JP4395743B2 - Image forming apparatus and positional deviation correction method - Google Patents

Description

The present invention includes a photosensitive drum for each color of Y (yellow), M (magenta), C (cyan), and K (black), and an optical scanning system that forms an electrostatic latent image on the surface of the photosensitive drum. The present invention relates to an image forming apparatus that forms a full-color image by superimposing and transferring toner images exposed on the respective photoreceptors onto the same recording medium, and a positional deviation correction method for the image forming apparatus.

  An image forming apparatus such as a color laser printer performs a printing operation at a high speed, so that a photosensitive drum is formed for each color image of Y, M, C, and K, and optical scanning for forming an electrostatic latent image on the surface of the photosensitive drum. System. The respective toner images are formed on the photosensitive drums of the respective colors, and transferred onto the recording medium at the transfer positions of the photosensitive drums.

  However, due to variations in the optical characteristics and mechanical accuracy of each color, misalignment tends to occur when the images of each color are superimposed. In order to obtain a high-quality image output, it is necessary to accurately align the colors so that they overlap at the same position.

  In particular, correction factors for alignment in the main scanning direction include a shift in the writing position in the main scanning direction (side registration), the length of one line in the main scanning direction (total magnification), center registration alignment (horizontal magnification), and the like. Various correction methods have been proposed.

  For example, the method of adjusting the optical scanning system of the laser beam mechanically, or adjusting the video clock frequency by controlling the voltage to the voltage controlled oscillator (VCO) of the PLL circuit, the total magnification and the horizontal magnification There are known methods for correcting the above.

The Patent Documents 1 and 2, in accordance with the shift amount of the desired printing position, is performed inserted or deleted to correct the integral multiple of the sub-pixels of 1 Pi Kuseru or 1 pixel. Further, when inserting pixels, the pixels are randomly shifted for each line in the main scanning direction.

JP 2001-5245 A JP 2001-71562 A

  However, the above-described method of mechanically adjusting the optical scanning system has a problem that it is expensive to correct with high accuracy. Further, in the method of controlling the voltage to the VCO of the PLL circuit, it is necessary to control the analog circuit on the time axis, and the analog circuit is adjusted with sufficient accuracy due to the response characteristics of the analog circuit and the variation of the analog circuit characteristics for each color. It is difficult to configure an analog circuit at a low cost.

  In Patent Documents 1 and 2, in order to suppress deterioration in image quality in a direction perpendicular to the main scanning direction, the pixel or sub-pixel insertion position in each line in the main scanning direction is randomly shifted. However, when random pixels or sub-pixels are inserted, there is a problem that the color of the entire image changes and a desired image quality cannot be obtained.

The present invention has been made in view of the above circumstances, and provides an image forming apparatus and a misregistration correction method that correct a misregistration of a color image and more appropriately prevent a change in color of the image at the time of correction. The purpose is to do.

In order to achieve such an object, the image forming apparatus of the present invention calculates a misregistration amount representing a misregistration amount of an image forming position between a reference color component and other color components among color components of a color image. And a signal of a predetermined frequency for correcting the positional deviation amount for each of the other color components based on the positional deviation amount calculated for each of the other color components by the positional deviation amount calculation unit. One pixel of the video clock signal by comparing the signal generating means to be generated, the frequency of the signal generated by the signal generating means and the frequency of the video clock signal defining the width of one pixel when forming an image The error calculation means provided for each of the other color components, and the error calculated by the error calculation means are accumulated, and when the error exceeds a predetermined value, the pixel width is calculated. Instruct the correction of The error accumulating means provided for each of the other color components, and when receiving the correction instruction, the target pixel to be corrected, and the image data of the target pixel are converted into pseudo multi-valued image data The density of the target pixel, which is a pixel in the screen matrix used at the time, is combined with the peripheral pixels of the target pixel, and the target pixel is sub-pixels so that the density of the target pixel does not change or becomes small. And a correction unit provided for each of the other color components, in which a sub-pixel is deleted from the target pixel.
ADVANTAGE OF THE INVENTION According to this invention, while changing the position shift of a color image, the change of the color of the image at the time of correction | amendment can be prevented more appropriately.

