JPH10276331A - Image processing unit and its method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress deterioration in image quality and to eliminate moire without deteriorating an edge of an image. SOLUTION: A main signal S as to a noted pixel is led to a mean density extract section 10 and a filter arithmetic section 40. Then the mean density extract section 10 extracts a mean density D in an area larger than a dot size and fed to an ID reference table 20. The ID reference table 20 provides an output of a reference value ID in response to the mean density value D. Then a weighting coefficient set selection section 30 decides an effective size of an image filter by selecting a set of a weight coefficient based on the reference value ID. The decided effective size is increased at a density area remarkable in moire. Then the filter arithmetic section 40 conducts calculation based on each weighting coefficient of the image filter obtained by the weighting coefficient set selection section 30 to generate a filtered signal S'.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for processing digital images obtained from a scanner, a digital camera, or another device.

[0002]

2. Description of the Related Art Conventionally, when a printed matter is generated, printing using halftone dots has been performed. Such a printed matter printed by halftone dots is, for example, 65 dpi to 200 dpi.
It has a resolution of about i.

FIG. 8 shows a case where a printed matter recorded with such halftone dots is read by an image input unit such as a scanner. FIG. 8 is a diagram showing a positional relationship between a halftone dot and a pixel to be read, and a halftone image as an original image has a density of 50%. That is, the hatched area shown in the figure is a part where the halftone dot is recorded. A case where such an original image is read by pixels P1 to P8 having a size indicated by a dotted frame will be described.

The size of one pixel shown in FIG. 8 is about 87.5% of the size of one halftone dot. In such a case, the density measured at the pixels P4 and P5 is 50%, which is equal to the density shown in the original image, so that the consistency is maintained, but the density measured at the pixels P1 and P8 is the pixel area. Contains a large number of blackened areas of halftone dots, so that it is larger than 50%. In practice, the density of pixels P1 and P8 is about 6
It is 2%. Pixels P2, P3, P6, P7
Are also greater than 50%. Thus, the density measured at the pixels P1,..., P8 periodically fluctuates between 50% and about 62%. This means that the image quality is degraded considering that the original image has a density of 50%.

This is because interference occurs because the halftone dot size of the original image and the pixel size to be read are different. Therefore, as the pixel size becomes smaller than the halftone dot size, the density of the halftone dot is read rather than the density of the original image, and the fluctuation of the density between pixels becomes larger. Further, when the pixel size is close to the halftone dot size, for example, the period of the change becomes long and becomes visually noticeable as moire.

[0006] Such moiré is conspicuous in a portion where the density level is flat, and is not so conspicuous in an intermediate density region where the density is 50%, but is high in a high density region or a low density region. Appears remarkably in the low-density region, causing deterioration in image quality.

The above phenomenon also occurs when the relationship between the pixel size and the pattern in the general original image or the pattern of the subject expressed by a method other than the halftone dot is the same as described above. That is, moire may occur in an image obtained by reading a transparent original or a reflective original with a scanner or an image obtained by a digital camera.

[0008] Conventionally, image processing using an image filter has been performed in order to remove such moiré and prevent deterioration in image quality.

FIG. 9 is a schematic configuration diagram of a conventional image processing apparatus. The pixel to be processed (hereinafter referred to as “target pixel”)
Is subjected to filter operation processing by an image filter of a predetermined size set in advance by the filter operation unit 400, and a filtered signal S 'is generated. FIG. 10 shows an example of an image filter used at this time.
Shown in In such a conventional image processing apparatus, the horizontal size and the vertical size of the image filter are set to be about twice or more of each halftone dot. Therefore, for example, in the case of the example of the image filter of FIG. 10, the size of one pixel is about "2/5" or more of the halftone dot size. In the conventional image processing apparatus, the filter operation unit 400 positions the center of the image filter at the target pixel, performs a weighted average of each density value based on the weighting coefficient assigned to the target pixel and its neighboring pixels, and performs filtering. The signal is output as a signal S '.

As described above, in the conventional image processing apparatus, by performing processing using an image filter as shown in FIG. 10, a weighted average of densities is obtained in an area (about twice) larger than a halftone dot size. The frequency component of the halftone dot pattern can be removed, and the influence of the halftone dot size can be eliminated. Therefore,
Moiré does not occur and the image quality does not deteriorate in the image subjected to the filter processing.

