JP2007311912A - Image processing apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image processing apparatus for reducing gray level without degrading the image quality due to block noise caused by applying JPEG compression to image data. <P>SOLUTION: When a JPEG companding circuit 32 compresses image data read by a read section 4, a selection circuit 31 selects the image data whose level is reduced by an ordered dither method gray-level reduction circuit 30 and gives the selected data to a recording section 6, when the image data are not compressed, the selection circuit 31 selects image data whose gray-level is reduced by an error diffusion method gray-level reduction circuit 29 and gives the selected data to the recording section 6 and the image data are recorded on a recording paper. Whether or not the image data are to be compressed is determined under conditions, e.g. as to whether or not a read original includes a photo image or whether or not a plurality of recordings are executed. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image processing apparatus such as a copier or a printer having a function of binarizing, for example, color multivalued image data and recording it on recording paper.

  Copiers that have been widely used in the past perform processing to reduce the number of gradations, such as binarizing the multi-valued image data obtained by optical reading, and record the image data with the reduced number of gradations It has a function of recording on paper. The binarization of the multi-value image data is performed by comparing the gradation value of each pixel of the image data with a threshold value. However, when the comparison is performed using a certain threshold value, a pseudo intermediate gradation value is obtained. Various algorithms for reducing the number of gradations have been proposed in order to reduce the image quality due to low reproducibility, and for example, a systematic dither method and an error diffusion method are known.

  The systematic dither method is a method in which, for example, a threshold value for binarization is systematically changed according to a certain rule in a small area of 4 × 4, and comparison is performed for each area to binarize image data. . The systematic dither method is an algorithm that has been widely used since it has a high reproducibility of halftones and is easy to process compared to simple binarization using a fixed threshold. There is a problem that the reproducibility of the part and the character part is not sufficient, and moire may occur.

  The error diffusion method is a quantization error that occurs when binarizing the gradation value of each pixel of image data, that is, the difference between the gradation value after binarization with respect to the gradation value of the pixel before binarization, This is a method of binarizing by distributing to pixels that have not been binarized yet. Compared with the systematic dither method, since the quantization error is distributed to the surrounding unprocessed pixels, the error diffusion method is less likely to cause moiré in the binarized image and has higher reproducibility of the intermediate gradation. There are advantages.

  In Patent Document 1, in the case of a communication mode in which data is transmitted by facsimile communication, multi-value image data is binarized by a centralized dither method, and in the case of a copy mode in which a read image is copied, an error occurs. There has been proposed a facsimile apparatus that can shorten the communication time to the maximum while maintaining the image quality of the copy image by binarizing the multi-value image data by the diffusion method.

In Patent Document 2, if the read image is a copy photo, the copy photo reproduction dither matrix is selected and binarized, and if it is any other photo mode, the recording characteristics of the communication destination device are judged. If the recording characteristics are not the same, the corresponding dither matrix is selected from the standard photo reproduction dither matrix and binarization is performed. If the recording characteristics are the same, the copy photo reproduction dither matrix is selected and binarized. Therefore, there has been proposed an image information processing apparatus that does not cause a large difference in recording density between when the read image is recorded by the own apparatus and when it is recorded by another apparatus.
JP 7-177350 A JP 2003-69820 A

On the other hand, in a copying machine, when outputting a plurality of copies of image data of a read document, particularly when using a so-called sort function that outputs a plurality of copies for each copy, the image data of the read document is temporarily stored in a memory. It is necessary to memorize it. In addition, for example, image data may be temporarily stored in a memory when, for example, a plurality of images are collected on one page and when images are recorded on both sides of a recording sheet. When storing image data temporarily in the memory, the memory capacity is limited, so the image data is often compressed and stored. For example, a JPEG compression algorithm is used to compress the image data. Can do.

  The JPEG method is a so-called irreversible compression in which compressed image data cannot be returned to the original image data because the compression efficiency is improved by truncating a part of the data when compressing the image data. In JPEG compression, image data in RGB format is first converted into three elements (so-called YCbCr format) of luminance, hue in the blue direction, and hue in the red direction (so-called YCbCr format), but the human eye is sensitive to the difference in luminance. The difference is that it is easy to overlook and the compression is performed after reducing the amount of data by thinning out part of the information about the hue. Discrete cosine transform is used for compression, and these processes are performed in units of 8 pixel square blocks. Therefore, depending on the compression ratio, color or density may be equalized for each block, and so-called block noise may occur. is there.

  In this way, when image data in which block noise has been generated by JPEG compression is binarized by systematic dithering, the boundaries of each block are weakened, so image quality deterioration due to block noise occurs. Although it is difficult, when binarization is performed by the error diffusion method, there is a problem that the boundary of each block is more emphasized and the image quality may be deteriorated.

  The facsimile apparatus described in Patent Document 1 only changes the binarization algorithm between a communication mode for transmitting data and a copy mode for copying an image. In the copy mode, an error diffusion method is used. Since binarization is performed, the above-described problem occurs. The image information processing apparatus described in Patent Document 2 changes the dither matrix used in the systematic dither method according to conditions, and since the error diffusion method is not used, there is no possibility that the above-described problem occurs. With only the systematic dither method, the reproducibility of the edge portion and character portion of the binarized image is not sufficient, and moire may occur.

