JP7339774B2 - IMAGE FORMING APPARATUS, CONTROL METHOD THEREOF, AND PROGRAM - Google Patents

Description

The present invention relates to an image forming apparatus, its control method, and a program.

In a tandem-type color image forming apparatus, misregistration of each color plate may occur. In order to suppress such deviations, for example, Japanese Patent Application Laid-Open No. 2002-200000 discloses that an optical sensor is used to measure the inclination and curvature of a scanning line, and an image is formed after correcting the image data so as to cancel them out. describes how to do it. Since this method performs electrical correction by processing image data, mechanical adjustment members and adjustment processes during assembly are not required. Therefore, it is possible to reduce the size of the color image forming apparatus, and it is possible to cope with the registration deviation at a low cost.

In laser scanners used in image forming apparatuses, a multi-beam technology is adopted in which a plurality of scanning lines are collectively exposed by combining a plurality of lasers to emit light. It has been realized.

JP-A-2004-170755

However, when the method disclosed in Japanese Patent Application Laid-Open No. 2002-300003, which corrects image data to offset distortion caused by main scanning curvature, is adopted in a multi-beam printer, if there is a large variation in the amount of light emitted from a plurality of lasers, the image will not be formed on the recording medium. There is a problem that unevenness appears in the printed image.

SUMMARY OF THE INVENTION It is an object of the present invention to solve at least one of the problems of the prior art described above.

SUMMARY OF THE INVENTION An object of the present invention is to provide a technique capable of reproducing equivalent density and color tone even when image data includes portions corrected for scanning line curvature correction.

In order to achieve the above object, an image forming apparatus according to one aspect of the present invention has the following configuration. Namely
An image forming apparatus,
halftone processing means for forming halftone image data including a plurality of halftone dots using a dither matrix for input image data;
correction means for correcting the image data so as to correct the curvature of the scanning line of the laser beam by shifting the line of the halftone image data formed by the halftone processing means in the sub-scanning direction ;
an image forming means for forming an image based on the image data corrected by the correcting means;
half of the plurality of halftone dots included in the halftone image data are formed in the first growth order, and the remaining half of the plurality of halftone dots are formed in the sub-scanning with respect to the first growth order; The halftone dots are formed in a second growth order that is directionally symmetrical.

According to the present invention, it is possible to reproduce the same density and color tone even when the image data includes a portion corrected for scanning line curvature.

Other features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings. In the accompanying drawings, the same or similar configurations are given the same reference numerals.

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
1 is a block diagram showing an example of the configuration of a printing system according to Embodiment 1 of the present invention; FIG. 3 is a block diagram for explaining the functional configuration of an image processing unit according to the first embodiment; FIG. FIG. 4 is a diagram showing an example of bending characteristics of a scanning line of a laser beam; FIG. 4 is a diagram showing an example of correcting the bending characteristics of scanning lines of a laser beam; FIG. 5 is a diagram showing an example of specific correction values (correction data) used in phase transfer processing according to the first embodiment; FIG. 10B is an enlarged view of part of the binary bitmap data of FIGS. 10A and 10B; 4 is a flowchart for explaining the flow of processing in an image processing unit according to the first embodiment; 4 is a diagram showing an example of a dither matrix according to the first embodiment; FIG. 4 is a diagram showing an example of a dither matrix according to the first embodiment; FIG. 4A and 4B are diagrams showing an example of the growth order of cells forming the dither matrix according to the first embodiment; FIG. FIG. 8B is a diagram showing an example of halftone dots formed using the dither matrix shown in FIG. 8A; FIG. 8B is a diagram showing an example of binary bitmap data obtained by performing halftone processing on different input pixel values using the dither matrix of FIG. 8B and then performing phase conversion processing; FIG. 11A is an enlarged view of part of the binary bitmap data of FIGS. 11A and 11B; FIG. FIG. 10 is a diagram showing an example of a dither matrix according to the second embodiment; FIG. FIG. 10 is a diagram showing an example of a dither matrix according to the second embodiment; FIG. FIG. 10 is a diagram showing an example of the order of growth of cells forming a dither matrix according to the second embodiment; 13B is a diagram showing an example of binary bitmap data obtained by performing halftone processing on different input pixel values and then performing phase conversion processing using the dither matrix of FIG. 13B according to the second embodiment; FIG. FIG. 15 is an enlarged view of part of the binary bitmap data of FIGS. 15A and 15B according to the second embodiment;

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that the following embodiments do not limit the invention according to the scope of claims. Although multiple features are described in the embodiments, not all of these multiple features are essential to the invention, and multiple features may be combined arbitrarily. Furthermore, in the accompanying drawings, the same or similar configurations are denoted by the same reference numerals, and redundant description is omitted.

