JP2011257636A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of performing a sub-scanning multiple-value variable-magnification processing which is not affected by environmental change such as temperature and reduces the deterioration of image quality.SOLUTION: An image forming apparatus comprises sub-scanning multiple-value variable-magnification means (S1101) to perform multiple-value variable-magnification of a binary image having an area subjected to halftone dot processing and the other areas in a sub-scanning direction, and N-value conversion means (S1102) to perform N-value conversion (N is a natural number) of the multiple-value image obtained by the sub-scanning multiple-value variable-magnification means by diffusing an error in the multiple-value variable-magnification processing to the sub-scanning direction in the area subjected to the halftone dot processing, and to perform N-value conversion of the multiple-value image obtained by the sub-scanning multiple-value variable-magnification means by diffusing the error in the multiple-value variable-magnification processing to a main scanning direction in the other areas.

Description

  The present invention relates to an electrophotographic image forming apparatus that forms an image using an electrostatic latent image.

  In an image forming process of an image forming apparatus using an electrophotographic system, a latent image formed on a photoreceptor is developed as a toner image, the toner image is transferred to a sheet, and heat or heat is applied to the sheet on which the toner image is transferred. The toner image is fixed on the sheet by applying pressure.

  Incidentally, it is known that in the process of fixing the toner image to the sheet, the sheet expands and contracts due to heat and pressure. Therefore, when images with the same magnification are formed on both surfaces of the sheet, the toner image is transferred to the back surface of the sheet that is expanded or contracted by heat or pressure in the fixing process of the toner image on the front surface side. Different images may be formed.

  Therefore, as shown in FIG. 16, a technique has been proposed in which the sheet size measuring unit measures the amount of variation in the sheet size and transfers the measured amount of variation to the controller (Patent Document 1). In this proposal, the controller corrects the magnification in the sub-scanning direction of the image transmitted from the host computer in accordance with the variation amount of the sheet size acquired from the sheet size measurement unit, and the image processing unit converts the magnification corrected image to a screen or the like. Then, image formation is performed.

Japanese Patent Application Laid-Open No. 2004-129069

  However, in Patent Document 1, the sheet size measurement unit measures the variation amount of the sheet size, and the process is started after the controller acquires the measured variation amount. Therefore, the control loop is large and the productivity is lowered. To do.

  On the other hand, as a general technique, it is conceivable to perform multi-valued sub-scan scaling processing. However, multi-value halftones are susceptible to environmental fluctuations such as temperature and are developed with toner. May not occur, which may cause image quality degradation.

  Referring to FIGS. 17 and 18, FIG. 17 (a) shows that when a binary screen process using halftone dots is subjected to multi-value scaling in the sub-scanning direction, the multi-value is halftone. It is a figure which shows the image currently reproduced. However, when this image is affected by environmental fluctuations such as temperature and is not developed with toner, as shown in FIG. 17B, a multi-value halftone is not reproduced and the halftone dot is in the sub-scanning direction. The desired density cannot be obtained due to thinning.

  FIG. 18A is a diagram showing an image in which multi-values are reproduced with halftones when a thin line is reduced and scaled with multi-values in the sub-scanning direction. However, when this image is affected by environmental fluctuations such as temperature and is not developed with toner, as shown in FIG. 18B, the multi-value halftone is not reproduced and the thin line becomes thin and the image deteriorates. Cause.

  Accordingly, an object of the present invention is to provide a mechanism capable of performing sub-scanning multi-value scaling processing with little image quality degradation without being affected by environmental changes such as temperature.

  In order to achieve the above object, the image forming apparatus of the present invention scans in the sub-scanning direction by rotating the photoconductor that irradiates the polarized light by performing main scanning by polarizing the light from the light emitting unit. An image forming apparatus for forming a latent image on a photoconductor, wherein a sub-scanning multi-value scaling unit that multi-values a binary image having a halftone-processed area and other areas in a sub-scanning direction; The image obtained by the sub-scanning multi-value scaling means is converted into an N-value (N is a natural number) by diffusing an error in the multi-value scaling process in the half-scanning region in the halftone process. In other areas, there is provided an N-value conversion means for diffusing an error in the multi-value scaling process in the main scanning direction into an N-value.