In the image forming apparatus, when the correction unit inserts a sub-pixel into the target pixel, an image of a target color component or a white image is applied to the target pixel based on the density of the target pixel. It is good to insert.

  In the image forming apparatus, the correction unit includes image information reference unit that determines whether or not the target pixel includes an edge of an image with reference to density information of the target pixel, and the target pixel is an image. When the edge is included, sub-pixel insertion or thinning may be performed so as to retain the edge. When the pixel of interest includes an edge of the image, the subpixel is inserted or thinned out so as to retain the edge, so that the edge of the image is not broken by the insertion of the subpixel.

  In the image forming apparatus, the image information reference unit may determine whether the pixel of interest includes an edge of an image with reference to density information of other colors. Since it is determined whether or not the target pixel includes the edge of the image with reference to the density information of the other colors, the edge can be held in the entire image.

  In the above-described image forming apparatus, the correction unit may control the insertion position of the sub-pixel so as to prevent an isolated point by referring to pulse position information indicating a coloring position of the image. Since the pulse position information indicating the coloring position of the image is referred to, the subpixel insertion position can be controlled so that no isolated point is formed.

  In the image forming apparatus, the correction unit may correct a density change caused by insertion or thinning of the subpixel in the target pixel by inserting or thinning the subpixel into a peripheral pixel of the target pixel. Since the density change caused by the insertion or thinning of the subpixel in the target pixel is corrected by inserting or thinning the subpixel into the peripheral pixel of the target pixel, the density change can be further reduced.

The misregistration correction method of the present invention includes a step of calculating a misregistration amount representing a misregistration amount of an image forming position between a reference color component and other color components among color components of a color image, and the other Generating a signal of a predetermined frequency for correcting the amount of misregistration for each of the other color components based on the amount of misregistration calculated for each color component, the frequency of the generated signal, and when forming an image Comparing the frequency of the video clock signal that defines the width of one pixel of the video clock signal, calculating an error in the scanning time of one pixel of the video clock signal for each of the other color components, and calculating the calculated error A step of instructing correction of the pixel width of the corresponding color component when the error exceeds a predetermined value, a target pixel of the color component to be corrected when receiving the correction instruction, The image data of the pixel of interest Obtain the density of the target pixel that is a pixel in the screen matrix used when converting into similar multi-valued image data, together with the peripheral pixels of the target pixel, and whether there is a density change of the target pixel, Or inserting a sub-pixel into the target pixel so as to be smaller, or deleting a sub-pixel from the target pixel.
According to the present invention, it is possible to correct a positional deviation of a color image and more appropriately prevent a change in the color of the image at the time of correction.

  The present invention can prevent a change in color due to color registration correction.

  The best embodiment of the present invention will be described with reference to the accompanying drawings.

  First, the configuration of the present embodiment will be described with reference to FIG. In FIG. 1, a document 2 placed on a document table 1 is R (red) and G (green) by an image scanner including a color CCD sensor 3 via a scanning optical system including a light source and a scanning mirror. , B (blue) image signals. The R, G, B image signals read by the color CCD sensor 3 are converted into Y (yellow), M (magenta), C (cyan), and K (black) colors by the image processing unit 25 in the printer controller unit 4. The image data of each color is sequentially output to ROS (Raster Output Scanner) 7C, 7M, 7Y, 7K of the image forming units 5C, 5M, 5Y, 5K, and the ROS 7C, 7M, 7Y, 7K according to the image data. The emitted laser beam is scanned and exposed on the surface of each of the photosensitive drums 6C, 6M, 6Y, and 6K to form an electrostatic latent image. The electrostatic latent images formed on the photosensitive drums 6C, 6M, 6Y, and 6K are developed as toner images of C, M, Y, and K colors by the developing devices 8C, 8M, 8Y, and 8K, respectively.

