Image forming apparatus

Abstract

To provide a technique for reducing moire.SOLUTION: An image formation device includes a photoreceptor, scanning means for repeatedly scanning the photoreceptor in a main scanning direction with one or more scanning light beams on the basis of an image signal, in a sub-scanning direction perpendicular to the main scanning direction, and thereby forming an electrostatic latent image on the photoreceptor, generation means for subjecting image data to halftone processing, and generating the image signal, and storage means for storing correction information for correcting a scanning start position of the photoreceptor with the one or more scanning light beams, wherein the correction information is set so that a scanning start position of a plurality of scanning lines continuous in the sub-scanning direction, which is formed on the photoreceptor with the one or more scanning light beams, according to a direction of a first vector with a small angle with respect to the sub-scanning direction in two vectors of the halftone processing is linearly deviated to the negative side or the positive side in the main scanning direction along the sub-scanning direction.SELECTED DRAWING: Figure 14

Description

Technical Field

[0001] The present invention relates to an image forming apparatus such as a laser beam printer, a digital copier, a digital FAX, etc.

Background Art

[0002] In an electrophotographic image forming apparatus, a photoreceptor that is rotationally driven is repeatedly scanned by scanning light based on image data to form an electrostatic latent image on the photoreceptor, and the electrostatic latent image is developed with toner to form an image. Here, the locus that the scanning light moves on the photoreceptor during one scan is called a scanning line, and the moving direction of the scanning light is called the main scanning direction. Also, the direction orthogonal to the main scanning direction and in which the scanning lines are sequentially formed is called the sub-scanning direction. Note that the main scanning direction is parallel to the rotation axis of the photoreceptor. Also, in the photoreceptor, the direction opposite to the rotation direction of the photoreceptor corresponds to the sub-scanning direction.

[0003] A rotating polyhedron is used to move the scanning light in the main scanning direction on the photoreceptor. Here, if the length of each scanning line and its position in the main scanning direction change periodically in the sub-scanning direction, moire may occur in the image due to interference with the period of the halftone process. Note that the periodic change of the scanning line in the sub-scanning direction may be caused by manufacturing errors of each reflecting surface of the rotating polyhedron or the like. Also, in a configuration in which the photoreceptor is scanned using a plurality of scanning lights emitted from a plurality of light sources, a periodic change of the scanning line in the sub-scanning direction may occur due to arrangement errors of the plurality of light sources or the like.

[0004] Patent Document 1 discloses a configuration in which, in order to make the lengths of the scanning lines by the scanning light reflected by each reflecting surface of the rotating polyhedron uniform, the error in the length of the scanning line for each reflecting surface is measured, and pixel pieces obtained by dividing one pixel are inserted or removed so as to correct the error.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

[0006] By correcting the scan line to suppress periodic changes in the sub-scanning direction based on pre-measured results, moiré patterns can be reduced. However, if there are measurement errors, periodic changes in the sub-scanning direction of the scan line may occur, resulting in insufficient moiré reduction.

[0007] This invention provides a technology for reducing moiré patterns. [Means for solving the problem]

[0008] According to one aspect of the present invention, an image forming apparatus comprises a photoreceptor, scanning means for forming an electrostatic latent image on the photoreceptor by repeatedly scanning the photoreceptor in a main scanning direction with one or more scanning lights in a sub-scanning direction orthogonal to the main scanning direction based on an image signal, generating means for generating the image signal by performing halftone processing on the image data, and storage means for storing correction information for correcting the scanning start position of the photoreceptor by the one or more scanning lights, wherein the correction information is set such that, according to the direction of the first vector of the two vectors of the halftone processing which has a small angle with respect to the sub-scanning direction, the scanning start positions of a plurality of scanning lines that are continuous in the sub-scanning direction and formed on the photoreceptor by the one or more scanning lights are linearly displaced along the sub-scanning direction to the negative or positive side of the main scanning direction. Ori , If the direction of the first vector is displaced along the sub-scanning direction to the positive side of the main scanning direction, the correction information is set so that the scanning start positions of the multiple consecutive scan lines are linearly displaced along the sub-scanning direction to the positive side of the main scanning direction. If the direction of the first vector is displaced along the sub-scanning direction to the negative side of the main scanning direction, the correction information is set so that the scanning start positions of the multiple consecutive scan lines are linearly displaced along the sub-scanning direction to the negative side of the main scanning direction. . [Effects of the Invention]

[0009] According to the present invention, moiré patterns can be reduced. [Brief explanation of the drawing]

[0010] [Figure 1] A schematic diagram of an image forming apparatus according to one embodiment. [Figure 2] Configuration diagram of the scanning unit according to an embodiment. [Figure 3] Control configuration diagram of the image forming apparatus according to an embodiment. [Figure 4] Block diagram of the image controller according to an embodiment. [Figure 5] Explanatory diagram of a pixel piece. [Figure 6] Explanatory diagram of the arrangement of a plurality of light sources and the positional relationship of the scanning light by the plurality of light sources on the photoreceptor. [Figure 7] Diagram showing a variation example of a scanning line by a plurality of light sources. [Figure 8] Diagram showing an image formed by halftone processing according to an embodiment. [Figure 9] Explanatory diagram of the generation principle of moiré. [Figure 10] Diagram showing correction information according to an embodiment. [Figure 11] Diagram showing an example of a deviation in the scanning start position that remains even after correction due to an error in the correction information. [Figure 12] Explanatory diagram showing that the moiré intensity varies depending on the halftone processing and the pattern of the deviation of the scanning start position. [Figure 13] Explanatory diagram showing that the moiré intensity varies depending on the halftone processing and the pattern of the deviation of the scanning start position. [Figure 14] Diagram showing the relationship between the halftone processing and the target scanning start position, and the relationship between the deviation amount of the scanning start position and the moiré intensity. [Figure 15] Diagram showing an image formed by halftone processing according to an embodiment. [Figure 16] Explanatory diagram showing that the moiré intensity varies depending on the halftone processing and the pattern of the deviation of the scanning start position. [Figure 17] Explanatory diagram showing that the moiré intensity varies depending on the halftone processing and the pattern of the deviation of the scanning start position.