  According to the present invention, it is possible to perform sub-scanning multi-value scaling processing with little deterioration in image quality without being affected by environmental changes such as temperature.

1 is a schematic cross-sectional view for explaining an image forming apparatus according to a first embodiment of the present invention. (A) is a side view of an optical unit, (b) is a top view of an optical unit. 2 is a schematic block diagram for explaining a control system of the image forming apparatus. FIG. It is a flowchart figure for demonstrating an example of the image process in a center image process part. It is the schematic diagram which represented the binarized image data D0y [y] [x] of Y color as an actual image. It is a flowchart for demonstrating the image correction process in an output type image process part. FIG. 7 is a flowchart for explaining a detailed example of sub-scanning multi-value scaling processing in step S1101 of FIG. It is explanatory drawing for demonstrating dx and dy in Formula (2). It is a flowchart figure for demonstrating the detailed example of the binarization process in FIG.6 S1102. FIG. 10 is a flowchart for explaining an example of binarization processing of a non-halftone area in step S1306 in FIG. FIG. 10 is a flowchart for explaining an example of binarization processing of a halftone dot region in step S1307 of FIG. (A) is a diagram showing one halftone dot composed of a plurality of pixels scaled in the sub-scanning direction, and (b) is an enlarged view of the vicinity of a halftone pixel above the halftone dot in (a). (A) is a figure which shows the original image of a thin line part, (b) is a figure which shows the image which performed the ideal subscanning multi-value scaling (enlarging) process with respect to the image of (a), (c) FIG. 5 is a diagram illustrating an image obtained by performing sub-scanning multi-value scaling (enlargement) processing of the present embodiment on the image of (a). FIG. 10 is a flowchart for explaining an example of binarization processing for a halftone dot area in step S1307 in FIG. 9 in the image forming apparatus according to the second embodiment of the present invention. (A) is an original image of a halftone dot image, (b) is a diagram obtained by subjecting the halftone dot image of (a) to conventional sub-scanning multi-value scaling, and (c) is a halftone dot image of (a). FIG. 6 is a diagram in which the sub-scanning multi-value scaling process of the present embodiment is performed. It is a schematic block diagram for demonstrating an example of the conventional image forming apparatus. It is explanatory drawing for demonstrating the conventional subscanning multi-value scaling process. It is explanatory drawing for demonstrating the conventional subscanning multi-value scaling process.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a schematic sectional view for explaining an image forming apparatus according to a first embodiment of the present invention.

  As shown in FIG. 1, the image forming apparatus of this embodiment includes a scanner unit 700 and a printer unit 701. First, the scanner unit 700 will be described. The scanner unit 700 forms an image on the color sensor 706 as color image information through the mirror groups 704 </ b> A to 704 </ b> C and the lens 705 through the reflected light emitted from the illumination lamp 703 onto the document 702.

  The color sensor 706 converts the formed color image information into an electrical image signal for each color separation light of, for example, blue (B), green (G), and red (R). Then, based on the image signal intensity levels of B, G, and R, a color conversion process is performed by a central image processing unit 904 (FIG. 3) described later, and black (K), cyan (C), magenta (M), yellow Color image data (Y) is obtained. The obtained color image data is output to output system image processing units 905 to 908 described later.

  Next, the printer unit 701 will be described. The printer unit 701 includes photoconductors 708M, 708C, 708Y, and 708K as image carriers.

