. If it is an interpolation section, the process advances to step S808, and the interpolation calculation unit 403 performs an interpolation calculation on the data of the pixels belonging to the interpolation section based on the interpolation section information and the weighting coefficient acquired from the correction calculation unit 404. The interpolation calculation method is as described with reference to FIGS. 6A to 6E, for example.

  On the other hand, if it is not the interpolation section, the process advances to step S809, and the correction calculation unit 404 determines whether or not the count value of the main scanning counter 405 means the number of the last pixel. If it is not the last pixel, the process returns to step S803. If it is the last pixel, the process returns to step S801.

  According to the present invention, when performing line transfer for each color and interpolation in the vicinity of the transfer point, it is possible to change the line by setting the interpolation section from the transfer point of the own color to the transfer point of the other color. Accordingly, occurrence of color misregistration and color unevenness between a plurality of colors is reduced.

  An interpolation section is generated before and after the transfer point. Therefore, the interpolation section positioned in front of the transfer point may be treated as the front section, and the interpolation section positioned behind the transfer point may be handled as the rear section. Such a division may be useful when the length of the section (the number of contained pixels) and the weighting factor are different between the front section and the rear section.

  In addition, it is preferable that the bending characteristic memory 406 holds the number of the pixel to be a transfer point among the first pixel to the last pixel constituting one line for each color. In general, an image forming measure has a function of counting a pixel number in the main scanning direction by a counter. Therefore, if the transfer point is expressed in units of pixel numbers, the present invention can be easily applied to existing image forming apparatuses. Note that information on different transfer points may be stored in the bending characteristic memory 406 in accordance with the temperature, humidity, or durability. In this case, the CPU 310 acquires the temperature, humidity, or durability year and passes it to the correction calculation unit 404. The correction calculation unit 404 reads information on the transfer point corresponding to the current temperature, humidity, or durability years from the curve characteristic memory 406. As a result, color misregistration and color unevenness may be alleviated according to environmental conditions such as temperature, humidity, and durability.

  As shown in FIGS. 6C and 6E, the interpolation calculation unit 403 performs interpolation processing using the data of the pixel to be interpolated and the data of the pixel located above or below the pixel to be interpolated in the sub-scanning direction. It is desirable to execute. Data of pixels adjacent in the sub-scanning direction often have a correlation. Therefore, if interpolation is performed using the data of these pixels, there is an advantage that the transfer point is not noticeable.

  The interpolation calculation unit 403 interpolates the sum of the product of the pixel data to be interpolated and the first weighting factor and the product of the pixel data located above or below the pixel to be interpolated and the second weighting factor. It may be calculated as pixel data. If these weighting factors are held in a storage device such as an LUT (Look Up Table), there is an advantage that it is possible to save time and effort for dynamic calculation. It should be noted that different coefficients may be stored in the LUT depending on the temperature, humidity, age, etc. In this case, the CPU 310 acquires the temperature, humidity, or durability year and passes it to the correction calculation unit 404. The correction calculation unit 404 reads a weighting factor corresponding to the current temperature, humidity, or durability from the LUT (bending characteristic memory 406). As a result, color misregistration and color unevenness may be alleviated according to environmental conditions such as temperature, humidity, and durability.

  The first weighting factor may be set to be relatively large and the second weighting factor may be set to be relatively small as the distance in the main scanning direction from the switching point increases. This is desirable from the viewpoint of alleviating color misregistration and color unevenness associated with line transfer.

  Note that the correction calculation unit 404 may update each weighting coefficient for n colors from information on each weighting coefficient for n colors and interpolation sections. For example, the CPU 310 may update the weighting coefficient so as to reduce the color misregistration and the color unevenness by measuring the color misregistration amount and the color unevenness amount. In this case, it is a case where the bending characteristic is changed due to a change with time, an environmental change or the like, and the color shift and the color unevenness can be alleviated. Also, the uniformity of the thickness of the thin line will be maintained. The amount of color misregistration and color unevenness can be measured, for example, by forming a patch pattern and reading the formed patch pattern with a reading device such as a CCD.

  In the embodiment, as an example, a polygon mirror is used as scanning means of the optical system. However, the present invention is not limited to this. For example, other scanning means such as a galvanometer mirror or EO (electro-optical element) may be employed.

