JP2017163178A - Image processing apparatus and averaging processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an averaging processing with high versatility and expandability.SOLUTION: An image processing apparatus comprises: an averaging processing part that inputs image data at a constant region unit as a center of each pixel, and outputs an average value of a pixel value of each pixel in an averaged pattern as a pixel value of a center pixel of the constant region by verifying the constant region with the averaged pattern; and a screen processing part that periodically verifies each pixel of the averaged image data to a dither matrix, and is made into multi-value data. The averaging processing part determines a shift amount up to the center pixel of the dither matrix to which each pixel belongs from a position of each pixel in the image data by using the averaged pattern of a size and a form similar to the dither matrix, shifts the position of the averaged pattern from the position where the center pixel of the averaged pattern and the center pixel of the constant region are matched by the determined shift amount, and averages the pixel value of each pixel of the post-shift averaged pattern.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an image processing apparatus and an averaging processing method.

  In an image forming apparatus such as a copying machine, screen processing using a dither method or the like is generally performed on image data in order to reproduce halftones. Since an image formed from image data after screen processing has a periodic structure and moire tends to occur, conventionally, moving average processing has been applied to image data for the purpose of breaking the periodic structure of the image.

However, since the moving average process averages not only the periodic structure of the image but also the detail of the image, the sharpness may deteriorate.
Therefore, by averaging the pixel values within the same average pattern as the one-dimensional or two-dimensional pattern periodically formed by the screen processing, the periodic structure of the image can be destroyed while maintaining the detail of the image. An averaging process has been proposed (see, for example, Patent Documents 1 and 2).

JP 2004-364084 A Japanese Patent No. 4189598

  In general, in moving average processing, image data of a pixel of interest and its surrounding pixels are input, and the average value thereof is output as the pixel value of the pixel of interest after moving average processing. As described above, it is common to use a first in first out (FIFO) that delays the input of the image data of each line in the processing circuit that inputs the image data of the target pixel and its peripheral pixels at a time.

On the other hand, the above-described conventional averaging process replaces the pixel values of each pixel in the average pattern at once with their average values. For pixels once averaged, the pixel values after the averaging process are overwritten, so the processing circuit uses the pixel values after the averaging process instead of the original pixel values as in the normal moving average process. A FIFO is used to input data with a delay. Since the use of the FIFO is different from the normal use in which the original pixel value is input after being delayed, it is necessary to prepare a dedicated circuit for the averaging process.
If the FIFO can be used so that the original pixel value is delayed and input as usual, the FIFO can be shared in other image processing other than the averaging processing, and the cost can be reduced.

  In order to reduce image noise, it is effective to extend the averaged area more than the periodic pattern formed by screen processing. Although the overlapping portions of the averaging patterns are generated by the expansion, in the conventional averaging process, the pixel values of the overlapping portions are already overwritten with the average values by the averaging in the previous averaging pattern. In order to obtain a more accurate average value, it is preferable to input the original pixel value instead of the average value after the averaging process.

  Furthermore, a supercell combining a plurality of types of dither matrices having different shapes and sizes may be used during screen processing. There is a demand for an averaging process that can also handle a supercell rather than a single type of dither matrix as in the conventional averaging process.

  An object of the present invention is to provide an averaging process with high versatility and expandability.

According to the invention of claim 1,
Image data is input in units of a fixed area centered on each pixel, the input fixed area is compared with the average pattern, the pixel values of each pixel in the average pattern are averaged, and the average value obtained is , An averaging processing unit that outputs as a pixel value of a central pixel of the fixed region;
A screen processing unit that periodically compares each pixel of the image data averaged by the averaging processing unit with a dither matrix, and compares the pixel value of each pixel with a threshold value in the dither matrix; With
The averaging processing unit
As the average pattern, using an average pattern having the same size and shape as the dither matrix,
Determine the shift amount from the position of each pixel in the image data to the center pixel of the dither matrix to which each pixel belongs when collated with the dither matrix,
The position of the average pattern is shifted by the determined shift amount from the position where the central pixel of the average pattern coincides with the central pixel of the certain region, and each pixel in the average pattern after the shift is shifted. An image processing apparatus is provided that averages pixel values.

