JP2020129729A - Image forming apparatus and control method therefor, and program - Google Patents

Abstract

To solve a problem in which, when screen processing using a dither matrix is performed on an image whose concentration has been adjusted according to the light amount characteristics at each main scanning position of adjacent light emitting elements, a change in dots of halftone dots becomes large at the boundary between pixels having different light amount characteristics, and the image quality deteriorates.SOLUTION: An image forming apparatus is provided that irradiates a photoreceptor with light modulated according to image data from a line head having a plurality of light emitting elements arranged therein to form an image. The image forming apparatus stores light amount characteristic data of the plurality of light emitting elements, obtains the light amount characteristic data of a light emitting element corresponding to a target area of screen processing of a dither matrix corresponding to a target pixel when performing screen processing on the target pixel, and obtains the light amount characteristic of the light emitting element corresponding to the target area on the basis of the light amount characteristic data of the target area. The mage forming apparatus then corrects the pixel value of the target on the basis of the light amount characteristic.SELECTED DRAWING: Figure 3

Description

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

Generally, an electrophotographic toner image forming unit includes a photoconductor as an image carrier having a photosensitive layer on its outer peripheral surface, a charging unit for uniformly charging the outer peripheral surface of the photosensitive member, and a uniformly charged outer peripheral surface. And an developing unit for selectively applying toner to the electrostatic latent image formed by the exposing unit to form a visible image (toner image). There is.

As a tandem type image forming apparatus for forming a color image, a plurality (for example, four) of toner image forming units as described above are arranged on the intermediate transfer belt. Then, the toner images on the photoconductor by the plurality of monochromatic toner image forming units are sequentially transferred to the intermediate transfer belt, and the toner images of a plurality of colors (for example, yellow, magenta, cyan, and black) are superimposed on the intermediate transfer belt. , Some form a color image on the intermediate transfer belt.

In the tandem type image forming apparatus having the above-described structure, there is known a line head using an LED or an organic EL element as a light emitting element. An electrostatic latent image is formed by arranging a line head on which a plurality of light emitting elements are arranged in parallel with a photoconductor, and irradiating a rotating photoconductor with light modulated in accordance with image data to expose it. Is formed. At this time, a direction parallel to the line head is called a main scanning direction, and a direction perpendicular to the line head is called a sub-scanning direction. In the optical writing line head used for such a light emitting element, the light amounts of the plurality of light sources (light emitting elements) are not uniform. Therefore, if writing is performed in that state, there is a problem that light and shade (streak and unevenness) depending on the amount of light occurs in the sub-scanning direction of the image formed thereby.

In order to avoid the occurrence of such a difference in light and shade, conventionally, a correction is made such that the light amount of each of a plurality of light sources provided corresponding to a pixel is corrected at the time of writing to make their densities uniform. A circuit was provided. Such correction of the light amount has been performed by changing the lighting time of the light source and the drive current. In order to correct the light intensity, the light intensity of each light source is measured at the time of shipment of the line head, and the lighting time and drive current correction values corresponding to each pixel are written in the memory built into the line head before use. That is, when writing an image, the correction value is read to correct the lighting time and the drive current.

However, in the conventional method, in addition to the lighting control of each pixel according to the image data to be printed, a circuit for controlling the lighting time and the driving current is required for each pixel in order to make the light amount of each pixel uniform. And the circuit scale was increasing. Therefore, in an image forming apparatus equipped with a line head having a plurality of these light emitting elements, Japanese Patent Laid-Open No. 2004-242242 describes a method of suppressing an increase in circuit scale and suppressing density unevenness due to non-uniformity of light amount. According to this, the density of the color-separated image of each color is corrected based on the light amount characteristic data for each pixel. It should be noted that the density correction is performed on the multi-valued image before the screen processing while changing the correction degree according to the density.

JP, 2002-237412, A

However, in the above-mentioned conventional method, when there is a large difference in the light amount characteristics of the adjacent light emitting elements, the degree of density correction greatly differs between the pixels adjacent in the main scanning direction. Then, when the screen processing using the dither matrix is performed on the image whose density is corrected according to the light amount characteristic of each main scanning position, the change in the dot of the halftone dot is large between the pixels having the difference in the light amount characteristic. It may become. Then, there is a problem that the dots of the halftone dots whose sizes are changed are continuous in the sub-scanning direction, so that streaks are generated in the sub-scanning direction.

An object of the present invention is to solve at least one of the problems of the above-mentioned conventional techniques.

One of the objects of the present invention is to provide a technique for suppressing the image influence on the density uniformity due to the light amount variation of the light emitting element.

Another object of the present invention is to provide a technique for improving the image quality of an output image in an image forming apparatus.

In order to achieve the above object, an image forming apparatus according to an aspect of the present invention has the following configuration. That is,
An image forming apparatus for forming an image by irradiating light modulated according to image data from a line head having a plurality of light emitting elements,
Storage means for storing light quantity characteristic data of the plurality of light emitting elements,
A first acquisition unit that acquires, from the storage unit, the light amount characteristic data of the light emitting element corresponding to the target area of the screen processing of the dither matrix corresponding to the target pixel when performing the screen processing on the target pixel;
Second acquisition means for acquiring the light amount characteristic of the light emitting element corresponding to the target region based on the light amount characteristic data of the target region;
And a correction unit that corrects the pixel value of the pixel of interest based on the light amount characteristic.

According to one aspect of the present invention, while suppressing streaks and unevenness due to continuous change in dot size of halftone dots in the sub-scanning direction, it is possible to achieve density uniformity due to light amount variations of light emitting elements. There is an effect that the influence of the image can be suppressed.