[Embodiment 1]
In the first embodiment, a multi-function peripheral (MFP) having multiple functions such as a copy function and a printer function will be described as an example of a color image forming apparatus. Also, this multifunctional processing apparatus forms an image by an electrophotographic method.

FIG. 1 is a block diagram showing an example of the configuration of a printing system according to Embodiment 1 of the present invention.

This printing system includes an MFP 100 and a PC 120, which are interconnected via a network 130 such as a LAN. MFP 100 includes CPU 101, memory 102, HDD (hard disk drive) 103, scanner unit 104, printer unit 105, PDL processing unit 106, RIP unit 107, image processing unit 108, display unit 109, and network I/F 110. there is These units are interconnected by an internal bus 111 .

A CPU 101 is a processor that controls the MFP 100 as a whole. The memory 102 has a ROM that stores various instructions (including application programs) and various data executed by the CPU 101 to control the MFP 100 and a RAM that functions as a work area for the CPU 101 . The HDD 103 is a large-capacity storage medium that stores various programs, image data, and the like. The scanner unit 104 optically reads a document set on a document table (not shown) or the like, and generates bitmap image data corresponding to the image of the document.

The PDL processing unit 106 analyzes PDL (page description language) data included in the print job received from the PC 120 and generates a DL (display list) as intermediate data. The generated DL is sent to RIP section 107 . A RIP (raster image processor) unit 107 executes rendering processing based on the received DL to generate contone (multi-value) bitmap image data. Incidentally, the bitmap image data of the contone means, for example, having a bit depth of 8 bits or 10 bits, multi-gradation, expressing colors in a color space such as RGB, and expressing these colors in units of discrete pixels. Image data with information. Specifically, the drawing bitmap data and the attribute bitmap data are generated respectively. Prior to generating these data, attribute information of the object to be rendered is generated for each pixel. The attribute information in this case is determined according to the following criteria.
・When specified by a character drawing command (character type or character code): Character attribute ・When specified by a line drawing command (coordinate point, length, thickness): Line attribute ・Figure drawing command (rectangle, shape, coordinate points): figure attributes, if specified by an image drawing command (a set of points): image attributes. , and generates drawing pit map data in which color information (multi-value) to be drawn is entered in each pixel. Further, the RIP unit 107 generates attribute bitmap data storing attribute information for each pixel so as to correspond to each pixel of the drawing bitmap. The drawing bitmap and attribute bitmap thus generated are temporarily stored in the memory 102 or HDD 103 or sent to the image processing unit 108 .

The image processing unit 108 performs necessary image processing on bitmap image data to be printed, which is related to a print job from the PC 120 or optically read by the scanner unit 104 . Details of the image processing unit 108 will be described later. The image data in bitmap format after image processing is sent to the printer unit 105 .

In the printer unit 105, a laser scanner (not shown) emits exposure light (laser beam) according to the image data generated by the image processing unit 108 to form an electrostatic latent image, and the electrostatic latent image is developed. to form a monochromatic toner image. The laser scanner according to the first embodiment has two lasers (light emitting elements) and irradiates laser beams every two rows in the sub-scanning direction. For example, if one of these two lasers is the A laser and the other is the B laser, the A laser draws the image data of the even-numbered lines, and the B laser draws the image data of the odd-numbered lines. Then, the single-color toner images are superimposed to form a multicolor toner image, and after this multicolor toner image is transferred to a recording medium (paper), this multicolor toner image is fixed by a fixing device, thereby forming an image on the recording medium. to form a color image.

The display unit 109 has, for example, a liquid crystal panel having a touch screen function, and displays various information. The network interface 110 is an interface for performing communication such as transmission and reception of print jobs with the PC 120 connected via the network 130 .

Note that the components of the image forming apparatus 100 described above are not limited to those described above. For example, instead of the touch screen, an input unit including a pointing device and a keyboard for the user to perform various operations may be provided. It is what you get.

FIG. 2 is a block diagram illustrating the functional configuration of the image processing unit 108 according to the first embodiment. In the first embodiment described below, the functions of the image processing unit 108 shown in FIG. 2 are described as being achieved by the CPU 101 executing a program developed in the memory 102, but these functions are implemented by hardware. may be implemented.

The image processing unit 108 has a color conversion unit 201 , a halftone processing unit 202 and a phase conversion unit 203 . Each processing unit will be described below.

