JP4366445B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that stabilizes image quality when the resolution of an image forming pattern is switched.

  Some recent copiers are connected to a network together with a printer or the like and used as an MFP (Multi Function Printer). In such an environment, color matching of images to be printed between devices connected to a network, or color of an image displayed on a display device such as a CRT and a color of a printed image are often performed. . Various color management techniques for this purpose are known.

  For example, in color management using an ICC (International Color Consortium) profile, calibration (color matching, also referred to as characterization) is performed by creating an ICC profile unique to an apparatus such as a printer or a copier. Using this, for example, color conversion is performed by a personal computer (PC) to create print data, which is output to an apparatus corresponding to the profile, and is represented by a color of an image to be printed and a display unit. Match the color of the image.

  Even for general users, software for creating a profile and a colorimeter are commercially available, so an environment for matching a color output from an image forming apparatus such as a printer with a target color is being prepared. As another calibration, the color conversion by the multi-dimensional LUT of the ICC profile is not used, and the calibration for obtaining the desired gradation characteristics by changing the content of the gamma LUT related to the gradation is also performed.

  As described above, color management is an effective technique in that it can suppress a difference in output colors between a plurality of apparatuses of the same model or between different models, and its application range is not limited to the above. For example, the present invention is also applied to a case where the printer is used for color calibration of printing by matching the color printed by the printer with the color printed by the offset printer. If the ICC profiles of the printing press and the printer are prepared, for example, color management as shown in FIG. 22 can be performed on the PC application.

  As shown in FIG. 22, the contents of the ICC profile 51 for printing and the ICC profile 52 for printer are based on the color measurement of the patch using the colorimeter, respectively, and are independent of the color space, for example, CIE L * A * b * is calibrated in association with the color space, so that the color printed by the printing press can be matched with the color printed by the printer. The color management module (CMM) 53 can create print data by performing color conversion using these profiles.

  As described above, since the color management environment such as the colorimeter, application, and profile creation software is in place, the electrophotographic image forming apparatus can be used for color proofing of the printing press as described above. It is spreading to the center.

  On the other hand, for color adjustment on the copier engine side, a monochrome gradation patch as described in Patent Document 1 is output, the density is calculated by a reader unit, and a desired target (density linearity, brightness linearity, etc.) is output. ) To create a one-dimensional LUT (tone correction table).

  Further, as described in Patent Document 2 and Patent Document 3, a patch image is formed on the transfer body, the amount (concentration) of toner is converted by a regular reflection sensor, and fed back to the LUT or ATR, and the user's hand. The color stability was maintained without trouble.

  On the other hand, compared with the offset print image quality, the difference in resolution appears clearly. The resolution of the CTP for creating the offset printing plate is at least 2400 dpi. On the other hand, the resolution of the electrophotographic printer is 600 dpi on average. For this reason, problems such as characters that are small points, character proportions, and families that are variations of typefaces cannot be formed.

  Further, when a dither matrix is assembled at 600 dpi, it is difficult to form a screen equivalent to printing because there are few control points.

  In this way, conventional color copiers have many problems due to resolution, and while users have a need for higher resolutions, color copiers are not used as color comps or proofers every time. Also used as That is, in order to increase the resolution, there are problems common to all users, such as an increase in the cost of the main body and an increase in processing time, and the resolution cannot be generally increased.

  In response to such problems, proposals have been made to meet the needs of both parties by switching the resolution without increasing the cost as much as possible.

  According to Patent Document 4, the dot density of the electrostatic latent image formed on the photosensitive drum can be changed. For this purpose, the frequency of the laser beam, that is, the frequency of the laser power modulation signal and the rotation speed of the polygon mirror are changed. It has become.

  Also, according to Patent Document 5, when performing pulse width modulation of a laser beam, high resolution is achieved by controlling the pulse rise time according to the pulse width and adjusting the dot interval.

  Further, according to Patent Document 6, the resolution is switched using a so-called twin beam laser.

  Patent Document 7 discloses an invention in which the process speed is halved between 600 dpi and 1200 dpi, and can be switched between a user who does not want high resolution and desires a printing speed and a user who likes high-definition images. It was made.

Japanese Patent Laid-Open No. 11-75067 JP 2002-72577 A JP 2002-72574 A JP 59-221165 A JP 60-7263 A JP 60-182868 A JP 2000-181275 A

  However, there is a problem that the output image becomes unstable due to the high resolution.

  For example, considering the current optical technology and cost, a laser spot diameter of about 35 μm is the limit. On the other hand, at 600 dpi and 1200 dpi, one pixel is about 42.3 μm and about 21.1 μm, respectively, and the former has a smaller laser spot diameter, while the latter has a larger laser spot diameter. A sharp and deep latent image cannot be formed, and unstable elements are generated due to overlapping of the bottom portions of the latent images. Further, in other processes, high-definition development technology, high transfer efficiency technology, and the like are necessary, and the stability is different between low resolution and high resolution.