  The present invention has been made in view of such circumstances, and the object of the present invention is to change the method of reducing the number of gradations of image data depending on whether the acquired image data is not compressed or compressed. Accordingly, it is an object of the present invention to provide an image processing apparatus capable of preventing a deterioration in image quality caused by a combination of image data compression and a gradation number reduction method.

  Another object of the present invention is to reduce the number of gradations of the image data by the error diffusion method when the acquired image data is not compressed, so that the image can be reproduced with high reproducibility of the intermediate gradation. An object of the present invention is to provide an image processing apparatus capable of reducing the number of gradations of data.

  Another object of the present invention is to reduce image quality due to block noise by reducing the number of gradations of image data using an organized dither method when compressing acquired image data. An object of the present invention is to provide an image processing apparatus that can prevent this.

  Another object of the present invention is to reduce the number of gradations of image data including a photographic image, so that the image data is not compressed, so that the photograph can be reproduced with high reproducibility of intermediate gradations. An object of the present invention is to provide an image processing apparatus capable of reducing the number of gradations of an image.

Another object of the present invention is to record, sort or consolidate a plurality of copies even when the image data includes a photographic image when recording image data with a reduced number of gradations on a recording sheet. For example, when recording after storing a plurality of image data in the memory, the image data is compressed, and the number of gradations is reduced by a method other than the error diffusion method. It is an object of the present invention to provide an image processing apparatus that can effectively use a memory that has been used and that can suppress deterioration in image quality.

  Another object of the present invention is that when image data including a photographic image is acquired and the acquired image data is compressed, the number of gradations is not reduced by the error diffusion method. By adopting a configuration in which the number of gradations is reduced by another method such as a dither method, it is possible to prevent image quality from being deteriorated due to block noise without reducing the number of gradations of the compressed image data by the error diffusion method. An object is to provide an image processing apparatus.

  Another object of the present invention is to provide an image processing apparatus capable of effectively using a memory mounted in the apparatus by adopting a configuration in which image data is compressed by the JPEG method.

  Another object of the present invention is to reduce the number of gradations of image data to two, so that a printer or the like records image data on a recording sheet or a facsimile apparatus or the like. An object of the present invention is to provide an image processing apparatus capable of reducing the number of gradations suitable for transmitting and receiving data.

  An image processing apparatus according to a first aspect of the present invention is an image processing apparatus that performs processing for reducing the number of gradations of image data, an acquisition unit that acquires image data, a compression unit that compresses the acquired image data, and a peripheral pixel First gradation number reduction means for reducing the number of gradations of the image data by a gradation number reduction method for reducing the number of gradations of the target pixel in accordance with the reduction result of the gradation value, and the first floor And a second gradation number reduction means for reducing the number of gradations of the image data by a method different from the key reduction means, and when the compression means does not compress the image data, the first floor When the logarithmic reduction means performs processing to reduce the number of gradations of the image data, and the compression means compresses the image data, the second gradation number reduction means determines the number of gradations of the compressed image data. It is characterized in that a decreasing process is performed.

  In the present invention, when the number of gradations of the image data is reduced, there are a case where the number of gradations is reduced after the image data is compressed and a case where the number of gradations is reduced without compressing the image data. Separate the log reduction method. For example, when there is a possibility that block noise may occur due to compression of image data, if compression is performed, the block noise is reduced or the number of gradations that does not enhance the block noise is reduced. If not, the number of gradations of the target pixel is reduced according to the result of reduction of the number of gradations of the surrounding pixels, and the image quality is improved by using a method of reducing the number of gradations with high reproducibility of the intermediate gradation. improves.

  The image processing apparatus according to the second invention is characterized in that the first gradation number reducing means reduces the gradation number of the image data by an error diffusion method.

  In the present invention, when the number of gradations is reduced without compressing the image data, the number of gradations of the image data is reduced by an error diffusion method. Compared with the systematic dither method or the like, image quality is improved because there is no moiré.

  The image processing apparatus according to the third invention is characterized in that the second gradation number reducing means reduces the gradation number of the image data by a systematic dither method.

In the present invention, when the number of gradations is reduced after the image data is compressed, the number of gradations of the image data is reduced by a systematic dither method. When block noise occurs due to compression of image data, the systematic dither method does not emphasize block noise or can reduce block noise compared to the error diffusion method. Reduction can be suppressed.

  The image processing apparatus according to the fourth invention is characterized in that the image data is not compressed by the compression means when the image data acquired by the acquisition means includes a photographic image.

  In the present invention, when the number of gradations of image data including a photographic image is reduced, the number of gradations is reduced by an error diffusion method or the like without compressing the image data. Image data including a photographic image is likely to include many portions having similar values such as color and density, and block noise is likely to occur due to compression in such portions. Therefore, the number of gradations of image data can be reduced with good reproducibility of intermediate gradations by reducing the number of gradations without compressing image data including a photographic image as much as possible.

  An image processing apparatus according to a fifth aspect of the present invention includes a recording unit that records image data with a reduced number of gradations on a recording sheet, and the image data acquired by the acquiring unit includes a photographic image. When recording is performed after the recording means accumulates a plurality of image data, the compression of the image data is performed by the compression means, and the number of gradations is reduced by the second gradation number reduction means. It is characterized by.