According to invention of Claim 2,
The position of each pixel in the block when each pixel of the image data is divided into rectangular blocks having the same cycle as the dither matrix, and the shift from the position of each pixel to the center pixel of the dither matrix to which each pixel belongs A storage unit that stores the amount in association with each other;
The averaging processing unit determines a position in a block of each pixel when each pixel of the image data is divided in units of the block, and acquires the shift amount corresponding to the determined position from the storage unit. An image processing apparatus according to claim 1 is provided.

According to invention of Claim 3,
The screen processing unit periodically collates each pixel of the image data with a supercell that is a combination of a plurality of types of dither matrices,
The averaging processing unit
Determine the shift amount from the position of each pixel in the image data to the center pixel of the dither matrix to which each pixel belongs when collated with the supercell, and the type of the dither matrix,
The average pattern having the same size and shape as the determined type of dither matrix is used, and the position of the average pattern is determined from the position where the central pixel of the average pattern matches the central pixel of the certain region. The image processing apparatus according to claim 1, wherein the image processing apparatus shifts by the shift amount and averages the pixel values of the pixels in the averaged pattern after the shift.

According to invention of Claim 4,
The said averaging process part expands and uses the average pattern of the same size and shape as the said dither matrix to a size larger than the said dither matrix, It is any one of Claims 1-3 characterized by the above-mentioned. The described image processing apparatus is provided.

According to the invention of claim 5,
Inputting image data in units of a fixed area centered on each pixel;
Collating the input fixed area with an average pattern, averaging the pixel values of each pixel in the average pattern, and outputting the obtained average value as the pixel value of the central pixel of the fixed area; Including
The step of averaging the pixel values of each pixel in the average pattern is performed by periodically comparing each pixel of the image data with a dither matrix as the average pattern, and setting the pixel value of each pixel to a threshold value in the dither matrix. Using an averaged pattern of the same size and shape as the dither matrix used for multi-valued screen processing by comparison with
Determining a shift amount from the position of each pixel in the image data to the center pixel of the dither matrix to which each pixel belongs when collated with the dither matrix;
The position of the average pattern is shifted from the position where the central pixel of the average pattern matches the central pixel of the certain region by the determined shift amount, and the pixel of each pixel in the average pattern after the shift A step of averaging the values;
An averaging processing method is provided.

  According to the present invention, it is possible to provide an averaging process with high versatility and expandability.

FIG. 2 is a block diagram illustrating a main configuration of an image forming apparatus for each function. It is a block diagram showing the main structures of the image processing apparatus of this Embodiment for every function. It is a flowchart which shows the process sequence when an averaging process part averages image data. It is a figure which shows an example of the circuit which can be used for the averaging process which concerns on this Embodiment. It is a figure which shows the example of a non-rectangular dither matrix. FIG. 6 is a diagram showing a rectangular dither matrix obtained by converting the non-rectangular dither matrix shown in FIG. 5. It is a figure which shows the dither matrix when collating with each pixel of image data. It is a figure which shows the relationship between the position of each pixel in a rectangular block, and the dither matrix to which each pixel belongs. It is a figure which shows the example of the table which matched the identification number of the dither matrix to which it belongs to the position number showing the position of each pixel in a rectangular block, and the shift amount to the center pixel of the said dither matrix. It is a figure which shows the fixed area | region before shifting the position of an average pattern. It is a figure which shows the fixed area | region after shifting the position of the average pattern. It is a figure which shows the example of the average pattern when it expands larger than the size of a dither matrix. In the conventional averaging process, it is a figure which shows the average pattern when 1 line scanning is repeated by shifting to the main scanning direction. In the conventional averaging process, it is a figure which shows the average pattern when shifting to the main scanning direction is repeated and 2 lines are scanned. In the conventional averaging process, it is a figure which shows the average pattern when seven lines are scanned by repeating the shift to the main scanning direction. It is a figure which shows the example of a circuit which can be used for the conventional averaging process. It is a figure which shows the pixel overwritten by an average value, when the averaged pattern extended in the conventional averaging process is used. It is a figure which shows the example of the non-rectangular supercell and the rectangular dither matrix obtained by converting the said supercell. It is a figure which shows the position of each pixel in a rectangular block, and the relationship between the identification number and kind number of the dither matrix to which each pixel belongs. The figure which shows the example of the table which matched with the position number showing the position of each pixel in a rectangular block the identification number and kind number of the dither matrix to which each pixel belongs, and the shift amount to the center pixel of the said dither matrix. It is.