  Furthermore, not only the basic resolution but also the screen line number, which is an image formation pattern at the time of area gradation, greatly affects the stability, and the stability cannot be ensured as the number of lines increases from the stability of the latent image described above. I was in

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an image forming apparatus capable of efficiently stabilizing image quality even at low resolution and at high resolution.

  According to the first aspect of the present invention, an image forming unit that forms a toner image on a photoconductor using a predetermined gradation reproduction table, and a toner image formed on the photoconductor by the image forming unit is transferred to a recording material. Image quality stabilization processing is performed by correcting the predetermined tone reproduction table based on the output of the transfer means, the density detection means for detecting the density of the patch image formed by the image forming means, and the density detection means. Image quality stabilizing means to be performed, and determination means for determining the resolution of the image forming pattern when an image forming pattern corresponding to the bitmap image is present in the image to be formed on the recording material. As the patch image is formed by the means, the frequency of executing the image quality stabilization process by the image quality stabilization means is increased at the time of high resolution than at the time of low resolution. Characterized in that the Gosuru.

  According to a second aspect of the present invention, an image forming unit that forms a toner image on a photoconductor using a predetermined gradation reproduction table, and a toner image formed on the photoconductor by the image forming unit is transferred to a recording material. Image quality stabilization processing is performed by correcting the predetermined tone reproduction table based on the output of the transfer means, the density detection means for detecting the density of the patch image formed by the image forming means, and the density detection means. Image quality stabilizing means to perform, and determination means for determining the resolution of an image forming pattern when an image forming pattern corresponding to a photographic image is present in an image to be formed on a recording material, the image forming means The frequency of executing the image quality stabilization process by the image quality stabilization means in association with the formation of the patch image is controlled so as to increase at the time of high resolution than at the time of low resolution. And wherein the door.

  According to the present invention, since the frequency of executing the image quality stabilization process is increased at the time of high resolution than at the time of low resolution, it is possible to efficiently stabilize the image quality at the time of high resolution as well as at the time of low resolution.

6 is a conceptual diagram showing a latent image profile at 600 dpi in Reference Example 1. FIG. It is a conceptual diagram which shows the latent image profile at the time of 1200 dpi in the reference example 1. FIG. 6 is a conceptual diagram illustrating a toner image at 1200 dpi in Reference Example 1. FIG. 6 is a conceptual diagram illustrating a toner image at 600 dpi in Reference Example 1. FIG. FIG. 6 is a diagram illustrating a relationship between the number of continuous image formation (output number) at 600 dpi and stability in Reference Example 1. FIG. 6 is a diagram illustrating a relationship between the number of continuous image formation (output number) at 1200 dpi and stability in Reference Example 1. It is a figure which shows the continuous stability output result (color reproduction index | exponent) in the reference example 1. FIG. 1 is a longitudinal sectional view illustrating a schematic configuration of an image forming apparatus. It is a figure which shows typically schematic structure of a density | concentration detection sensor. It is a figure which shows a real-time gradation correction table. It is a block diagram which shows the structure of the image process part in the reference example 1. FIG. 10 is a flowchart showing a control flow in Reference Example 1. It is a conceptual diagram which shows the latent image profile in the number of lines in the reference example 2. FIG. 10 is a flowchart showing a flow of control in Reference Example 2. FIG. 3 is a block diagram illustrating a configuration of an image processing unit according to the first embodiment. FIG. 3 is a user-specified conceptual diagram of an image formation pattern in Embodiment 1. 6 is a diagram illustrating a PDL description of a bitmap image in Embodiment 1. FIG. 6 is an explanatory diagram of a structure of a bitmap image according to Embodiment 1. FIG. 6 is a diagram illustrating a PDL description of a text image in Embodiment 1. FIG. 6 is a diagram illustrating a PDL description of a vector image in Embodiment 1. FIG. 2 is a flowchart illustrating a control flow in the first embodiment. It is a figure which shows the color management flow in a prior art example.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, what attached | subjected the same code | symbol in each drawing has the same structure or effect | action, The duplication description about these was abbreviate | omitted suitably.

<Reference Example 1>
[Stabilization control method when switching basic resolution]
First, the instability factors at high resolution will be described.

  The laser beam described in this reference example 1 is a red light emitting laser. The laser spot diameter is 40 μm in both the main scanning direction and the sub-scanning direction, and the state of the latent image in such a laser spot is as shown in FIGS. FIG. 1 shows the latent image distribution of the isolated dot pattern at 600 dpi, and FIG. 2 shows the isolated dot pattern at 1200 dpi. The horizontal axis represents coordinates (μm), and the vertical axis represents latent image potential (V).