A color conversion unit 201 performs color conversion processing to convert the color space of input image data into a color space compatible with the printer unit 105 . For example, if the printer unit 105 is a 4-color, 4-drum tandem system that uses four colors of toner, cyan (C), magenta (M), yellow (Y), and black (K), the CMYK color space Convert. A halftone processing unit 202 performs pseudo-halftone processing using a dither method for each color plate on image data converted into a color space corresponding to the printer unit 105 . The dither method uses a threshold matrix (dither matrix) in which different thresholds are arranged within a matrix of a predetermined size. This dither matrix is sequentially developed in tiles on multivalued bitmap data, which is input image data, and is compared in magnitude with input pixel values. As a result of this comparison, if the input pixel value is greater than the threshold, the pixel is turned on (“1”), and if the input pixel value is less than or equal to the threshold, the pixel is turned off (“0”), thereby creating a pseudo halftone image. express. Through this halftone processing, continuous tone input image data (multivalued bitmap data) is converted into area gradation halftone image data (binary bitmap data) composed of halftone dots. A different dither matrix may be used for each color plate. The embodiment is characterized by a dither matrix, and the details will be described later.

The phase transfer unit 203 performs line shift processing for shifting lines of image data (binary bitmap data in this case) after halftone processing in the sub-scanning direction, thereby correcting deviation (curvature) of the scanning lines of the CMYK color laser beams. ) is corrected. This line shift processing is also called "phase transfer processing".

FIG. 3 is a diagram showing an example of bending characteristics of a scanning line of a laser beam.

A curve 301 in FIG. 3A shows the characteristics when the scanning line of the laser beam shifts upward in the sub-scanning direction (paper transport direction) as it advances in the main scanning direction. A curve 302 in FIG. 3B shows the characteristics when the scanning line of the laser beam shifts downward in the sub-scanning direction as it advances in the main scanning direction. A straight line 300 in FIG. 3 represents the characteristics of an ideal scanning line when scanning is performed perpendicular to the sub-scanning direction so that the scanning line of the laser beam does not deviate in the sub-scanning direction even if it advances in the main scanning direction. showing.

FIG. 4 is a diagram showing an example of correcting the bending characteristics of the scanning line of the laser beam.

FIG. 4A is a diagram showing the bending characteristics (shift amount) of the scanning line of the laser beam, and a curve 401 represents the bending characteristics of the laser beam corresponding to the main scanning width.

FIG. 4B is a diagram showing the amount of correction (correction characteristics) when correcting the bending characteristics of FIG. It can be seen that the characteristics are opposite.

FIG. 5 is a diagram showing an example of specific correction values (correction data) used in phase transfer processing according to the first embodiment. In FIG. 5A, the vertical axis represents the amount of correction, and the horizontal axis represents the pixel position in the main scanning direction.

In FIG. 5A, P1, P2, . . . Pn denote points (changeover points) shifted by one pixel in the sub-scanning direction due to the curve characteristics described above. Note that the pixel position of the changeover point in the main scanning direction is sometimes called a "changeover position" or a "correction position". FIG. 5(B) shows directions in which scanning lines are shifted (shifted) to the next transfer point at each transfer point P1, P2, . . . , Pn. There are an upward direction and a downward direction in the shift direction at the transfer point. For example, the line change point P2 is a point at which the line should be further shifted by one pixel in the upward direction until the next line change point P3. Therefore, the transfer direction at P2 is upward (↑). Similarly, at P3, the transfer direction is upward (↑) up to the next transfer point P4. At the transfer point P4, the direction of transfer is downward (↓), unlike the direction up to that point. Next, the flow of processing in the image processing unit 108 during print processing will be described.

FIG. 7 is a flowchart for explaining the flow of processing in the image processing unit 108 according to the first embodiment. Here, the series of processes of the image processing unit 108 is achieved by reading a computer-executable program describing the following procedure from the ROM in the memory 102 onto the RAM, and then executing the program by the CPU 101. shall be However, the present invention is not limited to this, and the image processing unit 108 may be realized by hardware such as an ASIC (application specific integrated circuit).

Upon receiving a print instruction, in step S<b>701 , the CPU 101 first acquires the drawing bitmap and attribute bitmap data generated by the RIP unit 107 . Next, in step S702, the CPU 101 functions as the color conversion unit 201, converts the color space (here, RGB) of each pixel of the drawing bitmap into a color space corresponding to the printer unit 105 using a color conversion LUT and matrix operation. Here, convert to CMYK). Then, in step S703, the CPU 101 functions as the halftone processing unit 202 and selects a dither matrix according to the attribute information of each pixel in the attribute bitmap. For example, a high ruling dither matrix is selected for character attributes and line attributes, and a low ruling dither matrix is selected for graphic attributes and image attributes. Furthermore, the CPU 101 uses the selected dither matrix to apply halftone processing to each pixel of the drawing bitmap. As a result, bitmap data (halftone image data) is generated by converting each multi-valued pixel value of the drawing bitmap into binary data. Details of the dither matrix used in the first embodiment will be described later. Then, in step S704, the CPU 101 functions as the phase conversion unit 203, performs the above-described phase conversion processing on the binary bitmap data after the halftone processing, and corrects the curvature of the scanning line of the laser beam. The binary bitmap data corrected for scanning line curvature is sent to the printer unit 105 for printing.