  In the present invention, when image data with a reduced number of gradations is recorded on a recording sheet, even when the image data includes a photographic image, when recording is performed after storing a plurality of image data in a memory. After the image data is compressed, the number of gradations is reduced by a method other than the error diffusion method. For example, when a plurality of image data are recorded, when a plurality of image data are sorted and recorded, or when a plurality of images are collected and recorded on one page, a photographic image is included. Since it is necessary to temporarily store the image data in the memory, the image data is compressed. The compressed image data is subjected to a reduction in the number of gradations by a method such as a systematic dither method, thereby suppressing deterioration in image quality.

  An image processing apparatus according to a sixth aspect of the invention is an image processing apparatus that performs processing for reducing the number of gradations of image data, an acquisition means for acquiring image data, a compression means for compressing the acquired image data, and an error A gradation number reduction means for reducing the number of gradations of the image data by a diffusion method, the image data acquired by the acquisition means includes a photographic image, and the image data acquired by the compression means is compressed Is characterized in that the gradation number is not reduced by the gradation number reducing means.

  In the present invention, when image data including a photographic image is acquired and the acquired image data is compressed, the number of gradations is not reduced by the error diffusion method. A block noise image is not generated by a combination of compression and a reduction in the number of gradations by the error diffusion method, and the image quality is not deteriorated.

  An image processing apparatus according to a seventh aspect is characterized in that the compression means compresses image data by a JPEG method.

  In the present invention, the image data is compressed by the JPEG method, and the data size of the image data is reduced to effectively use the memory mounted in the apparatus. Since block noise may occur when compressing with the JPEG method, when the compression is performed, the number of tones is not reduced by the error diffusion method, but the number of tones is changed by a method such as an organized dither method. By performing the reduction, it is possible to suppress deterioration in image quality due to block noise.

The image processing apparatus according to the eighth invention is characterized in that the gradation number reducing means reduces the gradation number of the image data to two gradations.

  In the present invention, the number of gradations of the image data is reduced to two gradations. When a printer or the like performs recording on recording paper, it is necessary to reduce the number of gradations of image data to two gradations in order to reproduce an image with or without dots. In addition, when image data is transmitted and received, it is desirable that the amount of image data is small. Therefore, the amount of data can be reduced by reducing the number of gradations of image data to two.

  According to the first invention, when the number of gradations of the image data is reduced, the number of gradations is reduced after the image data is compressed, and the case where the number of gradations is reduced without compressing the image data. By adopting a configuration in which the method for reducing the number of gradations is changed, it is possible to prevent the deterioration in image quality caused by the combination of compression of image data and the method for reducing the number of gradations. For example, when compression is used, a method of reducing the number of gradations that does not reduce block noise or enhance block noise is used, and when compression is not performed, gradation with high reproducibility of intermediate gradations By using the method of reducing the number, it is possible to prevent the image quality from being deteriorated due to block noise, so that high-quality image data can be recorded on a recording sheet or displayed on a display.

  According to the second aspect of the invention, when the acquired image data is not compressed, a configuration in which the number of gradations of the image data is reduced by the error diffusion method is used, so that moire is reduced as compared with the systematic dither method. Since there is no occurrence and the number of gradations of image data can be reduced with good reproducibility of intermediate gradations, high-quality image data can be recorded on a recording sheet or displayed on a display.

  According to the third aspect of the present invention, when the acquired image data is compressed, block noise is generated due to the compression of the image data by adopting a configuration in which the number of gradations of the image data is reduced by a systematic dither method. In some cases, compared to the error diffusion method, the systematic dither method does not emphasize the block noise or can reduce the block noise. Even when data needs to be compressed, high-quality image data can be recorded on a recording sheet or displayed on a display.

  According to the fourth aspect of the present invention, when reducing the number of gradations of image data including a photographic image, a configuration in which the image data is not compressed is used to reduce the number of gradations of the photographic image in which block noise easily occurs. Since deterioration in image quality can be suppressed, high-quality photographic images can be recorded on recording paper or displayed on a display.

  According to the fifth aspect of the present invention, when image data with a reduced number of gradations is recorded on a recording sheet, even if the image data includes a photographic image, a plurality of image data are stored in the memory. When recording, the image data is compressed and the number of gradations is reduced by a method other than the error diffusion method, so that the memory installed in the apparatus can be used effectively and the image quality due to block noise or the like can be reduced. Since reduction can be suppressed, even when image data needs to be compressed, high-quality image data can be recorded on a recording sheet, displayed on a display, or the like.

According to the sixth aspect of the invention, when image data including a photographic image is acquired and the acquired image data is compressed, a systematic dither method or the like is used without reducing the number of gradations by the error diffusion method. By adopting a configuration that reduces the number of gradations by other methods, it is possible to suppress deterioration in the image quality of photographic images that are likely to generate block noise due to compression, so recording high-quality photographic images on recording paper Alternatively, display on a display can be performed.

  According to the seventh aspect of the invention, image data is compressed by the JPEG method, and when compression is performed, the number of gradations is not reduced by the error diffusion method, but by a method such as a systematic dither method. By reducing the image quality, it is possible to suppress deterioration of image quality due to block noise generated by JPEG compression and to effectively use the memory installed in the apparatus.