  As can be seen from these figures, the isolated dot pattern at 1200 dpi is a shallow latent image, that is, a latent image with little potential change, compared to a 600 dpi latent image. For this reason, if development is performed from a portion delimited by a certain development bias potential (Vdc), an output result as shown in FIG. 3 is obtained, and an isolated dot at the time of 1200 dpi (the upper diagram in the figure) shows a highlight portion. However, a phenomenon occurs in which the dots are stably formed when a plurality of dots (middle and lower figures in the figure) are formed. In other words, as shown in the figure, with independent dots, the image is far from the original, and only after a plurality of dots, the original image is approached. FIG. 4 shows image formation of independent dots (upper row) and a plurality of dots (middle / lower row) at 600 dpi.

  As described above, the higher resolution improves the character proportion, the degree of freedom of the dither matrix, etc., and the image quality is improved for these image quality items, but the highlight using the unstable dots at 1200 dpi as described above is used. -It tends to cause density fluctuations in the middle tone.

  FIG. 5 shows the relationship between continuous output and stability at 600 dpi. FIG. 6 shows the same relationship at 1200 dpi. In these figures, the vertical axis represents ΔE, indicating the color difference from the first sheet output, and the horizontal axis represents the number of continuous output sheets.

  As can be seen by comparing these, the stability at the time of continuous output is lower at 1200 dpi than at 600 dpi. These results are shown in FIG. 7 with ΔE on the horizontal axis and the cumulative occurrence frequency on the vertical axis. When this FIG. 7 is summarized from the viewpoint of the color reproduction index, the following results are obtained.

  That is, the color reproduction index (occurrence probability of ΔE ≦ 10) was 93% at 600 dpi and 87% at 1200 dpi.

  Therefore, in an image forming apparatus capable of switching between 600 dpi and 1200 dpi, it is necessary to increase the accuracy of the stabilization control when outputting at 1200 dpi.

Description of Image Forming Apparatus FIG. 8 shows an image forming apparatus according to Reference Example 1 as an example of the image forming apparatus according to the present invention. The image forming apparatus shown in FIG. 1 is an electrophotographic four-color full-color laser beam printer, which is a longitudinal sectional view showing a schematic configuration thereof.

  The laser beam printer shown in the figure (hereinafter referred to as “image forming apparatus”) is provided with four image forming stations for forming images of magenta, cyan, yellow, and black, respectively. Each image forming station includes electrophotographic photosensitive members (hereinafter referred to as “photosensitive drums”) 1 a, 1 b, 1 c, and 1 d that are image holders that are rotatably supported in the clockwise direction in FIG. The photosensitive drums 1a, 1b, 1c, and 1d are rotated counterclockwise in the drawing at a predetermined process speed (circumferential speed). Around the photosensitive drums 11a, 1b, 1c, and 1d, primary chargers (charging means) 2a, 2b, 2c, and 2d, and exposure devices (exposure means) 33a, 3b, 3c, and so on are arranged in almost the order along the rotation direction. 3d, developing devices (developing means) 4a, 4b, 4c, 4d, transfer chargers (transfer means) 5a, 5b, 5c, 5d, cleaning devices (cleaning means) 6a, 6b, 6c, 6d, and the like. In the following description, when the above-described members and devices are collectively referred to and when it is not necessary to distinguish colors, the photosensitive drum 1, the primary charger 2, the exposure device 3, the developing device 4, The transfer charger 5 and the cleaning device 6 are described.

  As shown in FIG. 9, for each image forming station, a density detection sensor (density detection means) 30 (30a, 30b, 30c, 30d) is provided so as to face each photosensitive drum 1 (1a, 1b, 1c, 1d). ) Is arranged. The density sensor 30 detects the toner amount of the toner image formed on the photosensitive drum 1.

  A transfer belt 17 is disposed below the photosensitive drums 1a, 1b, 1c, and 1d between the developing devices 4a, 4b, 4c, and 4d and the cleaning devices 6a, 6b, 6c, and 6d so as to be in contact therewith. ing. The transfer belt 17 carries a recording material P such as paper or transparent film as a recording medium on its surface and rotates in the direction of arrow R17, and sequentially conveys the recording material P to each of the photosensitive drums 1a, 1b, 1c, and 1d. The toner images formed on the photosensitive drums 1a, 1b, 1c, and 1d in each image forming station are sequentially transferred onto the recording material P on the transfer belt 17 by the copying chargers 5a, 5b, 5c, and 5d. .

  Further, the image forming apparatus is provided with a plurality of paper feed units, that is, paper feed cassettes 12, 13, and 14, a manual paper feed tray 11 that can be pulled out in the direction of arrow R11 in FIG. 8, and a large-capacity paper deck 15. It has been. The recording material P is supplied from one of these paper supply units to the conveyance belt 17 via a paper supply roller, a conveyance roller, and a registration roller 16.

