JP2019128484A - Image forming apparatus and method for controlling the same, and program - Google Patents

Abstract

To provide a technology to prevent a reduction in fixability of a toner image while reducing a processing load for detecting an isolated dot in an input image.SOLUTION: An image forming apparatus acquires, through dot analysis processing, the pixel value of a pixel array on a search path starting from a pixel at a dot growth start point (first pixel) to a pixel at a growth convergence point (second pixel) in an input image (S106). The image forming apparatus detects an isolated dot that is formed on the basis of the input image and not coupled to the other dots by using the pixel value of the pixel array acquired through the dot analysis processing (S107). The image forming apparatus controls a fixing temperature in formation of the input image on the basis a result of detection of the isolated dot.SELECTED DRAWING: Figure 6

Description

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

  In an electrophotographic image forming apparatus, generally, a toner image transferred to a sheet is heated and pressed to fix the toner image on the sheet. The image forming apparatus needs to control the fixing temperature of the fixing device so that the fixing device supplies a sufficient amount of heat for fixing the toner image. On the other hand, image forming apparatuses are required to perform fixing processing with a minimum amount of heat for energy saving control. Patent Document 1 describes an image forming apparatus that controls a fixing temperature based on the content of an image to be formed. Fixing is performed according to the size of a region in which pixels having pixel values equal to or greater than a predetermined value are continuous. The temperature is controlled.

  Further, in the image forming apparatus, in order to properly control the fixing temperature, it is also necessary to consider the shape of the image formed in the print image. In particular, if halftone dots representing low density portions of the image are formed, it may be necessary to increase the fixing temperature in order to improve the fixability of the image on the sheet. Patent Document 2 describes a halftone dot determination apparatus which detects halftone dots by pattern matching with reference to a pixel of interest and pixels in the vicinity thereof in an input image.

JP, 2014-074894, A JP 2017-85238 A

  In the image forming apparatus described above, when a toner image occupying a small area such as an isolated halftone dot (small area) is formed, if the fixing process is performed at the minimum fixing temperature for energy saving control, The fixability of the toner image may be reduced. Therefore, it is necessary to analyze the input image to detect such isolated halftone dots and control the fixing temperature. However, in the above-mentioned prior art, it is necessary to carry out processing with a very large load such as pattern matching in order to detect halftone dots.

  The present invention has been made in view of the above problems. SUMMARY OF THE INVENTION An object of the present invention is to provide a technique for preventing a toner image fixability from being lowered while reducing a processing load for detecting an isolated halftone dot in an input image.

  An image forming apparatus according to an aspect of the present invention is an acquisition unit that acquires pixel values of pixel columns on a path from a first pixel to a second pixel in an input image, wherein the first pixel is the input image. In the halftone process that forms a halftone dot by comparing the pixel value of each pixel and the threshold value corresponding to each pixel, the second threshold is a pixel to which the lowest threshold is applied. A pixel to which the highest threshold is applied, and each threshold is set to increase along the path, combined with the acquisition means and other halftone dots formed based on the input image A detection unit that detects an isolated halftone dot using a pixel value of the pixel row acquired by the acquisition unit, and a fixing temperature in image formation of the input image based on a detection result by the detection unit. Control Characterized in that it comprises a control means.

  According to the present invention, it is possible to prevent the fixing property of the toner image from being lowered while reducing the processing load for detecting isolated halftone dots in the input image.

FIG. 1 is a block diagram and a cross-sectional view showing an example of a hardware configuration of an image forming apparatus. FIG. 6 is a view showing an example of a threshold value table used for halftone processing of an input image. The figure which shows the example of application of the threshold value table to an input image. The figure which shows the example of the printing image obtained by printing the input image which has uniform density. The figure which shows the example of the search path | pass from the pixel of the growth start point of a halftone dot to the pixel of a growth convergence point. 5 is a flowchart showing the procedure of halftone dot analysis processing. FIG. 6 is a view showing an example of a part of a print image obtained by printing an input image. FIG. 7 is a view showing an example of control of a fixing temperature of the fixing device. The figure which shows the example of the read-out process of the data of an input image. The figure which shows the example of the read-out process of the data of an input image. The figure which shows the correspondence table of the bit string on a search path, and a density value and a feature value. 5 is a flowchart showing the procedure of halftone dot analysis processing.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following embodiments do not limit the invention according to the claims, and all combinations of features described in the embodiments are not necessarily essential to the solution means of the invention.

First Embodiment
<Configuration of Image Forming Apparatus>
FIG. 1A is a block diagram showing an example of the hardware configuration of the image forming apparatus 100 according to the first embodiment. The image forming apparatus 100 may be an image forming apparatus that forms a single-color image, but here, a multicolor image is formed with a predetermined resolution (for example, 600 dpi) using a plurality of colors of toner (developer). An image forming apparatus is assumed. The image forming apparatus 100 may be, for example, any of a printing apparatus, a printer, a copier, a multifunction peripheral (MFP), and a facsimile apparatus.

  The image forming apparatus 100 includes a control unit (controller) 120, a printer unit 107 connected to the control unit 120, an operation unit 109, a wireless communication unit 111, and a wired communication unit 113. The control unit 120 realizes various functions such as a printing function, a wired function, or a communication function in the image forming apparatus 100 by controlling each device connected to the control unit 120. The control unit 120 includes, as devices connected to the system bus 105, a CPU 101, a RAM 102, a ROM 103, a hard disk drive (HDD) 104, a printer I / F 106, an operation unit I / F 108, a wireless communication I / F 110, and a wired communication I / F 112. And an image processing unit 114.