  According to the eighth aspect of the invention, the configuration is such that the number of gradations of the image data is reduced to two, so that the printer or the like records the image data on the recording paper, or the facsimile apparatus or the like transmits and receives the image data. It is possible to reduce the number of gradations suitable for the case where the image quality is performed. In this case, it is possible to suppress the deterioration of the image quality due to block noise or the like.

  Hereinafter, the present invention will be specifically described with reference to the drawings showing embodiments thereof. In the present embodiment, an image processing apparatus will be described as an example of a multifunction machine having a copy function, a printer function, a scanner function, a facsimile function, and the like. FIG. 1 is a block diagram showing a configuration of a multifunction machine according to an embodiment of the present invention. In FIG. 1, reference numeral 1 denotes each part such as an NCU (Network Control Unit) 2, a modem 3, an image processing unit 5, a ROM 7, a RAM 8, a display unit 9, an operation unit 10, and a LAN (Local Area Network) interface 11 provided in the multifunction peripheral. These units are connected to the control unit 1 via a bus 12.

  Specifically, the control unit 1 is composed of a CPU (Central Processing Unit), controls each part of the facsimile machine, and processes described below and various other software functions according to a control program stored in the ROM 7. Is supposed to be executed.

In addition, NCU2 is a PSTN (Public Switched Telephone).
This is hardware for closing and opening a line with a network (public telephone line network), and has an interface circuit between the PSTN and the modem 3. When transmitting data by facsimile communication, the modem 3 converts digital image data into analog voice data, and transmits it to a device having another facsimile function via the PSTN. When data is received, an analog audio signal is converted into digital image data.

  The ROM 7 stores various software control programs necessary for the operation of the multifunction peripheral, and is configured by a nonvolatile memory element such as a mask ROM, an EEPROM, or a flash memory. The RAM 8 stores temporary data generated when the control unit 1 performs control processing, arithmetic processing, or the like, and includes a rewritable memory element such as SRAM, DRAM, or flash memory.

  The display unit 9 is constituted by, for example, a liquid crystal panel, and displays an operation state of the multifunction device or a warning message to the user. The operation unit 10 has a plurality of buttons, switches, or the like. The operation unit 10 receives operations given to the buttons or switches by the user and notifies the control unit 1 of the operations. The operation unit 10 includes a touch sensor (not shown) that detects a user's contact with the display screen of the display unit 9, and is operated when the user touches a menu or a button displayed on the display unit 9. Can be accepted. This is a so-called touch panel, and the operation unit 10 includes a touch panel. The control unit 1 performs processing according to the operation received by the operation unit 10.

  The LAN interface 11 has a connector for connecting a so-called LAN cable. The LAN interface 11 is connected to a LAN such as an in-house LAN or a home LAN via a LAN cable connected to the connector, and transmits data to and from a PC. Transmit and receive.

  In addition, the multi-function device records the image data on a recording sheet when using the copy function, the facsimile function, and the printer function, and the reading unit 4 that reads a document when using the copy function, the facsimile function, and the scanner function. And a recording unit 6 to be performed. The reading unit 4 reads an original with an optical system using, for example, a CCD (Charge Coupled Device), and outputs image data based on the read original. The image data output by the reading unit 4 is image processing. It is designed to be given to part 5. Although not shown, the reading unit 4 has an ADF (Auto Document Feeder), and can automatically read a plurality of pages of a document. The image processing unit 5 has an image memory 21 for temporarily storing image data given from the reading unit 4, image data received by the modem 3, and image data given from the PC via the LAN. In addition, the image processing described later is performed on the given image data and is given to the recording unit 6. The recording unit 6 is an electrophotographic printer and performs recording on a recording sheet based on given image data.

  For example, the copy function of the multifunction peripheral acquires image data by reading a document with the reading unit 4, the image processing unit 5 performs various image processing on the acquired image data, and the recording unit 6 records it on a recording sheet. It is realized by doing. The facsimile function reads a document by the reading unit 4 and gives image data from the image processing unit 5 to the modem 3, and transmits the image data to other devices via the PSTN by the modem 3 and the NCU 2. Is received from the modem 3 to the image processing unit 5 and is recorded on the recording paper by the recording unit 6. The scanner function is realized by transmitting image data of a document read by the reading unit 4 from the image processing unit 5 to the PC via the LAN interface 11. The printer function is realized by supplying image data received from the PC via the LAN interface 11 to the image processing unit 5 and recording it on a recording sheet by the recording unit 6.

  FIG. 2 is a block diagram illustrating a configuration of the image processing unit 5 of the multifunction peripheral according to the embodiment of the present invention. The image processing unit 5 amplifies the analog signals indicating the red component (R component), green component (G component) and blue component (B component) of the document read by the reading unit 4 to a predetermined magnitude, It has an AFE (Analog Front End) circuit 22 that samples and holds the amplified analog signal at a certain timing by a sample and hold circuit. The three values sampled and held by the AFE circuit 22 are A / D conversion circuits 23. To be given to. The A / D conversion circuit 23 converts the three analog values given from the AFE circuit 22 into, for example, 8-bit digital signals and outputs them.