  Around the photoreceptors 708M, 708C, 708Y, and 708K, charging units 709M, 709C, 709Y, and 709K, developing units 710M, 710C, 710Y, and 710K, and cleaning devices 725M, 725C, 725Y, and 725K are arranged. Further, above the photoconductors 708M, 708C, 708Y, and 708K, optical units 707M, 707C, and 707Y that convert color image data output from the scanner unit 700 into optical signals for each of M, C, Y, and K colors. , 707K are arranged.

  The chargers 709M, 709C, 709Y, and 709K apply charges having a uniform charge amount to the surfaces of the photoreceptors 708M, 708C, 708Y, and 708K that rotate counterclockwise in the drawing. Next, the optical units 707M, 707C, 707Y, and 707K irradiate the surface of the photoreceptors 708M, 708C, 708Y, and 708K with the converted optical signal as laser light or the like to form an electrostatic latent image.

  The developing units 710M, 710C, 710Y, and 710K visualize the electrostatic latent images formed on the surfaces of the photoreceptors 708M, 708C, 708Y, and 708K as toner images. The visualized toner image is transferred to an intermediate transfer belt 711 which is an example of an intermediate transfer member via first transfer bias blades 712M, 712C, 712Y and 712K in primary transfer areas Ta, Tb, Tc and Td. . The toner remaining on the surface of the photoreceptors 708M, 708C, 708Y, and 708K without being transferred to the intermediate transfer belt 711 is scraped off and removed by the cleaning devices 725M, 725C, 725Y, and 725K.

  The intermediate transfer belt 711 is stretched around a driving roller 713, a backup roller 714, and a driven roller 715 that are driven by a driving motor (not shown). A second transfer bias roller 716 is disposed at a position facing the backup roller 714 of the intermediate transfer belt 711, and a cleaning device 717 is disposed at a position facing the driven roller 715 of the intermediate transfer belt 711.

  The latent image formation on the surfaces of the above-described photoreceptors 708M, 708C, 708Y, and 708K is started from the photoreceptor 708M first. Thereafter, a latent image is started on the photoconductor 708C at a timing delayed from the rotational speed of the intermediate transfer belt 711 by the difference between the positions of the photoconductors 708M and 708C. Next, formation of a latent image on the photoconductor 708Y is started at a timing delayed from the rotational speed of the intermediate transfer belt 711 by the shift between the positions of the photoconductors 708C and 708Y. Next, formation of a latent image on the photoconductor 708K is started at a timing delayed from the rotational speed of the intermediate transfer belt 711 by the difference between the positions of the photoconductors 708Y and 708K. As a result, the toner images of the respective colors are sequentially superimposed on the intermediate transfer belt 711 and are primarily transferred.

  Next, a configuration example of the optical units 707M, 707C, 707Y, and 707K will be described with reference to FIG. 2A is a side view of the optical unit, and FIG. 2B is a plan view of the optical unit. Since the optical units 707M, 707C, 707Y, and 707K have the same configuration, only the optical unit 707M will be described.

  As shown in FIG. 2, the optical unit 707M irradiates the polygon mirror 802, which is driven to rotate by the polygon motor 802a, through the lens 801 with the laser light emitted from the light emitting unit 800. The laser light emitted from the light emitting unit 800 is polarized so as to scan six times while the polygon mirror 802 rotates once, and the polarized laser light is detected by a BD (BeamDetect) detection element 803 at the beginning of scanning. The Based on this detection signal, a BD signal serving as a trigger for starting exposure in the main scanning direction is generated.

  On the other hand, phase control is performed so that the polygon motor 802 is synchronized with the BD signal at the rising or falling edge of a signal (not shown) of a HP (Home Position) sensor provided on the intermediate transfer belt 711, and the exposure start timing in the sub-scanning direction is set. obtain. The fθ lens 804 corrects the scanning speed of the polarized laser light at the end in the main scanning direction, and the corrected laser light is polarized by the plane mirror 805 and applied to the photoreceptor 708M disposed below. Irradiated.