  As the recording material P is supported on the transfer belt 17 and passes through each image forming station, the toner images of the respective colors formed on the photosensitive drums 1a, 1b, 1c, and 1d are sequentially transferred. When this transfer process is completed, the recording material P is separated from the transfer belt 17 by the separation charger 18 and is transported to the fixing device 20 by the transport belt 19 serving as the recording material guiding means.

  The fixing device 20 includes a fixing roller 21 that is rotatably supported, a pressure roller 22 that rotates while being pressed against the fixing roller 21, a release agent application device 23 that is a release agent supply and application unit, and roller cleaning. Device. Heaters (not shown) such as halogen lamps are respectively disposed inside the fixing roller 21 and the pressure roller 22. The thermistor (not shown) is in contact with the fixing roller 21 and the pressure roller 22, respectively, and the voltage applied to each heater is controlled via a temperature adjusting device (not shown), so that the fixing roller 21 and the pressure roller 22 are heated. The surface temperature of the pressure roller 22 is adjusted. The pressure value of the pressure roller 22 and the surface temperature of the fixing roller 21 can be varied by the fixing control mechanism 25.

  The surface of the fixing roller 21 is in contact with a release agent application device 23 for applying silicon oil as a release agent to the surface of the fixing roller 21, and the recording material P is conveyed by the conveying belt 19, and the fixing roller 21, the pressure roller 22, The toner is prevented from adhering to the surface of the fixing roller 21 when passing between the two. The release agent coating device 23 is connected to a coating amount control device 26 that controls the coating amount of silicon oil applied to the surface of the fixing roller 21.

  A driving motor (not shown) that drives the fixing roller 21 and the pressure roller 22 includes a fixing roller 21 and a pressure roller 22 that pressurize and heat the conveying speed of the recording material P, that is, the front and back surfaces of the recording material P. A speed control device 27 is connected to control the rotation speed. As a result, the unfixed toner image on the surface of the recording material P is melted and fixed, and a full-color image is formed on the recording material P. The recording material P on which the full-color image is fixed is separated from the pressure roller 22 by a separation claw (not shown) and is discharged onto a paper discharge tray 24.

  In the upper part of the image forming apparatus shown in FIG. The document reading unit 28 optically scans and reads a document placed on a document table (not shown) to obtain image signals of each color. The operation display 29 is used for command input from an operator (user, serviceman), notification of the device status to the operator, and the like. Resolution switching at the time of copying, which will be described later, is instructed by this operation unit.

-Resolution switching method The resolution switching method employed in the present reference example is achieved, for example, by reducing the process speed to ½ as described in Japanese Patent Application Laid-Open No. 2000-181275. More specifically, the process speed at 600 dpi is 200 mm / sec, and at 1200 dpi, the process speed is 100 mm / sec, and the number of output sheets per minute is also halved.

  In recent years, various printer manufacturers have achieved high resolution by reducing the process speed to 1/2 as described above. This method was adopted in this reference example because the scale of parts change is small and the cost merit is high.

-Stabilization control Next, the stabilization control means will be described.

  A feature of the stabilization control method is that the stabilization control method is changed at 600 dpi and 1200 dpi. As mentioned in the topic, stability must be ensured strictly at higher resolutions. The following description focuses on control that is changed by switching the resolution.

[Density detection sensor]
The density detection sensor 30 (30a, 30b, 30c, 30d) includes a pair of light emitting elements 31 and light receiving elements 32, which are optical sensors, as shown in FIG. The light emitted from the light emitting element 31 is reflected by the surface of the patch image G (density detection toner image) formed on the photosensitive drum 1 (1a, 1b, 1c, 1d), and the reflected light is received by the light receiving element. 32 receives light. A light reception signal from the light receiving element 32 is input to the density detection sensor control unit 33. Then, the density is calculated from the received light signal based on the received light signal-density relationship calculated in advance and tabulated.

  There are various types of stabilization control.

[Reader gradation correction]
The first is related to automatic gradation correction, which is calibration performed by the user before important output. In this method, a gradation patch without an LUT (lookup table: gradation correction table) is output on the recording material P, the recording material P is placed on a reader unit by the user's hand, and the gradation patch is read, and the density is read. Convert to It is determined what the density is with respect to the input signal, and the gradation is corrected (rewrites the LUT) so as to become a predetermined target.

[Engine gradation correction]
Next, usability is improved by automatically performing the flow on the recording material P in the image forming apparatus main body. It is difficult for the user to carry out the stabilization control for reading the recording material P by the reader unit every day, and the user's need is to fully automate the above automatic gradation correction on paper, which is implemented in an actual image forming apparatus. There are many things.