  Embodiments of an image processing apparatus and an averaging processing method of the present invention will be described below with reference to the drawings.

FIG. 1 shows a main configuration of an image forming apparatus G including an image processing apparatus 19 according to an embodiment of the present invention for each function.
As shown in FIG. 1, the image forming apparatus G includes a control unit 11, a storage unit 12, an operation unit 13, a display unit 14, a communication unit 15, an image generation unit 16, an image reading unit 17, an image memory 18, and an image processing device. 19 and an image forming unit 20.

The control unit 11 includes a CPU (Central Processing Unit), a RAM (Random Access Memory), and the like, and controls each unit by reading and executing various programs from the storage unit 12.
For example, the control unit 11 causes the image processing device 19 to perform image processing on the original image data generated by the image generation unit 16 or the image reading unit 17 and held in the image memory 18, thereby converting the original image data into image-processed original image data. Based on this, an image is formed on the sheet by the image forming unit 20.

  The storage unit 12 stores a program that can be read by the control unit 11, a file that is used when the program is executed, and the like. As the storage unit 12, a large-capacity memory such as a hard disk can be used.

  The operation unit 13 generates an operation signal corresponding to a user operation and outputs the operation signal to the control unit 11. As the operation unit 13, a keypad, a touch panel configured integrally with the display unit 14, or the like can be used.

  The display unit 14 displays an operation screen and the like according to instructions from the control unit 11. As the display unit 14, an LCD (Liquid Crystal Display), an OELD (Organic Electro Luminescence Display), or the like can be used.

The communication unit 15 communicates with external devices on the network, such as a user terminal, a server, and other image forming apparatuses.
The communication unit 15 receives vector data in which an instruction content for forming an image is described in a page description language (PDL) via a network from a user terminal.

  The image generation unit 16 rasterizes the vector data received by the communication unit 15 and generates original image data in a bitmap format. In the original image data, each pixel has pixel values of four colors of C (cyan), M (magenta), Y (yellow), and K (black). The pixel value is a data value representing the density of an image. For example, an 8-bit data value represents a density of 0 to 255 gradations.

  The image reading unit 17 includes an automatic document feeder, a scanner, and the like, reads an original surface set on an original table, and generates original image data in a bitmap format. In the original image data generated by the image reading unit 17, each pixel has pixel values of three colors of R (red), G (green), and B (blue). The original image data is color-converted into original image data having four pixel values of C, M, Y, and K by a color converter (not shown).

  The image memory 18 is a buffer memory that temporarily holds original image data generated by the image generation unit 16 or the image reading unit 17. As the image memory 18, a DRAM (Dynamic RAM) or the like can be used.

The image processing device 19 reads the original image data from the image memory 18 and performs various image processing.
FIG. 2 shows the main configuration of the image processing device 19 for each function.
As shown in FIG. 2, the image processing device 19 includes an averaging processing unit 191, a γ correction processing unit 192, and a screen processing unit 193.

  The averaging processing unit 191 performs an averaging process on the image data. Details of the averaging process will be described later.

  The γ correction processing unit 192 performs γ correction processing on the image data after the averaging processing. The γ correction process is a process of correcting the pixel value of each pixel of the image data so that the density characteristic of the image formed by the image forming unit 20 becomes the target density characteristic.

  The screen processing unit 193 performs screen processing on the image data after the γ correction processing. The screen process is a process of reproducing a halftone in a pseudo manner using the dither method, and periodically compares and compares the pixel value of each pixel of the image data with the threshold value set for each pixel of the dither matrix, In accordance with the comparison result, the pixel value of each pixel is converted into a multivalued binary value.

  The image forming unit 20 converts an image having four colors of C, M, Y, and K into a color material such as toner according to the pixel values of the four colors of each pixel of the original image data processed by the image processing device 19 To form on a sheet.

FIG. 3 shows a processing procedure when the averaging processing unit 191 performs an averaging process on the image data.
As shown in FIG. 3, the averaging processing unit 191 inputs image data in units of a fixed area centered on the target pixel, with each pixel of the image data as the target pixel (step S1). The size of the fixed area is larger than the dither matrix used for screen processing.
Image data in a certain area centered on the target pixel can be input by using the FIFO to delay the input of the image data of each pixel to the averaging processing circuit.