Next, the dither matrix used in the first embodiment and its effect will be described in detail with reference to FIGS.

8A and 8B are diagrams showing examples of dither matrices according to the first embodiment.

The dither matrix 800 of FIG. 8A consists of four cells 801, 802, 803, 804 each forming one halftone dot. Here, the size of one cell is 6 vertical×12 horizontal. In addition, in FIG. 8A and FIG. 8B, common parts are indicated by the same reference numerals.

FIG. 9 is a diagram showing an example of the growth order of cells forming the dither matrix according to the first embodiment.

900 in FIG. 9A indicates the growth order of cells, and cells 801 to 804 in FIGS. 8A and 8B are obtained by multiplying the growth order 900 by the total number of cells by an integral number, and then multiplying the growth order 900 by the total number of cells. By adding different values, it is configured such that each cell has the same growth order but different thresholds. Each threshold value is normalized to be less than the maximum value of the input pixel value according to the input pixel value of the image data input to the halftone processing unit 202 .

In the first embodiment, the maximum input pixel value of the image data input to the halftone processing unit 202 is "255", so each threshold is normalized to a value of 0-254. A cell arranged such that the thresholds contained in the cells are arranged in the same order and have different thresholds is generally called a sub-matrix. The cells 801 to 804 are arranged so that halftone dots are at a predetermined angle and have a predetermined periodicity. This halftone dot period can be represented by two vectors. The first vector 805 in FIGS. 8A and 8B has 6 components in the main scanning direction and -6 components in the sub-scanning direction, and the second vector 806 has components in the main scanning direction. has 6 components in the horizontal direction and 6 components in the sub-scanning direction. The dither matrix 800 represented by these two vectors forms a halftone dot of 141 lines and 45 degrees at a resolution of 1200 dpi. As for the cell growth order 900, two continuous growth orders are set so as to be symmetrical with respect to the dashed line 901 in the sub-scanning direction.

FIG. 10 is a diagram showing an example of halftone dots formed using the dither matrix 800 shown in FIG. 8A.

FIG. 10A shows binary bitmap data (1200 dpi) obtained by performing halftone processing on a rendering bitmap whose input pixel values are all “27” using the dither matrix 800, and then performing phase conversion processing. is shown. In FIG. 10A, a dashed line 1001 is a transition point at which line shifting by one pixel is to be performed upward in the sub-scanning direction (see transition point P3 in FIG. 5B). It can be seen that halftone dots in the second area are shifted upward in the sub-scanning direction by one pixel (one line) from the transition point indicated by the dashed line 1001 . A halftone dot 1002 in which a plurality of ON pixels are periodically gathered in the first area, and a plurality of ON pixels are periodically gathered in the second area. Displaced halftone dots 1003 are lined up. As the input pixel value increases, the halftone dots 1002 and 1003 increase in area ratio and grow outward from the center of the halftone dot.

FIG. 10B shows binary bitmap data obtained by performing halftone processing on a drawing bitmap whose input pixel values are all "31" using the dither matrix 800, and then performing phase conversion processing ( 1200 dpi). As described above, in FIG. 10B, halftone dots 1004 and 1005, which have larger input pixel values than those in FIG. .

FIG. 6 is an enlarged view of part of the binary bitmap data of FIGS. 10A and 10B.

FIG. 6A is an enlarged view of a part of the binary bitmap data of FIG. 10A. A halftone dot 1002 exposes ON pixels of four A laser pixels and four B laser pixels in the first region. , halftone dot 1003 similarly exposes ON pixels of 4 pixels by A laser and 4 pixels by B laser in the second area. Since two halftone dots 1002 and two dots 1003 exist in each of the first and second regions of FIG. 6A, in the first and second regions of FIG. The B laser will expose the same number of ON pixels of 8 pixels.