  More specifically, an LUT-off gradation patch image is formed on the photosensitive drum 1 at a certain timing. The gradation patch image is detected by the density detection sensor 30, converted into density, and the table and LUT for correcting the gradation so as to become a predetermined target have been rewritten. The certain timing described above is a timing such as when the power is turned on, the front door is opened or closed, the specified number of sheets is finished, or the environment changes.

  The above two controls have a good reputation for stabilization for each job, but do not lead to ensuring stability during continuous jobs. During continuous jobs, it is difficult to ensure stability due to changes in toner characteristics in the developing device 4. In such a case, a patch image is formed between jobs in a continuous job, that is, between sheets (interval between the trailing edge of the preceding recording material P and the leading edge of the recording material P that follows it). The image quality stabilization process for correcting the gradation reproduction table (LUT) is controlled.

[Real-time gradation correction]
More specifically, a halftone patch image via the LUT is formed on the photosensitive drum 1, and it is confirmed whether the density is as prescribed. If not in accordance with the regulation, gradation correction as shown in FIG. 10 (hereinafter referred to as “real-time gradation correction table”) is registered before the LUT according to the amount of change. In other words, 30H (48 in decimal) of 0 to FFH (255 in decimal) is formed on the photosensitive drum 1, and the specified density is 0.3. The actually measured density value at that time is 0.2, which is less than 0.1. In such a case, an increase corresponding to 0.1 is added in the 30H portion, and the increase / decrease is controlled according to the change amount of 30H in the other portions. For other gradations, gradation correction is performed by changing the correction amount for each signal level, such as 30% for the density value and 100% correction for the shift amount, and for the 20H portion for 84% correction amount based on the table of FIG. I will go. At this time, since the reduction in stability from highlight to halftone is a problem due to resolution conversion, the patch forming the image is also devised to match the highlight area with 30H. Also, in terms of visual sensitivity, highlight to halftone variation is more conspicuous than shadow portion variation, and stricter stability is required. For this reason, formation and control of patch images with emphasis on halftone are performed from the above-described highlights. Note that the control at this time may be performed by forming a patch image having a lighter density than an intermediate point between the maximum density and the minimum density.

  As the flow of the image signal, it is necessary to perform the conventional LUT processing after passing through the above-described real-time gradation correction table. However, the above-described real-time gradation correction table and LUT may be a single LUT.

  With these controls, it is possible to correct the LUT with one patch image during a continuous job and perform real-time gradation correction in time for the next page printing.

  In this reference example, the control interval of the real-time gradation correction at the time of continuous output is changed between 1200 dpi and 600 dpi, and the accuracy is increased once every 10 sheets at 600 dpi and at 1200 dpi. Therefore, it is performed at a control interval of once every five sheets.

-Description of Image Processing Unit Next, the configuration of the image processing unit 209 will be described. FIG. 11 is a block diagram illustrating an outline of a configuration example of the image processing unit 209.

  In FIG. 11, the CCD 210 reads a document image at 600 dpi, and inputs the read image to the image processing unit 209 as an RGB signal. The RGB signal input to the image processing unit 209 is converted into a digital RGB signal by the A / D converter 102. The shading correction unit 103 corrects the illumination light amount, uneven light amount generated in the lens optical system, and uneven sensitivity of the pixels of the CCD 210. The scaling unit 104 enlarges or reduces the read image.

  The input direct mapping unit 105 has a multidimensional LUT close to an ICC profile, and converts an RGB signal into an L * a * b * signal that is a device-independent color space. The output direct mapping unit 106 converts the L * a * b * signal into a prescribed CMYK signal. The resolution conversion unit 107 converts 600 dpi reader information to 1200 dpi as necessary. In the case of a printout signal, conversion is performed based on information set by the user.

  The image forming pattern processing unit 108, a line growth type dither method, and a dot concentration type dither method (the method described in Japanese Patent Laid-Open No. 11-017947, etc.) have multi-value conversion functions of 600 dpi and 1200 dpi, respectively. A pattern is selected. The CMYK signals output from the image formation pattern processing unit 108 are sent to the printer unit 200. Note that the image forming pattern processing unit 108 also performs processing using an LUT for correcting the gamma characteristic of the printer unit 200. The LUT is prepared for each resolution and image formation pattern. The LUT processing is usually performed before pattern processing such as matrix calculation. The LUT included in the image forming pattern processing unit 108 is configured to be rewritable by an instruction from the CPU 110, and during real-time gradation correction during reader gradation correction, engine gradation correction, and continuous output. The LUT to be generated is generated by the LUT generation unit 121 described later and sent to the image forming pattern processing unit 108 for rewriting.

  The RGB signal from the reader unit and the signal from the density detection sensor 30 in the image forming apparatus are sent to the LUT generation unit 121. The function of the LUT generation unit 121 is to convert input RGB information and information from the density detection sensor 30 into density signals, respectively, so that the desired target is obtained by the relationship between the output patch signal information and the density converted density information. An LUT for correcting gradation is created. The created LUT can be uploaded to the image forming pattern processing unit 108.