Mode for Carrying Out the Invention

[0011] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the invention according to the claims. Although a plurality of features are described in the embodiments, not all of these plurality of features are essential for the invention, and the plurality of features may be arbitrarily combined. Further, in the accompanying drawings, the same or similar configurations are denoted by the same reference numerals, and redundant explanations are omitted.

[0012] <First Embodiment> FIG. 1 is a schematic configuration diagram of an image forming apparatus according to the present embodiment. In each of the following drawings, components that are not necessary for understanding the embodiment are omitted from the drawings for simplification. The image forming units 125Y, 125M, 125C, and 125K form yellow, magenta, cyan, and black toner images on the intermediate transfer member 128, respectively. Note that by overlapping and forming toner images on the intermediate transfer member 128 by each of the image forming units 125Y, 125M, 125C, and 125K, colors different from yellow, magenta, cyan, and black can be reproduced. The configurations of the image forming units 125Y, 125M, 125C, and 125K are the same, and each includes a photoreceptor 122, a charging roller 123, a scanning unit 124, a developing roller 126, and a primary transfer roller 127. In the following description, the image forming units 125Y, 125M, 125C, and 125K are also collectively referred to as the image forming unit 125.

[0013] The photoreceptor 122 is driven to rotate in a counterclockwise direction in the figure during image formation. The charging roller 123 charges the surface of the rotating photoreceptor 122 to a uniform potential. The scanning unit 124 repeatedly scans the rotating photoreceptor 122 in the main scanning direction with scanning light based on image data, thereby forming an electrostatic latent image on the photoreceptor 122. The main scanning direction is parallel to the rotation axis of the photoreceptor 122 and is the direction in which the scanning light moves. The sub-scanning direction is perpendicular to the main scanning direction and is the direction in which the scanning lines are formed sequentially. In the photoreceptor 122, the sub-scanning direction corresponds to the direction opposite to the rotation direction of the photoreceptor 122. The developing roller 126 develops the electrostatic latent image on the photoreceptor 122 with toner to form a toner image on the photoreceptor 122. The primary transfer roller 127 transfers the toner image on the photoreceptor 122 to the intermediate transfer unit 128. During image formation, the intermediate transfer body 128 is driven to rotate in a clockwise direction as shown in the figure. Therefore, the toner image on the intermediate transfer body 128 is transported to a position opposite the secondary transfer roller 129. The secondary transfer roller 129 transfers the toner image on the intermediate transfer body 128 to the sheet that has been transported along the transport path 130. The sheet is then transported to a fixing unit (not shown) where the toner image is fixed. After the toner image is fixed, the sheet is discharged to the outside of the image forming apparatus.

[0014] Figure 2 is a diagram of the scanning unit 124. The light source drive unit 207 controls the emission of light from the light source 209 based on the image signal under the control of the scanning control unit 230. The motor drive unit 208 controls the rotation speed and rotation phase of the rotating polyhedron 200 under the control of the scanning control unit 230. The scanning light emitted from the light source 209 is reflected and deflected by each reflective surface of the rotating polyhedron 200, which has multiple reflective surfaces, to scan the photoreceptor 122. The fθ lenses 204 and 205 are provided to maintain a constant scanning speed on the photoreceptor 122 of the scanning light deflected by the rotating polyhedron 200. The light sensor 202 detects the scanning light reflected in a predetermined direction by each reflective surface of the rotating polyhedron 200. The timing at which the light sensor 202 detects the scanning light is output to the scanning control unit 230 as a synchronization signal.

[0015] The scanning control unit 230 determines the rotation speed and rotation phase of the rotating polyhedron 200 based on the detection period of the synchronization signal from the optical sensor 202, and controls the rotation of the rotating polyhedron 200 so that the rotation speed and rotation phase reach target values.

[0016] In the configuration shown in Figure 2, the rotating polyhedron 200 has four reflective surfaces, but this is an example, and the number of reflective surfaces can be one or more. Also, in the configuration shown in Figure 2, the photoreceptor 122 is scanned by one scanning beam emitted from one light source 209, but it is possible to configure the system to scan the photoreceptor 122 with multiple scanning beams emitted from multiple light sources 209. Note that the multiple scanning beams illuminate different positions on the photoreceptor 122 in the sub-scanning direction. Even when using multiple scanning beams, the synchronization signal is generated based on one scanning beam.

[0017] Figure 3 shows the overall control configuration of the image forming apparatus. When the image controller 340 receives image data and a print request based on that image data, it generates an image signal based on the image data and transmits the image signal to the scanning unit 124. The timing of outputting the image signal to the scanning unit 124 is determined based on a synchronization signal from the scanning unit 124. The printer control unit 342 of the engine controller 341 controls each component shown in Figure 1 under the control of the image controller 340. The control performed by the printer control unit 342 includes the control of the scanning unit 124 via the scanning control unit 230.

[0018] Figure 4 is a diagram of the image controller 340. The memory 402 stores correction information to suppress "scan line fluctuations" in the image formed on a single sheet. In this embodiment, "scan line fluctuations" mean that the length of each scan line is fluctuating and that the "scan start positions" of each scan line are not aligned. Furthermore, the "scan start position" of a scan line means the position on the photoreceptor 122 in the main scanning direction where the formation of an electrostatic latent image is initiated by the scanning light. The correction information will be described later. The reading control unit 403 reads the correction information held in the memory 402. The image data control unit 401 performs various processing on the image data to generate a pulse width modulation (PWM) signal. The various processing on the image data includes halftone processing. Based on the correction information, the image data control unit 401 corrects the PWM signal so that the lengths of each scan line are aligned. The image data control unit 401 then adjusts the timing based on the correction information so that the scanning start positions of each scan line on the photoreceptor 122 are aligned, and outputs the corrected PWM signal as an image signal to the scanning unit 124. The scanning unit 124 controls the emission of light from the light source 209 based on the PWM signal. In this embodiment, the light source 209 emits light when the PWM signal is high level, and turns off when the PWM signal is low level. When multiple light sources are used, an image signal is generated for each light source and output to the scanning unit 124.