  Further, the misregistration amount measuring unit 9 shown in FIG. 1 calculates the misregistration amounts of the C, M, Y, and K color images forming the image. There are various methods for measuring the amount of misalignment by the misalignment measuring unit 9. For example, as shown in FIG. 2, a registration deviation measurement pattern P is formed over the entire circumference of the transfer belt 50, and this registration deviation measurement pattern P is repeated at a predetermined timing by the positional deviation measurement unit 9 (see FIG. 1). Sampling. From the sampling data obtained in this way, for example, the registration deviation amount of each color (yellow, magenta, cyan) with reference to black is detected, and these are divided by the number of samplings to calculate the average registration deviation amount R. Registration correction is performed based on the deviation amount R.

  The configuration of the printer controller unit 4 will be described with reference to FIG. As shown in FIG. 3, the printer controller unit 4 includes an I / O interface 21, a CPU 22, a bus bridge 23, a memory 24, an image processing unit 25, a PWM (Pulse Width Modulation) 26, a video clock, and a sub clock. And a pixel clock generation unit 27. The printer controller unit 4 operates under the control of the CPU 22.

  The R, G, B image signals read by the color CCD sensor 3 are input to the printer controller unit 4 by the I / O interface 21 and stored in the memory 24 by the transfer control of the bus bridge 23.

  The image processing unit 25 performs processing such as a halftone dot method, a spiral method, and a systematic dither method such as a Bayer on the multi-valued image data read from the memory 24, and artificially multi-values using the binary image data. Output. For example, a grid-like matrix such as 4 × 4 dots, 8 × 8 dots, and 16 × 16 dots is virtually regarded as one pixel, and each dot is set as a white or black dot that determines the gradation of the pixel, and its position The gradation of the pixel is changed depending on the number. The image processing unit 25 outputs density information of each pixel (pixel) and tag data indicating a position where a color is arranged in one pixel in order to express a multi-value (256 gradations) image. . For example, when the density information is 12.5% and the tag data indicates left justification, one pixel as shown in FIG. 5 is formed.

  The PWM 26 receives density information for each pixel and tag data from the image processing unit 25. A video clock, a subpixel clock, insertion position information, and color information (details will be described later) are input from the video clock generation unit 27. From these pieces of information, a PWM signal for switching on / off of the ROSs 7C, 7M, 7Y, 7K of the image forming unit 7 is generated.

  The video clock / sub-pixel clock generation unit 27 generates a video clock in which one cycle represents the scanning time of one pixel and a sub-pixel clock having a cycle equal to or shorter than the scanning time of one pixel, and outputs them to the PWM 26.

  FIG. 4 shows the configuration of the video clock / subpixel clock generator 27. As shown in FIG. 4, the video clock / subpixel clock generation unit 27 includes a subpixel clock generation unit 31, a reference clock count unit 32, an image information reference unit 33, a subpixel clock insertion position control unit 34, and a video. A clock generation unit 35, an error calculation unit 36, and an error accumulation unit 37 are included. The image information reference unit 33, the subpixel clock insertion position control unit 34, the error calculation unit 36, and the error accumulation unit 37 function as a correction unit 30 that corrects the positional deviation of the color image. The video clock and subpixel clock generation unit 27 shown in FIG. 4 is provided for each of the C, M, Y, and K color components that form an image.

  The subpixel clock generation unit 31 generates a subpixel clock having a period equal to or shorter than the scanning time of one pixel. A sub-pixel clock that defines a sub-pixel obtained by dividing one pixel into a predetermined number is generated.

  The reference clock count unit 32 counts the number of subpixel clocks generated by the subpixel clock generation unit 31. The count value is output to the video clock generator 35.

  The error calculator 36 calculates an error between the frequency of the video clock generated by the video clock generator 35 and the target frequency notified by the CPU 22. That is, an error between the scanning time of one pixel of the reference color component and the scanning time of one pixel of the video clock is obtained. Note that the CPU 22 obtains the target frequency of the video clock for correcting the positional deviation based on the registration deviation amount of each color (black, yellow, magenta, cyan) measured by the positional deviation amount measuring unit 9 shown in FIG. ing. This target frequency is notified to the error calculator 36.