Further, according to another aspect of the present invention, there is an effect that the image quality of the output image in the image forming apparatus can be improved.

Other features and advantages of the present invention will be apparent from the following description with reference to the accompanying drawings. Note that, in the accompanying drawings, the same or similar configurations are denoted by the same reference numerals.

The accompanying drawings are included in and constitute a part of the specification, illustrate embodiments of the invention, and together with the description, serve to explain the principles of the invention.
1 is a block diagram illustrating an example of a configuration of an image forming apparatus according to a first exemplary embodiment of the present invention. 3 is a block diagram illustrating an example of a configuration of a controller of the image forming apparatus according to the first embodiment. FIG. 3 is a block diagram illustrating an example of a configuration of an image processing unit according to the first embodiment. FIG. 6A and 6B are diagrams illustrating screen processing performed by the image forming processing unit according to the first embodiment. FIG. 3 is a diagram showing an example of a dither matrix used in the first embodiment. FIG. 6 is a diagram schematically showing screen processing performed on target image data using the dither matrix of FIG. 5. FIG. 6 is a diagram showing an example of a halftone dot image obtained as a result of performing screen processing using the dither matrix shown in FIG. 5. FIG. 3 illustrates an example of a printer engine of the image forming apparatus according to the first exemplary embodiment. FIG. 3 is a diagram showing an example of a configuration of an LED line head arranged in parallel with a photoconductor. FIG. 3 is a diagram showing an arrangement example of an LED chip according to Embodiment 1 and a light emitting element in the LED chip. FIG. 3 is a graph showing an example of light amount characteristics of each light emitting element of the LED chip according to the first embodiment. FIG. 6 is a diagram illustrating a light amount correction process using the pixel value correction unit and the light amount profile storage unit according to the first embodiment. FIG. 6 is a diagram illustrating a light amount correction process using the pixel value correction unit and the light amount profile storage unit according to the first embodiment. 6 is a flowchart illustrating an example of a pixel value correction process according to the light amount characteristic by the pixel value correction unit according to the first embodiment. FIG. 6 is a diagram showing that a light amount correction value is set for each pattern in the sub-scanning direction in the screen processing performed on target image data using the dither matrix according to the first embodiment. FIG. 16 is a diagram showing an example of setting a light quantity correction value pattern based on FIG. 15. The figure explaining the light amount correction value for every area|region of the main scanning direction. FIG. 3 is a diagram showing an example in which a light amount correction value is assigned to each block area in the first embodiment. 15 is a flowchart illustrating an example of a process of acquiring a light amount correction value GI(x, y) in S1402 of FIG. FIG. 3 is a diagram showing an example of a halftone dot image obtained according to the first embodiment. FIG. 6 is a block diagram illustrating a configuration of an image processing unit according to a second embodiment of the present invention. 6A and 6B are diagrams illustrating a dither matrix threshold value correction process based on a light amount characteristic in a dither matrix unit according to the second embodiment. 9 is a flowchart illustrating a pixel value correction process according to a light amount characteristic performed by a threshold value correction processing unit according to the second embodiment.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments do not limit the invention according to the claims. Although a plurality of features are described in the embodiment, not all of the plurality of features are essential to the invention, and the plurality of features may be arbitrarily combined. Further, in the accompanying drawings, the same or similar components are designated by the same reference numerals, and duplicated description will be omitted.

FIG. 1 is a block diagram illustrating the configuration of the image forming apparatus 101 according to the first embodiment of the present invention.

The image processing apparatus 101 includes a controller (control unit) 102 that controls the operation of the entire image processing apparatus 101, a plurality of keys for a user to give instructions, and an operation unit 103 that displays various kinds of information to be notified to the user. It has a scanner 104 which is an image reading device and a printer engine 105 which is an image output device. The controller 102 is connected to an external PC, an image processing device, or the like via the network 106. As a result, image data and device information can be input/output via the network 106. The operation unit 103 receives operation instructions from the user to the image forming apparatus 101 and displays a display screen. The scanner 104 scans a document to generate image data corresponding to the image of the document. In addition, the calibration image data can be generated by scanning the calibration image. The printer engine 105 forms an electrostatic latent image according to the image data after the image processing and develops the formed electrostatic latent image to form a monochromatic toner image. Then, the printer engine 105 forms a multicolor toner image by superposing the single color toner images, transfers the formed multicolor toner image to a recording medium (sheet), and fixes the multicolor toner image on the recording medium.

FIG. 2 is a block diagram illustrating the configuration of the controller 102 of the image forming apparatus 101 according to the first embodiment.

The CPU 201 controls the entire image forming apparatus 102. The image processing unit 202 converts the image data received via the network 106 or the image data obtained by the scanner 104 into a form that the printer engine 105 can form on a storage medium. The calibration unit 203 corrects various parameters used in the image processing performed by the image processing unit 202 based on the calibration data created by the scanner 104 reading the calibration image.

FIG. 3 is a block diagram illustrating the configuration of the image processing unit 202 according to the first embodiment.

The color conversion processing unit 301 converts the color space of the image data received via the network 106 or the image data obtained by the scanner 104 into a color space method that can be output by the printer engine 105. The color conversion processing unit 301 converts image data in the RGB color space acquired via the network 106 into image data in the CMYK color space, for example. The pixel value correction unit 302 corrects the pixel value of the image data according to the light amount characteristic of the LED (Light Emitting Diode) line head of each color plate used when the printer engine 105 performs the exposure. The light amount characteristic of each color plate is received from the light amount profile storage unit 304.