[0019] Figure 5 shows an example of exposure of one pixel using a PWM signal. For example, at 600 dpi, the length (width) of one pixel in the main scanning direction is approximately 0.042 mm. In Figure 5, one pixel is divided into three pixel pieces, and each pixel piece is exposed. One pulse of the PWM signal corresponds to one image piece. In this example, a high-level pulse exposes the corresponding pixel piece, and a low-level pulse leaves the corresponding image piece unexposed. In Figure 5, black pixel pieces indicate exposed pixels, and white pixel pieces indicate unexposed pixels. The number next to each pixel indicates the ratio of the exposed area to the area of ​​one pixel, and corresponds to the density. Note that the number of pixel pieces constituting one pixel is not limited to three, but can be any number of two or more. In this embodiment, pixel pieces are inserted and removed based on correction information to equalize the length of each scan line.

[0020] Figure 6(A) shows the arrangement of the four light sources 209-1 to 209-4 when using four light sources 209-1 to 209-4. As shown in Figure 6(A), the four light sources 209-1 to 209-4 are arranged in a straight line, and the spacing between them is E1. For example, E1 is approximately 0.03 mm. The scanning light emitted by the four light sources 209-1 to 209-4 arranged as shown in Figure 6(A) is expanded to a spacing of approximately 0.175 mm on the photoreceptor 122 by the action of fθ lenses 204 and 205. When forming an image at 600 dpi, the spacing between scan lines in the sub-scanning direction needs to be approximately 0.042 mm.

[0021] Therefore, as shown in Figure 6(B), the emission direction of the scanning light from each of the four light sources 209-1 to 209-4 is adjusted so that the spacing E2 in the sub-scanning direction of the scanning light 81-1 to 81-4 emitted by the four light sources 209-1 to 209-4 on the photoreceptor 122 is approximately 0.042 mm. In other words, the line connecting the scanning light 81-1 to 81-4 emitted by the four light sources 209-1 to 209-4 is rotated on the photoreceptor 122 in a direction intersecting the sub-scanning direction. Consequently, at any given moment, the position in the main scanning direction in which the scanning light 81-1 to 81-4 illuminate the photoreceptor 122 will be different. In order to align the scanning start position for each scanning light 81-1 to 81-4, the output timing of the image signal to the scanning unit 124 for generating each scanning light 81-1 to 81-4 is adjusted based on the correction information as described above.

[0022] Figures 7(A) and 7(B) show the state in which scan line fluctuations due to manufacturing errors, etc., occur when correction information based on design values ​​is used. In Figures 7(A) and 7(B), the scan lines produced by the four scan beams 81-1 to 81-4 are denoted as scan lines #1 to #4. In Figure 7(A), the lengths of scan lines #1 to #4 are the same, but the scanning start positions of scan lines #3 and #4 are shifted by a distance A from the target start position to the positive side of the main scanning direction. In Figure 7(B), the scanning start positions of scan lines #1 to #4 coincide with the target start position, but the lengths of scan lines #1 and #2 are B shorter than the ideal length, and the lengths of scan lines #3 and #4 are C longer than the ideal length. In Figure 7, the target start position is the target scanning start position, and the target end position is the position in the main scanning direction where the formation of the electrostatic latent image ends when the length of the scan line started at the target start position is the target value.

[0023] As shown in Figures 7(A) and 7(B), when four scanning beams are used, the variation in the scan lines in the sub-scanning direction may occur with a period of four scan lines (4 lines), equal to the number of scanning beams used. Furthermore, if the length of the scan lines differs for each reflective surface due to manufacturing tolerances of the reflective surfaces of the polyhedron 200, the variation in the scan lines may occur with a period of the number of scanning beams multiplied by the number of reflective surfaces. Also, if the length of the scan lines differs for each reflective surface due to manufacturing tolerances of the reflective surfaces of the polyhedron 200, even when using only one scanning beam, the variation in the scan lines may occur with a period of the number of reflective surfaces.

[0024] When the scan lines fluctuate periodically in the sub-scanning direction, periodic fluctuations occur in the halftone dots during halftone processing. If fluctuations occur in the halftone dots, the density of the dots will vary, and moiré patterns may appear.

[0025] The principle of how moiré patterns are generated by fluctuations in scan lines is explained below. Figures 8(A) and 8(B) show examples of images processed with halftone using a 146-line dither matrix. In Figures 8(A) and 8(B), and in similar figures shown below, the direction from left to right corresponds to the primary scanning direction, and the direction from top to bottom corresponds to the secondary scanning direction. Therefore, the direction of movement of the scanning light is from left to right in the figure, and scanning is performed from top to bottom in the figure. Furthermore, displacement in the primary and secondary scanning directions is considered a "positive" displacement, and displacement in the opposite direction to the primary and secondary scanning directions is considered a "negative" displacement.

[0026] Figures 8(A) and 8(B) show magnified portions of an image with a density of 25% of the maximum density (100%). Because the density is 25%, toner adheres to 1 / 4 of the area (shaded area in the figure). In the following explanation, the area where toner adheres in a cluster by exposing consecutive pixel pieces will be referred to as the "dot area". In halftone processing, multiple starting points are arranged, and as the image density increases, the dot area is increased based on these starting points. These starting points are arranged periodically on a two-dimensional plane. That is, the starting points are arranged periodically in the main scanning direction and the sub-scanning direction. Halftone processing is defined by a vector connecting the closest starting points in the sub-scanning direction and a vector connecting the closest starting points in the main scanning direction.