  The CPU 101 controls the overall operation of the image forming apparatus 100 by controlling each device connected via the system bus 105. The RAM 102 is used as a main memory or a work area of the CPU 101, or a temporary storage area such as an image development area. The RAM 102 is also used as a reception buffer for data received from an external device. The ROM 103 stores various programs such as a control program of the image forming apparatus 100 and an image generation program. The CPU 101 executes various processes for controlling the operation of the image forming apparatus 100 by reading out the program stored in the ROM 103 to the RAM 102 and executing the program. The HDD 104 is a nonvolatile storage device and stores various programs and various data such as setting data, print data, and image data. As the non-volatile storage device, for example, an SSD may be used instead of the HDD.

  In the present embodiment, an example in which one CPU 101 executes each process described later is described, but the present invention is not limited thereto. For example, the image forming apparatus 100 can be configured such that a plurality of CPUs cooperate to execute each process.

  The printer unit 107 performs an image forming process (print process) by electrophotography as described later with reference to FIG. 1B based on the image data output from the control unit 120 through the printer I / F 106. Run. The image forming apparatus 100 can execute print processing by the printer unit 107 based on print data received by the wireless communication unit 111 or the wired communication unit 113 from an external apparatus. The operation unit 109 includes a liquid crystal display unit having a touch panel function and a keyboard, and displays various screens. The user can input an instruction and information to the image forming apparatus 100 via the operation unit 109.

  The wireless communication unit 111 is a communication chip for realizing wireless communication with an external device, and executes wireless communication based on a wireless LAN standard (IEEE802.11a / b / g / n / ac, etc.), for example. The wireless communication I / F 110 controls the wireless communication unit 111 to realize wireless communication with an external device such as an external access point or a mobile terminal. The wired communication unit 113 is a communication chip that is connected to a wired LAN and realizes communication via the wired LAN. The wired communication I / F 112 controls the wired communication unit 113 to realize communication with an external device such as a PC terminal via the wired LAN.

  The image processing unit 114 performs various types of image processing on the image data stored in the image expansion area of the RAM 102. For example, the image processing unit 114 generates a laser drive signal for driving a laser light source provided in the exposure device 203 (FIG. 1B) of the printer unit 107 based on image data of an input image to be printed. , Output to the printer unit 107.

  FIG. 1B is a cross-sectional view showing an example of the hardware configuration of the image forming apparatus 100, and more particularly shows an example of the configuration of the printer unit 107. As shown in FIG. The image forming apparatus 100 includes four image forming stations that form toner images using toners of four colors of yellow (Y), magenta (M), cyan (C), and black (K). Yes. In FIG. 1B, reference numerals are given only to the components of the station of Y color, but all the four stations can adopt the same configuration. Each station functions as an image forming unit that forms an image using toner on an image carrier such as the photosensitive drum 201 and the intermediate transfer belt 207.

  A photosensitive drum (photosensitive member) 201 is a cylindrical image carrier that carries an electrostatic latent image and a toner image, and rotates in the direction of the arrow shown in FIG. 1B. A charger 202, an exposure device 203, a developing device 204, and a cleaning device 205 are disposed to face the outer peripheral surface of the photosensitive drum 201. Further, a transfer roller 206 is disposed at a position facing the photosensitive drum 201 via the intermediate transfer belt 207.

  The charger 202 uniformly charges the rotating photosensitive drum 201. The exposure device 203 outputs a laser beam (light beam) modulated based on image data (image information), and scans the surface of the photosensitive drum 201 with the laser beam. Thus, an electrostatic latent image is formed on the photosensitive drum 201. The developing device 204 develops the electrostatic latent image using toner and forms a toner image on the photosensitive drum 201. The transfer roller 206 transfers the toner image on the photosensitive drum 201 to the intermediate transfer belt 207. The toner remaining on the surface of the photosensitive drum 201 after the transfer of the toner image to the intermediate transfer belt 207 is removed by the cleaning device 205.

  The intermediate transfer belt 207 rotates in the direction of the arrow shown in FIG. The toner image on the intermediate transfer belt 207 is conveyed to a secondary transfer unit formed by the intermediate transfer belt 207 and the secondary transfer slurer 212. In the meantime, the toner images of the respective colors formed on the photosensitive drum 201 of each station are sequentially superimposed and transferred onto the intermediate transfer belt 207, so that a multicolor toner image is formed on the intermediate transfer belt 207.

  The sheets in the sheet feeding cassette 220 are fed to the sheet conveyance path by the sheet feeding roller. The sheet may be referred to as recording paper, recording material, recording medium, paper, transfer material, transfer paper or the like. The sheet fed to the sheet conveyance path is conveyed to the secondary transfer unit. At the secondary transfer portion, the toner image conveyed by the intermediate transfer belt 207 is transferred to the sheet. The fixing device 230 applies heat and pressure to the toner image to fix it on the sheet. Thereafter, the sheet on which the toner image is fixed is discharged to the discharge tray 240.

  In the present embodiment, the CPU 101 can control the amount of heat that the fixing device 230 supplies to the toner (toner image) on the sheet by controlling the temperature (fixing temperature) of the fixing device 230. The CPU 101 can realize energy saving control (energy saving control) of the image forming apparatus 100 by controlling the fixing device 230 so as to perform the fixing process with the minimum amount of heat to reduce power consumption.