1 is a schematic cross-sectional view of an image forming apparatus according to an embodiment. It is a schematic sectional drawing of the writing optical unit which concerns on an Example. It is a schematic plan view of the writing optical unit according to the example. It is an exemplary block diagram about a control system and an image processing system concerning an example. It is an exemplary block diagram of an output system image processing unit according to an embodiment. It is a figure for demonstrating the determination method of the interpolation area which concerns on an Example. , , It is a figure for demonstrating an example of the interpolation process of the pixel which concerns on an Example. , It is a figure for demonstrating an example of the interpolation process of the pixel which concerns on an Example. It is a flowchart which shows the determination method of the interpolation area which concerns on an Example. It is a flowchart which shows the interpolation method of the pixel which concerns on an Example. It is a side view which shows an example of an oblique incidence system. FIG. 6 is a diagram for explaining a difference between an actual scanning line formed on a photosensitive drum 903 in an oblique incidence method and an ideal scanning line. It is a figure which shows an example of an ideal optical system without a significant shift | offset | difference in a rotating shaft, and an actual optical system with a significant shift | offset | difference in a rotating shaft. It is a figure which shows an example of the method of correct | amending curvature by the transfer of a line. It is a figure which shows an example of the problem accompanying the transfer of a line. It is a figure for demonstrating the principle in which color shift and color nonuniformity generate | occur | produce in the vicinity of a transfer point.

Claims (7)

An image forming apparatus that forms a multicolor image by superimposing a plurality of colors,
Storage means for storing data of pixels included in each line for each color in a plurality of lines in the main scanning direction;
An image carrier provided for each color;
Scanning means provided for each color for scanning the corresponding image carrier using a beam modulated by pixel data read from the storage means for each color;
Correction means for correcting the curvature of the scanning line in the main scanning direction of the image carrier by changing lines read from the storage means according to a predetermined order when scanning one line by the scanning means; ,
Interpolating means for interpolating data of one or more pixels located before and after a transfer point when changing the read line;
Have
The interpolation interval corresponding to the color of the pixel of interest, the image forming apparatus to the transfer point corresponding to the color of the target pixel, characterized that you be up transfer point of the other color closest to said transfer point. Stores the number of the pixel that becomes the switching point of the color of the pixel of interest and the number of the pixel that becomes the switching point of the other color from the data of the first pixel constituting the line to the data of the last pixel Further comprising holding means for
2. The correction means and the interpolation means read out the number of a pixel that becomes a switching point of the color of the target pixel and the number of a pixel that becomes a switching point of the other color from the holding unit. The image forming apparatus described. The interpolation means comprises a data of a pixel to be interpolated, according to claim 1 or, characterized in that for performing interpolation processing using the data of pixels located above or below the pixel of the interpolation target in the sub-scanning direction Item 3. The image forming apparatus according to Item 2 . The interpolation means interpolates the sum of the product of the pixel data to be interpolated and the first weighting factor and the product of the data of the pixel located above or below the pixel to be interpolated and the second weighting factor. The image forming apparatus according to claim 3 , wherein the image forming apparatus calculates the subsequent pixel data. The image forming apparatus according to claim 4 , further comprising a storage unit that stores the first weighting factor and the second weighting factor. The first weighting coefficient is set to be relatively large and the second weighting coefficient is set to be relatively small as the distance in the main scanning direction from the color change point of the target pixel increases. the image forming apparatus according to claim 4 or 5, characterized in.   A plurality of photoreceptors for forming images corresponding to different colors;
  A plurality of exposure means for exposing each of the plurality of photoconductors;
  A transfer body onto which images formed on the plurality of photoconductors are transferred;
  Processing means for generating data for each of the plurality of exposure means,
  The processing means includes
  A transfer processing means for performing a line transfer process based on a transfer point according to a curve characteristic of a scanning line of a beam exposed by the exposure means;
  Interpolation means for performing interpolation so that dots reflecting the dot data of the target pixel are formed on adjacent pixels adjacent to the target pixel on a line adjacent to the target pixel line;
  The interpolation means is between a transfer point corresponding to the color of the pixel of interest and a transfer point corresponding to the color of the pixel of interest among transfer points corresponding to other colors, An image forming apparatus that interpolates dot data of one or more adjacent pixels.

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