[0027] In the halftone processing shown in Figure 8(A), the vector connecting the closest starting points in the sub-scanning direction is vector V1, and the vector connecting the closest starting points in the main scanning direction is vector V2. Similarly, in the halftone processing shown in Figure 8(B), the vector connecting the closest starting points in the sub-scanning direction is vector V3, and the vector connecting the closest starting points in the main scanning direction is vector V4. When these vectors are expressed in coordinates with the number of pixels in the main and sub-scanning directions as the unit, vectors V1, V2, V3, and V4 are (1,4), (4,-1), (-1,4), and (4,1), respectively. In the following explanation, the direction parallel to vector V will be referred to as the "V direction".

[0028] Figure 9 shows the case where the scanning start position of scan line #4 is shifted by one pixel to the right when forming the image shown in Figure 8(A). The numbers on the left indicate the scan line numbers. Lines 101, 102, 103, 104, and 101' in Figure 9 are lines in the V1 direction that connect the centroid positions of each dot region. The dot regions are periodic in the main scanning direction, and line 101' corresponds to line 101. If the scanning start positions of all scan lines are aligned, the distance in the V2 direction between two adjacent lines among lines 101 to 104 is equal. Hereafter, the positions of lines 101 to 104 when the scanning start positions of all scan lines are aligned will be referred to as "reference positions," and the distance in the V2 direction between two adjacent lines in that case will be referred to as "reference distances."

[0029] In the example in Figure 9, the scanning start position of scan line #4 is shifted one pixel to the right. Therefore, the centroid of the dot region formed by scan line #4 is also shifted to the right in the figure. As a result, lines 102 and 103, which pass through the dot region formed by scan line #4, are shifted to the right of their reference position in the figure. On the other hand, line 101 remains at its reference position. Therefore, the distance A in the V2 direction between line 101 and line 102 is greater than the reference distance. Since both line 102 and line 103 are shifted to the right of their reference position, and the amount of shift is the same, the distance B in the V2 direction between line 102 and line 103 is equal to the reference distance. On the other hand, since line 104 remains at its reference position, the distance C in the V2 direction between line 103 and line 104 is less than the reference distance. Since both line 104 and line 101' remain at their reference positions, the distance D in the V2 direction between line 104 and line 101' remains the reference distance. In other words, the distance relationship described above is A > B = D > C, and this change is repeated in the V2 direction. Similarly, the length of the blank areas outside the dot areas where toner does not adhere also changes periodically in the V2 direction. This periodic change can cause moiré patterns.

[0030] To suppress the occurrence of moiré patterns as explained using Figure 9, it is necessary to suppress fluctuations in the scan lines. For this reason, during the manufacturing of the image forming apparatus, fluctuations in the scan lines as shown in Figures 7(A) and 7(B) are measured, and correction information to suppress these fluctuations is created based on the measurement results and stored in the memory 402 of the image controller 340. For example, suppose the fluctuations in the scan lines are as shown in Figure 7(A), and the measured value A corresponds to four pixel segments. In this case, the correction information is as shown in Figure 10(A). According to Figure 10(A), the start timing for scan lines #3 and #4 is set to be advanced by four pulses of the PWM signal. In this case, the image controller 340 advances the output timing of the image signals (PWM signals) for generating scan rays 81-3 and 81-4 corresponding to scan lines #3 and #4 to the scanning unit 124 by a time corresponding to four pulses, according to the correction information in Figure 10(A). Note that in Figure 7(A), there is no need to correct the length of the scan lines, so the insertion and removal amounts are all 0.

[0031] Furthermore, for example, suppose the scan line fluctuations are as shown in Figure 7(B), and the measured values ​​of both B and C correspond to four pixel pieces. In this case, the correction information is as shown in Figure 10(B). Figure 10(B) shows that four pixel pieces are inserted into scan line #1 and scan line #2, and four pixel pieces are removed from scan line #3 and scan line #4. In this case, the image controller 340 generates a corrected image signal by inserting four pulses (pixel pieces) into the image signals (PWM signals) for scan line #1 and scan line #2 generated based on the image data. Similarly, the image controller 340 generates a corrected image signal by removing four pulses (pixel pieces) from the image signals (PWM signals) for scan line #3 and scan line #4 generated based on the image data. Whether to expose or not expose the pixel pieces to be inserted can be determined based on predetermined criteria. Also, the positions where the pixel pieces are inserted and removed can be dispersed on the scan line. Furthermore, the length of the scan lines can be adjusted not by inserting or removing pixel pieces, but by adjusting the image's clock signal.

[0032] If the correction information is ideal, fluctuations in the scan lines can be suppressed, and therefore, the occurrence of moiré patterns, as explained using Figure 9, due to interference with halftone processing can be suppressed. However, if there are errors in the measurements used to create the correction information, fluctuations in the scan lines will remain. For example, when using four light sources 209-1 to 209-4 as shown in Figure 6(A), the scanning rays 81-1 to 81-4 emitted by the four light sources 209-1 to 209-4 are rotated on the photoreceptor 122 so that the spacing between scan lines in the sub-scanning direction is a predetermined value, and the scanning rays 81-1 to 81-4 are arranged on the photoreceptor 122 as shown in Figure 6(B). In this case, correction information is created by measuring the amount of deviation of scanning rays 81-2 to 81-4 relative to scanning ray 81-1, as shown in Figure 6(B). Since the spacing between scanning beams 81-1 to 81-4 in the main scanning direction is approximately equal, the amount of deviation of scanning beams 81-2 to 81-4 relative to scanning beam 81-1 can be determined by measuring the amount of deviation of scanning beam 81-4 relative to scanning beam 81-1. For example, suppose the measured value of the deviation of scanning beam 81-4 relative to scanning beam 81-1 is X. In this case, the amount of deviation of scanning beam 81-2 relative to scanning beam 81-1 is determined to be X / 3, and the amount of deviation of scanning beam 81-3 relative to scanning beam 81-1 is determined to be 2X / 3. In this case, correction information is created based on these values.