Fixability of toner image
In the electrophotographic image forming apparatus, as described above, after transferring the toner image to the sheet, the transferred toner image is fixed to the sheet. The fixability of the toner image on the sheet after the fixing process depends on the number of toner particles constituting the toner image.

  In the fixing process, the heated and melted toner particles bond to each other. The greater the number of toner particles bound, the greater the cohesion. Also, the adhesion of the toner to the sheet depends on the cohesion of the toner particles. For this reason, toner images such as characters and lines formed of a large number of pixels are fixed to the sheet with a large adhesive force. On the other hand, a small and isolated toner image composed of several pixels to several tens of pixels (for example, a halftone dot representing a low density portion of the image) has a small number of toner particles constituting the toner image. It has weak adhesion and is easy to peel off from the sheet.

  In order to increase the adhesion of the toner to the sheet, it is necessary to increase the amount of heat supplied to the toner in the fixing process to increase the degree of melting of the toner. However, in the image forming apparatus that performs energy saving control, as described above, the fixing process can be performed with a minimum amount of heat. In this case, the fixability of the toner image that occupies a small area such as halftone dots (small area) may be reduced. The deterioration of the fixing property becomes remarkable particularly in a halftone dot corresponding to low density expression where the degree of isolation of the toner image is high. For this reason, in this embodiment, as will be described later, when a halftone dot corresponding to low density expression is included in the printed image more than a specified area, the fixing temperature is increased in order to improve the fixability of the toner image on the sheet. The fixing device 230 is controlled to make it

<Detecting isolated dots>
The detection of isolated dots contained in the input image to be printed generally requires a very large computational load to search for the position and size of the isolated dots. On the other hand, in the present embodiment, isolated dots are detected based on halftone dots included in the input image to be printed. The image data of the input image generated on the RAM 102 (image development area) by the CPU 101 for intermediate density representation is used for detection of isolated dots. The halftone dot phase and the growth start point included in the input image correspond to isolated dots that can be detected by the CPU 101. For this reason, in the present embodiment, it is possible to detect an isolated dot with less calculation load compared to general detection of an isolated dot.

<Gradation expression by halftone dots>
The gradation of the image to be displayed in a general computer corresponds to the light emission amount of the display device, and black is often expressed by the minimum gradation value. On the other hand, the gradation of the image to be printed in the image forming apparatus corresponds to the amount of toner, and the minimum gradation value represents a plain pixel (without toner adhesion). In the present embodiment, the image density in the image forming apparatus 100 is expressed by an 8-bit gradation value, that is, by a value in the range of 0 to 255.

  In gradation expression using halftone dots, an image is divided into a large number of small areas, and the density of each small area is expressed by the ratio of the area of the drawing pixel to the area of the non-drawing pixel. Each small area is composed of a plurality of pixels. The determination of whether each pixel in the input image to be printed is drawn (the toner is attached) is determined using the threshold value table corresponding to the small area, the tone value (density value) of each pixel, in the threshold value table It does this by comparing it to the corresponding threshold. For example, when printing an image having a uniform density, the higher the density, the more pixels having a density value equal to or higher than the threshold value, and the area of the drawn region increases.

  FIG. 2 is a diagram showing an example of a threshold value table corresponding to the above-described small area. In this example, the threshold value table corresponds to a small area having an L-shape. The reason why the L-shaped shape is adopted is that the lowest threshold value and the highest threshold value included in the threshold value table can be arranged in the vicinity. In the threshold table of FIG. 2, the threshold indicated by the white circle indicates the highest threshold, and the threshold indicated by the black circle indicates the lowest threshold.

  FIG. 3 shows an example of an input image in which small areas corresponding to the threshold table in FIG. 2 are arranged without gaps so that tiles are arranged. By the arrangement of the small areas corresponding to such a threshold value table, halftone dots (tilted) based on the L-shaped shape of the small areas are formed on the print image. When the density value of the corresponding pixel to be compared with each threshold value in the threshold value table is equal to or greater than the threshold value, the pixel is drawn. When the density value is less than the threshold value, the pixel is not drawn. For this reason, a pixel corresponding to a high threshold value is drawn when it has a high density value. Note that pixels shown by white circles and black circles in FIG. 3 correspond to pixels to which the highest threshold and the lowest threshold in the threshold table of FIG. 2 are respectively applied.

  FIG. 4 is a view showing an example of a print image obtained by printing an input image having uniform density. FIG. 4 shows a print image 400 corresponding to an input image in which each pixel has a density value 90, and a print image 410 corresponding to an input image in which each pixel has a density value 150. In FIG. 4, shaded pixels correspond to pixels to be drawn (toner adheres), and pixels indicated by white circles and black circles apply the highest threshold value and the lowest threshold value in the threshold table of FIG. 2, respectively. Corresponds to the pixel that has been

  The density value 90 exceeds the thresholds 1, 14, 26, 39, 52, 64, 75, 88 included in the threshold table of FIG. For this reason, in the print image 400, pixels corresponding to these threshold values are drawn. A dot pattern as shown in FIG. 4 is formed by the drawn pixels. In the printed image 400, since each halftone dot is independent of each other, each halftone dot is in a state where the fixing property to the sheet is weak.

  Further, the density value 150 exceeds the threshold values 100, 115, 128, and 141 in addition to the above-described threshold values included in the threshold value table of FIG. Therefore, in the print image 410, in addition to the pixels drawn in the print image 400, pixels corresponding to these threshold values are drawn. A dot pattern as shown in FIG. 4 is formed by the drawn pixels. In the printed image 410, each halftone dot is coupled to an adjacent halftone dot to form a larger toner image. For this reason, the fixability of the toner image on the sheet is higher than that of the print image 400 and is more stable.