[0011]

Generally, an edge portion of a printed image recorded with halftone dots is reproduced at a resolution higher than the halftone dot size. FIG. 11 is an explanatory diagram showing a conventional method for generating one halftone dot. As shown in FIG. 11A, an area indicating one dot size is divided into four blocks B1,..., B4, and each block is further subdivided for each blackened area. Then, a corresponding threshold value is set for each blackened area as shown in FIG. On the other hand, as shown in FIG.
1, the position corresponding to B4 is "20", and the block B
When the position corresponding to B2 and B3 is "10", blackening is performed sequentially from the center of the halftone dot in comparison with the threshold value shown in FIG. Then, a halftone dot as shown in FIG. 11C is recorded.

FIG. 12 shows a case where an edge portion of an image is recorded by such a recording method. FIG. 12 is a diagram illustrating an edge portion of the halftone dot image, and a dotted line in the diagram indicates an edge of the image. The right side of the dotted line indicates 50% density, and the left side indicates 0% density. As shown in FIG. 12, since the halftone dot is recorded in the edge portion of the halftone dot image according to the density value corresponding to each of the four divided blocks, the reproduction is performed at a high resolution. Further, the edge portion of the halftone dot image reproduced at such a high resolution is distributed in a relatively intermediate density region of the original image.

However, in the conventional image processing apparatus, when smoothing is performed using an image filter as shown in FIG. 10 as a countermeasure against moiré and image quality deterioration, the edge of an image reproduced at a high resolution as described above is obtained. The part is also smoothed,
There is a problem of blurring.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and provides an image processing method and apparatus capable of satisfactorily suppressing image quality deterioration and removing moiré without deteriorating image edges. The purpose is to do.

[0015]

According to an aspect of the present invention, there is provided a method for performing a predetermined process for each pixel on an original image, comprising the steps of: A step of obtaining an average density value from a pixel group including the neighboring pixels, (b) a step of specifying an effective size of the image filter according to the average density value obtained in the step (a), and (c) a step ( performing a filter operation on the main signal of the pixel of interest using the image filter of the effective size specified in b) to generate an output signal.

According to a second aspect of the present invention, there is provided a method for performing a predetermined process for each pixel on an original image obtained by reading a halftone dot original, comprising: Calculating an average density value from a pixel group including
(b) a step of specifying the effective size of the image filter according to the average density value obtained in the step (a), and (c) the pixel of interest using the effective size image filter specified in the step (b). Performing a filter operation on the main signal to generate an output signal. The effective size of the image filter specified in the step (b) is determined for each halftone dot forming the halftone original. It is characterized by being larger than the size.

According to a third aspect of the present invention, there is provided the first or second aspect.
In the method described in step (b), the effective size is set large when the average density value is in the high density region or the low density region, and the effective size is set when the average density value is in the intermediate density region. It is characterized by being set small.

According to a fourth aspect of the present invention, there is provided an apparatus for performing a predetermined process on an original image for each pixel, wherein (a) obtaining an average density value from a pixel group including a target pixel and its neighboring pixels. Means, (b) means for specifying an effective size of the image filter according to the average density value obtained by means (a), (c) means
means for performing a filter operation on the main signal of the target pixel using the image filter of the effective size specified in (b) to generate an output signal.

According to a fifth aspect of the present invention, there is provided an apparatus for performing a predetermined process for each pixel with respect to an original image obtained by reading a halftone dot original, comprising: Means for calculating an average density value from a pixel group including
(b) means for specifying the effective size of the image filter according to the average density value obtained by means (a), and (c) means for the pixel of interest using the effective size image filter specified by means (b). Means for performing a filter operation on the main signal and generating an output signal, wherein the effective size of the image filter specified by the means (b) is the size of each halftone dot constituting the halftone original. It is characterized by being larger than

The invention according to claim 6 is the invention according to claim 4 or 5
In the apparatus described in (1), the means (b) sets the effective size large when the average density value is in the high density region or the low density region, and increases the effective size when the average density value is in the intermediate density region. It is characterized by being set small.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION

<1. Overall Configuration of Apparatus> First, an example of the overall configuration of an apparatus to which an embodiment of the present invention is applied will be described. FIG. 1 is a schematic diagram showing an overall configuration of an apparatus to which an embodiment of the present invention is applied. The image input unit 100 optically reads a document, such as an input scanner, and generates original image data indicating a multi-value density value for each pixel. Then, the generated original image data is transferred to the image processing unit 200. The image processing unit 200 is a processing unit to which the image processing device according to the embodiment of the present invention is applied. In the image processing unit 200, in addition to the image processing using the image filter according to the embodiment of the present invention, after performing predetermined processing on the original image data, the image output unit 300 such as an output scanner or the like is used. Is output to And the image output unit 30
At 0, recording is performed on a recording medium such as a film. In order to cause the image input unit 100, the image processing unit 200, and the image output unit 300 to perform desired operations of the operator, an operation input unit 201 such as a keyboard and a mouse and an information display unit 202 such as a display unit are provided. Are provided.