FIG. 4 shows a circuit example of the averaging processing unit 191 that can input image data of a certain region.
As shown in FIG. 4, the same number of FIFOs 31 as the number of pixels in the sub-scanning direction of a certain region are connected in parallel to the processing circuit 32 that performs averaging, and image data input from the FIFO 31 in the previous stage to the processing circuit 32 is By inputting the data to the FIFO 31 in the subsequent stage, the image data of each pixel can be delayed and the image data of a plurality of lines can be input to the processing circuit 32 at a time. For example, if 11 FIFOs 31 are used, it is possible to input 11-pixel image data including the pixel of interest and peripheral pixels of 5 lines before and after the pixel of interest.

Next, the averaging processing unit 191 determines the position of the pixel of interest in the block when each pixel of the image data is divided in units of rectangular blocks having the same cycle as the dither matrix used by the screen processing unit 193 for screen processing. (Step S2).
The dither matrix conforms to the Holladay principle (TMHolladay, An optimum algorithm for halftone generation for displays and hard copies, Proc. SID, Vol. 21, pp. 185-192 (1980)), even if its shape is non-rectangular. Therefore, it can be converted to a rectangle. The averaging processing unit 191 determines the size and shape of the rectangular block so as to have the same size and shape as the dither matrix converted into the rectangle.

FIG. 5 shows an example of a non-rectangular dither matrix. In FIG. 5, the positions of all 10 pixels constituting the dither matrix are represented by position numbers e of 0-9.
FIG. 6 shows a rectangular dither matrix obtained by converting the non-rectangular dither matrix shown in FIG. 5 according to the Holladay principle.
When the length of the rectangular dither matrix in the main scanning direction is represented by M (pixel) and the length in the sub-scanning direction is represented by N (pixel), the rectangular dither matrix shown in FIG. 6 has M = 10 and N = 1. The size, that is, the total number of pixels M × N is 10 pixels, which is the same as the non-rectangular dither matrix.

FIG. 7 shows the position of each pixel when the non-rectangular dither matrix shown in FIG. 5 is periodically compared with each pixel of the image data by the position number e of each pixel of the dither matrix.
As shown in FIG. 7, at the time of screen processing, the non-rectangular dither matrix is shifted in units of M pixels in the main scanning direction x every time the image data is collated, and reaches the end of the main scanning direction x. 1 pixel is shifted to y and L pixels are shifted from the starting end in the main scanning direction x, and the shift and verification in the main scanning direction x are repeated. By this scanning, a non-rectangular dither matrix can be collated in a tile shape without overlapping each pixel of the image data.

  Similar collation can be realized using a rectangular dither matrix of M pixels × N pixels arranged in the order of position numbers e of 0 to 9. In that case, as shown in FIG. 7, when the scanning for shifting the rectangular dither matrix in units of M pixels in the main scanning direction x is repeated by shifting one pixel at a time in the sub-scanning direction y, and N pixels are shifted in the sub-scanning direction y. The shift in units of M pixels is started in the main scanning direction x from the position shifted by L pixels from the starting point in the main scanning direction x. Since the shift amount in the main scanning direction x is M pixels and the shift amount at the scanning start position after N pixel shifts in the sub-scanning direction y is also L pixels, the non-rectangular dither matrix and the rectangular dither matrix have the same period. become.

The averaging processing unit 191 determines the size of the rectangular block as the size of M pixels × N pixels, which is the same as the rectangular dither matrix.
Since the period does not change, the length of the rectangular block in the main scanning direction can be a multiple of M, and the length of the sub-scanning direction can be a multiple of N.

If the position coordinates of each pixel in the image data in the main scanning direction x and the sub-scanning direction y are expressed as (i, j), the position coordinates (Sx, Sy) can be obtained by the following equation. The position coordinate (i, j) is a coordinate system in which the position coordinate of the starting pixel of the image data is (0, 0), and the position coordinate (Sx, Sy) is the position of the starting pixel of the rectangular block. This is a coordinate system in which the coordinates are (0, 0).
Sx = (i−L × j / N)% M
Sy = j% N
When Sx <0, M is added to Sx to obtain a positive number.
In the above equation, M and N represent the length (number of pixels) of the main scanning direction x and the sub scanning direction y of the rectangular block, respectively, and L represents the main scanning direction every time the rectangular block is shifted N pixels in the sub scanning direction y. This represents the shift amount (number of pixels) to be shifted from the starting end of x. % Represents an operator for calculating the remainder.