<Analytical dot analysis process>
In the print image corresponding to the input image, halftone dots grow from the pixel corresponding to the lowest threshold value indicated by black dots in FIGS. 2 to 4 as the start point, as the density value of the input image increases. When the density value of the input image finally increases to the maximum density value (255), pixels corresponding to the maximum threshold indicated by white circles in FIGS. 2 to 4 are drawn, and the formation of halftone dots converges. For this reason, in the following description, the coordinates of the pixel corresponding to the minimum threshold value indicated by the black circle in FIGS. 2 to 4 are referred to as the “growth start point (or start point)” of the halftone dot, and are indicated by the white circle. The coordinates of the pixel corresponding to the threshold are referred to as “growth convergence point (or convergence point)” of the halftone dot.

  Since the pixel corresponding to the growth start point in the input image has the lowest threshold to be compared, the pixel is drawn as long as the drawing rate is the highest and the density value is not zero. On the other hand, since the pixel corresponding to the growth convergence point in the input image has the highest threshold value to be compared, the drawing rate is the lowest, and drawing is not performed unless the density value is 244 or more.

  As shown in FIG. 2, the threshold value of the threshold value table is not randomly arranged, but is low at pixels on the growth start point side and high at pixels on the growth convergence point side, and corresponds to adjacent pixels. It is arrange | positioned so that the difference of the threshold to be set may not become large too much. Due to the arrangement of such threshold values, at least when forming an input image having a uniform density, as shown in FIG. 4, splitting or merging of halftone dots does not occur, and screening with the same number of lines ( Halftone processing) can be realized. As a result, a monotonically increasing gradation characteristic in which gradation jump is suppressed can be obtained.

  In the present embodiment, as shown in FIG. 5, the threshold value table is set so that the threshold value monotonously increases along the straight line connecting the growth start point and the growth convergence point of the halftone dot in the threshold value table. Hereinafter, a path from the growth start point to the growth convergence point is referred to as a “search path”. The threshold values are arranged along the search path such that the threshold value is lower the closer to the start point and the threshold value is higher the closer to the convergence point. For this reason, it is possible to obtain an approximate value of the density of the print image by scanning the pixel row on the search path connecting the growth start point of the halftone dot and the growth convergence point in the input image. For example, when an input image having a uniform density is drawn from the growth start point to 3 pixels on the search path, the range of density values of the print image is estimated to be 141 or more and less than 206. It is possible. In particular, the determination based on such a search path can be performed with high accuracy when an image having a density value of 0% or 100% is printed.

  Further, the shape of the halftone dot can be estimated by the determination based on the search path described above. For example, as shown in FIG. 4, adjacent dots are mutually connected when drawing from the growth start point to three pixels on the search path for an input image having uniform density (density value 150) Formed in a state. On the other hand, when up to two pixels are drawn (density value 90), adjacent halftone dots are formed in an independent state. Note that objects such as characters and ruled lines included in the input image often have a larger configuration than the search path. Therefore, for such an object, a small number of plain areas or solid areas are randomly detected on the search path, and the halftone area has different characteristics.

<Analytical dot analysis procedure>
In the image forming apparatus 100 according to the present embodiment, at least one growth start point coordinate and a search path from a growth start point pixel (first pixel) to a growth convergence point pixel (second pixel) are determined in advance. , Are stored in the ROM 103 or the HDD 104. Since the screen line number and angle are known information, it is possible to acquire another growth start point from the coordinates of one growth start point in the input image.

  The process of searching all pixels of the input image for analysis of halftone dots requires a high computational load. On the other hand, in the present embodiment, first, scanning (searching) is discretely performed only on the growth start point. As a result, the amount of access to a storage device (such as the RAM 102) for referring to the pixels of the input image is significantly reduced (for example, to a few tenths of times), that is, the arithmetic load is significantly reduced. Enable.

  An area where pixels are not drawn at the growth start point corresponds to a plain area or a background portion of an area where an object such as a character or a ruled line is drawn. This area does not have to be considered for temperature control of the fixing device 230.

  On the other hand, a region where pixels are drawn at the growth start point is a halftone dot region where a halftone dot is formed or a solid region where a solid image is formed. In the case of a halftone dot region, the CPU 101 searches for pixels on the search path in order to determine whether the halftone dots are small halftone dots that are not connected to each other. The CPU 101 continues the search until non-drawing pixels are generated at the pixels on the search path or the growth convergence point is reached. In this way, by limiting the search of pixels for halftone analysis to the search path, even if the entire input image is configured in a halftone area, it is possible to refer to the pixels of the input image. It becomes possible to reduce the access amount to the storage device (for example, to a fraction).

  When all the pixels up to the growth convergence point on the search path are drawn, the CPU 101 determines that the area to be analyzed is a solid area. The solid area need not be considered for temperature control of the fixing device 230. However, in order to enable temperature control of the fixing device 230 based on the applied amount of toner in the area, the CPU 101 accumulates the density value of the toner image formed in the area.

  If a non-drawing pixel occurs before reaching the growth convergence point on the search path, the CPU 101 determines that the area to be analyzed is a halftone area. In the halftone dot area, halftone dots formed in the halftone dot area are formed based on the number of pixels from the growth start point until non-drawing pixels are generated on the search path for temperature control of the fixing device 230. It is determined whether the halftone dot is an isolated halftone dot or a halftone dot to be combined with an adjacent halftone dot. When the halftone dot formed in the halftone dot region is an isolated halftone dot, the CPU 101 controls the fixing device 230 so as to raise the fixing temperature above the target temperature at the time of energy saving control. Improve the fixability of halftone dots.

