JP2009060474A - Image processing apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To efficiently compress various image data with a higher compression ratio than the prior art while ensuring a proper resolution. <P>SOLUTION: A binarization unit 111 outputs binary data identifying a character region and a non-character region based on lightness/darkness of an image density of input image data. A character image production unit 117 discriminates image data in the character region of the input image data on the basis of the binary data and produces a character image file by performing irreversible compression on the image data using a quantization table for luminance and color differences. A background image production unit 116 discriminates image data in a background region of the input image data on the basis of the binary data and produces a background image file by performing irreversible compression on the image data using the quantization table for luminance and color differences. Between the character image production unit 117 and the background image production unit 116, characteristics of the quantization table to be used are different. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus used in an image forming apparatus such as a color digital copying machine or a scanner distribution apparatus, or an image reading apparatus. The present invention relates to an image processing apparatus that performs image processing to be compressed at a high compression rate.

In general, lossy compression techniques such as JPEG (Joint Photographic Experts Group) are known as data compression techniques for still image data. In JPEG, image data is divided into blocks of 8 × 8 pixels, and discrete cosine transform and Huffman coding are performed for each block, thereby reducing redundant data and high-frequency data to reduce the amount of data. ing.
However, since this JPEG is irreversible compression, if the compression rate is increased, the image quality is deteriorated when the compressed image data is decoded and reproduced. Therefore, various proposals have been made in the past in order to increase the compression rate while suppressing such deterioration of image quality as much as possible.

  For example, in the image processing apparatus described in Patent Document 1, a quantization table used when quantizing image data between a character area and a picture area is switched. Specifically, in the character area, priority is given to image quality, and a high-frequency component is held using a quantization table with a small coefficient to suppress the occurrence of character twist, mosquito noise, and the like due to loss of the high-frequency component. On the other hand, for the picture area, since the image quality deterioration is not noticeable even if the compression rate is increased, a quantization table having a large coefficient is used with priority on the compression rate.

  Also, in the image processing apparatus described in Patent Document 2, a picture part such as a photograph is separated from the image data as the first image data, and the color information is separated as the second image data from the character line drawing part. The shape information is selection data for selecting the first image data or the second image data, and the compression method is switched according to the respective attributes. In this case, since the selection data only selects the first image data or the second image data, it can be handled as binary data, and can be compressed at a high compression rate while maintaining the image quality at a high resolution.

Furthermore, in the image processing apparatus described in Patent Document 3, two-dimensional multi-value image data is separated into an edge region block and an image region block, and the edge region block generates an image using only high-frequency coefficient quantization means. The image region block generates an image using only low-frequency coefficient quantization means.
JP 2002-64712 A Japanese Patent Laid-Open No. 2002-368986 Japanese Patent No. 3644220

As described above, compression using irreversible compression such as a JPEG file has a very high compression rate and can create a small file, which is effective for a picture, but binary such as a character image. For typical images, the edges of the characters are blurred and the readability of the characters is poor.
Therefore, various proposals have been made as seen in Patent Documents 1 to 3. However, these various data compression methods that have been proposed in the past have not been able to sufficiently satisfy the demand for efficiently compressing all types of image data at a high compression rate while ensuring an appropriate resolution.
SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and an object of the present invention is to enable various types of commonly used image data to be efficiently compressed at a higher compression rate than before while ensuring an appropriate resolution. To do.

In order to achieve the above object, the present invention provides the following image processing apparatus.
The image processing apparatus includes: binarizing means for outputting binary data for identifying a character area and a non-character area based on brightness and darkness of the image density of the input image data; and a character of the input image data based on the binary data. Character image generation means for determining image data of a region and irreversibly compressing the image data using a quantization table of luminance and color difference to generate a character image file, and input image data based on upper binary data Background image generating means for discriminating image data of a background area and irreversibly compressing the image data using a quantization table of luminance and color difference to generate a background image file, and the character image generating means The quantization table used is different from the quantization table used by the background image generation means.

In the image processing apparatus, image file combining means for combining the character image file and the background image file generated by the character image generating means and the background image generating means into one image file may be provided.
In place of the binarization means, binarization means for outputting binary data and black character data for identifying character areas and non-character areas based on the contrast of the image density of the input image data is provided, and the character image generation is performed. In addition to the means and the background image generation means, the binary image generation means for reversibly compressing the binary data output from the binarization means to generate a binary image file, and the output from the binarization means It is further preferable to provide black image generation means for reversibly compressing the black character data to be generated to generate a black image file.

Both the binary image generating means and the black image generating means may be means for performing MMR compression.
Further, in this image processing apparatus, the binary image file, the black image file, the character image file, and the background image respectively generated by the black image generating means, the black image generating means, the character image generating means, and the background image generating means. It is preferable to provide an image file synthesizing means for synthesizing the file into one image file.
In these image processing apparatuses, both the character image generation means and the background image generation means may be JPEG compression means.

  Further, the character image generation means has means for rewriting the image data of the background area of the input image data to image data of a constant value based on the binary data output from the binarization means, and the background image The generating means may have means for rewriting the image data in the character area of the input image data to image data of a constant value corresponding to white based on the binary data output from the binarizing means.

In these image processing apparatuses, it is desirable that the quantization table used by the background image generation unit has a characteristic of storing more DC components than the quantization table used by the character image generation unit.
Of the quantization tables used by the character image generating means, the color difference quantization table preferably has a characteristic of storing more high-frequency components than the luminance quantization table.
Further, it is desirable that the color difference quantization table among the quantization tables used by the character image generation unit has a characteristic of storing high-frequency components as compared with the color difference quantization table used by the background image generation unit.

These image processing apparatuses can have means for transmitting the image data of the image processing result to an external device.
Alternatively, an image output unit that forms an image based on the image data obtained as a result of the image processing, forms the image on a sheet, and outputs the image may be provided.

  An image processing apparatus according to the present invention discriminates image data of a character area and image data of a background area of input image data, and performs irreversible compression using different quantization tables suitable for the respective character image files and backgrounds. Since the image file is generated, various image data can be efficiently compressed at a higher compression rate than before while ensuring an appropriate resolution.

The best mode for carrying out the present invention will be specifically described with reference to the drawings.
FIG. 1 is a block diagram showing a schematic configuration of a digital color image processing apparatus according to an embodiment of the present invention.
The color image processing apparatus shown in FIG. 1 includes an image reading system including a scanner 1, a scanner correction unit 2, and a compression processing unit 3, an expansion processing unit 7, a printer correction unit 8, and a printer 9. An image printing system and a controller 5 for controlling them are connected by a general-purpose bus 4. A network interface controller that transmits and receives data between a hard disk drive (abbreviated as HDD) 6 that stores image data and an external personal computer (abbreviated as PC) 11 via the network. (Abbreviated as NIC) 10 is connected.
Hereinafter, the functions of this color image processing apparatus will be described separately for each type of operation.

(1) When operating as a copying machine When this color image processing apparatus operates as a copying machine, the scanner 1 reads an image from a document D, converts the image data (analog data) into digital data, and performs scanner correction. Output to part 2.
As will be described in detail later, the scanner correction unit 2 classifies the image area of the image data (digital data) read by the scanner 1 into a character / line drawing, a photograph, or the like, or an RGB (red green blue) image of the original image. The data is subjected to image processing such as filter processing and output to the compression processing unit 3.

The compression processing unit 3 includes RGB data (8-bit image data of each of R, G, and B) and image area separation data (1-bit character edge region data and 1) after image processing is performed by the scanner correction unit 2. (Composed of bit color area data) is compressed and sent to the controller 5 via the general-purpose bus 4.
The controller 5 temporarily stores the image data sent from the compression processing unit 3 in a semiconductor memory (not shown), and then sends it to the HDD 6 for storage.
When image data is stored in the HDD 6, bibliographic information including the image size of the image data and the type of the read original is also recorded.
Although the case where the image data is compressed has been described here, the image data may be handled in an uncompressed state as long as the bandwidth of the general-purpose bus is sufficiently wide and the storage capacity of the HDD 6 is large.