Similar to the rectangular dither matrix shown in FIG. 6, the position of each pixel in the rectangular block is represented by consecutive position numbers e (0 ≦ e ≦ M × N) in which the position number of the starting pixel of the rectangular block is 0. And the position number e can be calculated | required by a following formula.
e = Sx + Sy × M
That is, the averaging processing unit 191 can determine the position number e representing the position of the target pixel in the rectangular block from the position coordinates (i, j) of the target pixel in the image data.

  Next, the averaging processing unit 191 determines a shift amount from the position of the target pixel to the center pixel of the dither matrix to which the target pixel belongs according to the determined position of the target pixel in the rectangular block (step S3).

  The shift amount for each pixel may be determined each time. However, since the period of the rectangular block is the same as the period of the dither matrix, if the position of the pixel in the rectangular block is the same, the position of the pixel The shift amount to the center pixel of the dither matrix to which the pixel belongs is also the same. In order to increase the processing efficiency, a table in which the position of each pixel in the rectangular block and the shift amount from the position of each pixel to the center pixel of the dither matrix to which each pixel belongs is stored in the storage unit 12 in advance. The averaging processing unit 191 preferably acquires the shift amount corresponding to the position of the target pixel in the rectangular block from this table.

FIG. 8 shows each pixel of the rectangular block and the identification number of the dither matrix to which each pixel belongs.
As shown in FIG. 8, each pixel whose position number e is 0 to 9 in the rectangular block belongs to one of four dither matrices having identification numbers d1 to d4. Since the pixel at position number 0 is located at the center of each dither matrix, a table of shift amounts from the pixels at position numbers 0 to 9 in the rectangular block to the pixel at position number 0 in the associated dither matrix Should be created.

FIG. 9 shows an example of the table.
As shown in FIG. 9, the position number e of each pixel in the rectangular block is associated with the identification number of the dither matrix to which each pixel belongs, the shift amount Δx in the main scanning direction, and the shift amount Δy in the sub scanning direction. It has been.
For example, as shown in FIG. 8, the pixel with position number 0 belongs to the dither matrix with identification number d1 and is the center pixel thereof, so that the shift amounts Δx and Δy are both 0 pixels.
Further, as shown in FIG. 8, the pixel of position number 4 belongs to the dither matrix of identification number d2, and the shift amounts to the center pixel are Δx = −1 pixel and Δy = −1 pixel.

  Next, the averaging processing unit 191 calculates the average pattern by the determined shift amount from the position at which the pixel (target pixel) positioned at the center of the input fixed area matches the pixel positioned at the center of the average pattern. Is shifted (step S4). The averaging pattern is a pattern indicating pixels to be averaged.

  The averaging processing unit 191 uses an average pattern having the same size and shape as the dematrix as the average pattern. The size and shape of the averaging pattern can be specified by setting a value of 1 for pixels to be averaged in a rectangular area and a value of 0 for pixels not to be averaged in a rectangular area, for example. The averaging processor 191 acquires this setting and collates the average pattern with a certain area. Note that the rectangular area is set to a size larger than the dither matrix and smaller than the fixed area, and a value of 1 representing the pixel to be averaged is such that the pixel located at the center of the averaging pattern is located at the center of the rectangular area. Is set. When changing the dither matrix used in the screen processing, the average pattern to be used can be changed in accordance with the dither matrix by rewriting the register settings according to the size and shape of the dither matrix after the change.

FIG. 10 shows an average pattern before shifting.
As shown in FIG. 10, when the central pixel of the constant region R1 of 13 × 11 pixels and the central pixel of the rectangular region R2 of 7 × 6 pixels are matched, an average pattern R3 in which a value of 1 is set in the rectangular region R2 Can be matched with the central pixel of the fixed region R1, that is, the target pixel. The average pattern R3 has the same size and shape as the dither matrix.

Here, when the target pixel is the pixel at position number 2 in the rectangular block, the averaging processing unit 191 shifts the average pattern R3 for each rectangular region R2 by a shift amount of Δx = + 1 pixel and Δy = −1 pixel. .
FIG. 11 shows the averaged pattern after the shift.
As shown in FIG. 11, the position of the pixel of interest in the averaged pattern R3 after the shift matches the position in the dither matrix to which the pixel with position number 2 belongs.