<Processing procedure>
FIG. 6 is a flowchart showing a procedure of the above-described analysis process executed in the image forming apparatus 100. The process of each step in FIG. 6 is realized in the image forming apparatus 100 by the CPU 101 reading out and executing a program stored in the ROM 103 or the HDD 104. FIG. 7 is a diagram illustrating an example of a part of a print image obtained by printing an input image. The print image of FIG. 7 has a size of 13 × 18 pixels as the size in the main scanning direction (X direction) and the sub scanning direction (Y direction) orthogonal to the main scanning direction. The size of the entire image is about 5000 × 7000 pixels when printing on an A4 size sheet at a resolution of 600 dpi, for example. In FIG. 7, the growth start point is indicated by a black circle, and the growth convergence point is indicated by a white circle.

  In the present embodiment, the CPU 101 divides the input image into one or more areas, and executes analysis processing according to the procedure of FIG. 6 for each of the divided areas. In the following, analysis processing for one specific area in an input image will be described. The CPU 101 sequentially scans the growth start points of the halftone dots in the region to be processed (target region) in the input image, searches for pixels on the search path from each growth start point, and starts each growth start point. The number of pixels until non-drawing pixels are detected on the search path is acquired. When a non-drawing pixel is detected, the CPU 101 updates the cumulative density value and the cumulative feature value for the target region based on the detection result. The cumulative density value is the sum of approximate density values of the images of the respective colors formed in the target area. The cumulative feature value is a sum of feature values corresponding to the shapes of halftone dots in the target area.

  First, in S <b> 101, as an initialization operation, the CPU 101 initializes the accumulated density value and the accumulated feature value for the target region to 0, and acquires the coordinates of the growth start point of the first search target from the ROM 103 or the HDD 104. In this example, the coordinates (0, 0) shown in FIG. 7 are acquired as the coordinates of the growth start point. In the example of FIG. 7, when the growth start point exists at the coordinates (x, y), the coordinates (x + 2, y + 4), (x-2, y-4), (x-4, y + 2) and (x + 4, y) -2) The growth starting point is regularly present. For this reason, if the coordinates of any one growth start point can be acquired, the coordinates of all other growth start points can be grasped.

  Next, in step S102, the CPU 101 initializes the Y coordinate (coordinate in the Y direction) indicating the scan line to be scanned in the input image to 0, and advances the process to step S103. In S103, the CPU 101 acquires the coordinates of the top growth start point in the X direction among the growth start points present in the current Y coordinate. Note that the coordinates (0, 0) described above are acquired at the start of analysis processing. The coordinates of the top growth start point are held to specify the coordinates of the top growth start point in the X direction on the next scan line after the Y coordinate is updated. Next, in S104, the CPU 101 acquires the pixel value of the pixel at the growth start point from the input image based on the coordinate information indicating the current X coordinate and Y coordinate, and advances the process to S105.

  In step S <b> 105, the CPU 101 determines whether a pixel at the growth start point is drawn by comparison with the threshold values included in the threshold value table of FIG. 2. When the pixel at the growth start point is a non-drawing pixel not to be drawn, the process proceeds to S109, and the CPU 101 determines that the small area including the growth start point is a solid area. In the example of FIG. 7, the growth start point present at the coordinates (8, 16) corresponds to such a growth start point. In this case, the CPU 101 advances the process to S111 and updates the X coordinate so that the process advances to the next growth start point in the X direction.

  On the other hand, if the pixel at the growth start point is a drawing pixel to be drawn in S105, the CPU 101 proceeds to S106. In step S106, the CPU 101 acquires, from the input image, the pixel values of the pixel array on the search path starting from the growth start point. In step S <b> 107, the CPU 101 determines the number of pixels to be continuously drawn on the search path from the growth start point based on the obtained pixel value of the pixel row and the threshold value table in FIG. It is determined whether the number is less than or equal to a predetermined number. In this example, the prescribed number is three.

  In step S107, when the number of drawn pixels is equal to or less than the specified number, the CPU 101 has a small dot corresponding to the growth start point (is not connected to another halftone dot) or connected. Even if it is determined that the degree of coupling is weak. In this case, in step S108, the CPU 101 determines a feature value indicating the degree of isolation of the halftone dot corresponding to the shape of the halftone dot, and adds it to the accumulated feature value. The feature value is determined in accordance with the degree of isolation of the halftone dot, and is a feature value indicating the area of the halftone dot corresponding to the pixel of the growth start point. This feature value is the maximum value when the halftone dots are isolated, and is zero when the halftone dots are firmly connected to other halftone dots or when the halftone dots correspond to a solid image. Determined. The feature value is smaller than the maximum value if the degree of coupling is weak even if the halftone dot is coupled to another halftone dot. That is, the lower the halftone density, the larger the feature value, and the higher the halftone density, the smaller the feature value.

  For example, when the number of consecutive drawing pixels on the search path is 2, the feature value is determined as the maximum value, and the determined feature value is added to the accumulated feature value. In the example of FIG. 7, the halftone dots corresponding to the growth start points present at the coordinates (6, 12), (2, 14), etc. are examples of such halftone dots. Further, for example, when the number of consecutive drawing pixels on the search path is 3, since the halftone dots are connected to each other but the degree of the connection is weak, the feature value is determined to be smaller than the maximum value, The determined feature value is added to the cumulative feature value. In the example of FIG. 7, a halftone dot corresponding to the growth start point present at coordinates (4, 8), (8, 6), etc. is an example of such a halftone dot.