  Three digital signals of R component, G component, and B component output from the A / D conversion circuit 23, that is, RGB image data, are supplied to the shading correction circuit 24. The shading correction circuit 24 performs correction to remove various distortions generated in the illumination system, the imaging system, the imaging system, or the like of the given image data, and the corrected image data is input to the input color space. This is given to the conversion circuit 25. The input color space conversion circuit 25 converts the given RGB image data into YCbCr image data composed of three components of luminance, blue hue, and red hue, and sends them to the memory controller 26. To give.

The memory controller 26 stores the image data from the input color space conversion circuit 25 or the JPEG compression / decompression circuit 32 in the image memory 21 under the control of the control unit 1 and reads out the image data stored in the image memory 21 to read the JPEG. Compression / decompression circuit 32 or image processing circuit 2
7 is given. The memory controller 26 stores the image data after compressing the image data by the JPEG compression / decompression circuit 32 when storing the image data in the image memory 21, and reads the image data when reading the image data from the image memory 21. The image data is expanded by the JPEG compression / decompression circuit 32 and supplied to the image processing circuit 27.

  When image data is given from the input color space conversion circuit 25 and transmitted to another apparatus by the facsimile function or to the PC via the LAN, the memory controller 26 uses the JPEG compression / decompression circuit 32. After the image data is compressed, the image data is stored in the image memory 21, and the control unit 1 supplies the image data stored in the image memory 21 to the modem 3 or the LAN interface 11. Also, image data received from another apparatus by the facsimile function or image data received from the PC via the LAN is stored in the image memory 21 by the control unit 1 and is read out by the memory controller 26 and is read by the JPEG compression / decompression circuit 32. The image data is decompressed and then applied to the image processing circuit 27 for recording.

  When the image data of the document read by the reading unit 4 is recorded by the recording unit 6, that is, when copying the document, the memory controller 26 receives the image data supplied from the input color space conversion circuit 25. Is supplied to the image processing circuit 27. Further, when copying a plurality of originals or when copying originals together, the image data given from the input color space conversion circuit 25 is compressed by the JPEG compression / decompression circuit 32 and stored in the image memory 21. The image data is supplied to the image processing circuit 27 after temporarily storing the image data.

  For example, when a plurality of originals are aggregated and copied as described above, the image processing circuit 27 performs a process of aggregating a plurality of given image data to create one image data. For example, image processing such as enlargement and reduction of image data can be performed, and the image data subjected to the image processing is supplied to the output color space conversion circuit 28. The output color space conversion circuit 28 converts the supplied YCbCr image data into CMYK image data composed of four components of cyan, magenta, yellow, and black, and outputs the converted data.

  The CMYK image data output from the output color space conversion circuit 28 is supplied to a systematic dither method gradation number reduction circuit 30 and an error diffusion method gradation number reduction circuit 29 for reducing the number of gradations. The systematic dither method gradation number reduction circuit 30 performs a process for reducing the number of gradations of image data, for example, a binarization process using an algorithm of a systematic dither method, and outputs CMYK image data with a reduced number of gradations. The signal is supplied to the selection circuit 31. The error diffusion method gradation number reduction circuit 29 performs a process of reducing the number of gradations of the image data by an error diffusion method algorithm so that CMYK image data with the reduced number of gradations is supplied to the selection circuit 31. is there. The selection circuit 31 selects and records image data given from either the systematic dither method gradation number reduction circuit 30 or the error diffusion method gradation number reduction circuit 29 in accordance with a command from the control unit 1. The image data is recorded on the recording paper by the recording unit 6.

  FIG. 3 is a schematic diagram for explaining an outline of the JPEG compression processing performed by the JPEG compression / decompression circuit 32. The YCbCr system image data (see FIG. 3A) output from the input color space conversion circuit 25 is given from the memory controller 26 to the JPEG compression / decompression circuit 32. The JPEG compression / decompression circuit 32 divides the given image data into blocks of 8 pixels × 8 pixels (see FIG. 3B), and performs DCT (Discrete Cosine Transform) for each block to generate a frequency. After conversion into components and quantization based on the quantization table, compression using a Huffman code is performed.

As described above, since compression processing is performed for each block of 8 pixels × 8 pixels in the JPEG method, block noise (FIG. 3 (FIG. 3 (FIG. 3C) in which boundary lines between blocks are visually recognized in the compressed image data (see FIG. 3C)). c) may occur). This is because the color density gradually increases or decreases in the non-compressed part of the image before compression, but in the post-compression image, the color density changes greatly at the boundary part between blocks, and this density difference is visually recognized. It is.

  FIG. 4 is a schematic diagram for explaining the outline of the gradation number reduction process performed by the systematic dither gradation number reduction circuit 30. The example shown in this figure is a case where each pixel of the image data input to the systematic dither gradation number reduction circuit 30 has an 8-bit information amount, that is, each pixel has a gradation number of 256 gradations. (See FIG. 4 (a)). In the systematic dither method, 16 threshold values are set in advance in a 4 × 4 dither matrix (see FIG. 4B), and the dither matrix threshold value is set for each 4 pixel × 4 pixel region of the input image data. When the gradation value of each pixel is smaller than the threshold value, this pixel is converted to the lowest gradation value (ie, 0), and when it is larger than the threshold value, it is converted to the highest gradation value (ie, 255). By doing so, binarization is performed (see FIG. 4C).