  Returning to FIG. 1, the sheets stored in the sheet feeding cassette 718 are fed one by one to the conveying path by the sheet feeding roller 719, and the sheets fed to the conveying path are fed by the conveying rollers 722, 721 and 720. The sheet is conveyed to the registration roller 723 and skew is corrected. The registration roller 723 sends out the sheet to the secondary transfer region Te so that the toner images of the respective colors primarily transferred to the intermediate transfer belt 711 are transferred to the sheet in the secondary transfer region Te. In the secondary transfer region Te, a predetermined transfer bias is applied to the second transfer bias roller 716, whereby the toner images of the respective colors primarily transferred to the intermediate transfer belt 711 are collectively transferred to the sheet. The

  The sheet on which the toner images of the respective colors are secondarily transferred in the secondary transfer region Te is conveyed to the fixing device 724, and a fixing roller having a heat source therein and a pressure applied to the fixing roller. Passes through the nip with the roller. As a result, the toner image on the sheet is fixed, and the fixed sheet is discharged to the outside by a discharge roller (not shown). Note that toner remaining on the surface of the intermediate transfer belt 711 without being transferred onto the sheet is scraped off and removed by the cleaning device 717.

  In the case of duplex printing, the sheet discharged from the fixing device 724 is reversed by the flapper 727 and conveyed to the reversing path 728. The sheet conveyed to the reversing path 728 is conveyed to the right side of the drawing by the reversing roller 729, and then the flapper 730 is switched to reversely rotate the reversing roller 729, so that the reversed sheet is downstream by the conveying roller 731. It is conveyed to.

  Then, the sheet is guided to the conveying roller 721 side by the flapper 732 and conveyed to the registration roller 723. Thereafter, in the same manner as described above, the toner images of the respective colors are secondarily transferred collectively to the back surface of the sheet by the second transfer bias roller 716, and the secondly transferred sheet is discharged to the outside via the fixing device 724. .

  Here, the image forming apparatus of the present embodiment includes a user interface for setting the magnification of the image with respect to the front surface of the sheet by a user operation, and at the time of duplex printing, an image is formed on the back surface of the sheet at the set magnification. The

  FIG. 3 is a schematic block diagram for explaining a control system of the image forming apparatus of the present embodiment.

  In FIG. 3, the reading system image processing unit 900 performs shading correction and the like on the image signal of the original 702 read by the scanner unit 700 and outputs the image signal to the central image processing unit 904.

  The central image processing unit 904 includes a CPU 909 and memories 910 to 914. The memories 911 to 914 store image data for each color of Y, M, C, and K. The central image processing unit 904 transmits / receives external input data such as a telephone line and a network via the external interface 902, and when the received data is PDL (Page Description Language), the PDL processing unit 901 develops the image information.

  The output system image processing units 905 to 908 perform image processing for each color of Y, M, C, and K. Since the output system image processing units 905 to 908 have the same configuration, only the output system image processing unit 905 will be described.

  The output system image processing unit 905 includes a DMAC 915, a CPU 916, memories 918 and 919, a modulation circuit 917, and a laser drive circuit 920. The DMAC 915 acquires image data from the memory 911 in which Y color image data is stored, and stores the image data in the memory 918.

  The image data stored in the memory 918 is modulated by the modulation circuit 917 according to the BD signal, and a pulse signal is sent to the laser drive circuit 920. The laser drive circuit 920 drives the laser according to this pulse signal, and causes the light emitting unit 800 (FIG. 2) to emit light.

  Next, an example of image processing in the central image processing unit 904 will be described with reference to FIG. Each process in FIG. 4 is executed by the CPU 909 by loading a program stored in the memory 910 or the like into the RAM.

  In step S1001, the CPU 909 performs processing for color-converting the RGB image data for each pixel acquired from the reading system image processing unit 900 into YMCK image data, and the process proceeds to step S1002.