  After the process of S108, the CPU 101 advances the process to S110. In addition, when the number of consecutive drawing pixels on the search path exceeds the specified number (“NO” in S107), the CPU 101 combines the halftone dots corresponding to the growth start points with the adjacent halftone dots. It is determined that there is, and the process proceeds to S110. In S110, the CPU 101 adds an approximate density value based on the number of consecutive drawing pixels on the search path to the accumulated density value, and advances the process to S111. In the example of FIG. 7, a halftone dot corresponding to the growth start point existing at the coordinates (6, 2) (halftone dot whose drawing pixel column is 4) is an example of such a halftone dot. Note that, in the halftone dot corresponding to the growth start point existing at the coordinates (0, 0), the pixels up to the growth convergence point existing at the coordinates (3, 1) on the search path are drawn. It is determined that the included small area corresponds to a solid image.

  In S111, the CPU 101 updates the X coordinate so that the process proceeds to the next growth start point in the X direction in the current Y coordinate (X = X + ΔX). In the example of FIG. 7, since the next growth start point in the same Y coordinate exists at the coordinates (x + 2 + 4 + 4, y + 4-2-2) with respect to the growth start point at the coordinates (x, y), ΔX = 10 It is determined. Next, in step S112, the CPU 101 determines whether the updated X coordinate is a coordinate outside the target area. If the updated X coordinate is not a coordinate outside the target area, the CPU 101 returns the process to S104. If the updated X coordinate is a coordinate outside the target area, the CPU 101 proceeds to S113, and the Y coordinate. Update (increase by 1). Thereafter, in S114, the CPU 101 determines whether or not scanning has been completed for all the Y coordinates in the target area. If scanning has not been completed, the process returns to S103, and if scanning has been completed. Ends the process.

  It should be noted that in the generally used screen line number and angle, there is a growth start point on all the Y coordinates. As an exception, when a halftone dot having an integral multiple of screen line number can exist, the growth start point There is a Y coordinate that does not exist. For example, in the case of an image forming apparatus which performs printing at a resolution of 600 dpi, halftone dots of 268 lines of a double cycle can be established with respect to halftone dots of 134 lines, and every two growth start points are on the Y coordinate. Only exists. FIG. 7 shows such an example, in which the growth start points exist only at the even-numbered Y coordinates. When there is no halftone dot growth start point at some of these Y coordinates, it is possible to increase the processing speed by skipping scanning of the Y coordinate where no growth start point exists.

<Fixing temperature control of fixing device>
In the image forming apparatus 100 of this embodiment, the CPU 101 controls the fixing temperature of the fixing device 230 in the image formation of the input image based on the detection result of the isolated halftone dot in the above-described halftone dot analysis process. Specifically, the CPU 101 increases the target temperature of the fixing temperature and increases the target temperature of the input image when the accumulated feature value exceeds a predetermined specified value in any one or more of the divided areas in the input image. Image formation is performed. For example, in the energy-saving control of the image forming apparatus 100, the CPU 101 sets the target temperature of the fixing temperature to the temperature T1, and changes the target temperature to a temperature T2 higher than the temperature T1 when the accumulated feature value exceeds a specified value. . As a result, the fixability of the isolated halftone dots, which are low density halftone dots, to the sheet is enhanced, and the toner forming the halftone dots is prevented from peeling from the sheet.

  Further, as shown in FIG. 8A, the CPU 101 may use a value obtained by dividing the accumulated density value by the accumulated feature value as the halftone dot size estimated value as an auxiliary index. In this case, the CPU 101 may continuously change the target temperature of the fixing temperature between the temperature T1 and the temperature T2 in accordance with the estimated size of the halftone dot. Thus, when a size estimation value indicating that the halftone dot is close to a solid image is obtained, a target temperature close to the temperature T1 is set. On the other hand, when a size estimate value is obtained which is coupled to another halftone dot but indicates that the coupling is weak, a target temperature close to the temperature T2 is set.

  Further, the CPU 101 prevents the amount of heat supplied from the fixing device 230 to the toner image from being insufficient with respect to the amount of applied toner when multiple color toner images are superimposed in the printing of a multicolor image. Control of the fixing temperature may be performed. Specifically, the CPU 101 accumulates the density values of the images of the respective colors. When the above-described accumulated density value exceeds a predetermined specified value, the CPU 101 increases the target temperature of the fixing temperature and forms an image of the input image. You may go. Thereby, when a multi-color image with a high amount of applied toner is formed, it is possible to improve its fixing property. Note that the CPU 101 may switch the target temperature of the fixing temperature from the temperature T1 to a temperature T3 higher than the temperature T1 in accordance with the accumulated density value (toner applied amount) as indicated by reference numeral 801 in FIG. 8B. . Alternatively, the CPU 101 may continuously change the target temperature of the fixing temperature from the temperature T1 to the temperature T3 according to the accumulated density value, as indicated by reference numeral 802 in FIG. 8B.