  As described above, the systematic dither method is a method in which the number of gradations is reduced by comparison with a threshold value, so that even when block noise occurs due to JPEG compression, the block noise does not exceed the threshold value. If it is, the block noise can be suppressed after the number of gradations is reduced. However, since the comparison is repeatedly performed using a 4 × 4 dither matrix, there is a problem that a so-called moire in which a periodic pattern is visually recognized with the size of the dither matrix may occur.

FIG. 5 is a schematic diagram for explaining the outline of the gradation number reduction process performed by the error diffusion method gradation number reduction circuit 29. The example shown in this figure is a case where each pixel of the image data input to the error diffusion method gradation number reduction circuit 29 has 256 gradations and the threshold for binarization is 128. is there. Further, the horizontal direction of the image data, that is, the right direction in FIG. 5 is the x direction, and the vertical direction, that is, the lower direction in FIG. 5 is the y direction. , Y), the pixel f (x + 1, y) on the right side of the target pixel f (x, y), the pixel f (x + 1, y + 1) on the lower right side, and the pixel f (x, y on the lower side). In this example, the error Er (x, y) generated when the pixel of interest f (x, y) is binarized is diffused to y + 1) based on the following equation.
f (x + 1, y) = f (x + 1, y) + Er (x, y) * (3/8)
f (x + 1, y + 1) = f (x + 1, y + 1) + Er (x, y) * (1/8)
f (x, y + 1) = f (x, y + 1) + Er (x, y) * (2/8)
Er (x, y) = f (x, y) − {0 or 255}

First, when the pixel of interest is the upper left pixel f (0, 0) = 143 in the image data (see FIG. 5A), the binary value of f (0, 0) = 255 is obtained by comparison with the threshold value of 128. The error at this time is Er (0, 0) = 143-255 = −112. This error Er (0, 0) is determined by changing the right adjacent pixel f (1, 0) = 51, the lower right adjacent pixel f (1, 1) = 99, and the lower adjacent pixel f (0, 1) = 86. When diffusing, it becomes as follows.
f (1,0) = 51-112 * (3/8) = 9
f (1,1) = 99−112 * (1/8) = 84
f (0, 1) = 86−112 * (2/8) = 56

Next, when the pixel of interest is shifted by one in the x direction to be f (1, 0) = 9 (see FIG. 5B), a binary value of f (1, 0) = 0 is obtained by comparison with a threshold value of 128. The error at this time is Er (1, 0) = 9−0 = 9. The error Er (1, 0) is determined by changing the right adjacent pixel f (2, 0) = 63, the lower right adjacent pixel f (2, 1) = 55, and the lower adjacent pixel f (1, 1) = 84. When diffusing, it becomes as follows.
f (2, 0) = 63 + 9 * (3/8) = 66
f (2,1) = 55 + 9 * (1/8) = 56
f (1,1) = 84 + 9 * (2/8) = 86

Next, when the pixel of interest is shifted by one in the x direction to be f (2, 0) = 66 (see FIG. 5C), a binary value of f (2, 0) = 0 is obtained by comparison with a threshold value of 128. The error at this time is Er (2, 0) = 66−0 = 66. This error Er (2, 0) is determined by changing the right adjacent pixel f (3, 0) = 177, the lower right adjacent pixel f (3, 1) = 30, and the lower adjacent pixel f (2, 1) = 56. When diffusing, it becomes as follows.
f (3,0) = 177 + 66 * (3/8) = 202
f (3, 1) = 30 + 66 * (1/8) = 38
f (2,1) = 56 + 66 * (2/8) = 73

  As described above, it is possible to binarize image data by repeatedly moving the pixel of interest sequentially in the x direction and further in the y direction. Since the binarization error can be diffused to the right adjacent pixel, the lower right adjacent pixel, and the adjacent lower pixel, moire does not occur unlike the systematic dither method, and the reproducibility of the intermediate gradation is higher. There are advantages. However, when block noise occurs due to JPEG compression of image data, the influence of block noise may spread to surrounding pixels, and image quality may deteriorate.

  6 and 7 are flowcharts showing a copy processing procedure performed by the control unit 1 of the multifunction peripheral according to the embodiment of the present invention. In the example shown in this flowchart, the user can set whether to copy a plurality of copies and whether to copy a document including a photo (whether it is a photo mode). And First, the control unit 1 checks whether or not a plurality of copies are set by the user operating the operation unit 10 (step S1), and if a plurality of copies are not set (S1: NO) Further, it is checked whether or not a photo mode copy is set (step S2).

  When copying in the photographic mode is not set (S2: NO), the original is read by the reading unit 4 (step S3), and after the read RGB image data is converted into YCbCr image data, the JPEG compression / decompression circuit The image data is compressed at 32 (step S4) and stored in the image memory 21. Next, the compressed image data is read out from the image memory 21 and decompressed, and the YCbCr system image data is converted into CMYK system image data, and then the gradation number reduction process is performed. The selection circuit 31 selects the image data that is the result of the gradation number reduction process by (Step S5), and the image data with the reduced gradation number is recorded on the recording paper by the recording unit 6 (Step S6). Next, it is checked whether or not copying of all the originals has been completed (step S7). If not completed (S7: NO), the process returns to step S3 to repeat the processes from reading to recording of the original. When copying has been completed for all the originals (S7: YES), the copying process ends.