The image formation processing unit 303 receives the dither matrix from the dither matrix storage unit 305 and executes the screen processing on the input image data so that the printer engine 105 can transfer the image onto the recording paper. As a result, the input image is converted into image data represented by the density of the four CMYK color dots. The light amount profile storage unit 304 stores the light amount characteristic data of the LED line heads of CMYK colors. The dither matrix storage unit 305 stores dither matrices of CMYK colors. In the first embodiment, it is assumed that the light amount characteristic data is measured at the time of factory shipment and stored in the light amount profile storage unit 304. However, the present invention is not limited to this, and the light amount characteristic data may be periodically remeasured or corrected in consideration of the deterioration of the LED over time.

In the first embodiment, it is assumed that the image processing unit 202 is realized by a hardware circuit such as an ASIC, but the present invention is not limited to this. For example, a general-purpose processor such as the CPU 201 and a hardware circuit can cooperate to realize various image processing. In addition, for example, a general-purpose processor such as the CPU 201 reads and executes an instruction that constitutes an image processing program, so that various kinds of image processing can be realized.

FIG. 4 is a diagram illustrating screen processing performed by the image forming processing unit 303 according to the first embodiment. Here, binarization will be described in order to simplify the description. The screen process is a process of expressing image data expressed in continuous gradation by area gradation, that is, gradation expressed by a ratio of a colored area and a non-colored area per unit area. A dither matrix representing the growth of the screen is used for this screen processing. This dither matrix points to a threshold table that controls the growth of the screen.

In FIG. 4, the screen image indicates the screen image data obtained as a result of the screen processing by applying the dither matrix to the processing target area of the input image data. When the image data is input, the image forming processing unit 303 pays attention to the pixel value of each pixel of the image data and compares it with the threshold value of the dither matrix. As a result of this comparison, if the pixel value of the target pixel is larger than the threshold value of the dither matrix, the pixel corresponding to the target pixel of the screen image data has a value. For example, the image data is 9 pixels of 3×3, and the pixel value of each pixel is “130”. By using a 3×3 dither matrix, the image data is binarized into “0” and “255”. Screen processing. Here, if the pixel value of the input pixel is greater than or equal to the threshold value of the corresponding dither matrix, the pixel value of the corresponding pixel of the screen image data is “255”. Further, when the pixel value of the input pixel is less than or equal to the threshold value of the dither matrix, the pixel value of the corresponding pixel of the screen image data becomes “0”.

FIG. 5 is a diagram showing an example of the dither matrix used in the first embodiment. FIG. 5 shows the threshold value of the dither matrix having a resolution of 1200 dpi and a line count of 134 lines. The number of lines is a unit indicating how many dots are present in one inch. The dither matrix of the first embodiment has a shape in which an 8×8 square 500 and a 4×4 square 501 are combined.

FIG. 6 is a diagram schematically showing screen processing performed on target image data using the dither matrix of FIG. As shown in FIG. 6, when performing the screen processing using the dither matrix, the dither matrix is repeatedly used without a gap so that the screen processing can be performed on all pixels.

FIG. 7 is a diagram showing an example of a halftone dot image obtained as a result of screen processing using the dither matrix shown in FIG. FIG. 7 shows a halftone dot image obtained as a result of the screen processing using the dither matrix of FIG. 5 on the image having the entire surface signal value of “100”. As shown in FIG. 6, since the screen processing is performed by repeatedly using the dither matrix without a gap, the pattern of halftone dots also repeats in the main scanning direction and the sub scanning direction.

FIG. 8 is a diagram illustrating the printer engine 105 of the image forming apparatus 101 according to the first embodiment. Here, a cross-sectional view of a printer engine 105 that uses a tandem electrophotographic method that employs the intermediate transfer member 28 is shown. In the drawings, members provided for each color are shown by adding alphabets (Y/M/C/K) indicating the respective colors to the end of the reference numerals, but the description will be made without distinguishing the colors. In this case, the alphabet at the end of this code will be omitted in the explanation.

The printer engine 105 forms an electrostatic latent image by exposing the photoconductor 22 according to the image data processed by the image forming processing unit 303. Then, the electrostatic latent image is developed to form a monochromatic toner image. A multicolor toner image is formed by superposing the single color toner images on the intermediate transfer member 28. This multicolor toner image is transferred to the recording medium (sheet) 11, and the fixing device 31 fixes the multicolor toner image on the recording medium. The injection charger 23 is for uniformly charging the surface of the photoconductor 22 to a predetermined potential, and includes a sleeve 23S. The photoconductor 22 is rotated by the driving force of a drive motor (not shown) being transmitted, and the drive motor rotates the photoconductor 22 counterclockwise in accordance with an image forming operation. The exposure unit performs exposure with an LED from a line head unit 24 arranged in parallel with the photoconductor 22, and selectively exposes the surface of the photoconductor 22 to form an electrostatic latent image. The printer engine 105 according to the first embodiment drives at a resolution of 1200 dpi in a direction parallel to the line head unit 24 (hereinafter, main scanning direction) and at a resolution of 1200 dpi in a sub-scanning direction orthogonal to the main scanning direction.

