JP3929030B2 - Image processing device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for performing image processing including compression / decoding processing of image data, and in particular, includes a binary image region composed of characters and lines and a halftone image region such as a photograph or halftone printing. The present invention relates to an image processing apparatus that appropriately reproduces a mixed document image according to an image area.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in an image processing apparatus such as a digital color copying machine, a color image is printed by overlapping and printing four color plates of K (black), C (cyan), M (magenta), and Y (yellow). It has gained.
At that time, in order to control the timing of printing each color plate, it is necessary to temporarily store the image in the memory. However, in order to prevent the necessary memory capacity from becoming enormous, the image data is compressed to the memory. The method of storing in is taken.
In recent years, there has been a demand for not only outputting captured images but also storing them as digital data and using them on a PC or the like. In this case as well, it is necessary to compress the image data to obtain an appropriate amount of data.
In any case, in a digital color copying machine, compression processing is generally performed at a certain stage of image processing.
[0003]
On the other hand, when performing image processing, by using a technique called image area separation for determining whether each pixel area of the captured image is a character area or a halftone area such as a photographic image, In order to improve image quality, area data indicating whether the area is a character area or a halftone area is generated for each pixel, and image processing is performed in accordance with the area data.
For example, the spatial filter process, the halftone process, and the like are switched according to the area.
[0004]
FIG. 9 shows an example of the flow of image processing.
In this case, there is a compression and decoding process between the filter process and the halftone process. In order to use the image area separation data in the processing after compression / decoding, the image area separation data must naturally be compressed and stored in the memory, which causes a problem that the amount of necessary memory increases accordingly.
[0005]
As a prior art for solving this problem, there is a method described in Japanese Patent No. 3134756, which has a second image area separation means for identifying the presence or absence of an edge after compression / decoding.
In this method, a binary image area composed of characters and lines and a halftone image area such as a photograph or halftone printing are separated from input image data by a first image area separation means, and separated data is obtained. The second image area separation means for performing the first image processing, compressing and storing the processed image signal, expanding the stored signal, and identifying the presence or absence of an edge in the expanded image data The non-edge region is determined, and the second image processing is performed based on the determination result.
[0006]
FIG. 10 shows the configuration.
This eliminates the need to have image area separation data by the first image area separation after compression / decoding, and the subsequent processing is an area generated by performing second image area separation on the compressed / decoded image. Depending on the data.
[0007]
[Problems to be solved by the invention]
By the way, in many digital color copiers and the like, a user can designate a mode according to preference and necessity.
For example, color reproduction or monochrome reproduction can be designated, the intensity level of spatial filter processing can be designated, or the document type (character document / photograph document / halftone document) can be designated.
[0008]
By this mode designation, processing methods and parameters such as a spatial processing filter and compression are switched, and an image suitable for the mode is obtained.
For this reason, in the above processing flow, when the mode is switched, the decoded image is different even if the same image is input.
Therefore, in the configuration of FIG. 10, the region data generated by the second image region separation differs depending on the mode, and there is a problem that appropriate region data cannot be generated.
[0009]
In order to solve the above problem, the present invention generates appropriate region data by switching the second image region separation method and parameters according to a designated mode, and performs processing after compression / decoding. An object of the present invention is to provide an image processing apparatus capable of preventing image quality deterioration.
[0010]
[Means for Solving the Problems]
According to the first aspect of the present invention, there is provided a mode designating unit for designating a mode by a user, a filter processing unit for performing a spatial filter process on input image data, and an irreversible compression process for the image data after the spatial filter process. Image processing unit, a decoding unit that performs decoding processing on image data after compression, and a unit that performs edge detection on the decoded image data and adaptively switches processing based on the edge detection result The apparatus further includes means for changing a spatial filter or compression / decoding processing and changing a criterion for edge detection according to a mode designated by the mode designation means.
[0011]
According to the second aspect of the present invention, there is provided a mode designating unit for designating a mode by a user, a filter processing unit for performing a spatial filter process on input image data, and an irreversible compression process for the image data after the spatial filter process. Compression means for performing the decoding, decoding means for performing decoding processing on the compressed image data, and means for performing halftone dot detection on the decoded image data and adaptively switching the processing based on the halftone detection result The image processing apparatus further includes means for changing a spatial filter or compression / decoding process and changing a criterion for detecting halftone dots according to a mode designated by the mode designation means.