On the other hand, FIG. 6B is an enlarged view of part of the binary bitmap data of FIG. 10B, and corresponds to a case where the input pixel value is larger than that of FIG. 6A. In FIG. 6B, a halftone dot 1004 exposes four ON pixels for the A laser and five pixels for the B laser in the first region, and a halftone dot 1005 for the second region exposes five pixels for the A laser and five pixels for the B laser. 4 ON pixels are exposed. That is, since two halftone dots 1004 and 1005 exist in each of the first area and the second area in FIG. 6B, in the first area in FIG. will expose 8 ON pixels, and in the second region, the A laser will expose 8 pixels and the B laser will expose 10 ON pixels.

At this time, if there is a difference in the light amount of the laser beam between the A laser and the B laser, the number of ON pixels exposed by each laser is switched between the first area and the second area. Therefore, different halftone dots are formed on the recording medium between the first area and the second area bordering on the transfer point. As a result, when the dither matrix 800 of FIG. 8A is used, the input pixel values of FIG. 10B are reproduced with different densities and colors in the first area and the second area.

The dither matrix 810 of FIG. 8B includes four cells 801, 804, 811, 812 each forming one halftone dot. The dither matrix 810 has the same number of cells as the dither matrix 800, but the threshold values included in the cells 811 and 812 are switched in the continuous growth order in the sub-scanning direction with respect to the cells 801 and 804. FIG. The growth order of the cells at this time is indicated by the growth order 902 in FIG. 9B. FIG. 9B is a diagram showing the growth order 902 of the cells 811 and 812, and it can be seen that the growth order 900 in FIG. As described above, the growth order 900 is set so as to be linearly symmetrical with respect to the dashed line 901, so the growth order 902 is similarly set to be linearly symmetrical with respect to the dashed line 903. Two consecutive growth orders are set so that

Since the arrangement of cells 801, 804, 811, 812 of FIG. 8B is similar to the arrangement of cells 801-804 of dither matrix 800 of FIG. 8A, dither matrix 810 of FIG. form halftone dots. However, in FIG. 8B, the cells 801 and 811, and the cells 804 and 812 are alternately arranged in the main scanning direction.

FIG. 11 is a diagram showing an example of binary bitmap data obtained by performing halftone processing on different input pixel values using the dither matrix 810 of FIG. 8B and then performing phase conversion processing. Here, an example of arrangement of halftone dots in two predetermined areas (first and second areas) positioned before and after the transition point 1101 in the main scanning direction is shown.

FIG. 11A is binary bitmap data obtained by performing halftone processing on a drawing bitmap whose input pixel values are all "27" using the dither matrix 810 of FIG. 8B, and then performing phase conversion processing. (1200 dpi).

In FIG. 11A, a dashed line 1101 is a transition point at which line shifting by one pixel is to be performed upward in the sub-scanning direction (see transition point P3 in FIG. 5B). It can be seen that the halftone dots in the second area are shifted upward in the sub-scanning direction by one pixel (one line) from the transition point 1101 as a boundary. A halftone dot 1102 in which a plurality of ON pixels are periodically gathered is in the first area, and a plurality of ON pixels are periodically gathered in the second area. Displaced halftone dots 1103 are lined up. These halftone dots 1102 and 1103 are the same as the halftone dots 1002 and 1003 in FIG. 10A.

FIG. 11B shows binary bitmap data obtained by performing halftone processing on a drawing bitmap whose input pixel values are all "31" using the dither matrix 810 of FIG. 8B, and then performing phase conversion processing. (1200 dpi). As described above, in FIG. 11B, halftone dots 1104, 1105, 1106, and 1107, which have larger input pixel values than those in FIG. I know there is.

Further, FIG. 11C is a binary bit map obtained by performing halftone processing on a rendering bitmap whose input pixel values are all “35” using the dither matrix 810 of FIG. 8B, and then performing phase conversion processing. It is a figure which shows map data (1200 dpi). In FIG. 11C, since the input pixel value is larger than that in FIG. You can see they are lined up.

In FIG. 11B, two types of halftone dots 1104 and 1105 are present in the first area, and two types of halftone dots 1106 and 1107 are also present in the second area. On the other hand, in FIG. 11C, only one kind of halftone dot 1108 exists in the first area, and only one kind of halftone dot 1109 exists in the second area. That is, when using the dither matrix 810 of FIG. 8B according to the first embodiment, one type of halftone dot is provided in each of the first region and the second region for a certain input pixel value, and two halftone dots are provided in each region for a larger input pixel value. Kinds of axisymmetric halftone dots repeat that at higher input pixel values there is one kind of halftone dot in each region.

FIG. 12 is an enlarged view of part of the binary bitmap data of FIGS. 11(A) and 11(B).