Then, the controller 5 reads out the image data stored in the HDD 6 and sends it to the expansion processing unit 7 via the general-purpose bus 4.
The decompression processing unit 7 decompresses the compressed image data into the original RGB data, character edge region data, and color region data, and sends them to the printer correction unit 8.
The printer correction unit 8 converts the RGB data into YMCBk data, and replaces the portion that is character edge area data and not color area data with black Bk data as black characters.

Further, a gamma correction process that is a process for correcting the light / dark characteristics of the plotter and a halftone process that is a gradation number conversion process are performed and output to the plotter 9.
In the gradation number conversion process, image data is converted from 8-bit to 2-bit for each color by error diffusion processing or dither processing.
The plotter 9 is a transfer paper printing unit that uses a laser beam writing process. The plotter 9 draws 2-bit image data as a latent image on a photoconductor, develops it with toner, transfers it to transfer paper, and outputs a copy image C. .
These correspond to image output means for forming an image based on image data obtained as a result of image processing according to the present invention, and forming and outputting the formed image on a sheet.

(2) When operating as a distribution scanner When this color image processing apparatus operates as a distribution scanner that distributes image data to the PC 11 via a network, the scanner operates in the same manner as when operating as a copying machine described above. The image data input from 1 is subjected to the above-described image processing by the scanner correction unit 2, compressed by the compression processing unit 3, and then sent to the controller 5 via the general-purpose bus 4.
The controller 5 decompresses the image data sent from the compression processing unit and performs format processing including color conversion processing from RGB data to sRGB data and general-purpose image format conversion processing to JPEG or TIFF format.
Thereafter, the image data is distributed from the NIC 10 to the external PC 11 via a network (not shown). This is means for transmitting the image processing result image data according to the present invention to an external device.

(3) When operating as a printer When this color image processing apparatus operates as a printer that receives image data from an external PC via a network and prints it out, the controller 5 is transmitted from the external PC 11. The NIC 10 receives the data, analyzes the image data and a command for instructing printing from the data, develops the bitmap into a state where the image data can be printed, and compresses the decompressed data into the controller 5. Accumulate once.
The accumulated data is written to the HDD 6 as needed. When image data is stored in the HDD 6, bibliographic information of the image data is also written.

Then, this controller reads the image data of the HDD and sends it to the expansion processing unit 7 via the general-purpose bus 4.
The decompression processing unit 7 decompresses the compressed image data to the original 8-bit data and sends it to the printer correction unit 8.
The printer correction unit 8 converts the data received from the decompression processing unit 7 into YMCBk data if the data is RGB data. In addition, YMCBk data was independently subjected to γ correction processing, and halftone processing, correction processing of light / dark characteristics of plotter, error diffusion processing and dither processing were performed to convert the number of gradations from 8-bit data to 2-bit data. Thereafter, the data is sent to the plotter 9.
The plotter 9 is a transfer paper printing unit that uses a laser beam writing process. The plotter 9 draws 2-bit image data as a latent image on a photoconductor, develops it with toner, transfers it to transfer paper, and outputs a copy image C. To do.

  This color image processing apparatus reads an original D with the scanner 1 and converts the image data into digital data, and separates the image area (image area) of the original into areas having different characteristics. It is determined to which region each pixel of interest belongs, and various image processing is performed on the image data according to the determination result. This greatly improves the image quality of the output image.

Next, details of the scanner correction unit will be described with reference to FIG.
FIG. 2 is a block diagram showing an internal configuration of the scanner correction unit 2 of the color image processing apparatus shown in FIG.
The scanner correction unit 2 includes an image area separation unit 21, a scanner γ unit 22, a filter processing unit 23, and a document type determination unit 24.
In the image area separation unit 21, based on the reflectance linear image data img input from the scanner 1, an image area separation process is performed for separating the image area of the document into areas having different characteristics. Regarding this image area separation processing, for example, a known technique disclosed in detail in Japanese Patent Laid-Open No. 2003-259115 may be used, and therefore detailed description of the processing is omitted.

In the image area separation unit 21 in this embodiment, the image area of the document is separated into three areas: a black character edge area, a color character edge area, and other areas (photograph area and the like). Then, the image area separation unit 21 separates the image area of the document as described above, and image area separation data (character edge area data, color area data, and other areas) for each pixel of the image data of the document. One of the data).
Further, based on the image area separation signal, the image area of the document is divided into a black character edge area (character edge area and not a color area), a color character edge area (character edge area and a color area), and other Each area is classified into an area (an area other than the above such as a photograph area). The character edge area here is an area where a character edge on the white background is detected.
The scanner γ unit 22 converts the image data from reflectance linear to density linear data.

The filter processing unit 23 switches the filter processing according to the image area separation signal. For example, in a character edge (black character edge and color character edge) region, sharpening processing is performed with emphasis on legibility. In other areas (photo areas, etc.), a sharp density change in the image data is used as an edge amount, and smoothing processing or sharpening processing is performed according to the edge amount. The sharp edges are sharpened in order to make the characters in the picture easier to read.
Note that the threshold value for binarization can be changed between the color character edge region and the black character edge region by combining the character edge region with the color region.

The document type determination unit 24 may determine whether the document type is a text-only document or a color document, and record the determination result as bibliographic information when accumulating images.
In this case, four types of determinations are performed: character original determination, chromatic original determination, photographic paper photo determination, and print photo determination.
For the four types of determinations, for example, a known technique as described in JP 2000-324338 A may be used.
A specific example of the document type determination (recognition) technique will be briefly described below.

The techniques described in paragraphs 0023 to 0025 of the above publication, character determination of chromatic originals in paragraphs 0026 to 0027, determination of photographic paper photographs in paragraph 0028, and determination of printed photographs in paragraphs 0029 to 0031 of the above publication are described. Since these may be used, detailed description of these processes is omitted.

[Detection of presence / absence of character area]
First, the document type recognition apparatus for detecting “with / without character area” will be described with reference to FIGS.
FIG. 29 is a block diagram of the document type recognition apparatus. The character pixel detection circuit 401 detects pixels belonging to a character pixel (more precisely, a character edge) based on the input image signal. The input image signal is a signal from an image input device such as a scanner, and is, for example, an 8-bit signal (white = 0, black = 255) having a linear density of 400 dpi or a G signal among RGB signals. Depending on the document type recognition accuracy required, a max (R, G, B) signal, a luminance signal Y that can be converted in a linear format, or an L signal in the Lab space may be used.

  The counter / reset circuit 402 divides the document, counts the character pixels detected by the character pixel detection circuit 401, and sets the count value of each area in the count value holding circuit 403 in the next stage. For example, as shown in FIG. 30, when counting the character pixels of the A3 document, the first half count value in the sub-scanning direction of the A3 document is set in the count value holding circuit 403, and then this count value is reset. By counting the remaining second half, the count values of the character pixels in the first half and the second half of the A3 document are set in the count value holding circuit 403.

The maximum value calculation circuit 404 detects the maximum value of a plurality of (two in the example shown in FIG. 30) count values set in the count value holding circuit 403, and the subsequent threshold value determination circuit 405 calculates the maximum value. The maximum value of the count value calculated by the circuit 404 is compared with the threshold value TH, and if the maximum value of the count value> the threshold value TH, it is determined that “document with text area”, and in other cases, the “character area” “No original”.
In the example shown in FIG. 30, the document area is divided into two only in the sub-scanning direction, but the document area may be divided into, for example, a 4 × 4 matrix.

A character pixel detection circuit 401 in FIG. 29 is configured as shown in FIG. 31, for example, and a binarization circuit 411 binarizes a multi-gradation image signal using a predetermined threshold. Next, the black pixel pattern matching circuit 412 and the white pixel pattern matching circuit 413 detect the areas where the binarized black pixels and white pixels are connected by pattern matching, respectively.
FIGS. 32 and 33 show patterns for detecting the connection of black pixels or white pixels in an oblique direction, a horizontal direction, a vertical direction, etc., and the black pixel pattern matching circuit 412 and the white pixel pattern matching circuit 413 are respectively When matching these connection patterns, the target pixel is output as a connected black pixel or a connected white pixel.