The averaging processing unit 191 averages the pixel values of each pixel in the shifted average pattern R3, and outputs the obtained average value as the pixel value after the averaging process of the target pixel (step S5). Specifically, the averaging processing unit 191 obtains an average value by accumulating the pixel values of each pixel in the average pattern and then dividing by the number of pixels.
Note that a weighting coefficient may be set for each pixel of the average pattern, and an average value obtained by weighted averaging may be calculated. Thereby, the shape of the periodic pattern formed by the dither matrix can be shaped, and an image with high sharpness can be formed.

If the averaging process has not been completed for all pixels (step S6: N), the averaging processing unit 191 shifts the position of the target pixel (step S7), returns to the process of step S1, and sets a new target pixel. The averaging process described above is repeated.
Then, when the averaging process for all pixels is finished (step S6: Y), this process is finished.

Note that the averaging processing unit 191 can use an averaged pattern having the same size and shape as the dither matrix by expanding it to a size larger than the dither matrix.
FIG. 12 shows an example of an average pattern when an average pattern having the same size and shape as the dither matrix is expanded to a size larger than the dither matrix.
By using the extended averaging pattern, the area to be averaged can be extended to reduce image noise. Since the averaging processing unit 191 inputs not the pixel value after the averaging process but the original pixel value of each pixel, even if the area to be averaged is expanded, the original pixel value may be used for averaging. And an accurate average value can be obtained. Further, since the period of the rectangular block does not change just by expanding the size of the average pattern to be used, the averaging process can be performed with the same period as the dither matrix.

As described above, the image processing apparatus 19 according to the present embodiment inputs image data in units of a fixed area centered on each pixel, compares the input fixed area with the average pattern, and stores the average data in the average pattern. An average processing unit 191 that averages the pixel values of each pixel and outputs the obtained average value as a pixel value of a central pixel in a certain region, and each pixel of the image data averaged by the averaging processing unit 191 A screen processing unit 193 that periodically collates with the dither matrix and multi-values the pixel value of each pixel by comparing with a threshold value in the dither matrix.
The averaging processing unit 191 uses an average pattern having the same size and shape as the dither matrix as an average pattern, and the dither matrix to which each pixel belongs when compared with the dither matrix from the position of each pixel in the image data The shift amount to the center pixel is determined, and the position of the average pattern is shifted by the determined shift amount from the position where the center pixel of the average pattern matches the center pixel of the fixed area, and the average after the shift The pixel values of each pixel in the pattern are averaged.

  Compared with the conventional averaging process in which the average pattern having the same size and shape as the dither matrix is periodically checked and the pixel value of each pixel in the average pattern is replaced with the average value at one time. The averaging process in is highly versatile and scalable.

13A to 13C show a conventional averaging process.
As shown in FIG. 13A, in the conventional averaging process, the average pattern is scanned in the same manner as the dither matrix. That is, the average pattern having the same size and shape as the dither matrix shown in FIG. 5 is shifted by one line in the main scanning direction x in units of M pixels, and the pixel value of each pixel in the average pattern is changed for each shift. Replace with the average value. When the shift reaches the end of the main scanning direction x, as shown in FIG. 13B, the pixel is shifted by one pixel in the sub-scanning direction y and L pixels are shifted from the starting point in the main scanning direction x, and the main scanning direction x in M pixel units. Repeat shift to, and replacement with average value. When scanning 1 to 7 lines in the main scanning direction x is repeated, each pixel of the image data can be averaged in the same area unit as the dither matrix as shown in FIG. 13C. The numbers in FIGS. 13A to 13C represent the line order in the main scanning direction x scanned by shifting the average pattern.

  In such a conventional averaging processing circuit, pixel values in the averaging pattern are replaced by the average value at a time, so that the processing circuit 32 replaces the pixel value instead of the original pixel value as shown in FIG. Each FIFO 31 is used so that the later average value is delayed and input to the processing circuit 32. Since the use of the FIFO 31 is different from the normal use of the FIFO that only delays the original pixel value, it is necessary to prepare a dedicated circuit specialized for the averaging process.

  On the other hand, in the averaging process of the above embodiment, the pixel values in the average pattern are not replaced at once, but the average value of the pixel values in the average pattern is calculated and output for each pixel. Therefore, the processing circuit of the averaging processing unit 191 may use each FIFO 31 so that the original pixel value is delayed and input to the processing circuit 32 as shown in FIG. Since it is the same as the normal use of FIFO, even in image processing other than the averaging processing, each FIFO 31 can be shared to input a fixed area, and averaging processing with high versatility can be performed. it can.