[0012]
According to a third aspect of the present invention, in the image processing apparatus according to the first or second aspect, the mode designation means is means for designating color reproduction or monochrome reproduction.
[0013]
According to a fourth aspect of the present invention, in the image processing apparatus according to any one of the first to third aspects, the mode designating unit is a unit for designating a document type (character document / photo document / halftone document). It is characterized by that.
[0014]
According to a fifth aspect of the present invention, in the image processing apparatus according to any one of the first to fourth aspects, the mode designating unit is a unit for designating an edge enhancement level of filter processing.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
[Example 1] (Regarding claims 1 and 2)
FIG. 1 is an overall block diagram.
Hereinafter, the operation of each means will be briefly described by taking a color copying machine as an example.
[0016]
Specifically, the image input means 1 is a scanner.
This scanner reads image information of a document by dividing it into small areas.
For the image information of each area, the RGB (red, green, blue) components of the reflected light of the light emitted from the light source are read by the CCD, and the signal is digitized to read the image information proportional to the reflectance.
[0017]
The image area separation means 2 is a means for determining whether the image data is a character / picture area for each pixel area. For example, the electronic information communication society journal “1992/1 Vol. J75-D-II NO .1, pp. 39-47 ”, etc.,“ image area separation method for mixed images of characters / pictures (halftone dots / photos) ”.
In this method, edge detection and halftone dot detection are combined to separate an image area, and a non-halftone dot and edge area is determined as a character area, and the other area is determined as a picture area.
The edge detection and halftone dot detection methods will be described in detail later together with the second image area separation.
Further, as areas to be determined, there are methods of dividing areas such as character / halftone dots / photograph areas and black characters / colored characters / design areas, and image area separation methods corresponding to these may be used.
[0018]
The scanner γ correction unit 3 converts an image signal proportional to the reflectance read by the scanner into an image signal proportional to the density.
[0019]
The filter processing means 4 performs conversion for improving image quality such as noise removal and character image sharpening by performing smoothing, character edge enhancement, and the like.
In addition, using the image area separation data, appropriate processing is performed on each of the character / picture areas.
For example, processing that prioritizes sharpness such as emphasizing high-frequency components is performed on the character region, and smoothing processing that emphasizes gradation is performed on the pattern region.
[0020]
The color correction unit 5 converts the image signal read as the RGB signal into a density of CMY (cyan, magenta, yellow) as toner components.
[0021]
The under color removal / black generation means 6 removes an amount equal to or less than the minimum density among the CMY component densities as an under color by an equal amount from the CMY density, and replaces it with a K (black) toner density component.
This process can also be improved by switching according to the image area separation data.
If the ratio with respect to the minimum density is expressed in% as a scale representing the degree of replacement, for example, by replacing 100% of the character area, black characters can be reproduced with almost K colors, and defects such as coloring can be suppressed. it can.
On the contrary, the pattern area can be reduced in the replacement rate to suppress problems such as roughness of the achromatic portion.
[0022]
The compression method used in the compression means 7 is an irreversible method. For example, the existing method can use the JPEG method or the like.
Further, the method described in the prior art (Japanese Patent No. 3134756) may be used.
[0023]
The image data compressed to reduce the data size is stored in the memory by the storage means 8.
[0024]
Then, the decoding unit 9 decodes each color according to a predetermined timing.
[0025]
Image region separation determination is performed by the second image region separation means 10 on the decoded image. The second image area separation means 10 will be described later.
[0026]
The printer γ correction unit 11 converts the image signal in consideration of the output characteristics of the image output unit and the gradation characteristics of the halftone process described below.
Therefore, when the halftone processing switches processing depending on the area, it is necessary to switch the printer γ correction accordingly.
In addition, for example, binary density conversion may be performed in a character area in order to clarify the contrast of characters, and on the contrary, gentle density conversion may be performed in a picture area.
[0027]
The halftone processing means 12 performs processing for expressing the multi-valued input gradation number with the output gradation number normally having a smaller value by using a dither method, an error diffusion method, or the like.
In this process, for example, an error diffusion method that emphasizes resolution is used in a character area, and a dither method that emphasizes gradation is used in a picture area.
[0028]
The image output means 13 is a so-called printer unit, forms an output image according to the input image signal, and prints it on paper or the like. In addition to a color reproduction function using four-color toners of CMYK (cyan, magenta, yellow, and black), a monochrome reproduction function using one black color is also provided.