The developing device 26 is used to visualize the electrostatic latent image on the photoconductor 22 with a single color toner, and includes a sleeve 26S. The developing device 26 can be attached to and detached from the photoconductor 22. The intermediate transfer body 28 rotates in the clockwise direction to receive the monochromatic toner image from the photoconductor 22, and the monochromatic toner image is transferred as the photoconductor 22 and the primary transfer roller 27 located opposite thereto rotate. .. By applying an appropriate bias voltage to the primary transfer roller 27 and making the rotational speed of the photoconductor 22 different from the rotational speed of the intermediate transfer body 28, the monochromatic toner image is efficiently transferred onto the intermediate transfer body 28. This is called primary transfer. Further, the monochromatic toner images of the CMYK stations are superposed on the intermediate transfer member 28. The superposed multicolor toner images are conveyed to the secondary transfer roller 29 as the intermediate transfer member 28 rotates. At the same time, the recording medium 11 is nipped and conveyed from the paper feed tray 21 to the secondary transfer roller 29, and the multicolor toner image on the intermediate transfer body 28 is transferred to the recording medium 11. At this time, by applying an appropriate bias voltage to the secondary transfer roller 29, the toner image is electrostatically transferred. This is called secondary transfer. The secondary transfer roller 29 contacts the recording medium 11 at the position 29a while transferring the multicolor toner image onto the recording medium 11, and is separated to the position 29b after the transfer process.

The fixing device 31 fixes the multicolor toner image transferred on the recording medium 11 to the recording medium 11 by melting and fixing, and the fixing roller 32 for heating the recording medium 11 and the pressing roller for pressing the recording medium 11 to the fixing roller 32. The pressure roller 33 is provided. The fixing roller 32 and the pressure roller 33 are formed in a hollow shape and have heaters 34 and 35 built therein. The fixing device 31 conveys the recording medium 11 holding the multicolor toner image by the fixing roller 32 and the pressure roller 33 and applies heat and pressure to fix the toner on the recording medium 11. The recording medium 11 on which the toner has been fixed in this manner is then discharged by a discharge roller (not shown) to a discharge tray (not shown), and the image forming operation is completed. The cleaning unit 30 cleans the toner remaining on the intermediate transfer body 28, and the waste toner remaining after the multicolor toner image of four colors formed on the intermediate transfer body 28 is transferred to the recording medium 11. , Stored in cleaner container.

FIG. 9 is a diagram showing an example of the configuration of the LED line head 24 arranged in parallel with the photoconductor 22.

In the first embodiment, the LED line head 24 includes a printed circuit board 40 on which a circuit for supplying various signals for controlling the driving of the LED line head 24 is formed, a lens array 41, and a plurality of LED chips 42 in a zigzag manner. It is arranged and configured. Here, each LED chip 42 is configured by arranging a large number (for example, 256) of LED light emitting elements 43 having the same size in a line shape at equal intervals as shown in FIG. The LED chips 42 may be arranged in a staggered manner so that the light emitting elements at the main scanning end portion overlap (in FIG. 10, two light emitting elements are arranged in an overlapping state). In the first embodiment, the LED chip 42 uses a self-scanning LED (SLED: Self-scanning LED) array chip, but the LED chip 42 is not limited to this.

The lens array 41 is provided as an imaging lens between the LED chip 42 and the photoconductor 22. The lens array 41 is configured by arranging LED refractive index distribution type rod lenses at a pitch corresponding to each pixel according to the resolution, for example, and a light beam emitted from each LED light emitting element 43 is a photoconductor. The image is formed on 22. As described above, the LED line head 24 has a large number of light emitting elements arranged in the main scanning direction, and has a configuration in which a solid variation in the light amount occurs for each light emitting element at each main scanning position.

FIG. 11 is a graph showing an example of light amount characteristics of each light emitting element in the LED chip 42 according to the first embodiment.

In the first embodiment, the LED chip 42 has 256 LED elements. Of all the light emitting elements of each color plate, the light amount characteristic of the light emitting device having the minimum light amount is normalized as "1", and the normalized light amount characteristic is shown in the graph.

FIG. 12 is a diagram illustrating a light amount correction process using the pixel value correction unit 302 and the light amount profile storage unit 304 according to the first embodiment.

FIG. 12A is a graph showing the light amount characteristics of the target dither matrix region 1201 of FIG. 12B and its peripheral region (main scanning position x=1 to 20). FIG. 12B is a diagram showing the light amount characteristic values of FIG. 12A corresponding to the target dither matrix region 1201 and each pixel (main scanning position x=1 to 20) in the peripheral region thereof.

As shown in FIG. 12B, the light amount characteristic at each main scanning position appears continuously in the sub scanning direction. For this reason, if the printer engine 105 performs printing without performing light amount correction and printing is performed, density unevenness occurs in accordance with the light amount characteristic, which causes streaks in the sub-scanning direction. Further, as mentioned in the above problem, when the pixel value is corrected for the input image in accordance with the light amount characteristic of the main scanning position, the pixel value changes in the dither matrix area, and as a result, the dot of the halftone dot There is a problem that the size of is changed.

Therefore, in the first embodiment, the light amount characteristic Ld of the entire dither matrix region of interest is calculated, and the pixel value of the pixel in the dither matrix region of interest is corrected based on the calculated light amount characteristic Ld. In the first embodiment, the light amount characteristic Ld of the entire dither matrix region is the average value of the light amount of each pixel, but it may be the sum of the light amounts or the weighted average of the light amount based on the pixel position.