The subsequent counting circuits 414 and 415 respectively count the connected black pixels and the connected white pixels existing in the 3 × 3 matrix centered on the target pixel, and when the count value becomes 2 or more, for example, “1” is set. Output.
The subsequent AND circuit 416 becomes active when two or more connected black pixels and connected white pixels exist simultaneously, and outputs the target pixel as a character portion pixel candidate. The subsequent determination circuit 417 sets the target pixel as the character part pixel when, for example, one or more character part pixel candidates exist in the 5 × 5 matrix centered on the target pixel, and the determination result is shown in the counter / counter shown in FIG. Output to the reset circuit 402.

[Detection of chromatic / achromatic]
The document type recognition device for detecting chromatic / achromatic has a configuration in which a chromatic / achromatic pixel detection circuit is provided in place of the character pixel detection circuit 401 in the document type recognition device shown in FIG.
The chromatic / achromatic pixel detection circuit determines that a pixel satisfying the following expression is a chromatic pixel, and outputs it to the counter / reset circuit 402 shown in FIG.
max (| RG |, | GB |, | BR |)> TH
As another determination method, for example, as described in Japanese Patent Publication No. 7-22330, a color block is determined, and the determination result is output to the counter / reset circuit 402 shown in FIG. You may make it determine whether it is a manuscript.

[Detection of halftone]
The document type recognizing apparatus for detecting a halftone is shown in FIG. 29, and has a configuration in which a halftone pixel detection circuit is provided instead of the character pixel detection circuit 401 in the document type recognition apparatus shown in FIG.
Here, when a photographic original is read, there are many pixels taking intermediate levels, and these pixels have a certain amount of chunks. The halftone pixel detection circuit detects such a pixel (photo pixel) that is a part of the photo by using such characteristics of the photo original, and whether or not the photo is included in the target original by the count value. Determine whether it is included. There are two types of photographic originals: photographic paper photographs (silver lead photographs) and printed photographs (halftone dot images).

(1) Photographic paper photo (silver lead photo)
FIG. 34 shows a configuration of a halftone detection circuit 420 for detecting a photographic paper photograph. The halftone detection circuit 420 includes a ternary circuit 421 and a pattern matching circuit 422.
The ternary circuit 421 ternarizes the G (green) signal with two threshold values α and β (α> β), and then the pattern matching circuit 422 applies to the intermediate level pixel X (α>X> β). Matching with the 7 × 3 pixel pattern shown in FIG. 35 is performed. When all the 7 × 3 pixels are at the intermediate level, the target pixel (center pixel) of the 7 × 3 size image is determined as a photographic document pixel, and the determination result is shown in the counter / reset circuit shown in FIG. It is output to 402 and it is determined whether it is a photographic paper photograph original.

(2) Print photo (halftone image)
A halftone detection circuit that detects a printed photograph detects a pixel (halftone pixel) that is a part of a halftone dot, and determines whether or not a halftone dot is included in the target document based on the count value. As a halftone dot detection method, for example, a halftone dot region separation method by “pole pixel” detection described in Japanese Patent Laid-Open No. 2-115988, or a paper “character / picture (halftone dot) previously proposed by the present applicant. , Photo) Image area separation method of mixed image "(see IEICE Transactions Vol. J75-DI1 No. 1pp39-47 January 1992) Can be used.

  The peak pixel is detected by the following calculation. As shown in FIG. 36, in the 3 × 3 block, the density level L of the center pixel is higher or lower than that of all the surrounding pixels, and the counter pixels existing on the diagonal line with L and the center pixel in between. When the density levels a and b of the four pairs are | 2 × L−a−b |> TH (fixed threshold), the center pixel is set as the peak pixel.

After detecting halftone pixels, halftone dots are counted for each block of a predetermined size (for example, 8 × 8 pixels), and when the counted value is a predetermined number or more, the target block is determined as a “halftone block”. The determination result may be output to the counter / reset circuit 402 shown in FIG. 29 to determine whether the target document is a halftone document.
An example of the determination result of the document type determined in this way is shown in Table 1.

Here, the “character-only document” is a document in which only characters are present in the document, and as shown in the determination result of Table 1, a document with characters (present), a photographic paper photo document (none), and a printed photo document This is a document for which a determination result of (None) is obtained.
A “color original” is an original from which a chromatic original (presence) determination is obtained.
Here, a pattern such as a copy original or an ink jet original is subjected to gradation processing, and is classified into either a photographic paper photograph or a printed photograph original.

Next, details of the printer correction unit will be described with reference to FIG.
FIG. 3 is a block diagram showing an internal configuration of the printer correction unit 8 of the color image processing apparatus shown in FIG.
The printer correction unit 8 shown in FIG. 3 includes a color correction processing unit 81, a γ correction processing unit 82, a halftone processing unit 83, and an edge amount detection unit 84.
The color correction processing unit 81 converts the RGB data into CMY data by using a primary density masking method or the like in areas other than the black character edge region with respect to the image data that has undergone the compression processing unit 3 and the expansion processing unit 7 in FIG. In order to improve the color reproduction of the image data, UCR (additional color removal) processing is performed on the common part of the CMY data to generate Bk data, and the CMYBk data is output to the γ correction processing unit 82 and the edge amount detection unit 83.

Here, since the black character edge area is not legible if the black character of the original is colored due to the RGB data reading position shift in the scanner 1 or the overlay position shift when printing the YMCBk data in the plotter 9 is not good. It outputs as Bk single color data (image data not including C, M, Y) which is a signal corresponding to luminance.
The γ correction processing unit 82 performs γ correction (input signal and actual value) on the image data that has passed through the compression processing unit 3 and the expansion processing unit 7 in FIG. 1 according to the γ frequency characteristics (γ characteristics) of the plotter 9. (Adjustment of relative relationship with the output) and output to the halftone processing unit 83.

The halftone processing unit 83 performs quantum processing such as dither processing and error diffusion processing on the image data received from the γ correction processing unit 82 according to the gradation characteristics of the plotter 9 and the edge amount input from the edge amount detection unit 84. The gradation correction is performed to output the result to the plotter 9. When the quantization process is performed, it is possible to enhance the contrast of the black character with respect to the black character signal (a signal obtained by a black character extraction process described later). Thus, by enhancing the contrast of black characters, the legibility of the characters is improved.
The edge amount detection unit 84 detects a steep density change in the image data as an edge amount and outputs it to the halftone processing unit.

Next, details of the controller will be described with reference to FIG.
FIG. 4 is a block diagram showing an internal configuration of the controller 5 of the color image processing apparatus shown in FIG.
The controller 5 shown in FIG. 4 includes a page memory 51, a compression / decompression processing unit 52, an output format conversion unit 53, an input format conversion unit 54, and a data I / F unit 55.
Although not shown, the controller 5 includes a microcomputer including a CPU, a ROM, a RAM, and the like, and the compression / decompression processing unit 52, the output format conversion unit 53, and the input format conversion unit. Many of the functions such as 54 are executed by the microcomputer.

First, a process when the controller 5 outputs image data stored in the page memory 51 to an external device will be described.
The compressed RGB image data stored in the page memory 51 is decompressed to the original 8-bit data for each color by the compression / decompression processing unit 52 and output to the output format conversion unit 53.
The output fort conversion unit 53 performs color conversion of image data of RGB data to sRGB data that is a standard color space, and at the same time, performs general-purpose image format conversion processing to JPEG or TIFF format, and outputs it to the data I / F unit 55. .
The data I / F unit 55 outputs the image data output from the output format conversion unit 53 to the NIC 10 shown in FIG.

Next, processing for outputting image data input from an external device to a plotter will be described.
First, a CPU of a microcomputer (not shown) analyzes a command instructed from an external device and writes it in a page memory.
The data I / F unit 55 outputs image data input from an external device to the input format conversion unit 54, and the input format conversion unit 54 expands the bitmap data to output to the compression / decompression processing unit 52 for compression / decompression. The processing unit 52 compresses it and writes it in the page memory 51.
The image developed by the input format conversion unit 54 is a natural image such as JPEG (abbreviated as “JPG”) or TIFF (abbreviated as “TIFF”).