[0029]
Specifically, the second image area separation means 10 performs image area separation using an edge detection means 16, a halftone dot detection (a halftone dot pattern portion of a printed matter) 17, and the like.
That is, only the edge detection means 16 or the halftone dot detection means 17 may be used, or image edge separation may be performed by combining the edge detection means 16 and the halftone dot detection means 17.
[0030]
Examples of specific configurations are shown in FIGS. 2 (a), 2 (b), and 2 (c).
[0031]
First, the case where the edge detecting means 16 is used as the second image area separating means 10 as shown in FIG.
Edge detection is performed on the compressed and decoded image, and data for switching the printer γ correction and halftone processing depending on the size of the edge amount is generated.
At this time, the processing switching by the edge detection means 16 may be binary switching of (edge = character part) / (non-edge = picture part) or multi-value switching based on the edge amount.
[0032]
The edge detection method will be described below.
FIG. 3 shows a block diagram of the edge detection means 16.
The edge amount is calculated by performing a masking process using the 5 × 5 or 7 × 7 edge detection mask shown in FIG.
The four types of masks are for detecting edges in two vertical, horizontal, and diagonal directions, respectively, and the four edge amounts are converted into absolute values and the largest one is adopted.
Data for performing adaptive processing according to the region is generated by binarizing or multi-leveling the edge amount obtained in this way using a threshold value.
Note that other methods may be used as the edge detection method.
[0033]
Next, the case where the halftone dot detection means 17 is used as the second image area separation means 10 as shown in FIG.
The processing switching by the halftone dot detection means 17 is generally binary switching between halftone dots / non-halftone dots, but multi-value switching is also possible.
In the case of binary switching of halftone dots / non-halftone dots, for example, the printer γ correction means 11 performs a gradual density conversion in the halftone dot region to make the periodic halftone pattern inconspicuous, while the non-halftone dots are medium. It is possible to perform a process of clarifying the contrast by performing density conversion with a sudden change in density.
[0034]
The reason why the periodic net pattern is inconspicuous is to suppress moire. Moire refers to periodic patterns such as halftone dots that are inherent to the original, and periodic patterns generated by processing, such as the dither mask cycle when using the dither method in halftone processing, resulting in an output image This is a cause of image degradation.
[0035]
The halftone dot detection method will be described below.
First, halftone dot region detection is performed, for example, by the IEICE Transactions [1992/1 Vol. J75-D-II NO. 1 pp. 39-47] “Peak pixel detection method” described in “Character / Picture (Half Dot, Photo) Mixed Image Image Area Separation Method”.
[0036]
Next, in the peak pixel detection, it is determined from the density relationship with surrounding pixels whether or not the target pixel is an extreme point indicating a density change peak.
For example, as shown in FIG. 5, when the density level of the central pixel is higher or lower than all other density levels in a block composed of M × M pixels, whether or not it is a pole as shown in Equation 1 or Equation 2 Determine.
(1) When M = 3 [FIG. 5 (a)]
| 2m ₀ -m ₁ -m ₈ | ≧ Δm _TH and
| 2m ₀ -m ₂ -m ₇ | ≧ Δm _TH and
| 2m ₀ -m ₃ -m ₆ | ≧ Δm _TH and
| 2m ₀ -m ₄ -m ₅ | ≧ Δm _TH (Formula 1)
(2) When M = 5 [FIG. 5 (b)]
| 2m ₀ -m ₃ -m ₂₂ | ≧ Δm _TH and
| 2m ₀ -m ₈ -m ₁₇ | ≧ Δm _TH and
| 2m ₀ -m ₁ -m ₂₄ | ≧ Δm _TH and
| 2m ₀ -m ₇ -m ₁₈ | ≧ Δm _TH (Formula 2)
[0037]
That is, the center pixel is detected as a peak when the average value of the two pixel levels located symmetrically with respect to the center pixel and the absolute value of the density difference between the center pixels are larger than the threshold value Δm _TH .
Then, based on the peak pixel information, it is determined whether the area is a halftone dot area.
For example, in the simplest case, peak pixels are counted for each block of a predetermined size, and when the counted value is a predetermined number n or more, the target block is determined as a halftone dot region.
When this region determination is multivalued, a plurality of threshold values Δm _TH , n, etc. may be used.
It should be noted that other methods may be used for halftone dot detection.
However, as will be described later, since the determination parameter of the second image area separation means 10 is switched according to the designated mode, it is disclosed in JP-A-7-107275 and JP-A-7-123253. It is necessary to be able to change various criteria.