  The CPU 110 comprehensively controls each component of the image processing unit 209 using the RAM 112 as a work memory based on a control program held in the ROM 111. For example, the resolution conversion unit 107 and the image formation pattern processing unit 108 are controlled. It also performs control to set parameters to. The CPU 110 controls the operation / display unit 114 and the network I / F 113 for communicating with an external device, and inputs and outputs image information and device information. That is, the CPU 110 is a processor that controls the entire system.

  The HDD 115 is a hard disk drive that stores system software, general image data, and output image data (user setting is possible). Also, it serves to convey information input from the operation / display unit 114 by the system user to the CPU 110. A raster image processor (RIP) 116 expands the PDL code into a bitmap image, and sends L * a * b * or CMYK signals before and after the output direct mapping unit 106.

-Explanation of Flowchart FIG. 12 shows a flow when an instruction for resolution switching output is received from the user, which is a feature of this reference example.

  The image forming apparatus that has received an instruction for resolution switching output determines whether the resolution is instructed from the operation / display unit 114 or the printer driver by the user, that is, whether the output is 600 dpi or 1200 dpi (S1, S2).

  Here, in the case of 600 dpi (Yes in S3), the real-time gradation correction interval is once every 10 sheets (S4, S5, S6), and in the case of 1200 dpi (No in S3), the frequency is once every 5 sheets. (S7, S8, S9).

  In this reference example, in order to improve productivity, when the next job is received (Yes in S10), the image formation is not completed, and the process returns to step S3 to perform an image formation sequence operation as if it were continuous output. Therefore, regardless of whether or not a plurality of sheets are output, it is determined how many sheets have passed since the previous real-time gradation correction, and is executed once for 10 sheets for 600 dpi and once for 5 sheets for 1200 dpi. .

  If the next job has not come (No in S10), it is determined how many sheets have been output from the previous engine gradation control (100 sheets or more in the figure), and the engine gradation control is performed as necessary. (S11, S12) to prepare for the next image formation.

  As described above, unstable output caused by resolution switching can be avoided by changing the stabilization control (image quality stabilization processing). Since the user can ensure a high-resolution output of 1200 dpi and stability as low as 600 dpi, an image forming apparatus that meets various user needs can be provided.

<Reference Example 2>
[Stabilization control switching according to the number of screen lines]
The characteristic point of the reference example 2 is that the resolution switching determination used in the reference example 1 is applied to the screen line number.

  In the printing industry, newspaper printing is 105 Line / Inch (hereinafter referred to as “lpi”. This indicates how many lines are included per inch. Also called the number of lines or screen resolution). High-quality paper output as the recording material P is 133 to 150 lpi. Art paper and coated paper are 150-200 lpi. In gravure printing, the number of lines is changed depending on the printing application such as 300 lpi.

  In today's color copiers that are output as color comps and proofs, the number of lines can be switched to accommodate such output. This is a so-called halftone dot simulation.

  As in Reference Example 1, it becomes difficult to ensure stability by reducing the number of screen lines. FIG. 13 shows the appearance of a latent image when a pattern having the same area ratio is output. As in Reference Example 1, it can be seen that fine dots are not stably formed.

  Therefore, the present reference example is characterized in that the stabilization control method is changed when a screen line number switching output is instructed.

-Number of screen lines In this reference example, 133 lines and 166 lines that are conscious of high-quality paper printing, 200 lines that are conscious of high-quality printing, and 268 lines that are optimal for character thin line images are prepared.

  There are 268 lines available for reducing jaggy in character parts, CAD drawings, etc., but there are few skin colors, process black, etc. that have memory colors, and it performs stabilization control until the toner consumption is reduced and the life of various parts is reduced. Since there is no necessity, the control is the same as that of the 166 line.

  The relationship between the number of screen lines and stabilization control performed in this reference example is shown below.

133 lines: once every 15 lines 166 lines: once every 10 lines 200 lines: once every 5 lines 268 lines: once every 10 sheets Of course, LUTs are prepared separately for each number of lines mentioned above, and real-time gradation A patch image corresponding to the patch image at the time of correction is created.

  Since there is no significant change in the configuration of the image forming apparatus, density detection sensor, and image processing unit, description thereof is omitted. However, in this embodiment, the resolution conversion unit 107 in FIG. 11 is not provided.

Flowchart FIG. 14 shows a flow when the number of screen lines is specified by the user, which is a feature of this reference example.

  The image forming apparatus that has received the output instruction determines whether the number of screen lines is instructed by the user from the operation / display unit 114 (see FIG. 11) or the printer driver (S11, S12), that is, 133 lines, 166 lines, or 200 lines. Or 268 line (S13, S17, S21).