Next, details of the output format conversion unit will be described with reference to FIG.
FIG. 5 is a block diagram showing an internal configuration of the output format conversion unit 53 in the controller 5 shown in FIG.
The output format conversion unit 53 shown in FIG. 5 includes a color conversion unit 101, a resolution conversion unit 102, a TIF format generation unit 103, a JPEG format generation unit 104, a compression format generation unit 105, and an output selection unit 106.
The color conversion unit 101 converts the image data from RGB data to sRGB data and outputs the data to the resolution conversion unit 102.

The resolution conversion unit 102 performs pixel density conversion such as 300 dpi and 200 dpi on the image data converted into the sRGB data, and each format generation unit of the TIF format generation unit 103, the JPEG format generation unit 104, and the compression format generation unit 105. Output to. In this example, the pixel density degree when converted at 300 dpi will be described.
The format generation units of the TIF format generation unit 103, the JPEG format generation unit 104, and the compression format generation unit 105 convert the resolution-converted image data format into a TIF format format, a JPEG format, and a compression format, respectively. Then, the output selection unit 106 selects image data of a format to be output to the NIC 10 via the data I / F unit 55 shown in FIG. 1 from the image data of each format generated by each of the format generation units. Output.

Next, processing in the compression format generation unit, which is a main part of the present invention, will be described in detail.
The compression format generation unit 105 of the output format conversion unit 53 illustrated in FIG. 5 includes a binarization unit 111, a binary image generation unit 112, a black image generation unit 113, a first resolution conversion unit 114, and a second resolution conversion. Section 115, background image generation section 116, character image generation section 117, and image file composition section 118.
A binarization unit 111 that is a binarization unit outputs binary data and black character data of a character region and a non-character region from the image data subjected to resolution conversion by the resolution conversion unit 102 based on lightness and darkness of the image density.

  The binary image generation unit 112 that is a binary image generation unit performs lossless compression on the binary data output from the binarization unit 111 to generate a binary image file, and outputs the binary image file to the image file synthesis unit 118. The black image generation unit 113 serving as a black image generation unit performs lossless compression on the black character data output from the binarization unit 111 to generate a black image file, and outputs the black image file to the image file composition unit 118. These lossless compressions may be performed by MMMR compression. This MMMR compression is compression by an MMR encoding method (modified MR encoding method) which is a standard encoding method of G4 facsimile, and is a known technique for compressing two-dimensional binary image data. In addition, compression using an MH encoding method or an MR encoding method may be performed.

The first resolution conversion unit 114 and the second resolution conversion unit 115 further perform resolution conversion on the image data subjected to resolution conversion by the resolution conversion unit 102 to lower the resolution (150 dpi).
Then, the background image generation unit 116 which is a background image generation unit converts the image data of the background area of the image data input from the first resolution conversion unit 114 based on the binary data output from the binarization unit 111 unit. Then, the image data is irreversibly compressed using a luminance and color difference quantization table to generate a character image file, which is output to the image file composition unit 118. At that time, of the image data input from the first resolution conversion unit 114, only the region where the binary data indicating the character region is input from the binarization unit 111 is rewritten to image data of a constant value corresponding to white. Good. As described above, the reason why the character area should be set to a constant value for the background image is that the compression ratio is improved by setting the character area to a constant value.

  A character image generation unit 117 serving as a character image generation unit determines image data of a character area of image data input from the first resolution conversion unit 114 based on binary data output from the binarization unit 111, and The image data is irreversibly compressed using a luminance and color difference quantization table to generate a character image file, which is output to the image file composition unit 118. At that time, among the image data input from the first resolution conversion unit 114, only the region where the binary data indicating the background region is input from the binarization unit 111 may be rewritten to image data having a constant value. The reason why the background area of the character image is preferably set to image data having a constant value is also for improving the compression rate.

The irreversible compression of the image data in the background image generation unit 116 and the character image generation unit 117 may be performed by JPEG compression.
The quantization table used by the character image generation unit 117 and the quantization table used by the background image generation unit 116 are preferably different quantization tables, which will be described in detail later.
The resolution conversion unit 102 may be about 75 dpi because the character image and the background image do not require the same resolution as the background image.
If it can be determined from the bibliographic information that the document is “character only document”, both the background image and the character image are created with a resolution of 75 dpi.

Decreasing the resolution of the character is guaranteed by the MMR resolution as the character resolution, so even if the resolution of the JPEG image is decreased, the gradation is deteriorated, but there is no problem. The file size can be reduced by reducing the resolution.
In this embodiment, the file size is reduced by reducing the image resolution. However, the file size may be reduced by reducing the image quality such as the number of gradations other than the image resolution.
Since the stored data includes “character only original / not present” as the bibliographic information, it is possible to increase the compression ratio of “character only original” with respect to the stored image data.

The JPEG compression in the background image generation unit 116 and the character image generation unit 117 is performed using a quantization table having different characteristics.
For the JPEG compression, a known technique as disclosed in paragraphs 0069 to 0072 (see FIGS. 3 to 5) of Japanese Patent Laid-Open No. 2005-303979 is used.

FIG. 7 is a functional block diagram of the JPEG compression unit. FIG. 8 is a diagram for explaining processing for cutting out image data in units of blocks. The JPEG method is an international standard method, and an image is cut out in units of 8 × 8 pixels as shown in FIG. The JPEG compression unit 300 includes a blocking unit 301 that cuts out image data in units of 8 × 8 pixels.
Further, a DCT unit 302, a quantizing unit 303, and a Huffman coding unit 304 are provided that perform discrete cosine transform on the extracted DCT block data and convert the data into a frequency space.
For the quantization unit 303 that performs quantization processing that affects the compression level, as the reference quantization table 306, as shown in FIG. 9, for the luminance component (Y component) and for the color difference component (CbCr component), different values are used. Has a reference quantization table.

FIG. 9 is a diagram illustrating an example of the reference quantization table Qij. The 8 × 8 upper left corner Q00 is a parameter used for quantization of a DC component (DC component), and the rest are parameters used for quantization of an AC component (AC component). The upper left is a low frequency component, and the lower right is a high frequency component.
The quantization is performed by dividing the 8 × 8 DCT coefficient, which is the output value from the DCT unit 202, by the quantization table value Q′ij. The larger the quantization table value Q′ij is, the larger the loss due to quantization (= large quantization), and the most is to increase the compression rate by increasing the quantization of high frequency components as shown in FIG. This is a conventional means with little influence on image quality.

With respect to the reference quantization table value Qij, a quantization table value Q′ij that is actually used for quantization is obtained by the following equation by the calculator 305.
For qf <50:
Q′ij = Qij × 50 ÷ qf
When qf ≧ 50:
Q′ij = Qij × (100−qf) ÷ 50
Here, qf is a fixed parameter set in the quality factor setting unit 307 and is set in the range of 0 to 100. The smaller this qf, the lower the compression rate and the higher the image quality.

The general quantization table shown in FIG. 9 is based on the fact that human vision is sensitive to low frequencies and insensitive to high frequencies. Furthermore, since color information has a lower discrimination ability than luminance information, more high frequency components are cut than for luminance information.
Since the JPEG compression unit of the background image generation unit 116 in FIG. 5 compresses the image with qf = 20 to about 30, block noise is likely to occur when a general reference quantization table is used. In particular, the block noise of the basic unit (8 × 8) of JPEG is greatly affected by the value of the DC component representing the average density in the block.

In order to reduce the block noise of the basic unit of JPEG, it is preferable to save the DC component by using the reference quantization table shown in FIG.
FIG. 11 shows an example of a quantization table with qf = 25, that is, an example of a quantization table that stores DC components that are actually used. The character image generation unit 117 in FIG. 5 performs JPEG compression on an image in which a plurality of colors (red, blue) exist in one 8 × 8 block shown in FIG. 14 using a general reference quantization table. Then, since the high frequency component is cut, the influence of aliasing distortion comes out and red and blue colors are likely to be mixed.