[0038]
When the combination of the edge detection unit 16 and the halftone detection unit 17 is used as the second image area separation unit 10 as shown in FIG. A determination such as halftone dot = character, otherwise = picture) can be performed, and the process can be switched.
[0039]
The mode designation means 14 can be designated by a user by inputting a mode, and switches each processing method and parameters according to the designated mode.
As modes that can be specified, a color / monochrome mode, a document type (text document / photo document / halftone document) mode, a mode for specifying an edge emphasis level for filter processing, a mode for specifying a compression rate, and the like can be considered.
[0040]
In any case, when a certain mode is designated, the processing of the filter processing means 4, the under color removal / black generation means 6, the compression / decoding means 7, 9 and the like is switched to the optimum one according to the mode.
Specifically, the processing is optimized by increasing the degree of edge enhancement of the filter, changing the replacement rate by the under color removal / black generation means 6, or changing the compression rate in the compression processing.
At this time, the image after compression / decoding differs in the way the edges stand depending on the mode.
At this time, the determination parameter reading means 15 changes the determination parameter for the second quadrant separation according to the mode.
Specifically, the threshold value for binarization or multi-value conversion is used for edge detection, and the threshold values Δm _TH , n, etc. are changed for halftone detection.
Therefore, optimal image data can be generated by performing the second image area separation on the compressed / decoded image.
Note that, in a copying machine or the like, a scaling process is generally performed. Depending on a model, a filtering coefficient of a filter process is switched according to a scaling ratio.
Even in such a case, it is effective to change the second image area separation criterion, which can be implemented using the mechanism of the present invention.
[0041]
In this embodiment, the compression / decoding means 7 and 9 are provided after the undercolor removal / black generation means 6 and before the printer γ correction means 11, but the compression / decoding means 7 and 9 are provided somewhere in the image processing. If it is.
For example, there may be compression / decoding means 7, 9 after the filter processing means 4 and before the color correction means 5. In this case, the lower color removal / black generation means 6 is also processed by the second image area separation means 10. Will be switched.
As described above, the image processing after compression / decoding can be optimized by changing the determination parameter of the second image area separation unit 10 according to the mode.
[0042]
[Example 2] (Regarding claim 3)
FIG. 6 shows an overall block diagram.
This embodiment is provided with means 19 capable of specifying a color / monochrome mode as the mode specifying means 14 in the configuration of the first embodiment.
The color / monochrome mode specifying means 19 will be described below.
[0043]
An original printed in the monochrome mode is often an original with many character portions, and the image quality of the pattern is not so important as compared with the color mode, and good character image quality is required. Therefore, when the monochrome mode is designated, for example, in the filter processing, the degree of edge enhancement is enhanced with emphasis on sharpness.
[0044]
Further, the compression means 7 performs processing switching such that the compression method and parameters are changed at a high compression rate and the decoding means is switched correspondingly without changing the image quality of the pattern as much as color.
The reason why the compression rate is increased is that it is convenient that the data size is as small as possible in consideration of sending the compressed image to the outside for use on a PC or the like.
[0045]
In this case, the image after compression / decoding differs from the image processed in the color mode in the way of standing edges of each part, apart from color information.
Correspondingly, the determination parameter for the second image area separation is switched by the determination parameter reading means 15 in order to perform the optimum second image area separation.
Therefore, as the area data necessary for the printer γ correction and halftone processing after compression / decoding, optimum data is obtained according to the mode.
[0046]
[Example 3] (Regarding claim 4)
FIG. 7 shows an overall block diagram.
This embodiment is provided with means 20 capable of designating a document type (character document / photo document / halftone document) as the mode designating unit 14 in the configuration of the first embodiment.
The document type (text document / photo document / halftone document) mode designating unit 20 will be described below.
[0047]
As described above, the text manuscript does not require much gradation as required by the design, and can perform resolution-oriented processing.
Therefore, when the character document mode is designated, character image quality improvement processing is performed such that the edge enhancement degree is increased in the filter processing, and the replacement rate is set to 100% in the under color removal / ink generation.
[0048]
In compression, process switching such as increasing the compression rate is performed. When the photo original mode is specified, the smoothness of the image is emphasized rather than the resolution, that is, the gradation, and the edge enhancement is weakened in the filter process. Processing such as reducing the rate is performed.