  If copying in the photographic mode is set in step S2 (S2: YES), the original is read by the reading unit 4 (step S8), and the gradation number reduction process is performed without compressing the read image data. Then, the selection circuit 31 selects image data as a result of the gradation number reduction process by the error diffusion method gradation number reduction circuit 29 (step S9), and the image data with the reduced gradation number is stored in the recording unit 6. To record on the recording sheet (step S10). Next, it is checked whether or not copying of all the originals has been completed (step S11). If not completed (S11: NO), the process returns to step S8 to repeat the processes from reading to recording of the original. When copying is completed for all the originals (S11: YES), the copying process is ended.

If multiple copies are set in step S1 (S1: YES), the document is read by the reading unit 4 regardless of whether or not photo mode copying is set (step S1).
In step S12), the read image data is compressed by the JPEG compression / decompression circuit 32 (step S13) and stored in the image memory 21 (step S14). Next, after the compressed image data is read out from the image memory 21 and decompressed, the number of gradations is reduced, and the image data resulting from the gradation number reduction processing by the systematic dither method gradation number reduction circuit 30 is selected. 31 is selected (step S15), and the image data with the reduced number of gradations is recorded on the recording paper by the recording unit 6 (step S16). Next, it is checked whether or not copying of all the originals has been completed (step S17). If not completed (S17: NO), the process returns to step S12 to repeat the processes from reading to recording of the original.

  When all the copies have been completed for all the originals (S17: YES), the stored image data is read from the image memory 21 (step S18) and decompressed by the JPEG compression / decompression circuit 32, and then the gradation number reduction processing is performed. The selection circuit 31 selects image data that is the result of the gradation number reduction process by the systematic dither gradation number reduction circuit 30 (step S19), and the image data with the reduced number of gradations is recorded in the recording unit 6. To record on a recording sheet (step S20). Next, it is checked whether or not all recording has been completed for the number of copies set by the user (step S21). If not completed (S21: NO), the process returns to step S18, and the image stored in the image memory 21 is stored. The processing from data reading to recording is repeated, and when all recording is completed for the set number of copies (S21: YES), the copy processing is ended.

  In the MFP with the above configuration, if the photo mode is set when performing the copy process, the number of gradations is reduced by the error diffusion method without compressing the image data, and in other cases Since the image data is compressed and the number of gradations is reduced by the systematic dither method, the JPEG compression reduces the image quality due to the occurrence of block noise in the image data. Image data can be recorded on recording paper with high quality.

  In this flowchart, when the user performs copying, only two are set: whether or not to copy a plurality of copies and whether or not to copy a document including a photograph (whether or not the photograph mode is set). In other configurations, for example, it is possible to set conditions such as whether to copy a plurality of originals together or whether to copy originals with enlargement / reduction. In this case, this flowchart is expanded as appropriate. When image data is compressed by the JPEG method, the number of gradations is reduced by the systematic dither method. When compression is not performed, the error diffusion method is used. A configuration in which the number of gradations is reduced may be employed. Further, the configuration is such that the user sets the photo mode. However, the present invention is not limited to this, and it may be configured to automatically detect whether the image data read by the reading unit 4 includes a photo. In addition, the case where the present invention is applied to the copy function of the multifunction peripheral has been described, but the present invention is not limited to this. Similarly, whether the image data is compressed by the JPEG method for the facsimile function or the printer function. It is possible to apply the configuration of the present invention in which the method of reducing the number of gradations is changed according to the above. The same configuration can also be applied to a device having only a copy function, that is, a so-called copy machine, instead of a multi-function device.

(Modification)
In the copy processing shown in FIG. 6 and FIG. 7, it is configured to determine whether or not to compress JPEG image data depending on whether or not the mode is a photo mode. If the copy process is not in the photo mode, for example, in the character mode for copying a text document, the number of gradations by the error diffusion method is not performed without performing the compression by the JPEG method unless multiple copies are made. It is the structure which performs reduction.

FIG. 8 is a flowchart illustrating a copy processing procedure performed by the control unit 1 of the multifunction peripheral according to the modification of the embodiment of the present invention. First, the control unit 1 checks whether or not a plurality of copies have been set by the user operating the operation unit 10 (step S41). If a plurality of copies have not been set (S41: NO), the original is read by the reading unit 4 (step S42), the gradation number reduction processing is performed without compressing the read image data, and the result of the gradation number reduction processing by the error diffusion type gradation number reduction circuit 29 is obtained. Is selected by the selection circuit 31 (step S43), and the image data with the reduced number of gradations is recorded on the recording paper by the recording unit 6 (step S44). Next, it is checked whether or not copying of all the originals has been completed (step S45). If not completed (S45: NO), the process returns to step S42, and the processes from reading to recording of the original are repeated. When copying has been completed for all the originals (S45: YES), the copying process ends.

