JP2001119575A - Image processing unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect only a dot image area having frequencies at which moire is generated and to suppress generation of moire in the detected image area. SOLUTION: The image processing unit is provided with a conventional image processing section 3 that processes an image area other than areas of dots with specific line number, a specific line number dot purpose image processing section 4 that processes an area of dot images with specific line number, a specific line number dot decision processing section 5 and a selector 6. The specific line number dot decision processing section 5 detects only the dot area with frequencies at which moire is caused from image data received from a scanner section 1, the specific line number dot purpose image processing section 4 adopting a strong smoothing processing for its filter processing applies the processing to the specific line number where a problem such as generation of moire so as to suppress deterioration in the image quality due to moire or the like and the conventional image processing section 3 applies a weak smoothing processing to the dot image consisting of a line number other than the specific line number so as not to deteriorate the image.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a digital copying machine,
The present invention relates to an image processing apparatus used for a facsimile and a printer, which is characterized by a halftone process for an input image signal.

[0002]

2. Description of the Related Art In general printed matter, a halftone image is used in many cases, and the number of lines is various although there are representative ones. The spatial frequency characteristics vary depending on the number of lines. Generally, when a halftone image is processed in a digital copying machine, smoothing processing and edge enhancement processing are performed on input image data. Although it depends on the resolution of the digital copying machine and the filter size of the above processing, in the normal processing, the halftone dots having a high ruling tend to be crushed by the smoothing process, and the halftone dots having the low ruling are directly replaced with the halftone dots. In many cases, it does not cause a problem. Therefore, what actually matters is a halftone image having an intermediate number of lines.
A halftone image having such an intermediate line frequency is often reproduced as a poor moiré image. For example, if a very strong smoothing process is performed so as to crush a halftone dot having a low screen ruling, the above-mentioned problem is eliminated. However, there are problems such as a blurred character image on a halftone dot due to strong smoothing, an increase in hardware scale, and an increase in cost.

To solve the problem of halftone images and moiré, Japanese Patent Laid-Open No.
The inventions disclosed in JP-A-8894 and JP-A-6-178097 are known. The former is intended to effectively suppress moiré noise generated when performing halftone processing on a halftone dot photograph, and to suppress blurring of edges in the halftone dot photograph, and to convert an image into blocks of a predetermined size. A dividing circuit for dividing, a halftone dot matching degree calculating circuit for calculating a halftone dot matching degree from pixel data in the block, and a line number detecting circuit for detecting a line number of a halftone picture in the block; A smoothing process of a characteristic according to the halftone dot conformity calculated by the degree calculation circuit and the number of lines detected by the number of lines detection circuit is performed on the image in the block.

In the latter, it is possible to two-dimensionally detect the period of the density generated by a halftone dot with a simple configuration, to accurately separate and extract halftone pixels in an image, and to detect the halftone frequency. With the aim of being able to, by having a mechanism to detect the end point of the density in the main scanning direction and the sub-scanning direction, and a mechanism to count the number of pixels between the end points of the halftone dot generated in the horizontal and vertical directions by the halftone dot , By extracting a halftone pixel, two-dimensionally detecting the end of the density of the halftone dot by a simple configuration, and configured so that halftone pixels in an image can be accurately separated and circulated. I have.

[0005]

As described above, Japanese Patent Application Laid-Open No.
No. 048894, JP-A-6-178097
In the invention described in the publication, the halftone frequency is detected together with the halftone area detection. Currently, the halftone screen ruling used in general manuscripts is 65 to 200 including shaded characters.
Detecting the halftone frequency in a wide range of lines and halftone dots in these general manuscripts may lead to an increase in hardware, or may erroneously detect dense portions of characters as halftone dots.

The present invention has been made in view of such a situation of the prior art, and has as its object to detect only a halftone dot image region having a frequency at which moire occurs.

It is another object of the present invention to control a spatial filter means for a halftone dot image region having a frequency at which moiré occurs, thereby suppressing the occurrence of moiré.

Another object is to add noise to a dot image area having a frequency at which moiré occurs,
The purpose is to suppress the occurrence of moire.

[0009]

In order to achieve the above object, the present invention provides an image processing apparatus for detecting a halftone dot region from an input image signal, wherein only a halftone dot region having a spatial frequency of a specific band is detected. It is characterized by comprising a specific frequency halftone dot area detecting means for detecting.

In this case, it is preferable to further provide a spatial filter processing means and a control means for controlling the filter means based on the detection result of the specific frequency halftone dot area detecting means.

It is to be noted that the control means includes a filter characteristic,
For example, the filter means is controlled by changing a strong smoothing characteristic and a weak smoothing characteristic.

Further, in addition to the spatial filter processing means and the specific frequency halftone dot area detecting means, processing noise generating means and noise are added to the input image signal based on the detection result of the specific frequency halftone dot area detecting means. It is preferable to provide noise adding means.

In the above case, the spatial frequency of the specific band is a frequency at which moire occurs.

[0014]

Embodiments of the present invention will be described below with reference to the drawings.