In order to avoid this problem, the character image generation unit 117 may use a reference quantization table having characteristics for storing the color difference components as shown in FIG. 12 so as to store the entire frequency components of the color difference components.
FIG. 13 shows an example of a quantization table with qf = 20, that is, an example of a quantization table that stores color difference components that are actually used.

That is, the background image generation unit 116 reduces block noise in a smooth portion of an image by using a quantization table having a characteristic of storing more DC components than the quantization table of a character image.
The character image color difference table used by the character image generation unit 117 uses a quantization table having characteristics that store more high-frequency components than the color difference table of the background image and the luminance table of the background image. It is better to prevent color mixing. Since the resolution of characters is determined by the resolution of MMR, the luminance component is not so important. Therefore, a standard quantization table may be used as the luminance table.

Here, the relationship of the quantization table is summarized as shown in Table 2.

  Returning to FIG. 5, the image file composition unit 118, which is an image file composition unit in the compression format generation unit 105, generates a binary image file (MMR) generated by the binary image generation unit 112 and a black image generation unit 113. Four image files of the black image file (MMR), the background image file (JPEG) generated by the background image generation unit 116, and the character image file (JPEG) generated by the character image generation unit 117 are combined into one image. Composite to file. As the file format at this time, a general-purpose format (such as a PDF file) may be used.

The binarization unit 111 in FIG. 5 performs binarization by extracting a plurality of feature amounts.
In the following processing, when one processing is completed in the sequential processing, the processing of the adjacent pixel is performed, and when the processing of one line is completed, the processing is performed from the beginning of the next line, and the processing is continued until the end of the image. It is assumed that the RGB image data becomes black when the number increases and becomes white when the number decreases.

FIG. 15 is a flowchart showing the flow of processing in the binarization unit 111. Each processing shown in FIG. 15 will be described in detail below.
1. Character Resolution Conversion, Color Conversion, and Resolution Conversion Resolution conversion (300 dpi) at the same resolution as the character edge region and color region signal (RGB) that are data expanded by the compression / decompression processing unit 52 shown in FIG. To make the image data and the pixel density the same. The output here is “character edge” and “color”.
Then, the binarization unit and the mask unit are processed.

2. Binarization unit The binarization unit processes adaptive binarization by changing the binarization threshold in the character edge area and other areas, and removes isolated points from the output of the adaptive binarization. The isolated point removal 1 is performed.
2.1. Adaptive Binarization As shown in FIG. 16, adaptive binarization processing includes threshold selection for detecting the color of an edge and changing the threshold for each line in order to extract color ground characters and white ground characters. , And a binarization process for binarizing the image data using the threshold value.

For threshold selection, two threshold values are prepared for the character edge region and other than the character edge region, and the threshold values are switched based on the character edge region. Parameters other than the character edge region and the character edge region are set so that the character edge region is more easily determined as a character.
In binarization, if at least one RGB value of the input image data exceeds the threshold value (if it is larger) than the threshold value selected in the threshold value selection process, it is determined as black (ON) and black (ON) Otherwise, it is output as white (OFF).
For example, if the threshold is R = 128, G = 120, B = 118, and the input image data is R = 121, G = 121, B = 121, “black” is set, and R = 110, G = 110, B If “= 110”, “white” is set.

The threshold value is switched in areas other than the character edge area and the character edge area. Characters described on the color ground (other than the white background) are darker (characters on the ground color), and characters written on the white ground (white ground characters) are lighter. Because there are letters. Therefore, if binarization is performed with a fixed threshold value, the color ground of the character on the color ground becomes a character when attempting to extract the light character on the white background as black, and conversely, if the character on the color ground is extracted as black, the character on the white ground is light. Characters can be extracted as black.
Therefore, by switching the parameters other than the character edge region and the character edge region, the white ground character is binarized with a threshold value for the character edge region, and the colored ground character is binarized with a threshold value for other than the character edge region.

That is, since a character with a white background is a white ground character edge, it is binarized with a threshold value for the character edge region, and since a color ground character is not a white ground character edge, it is binarized with a threshold value for other than the character edge region. Thus, by binarizing white ground (light) characters and colored ground characters with different threshold values, it is possible to binarize both colored ground characters and white ground characters well.
Since the image data to be binarized is subjected to sharpening processing in the character edge region, processing other than the character edge region is performed according to the edge amount, and smoothing processing is applied to the picture portion. Since the background of the color ground character is smoothed, good binarization is possible.

2.2 Isolated point removal 1
In the isolated point removal 1 process, since there are many isolated points in the adaptive binarization processing result, the isolated points are removed. When the patterns shown in FIG. 17 coincide with each other, the isolated point is removed by inverting the target pixel.

3. Mask part 3.1 N-value conversion This part is used in common for the features of halftone detection and gray detection described later and the value of N value.
When the data (bk) having a small RGB difference has a value larger than thabk, the color is divided into six hues of the data YMCBGR having a small RGB difference as black characters, and threshold values (thay, tham, thac, thab, thag, tha), and binarization processing is performed using dark pixels as active pixels (characters). The method of dividing the hue may be simply based on the magnitude relationship of RGB or may be determined by the ratio of RGB colors. This is determined by the characteristics of the input image.
However, the output result is held for each hue.

Here, it is defined as follows. The parentheses are bit display.
Dtah = 0 (000): Not applicable Dtah = 1 (001): Yellow Dtah = 2 (010): Magenta Dtah = 3 (011): Red Dtah = 4 (100): Cyan Dtah = 5 (101): Green Dtah = 6 (110): Blue Dtah = 7 (111): Black

Further, the white level is also binarized in the same manner.
When the data with a small RGB difference (bk) has a value smaller than thbbk, the color is divided into six hues of the data YMCBGR with a small RGB difference as a white pixel, and threshold values (thby, thbm, thbc, thbb, thbg) for each hue. , Thbr), and binarization processing is performed with light-colored pixels as active pixels (white pixels).
Moreover, what is contained in black and further extracted from a black (dark) color is made dark black and used when extracting black.

Further, extraction of the background for background detection is performed. The background is the same as the white level, but the threshold value is an intermediate value between the white level and the character threshold value.
The relationship between the ground area, gray area, and character area is as shown in FIG.
There is a density range that spans two areas, the darker of the background area and the thinner gray area. The reason is that both the background area and the gray area are determined by pattern matching unless there is an area of a certain size, so if there is no overlap, the background area and the density area of the uniform image near the gray determination This is also to prevent a situation that does not correspond to either the background area or the gray area when it extends over the background area and the gray area.

3.2 Halftone detection 1
Although the halftone dot shape of fine halftone dots has been eliminated by the smoothing process in the scanner correction unit 2 (FIG. 2), rough halftone dots such as newspaper photographs cannot be sufficiently smoothed. A halftone dot shape remains. The halftone dot detection 1 is intended to detect this rough halftone dot.
Here, halftone dot pattern matching is performed. For example, when Dtah ≠ 0, a black pixel is used, and an N-valued white pixel is a white pixel.

This halftone dot detection 1 process will be described with reference to the flowchart of FIG.
When this process is started, it is first determined in step S1 whether or not the pattern is a white pattern. The white pattern is determined by matching with the pattern of the white pixel shown in FIG. 20, and if matching is determined as “white pattern”, the process proceeds to step S2, and if not matched, it is determined that the pattern is not “white pattern”. Then, the process proceeds to step S3.
In step S2, the following variables are initialized by halftone dot count initialization, and the process proceeds to step S8.
count_bk = tha_count
state = 0
SS [I] = 0
tha_count: distance between white areas (threshold)
SS [I]: Information before one line

In step S3, a halftone dot determination process is performed.
Set sat to 1 under the following conditions.
・ SS [I] = 1
-The result one pixel before is a halftone dot.
-Count_bk is 0.
Furthermore, SS [I] is set to 1 under the following conditions.
Count_bk is 0.