[0033] However, if there is a measurement error in the measured amount of deviation X, the actual scanning start positions of each scan line will be uneven. This is shown in Figures 11(A) and 11(B). In Figures 11(A) and 11(B), #1 to #4 indicate the scan line numbers, and are assumed to be formed by scan beams 81-1 to 81-4, respectively. Figure 11(A) shows the case where the measured value X is greater than the actual deviation Y. In this case, the correction is excessive, and the scanning start position for scan line #k (k is 2 to 4) is shifted to the right of scan line #(k-1) in the figure, that is, to the positive side of the main scanning direction. On the other hand, Figure 11(B) shows the case where the measured value X is less than the actual deviation Y. In this case, the correction is insufficient, and the scanning start position for scan line #k (k is 2 to 4) is shifted to the left of scan line #(k-1) in the figure, that is, to the negative side of the main scanning direction. As shown in Figure 6(B), the spacing between the scanning rays 81-1 to 81-4 in the main scanning direction is approximately equal, and the correction amount is also determined based on the measured value X. Therefore, the displacement amounts D1 to D3 shown in Figure 11 are approximately the same, and displacement amount D4 is approximately three times the displacement amounts D1 to D3. Consequently, the relative displacement amount D4 between the scanning start positions of scan line #4 and scan line #1 is greater than the relative displacement amounts D1 to D3 between the scanning start positions of the other two adjacent scan lines, resulting in a scan line fluctuation in the sub-scanning direction with a period of 4 lines (scan lines).

[0034] Figures 12(A) and 12(B) show the images that would actually be formed if the scanning start position shifts shown in Figures 11(A) and 11(B) occurred during the formation of the image in Figure 8(A), respectively. Note that the shift amounts D1 to D4 in Figure 11(A) and Figure 11(B) are assumed to be the same.

[0035] In Figures 12(A) and 12(B), reference numerals 145a to 148a and 145b to 148b indicate blank areas between dot regions. The arrangement direction of blank areas 145a to 148a and blank areas 145b to 148b correspond to the V2 direction, respectively. In Figure 12(A), scan line #1 is shifted by D4 = D1 + D2 + D3 on the negative side of the main scan direction relative to the scan line #4 above it. As shown in Figure 12(A), the length of blank area 145a in the V2 direction is determined by the positions of the dot regions of scan lines #1 and #2. Similarly, the length of blank area 146a in the V2 direction is determined by the positions of the dot regions of scan lines #3 and #4. Furthermore, the length of blank area 147a in the V2 direction is determined by the positions of the dot regions of scan lines #2 to #4. Furthermore, the length of blank area 148a in the V2 direction is determined by the position of the dot area on scan lines #1 to #3. Thus, since the length of blank areas 145a to 148a is not defined by the position of the dot area on scan line #1 and the position of the dot area on scan line #4, the lengths of blank areas 145a to 148a in the V2 direction are approximately the same.

[0036] On the other hand, in Figure 12(B), scan line #1 is shifted by D4 = D1 + D2 + D3 on the positive side of the main scanning direction relative to the scan line #4 above it. As shown in Figure 12(B), the length of blank area 145b in the V2 direction is determined by the positions of the dot areas of scan lines #1 and #4. Similarly, the length of blank area 146b in the V2 direction is determined by the positions of the dot areas of scan lines #1 and #4. The length of blank area 147b in the V2 direction is determined by the position of the dot area of ​​scan line #3. Furthermore, the length of blank area 148b in the V2 direction is determined by the position of the dot area of ​​scan line #2. Because scan line #1 is shifted significantly on the positive side of the main scanning direction relative to scan line #4, the lengths of blank areas 145b and 146b in the V2 direction are shorter than the lengths of blank areas 147b and 148b in the V2 direction.

[0037] In the image of Figure 12(B), moiré patterns can be seen due to periodic fluctuations in the length of the blank area 145b to 148b in the V2 direction. On the other hand, in the image of Figure 12(A), moiré patterns are less visible because the length of the blank area 145b to 148b in the V2 direction is approximately the same. Therefore, when performing halftone processing with vectors V1 and V2 as shown in Figure 8(A), if there is a shift in the scanning start position as shown in Figure 11(B), moiré patterns will be more easily visible than when there is a shift in the scanning start position as shown in Figure 11(A).

[0038] Figures 13(A) and 13(B) show the images that would actually be formed if the scanning start position shifts shown in Figures 11(A) and 11(B) occurred during the formation of the image in Figure 8(B), respectively. Note that the shift amounts D1 to D4 in Figure 11(A) and Figure 11(B) are assumed to be the same.

[0039] In Figures 13(A) and 13(B), reference numerals 145c to 148c and 145d to 148d indicate blank areas between dot regions. The arrangement direction of blank areas 145c to 148c and blank areas 145d to 148d correspond to the V4 direction, respectively. In Figure 13(A), scan line #1 is shifted by D4 = D1 + D2 + D3 on the negative side of the main scan direction relative to the scan line #4 above it. As shown in Figure 13(A), the length of blank area 145c in the V4 direction is determined by the positions of the dot regions of scan lines #1 and #4. Similarly, the length of blank area 146c in the V4 direction is determined by the positions of the dot regions of scan lines #1 and #4. The length of blank area 147c in the V4 direction is determined by the position of the dot region of scan line #2. Furthermore, the length of blank area 148c in the V4 direction is determined by the position of the dot area of ​​scan line #3. Because scan line #1 is significantly shifted to the negative side of the main scan direction relative to scan line #4, the lengths of blank areas 145c and 146c in the V4 direction are shorter than the lengths of blank areas 147c and 148c in the V4 direction.

[0040] On the other hand, in Figure 13(B), scan line #1 is shifted by D4 = D1 + D2 + D3 on the positive side of the main scanning direction relative to the scan line #4 above it. As shown in Figure 13(B), the length of blank area 145d in the V4 direction is determined by the positions of the dot areas on scan lines #3 and #4. Similarly, the length of blank area 146d in the V4 direction is determined by the positions of the dot areas on scan lines #1 and #2. Furthermore, the length of blank area 147d in the V4 direction is determined by the positions of the dot areas on scan lines #1 to #3. Finally, the length of blank area 148d in the V4 direction is determined by the positions of the dot areas on scan lines #2 to #4. Thus, since the length of blank areas 145d to 148d is not defined by the dot areas on scan line #1 and scan line #4, the lengths of blank areas 145d to 148d in the V4 direction are approximately the same.