In FIG. 12A, as in FIG. 6A, the number of ON pixels exposed by the A laser and the B laser is the same between the first region and the second region. On the other hand, in FIG. 12B where the input pixel value is large, for the halftone dot 1104 in the first area, 5 pixels are exposed by the A laser and 4 pixels by the B laser. For halftone dot 1106 in the second area, the A laser exposes four pixels and the B laser exposes five ON pixels. If only the halftone dot 1104 exists in the first area, and if there is a difference in the amount of laser beams between the A laser and the B laser, the same problem as in the dither matrix 800 described above occurs. However, halftone dots 1105 and halftone dots 1107 which are reversed in the sub-scanning direction with respect to the halftone dots 1104 and 1107 exist in the first area and the second area, respectively. Therefore, both the A laser and the B laser expose 9 pixels in the first area, and the A laser and the B laser both expose 9 pixels in the second area. Since the A laser and the B laser both emit light the same number of times in the first region and the second region, even if there is a difference in the amount of laser beams between the A laser and the B laser, the above problem does not occur.

In the first embodiment, at 1200 dpi, the first vector has 6 components in the main scanning direction and -6 components in the sub-scanning direction, and the second vector has 6 components in the main scanning direction and 6 components in the sub-scanning direction. , 141 lines and 45 degrees, but the present invention is not limited to this.

Also, the dither matrix according to the first embodiment is configured by combining four cells, but the present invention is not limited to this. For example, 8 or 16 cells may be combined, and half of the cells may be arranged in line-symmetric growth order with respect to the remaining half of the cells.

Further, although the dither matrix used in the first embodiment is binary, it goes without saying that a multi-value dither matrix may be used as long as the above two growth orders are combined. Further, although the laser scanner used in the first embodiment has been described as drawing using two lasers, it is not limited to this, and needless to say, a plurality of lasers other than two, such as four lasers, may be used. .

As described above, according to the first embodiment, a dither matrix is used in which the growth order of the cells is line-symmetrically reversed in the sub-scanning direction in a plurality of cells forming the dither matrix. As a result, even when there is a difference in the light intensity of the laser beams of a plurality of lasers, the same halftone dot can be formed in the first area and the second area bordering on the transfer point. It is possible to reproduce the appropriate density and color tone.

[Embodiment 2]
In the first embodiment described above, in the halftone processing unit 202, two consecutive growth orders are set so as to be line-symmetrical in the sub-scanning direction, and two different growth orders are set line-symmetrically reversed in the sub-scanning direction. The case of using a dither matrix in which cells are combined has been described.

On the other hand, in the second embodiment, a case will be described in which dither matrices of cells in which two consecutive growth orders are set so as to be point symmetrical are used. The second embodiment differs from the first embodiment only in the configuration of the dither matrix used by the halftone processing unit 202. Therefore, the same parts as those in the first embodiment are given the same reference numerals, and the description thereof is omitted. only are described below.

The dither matrix used in the second embodiment and its effect will be described in detail with reference to FIGS. 13A, 13B, 14, 15, and 16. FIG.

13A and 13B are diagrams showing examples of dither matrices according to the second embodiment.

The dither matrix 1300 of FIG. 13A consists of four cells 1301, 1302, 1303, 1304 each forming one halftone dot. 13A and 13B are denoted by the same reference numerals.

FIG. 14 is a diagram showing an example of the growth order of cells forming a dither matrix according to the second embodiment.

FIG. 14A shows a cell growth order 1400. FIG. Cells 1301 to 1304 in FIG. 13A are obtained by multiplying the growth order 1400 by the total number of cells and then adding a different value less than the total number of cells to each cell, thereby obtaining the same growth order but different cells. Configured to have a threshold. Each threshold value is normalized to be less than the maximum value of the input pixel value according to the input pixel value of the image data input to the halftone processing unit 202 .

In the second embodiment, the maximum input pixel value of the image data input to the halftone processing unit 202 is “255”, so it is normalized to a value of 0-254. The halftone dot period of the second embodiment is the same as that of the first embodiment, and halftone dots of 141 lines and 45 degrees are formed at a resolution of 1200 dpi. The cell growth order 1400 is set such that two consecutive growth orders are point symmetrical. Since halftone dots are formed so that ON pixels are gathered, a point 1401 serving as a point-symmetrical reference is set between the two lowest growth orders.

Binary bitmap data when the input pixel values are "27" and "31" using the dither matrix 1300 shown in FIG. 13A are the same as those shown in FIGS. omitted. When the dither matrix 1300 is used in this way, when there is a difference in the light amount of the laser beams between the two lasers, and when the input pixel values are different, there is a degree of ON pixels between the first region and the second region. areas are reproduced with different densities and colors.