Further, in compression, processing switching is performed such as reducing the compression rate to prevent image quality deterioration. When the halftone original mode is specified, the same processing as that for a photographic original is basically performed, but in order to prevent moiré, a periodic mesh pattern is crushed by strong smoothing in the filter processing. Is called.
Moire refers to periodic patterns such as halftone dots that are inherent to the original, and periodic patterns generated by processing, such as the dither mask cycle when using the dither method in halftone processing, resulting in an output image This is a cause of image degradation.
[0049]
As a result of the process switching as described above, the image after compression / decoding has different differences in how the edges of each part rise depending on the mode. Therefore, the determination parameter reading means 15 switches the determination parameter for the second image area separation. Thus, the image processing after compression / decoding can be optimized.
[0050]
[Example 4] (Regarding claim 5)
FIG. 8 shows an overall block diagram.
In this embodiment, in the configuration of the first embodiment, the mode specifying means 14 is provided with means 21 capable of specifying the edge enhancement level of the filter processing. In this case, the determination parameter for the second image area separation is switched according to the designated edge enhancement strength.
The filter processing emphasis level may be two levels of software / hardware, or may be specified in multiple stages.
[0051]
【The invention's effect】
According to the present invention, appropriate region data is generated by switching the determination parameter for edge detection or halftone detection used as the second image region separation according to the designated mode, and after compression / decoding Image processing can be optimized.
[Brief description of the drawings]
FIG. 1 is an overall block diagram of the present invention.
FIG. 2 is a specific configuration diagram of a second image area separation unit of the present invention.
FIG. 3 is a block diagram of edge detection means of the present invention.
FIG. 4 is a diagram showing a specific example of masking processing using the edge detection mask of the present invention.
FIG. 5 is a diagram showing a density level of a central pixel in a block composed of M × M pixels according to the present invention.
FIG. 6 is a diagram showing an image processing apparatus having color / monochrome mode designating means of the present invention.
FIG. 7 is a diagram showing an image processing apparatus having a document type (character document / photo document / halftone document) mode designating unit of the present invention.
FIG. 8 is a diagram showing an image processing apparatus having filter processing enhancement level designation means of the present invention.
FIG. 9 is a diagram illustrating an example of the flow of general image processing.
FIG. 10 is a diagram showing an image processing apparatus having a second image area separation unit for identifying the presence or absence of an edge after compression / decoding.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Image input means 2 Image area separation means 3 Scanner gamma correction means 4 Filter processing means 5 Color correction means 6 Under color removal / black generation means 7 Compression means 8 Accumulation means 9 Decoding means 10 Second image area separation means 11 Printer γ Correction means 12 Halftone processing means 13 Image output means 14 Mode designation means 15 Determination parameter reading means 16 Edge detection means 17 Halftone detection means 18 Area determination means 19 Color / monochrome mode designation means 20 Document type (text original / photo original) / Halftone original) mode designation means 21 filter processing emphasis level designation means

Claims (5)

A mode specifying means for the user to specify the mode;
Filter processing means for performing spatial filtering on the input image data;
Compression means for performing irreversible compression processing on the image data after the spatial filter processing;
Decoding means for performing decoding processing on the compressed image data;
In the image processing apparatus having means for performing edge detection on the decoded image data and adaptively switching processing based on the edge detection result,
An image processing apparatus according to claim 1, further comprising means for changing the spatial filter or the compression / decoding process and also changing the determination criterion for the edge detection according to the mode specified by the mode specifying means. A mode specifying means for the user to specify the mode;
Filter processing means for performing spatial filtering on the input image data;
Compression means for performing irreversible compression processing on the image data after the spatial filter processing;
Decoding means for performing decoding processing on the compressed image data;
In the image processing apparatus having means for performing halftone dot detection on the decoded image data and adaptively switching processing based on the halftone dot detection result,
An image processing apparatus comprising: means for changing the spatial filter or the compression / decoding process according to the mode designated by the mode designating means, and changing a criterion for detecting the halftone dot. The mode specifying means includes
3. The image processing apparatus according to claim 1, wherein the image processing apparatus is means for designating color reproduction or monochrome reproduction. The mode specifying means includes
4. The image processing apparatus according to claim 1, wherein the image processing apparatus is means for designating a document type (character document / photo document / halftone document). The mode specifying means includes
The image processing apparatus according to claim 1, wherein the image processing apparatus is means for designating an edge enhancement level of the filter processing.

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