  The error accumulating unit 37 accumulates the error calculated by the error calculating unit 36, and when the accumulated error exceeds the threshold value, the correction instruction information for instructing correction is supplied to the subpixel clock insertion position control unit 34, the image The information is output to the information reference unit 33. The correction instruction information includes an instruction for the number of subpixels to be inserted or thinned out. Further, the error accumulating unit 37 initializes the accumulated error according to the correction amount of the correction target pixel notified by the subpixel clock insertion position control unit 34.

  The image information reference unit 33 refers to the image information of the target pixel and its surrounding pixels from the image processing unit 25. The image information includes density information for each color (C, M, Y, K) and tag data (pulse position information). The peripheral pixel is a block including the pixel of interest, and can include, for example, a screen matrix used by the image processing unit 25 for screen processing. The image information reference unit 33 refers to the density information of the target pixel to determine whether the target pixel includes the edge of the image. The density information to be noticed at this time refers not only to the density information of the color image being corrected, but also to the density information of other colors. When the target pixel includes an edge of the image, the sub-pixel clock insertion position control unit 34 is notified of edge position information so that the edge is not broken by insertion or thinning of the sub-pixel. By referring to density information of other colors as well, it is possible to prevent a change in color in the entire image.

  The subpixel clock insertion position control unit 34 controls the position where the subpixel clock is inserted based on the density information of the target pixel and its surrounding pixels acquired by the image information reference unit 33 and the correction instruction information from the error accumulation unit 37. To do. In addition, when a sub-pixel is inserted, it is determined whether to insert a color image or a white image from the density information of the target pixel and its surrounding pixels. The insertion position information and color information output from the subpixel clock insertion position control unit 34 are input to the video clock generation unit 35 and also output to the PWM 26.

  Edge position information indicating whether the target pixel corresponds to an edge of the image is also output from the image information reference unit 33 to the subpixel clock insertion position control unit 34. The subpixel clock insertion position control unit 34 controls the position of the subpixel to be inserted so as not to break the edge of the image based on the edge position information. Furthermore, since the subpixel clock insertion position control unit 34 can recognize the coloring position of the image based on the tag data, the subpixel clock insertion position control unit 34 performs control so that the inserted subpixel does not become an isolated point.

  The video clock generator 35 receives the subpixel clock from the subpixel clock generator 31. Further, the count value of the subpixel clock is input from the reference clock count unit 32. Further, an insertion instruction or deletion instruction signal (insertion position information) is input from the subpixel clock insertion position control unit 34. The video clock generation unit 35 receives these signals and generates a video clock that defines the width of one pixel of the print image. For example, a video clock composed of N subpixel clocks (N is an arbitrary natural number) is generated based on the count value of the subpixel clocks from the reference clock count unit 32. In addition, when the subpixel clock insertion position control unit 34 receives a subpixel clock insertion instruction or deletion instruction, the video clock generation unit 35 performs thinning or insertion of the subpixel clock, and other than N (for example, N + 1, N + 2, etc.). N−1, N−2, etc.) is generated.

  The operation of the video clock / subpixel clock generator 27 will be described with reference to FIG. The subpixel clock generated by the subpixel clock generation unit 31 is output to the video clock generation unit 35 and the reference clock count unit 32. The reference clock count unit 32 counts the number of subpixel clocks and outputs the count value to the video clock generation unit 35.

  The error calculator 36 calculates an error between the video clock frequency output from the video clock generator 35 and the target frequency. The error information is output to the error accumulating unit 37. The error accumulating unit 37 accumulates the error between the video clock frequency and the target frequency from the error information, and outputs correction instruction information to the subpixel clock insertion position control unit 34 when the accumulated result exceeds the threshold value. To do. Receiving the correction instruction information, the sub-pixel clock insertion position control unit 34 provides an insertion instruction (insertion position information) for inserting the sub-pixel clock at a predetermined position and a deletion instruction (insertion position information) for thinning out the sub-pixel clock at the predetermined position. The data is output to the clock generator 35. The subpixel clock insertion position control unit 34 determines color information indicating which of the color image and the white image is to be inserted from the density information of the target pixel and its surrounding pixels when the subpixel is inserted. The color image to be inserted is determined based on the density information referred to by the image information reference unit 33 so that there is no change in the density of the target pixel, or the target pixel and its surrounding pixels, or is small. The determined color information is output to the video clock generator 35 and the PWM 26.