As shown in FIG. 5, the dither matrix in the first embodiment is a dither matrix of 80 pixels in total of 8 pixels×8 pixels and 4 pixels×4 pixels, and therefore the average of 80 pixels is obtained. The result is shown in FIG. FIG. 13A shows the light quantity characteristic Ld in the dither matrix area 1201 of interest, which is shown in association with each pixel of the dither matrix area 1201 of interest. The light amount characteristics for each main scanning position in the dither matrix area of interest 1201 are [1.11, 1.02, 1.05, 1.05, 1.08, 1.03, 1.1, 1.06] from the left. Among them, the four light quantity characteristics in the first half are 8 pixels in the sub-scanning direction, and the four light quantity characteristics in the latter half are 12 pixels in the sub-scanning direction. As a result of calculating the sum of the light amount characteristics for these 80 pixels and calculating the average in the dither matrix region of interest, the light amount characteristic Ld becomes "1.06".

Then, FIG. 13B is a diagram showing a processing result in which the pixel value of the pixel corresponding to the dither matrix area of interest is corrected based on the obtained light amount characteristic Ld of the dither matrix area of interest. In this example, the correction result of the pixel value of the pixel corresponding to the dither matrix area 1201 of interest is shown when an image having a pixel value of “100” is input. In the first embodiment, the minimum light amount of all pixels is used as a reference, and the pixel value is corrected so that the product of the light amount characteristic Ld in the dither matrix region of interest and the referred pixel value becomes equal to the reference light amount characteristic. However, the correction method is not limited to this. Since the reference light amount characteristic is set to "1" in the first embodiment, the correction value GI(x, y) in the dither matrix region of interest is "1/6" obtained by dividing the reference light amount characteristic by "1.06". 1.06". Then, the pixel value after the correction processing in the dither matrix area of interest 1201 is 100×1/1.06=94 (rounded up after the decimal point).

As described above, the light amount characteristic Ld of the entire dither matrix region of interest is obtained, and the pixel value is corrected based on the obtained light amount characteristic Ld, so that the light amount characteristic of the region of interest is suppressed while suppressing a sharp change in the pixel value in the dither matrix region. It is possible to correct the pixel value according to the above.

The light amount correction value used for the image correction processing according to the light amount characteristic is uniquely determined for each pixel when the dither matrix used is constant. Therefore, the light amount characteristic may be called and calculated each time in accordance with the coordinates of the pixel of interest or the coordinates of the dither matrix region of interest. Alternatively, a light amount correction value corresponding to each coordinate may be calculated in advance from the light amount characteristic and stored in the light amount profile storage unit, and the stored light amount correction value may be called during the light amount correction process. The first embodiment employs a format in which a light amount correction value created and stored in advance is called and a pixel value correction process is performed according to the light amount characteristic.

With reference to FIG. 14, a flow of the pixel value correction processing according to the light amount characteristic performed by the pixel value correction unit 302 according to the first embodiment will be described.

FIG. 14 is a flowchart illustrating a pixel value correction process according to the light amount characteristic by the pixel value correction unit 302 according to the first embodiment.

First, in step S1401, the pixel value correction unit 302 acquires the pixel value I(x, y) of the target pixel. Next, proceeding to step S1402, the pixel value correction unit 302 acquires the light amount correction value GI(x, y) for the pixel value corresponding to the coordinates (x, y) of the pixel of interest from the light amount profile storage unit 304. The process advances to step S1403, and the pixel value correction unit 302 multiplies the pixel value I(x, y) of the pixel of interest by the light amount correction value GI(x, y) of the pixel value to obtain the light amount corrected pixel value I of the pixel of interest. Get'(x,y). Then, the processing proceeds to step S1403, in which the pixel value correction unit 302 determines whether or not the pixel value correction processing according to the light amount characteristic has been performed on all pixels of the input image, and the pixel value correction processing according to the light amount characteristic is performed on all pixels. If not, the process advances to step S1405, the pixel value correction unit 302 updates the coordinates (x, y) of the pixel of interest, and the process advances to step S1401. In this way, in S1404, when the pixel value correction processing according to the light amount characteristic is completed for all the pixels, this processing is completed and the image data after the correction processing is output to the image formation processing unit 303.

Here, the light amount correction value GI(x, y) for the pixel value is uniquely determined for each pixel when the dither matrix used as described above is constant. However, if a light amount correction value is prepared for each pixel, for example, if the light amount correction values for all pixels of 1200 dpi and A4 size are stored in advance, it becomes about 9000 pixels×14000 pixels, which requires a huge memory. Therefore, in the first embodiment, the amount of memory used for storing the light amount correction value is reduced by performing the light amount correction value storing and calling process based on the above-described repeating pattern of the Dizen matrix.

Next, with reference to FIGS. 15 to 18, a method of storing the light amount correction amount according to the first embodiment will be described.

FIG. 15 is a diagram schematically showing screen processing performed on target image data by using the dither matrix of the first embodiment shown in FIG. 6, in which patterns appearing in the sub-scanning direction are separated by dotted lines. Is an image showing.

For example, it can be seen that the regions 1501 and 1502 in the figure have the same dither matrix arrangement at each main scanning position. If the main scanning positions of the dither matrix region of interest are the same, the light amount characteristics to be referred to are also the same. Therefore, since the obtained light amount correction values are also equal, the same light amount correction value can be used in the regions 1501 and 1502 if the main scanning positions are the same. Similarly, areas 1503 and 1504, areas 1505 and 1506, and areas 1507 and 1508 have the same pattern. Although not shown in the drawing, the same pattern is also present in the region 1509.

Therefore, in the first embodiment, the light amount correction value pattern Gj(x) (j=0 to 4) is created in advance as shown in FIG. 16, and the light amount correction value pattern to be used is switched according to the sub-scanning position of the coordinate of interest. Ylen(j) is set to each light quantity correction value pattern, light quantity correction is performed using Gj(x) in the range of Ylen(j), and the value of pattern number j is changed when the range of Ylen(j) is exceeded. The next light amount correction value set is used by updating. Further, by setting the light amount correction value for each area also in the main scanning direction, the held light amount correction value can be reduced.