  As described above, in the present embodiment, the CPU 101 uses the pixel array on the search path from the halftone dot growth start point pixel (first pixel) to the growth convergence point pixel (second pixel) in the input image. The pixel value of is acquired by halftone dot analysis processing. The first pixel is a pixel to which the lowest threshold value is applied in halftone processing for forming a halftone dot by comparing the pixel value of each pixel of the input image with the threshold value corresponding to each pixel. The second pixel is a pixel to which the highest threshold is applied in halftone processing. Each threshold value used in the halftone process and included in the threshold value table of FIG. 2 is set so as to increase along the search path. The CPU 101 detects isolated halftone dots which are formed based on the input image and which are not combined with other halftone dots, using the pixel values of the pixel array acquired by the halftone dot analysis processing. Further, the CPU 101 controls the fixing temperature in the image formation of the input image based on the detection result of the isolated halftone dot. This makes it possible to prevent the fixing property of the toner image from being lowered while reducing the processing load for detecting isolated halftone dots in the input image.

Second Embodiment
In the second embodiment, an image forming apparatus capable of more efficient hardware implementation than the first embodiment will be described. In addition, below, description is abbreviate | omitted about the part which is common in 1st Embodiment.

  The image forming apparatus 100 according to this embodiment has a 32-bit width as the width of the system bus 105. The CPU 101 executes an operation in units of bit strings of the width of the system bus 105 as in a general computer system. In general, in the input image for printing, the data order required by the printer unit 107 and the arrangement order of the pixels in the input image are matched, thereby saving hardware resources necessary for image processing. Yes. Also, the pixels that are simultaneously read and written are continuously arranged in the main scanning direction or the sub scanning direction in the input image, and are continuously used in the printer unit 107.

  The data of one pixel of the input image is composed of 1 to several bits, and one data exchange via the system bus 105 includes data of a plurality of pixels. For example, when 1 pixel data is 1 bit and the system bus 105 has a 16-bit width, 16 pixel data can be read simultaneously via the system bus 105. Note that when the pixel data is multi-value data or includes attribute information, the number of pixels read out simultaneously becomes smaller.

  FIG. 9 is a diagram showing an example of processing of an input image when the system bus 105 has a 32-bit width and one pixel is represented by 4 bits (16 values). In this case, the CPU 101 can simultaneously read 8-pixel data via the system bus 105. In multi-valued input images, pixels of light quantity that is not saturated in an electrophotographic image forming apparatus generally become unstable, which causes deterioration in print quality. However, since multi-value halftone drawing is effective to generate fine gradation adjustment and smooth contours, it is used in an input image for printing. In the processing of halftone dots, the portion where multi-value intermediate values are generated is limited, and is limited to the outer peripheral contour of the halftone dots. In a region where a halftone dot is formed instead of a solid image, the pixels on the search path are always saturated pixels on the growth start point side, the saturated pixels are continuous, and the growth convergence point is a non-drawing pixel. The intermediate value may be one pixel at the boundary between the non-drawing pixel and the saturated pixel, or none at all. Therefore, it is determined whether the pixel is a saturated pixel or not even if the pixel value is multi-valued.

  Therefore, when the bit data string indicating the pixel string on the search bus is “nnnnooopopppppqqqqrrrr” as shown in FIG. 10 (the bit string “zzzz” outside the search path is not relevant to the determination and is therefore excluded), pppp = In the case of 1111, in the halftone dot region, nnnn = 1111 and oooo = 1111 for the pixels on the growth start point side. When an intermediate value pixel is present at the boundary between the non-drawing pixel and the saturated pixel, the density value can be accurately estimated.

  Depending on the number of screen lines and the angle, as in the search path 901 in FIG. 9, it is possible to simultaneously read out pixel column data on the search path from the input image. In this case, the CPU 101 can determine the number of continuous drawing pixels on the search path by one memory access and one group of operations, and the processing speed can be increased.

  In the halftone dot having a tilt angle of 45 degrees as shown in FIG. 9, the growth start point and the growth convergence point are aligned on the same coordinate in the X direction (the same applies to the Y direction). For this reason, it is easy to arrange a search path for pixel columns that can be accessed simultaneously by the CPU 101, and it is easy to reduce the calculation amount of the CPU 101. However, even with such a halftone dot, depending on the phase, two memory accesses are required to read a pixel column on the search path, as in the search path 902. In general, the bus width and the dot period do not match, and therefore, multiple memory accesses are required to read a pixel string for many search paths.

  FIG. 11 is a diagram showing a correspondence table between bit strings on the search path and density values and feature values. As in the first embodiment, the density value is an approximate density value of an image corresponding to halftone dots, and the feature value is a feature value corresponding to the shape of halftone dots. In the figure, the maximum value 10 of the feature value corresponds to the isolated halftone dot, and corresponds to a low density halftone dot. As the dot density increases (dots grow), the feature value decreases to 5, 2, 0. Most of the pixels constituting each halftone dot are pixels having the maximum value (1111), and only a small number of pixels having intermediate values are present at halftone dot boundaries. Therefore, on the search path, normally, only the pixel having the maximum value (1111) and the non-drawing pixel are present, and the probability that the pixel having the intermediate value is present is low. When an intermediate value pixel is present on the search path, it is possible to accurately obtain the approximate density value, but as shown in FIG. 11, there is no change in the feature value. Note that the processing load is reduced by making the determination by binarizing each pixel instead of referring to 4 bits for the determination for one pixel.

  FIG. 12 is a flowchart showing the procedure of halftone dot analysis processing executed in the image forming apparatus 100 of the present embodiment. The process of each step in FIG. 12 is realized in the image forming apparatus 100 by the CPU 101 reading out and executing a program stored in the ROM 103 or the HDD 104.