On the other hand, the dither matrix 1310 in FIG. 13B is composed of four cells 1301, 1302, 1311 and 1312 each forming one halftone dot. The dither matrix 1310 has the same number of cells as the dither matrix 1300, but the threshold values included in the cells 1311 and 1312 are arranged symmetrically with respect to the reference point. .

The cell growth order 1402 shown in FIG. 14B shows the growth order of the cells 1311 and 1312, and the growth center 1403 is similar to the growth center 1401 of the cell growth order 1400 shown in FIG. 9A. It is the same. Comparing FIG. 14B and FIG. 14A, in FIG. 14B, for example, thresholds 0 and 1, 2 and 3, 4 and 5 (continuous even and odd numbers) are interchanged. Recognize. Since the arrangement of cells 1301, 1302, 1311, 1312 is similar to dither matrix 1300, dither matrix 1310 of FIG. 13B forms a 141-line 45 degree halftone dot at a resolution of 1200 dpi.

FIG. 15 shows an example of binary bitmap data obtained by performing halftone processing on different input pixel values using the dither matrix 1310 of FIG. 13B according to the second embodiment, and then performing phase conversion processing. FIG. 4 is a diagram showing;

FIG. 15A shows binary bitmap data (1200 dpi) obtained by performing halftone processing on a rendering bitmap whose input pixel values are all “27” using the dither matrix 1310 and then performing phase conversion processing. is shown. In FIG. 15A, a dashed line 1501 indicates a transition point at which line shifting by one pixel is to be performed upward in the sub-scanning direction (see transition point P3 in FIG. 5B). It can be seen that halftone dots in the second area are shifted upward in the sub-scanning direction by one pixel (one line) from this transition point 1501 as a boundary. A halftone dot 1502 in which a plurality of ON pixels are periodically gathered is in the first area, and a plurality of ON pixels are periodically gathered in the second area. Displaced halftone dots 1503 are lined up. The halftone dots 1502 and 1503 are the same as the halftone dots 1002 and 1003 in FIG. 10A.

FIG. 15B shows binary bitmap data (1200 dpi) obtained by performing halftone processing on a drawing bitmap whose input pixel values are all “31” using the dither matrix 1310 and then performing phase conversion processing. is shown. As described above, in FIG. 15B, halftone dots 1504, 1505, 1506, and 1507, which have larger input pixel values than those in FIG. I know there is.

Further, FIG. 15C shows binary bitmap data (which is obtained by performing halftone processing on a drawing bitmap whose input pixel values are all “35” using a dither matrix 1310 and then performing phase conversion processing). 1200 dpi).

In this way, in FIG. 15C, halftone dots 1508 and 1509, which have larger input pixel values than those in FIG. I understand. In FIG. 15B, there are two types of halftone dots 1504 and 1505 in the first region, and two types of halftone dots 1506 and 1507 in the second region. There is only one kind of halftone dot 1508 in the second area, and only one kind of halftone dot 1509 in the second area.

As described above, when the dither matrix 1310 according to the second embodiment is used, one type of halftone dot is placed in each of the first and second regions for a certain input pixel value, and two types of halftone dots are placed in each region for a larger input pixel value. Point-symmetric halftone dots repeat that there is one type of halftone dot in each region for larger input pixel values.

FIG. 16 is an enlarged view of part of the binary bitmap data of FIGS. 15A and 15B according to the second embodiment.

In FIG. 16A, as in FIG. 6A, the number of ON pixels exposed by the A laser and the B laser is the same between the first region and the second region. On the other hand, in FIG. 16B where the input pixel value is large, the halftone dot 1504 exposes 5 pixels of the A laser and 4 pixels of the B laser in the first region, and the halftone dot 1506 exposes the A laser in the second region. will expose 4 pixels and the B laser will expose 5 ON pixels. With only halftone dots 1504 in the first area, the same problem as in the dither matrix 1300 described above occurs when there is a difference in the amount of laser beams between the A laser and the B laser. However, in the first area there is a halftone dot 1505 which is symmetrical with respect to the halftone dot 1504, and in the second area there is a halftone dot 1507. FIG. In this way, in each of the first region and the second region, since halftone dots that are point-symmetrical exist, both the A laser and the B laser expose 9 ON pixels in the first region, and the A laser in the second region. Both laser and B laser expose 9 ON pixels. Therefore, even if there is a difference in the amount of laser beams between the A laser and the B laser, the above problem does not occur.

In the second embodiment, at 1200 dpi, the first vector has 6 components in the main scanning direction and -6 components in the sub-scanning direction, and the second vector has 6 components in the main scanning direction and 6 components in the sub-scanning direction. , 141 lines and 45 degrees, but the present invention is not limited to this.