Next, halftone dot interval determination is performed in step S4.
1) If state = 0 and count> tha_count_s, the following variables are initialized.
count_bk = tha_count
SS [I] = 0
tha_count_s: distance between halftone dot patterns on a non-halftone dot area (threshold value)

2) If any of count_bk, count_c, count_m, count_y is 0 and count> tha_count_e, the following variables are initialized.
count_bk = tha_count
state = 0
SS [I] = 0
tha_count_e: distance (threshold value) between halftone dot patterns on a non-halftone dot area
3) After the above determination, count = count + 1 is performed.

Next, halftone dot count is performed in step S5. The halftone dot pattern is the pattern shown in FIG. 21, where ● is a black pixel and ◯ is a non-black pixel. If it matches any one of these patterns, count is set to 0 and count_bk is set to -1.
Then, the process proceeds to step S6 to determine the state. In this state determination, if state = 1, it is determined as a halftone dot, and the process proceeds to step S7. Otherwise, the process proceeds to step S8.

In step S7, fine line determination is performed. If “white pattern” exists on both sides of the pixel of interest (12 pixels) on both sides and count_x is an initial value, it is determined as a thin line and SS [I] = 0 is set. . Then, the process proceeds to step S9.
In step S8, non-halftone pixel processing is performed, state = 0 is set, and the result is output as a non-halftone dot. In step S9, halftone pixel processing is performed, state = 1 is set, and the result is output as a halftone dot.

3.3 Halftone detection 2
Halftone detection 2 is the same processing as halftone detection 1 described above, but halftone detection 1 processed image data in the forward direction, while halftone detection 2 processed image data in the reverse direction. Thus, in halftone dot detection 1, the halftone dot is converted to a halftone dot by reverse-reading the portion that does not become a halftone dot.

By performing halftone dot detection 1 and 2, if there is a certain halftone dot pattern between the white pattern and the white pattern, a halftone dot is detected.
Further, by setting the non-halftone dot when the interval between the halftone dot pattern is wide, it is possible to make a non-halftone dot a white character in a dark background with a black gradation as shown in FIG. A dark background has almost no halftone dot shape and is a non-halftone dot.
Furthermore, as shown in FIG. 23, dark characters in a light gradation background are almost non-halftone dots with almost no halftone dot shape.

Or, since the characters are generally white ground characters, the white ground characters are rarely erroneously detected because the halftone dot pattern is only near the edges of the characters.
The pattern matching in this embodiment is performed for white and other than white, but the Y component, M component, and C component may be performed independently.
If the Y, M, and C components are expanded, the ink ink component for printing is YMC, so that the ink dot reproduction can be accurately extracted.

3.4 Gray detection 1
In the gray detection 1, using the fact that the character region is dark and the region around the character is thin, gray determination is performed by setting the portion darker than the character region and darker than the region around the character as medium density. The white background used in the gray determination is an N-valued white pixel.
The gray detection 1 process will be described with reference to the flowchart of FIG.
When this process is started, pre-processing is first performed in step S11. In this preprocessing, by comparing the MS that is the processing result of the previous pixel and the SS [i] that is the processing result of the previous line, the processing result of the previous line and the processing result of the previous pixel are many. Get the threshold value. MS is the number of white pixels after gray detection described later.

Next, in step S12, it is determined whether or not the pattern is a gray pattern. This determination is performed by matching with the gray pattern shown in FIG. 25, and if the medium density matches (matches) the gray pattern, MS = 5 and S [i] is set as a gray pixel (gray pattern). The medium density is when N-value conversion and Dtah = 0 and a non-white pixel.
If the pattern matches the gray pattern, the process proceeds to step 17; otherwise, the process proceeds to step S13.

In step S13, it is determined whether or not the pixel is a white pixel. The white pixel is an N-valued white pixel. If the target pixel is a white pixel, the process proceeds to step S15. If not, the process proceeds to step S14.
In step S14, it is determined whether MS> 0. If MS> 0, the process proceeds to step S17, and if MS> 0, the process proceeds to step S18.
In step S15, it is determined whether MS> 0. If MS> 0, the process proceeds to step S16, and if MS> 0, the process proceeds to step S18.
In step S16, MS = MS-1 is set, and the process proceeds to step S14.
In S17, the result is output as a gray pixel. In step S18, the result is output as a non-gray pixel.

Thereafter, the next post-processing is performed in step S10.
Update SS [I] If the color is white or MS> 1, then SS [I] = MS-1 is performed.
If it is a Bk pixel and MS> 0, SS [I] = MS−1 is performed.
Bk count When the number of consecutive bk pixels is counted and the number of consecutive is greater than or equal to thg_count1, it is determined that there is bk continuous.
For example, when thg_count is 12 and the number of continuations is N, bk continuation is N-thg_count + 1 times.
When there is a continuous number, MS = MS-1 is performed.

3.5 Gray detection 2
The gray detection 2 process is the same as the gray detection 1 process described above, but the gray detection 1 process processed image data in the forward direction, whereas the gray detection 2 process processed image data in the reverse direction. Process. By processing in the reverse direction _, the portion where the tip of the gray area did not turn gray is made gray.
The character portion is generally composed of dark data and light (white) data, and a medium-density block, which is a feature of a photograph that does not exist in the character portion, is detected as gray. Since this peripheral pixel is a gray pixel until the number of white pixels exceeds a certain value, even a dark color is a gray pixel.
However, if more than a predetermined number of blacks are continuous in the gray area, non-gray pixels are set. Here, the result of gray pixels is defined as a gray region.
As a result, it is possible to make white characters with a black background a non-gray area.

3.6 Background detection When the threshold value is switched for each line in the adaptive binarization process, there is no problem if character determination is performed even in a low-density area if all low-density areas are determined as characters. If the threshold value extends over an image of a region with a low density, the number of black and white change points increases in the result of the binarization unit, and the file size increases in the final image.
If the lower limit (white side) of the simple gray determination area is set closer to white, characters on the color ground cannot be determined, so the background is detected here.
Add 4x5 AND to the ground, then 2x3 OR. This reduces the background for the input data.
Thus, the background around the character is also the boundary between the character and the background, and the background area is reduced, so that it is difficult to include the character in the background, so that the background can be detected satisfactorily. FIG. 18 shows a conceptual diagram thereof.

5). Character determination + character determination 5.1 Character determination This is a character area (output of isolated point removal 1), not a halftone area (output of halftone detection 1, 2), but a gray area (output of gray detection 1, 2) ), Not a background area (background detection output), and if it is not a character (a character determination result to be described later), it is determined as a character.
The reason why the character area is logically calculated with the background area, the halftone dot area, and the gray area is that the character determination result is a photographic area that originally does not require resolution, and is corrected as a photographic area. .
In particular, the photographic region contains a large number of isolated points, and by performing this correction, the compression rate of the character image is improved, and the image and the character non-character mixture are reduced, so that the image quality can be improved.
The reason why the character determination result is excluded from the character determination result is that the black character is output in a single black color, so that the character image is fixed to improve the compression rate.

5.2 Determining whether Character Is Character The character determination process is performed by two mirrors: character determination, three line OR, picture determination, black determination, extraction 1, extraction 2, and determination, as shown in FIG.
(1.1) Character Determination A logical operation is performed on the character determination result and the color determination result, which are the output results of the image area separation in the previous stage, and if it is a character and is not a color, a black character edge is determined.

(1.2) 3-line OR
OR the 3 lines × 1 pixel of the black character edge which is the result of character determination. Originally, instead of this three-line OR, it is necessary to perform a two-line delay after the extraction 1 to match the line delay with the extraction 2, but if a line delay is performed after the extraction unit 1, it is necessary to delay the image data. Therefore, the line delay is absorbed by ORing three lines here.

(1.3) Picture Determination One of the color determination result, the gray detection 1 result, the gray detection 2 result, the halftone detection 1 result, and the halftone detection 2 result, which is the output result of the image area separation in the previous stage. If it is ON at any time, it is determined as a picture.
(1.4) Black determination If the color determination result, which is the output result of the image area separation in the previous stage, is non-colored and black (Bk or dark black) in N-value, it is determined as black.