  If multiple copies are set in step S41 (S41: YES), the document is read by the reading unit 4 (step S46), and the read image data is compressed by the JPEG compression / decompression circuit 32. (Step S47), the image is stored in the image memory 21 (Step S48). Next, after the compressed image data is read out from the image memory 21 and decompressed, the number of gradations is reduced, and the image data resulting from the gradation number reduction processing by the systematic dither method gradation number reduction circuit 30 is selected. 31 is selected (step S49), and the image data with the reduced number of gradations is recorded on the recording paper by the recording unit 6 (step S50). Next, it is checked whether or not copying for all the originals has been completed (step S51). If not completed (S51: NO), the process returns to step S46, and the processes from reading to recording are repeated.

  When all the copies have been completed for all the originals (S51: YES), the stored image data is read from the image memory 21 (step S52) and decompressed by the JPEG compression / decompression circuit 32, and then the gradation number reduction processing is performed. The selection circuit 31 selects image data that is the result of the gradation number reduction processing by the systematic dither gradation number reduction circuit 30 (step S53), and the image data with the reduced number of gradations is recorded in the recording unit 6. To record on the recording sheet (step S54). Next, it is checked whether or not all recording has been completed for the number of copies set by the user (step S55). If not completed (S55: NO), the process returns to step S52, and the image stored in the image memory 21 is stored. The processing from data reading to recording is repeated, and when all recording has been completed for the set number of copies (S55: YES), the copy processing is terminated.

  As described above, it is not a configuration that determines whether or not to compress image data by the JPEG method depending on whether or not it is in the photographic mode, that is, whether or not a photographic image is included in the original to be copied. Alternatively, at least when the image data is compressed by the JPEG method, the number of gradations may not be reduced by the error diffusion method. As a result, it is possible to suppress deterioration in the quality of the image recorded by the recording unit 6 due to block noise caused by JPEG compression.

1 is a block diagram illustrating a configuration of a multifunction machine according to an embodiment of the present invention. FIG. 2 is a block diagram illustrating a configuration of an image processing unit of the multifunction peripheral according to the embodiment of the present invention. It is a schematic diagram for demonstrating the outline | summary of the JPEG compression process which a JPEG compression / decompression circuit performs. It is a schematic diagram for demonstrating the outline | summary of the gradation number reduction process which a systematic dither system gradation number reduction circuit performs. It is a schematic diagram for explaining an outline of gradation number reduction processing performed by an error diffusion method gradation number reduction circuit. 6 is a flowchart illustrating a copy processing procedure performed by the control unit of the multifunction peripheral according to the embodiment of the present invention. 6 is a flowchart illustrating a copy processing procedure performed by the control unit of the multifunction peripheral according to the embodiment of the present invention. 10 is a flowchart illustrating a copy processing procedure performed by a control unit of a multifunction peripheral according to a modification of the embodiment of the present invention.

Explanation of symbols

1 control unit 4 reading unit (acquisition means)
5 Image processing section 6 Recording section (recording means)
21 Image memory 26 Memory controller 29 Error diffusion method gradation number reduction circuit (first gradation number reduction means)
30 Systematic dither method gradation number reduction circuit (second gradation number reduction means)
31 selection circuit 32 JPEG compression / decompression circuit (compression means)

Claims (8)

In an image processing apparatus that performs processing for reducing the number of gradations of image data,
Acquisition means for acquiring image data;
Compression means for compressing the acquired image data;
First gradation number reduction means for reducing the number of gradations of the image data by a gradation number reduction method for reducing the number of gradations of the target pixel according to the result of reduction of the gradation values of the peripheral pixels;
A second gradation number reduction means for reducing the number of gradations of the image data by a method different from the first gradation number reduction means,
When the compression means does not compress the image data, the first gradation number reduction means performs a process of reducing the gradation number of the image data,
An image processing apparatus, wherein when the compression unit compresses image data, the second gradation number reduction unit performs a process of reducing the gradation number of the compressed image data.   The image processing apparatus according to claim 1, wherein the first gradation number reduction means reduces the gradation number of the image data by an error diffusion method.   The image processing apparatus according to claim 1, wherein the second gradation number reduction unit reduces the gradation number of the image data by a systematic dither method.   The image processing apparatus according to any one of claims 1 to 3, wherein when the image data acquired by the acquisition unit includes a photographic image, the compression of the image data by the compression unit is not performed. Comprising recording means for recording image data having a reduced number of gradations on recording paper;
Even when the image data acquired by the acquisition means includes a photographic image, when the recording means performs recording after accumulating a plurality of image data, the image data is compressed by the compression means, and the second 5. The image processing apparatus according to claim 1, wherein the number of gradations is reduced by the number of gradation reduction means. In an image processing apparatus that performs processing for reducing the number of gradations of image data,
Acquisition means for acquiring image data;
Compression means for compressing the acquired image data;
A gradation number reducing means for reducing the number of gradations of the image data by an error diffusion method,
When the image data acquired by the acquisition unit includes a photographic image, and the image data acquired by the compression unit is compressed, the gradation number is not reduced by the gradation number reduction unit. An image processing apparatus characterized by the above.   The image processing apparatus according to claim 1, wherein the compression unit compresses image data by a JPEG method.   The image processing apparatus according to claim 1, wherein the gradation number reducing unit is configured to reduce the gradation number of the image data to two gradations.

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