  As shown in FIG. 6, the video clock generation unit 35 that generates a video clock composed of three subpixel clocks normally inserts a subpixel clock according to an insertion instruction from the subpixel clock insertion position control unit 34, A video clock composed of four subpixel clocks is generated.

  Note that the density change caused by the insertion or thinning of the subpixel in the target pixel may be corrected by inserting or thinning the subpixel into the peripheral pixel of the target pixel. Since the density change caused by the insertion or thinning of the subpixel in the target pixel is corrected by inserting or thinning the subpixel into the peripheral pixel of the target pixel, the density change can be further reduced.

  The control of the subpixel clock insertion position by the subpixel clock insertion position control unit 34 will be described with reference to FIGS. One pixel shown in FIG. 7 is composed of eight sub-pixels, and only the leftmost sub-pixel is black, the others are white, and the density is 12.5%. When performing registration correction for increasing one subpixel to one pixel, if a white subpixel is inserted, the density of one pixel becomes 11.1%. When a black subpixel is inserted, the density of one pixel is 22.2%. In conventional registration correction, it has not been possible to control which of 11.1% and 22.2%, but in this embodiment, subpixels are inserted so that there is no change in density or the minimum. Therefore, a white sub-pixel is inserted so that the density becomes 11.1%.

  In addition, when performing registration correction for increasing one subpixel to one pixel of 62.5% density shown in FIG. 8, if a white subpixel is inserted, the density of one pixel becomes 55.5%, and the black subpixel becomes dark. When a pixel is inserted, the density of one pixel is 66.6%. For this reason, the sub-pixel clock insertion position control unit 34 inserts black sub-pixels so as to minimize the density change, and sets the density of one pixel to 66.6%.

  In addition, when registration correction is performed on one pixel having a 50.0% density shown in FIG. 9, the density of one pixel is 55.5% when a black subpixel is inserted, and 1 when a white subpixel is inserted. The pixel density is 44.4%. In such a case, the subpixel clock insertion position control unit 34 can also insert two white and black subpixels in one pixel so that the density change is minimized.

  Further, instead of referring only to the target pixel to be corrected, the position of the sub-pixel to be inserted or thinned out may be determined according to the situation of the pixels around the target pixel. For example, consider a case where four 2 × 2 pixels shown in FIG. 10 are referred to. As shown in FIG. 10, the density of the entire four pixels is 68.8%, and the density of the target pixel is 12.5%. Looking at the change in density of the pixel of interest, the density change is smaller when the white subpixel is inserted. However, when the density change of the entire four referenced pixels is seen, the change in density can be obtained by inserting the black subpixel. Can be suppressed.

  In this way, in this embodiment, since the pixels are inserted or thinned out so that there is no change in the density of the target pixel or the target pixel and its surrounding pixels, or the thinning is reduced, it is possible to prevent a change in the color of the image. .

  In addition, the video clock frequency can be adjusted with an extremely simple and inexpensive circuit configuration to correct the total magnification and the left / right magnification.

  The embodiment described above is a preferred embodiment of the present invention. However, the present invention is not limited to this, and various modifications can be made without departing from the scope of the present invention. For example, in the above-described embodiment, the processing for the image read by the scanner has been described. However, a good image can be formed by performing the above-described processing on the image data created by a personal computer or the like.