FIG. 17 is a diagram illustrating a light amount correction value for each area in the main scanning direction.

As indicated by areas 1701 and 1702, within the range of the sub-scanning direction Ylen(0), the length in the main scanning direction is 3 of Xlen(0,0), Xlen(1,0), and Xlen(2,0). It can be seen that one block area appears repeatedly. This also applies to other sub-scanning positions. For example, even in the areas 1703 and 1704 existing within the range of the sub-scanning direction Ylen(3), the lengths in the main scanning direction are Xlen(0,3) and Xlen( 1,3) and Xlen(2,3) blocks are repeated. At this time, the first block area of the area 1703 starts halfway. The start position of the block at this time is defined as Shift(j).

By knowing the length Xlen(i,j) of the block area in the main scanning direction and the start position Shift(j) of the block, it is possible to estimate in which area the pixel of interest exists from the main scanning position of the pixel of interest. The corrected light amount correction value can be acquired. FIG. 18 shows the light quantity correction value assigned to each block area.

FIG. 18 is a diagram showing an example in which a light amount correction value is assigned to each block area in the first embodiment.

From the coordinates of the target pixel, it is specified that the target pixel corresponds to the i-th block in the main scanning direction in the j-th pattern in the sub-scanning direction, and the light amount correction value G(i,j) corresponding to the block area is acquired. Therefore, the light amount of the pixel of interest is corrected.

Next, with reference to FIG. 19, a process of calculating the pattern number j in the sub-scanning direction and the block number i in the main scanning direction from the coordinates of the pixel of interest and acquiring the light amount correction value G(i,j) will be described.

FIG. 19 is a flowchart illustrating in more detail the process of acquiring the light amount correction value GI(x, y) in S1402 of FIG.

First, in step S1901, the pixel value correction unit 302 obtains a remainder by dividing the sub-scanning position y of the pixel of interest by the sum SumYlen of the lengths Ylen(j) in the sub-scanning direction of each pattern, and sets it as Ynow. Next, proceeding to step S1902, the pixel value correction unit 302 subtracts Ylen(j) from Ynow, and sets the result as a new Ynow. Next, proceeding to step S1903, the pixel value correction unit 302 determines whether Ynow is less than 0. If Ynow is not less than 0, the process advances to step S1904, and the pixel value correction unit 302 adds 1 to the pattern number j and advances to step S1902. On the other hand, if Ynow is less than 0 in step S1903, the processing proceeds to step S1905, and the pixel value correction unit 302 adds shift(j) to the main scanning position x of the pixel of interest, sets the result as Xnow, and proceeds to step S1906. In step S1906, the pixel value correction unit 302 subtracts Xlen(i) from Xnow, and sets the result as a new Xnow. Then, the processing proceeds to step S1907, and the pixel value correction unit 302 determines whether Xnow is less than 0. If Ynow is not less than 0, the process advances to step S1908, the pixel value correction unit 302 adds 1 to the block number i, and the process advances to step S1906. If Ynow is less than 0 in step S1907, the process advances to step S1909, and the pixel value correction unit 302 acquires the light amount correction value G(i,j) from the light amount profile storage unit 304 based on the pattern number j and the block number i, and the pixel number The light amount correction value GI(x, y) is set.

FIG. 20 shows a halftone image obtained as a result of the pixel value correction unit 302 performing the pixel value light amount correction processing and the image forming processing unit 303 performing the screen processing on the obtained image.

FIG. 20A is a diagram showing an example of a halftone dot image obtained by performing screen processing on an image in which pixel values are corrected based on the light amount characteristic for each main scanning position shown in the above problem.

FIG. 20B is a diagram showing an example of a halftone dot image obtained by performing screen processing on an image in which pixel values are corrected based on the light amount characteristic of the dither matrix shown in the first embodiment.

In FIG. 20A, since the pixel value is corrected for each main scanning position, the dot size of the halftone dots is not constant. On the other hand, in the correction process for each dither matrix region shown in FIG. 20B according to the first embodiment, it can be seen that the image forming process is performed without the dots of the halftone dots changing significantly.

As described above, according to the first embodiment, the light amount characteristic in the entire dither matrix region corresponding to the pixel of interest is obtained based on the light amount information of the line head, and the light amount correction value is acquired based on the light amount characteristic. Then, by correcting the pixel value using the light amount correction value, it is possible to suppress the generation of light and shade (streaks and unevenness) due to the variation in the light amount for each pixel while suppressing the change in the dot size of the halftone dots. There is an effect that you can.

[Embodiment 2]
In the above-described first embodiment, an example has been described in which the light amount correction value is calculated for each dither matrix used for the screen processing, and the pixel value of the input image is corrected using the light amount correction value. However, in general screen processing, the dither matrix may be switched within the image according to the image attributes such as characters and photographs. In such a case, it is necessary to grasp the switching position of the dither matrix in advance before the image forming process to correct the pixel value, which increases the calculation cost. Therefore, in the second embodiment, a method of correcting the threshold value of the dither matrix used for the screen processing based on the light amount information of the line head will be described. The hardware configuration and the like of the image forming apparatus 101 according to the second embodiment is the same as that of the above-described first embodiment, and thus the description thereof will be omitted.