  First, in step S <b> 201, the CPU 101 initializes the cumulative density value and the cumulative feature value for the target region to 0 as an initialization operation, and acquires the coordinates of the growth start point of the first search target from the ROM 103 or the HDD 104. Next, in step S202, the CPU 101 initializes the read address of the input image.

  After that, the CPU 101 reads the pixel string data from the read address in S203, and cuts out a bit pattern on the search path from the read pixel string data in S204. In step S205, the CPU 101 determines whether the bit pattern on the search path has been acquired. If the bit pattern has not been acquired, the CPU 101 adds a value to the read address in step S206, and the remaining bit patterns on the search path. To get. If the CPU 101 has acquired a bit pattern on the search path, the process proceeds to S207.

  In step S207, the CPU 101 acquires the density value and the feature value using the table of FIG. 11 corresponding to the search pattern based on the acquired bit pattern. Further, the CPU 101 adds the acquired feature value to the accumulated feature value in S208, and adds the acquired density value to the accumulated density value in S209.

  Thereafter, in 210, the CPU 101 determines whether or not the current read address indicates the outside of the scanning area of the input image. If the reading address does not indicate the outside of the scanning area, the process returns to S204, and the outside of the scanning area is detected. If it indicates, the process proceeds to S211. In S211, the CPU 101 updates the read address to the address of the scanning line where the next growth start point and growth convergence point exist. In step S212, the CPU 101 determines whether the scanning is completed based on whether there is still a scanning line where the growth start point and the growth convergence point exist. If the scan has not been completed, the CPU 101 returns the process to S203, and if the scan has been completed, the CPU 101 ends the process.

  According to the present embodiment, the necessary processing can be further speeded up, so that the same effects as the first embodiment can be obtained while enabling more efficient hardware implementation than the first embodiment. Is possible.

[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 storage medium, and one or more processors in a computer of the system or apparatus read and execute the program. Can also be realized. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

  100: Image forming apparatus, 120: Control unit (controller), 101: CPU, 102: RAM, 103: ROM, 104: HDD, 107: Printer unit, 109: Operation unit, 111: Wireless communication unit, 113: Wired communication Part 114: Image processing part

Claims (10)

Acquisition means for acquiring pixel values of pixel columns on a path from a first pixel to a second pixel in an input image, wherein the first pixel corresponds to the pixel value of each pixel of the input image and each pixel In a halftone process that forms a halftone dot by comparing with a threshold value, the pixel to which the lowest threshold value is applied, and the second pixel is a pixel to which the highest threshold value is applied in the halftone process, The acquisition means, wherein each threshold is set to increase along the path;
A detection unit that detects isolated halftone dots that are formed based on the input image and that are not combined with other halftone dots using the pixel values of the pixel row acquired by the acquisition unit;
Control means for controlling a fixing temperature in image formation of the input image based on a detection result by the detection means;
An image forming apparatus comprising: The detection means is a drawing pixel drawn using toner in the pixel row, and detects the isolated halftone dot based on the number of pixels of one or more continuous drawing pixels. The image forming apparatus according to claim 1. If the first pixel is a drawing pixel and the number of continuous drawing pixels in the pixel column is equal to or less than a predetermined number, the detection means detects the halftone dot corresponding to the first pixel as the isolated pixel. The image forming apparatus according to claim 2, wherein the image forming apparatus is detected as a halftone dot. The image forming apparatus according to claim 2, wherein the detection unit determines that a halftone dot is not formed in a region corresponding to the first pixel if the first pixel is not a drawing pixel. . The detection means determines a feature value indicating an area of a halftone dot corresponding to the first pixel in accordance with the number of consecutive drawing pixels in the pixel row, and detects the halftone dot with respect to the input image. Accumulate feature values for each
3. The control unit performs image formation of the input image by increasing a target temperature of the fixing temperature when the accumulated feature value accumulated by the detection unit exceeds a specified value. The image forming apparatus according to any one of 4. In the energy saving control of the image forming apparatus, the control unit sets the target temperature of the fixing temperature to a first temperature, and when the accumulated feature value exceeds the specified value, the target temperature is higher than the first temperature. The image forming apparatus according to claim 5, wherein the image forming apparatus is changed to a second temperature. The detection means may further estimate a density value indicating a density of an image formed in an area corresponding to the first pixel, in accordance with the number of consecutive drawing pixels in the pixel row. Item 7. The image forming apparatus according to any one of Items 2 to 6. When the cumulative density value obtained by accumulating the density values of the images of the respective colors estimated by the detection means exceeds a specified value, the control means increases the target temperature of the fixing temperature to form an image of the input image. The image forming apparatus according to claim 7, wherein the image forming apparatus is performed. A control method of the image forming apparatus,
It is an acquisition process of acquiring the pixel value of the pixel sequence on the path from the first pixel to the second pixel in the input image, wherein the first pixel corresponds to the pixel value of each pixel of the input image and each pixel. In a halftone process that forms a halftone dot by comparing with a threshold value, the pixel to which the lowest threshold value is applied, and the second pixel is a pixel to which the highest threshold value is applied in the halftone process, The acquisition step wherein a threshold is set to increase along the path;
A detection step of detecting isolated halftone dots which are formed based on the input image and which are not combined with other halftone dots, using the pixel values of the pixel array acquired in the acquisition step;
A control step of controlling a fixing temperature in image formation of the input image based on a detection result in the detection step;
And controlling the image forming apparatus.   A program for causing a computer to execute each step of the control method for an image forming apparatus according to claim 9.