  In step S1002, the CPU 909 performs halftone processing for improving halftone reproducibility by halftone halftone processing on the YMCK image data color-converted in step S1001. Here, the CPU 909 performs halftone processing when performing halftone processing of a photograph or the like, and does not perform halftone processing for characters or fine lines. Further, the CPU 909 stores the Y-color binarized image data subjected to the halftone process in the memory 911 as a two-dimensional array D0y [y] [x].

  FIG. 5 is a schematic diagram showing D0y [y] [x] as an actual image. In FIG. 5, y represents coordinates in the sub-scanning direction, and x represents coordinates in the main scanning direction. D0y [y] [x] is stored in the memory 911 as a two-dimensional array having the coordinates (y, x).

  Similarly, in the memories 912 to 914, image data of each color of M, C, and K is stored as D0m [y] [x], D0c [y] [x], and D0k [y] [m]. Also, binary region determination data A [y] [x] for distinguishing the region by setting the pixel of the region subjected to halftone processing at halftone processing to 1 and the pixel of the other region such as a character or thin line as 0 Are stored in the memories 911 to 914 for each color.

  Thereafter, the CPU 916 of the output image processing unit 905 transfers the image data D0y [y] [x] and the area determination data A [y] [x] stored in the memory 911 to the memory 919 by the DMAC 915. Although not described, the output system image processing units 906 to 908 are also processed in the same manner as the output system image processing unit 905.

  FIG. 6 is a flowchart for explaining the image correction processing in the output system image processing unit 905. Each process in FIG. 6 is executed by the CPU 916 by loading a program stored in the memory 918 or the like into the RAM.

  In step S1101, the CPU 916 performs sub-scanning multi-value scaling on the image data D0y [y] [x] stored in the memory 919 at a magnification (for example, 100%) set by the user, and D1y [y ] [X] image data is generated. Then, the CPU 916 holds the generated image data of D1y [y] [x] in the memory 911, and proceeds to step S1102. Here, when forming an image on the sheet surface, the image data of D0y [y] [x] held in the memory 919 is used at a magnification set by the user. The image data of D1y [y] [x] subjected to the double processing is used. Details of the sub-scanning multi-value scaling process will be described later with reference to FIG.

  In step S1102, the CPU 916 performs N-value conversion processing (N is a natural number: binarization processing in the present embodiment) on the image data D1y [y] [x] held in the memory 919 in step S1101. Thus, the image data of D2y [y] [x] is generated. Details of the binarization process will be described later with reference to FIG.

  FIG. 7 is a flowchart for explaining a detailed example of the sub-scanning multi-value scaling process in step S1101 of FIG.

  In step S1201, the CPU 916 sets the coordinates of the processing target of D1y [y] [x] to (yc, xc), sets the coordinate position yc in the sub-scanning direction to 0, and proceeds to step S1202.

  In step S1202, the CPU 916 sets the coordinate position xc in the main scanning direction to 0, and proceeds to step S1203.

  In step S1203, the CPU 916 calculates the coordinates (Xorg, Yorg) of the D0y image corresponding to the (yc, xc) coordinates on the D1y image being processed using the affine transformation formula (1), and the process proceeds to step S1204. .

  Here, a, b, c, d, e, and f in the above formula (1) can be coordinate-converted by making them arbitrary values. However, in this embodiment, since the sub-scanning direction is scaled, a = 1, b = 0, c = 0, d = 0, f = 0, and e is a value corresponding to the magnification. For example, when an image is formed on the front surface of the sheet at a magnification of 100%, e = 1, and when an image is formed on the back surface of the sheet at a magnification of 99%, e = 1.01. In this way, it is possible to obtain the resampling coordinates (Yorg, Xorg) for the image of D0y [y] [x].

  In step S1204, the CPU 916 calculates the density value D1y [yc] [xc] at the re-sampling coordinates (Yorg, Xorg) obtained in step S1203, and the process proceeds to step S1205. In the present embodiment, the density value D1y [yc] [xc] is calculated from the four pixels around the resampling coordinates (Yorg, Xorg) by linear interpolation using the following equation (2).