FIG. 21 is a block diagram illustrating the configuration of the image processing unit 202 according to the second embodiment of the present invention. Note that, in FIG. 21, portions common to those in FIG. 3 described above are denoted by the same reference numerals, and description thereof will be omitted.

The image formation processing unit 303 receives the threshold value of the correction dither matrix that has been subjected to the light amount correction processing from the threshold value correction unit 2101, and performs screen processing on the input image using the correction dither matrix threshold value. The threshold value correction unit 2101 is a processing unit that corrects the threshold value of the dither matrix in accordance with the light amount characteristic of the LED line head of each color plate used when performing exposure in the printer engine 105. The light amount characteristic of each color plate is received from the light amount profile storage unit 304.

Next, with reference to FIG. 22, a dither matrix threshold value correction process based on the light amount characteristic in units of dither matrix will be described. Here, it is assumed that screen processing is performed on the target dither matrix area 1201 of FIG. 12B using the threshold value of the dither matrix of FIG. 22A. The threshold values of the dither matrix in FIG. 22A are the same as those in FIG. 5 according to the first embodiment described above. As described above with reference to FIG. 13A, the light amount characteristic Ld in the dither matrix area 1201 of interest is 1.06. Therefore, when the screen processing is performed using the same dither matrix threshold as the reference light amount characteristic (=1), the light amount characteristic increases by 0.06 each time one pixel dot is turned on. Therefore, the light amount of the entire dither matrix region of interest increases faster than the pixel value of the input image.

Therefore, the threshold value of the dither matrix is increased by the light amount characteristic of the dither matrix region and the dot lighting is delayed to perform the light amount correction process. In the second embodiment, the light amount correction value GD(x, y)=Ld for the dither matrix threshold value is set, and the light amount correction value GD(x, y) is multiplied by the base dither matrix threshold value D(x, y). A correction threshold D'(x, y) is calculated.

Here, when the threshold value of the dither matrix of FIG. 22A is used for the dither matrix area of interest 1201 of FIG. 12B, the light amount characteristic Ld of the dither matrix area of interest 1201 is “1.06”, and thus the dither matrix of FIG. The light amount correction value GD(x, y)=1.06 is applied to the threshold value. Then, FIG. 22B shows a correction threshold G′(x, y) obtained as a result of multiplying each threshold by GD(x, y). In the second embodiment, the correction threshold G′(x, y) is a value obtained by rounding off the fractional part of the value obtained by multiplying each threshold by GD(x, y).

As described above, the light amount is corrected by correcting the threshold value of the dither matrix according to the light amount characteristic and adjusting the dot growth order. In the second embodiment, GD(x, y)=Ld is set and all dither matrix threshold values are multiplied by the light amount correction value GD(x, y). However, the present invention is not limited to this, and weighting is performed for each threshold value. May be.

FIG. 23 is a flowchart illustrating a pixel value correction process according to the light amount characteristic performed by the threshold value correction processing unit 2101 according to the second embodiment.

First, in step S2301, the threshold value correction processing unit 2101 acquires the threshold value D(x, y) of the dither matrix corresponding to the target pixel that is the screen processing target. Next, the processing proceeds to step S2302, and the threshold value correction processing unit 2101 acquires the light amount correction value GD(x, y) to the threshold value corresponding to the coordinates (x, y) of the pixel of interest from the light amount profile storage unit 304. Then, the processing proceeds to step S2303, and the threshold value correction processing unit 2101 multiplies the dither matrix threshold value D(x, y) by the light amount correction value GD(x, y) to obtain the threshold value, thereby obtaining the light amount correction threshold value D'(x, y). To get.

Then, the light amount correction threshold value D′(x, y) is output to the image forming processing unit 303, and the image forming processing unit 303 performs screen processing by using the corrected threshold value of the dither matrix, whereby the dither matrix is obtained. It is possible to perform image forming processing according to the unit light amount characteristic. The multiplication process shown in FIG. 23 may be implemented by a multiplication circuit or the like that outputs the multiplication result by using the threshold data forming the dither matrix and the light amount profile as an input signal, or by a general-purpose processor. ..

As described above, according to the second embodiment, the light amount characteristic in the entire dither matrix region corresponding to the pixel of interest is obtained based on the light amount information of the line head, and the light amount correction value is obtained based on the light amount characteristic in the entire dither matrix region. get. Then, by correcting the threshold value of the dither matrix using this light amount correction value, it is possible to suppress the generation of light and shade (streaks and unevenness) due to the light amount variation between pixels while suppressing the change in the dot size of the halftone dots. The effect that it can be suppressed is obtained.

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

The present invention is not limited to the above embodiments, and various modifications and changes can be made without departing from the spirit and scope of the present invention. Therefore, in order to make the scope of the present invention public, the following claims are attached.

301... Color conversion processing unit, 302... Pixel value correction unit, 303... Image formation processing unit, 304... Light intensity profile storage unit, 305... Dither matrix storage unit

Claims (21)