In order to reduce the noise of the image, it is effective to expand the area to be averaged. However, in the conventional averaging process, since the averaged pattern is scanned without overlapping, the averaged pattern Pixels that have already been subjected to averaging processing by extension are subjected to averaging again.
For example, when an averaging process is performed as shown in FIGS. 13A to 13C using an extended averaging pattern as shown in FIG. 12, pixels already averaged are also subject to averaging, as shown in FIG. The already averaged pixels are overwritten with the average values, not the original pixel values.

  On the other hand, as shown in FIG. 4, the processing circuit of the averaging processing unit 191 of the above embodiment inputs the original pixel value regardless of whether or not it is an averaged pixel. Even when is expanded, the original pixel values can be used for averaging, and an accurate average value can be obtained. That is, the size of the averaging pattern can be freely designed, and an averaging process with high expandability can be performed.

[Other Embodiments]
The screen processing unit 193 can use a supercell that is a combination of a plurality of types of dither matrices as a dither matrix used for screen processing (P.Fink, PostScript Screening: Adobe AccurateScreens, Adobe_Press, Mountain View, CA, 1992). , Chapter4, pp.43-61). According to the supercell, it is possible to form a pattern having a specific number of lines and angle within a limited resolution. When a supercell is used, the averaging process can be performed by the same processing procedure as described above by identifying the type of dither matrix to which each pixel belongs.

FIG. 16 shows an example of a supercell in which two types of dither matrices having different sizes and shapes are combined. Each dither matrix is assigned type numbers a1 and a2 indicating the type.
The supercell can also be converted into a rectangular dither matrix of M × N pixels according to the Holladay principle. As shown in FIG. 16, the rectangular dither matrix obtained by converting the supercell has a length M in the main scanning direction of M = 7, a length N in the sub scanning direction of N = 7, The shift amount L in the main scanning direction x when N pixels are shifted in the scanning direction is L = 0.

The averaging processing unit 191 uses each pixel of the image data as a pixel of interest and divides the pixel data in units of rectangular blocks having the same size and shape as the 7 × 7 pixel rectangular dither matrix. To decide.
The averaging processing unit 191 determines the identification number and type number of the dither matrix to which the target pixel belongs according to the determined position, and the center pixel of the dither matrix having the determined identification number and type number from the position of the target pixel Shift amounts Δx and Δy are determined.

FIG. 17 shows the 0 to 48 position number e of each pixel in the 7 × 7 pixel rectangular block, and the identification number and type number of the dither matrix to which the pixel of each position number e belongs.
For example, the pixel with e = 30 belongs to the dither matrix with the identification number d1 and the type number a1, and the shift amount from the position of this pixel to the central pixel of the belonging dither matrix is Δx = + 1 pixel, Δy = −1. Pixel.

  Similar to the normal dither matrix, the averaging processing unit 191 creates a table in which the position number e of each pixel in the rectangular block is associated with the identification number and type number of the belonging dither matrix and the shift amounts Δx and Δy. The data is stored in the storage unit 12, and the averaging processing unit 191 can acquire the identification number and type number of the dither matrix to which the target pixel belongs and the shift amounts Δx and Δy from this table.

FIG. 18 shows an example of a table that can be used in the case of a supercell.
As shown in FIG. 18, the position number e indicating the position of each pixel in the rectangular block, the identification number and type number of the dither matrix to which each pixel in the rectangular block belongs, and the shift amount Δx to the center pixel of the dither matrix And Δy are associated with each other.

  The averaging processing unit 191 uses an averaging pattern having the same size and shape as the dither matrix of the determined type number in the averaging process of the target pixel. The averaging processing unit 191 shifts the position of the average pattern by the determined shift amount from the position where the input center pixel (target pixel) of the certain area matches the center pixel of the average pattern. The averaging processing unit 191 averages the pixel values of each pixel in the averaged pattern after the shift, and outputs the obtained average value as the pixel value of the target pixel after the averaging process.