[0041] In the image in Figure 13(A), moiré patterns can be seen due to periodic fluctuations in the length of the blank area 145c to 148c in the V4 direction. On the other hand, in the image in Figure 13(B), moiré patterns are less visible because the length of the blank area 145d to 148d in the V4 direction is approximately the same. Therefore, when performing halftone processing with vectors V3 and V4 as shown in Figure 8(B), a shift in the scanning start position as shown in Figure 11(A) makes moiré patterns more easily visible than when a shift in the scanning start position as shown in Figure 11(B).

[0042] As explained using Figures 12 and 13, the strength of the moiré pattern differs depending on the combination of the direction of the vector with the smaller angle to the sub-scanning direction among the two vectors of the halftone processing, and the direction connecting the scanning start positions of each scan line. Specifically, among the two vectors that define the halftone processing, the vector with the smaller angle to the sub-scanning direction is called the first vector. In the halftone processing of Figure 8(A), the first vector is vector V1, and in the halftone processing of Figure 8(B), the first vector is vector V3. Also, as shown in Figure 11(A), a pattern of scanning start positions in which the scanning start position is linearly displaced to the positive side of the main scanning direction along the sub-scanning direction is referred to as the "positive pattern". Conversely, as shown in Figure 11(B), a pattern of scanning start positions in which the scanning start position is linearly displaced to the negative side of the main scanning direction along the sub-scanning direction is referred to as the "negative pattern". When the direction of the first vector is along the sub-scanning direction and towards the positive side of the main scanning direction, setting the scanning start position to a "positive pattern" makes the moiré pattern less visible, while setting the scanning start position to a "negative pattern" makes the moiré pattern more visible. On the other hand, when the direction of the first vector is along the sub-scanning direction and towards the negative side of the main scanning direction, setting the scanning start position to a "positive pattern" makes the moiré pattern more visible, while setting the scanning start position to a "negative pattern" makes the moiré pattern less visible.

[0043] Therefore, in this embodiment, correction information is set so that the pattern of the scanning start position becomes a "positive pattern" or a "negative pattern" depending on the direction of the first vector of the halftone processing. In other words, instead of making the line connecting the scanning start positions of each scan line parallel to the sub-scanning direction, the correction information is set so that it intersects with the sub-scanning direction depending on the direction of the first vector. Figure 14(C) shows the relationship between the value of D4 in Figure 11(A) and the moiré intensity. The solid line shows the case when the halftone processing in Figure 8(A) is used, and the dotted line shows the case when the halftone processing in Figure 8(B) is used. According to Figure 14(C), when the halftone processing in Figure 8(A) is used, no moiré is visible in the range of D4 value from 0 to DT1. Note that DT1 is a positive value. Also, according to Figure 14(C), when the halftone processing in Figure 8(B) is used, no moiré is visible in the range of D4 value from DT2 to 0. Note that DT2 is a negative value.

[0044] Therefore, in this embodiment, when using the halftone processing shown in Figure 8(A), the correction information is set so that the scanning start positions of each scanning beam 81-1 to 81-4 are arranged as shown by the solid circles in Figure 14(A). In Figure 14(A), the scanning start position of scanning beam 81-4 is set to be shifted by |DT1 / 2| to the positive side of the main scanning direction from the scanning start position of scanning beam 81-1. In other words, the correction information is set to be the median value of 0 to DT1, which is the range of D4 in which the moiré shown in Figure 14(C) is not visible. Accordingly, the scanning start position of scanning beam 81-2 is set to be shifted by |DT1 / 6| to the positive side of the main scanning direction from the scanning start position of scanning beam 81-1. Also, the scanning start position of scanning beam 81-3 is set to be shifted by |DT1 / 3| to the positive side of the main scanning direction from the scanning start position of scanning beam 81-1. This configuration makes it possible to suppress the visibility of moiré patterns even if the scanning start position shifts to the position indicated by the dotted circle in Figure 14(A) due to measurement errors.

[0045] Similarly, when using the halftone processing shown in Figure 8(B), the correction information is set so that the scanning start positions of each scanning beam 81-1 to 81-4 are arranged as shown by the solid circles in Figure 14(B). In Figure 14(B), the scanning start position of scanning beam 81-4 is set to be shifted by |DT2 / 2| to the negative side of the main scanning direction from the scanning start position of scanning beam 81-1. In other words, the correction information is set so that it is the median value of DT2 to 0, which is the range of D4 in which the moiré shown in Figure 14(C) is not visible. Therefore, the scanning start position of scanning beam 81-2 is set to be shifted by |DT2 / 6| to the negative side of the main scanning direction from the scanning start position of scanning beam 81-1. Also, the scanning start position of scanning beam 81-3 is set to be shifted by |DT2 / 3| to the negative side of the main scanning direction from the scanning start position of scanning beam 81-1. This configuration makes it possible to suppress the visibility of moiré patterns even if the scanning start position shifts to the position indicated by the dotted circle in Figure 14(B) due to measurement errors. Note that the specific values ​​of DT1 and DT2 change depending on the screen ruling of the halftone processing. Therefore, the amount of displacement of the scanning start position of scanning light 81-4 relative to the scanning start position of scanning light 81-1 set in the correction information is determined based on the screen ruling of the halftone processing.

[0046] In this embodiment, when fluctuations in the scan lines occur with a period of N scan lines (where N is an integer of 2 or more) in the sub-scanning direction, correction information is set so that the scanning start position of each of the N scan lines is linearly displaced along the sub-scanning direction to the positive or negative side of the main scanning direction. The direction of the lines is determined based on the direction of the vector with the smaller angle to the sub-scanning direction among the two vectors of the halftone processing used. This configuration makes it difficult to see moiré patterns.

[0047] In this embodiment, we have described the misalignment of the scan lines in the main scanning direction, but the same method can also be applied when there is a misalignment in the spacing of the scan lines in the sub-scanning direction.