  In the case of 133 lines (Yes in S21), the interval of real-time gradation correction is once every 15 sheets (S22, S23, S24). In the case of 166 lines (Yes in S13), once every 10 sheets (S14, S15, In the case of S16), 200 lines (Yes in S17), once every 5 sheets (S18, S19, s20), and in the case of 268 lines (No in S21), frequency once every 10 sheets (S25, S26, s27) To run.

  If the next job has arrived (Yes in S28), the process returns to S13. On the other hand, if the next job has not come (No in S28), the process proceeds to step S29. Then, when there is no output of 100 sheets or more from the previous engine gradation control (No in S29), the process is finished, and when there is an output of 100 sheets or more (Yes in S29), engine gradation control is performed. After (S30), the process ends.

  As described above, unstable output caused by switching the number of screen lines can be avoided by changing the stabilization control (image quality stabilization processing). For the user, stability can be ensured even at the time of outputting a high number of lines, so that it can be used not only for a comp application but also as a proof output comparable to the color proofing of a printing press, and the use application is widened.

<Example 1>
[When there are multiple processing patterns in the surface]
This embodiment describes a stabilization control method when there are a plurality of processing patterns (image formation patterns) in the plane, that is, when text information, vector information, and bitmap information exist using tag bits. .

  In the reference example 2 described above, the same screen line number in the plane has been described. However, considering DTP (Desktop Publishing), text information used for headings, explanatory texts, vector information represented by boxes and circles, There are bitmap information such as photographs and various objects.

  In recent image processing apparatuses, such information is analyzed by PDL (Page Description Language) and optimal processing is performed on the object.

  For example, as described in Reference Example 2, since characters and the like have many curves as well as straight lines, jaggies are conspicuous unless the number of lines is increased.

  On the other hand, in the case of a photographic image, a phenomenon in which not only the color stability but also the graininess deteriorate due to the unstable factor of the latent image by increasing the number of lines. For this reason, a line number lower than the screen line number of characters is generally adopted.

  In the case of an image forming apparatus that automatically determines such a process and selects a screen process, it cannot be determined which process the stabilization control method is focused on.

  In addition, there are image processing apparatuses that are automatically switched by the above-described objects, and there are image forming apparatuses that can be manually specified, which causes a problem in stabilization control with a fixed number of lines.

  The present embodiment solves the above-described problems, and is characterized in that only the number of lines for recording bitmap images such as photographic images is targeted as the number of lines for real-time gradation correction.

Image Processing Unit FIG. 15 shows a schematic configuration diagram of an image processing unit in the present embodiment.

  Since this embodiment does not perform basic resolution conversion as in Reference Example 2, the resolution conversion unit 107 is deleted from Reference Example 1. In addition, the internal function of the RIP unit 116 that analyzes PDL information and develops it into a bitmap image includes a function of automatically distributing as described below, and a mechanism for sending an instruction to the image forming pattern processing unit 108 is provided. As shown in FIG. 16, the number of lines can be designated by the user, and the RIP unit 116 sends an instruction to the image forming pattern processing unit (determination unit) 108 based on the designated information.

Text: 268 lines Vector: 200 lines Bitmap: 166 lines On the other hand, the object determination in the RIP unit 116 is determined by the following PDL description.

  FIG. 17 shows a PDL format (postscript language) when an image file is instructed to be printed. In this example, the application software Photoshop prints a CMYK image with a resolution of 72 dpi and a 7 × 11 pixel image. In the image file, an image data value is described for each pixel (see FIG. 18), and it may be determined that a photographic image is included using the information. On the other hand, FIG. 19 shows the contents of a Postscript file when a simple character is printed.

  At this time, / TimesNewRoman is the font name, findfont plays the role of searching for the above-mentioned fonts in the HDD or ROM connected to the image forming apparatus, 15 scalefont is 15 times the found font, that is, 15 points (Font is registered at one point), and setfont is an instruction to register the font created in the above flow.

  150 150 move is an instruction to write at the position of (150, 150) point (point is 1/72 inch) when the lower left is the origin (0, 0), (itagaki tomohisa) is the character that is actually output, This is a command for drawing characters, and the showpage actually performs output work.

  Thus, the RIP unit 116 analyzes the PDL code as shown in FIG. 19 and determines the coordinates of the character information.

  Unlike an image image, a vector image is characterized in that it has a smaller capacity than a bitmap image in which all color information of one pixel must be entered because a mathematical expression, coordinates, and code are described and drawn.