  Here, dx and dy in the above equation (2) will be described with reference to FIG. In FIG. 8, the pixels of the resampling coordinates are displayed with diagonal lines, and the surrounding four pixels are displayed as white pixels. When the coordinates of the hatched pixels are expressed as (Yorg, Xorg), the coordinates of the upper left pixel among the four surrounding pixels are (<Yorg>, <Xorg>), and the coordinates of the upper right pixel are (<Yorg>, <Xorg + 1). >). The lower left coordinates are (<Yorg + 1>, <Xorg>), and the lower right coordinates are (<Yorg + 1>, <Xorg + 1>). Here, <> represents that the decimal part is rounded down. Thereby, the density value D1y [yc] [xc] in the re-sampling coordinates (Yorg, Xorg) can be obtained.

  In step S1205, the CPU 916 increments xc and proceeds to step S1206.

  In step S1206, the CPU 916 determines whether or not the value of xc matches the maximum number of main scanning pixels XMax of the image. If they match, the process proceeds to step S1207, and if they do not match, the process returns to step S1203.

  In step S1207, the CPU 916 increments yc and proceeds to step S1208.

  In step S1208, the CPU 916 determines whether or not yc matches the maximum number of sub-scanning pixels YMax of the image. If they do not match, the process returns to step S1202, and if they match, the process ends. As a result, an image that has been subjected to multi-value scaling in the sub-scanning direction is obtained.

  FIG. 9 is a flowchart for explaining a detailed example of the binarization processing in step S1102 of FIG.

  In step S1301, the CPU 916 initializes yc, which is a coordinate counter in the sub-scanning direction, of the image to be processed to 0, and proceeds to step S1302.

  In step S1302, xc which is a coordinate counter in the main scanning direction of the image to be processed is initialized to 0, and the process proceeds to step S1303.

  In step S1303, the CPU 916 calculates resampling coordinates (Xorg, Yorg) using the above equation (1), as in step S1203 of FIG. 7, and proceeds to step S1304.

  In step S1304, the CPU 916 calculates area determination data Aorg using the following equation (3), and the process proceeds to step S1305.

  The above formula (3) is a formula for rounding off the re-sampling coordinates (Yorg, Xorg) and selecting the area determination data at the closest integer coordinate position.

  In step S1305, if the Aorg value Aorg obtained in step S1304 is 0, the CPU 916 determines that the image is a non-halftone halftoning area and proceeds to step S1306. On the other hand, when the value of Aorg is 1, the CPU 916 determines that the image is a halftone halftone area image and proceeds to step S1307.

  In step S1306, the CPU 916 performs binarization processing on the image of the non-halftone halftoning area, and the process advances to step S1308. Note that the processing from step S1308 to step S1310 is the same as the processing from step S1206 to step S1208 in FIG.

  FIG. 10 is a flowchart for explaining an example of the binarization processing of the non-halftone area in step S1306 in FIG.

  In FIG. 10, in step S1401, the CPU 916 obtains pix, which is a value obtained by adding e1 [yc] [xc] and D1y [yc] [xc], and then proceeds to step S1402. The value of e1 [yc] [xc], which is not calculated, is an error at the time of binarization in the non-halftone area, which will be described later, and is held in the memory 919.

  In step S1402, the CPU 916 determines whether or not the value of pix is less than the threshold value 0.5, the process proceeds to step S1403 if the value is less than the threshold value 0.5, and the process proceeds to step S1404 if the value is greater than or equal to the threshold value 0.5. .

  In step S1403, the CPU 916 sets the value of D2y [yc] [xc] to 0, and proceeds to step S1405.

  In step S1404, the CPU 916 sets the value of D2y [yc] [xc] to 1, and proceeds to step S1405.