Also, although the dither matrix of the second embodiment is configured by combining four cells, it is not limited to this. For example, 8 or 16 cells may be combined, and half of the cells may be arranged in order of growth point-symmetrically with respect to the remaining half of the cells.

Further, although the dither matrix used in the second embodiment is binary, it goes without saying that a multi-value dither matrix may be used as long as the two growth orders described above are combined.

In addition, although the laser scanner used in the second embodiment has been described as drawing using two lasers, it is not limited to this, and needless to say, a plurality of lasers other than two, such as four lasers, may be used.

As described above, according to the second embodiment, a dither matrix that is point-symmetrical with respect to a point based on the growth order of the cells is used in a plurality of cells forming the dither matrix. As a result, even when there is a difference in the light intensity of the laser beams of a plurality of lasers, the same halftone dot can be formed in the first area and the second area bordering on the transfer point. It is possible to reproduce the same density and color tone.

(Other embodiments)
The present invention supplies a program that implements one or more functions of the above-described embodiments to a system or apparatus via a network or a storage medium, and one or more processors in the computer of the system or apparatus reads and executes the program. It can also be realized by processing to It can also be implemented by a circuit (for example, ASIC) that implements one or more functions.

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, to publicize the scope of the invention, the following claims are included.

DESCRIPTION OF SYMBOLS 100...MFP (complex machine) 101...CPU 102...Memory 108...Image processing part 201...Color conversion part 202...Halftone processing part 203...Phase transfer part 800, 810, 1300, 1301...Dither matrix

Claims (10)

An image forming apparatus,
halftone processing means for forming halftone image data including a plurality of halftone dots using a dither matrix for input image data;
correction means for correcting the image data so as to correct the curvature of the scanning line of the laser beam by shifting the line of the halftone image data formed by the halftone processing means in the sub-scanning direction;
an image forming means for forming an image based on the image data corrected by the correcting means;
half of the plurality of halftone dots included in the halftone image data are formed in the first growth order, and the remaining half of the plurality of halftone dots are formed in the sub-scanning with respect to the first growth order; An image forming apparatus, wherein halftone dots are formed in a second growth order that is symmetrical in a direction. The image forming means forms an image by an electrophotographic method, and has two light emitting elements for irradiating the laser beam for every plurality of rows in the sub-scanning direction,
The correcting means sub-scans the halftone image data at a scan line change point shifted by one pixel in the sub-scanning direction due to the bending characteristics of the laser beams so as to correct the bending of the laser beams emitted by the two light emitting elements. 2. The image forming apparatus according to claim 1, wherein the image forming apparatus is shifted in the scanning direction. In each of the first area and the second area located before and after the scan line changing point in the main scanning direction, one or two types of 3. The image forming apparatus according to claim 2, wherein the line-symmetrical halftone dots are present. 4. The image forming apparatus according to claim 3, wherein said two light emitting elements emit light the same number of times in said first area and said second area. 5. The dither matrix according to any one of claims 2 to 4, wherein said dither matrix has a plurality of cells, half of said plurality of cells being linearly symmetrical growing order cells with respect to the remaining half of the cells. The image forming apparatus according to . The dither matrix has at least first cells in which a plurality of threshold values are arranged in the first growth order and second cells in which a plurality of threshold values are arranged in the second growth order, 5. The growth order according to any one of claims 2 to 4 , wherein the growth order and the second growth order are symmetrical with respect to a reference point set between the two lowest growth orders. The described image forming apparatus. 7. The image forming apparatus according to claim 6, wherein the first cells and the second cells are alternately arranged in the dither matrix in a direction corresponding to the main scanning direction. 8. The method according to claim 6, wherein the total number of cells included in said dither matrix is an even number, and the total number of said first cells and said second cells included in said dither matrix is the same. Image forming device. An image forming method in an image forming apparatus,
a halftone processing step of forming halftone image data including a plurality of halftone dots using a dither matrix for input image data;
a correction step of correcting the image data so as to correct the curvature of the scanning line of the laser beam by shifting the line of the halftone image data formed in the halftone processing step in the sub-scanning direction;
an image forming step of forming an image based on the image data corrected in the correcting step;
half of the plurality of halftone dots included in the halftone image data are formed in the first growth order, and the remaining half of the plurality of halftone dots are formed in the sub-scanning with respect to the first growth order; A method of forming an image, wherein halftone dots are formed in a second growth order that is symmetrical in direction. A program for causing a computer to function as each means of the image forming apparatus according to any one of claims 1 to 8.

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