  FIG. 20 introduces a PDL description of a square image. newpath is a declaration for creating a new path, and the first position is specified by moveto. The numerical value 100 100 represents coordinates, and means the position of 100 points in the X direction and 100 points in the Y direction with respect to the lower left. The point is 1/72 inch. “lineto” is a code for drawing the trajectory that has been moved, and is a trajectory that has moved from (100 100) to (200 100), that is, a line. Similarly, move to the remaining two points and draw a line. Then, connect the start point and end point with closepath, and fill with fill. At this time, the color designation is 50% gray. Finally, as with text, it is output by showpage.

  In this way, a vector image may be determined using codes such as closepath, lineto, and fill.

  As described above, the PDL code is analyzed, the object classification and the area are determined, and the information is passed to the image forming pattern processing unit 108.

Flowchart Real time gradation control when using tag bits, which is a feature of the present embodiment, will be described with reference to FIG.

  The image forming apparatus that has received the output instruction determines whether or not the screen line number at the time of the bitmap image (photograph part) instructed by the user from the operation / display unit (see FIG. 15) or the printer driver (S31, S32), that is, It is determined whether the line is 133, 166, 200, 268 or AUTO (S33, S37, S41, S45). In the case of 133 lines (Yes in S41), the interval of real-time gradation correction is once every 15 sheets (S42, S43, S44). In the case of 166 lines, once every 10 sheets (S34, S35, S36), 200 lines In the case of (1), it is executed once every 5 sheets (S38, S39, S40), and in the case of 268 lines, it is executed once every 10 sheets (S45, S46, S47). On the other hand, in the case of AUTO, image formation and real-time gradation correction are performed on line 166 (No in S32), and the frequency is set to once per 10 sheets (S34, S35, S36).

  As described above, unstable output caused by switching the number of screen lines is avoided by changing the stabilization control (image quality stabilization processing). Even when the user designates the number of screen lines of the photographic part separately, the stability of the color can be ensured by paying attention to the image forming pattern of the photographic part and performing the stabilization control.

  Further improvements in usability and higher image quality can be achieved by making the following modifications to the embodiments and reference examples described above.

-Change of stabilization control In the above-described embodiments and reference examples, the example in which the frequency of real-time gradation correction is increased when the resolution and the number of lines are increased has been described. The purpose is to improve resolution and to ensure stability when the number of lines is increased, and frequency is not the only means to achieve it.

  For example, although 1/2 speed is adopted to double the basic resolution, it means that the theoretical paper interval during continuous jobs is doubled. For this reason, a plurality of patch images may be formed and combined with high accuracy.

  In addition, the accuracy may be improved by increasing the number of samplings with one patch image and averaging from many points.

  Further, regarding the stability, the amount of toner in the developing device 4 is greatly related, so the frequency of ATR toner replenishment control may be increased. The density detection sensor is common, but the processing for ATR (image formation pattern) is different, so it cannot be performed at the same timing. However, by increasing the frequency of ATR control, high-precision toner supply is performed to solve the problem. Good. The image formation pattern at this time is preferably executed using a patch image that is as close to analog as possible, for example, an image formation pattern such as 600 lines. This is because this is highly correlated with the toner amount in the developing device 4 and can be controlled with high accuracy.

1 (1a, 1b, 1c, 1d) Image carrier (photosensitive drum)
30 (30a, 30b, 30c, 30d) Concentration detection means (concentration detection sensor)
110 Image quality stabilizing means (control means: CPU)
116 Pattern switching means (RIP part)
121 Resolution switching means (LUT generator)
G Patch image P Recording material

Claims (2)

Image forming means for forming a toner image on the photoreceptor using a predetermined gradation reproduction table;
Transfer means for transferring a toner image formed on the photoreceptor by the image forming means to a recording material;
Density detecting means for detecting the density of the patch image formed by the image forming means;
Image quality stabilization means for performing image quality stabilization processing by correcting the predetermined gradation reproduction table based on the output of the density detection means;
Determining means for determining the resolution of the image formation pattern when an image formation pattern corresponding to the bitmap image is present in the image to be formed on the recording material;
The image is controlled such that the frequency at which the image quality stabilization process is executed by the image quality stabilization unit is increased at the time of high resolution than at the time of low resolution in association with the formation of the patch image by the image forming unit. Forming equipment. Image forming means for forming a toner image on the photoreceptor using a predetermined gradation reproduction table;
Transfer means for transferring a toner image formed on the photoreceptor by the image forming means to a recording material;
Density detecting means for detecting the density of the patch image formed by the image forming means;
Image quality stabilization means for performing image quality stabilization processing by correcting the predetermined gradation reproduction table based on the output of the density detection means;
Determination means for determining the resolution of the image formation pattern when an image formation pattern corresponding to the photographic image is present in the image to be formed on the recording material,
The image is controlled such that the frequency at which the image quality stabilization process is executed by the image quality stabilization unit is increased at the time of high resolution than at the time of low resolution in association with the formation of the patch image by the image forming unit. Forming equipment.

Images