  In step S1405, the CPU 916 sets a value e1 [yc] [xc + 1] obtained by subtracting the binarized value D2y [yc] [xc] in step S1403 or step S1404 from the value pix before binarization as an error. Then, the CPU 916 propagates the error from the currently processed pixel (yc, xc) to (yc, xc + 1). That is, the error is diffused in the main scanning direction.

  Returning to FIG. 9, in step S <b> 1307, the CPU 916 performs binarization processing of the image of the halftone dot halftone area, and proceeds to step S <b> 1308. Note that the processing from step S1308 to step S1310 is the same as the processing from step S1206 to step S1208 in FIG.

  FIG. 11 is a flowchart for explaining an example of the binarization processing of the dot area in step S1307 of FIG.

  In FIG. 11, in step S1501, the CPU 916 obtains a value pix obtained by adding e2 [yc] [xc] and D1y [yc] [xc], and proceeds to step S1502. The value of e2 [yc] [xc] is an error at the time of binarization in the halftone dot region, and is held in the memory 919.

  In step S1502, the CPU 916 determines whether or not the value of pix obtained in step S1501 is greater than 0 and less than 1. If the value of pix exceeds 0 and is less than 1, the CPU 916 determines that it is a halftone, and proceeds to step S1504. If the value of pix exceeds 0 and is not less than 1, If it is not halftone, the process advances to step S1503.

  In step S1504, the CPU 916 calculates the value of yk using the following equation (4), and proceeds to step S1505.

yk = yc-1 (4)
In step S1505, the CPU 916 determines whether or not the density value D1y [yk] [xc] is 0. If it is 0, the process proceeds to step S1506, and if not, the process proceeds to step S1507. Here, the reason why the density value D1y [yk] [xc] is 0 is to determine whether the halftone is the upper edge or the lower edge of the halftone dot.

  Referring to FIG. 12, FIG. 12 (a) is a diagram showing one of halftone dots composed of a plurality of pixels scaled in the sub-scanning direction. When a binary halftone dot is subjected to multi-value scaling in the sub-scanning direction, a halftone is generated at the upper or lower edge of the contour.

  FIG. 12B is an enlarged view of the vicinity of the halftone pixel above the halftone dot in FIG. Here, for example, when D1y [yc] [xc] is a halftone, D1y [yk] [xc] is a pixel in the reverse direction of the sub-scanning direction. Therefore, when D1y [yk] [xc] is 0, it can be determined that this halftone is the upper edge of a halftone dot.

  FIG. 12C is an enlarged view of the vicinity of the halftone pixel in the lower part of FIG. Similarly to the above, when D1y [yk] [xc] is not 0, it can be determined that the halftone is the lower edge of the halftone dot.

  Thus, in step S1505, it is determined whether the halftone is the upper edge or lower edge of the halftone dot. In FIG. 11, when the halftone is the upper edge of the halftone dot, the CPU 916 sets D2y [yc] [xc] to 0 in step S1506. If the halftone is the lower edge of the halftone dot, the CPU 916 binarizes the halftone of the halftone dot by setting D2y [yc] [xc] to 1 in step S1507.

  On the other hand, the processing after step S1503 is binarization processing other than the halftone area of the halftone dot. In step S1503, the CPU 916 determines whether or not pix is 1 or more.

  If pix is 1 or more, the CPU 916 sets D2y [yc] [xc] to 1 in step S1508, and if pix is less than 1, sets D2y [yc] [xc] to 1 in step S1509.

  After executing any of steps S1506 to S1509, the CPU 916 proceeds to step S1510.

  In step S1510, the CPU 916 subtracts the value of D2y [yc] [xc] after binarization from the value of pix before binarization and sets the error as the pixel (yc, xc) currently being processed. To (yc + 1, xc). That is, the error is diffused in the sub-scanning direction. This makes it possible to perform binarization processing without losing the halftone dot shape as much as possible.