An image forming apparatus for forming an image by irradiating light modulated according to image data from a line head having a plurality of light emitting elements,
Storage means for storing light quantity characteristic data of the plurality of light emitting elements,
A first acquisition unit that acquires, from the storage unit, the light amount characteristic data of the light emitting element corresponding to the target area of the screen processing of the dither matrix corresponding to the target pixel when performing the screen processing on the target pixel;
Second acquisition means for acquiring the light amount characteristic of the light emitting element corresponding to the target region based on the light amount characteristic data of the target region;
Correction means for correcting the pixel value of the pixel of interest based on the light quantity characteristic;
An image forming apparatus comprising: The image forming apparatus according to claim 1, wherein the dither matrix has a threshold value for controlling the growth of the screen in the screen processing. The image forming apparatus according to claim 2, wherein the dither matrix has a shape obtained by combining a plurality of matrices in which the threshold values are arranged in square shapes having different sizes. The image forming apparatus according to claim 3, wherein the dither matrix is repeatedly used without a gap so that screen processing can be performed on all pixels of the image data. Further comprising storage means for storing light quantity characteristics corresponding to each coordinate of the pixel of interest based on the light quantity characteristic data,
5. The image forming apparatus according to claim 1, wherein the correction unit corrects the pixel value of the target pixel based on the light amount characteristic stored in the storage unit. When the dither matrix is repeatedly used for the image data, when the same pattern is repeated at a position in the main scanning direction of the image data in the sub-scanning direction to which the dither matrix is applied, Save the light quantity characteristics in association with the same pattern,
The image forming apparatus according to claim 5, wherein the correction unit corrects the pixel value of the target pixel based on the light amount characteristic corresponding to the position in the sub-scanning direction. When the dither matrix is repeatedly used for the image data and the same pattern is repeated in the main scanning direction to which the dither matrix is applied, the storage means has the same length in the main scanning direction as the same pattern. Save the light intensity characteristics in association with the pattern,
The correction unit corrects the pixel value of the pixel of interest based on the light amount characteristic corresponding to the length of the same pattern in the main scanning direction and the position of the pixel of interest in the main scanning direction. The image forming apparatus according to 5 or 6. The image forming apparatus according to claim 1, wherein the light amount characteristic is an average value of light amount characteristic data of the light emitting elements included in the target area. 9. The correction unit corrects the pixel value of the pixel of interest by multiplying the pixel value of the pixel of interest by a value obtained by dividing a reference light amount characteristic by the light amount characteristic. The image forming apparatus according to any one of items. An image forming apparatus for forming an image by irradiating light modulated according to image data from a line head having a plurality of light emitting elements,
Storage means for storing light quantity characteristic data of the plurality of light emitting elements,
A first acquisition unit that acquires, from the storage unit, the light amount characteristic data of the light emitting element corresponding to the target area of the screen processing of the dither matrix corresponding to the target pixel when performing the screen processing on the target pixel;
Second acquisition means for acquiring the light amount characteristic of the light emitting element corresponding to the target region based on the light amount characteristic data of the target region;
Correction means for correcting the threshold value of the dither matrix based on the light amount characteristic;
An image forming apparatus comprising: The image forming apparatus according to claim 10, wherein the dither matrix has a threshold value for controlling the growth of the screen in the screen processing. The image forming apparatus according to claim 11, wherein the dither matrix has a shape obtained by combining a plurality of matrices in which the threshold values are arranged in square shapes having different sizes. The image forming apparatus according to claim 12, wherein the dither matrix is repeatedly used without a gap so that screen processing can be performed on all pixels of the image data. Further comprising storage means for storing light quantity characteristics corresponding to each coordinate of the pixel of interest based on the light quantity characteristic data,
14. The image forming apparatus according to claim 10, wherein the correction unit corrects the threshold value of the dither matrix based on the light amount characteristic stored in the storage unit. When the dither matrix is repeatedly used for the image data, when the same pattern is repeated at a position in the main scanning direction of the image data in the sub-scanning direction to which the dither matrix is applied, Save the light quantity characteristics in association with the same pattern,
The image forming apparatus according to claim 14, wherein the correction unit corrects the threshold value of the dither matrix based on the light amount characteristic corresponding to the position in the sub-scanning direction. When the dither matrix is repeatedly used for the image data and the same pattern is repeated in the main scanning direction to which the dither matrix is applied, the storage means has the same length in the main scanning direction as the same pattern. Save the light intensity characteristics in association with the pattern,
The correction unit corrects the threshold value of the dither matrix based on the light amount characteristic corresponding to the length of the same pattern in the main scanning direction and the position of the pixel of interest in the main scanning direction. The image forming apparatus according to item 15. The image forming apparatus according to claim 10, wherein the light amount characteristic is an average value of light amount characteristic data of the light emitting elements included in the target region. 18. The correction unit corrects the threshold value of the dither matrix by multiplying the pixel value of the target pixel by a value obtained by dividing a reference light amount characteristic by the light amount characteristic. 2. The image forming apparatus according to item 1. A control method for controlling an image forming apparatus that forms an image by irradiating light modulated according to image data from a line head in which a plurality of light emitting elements are arranged,
A first acquisition step of acquiring light amount characteristic data of a light emitting element corresponding to a target area to be subjected to screen processing of the dither matrix corresponding to the target pixel, in the screen processing on the target pixel;
A second acquisition step of acquiring the light amount characteristic of the light emitting element corresponding to the target region based on the light amount characteristic data of the target region;
A correction step of correcting the pixel value of the target pixel based on the light amount characteristic,
A control method comprising: A control method for controlling an image forming apparatus that forms an image by irradiating light modulated according to image data from a line head in which a plurality of light emitting elements are arranged,
A first acquisition step of acquiring light amount characteristic data of a light emitting element corresponding to a target area to be subjected to screen processing of the dither matrix corresponding to the target pixel, in the screen processing on the target pixel;
A second acquisition step of acquiring the light amount characteristic of the light emitting element corresponding to the target region based on the light amount characteristic data of the target region;
A correction step of correcting the threshold value of the dither matrix based on the light amount characteristic,
A control method comprising: A program for causing a computer to execute each step of the control method according to claim 19 or 20.

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