(1.5). Extraction 1
The process of extracting characters will be described with reference to the flowchart of FIG.
In the following description, the output of the 3-line OR is described as a black character edge, the pattern determination result is a pattern, and the black determination result is black.
When the processing of FIG. 27 is started, the target pixel determination is first performed in step S21. If it is a black character edge, it is set as an area within a character. If the result of the pattern determination is a pattern, the pattern area is set. In the image area separation algorithm, neither the character nor the picture is turned on. If it is neither a character nor a picture (others), the process proceeds to step S22 and black pixel determination is performed.

If the target pixel is not a black pixel, the intermediate region is set. If it is a black pixel, the pattern determination of the previous line is performed in step S23. If the pattern determination result one line before the line segment processing is a pattern, the pattern area is set. If it is not a pattern, the pattern determination for the previous pixel is performed in step S24.
As a result, if it is a picture, it is set as a picture area. If it is not a picture, it is determined whether the character is one line before in step S25. If the determination result is not a character one line before the line segment processing, the character is a region. If it is not a character, it is determined whether the character is one pixel before.
If the determination result is a character one pixel before, the character is a region. If it is not a character, it is set as an intermediate area.
Finally, the line segment processing in step S27 is performed. Here, if the color determination result is not a color and if high-density pixels are 128 pixels or more, the pattern area is corrected.

(1.6) Extraction 2 and mirror In extraction 2, it is determined whether the character is a reverse image (mirror) image. In order to realize the reverse image processing by pipeline processing, mirroring is performed before and after the extraction 2. Since the processing content of extraction 2 is the same as that of extraction 1, description thereof is omitted.
(1.7) Judgment If both the extraction 1 and the output of the extraction 2 mirror are in a character region, and the output of the binarization unit is a character, the character is a black character.

In this way, it is possible to determine whether a character is in a large image area by referring to the state of the image of the surrounding pixels to determine whether the character is a character.
The character edge area and the color area can accurately detect colors and halftone dots in the read image. When smoothed data is used for density information for detection of characters, a halftone dot area is small for a halftone dot character and may not become a character edge area. For this reason, not the halftone dot information but the data after filtering (smoothed because it is not a character edge region), only the dark portion is made into a character.

A halftone dot with a low screen line number, such as a newspaper photograph, is detected by halftone dot detection because a halftone dot shape remains even if it is smoothed. A halftone dot having a high screen line number in a general document such as a catalog is smoothed and the halftone dot shape is lost, and a halftone dot area ratio of about 30% to 60% is detected as gray by gray determination.
In particular, since the leading edge of a character is narrow, it is difficult to form a halftone dot (non-character area). This is to avoid becoming unsightly and unsightly.

The above-described compression format generation unit of the output format conversion unit 53 shown in FIG. 5 is an embodiment constituting the characteristic part of the image processing apparatus according to the present invention. However, all the parts constituting this embodiment are not essential, and for example, the first resolution converter 114 and the second resolution converter 115 may be omitted.
The binarization unit 11 is a means for only outputting binary data for identifying a character area and a non-character area based on the lightness and darkness of the image density of the input image data, and omits the black image generation unit 113. Alternatively, the binary image generation unit may be omitted.

In that case, the character image generation unit 117 determines the image data of the character area of the input image data based on the binary data output from the binarization unit 111, and uses the image data to quantize the luminance and color difference. Is used to generate a character image file by irreversible compression such as JPEG compression. Further, the background image generation unit 116 determines the image data of the background area of the input image data based on the binary data output from the binarization unit 111, and uses the brightness and color difference quantization table for the image data. To generate a background image file by irreversible compression such as JPEG compression.
Then, the image file composition unit 118 combines the character image file and the background image file generated by the character image generation unit 117 and the background image generation unit 116, respectively, into one image file.
Also in this case, the quantization table used by the character image generation unit 117 and the quantization table used by the background image generation unit 116 are quantization tables having different characteristics as described above.

Next, details of the input format conversion unit 54 in FIG. 4 will be described with reference to FIG.
The input format conversion unit 54 shown in FIG. 6 includes a TIF format development unit 201, a JPEG format development unit 202, a compression format development unit 203, and an output selection unit 204. The TIF format developing unit 201, the JPEG format developing unit 202, and the compressed format developing unit 203 each have a function of developing image data into a bitmap of each format, and the output selecting unit 204 selects one of the three formats. At the same time, the RGB data is converted into YMCBk.
If the input image data is in the TIFF format, the TIFF format developing unit 201 develops it with bitmap data. If the input image data is in the JPEG format, the JPEG format developing unit 202 develops it into bitmap data.
Further, if the input image data is a compression format, the compression format expansion unit 203 expands it into bitmap data.

Here, the compression format expansion unit 203 which is a main part of the present invention will be described.
The compression format expansion unit 203 includes an image file expansion unit 211, a black image expansion unit 212, a binary image expansion unit 213, a background image expansion unit 214, a character image expansion unit 215, and an image file composition unit 216.
The image file development unit 211 converts the image data of four files in the file generated by the compression format generation unit 105 shown in FIG. 5 into a black image development unit 212, a binary image development unit 213, and a background image development unit in the subsequent stage. 214 and the character image development unit 215 output the corresponding images.

The binary image development unit 213 decompresses the MMR of the binary image into a bitmap, the black image development unit 212 decompresses the MMR of the black image into a bitmap, and the background image development unit 214 performs JPEG of the background image. Is expanded into a bitmap, and the character image expansion unit 215 expands the JPEG of the character image into a bitmap.
The developed four bitmap data are synthesized into one piece of bitmap data by the image file synthesis unit 216. In this image file composition unit 216, if the output of the binary image development unit 213 is a character region, the image data that is the output of the character image development unit 215 is output, and the output of the binary image development unit 213 is a non-character region. If so, the image data output from the background image development unit 214 is output.

Further, if the output of the black image development unit 212 is a black character, it is output in black. Thus, one image is generated. The resolution of characters and non-characters is the resolution of the binary image.
The output selection unit 204 selects the output image data from the compression format development unit 203 and one of the image data developed by the TIF format development unit 201 and the JPEG format development unit 202, as shown in FIG. The data is output to the compression / decompression processing unit 52.
FIG. 28 shows an image diagram of an input image, a file image, and an output image.

  According to the present invention, a storage medium storing software (program) for realizing the above-described embodiment is installed in a storage device of a computer constituting the image processing system, and the computer is caused to execute each function by software. Also good.

According to the above-described embodiment of the present invention, since the image characteristics of the character image and the background image are different from each other, the character image and the background image are quantized using different quantization tables. Image characteristics suitable for the image and the background image can be obtained, and efficient compression becomes possible. The black character is compressed as binary data, and the compression rate is increased by making the black character 1 bit.
The background image is a block of a smooth portion of the image by using a quantization table that stores the DC component (average density) of the background image in accordance with the characteristics of the background image to reduce block noise at the flat end. Noise can be reduced.
The character image is matched to the characteristics of the color difference component of the character image to prevent color mixing, and the quantization table that stores the high frequency part of the color difference component of the character image from the luminance component of the character image is used. It becomes possible to reduce color mixing.

In order to prevent color mixing, the character image uses a quantization table that stores the high-frequency part of the color difference component of the character image from the color difference component of the background image in accordance with the characteristics of the color difference component of the character image. It is possible to reduce the color mixture.
By using different JPEG compression quantization tables for character images and background images, compression can be performed efficiently.
The present invention can be applied to scanner distribution apparatuses and image forming (copying) apparatuses.
A program for realizing the image processing function according to the present invention by software using a personal computer and a computer-readable storage medium storing the program can be provided.

The image processing apparatus according to the present invention can compress image data including characters and pictures at a high compression rate without reducing the resolution of the characters. Therefore, the image processing apparatus can be used for various types of digital multifunction peripherals, color digital copying machines, scanner distribution apparatuses, and the like. It can be used for an image forming apparatus or an image reading apparatus.
The present invention can also be applied to the case where image processing is performed by software using a personal computer such as a desktop personal computer or a notebook personal computer.