<First Embodiment> FIG. 1 is a block diagram showing the configuration of an image processing apparatus according to this embodiment. Hereinafter, the processing operation in this embodiment will be described with reference to FIG.
In all the embodiments described below, the processing of image data proceeds in the main scanning (horizontal) direction starting from the upper left portion of the image data, and when the processing of one line is completed, the processing of the next line is completed. (Processing in the sub-scanning direction) will be described. It is assumed that the image data is 600 dpi.

The image processing apparatus includes a normal image processing unit 3 for processing an image area other than a specific screen frequency halftone dot, an image processing unit 4 for a specific screen frequency halftone image processing a specific frequency screen halftone image area, The image processing apparatus includes a screen frequency halftone dot determination processing unit 5 and a selector 6. Image data is input from the scanner unit 1 and output to the printer unit 2.

The 600 dpi multi-valued image data (256-value (8-bit signal)) input from the scanner unit 1 is converted into a normal image processing unit 3, an image processing unit 4 for a specific screen ruling, and a specific screen ruling screen. Each is input to the determination processing unit 5.

FIG. 2 is a block diagram showing the configuration of the specific screen frequency halftone dot determination processing section 5 in the present embodiment. The input data is first determined by the halftone dot center determination unit 11 as to whether it is the center of a halftone dot. FIG. 5 is a schematic diagram for explaining the halftone dot center determination performed by the halftone dot center determination unit 11. For explanation, a pixel group adjacent to the target pixel in a 5 × 5 pixel window centered on the target pixel (* in the figure)
(A to D in the figure) G1 and a pixel group outside thereof (a to a in the figure)
f) Divide into G2. Pixels in the hollow column are arbitrary.

The halftone dot center determination section 11 first determines whether all the G1 pixel groups are higher or lower than the target pixel value (L). In this determination, if all of the G1 pixel groups are higher or lower than the target pixel value (L), the target pixel is located diagonally across the measure (L) and the target pixel * (the pixel indicated by the same notation in the drawing). -It is determined whether or not the pixel value (H, I) of Example A) satisfies the following relationship for all pairs (A to D), and if so, it is determined to be a dot center pixel.

| 2 × L−HI |> TH1 where TH1 is a predetermined constant. On the other hand, if the above condition is not satisfied, then all the G2 pixel groups have a value greater than the target pixel value (L). Determine if it is high or low. If the values of the pixel group G2 are all higher or lower than the pixel value of interest (L), pixels of a paired pixel (shown in the same notation in the drawing—example a) sandwiching L and the pixel of interest * It is determined whether or not the value (J, K) satisfies the following relationship for all pairs (a to f).

| 2 × LKJ |> TH2 where TH2 is a predetermined constant. Next, the data determined as to whether or not the pixel is the center pixel of the halftone dot is determined by the specific number of lines for which the halftone dot is a target. The specific line number determination unit 12 determines whether or not there is a line. Here, it is assumed that the halftone frequency of about 100 lines is determined.

FIG. 3 shows the distribution of halftone dot center pixels by halftone dots of 100 lines when the pixel of interest (* in the figure) is the halftone dot center pixel. In the figure, C is a Cyan dot (screen angle 15
), M is the center of the dot when it is a Magenta halftone (screen angle of 75 degrees), Y is the center of the halftone dot when it is a Yellow halftone (screen angle of 0 degrees), and K is Black dot
(Screen angle 45 degrees) shows the center of the halftone dot. As can be seen from the figure, in the image processing system of 600 dpi, the center of the halftone dot of 100 lines is 5
It can be seen that it does not exist in the area of × 5, but exists more outside and in the area of 8 × 8. Therefore, in the present embodiment, the specific line number determination processing is performed as follows.

As shown in FIG. 3, for a pixel window in which the pixel of interest * is located at the lower right, there is no halftone dot center in a 5 × 5 area and one or more halftone dot centers in an 8 × 8 area. Is determined to be a halftone dot of 100 lines.

The data which has been determined as to whether or not it is a specific screen ruling is subjected to the following determination by the block determination unit 13 in order to reduce the influence of a determination error. In the present embodiment, a 5 × 5 pixel block is defined as one block.
As shown in FIG. 4, 3 ×
The following determination is made by three blocks. If there is one or more pixels determined to be a specific screen frequency halftone dot in the pixel block, the block is regarded as a specific frequency screen halftone block.

On the other hand, among the 3 × 3 blocks shown in FIG.
If there are three or more, it is determined that the pixel of interest * is a specific line frequency halftone pixel area. Note that TH3 is a predetermined constant. The determination result thus determined is output as an AMI signal in pixel units.

In the present embodiment, the specific screen frequency halftone dot area determination processing is executed in this manner. The determination of the specific line frequency halftone dot region in the present invention is not limited to the present embodiment only. For example, the specific line frequency is determined by performing frequency analysis or the like using FFT or the like. There is also a method of determining the number.

Next, the normal processing image processing section 3 and the image processing section 4 for halftone dots will be described. In this embodiment,
In the halftone dot area, the smoothing filter is switched. FIG. 6A shows an example of a smoothing filter for a specific screen ruling, in which strong smoothing is performed to remove moiré.
FIG. 6B shows a smoothing filter for the normal image processing unit 3, which has a halftone dot with no problem such as character image quality and moiré.
Weak smoothing is performed in consideration of photographic paper image quality and the like.