1 is a diagram illustrating a configuration of an image forming apparatus. It is a figure for demonstrating the measuring method of the positional offset amount by the positional offset amount measurement part 9. FIG. 2 is a diagram illustrating a configuration of a printer controller unit 4; FIG. 3 is a diagram illustrating a configuration of a video clock and subpixel clock generation unit 27. It is a figure for demonstrating density information and tag data. 6 is a diagram for explaining control of a subpixel clock insertion position control unit 34 and an error accumulation unit 37. FIG. It is a figure explaining the insertion position of a sub pixel. It is a figure explaining the insertion position of a sub pixel. It is a figure explaining the insertion position of a sub pixel. It is a figure explaining the insertion position of a sub pixel.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Original stand 2 Original 3 Color CCD sensor 4 Printer controller part 5C, 5M, 5Y, 5K Image forming unit 6C, 6M, 6Y, 6K Photosensitive drum 7C, 7M, 7Y, 7K ROS
8C, 8M, 8Y, 8K Developer 9 Position shift amount measurement unit 21 I / O interface 22 CPU 23 Bus bridge 24 Memory 25 Image processing unit 26 PWM
27 Video clock and subpixel clock generation unit 30 Correction unit 31 Subpixel clock generation unit 32 Reference clock count unit 33 Image information reference unit 34 Subpixel clock insertion position control unit 35 Video clock generation unit 36 Error calculation unit 37 Error accumulation unit

Claims (7)

A positional deviation amount calculating means for calculating a positional deviation amount representing a deviation amount of an image forming position between a reference color component and other color components among the color components of the color image;
A signal generation unit configured to generate a signal of a predetermined frequency for correcting the positional shift amount for each of the other color components based on the positional shift amount calculated for the other color component by the positional shift amount calculation unit;
Comparing the frequency of the signal generated by the signal generating means with the frequency of the video clock signal that defines the width of one pixel when forming an image, the error of the scanning time of one pixel of the video clock signal is calculated. Error calculating means provided for each of the other color components;
An error accumulating unit provided for each of the other color components, which accumulates the error calculated by the error calculating unit and instructs correction of a pixel width when the error exceeds a predetermined value;
The pixel of interest, which is a pixel in the screen matrix used when converting the pixel data of the pixel of interest into pseudo multivalued image data when receiving the correction instruction Determining the density of the target pixel combined with the peripheral pixels of the target pixel, and inserting or subtracting the subpixel from the target pixel so that the density of the target pixel does not change or becomes small, Correction means provided for each of the other color components ;
An image forming apparatus comprising: The correction means inserts an image of a target color component or a white image into the target pixel based on the density of the target pixel when a sub-pixel is inserted into the target pixel. The image forming apparatus according to claim 1. The correction means has image information reference means for referring to density information of the target pixel to determine whether the target pixel includes an edge of an image,
3. The image forming apparatus according to claim 1, wherein when the target pixel includes an edge of an image, subpixel insertion or thinning is performed so as to hold the edge. The image forming apparatus according to claim 3, wherein the image information reference unit determines whether the target pixel includes an edge of an image with reference to density information of another color. The correction means refers to the pulse position information indicating the coloring position of an image, according to any one claim of 4 claim 1, characterized in that for controlling the insertion position of the sub-pixels so as not to isolated point Image forming apparatus. Said correction means, 5 a density change caused by insertion or thinning of the sub-pixels to the target within the pixel, the claim 1, characterized in that the correction by the subpixel insertion or thinning to the surrounding pixels of the pixel of interest The image forming apparatus according to claim 1. Calculating a positional deviation amount representing a deviation amount of an image forming position between a reference color component and other color components among the color components of the color image;
Generating a signal of a predetermined frequency for correcting the positional deviation amount for each of the other color components based on the positional deviation amount calculated for each of the other color components;
By comparing the frequency of the generated signal with the frequency of the video clock signal that defines the width of one pixel when forming an image, the error of the scanning time of one pixel of the video clock signal is determined as the other color component. Calculating each step,
Instructing correction of the pixel width of the corresponding color component when the calculated error is accumulated and the error exceeds a predetermined value;
When receiving the correction instruction, the target pixel of the color component to be corrected, and the pixel in the screen matrix used when converting the image data of the target pixel into pseudo multi-valued image data. The density of the target pixel combined with the surrounding pixels of the target pixel is obtained, and a sub-pixel is inserted into the target pixel so that the density change of the target pixel is not reduced, or a sub-pixel is extracted from the target pixel. A step to delete,
A misregistration correction method characterized by comprising:

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