1 is a block diagram illustrating a schematic configuration of a digital color image processing apparatus according to an embodiment of the present invention. FIG. 2 is a block diagram illustrating an internal configuration of a scanner correction unit 2 of the color image processing apparatus illustrated in FIG. 1. FIG. 2 is a block diagram illustrating an internal configuration of a printer correction unit 8 of the color image processing apparatus illustrated in FIG. 1. FIG. 2 is a block diagram illustrating an internal configuration of a controller 5 of the color image processing apparatus illustrated in FIG. 1. FIG. 5 is a block diagram showing an internal configuration of an output format conversion unit 53 in the controller 5 shown in FIG. 4. FIG. 5 is a block diagram showing an internal configuration of an input format conversion unit 54 in FIG. 4.

It is a functional block diagram of a JPEG compression part. It is a figure explaining the process which cuts out image data per block by the block forming part 301 in FIG. It is a figure which shows an example of Qij of the reference | standard quantization table 306 in FIG. It is a figure which shows an example of the reference | standard quantization table which preserve | saved DC component. It is a figure of the quantization table which preserve | saved the DC component actually used of qf = 25. It is a figure which shows an example of the reference | standard quantization table which preserve | saved the color difference component. It is a figure of the quantization table which preserve | saved the color difference component actually used of qf = 20. It is a figure which shows the example of the image in which several color (red, blue) exists in one block of 8x8.

It is a flowchart which shows the flow of a process by the binarization part 111 in FIG. It is explanatory drawing of the process of adaptive binarization in FIG. It is a figure which shows each pattern used for the process of the isolated point removal 1 in FIG. FIG. 16 is a diagram illustrating a relationship among a background area, a gray area, and a character area for background detection in FIG. 15. It is a flowchart which shows the flow of a process of the halftone detection 1 in FIG. It is a figure which shows the matching pattern of the white pixel used for determination of a white pattern. It is a figure which shows the matching pattern of the halftone dot used for a halftone dot determination.

It is a figure which shows the example of the outline character in a dark background of black gradation. It is a figure which shows the example of the dark character in the background with a light gradation. It is a flowchart which shows the flow of a process of the gray detection 1 in FIG. It is a figure which shows the gray pattern for a matching for determining whether it is a gray pattern by step S12 of FIG. It is a figure which shows the detail of the character determination processing which performs character determination in FIG. It is a flowchart which shows the flow of the process which extracts a character. It is an image figure of the input image by the image processing apparatus of this invention, a file image, and an output image.

FIG. 3 is a block configuration diagram of a document type recognition device for explaining an example of a document type recognition technique used in a document type determination unit 24 in FIG. 2. FIG. 10 is an explanatory diagram when a character pixel is counted by dividing an A3 document into first and second half regions. It is a block diagram which shows the structure of the character pixel detection circuit 401 in FIG. It is a figure which shows the connection pattern used with the black pixel pattern matching circuit 412 in FIG. It is a figure which shows the connection pattern used with the white pixel pattern matching circuit 413 in FIG. It is a block diagram showing a configuration of a halftone detection circuit for detecting a photographic paper photograph. FIG. 35 is a diagram showing a 7 × 3 pixel pattern used in the pattern matching circuit 422 in FIG. 34. It is a figure which shows the direction of each diagonal in a 3x3 block for detecting a peak pixel.

Explanation of symbols

1: Scanner 2: Scanner correction unit 2 3: Compression processing unit 4: General-purpose bus
5: Controller 6: Hard disk drive (HDD) 7: Decompression processing unit 8: Printer correction unit 9: Printer
10: Network interface controller (NIC)
11: Personal computer (PC)
21: Image area separation unit 22: Scanner γ unit 23: Filter processing unit
24: Document type determination unit 51: Page memory 52: Compression / decompression processing unit 53: Output format conversion unit 54: Input format conversion unit 55: Data I / F unit 55
81: Color correction processing unit 82: γ correction processing unit 83: Halftone processing unit 84: Edge amount detection unit

101: Color conversion unit 102: Resolution conversion unit 103: TIF format generation unit 104: JPEG format generation unit 105 Compression format generation unit 106: Output selection unit 111: Binarization unit 112: Binary image generation unit 113: Black image generation Unit 114: first resolution conversion unit 115: second resolution conversion unit 116: background image generation unit 117: character image generation unit 118: image file composition unit 201: TIF format development unit 202: JPEG format development unit 203: compression Format development unit 204: output selection unit 211: image file development unit 212: black image development unit 213: binary image development unit 214: background image development unit 215: character image development unit 216: image file composition unit 300: JPEG compression unit 301: Blocking unit 302: DCT unit 303: Quantizing unit 04: Huffman encoding section 305: arithmetic unit 306: standard quantization table 307: Quality factor setting unit

Claims (12)

Binarization means for outputting binary data for identifying a character region and a non-character region based on lightness and darkness of image density of input image data;
Based on the binary data output from the binarization means, the image data of the character area of the input image data is discriminated, and the image data is irreversibly compressed using a quantization table of luminance and color difference to obtain a character image. Character image generation means for generating a file;
Based on the binary data output from the binarization means, image data in the background area of the input image data is determined, and the image data is irreversibly compressed using a quantization table of luminance and color difference to obtain a background image Background image generation means for generating a file,
An image processing apparatus, wherein the quantization table used by the character image generation unit and the quantization table used by the background image generation unit are quantization tables having different characteristics. The image processing apparatus according to claim 1.
An image processing apparatus comprising image file composition means for combining a character image file and a background image file generated by the character image generation means and the background image generation means, respectively, into one image file. Binarizing means for outputting binary data and black character data for identifying a character region and a non-character region based on lightness and darkness of image density of input image data;
Binary image generation means for reversibly compressing binary data output from the binarization means to generate a binary image file;
Black image generation means for generating a black image file by reversibly compressing black character data output from the binarization means;
Based on the binary data output from the binarization means, the image data of the character area of the input image data is discriminated, and the image data is irreversibly compressed using a quantization table of luminance and color difference to obtain a character image. Character image generation means for generating a file;
Based on the binary data output from the binarization means, image data in the background area of the input image data is determined, and the image data is irreversibly compressed using a quantization table of luminance and color difference to obtain a background image Background image generation means for generating a file,
An image processing apparatus, wherein the quantization table used by the character image generation unit and the quantization table used by the background image generation unit are quantization tables having different characteristics. The image processing apparatus according to claim 3.
The binary image generating means and the black image generating means are both means for performing MMR compression. The image processing apparatus according to claim 3 or 4,
The binary image file, the black image file, the character image file, and the background image file generated by the black image generation unit, the black image generation unit, the character image generation unit, and the background image generation unit, respectively. An image processing apparatus comprising image file combining means for combining with an image file. In the image processing device according to any one of claims 1 to 4,
The character processing unit and the background image generation unit are both units that perform JPEG compression. The image processing apparatus according to any one of claims 1 to 6,
The character image generation means has means for rewriting image data of a background area of the input image data to image data of a constant value based on binary data output from the binarization means,
The background image generation means has means for rewriting image data of a character area of the input image data to image data of a constant value corresponding to white based on the binary data output from the binarization means. A featured image processing apparatus. In the image processing device according to any one of claims 1 to 7,
The image processing apparatus according to claim 1, wherein the quantization table used by the background image generation unit has a characteristic of storing more DC components than the quantization table used by the character image generation unit. The image processing apparatus according to any one of claims 1 to 8,
The color difference quantization table of the quantization tables used by the character image generation means has a characteristic of storing more high-frequency components than the luminance quantization table. The image processing apparatus according to any one of claims 1 to 9,
Of the quantization tables used by the character image generation means, the color difference quantization table has a characteristic of storing high-frequency components as compared with the color difference quantization table used by the background image generation means. Processing equipment. The image processing apparatus according to any one of claims 1 to 10,
An image processing apparatus comprising means for transmitting the image data of the image processing result to an external device. The image processing apparatus according to any one of claims 1 to 11,
An image processing apparatus comprising: an image output unit configured to form an image based on the image data obtained as a result of the image processing, and to form and output the formed image on a sheet.

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