In this embodiment, an example in which the smoothing filter is switched is shown. However, the same result can be obtained by using an edge emphasis filter, and the same applies to a method of switching both. The invention is not limited to these methods.

The two types of processing data are switched by the selector 6 according to the AMI signal and output, and output to the printer unit 2.

As described above, according to the present embodiment,
In particular, only the specific number of lines where a problem such as moiré occurs is determined, and in the case of the specific number of lines where a problem such as moiré occurs, deterioration of image quality due to moiré or the like is reduced by changing the filtering process to a strong smoothing process. In addition to the suppression, the halftone image with the number of lines other than the specific number of lines is not unnecessarily deteriorated.

<Second Embodiment> FIG. 7 is a block diagram showing a configuration of an image processing apparatus according to a second embodiment. In this embodiment, the normal image processing unit 3 in the first embodiment is omitted, and the image processing unit 4 for a specific screen frequency halftone comprises a random number generation unit 7 and an addition processing unit 8. The configuration is the same as that of the first embodiment.

In this embodiment, noise is added to the input image data by the image processing unit 4 for the specific screen frequency halftone,
It is configured to suppress moiré. That is, for each pixel of the input image data input from the scanner unit 1, a random number (for example, +20
To -20), and the addition unit 8 adds the random number to the input image data. This makes it possible to suppress the moiré periodicity and suppress the moiré.

Other parts not particularly described are the same as those described in the first embodiment.
It functions in the same way as the embodiment.

[0034]

As described above, according to the first aspect of the present invention, since only the halftone dot region having the spatial frequency of the specific band is detected, the spatial frequency of the specific band is relatively low. It becomes possible to detect a dot area having a frequency.

According to the second aspect of the present invention, since there are provided spatial filter processing means and control means for controlling the filter means based on the detection result of the specific frequency halftone dot area detecting means, for example, moire For the halftone dot image area having the frequency at which the moire occurs, the spatial filter means can be controlled to suppress the occurrence of moire.

According to the third aspect of the invention, since the filter means is controlled by changing the characteristics of the filter, the effect of the second aspect of the invention can be obtained with simple control.

According to the fourth aspect of the present invention, since the characteristics of the filter are strong smoothing characteristics and weak smoothing characteristics, generation of moire can be suppressed by using a filter having strong smoothing characteristics. .

According to the fifth aspect of the present invention, there is provided the noise generating means and the noise adding means for adding noise to the input image signal based on the detection result of the specific frequency halftone dot area detecting means. By adding noise to a halftone dot image region having a frequency at which moiré occurs, the occurrence of moiré can be suppressed.

According to the sixth aspect of the present invention, since the spatial frequency of a specific band is a frequency at which moiré occurs, it is possible to detect only a halftone dot region at which moiré occurs.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an image processing apparatus according to a first embodiment.

FIG. 2 is a block diagram illustrating a specific screen frequency halftone dot determination processing unit according to the first embodiment.

FIG. 3 is a diagram showing a distribution of halftone dot center pixels by halftone dots of 100 lines when the target pixel is a halftone dot center pixel.

FIG. 4 is a diagram showing a block including a target pixel.

FIG. 5 is a schematic diagram illustrating halftone dot center determination.

FIG. 6 is an explanatory diagram illustrating an example of a smoothing filter.

FIG. 7 is a block diagram illustrating an image processing apparatus according to a second embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Scanner part 2 Printer part 3 Normal image processing part 4 Image processing part for specific number of halftone dots 5 Characteristic number halftone part judgment processing part 6 Selector 7 Random number generation part 11 Halftone dot center judgment part 12 Specific number of line judgment part 13 Judgment unit

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tokiko Miyagi 1-3-6 Nakamagome, Ota-ku, Tokyo F-term in Ricoh Co., Ltd. 5B057 AA11 CA01 CA08 CA12 CA16 CB01 CB08 CB12 CB16 CC01 CE05 CE06 5C077 LL03 MP02 PP02 PP49

Claims (6)

[Claims] 1. An image processing apparatus for detecting a halftone dot region from an input image signal, comprising: a specific frequency halftone dot region detecting means for detecting only a halftone dot region having a spatial frequency of a specific band. Characteristic image processing device. 2. The apparatus according to claim 1, further comprising: spatial filter processing means; and control means for controlling said filter means based on a detection result of said specific frequency halftone dot area detection means. Image processing device. 3. The image processing apparatus according to claim 2, wherein the control unit controls the filter unit by changing a characteristic of the filter. 4. The image processing apparatus according to claim 3, wherein the characteristics of the filter are strong smoothing characteristics and weak smoothing characteristics. 5. The apparatus according to claim 1, further comprising: noise generating means; and noise adding means for adding noise to the input image signal based on a detection result of the specific frequency halftone dot area detecting means. 2. The image processing device according to 1. 6. The image processing apparatus according to claim 1, wherein the spatial frequency of the specific band is a frequency at which moire occurs.