  As described above, in the image processing device 19 according to another embodiment, the screen processing unit 193 periodically collates each pixel of image data with a supercell that is a combination of a plurality of types of dither matrices, and performs an averaging process. The unit 191 determines the shift amount from the position of each pixel in the image data to the center pixel of the dither matrix to which each pixel belongs when collated with the supercell, and the type of the dither matrix. Using an averaged pattern of the same size and shape as the dither matrix, shifting the position of the averaged pattern from the position where the central pixel of the averaged pattern and the central pixel of the fixed region coincide with each other by the determined shift amount, The pixel values of each pixel in the averaged pattern after shifting are averaged.

  Thereby, even when a supercell is used for the screen processing, it is possible to perform an averaging process in the same area unit as a plurality of types of dither matrices having different shapes and sizes in the supercell. Averaging processing with high versatility and expandability that can be applied to a supercell is possible.

The above embodiment is a preferred example of the present invention, and the present invention is not limited to this. Modifications can be made as appropriate without departing from the spirit of the present invention.
For example, when the control unit 11 reads a program, the control unit 11 can execute the above processing procedure. In addition to the image forming apparatus G, the program can be read by a computer such as a general-purpose PC, and the above processing procedure can be executed.
Further, as a computer-readable medium for the program, a non-volatile memory such as a ROM and a flash memory, and a portable recording medium such as a CD-ROM can be applied. A carrier wave is also used as a medium for providing program data via a communication line.

G Image forming device 11 Control unit 12 Storage unit 19 Image processing device 191 Averaging processing unit 192 γ correction processing unit 193 Screen processing unit

Claims (5)

Image data is input in units of a fixed area centered on each pixel, the input fixed area is compared with the average pattern, the pixel values of each pixel in the average pattern are averaged, and the average value obtained is , An averaging processing unit that outputs as a pixel value of a central pixel of the fixed region;
A screen processing unit that periodically compares each pixel of the image data averaged by the averaging processing unit with a dither matrix, and compares the pixel value of each pixel with a threshold value in the dither matrix; With
The averaging processing unit
As the average pattern, using an average pattern having the same size and shape as the dither matrix,
Determine the shift amount from the position of each pixel in the image data to the center pixel of the dither matrix to which each pixel belongs when collated with the dither matrix,
The position of the average pattern is shifted by the determined shift amount from the position where the central pixel of the average pattern coincides with the central pixel of the certain region, and each pixel in the average pattern after the shift is shifted. An image processing apparatus that averages pixel values. The position of each pixel in the block when each pixel of the image data is divided into rectangular blocks having the same cycle as the dither matrix, and the shift from the position of each pixel to the center pixel of the dither matrix to which each pixel belongs A storage unit that stores the amount in association with each other;
The averaging processing unit determines a position in a block of each pixel when each pixel of the image data is divided in units of the block, and acquires the shift amount corresponding to the determined position from the storage unit. The image processing apparatus according to claim 1, wherein: The screen processing unit periodically collates each pixel of the image data with a supercell that is a combination of a plurality of types of dither matrices,
The averaging processing unit
Determine the shift amount from the position of each pixel in the image data to the center pixel of the dither matrix to which each pixel belongs when collated with the supercell, and the type of the dither matrix,
The average pattern having the same size and shape as the determined type of dither matrix is used, and the position of the average pattern is determined from the position where the central pixel of the average pattern matches the central pixel of the certain region. The image processing apparatus according to claim 1, wherein the image processing apparatus shifts by the shift amount and averages the pixel values of the pixels in the averaged pattern after the shift.   The said averaging process part expands and uses the average pattern of the same size and shape as the said dither matrix to a size larger than the said dither matrix, It is any one of Claims 1-3 characterized by the above-mentioned. The image processing apparatus described. Inputting image data in units of a fixed area centered on each pixel;
Collating the input fixed area with an average pattern, averaging the pixel values of each pixel in the average pattern, and outputting the obtained average value as the pixel value of the central pixel of the fixed area; Including
The step of averaging the pixel values of each pixel in the average pattern is performed by periodically comparing each pixel of the image data with a dither matrix as the average pattern, and setting the pixel value of each pixel to a threshold value in the dither matrix. Using an averaged pattern of the same size and shape as the dither matrix used for multi-valued screen processing by comparison with
Determining a shift amount from the position of each pixel in the image data to the center pixel of the dither matrix to which each pixel belongs when collated with the dither matrix;
The position of the average pattern is shifted from the position where the central pixel of the average pattern matches the central pixel of the certain region by the determined shift amount, and the pixel of each pixel in the average pattern after the shift A step of averaging the values;
An averaging processing method comprising:

Images