[0048] <Second Embodiment> Next, the second embodiment will be described, focusing on the differences from the first embodiment. Figures 15(A) and 15(B) show examples of images that have been halftoned using a 134-line dither matrix. Figures 15(A) and 15(B) are enlarged views of the image portion at 25% density of the maximum density (100%). The two vectors defined in the halftoning process in Figure 15(A) are vectors V5 and V6, and the two vectors defined in the halftoning process in Figure 15(B) are vectors V7 and V8. As with the first embodiment, the coordinates of vectors V5 to V8 are (2, 4), (4, -2), (-2, 4), and (4, 2), respectively.

[0049] Figures 16(A) and 16(B) show the images that would actually be formed if the scan start position shift shown in Figures 11(A) and 11(B) occurred when forming the image in Figure 15(A), respectively. Note that the values ​​of the shift amounts D1 to D4 in Figure 11(A) and Figure 11(B) are the same. Lines 151a, 152a, 153a, 154a, and 151'a in Figure 16(A) are lines in the V5 direction connecting the centroid positions of each dot region. The dot regions are periodic in the main scan direction, and line 151'a corresponds to line 151a. Reference numerals 151b, 152b, 153b, 154b, and 151'b in Figure 16(B) are lines in the V5 direction connecting the centroid positions of each dot region, respectively. The dot region is periodic in the main scanning direction, and line 151'b corresponds to line 151b. Also, as explained in Figure 9, the position of each line when the scanning start positions of all scan lines are aligned is denoted as the "reference position," and the distance in the V6 direction between two adjacent lines in that case is denoted as the "reference distance."

[0050] In Figure 16(A), lines 152a and 154a connect dot regions formed only by scan line #1 and scan line #2. Therefore, these lines move from the reference position to the positive side of the main scanning direction by D1 / 2 = 0.5D1, which corresponds to the average value of the displacement amount of scan line #1 (value is 0) and the displacement amount D1 of scan line #2. On the other hand, lines 151a, 153a, and 151'a connect dot regions formed only by scan line #3 and scan line #4. Therefore, these lines move from the reference position to the positive side of the main scanning direction by D1 + D2 + D3 / 2, which corresponds to the average value of the displacement amount D1 + D2 of scan line #3 and the displacement amount D1 + D2 + D3 of scan line #4. As mentioned above, since D1 ≈ D2 ≈ D3, if we assume D1 = D2 = D3, then lines 151a, 153a, and 151'a move 2.5D1 from the reference position towards the positive side of the main scanning direction. Therefore, the distance 155a in the V6 direction between line 151a and line 152a, and the distance 157a in the V6 direction between line 153a and line 154a, become 2D1 smaller than the reference distance. On the other hand, the distance 156a in the V6 direction between line 152a and line 153a, and the distance 158a in the V6 direction between line 154a and line 151'a, become 2D1 larger than the reference distance. In this way, in the image of Figure 16(A), the distance in the V6 direction between dot regions changes periodically, making moiré patterns more easily visible.

[0051] In Figure 16(B), lines 152b and 154b connect dot regions formed only by scan lines #1 and #2. Lines 151b, 153b, and 151'b connect dot regions formed only by scan lines #3 and #4. Therefore, similar to Figure 16(A), the distance in the V6 direction between each line changes periodically. As a result, in the image of Figure 16(B), the distance in the V6 direction between dot regions changes periodically, making moiré patterns more easily visible. However, because the direction of the scan line misalignment is opposite to that in Figure 16(A), the distance in the V6 direction between line 151b and line 152b (155b) and the distance in the V6 direction between line 153b and line 154b (157b) are greater than the reference distance. Then, the distance 156b in the V6 direction between line 152b and line 153b, and the distance 158b in the V6 direction between line 154b and line 151'b are smaller than the reference distance.

[0052] Figure 17(A), like Figure 16(A), shows the image that would actually be formed if the scan start position shift shown in Figure 11(A) occurred when forming the image in Figure 15(A). However, in Figure 17(A), the starting point of the halftone processing is moved upward by one scan line segment compared to the halftone processing in Figure 16(A). In other words, the phase of the periodic halftone processing in the sub-scanning direction is shifted upward by a value corresponding to one pixel.

[0053] In Figure 17(A), lines 152c and 154c connect dot regions formed only by scan line #1 and scan line #4. Therefore, these lines move from the reference position to the positive side of the main scanning direction by (D1+D2+D3) / 2, which corresponds to the average value of the displacement amount of scan line #1 (value is 0) and the displacement amount of scan line #4 D1+D2+D3. On the other hand, lines 151c, 153c, and 151'c connect dot regions formed only by scan line #2 and scan line #3. Therefore, these lines move from the reference position to the positive side of the main scanning direction by D1+D2 / 2, which corresponds to the average value of the displacement amount D1 of scan line #2 and the displacement amount D1+D2 of scan line #3. Since D1 ≈ D2 ≈ D3, if we assume D1 = D2 = D3, then the displacement of lines 152c and 154c from their reference positions is 1.5D1, and the displacement of lines 151c, 153c, and 151'c from their reference positions is also 1.5D1. In this way, all lines 151c to 154c and 151'c are shifted by 1.5D1 towards the positive side of the main scanning direction, and therefore the distance between each line in the V6 direction is equal. Consequently, moiré patterns are difficult to see in the image of Figure 17(A).

[0054] Figure 17(B), like Figure 16(B), shows the image that would actually be formed if the scan start position shift shown in Figure 11(B) occurred when forming the image in Figure 15(A). However, in Figure 17(B), the starting point of the halftone processing is moved upward by one scan line compared to the halftone processing in Figure 16(B). In Figure 17(B), lines 152d and 154d connect dot areas formed only by scan lines #1 and #4. Also, lines 151d, 153d, and 151'd connect dot areas formed only by scan lines #2 and #3. Therefore, similar to the image in Figure 17(A), the distances 155d to 158d in the V6 direction of each line are approximately equal, making moiré patterns difficult to see. Similarly, in the halftone processing shown in Figure 14(B), moiré patterns can be made less visible by adjusting the phase of the sub-scan direction of the halftone processing.