  The binarized image is held in the memory 919, and the modulation circuit 917 reads it from the memory 919 at a predetermined timing based on the BD signal and sends it to the laser drive circuit 920. By performing this processing in the output system image processing units 905 to 908, desired latent image images are formed on the surfaces of the photoreceptors 708M, 708C, 708Y, and 708K.

  FIG. 13A shows an original image of a thin line portion, and FIG. 13B shows an ideal sub-scanning multi-value scaling (enlargement) process performed on the image of FIG. It is a figure which shows an image. FIG. 13C is a diagram illustrating an image obtained by performing the sub-scanning multi-value scaling (enlargement) process of the present embodiment on the image of FIG. The ideal enlarged image in FIG. 13B cannot actually change the resolution in the sub-scanning direction. However, if the image data in FIG. 13C is a latent image, the ideal image in FIG. An image close to general enlargement can be obtained.

  As described above, in this embodiment, when an image is formed on the back side of a sheet, a binary image including a halftone-processed region and a character / thin line region is subjected to multivalue scaling in the sub-scanning direction. At the time of binarization, the binarization method is changed between the character / thin line area and the halftone dot area. As a result, the sub-scanning multi-value scaling process with little image quality degradation can be performed without being affected by environmental changes such as temperature. As a result, when performing double-sided printing, images of approximately the same magnification are formed on both sides of the sheet without being affected by the expansion and contraction of the sheet that occurs when fixing the toner image on the sheet surface side, and without reducing productivity. can do.

(Second Embodiment)
Next, an image forming apparatus according to the second embodiment of the present invention will be described with reference to FIGS. In the present embodiment, only the portions that differ from the first embodiment will be described.

  FIG. 14 is a flowchart for explaining an example of the binarization processing of the dot area in step S1307 of FIG.

  14 is the same as the process of FIG. 11 except that the processes after step S1507 are different. That is, in this embodiment, after the binarization process (step S1507) when the halftone is the lower edge of the halftone dot (step S1507), error propagation in the sub-scanning direction is not performed in step S1510. In this way, the halftone binarization of the lower edge of the halftone dot is performed in consideration of the binarization error of the halftone of the upper edge of the halftone dot. Since it is not propagated in the scanning direction, useless texture is not generated. For this reason, the binarization process which does not destroy the shape of a halftone dot more can be performed.

  FIG. 15A shows a halftone image original image, and FIG. 15B shows a conventional sub-scanning multi-value scaling process applied to the halftone image of FIG. 15A. FIG. 15C is a diagram in which the halftone image of FIG. 15A is subjected to the sub-scanning multi-value scaling process of the present embodiment. From the comparison of FIG. 15, it can be seen that the sub-scanning multi-value scaling process with little image quality degradation can be performed in this embodiment. Other configurations and operational effects are the same as those of the first embodiment.

700 Scanner unit 701 Printer unit 708 Photoconductor 800 Light emitting unit 802 Polygon mirror 805 Deflection mirror 909 CPU
916 CPU

Claims (3)

An image forming apparatus that performs main scanning by polarizing light from a light emitting unit and scans in a sub-scanning direction by rotating a photosensitive member that emits polarized light to form a latent image on the photosensitive member.
Sub-scanning multi-value scaling means for multi-value scaling a binary image having a halftone-processed area and other areas in the sub-scanning direction;
The image obtained by the sub-scanning multi-value scaling means is converted into an N-value (N is a natural number) by diffusing an error in the multi-value scaling process in the half-scanning region in the halftone process. An image forming apparatus comprising: an N-value converting unit that diffuses an error in the multi-value scaling process in the main scanning direction to form an N-value in other areas.   The N-value conversion unit determines whether the halftone-processed area or any other area based on area determination data for distinguishing the halftone-processed area from other areas. The image forming apparatus according to claim 1.   3. The image formation according to claim 1, wherein the sub-scanning multi-value scaling unit multi-scales the binary image when forming an image on the back side of the sheet by duplex printing. apparatus.

Images