[0055] In this way, when the scanning start positions of N scan lines (where N is an integer greater than or equal to 2) are repeatedly displaced in the main scanning direction along the sub-scanning direction, the relative displacement in the main scanning direction between the last scan line of the N scan lines and the first scan line of the next N scan lines becomes larger than the relative displacement in the main scanning direction between two other adjacent scan lines. In this case, moiré patterns can be made less visible by setting the phase of the sub-scanning direction of the halftone processing so that the last scan line and the start scan line form the same dot region. The dot region corresponds to the exposure region of the photoreceptor 122. In other words, moiré patterns can be made less visible by setting the phase of the sub-scanning direction of the halftone processing so that the last scan line and the start scan line expose the same exposure region with the same starting point for the halftone processing. For example, in Figures 17(A) and 17(B), the last scan line is scan line #4, and the start scan line is scan line #1. In Figures 17(A) and 17(B), scan line #1 and scan line #4 form the same dot area. Furthermore, as described in the first embodiment, for example, the system can be configured such that the scanning start positions of N scan lines are linearly displaced in the main scanning direction along the sub-scanning direction by correction information.

[0056] [Other embodiments] The present invention can also be realized by supplying a program that implements one or more of the functions of the above-described embodiments to a system or device via a network or storage medium, and by having one or more processors in the computer of that system or device read and execute the program. It can also be realized by a circuit (e.g., an ASIC) that implements one or more functions.

[0057] The invention is not limited to the embodiments described above, and various modifications and variations are possible without departing from the spirit and scope of the invention. Accordingly, claims are attached to disclose the scope of the invention. [Explanation of Symbols]

[0058] 122: Photoconductor, 124: Scanning unit, 340: Image controller, 402: Memory

Claims

1. Photoreceptor and A scanning means that, based on an image signal, scans the photoreceptor in the main scanning direction with one or more scanning lights, and repeats this process in a sub-scanning direction perpendicular to the main scanning direction, thereby forming an electrostatic latent image on the photoreceptor. A generation means that generates the image signal by performing halftone processing on the image data, A storage means for storing correction information for correcting the scanning start position of the photoreceptor by one or more scanning lights, Equipped with, The correction information is set such that, according to the direction of the first vector among the two vectors of the halftone processing which has a small angle with respect to the sub-scanning direction, the scanning start position of a plurality of scanning lines that are continuous in the sub-scanning direction and formed on the photoreceptor by the one or more scanning lights is linearly displaced along the sub-scanning direction to the negative or positive side of the main scanning direction. When the direction of the first vector is displaced along the sub-scanning direction to the positive side of the main scanning direction, the correction information is set such that the scanning start positions of the multiple consecutive scan lines are linearly displaced along the sub-scanning direction to the positive side of the main scanning direction. Image forming apparatus, wherein when the direction of the first vector is displaced to the negative side of the main scanning direction along the sub-scanning direction, the correction information is set such that the scanning start positions of the plurality of consecutive scan lines are linearly displaced to the negative side of the main scanning direction along the sub-scanning direction.

2. The image forming apparatus according to claim 1, wherein the amount of displacement of the scanning start position of the plurality of consecutive scan lines is determined according to the number of screen lines of the halftone processing.

3. The one or more scanning beams mentioned above are a single scanning beam. The image forming apparatus according to claim 1 or 2, wherein the number of consecutive scan lines is equal to the number of reflective surfaces of a rotating polyhedron that reflects one scan light in the scanning means.

4. The one or more scanning beams are multiple scanning beams, The image forming apparatus according to any one of claims 1 to 3, wherein the number of consecutive scan lines is equal to the number of scan lights.

5. The one or more scanning beams are multiple scanning beams, The image forming apparatus according to any one of claims 1 to 3, wherein the number of consecutive scan lines is equal to the number obtained by multiplying the number of reflective surfaces of a rotating polyhedron that reflects the plurality of scan lights in the scanning means by the number of scan lights.

6. Photoreceptor and A scanning means that, based on an image signal, scans the photoreceptor in the main scanning direction with one or more scanning lights, and repeats this process in a sub-scanning direction perpendicular to the main scanning direction, thereby forming an electrostatic latent image on the photoreceptor with multiple scanning lines. A generation means that generates the image signal by performing halftone processing on the image data, Equipped with, In the aforementioned halftone processing, the exposure area of ​​the photoreceptor is enlarged from starting points periodically arranged in the sub-scanning direction and the main scanning direction, depending on the density of the image to be formed. The scanning start position of the photoreceptor by the plurality of scanning lines is periodically displaced along the sub-scanning direction in the main scanning direction, An image forming apparatus in which the phase of the halftone processing in the sub-scanning direction is set such that the starting point of the exposure region exposed by the last first scan line of one cycle of the periodically displaced scanning start position is the same as the starting point of the exposure region exposed by the second scan line following the first scan line in the sub-scanning direction.

7. The one or more scanning beams mentioned above are a single scanning beam. The image forming apparatus according to claim 6, wherein the number of scan lines included in one period is equal to the number of reflective surfaces of a rotating polyhedron that reflects one scan light in the scanning means.

8. The one or more scanning beams are multiple scanning beams, The image forming apparatus according to claim 6, wherein the number of scan lines included in one period is equal to the number of scan lights.

9. The one or more scanning beams are multiple scanning beams, The image forming apparatus according to claim 6, wherein the number of scan lines included in one period is equal to the number obtained by multiplying the number of reflective surfaces of a polyhedron that reflects the plurality of scan lights in the scanning means by the number of the plurality of scan lights.

10. The system further includes storage means for storing correction information for correcting the scanning start position using one or more scanning lights, The image forming apparatus according to any one of claims 6 to 9, wherein the correction information is set such that the scanning start position by the scanning line included in one period is linearly displaced in the main scanning direction along the sub-scanning direction.

Images