<2. Outline of Image Processing> Next, a processing mode in an image processing apparatus according to an embodiment of the present invention will be described. In the image processing device according to the embodiment of the present invention, image processing using an image filter is performed. That is, a process such as image smoothing is performed by scanning an M × N (M, N is an odd number of 3 or more) matrix image filter in the image plane.

FIG. 2 is an explanatory diagram showing an outline of image processing in the embodiment of the present invention. The image I shown in FIG. 2A is sequentially processed while scanning the target pixel OP pixel by pixel with the X direction (horizontal direction) as the main scanning direction and the Y direction (vertical direction) as the sub-scanning direction. I will go. At the time of processing by the image filter, the target pixel OP is positioned at the center of the image filter F as shown in FIG. FIG.
In the example of (b), a 5 × 5 (M = N = 5) matrix image filter is used. Then, based on the weighting coefficient assigned to each component of the image filter, a weighted average of the density value of the target pixel OP and the density values of the neighboring pixels is derived by calculation, and the obtained value is calculated as the filtered signal of the target pixel OP. I do.

[0024] That is, image filter F is the size of the matrix as shown in FIG. 3 (2m + 1) × ( 2n + 1), the weighting coefficient of each component k _-mn to k _mn is assigned . A filtered signal S ′ _xy obtained by performing a filter operation on the pixel of interest OP located at the coordinates (x, y) on the image plane using the image filter F is

[0025]

(Equation 1)

## EQU1 ## Here, S _xy is the density value of the pixel located at the coordinates (x, y) on the image plane of the original image.

In the image filter shown in FIG.
When all the weighting coefficients corresponding to the target pixel and its neighboring pixels are positive numbers, it is an image filter for performing image smoothing, and the frequency characteristic of the weighting coefficient is the characteristic of the image filter.

<3. First Embodiment> Next, an image processing apparatus according to a first embodiment of the present invention will be described. FIG. 4 is a schematic configuration diagram showing an image processing device 250 according to an embodiment of the present invention. The main signal S for the target pixel is obtained by the average density value extraction unit 10 and the filter operation unit 40.
It is led to.

The average density extracting section 10 calculates an average density value D from a pixel group including a pixel of interest and its neighboring pixels. Here, the area formed by the pixel of interest for obtaining the average density value D and its neighboring pixels is set to an area larger than the dot size of the original image so as not to detect the dot shape. That is, if the size to be averaged is not sufficiently large, the density value may be different from the original density value of the original image depending on whether it is close to the center position of the halftone dot or not. As a result, the average density value D becomes the density value originally indicated by the original image that is not affected by the halftone dots. The method of obtaining the average density value D may be a simple average of a pixel group including the target pixel and its neighboring pixels, or a weighted average. Then, the average density value D obtained by the average density extraction unit 10 is sent to the ID reference table 20.

The ID reference table 20 outputs a reference value ID according to the average density value D to be inputted. FIG. 5 shows the relationship between the average density value D and the reference value ID. As shown in FIG. 5, possible values of the reference value ID are “1”, “2”,
"3". Then, as the average density value D becomes higher or lower, the reference value ID becomes smaller. When the average density value D becomes an intermediate density range, the reference value ID becomes the maximum value “3”. In the ID reference table 20, a table in which the relationship of FIG. 5 is associated is stored in a storage member such as a memory, and when the average density value D is input, the corresponding reference value ID is obtained by referring to the table. . The obtained reference value ID is used as the weighting coefficient set selection unit 30.
Sent to

[0031] In the weighting coefficient set selection section 30, based on the reference value ID to be input, the determination of all the weighting coefficient k _-mn to k _mn used for image filter. The weighting coefficient set selection unit 30 is also provided with a storage member such as a memory, in which a weighting coefficient corresponding to each value of the reference value ID as shown in FIGS. The set image filter is stored.

As shown in FIG. 6A, when the input reference value ID is "1", effective weighting coefficients are assigned to all components of the 5.times.5 image filter. As shown in FIG. 6B, when the input reference value ID is “2”, an invalid weighting coefficient “0” is assigned to the four corner components of the 5 × 5 image filter. Further, as shown in FIG. 6C, when the input reference value ID is “3”, an invalid weighting coefficient “0” is assigned to the component located at the outer edge of the 5 × 5 image filter. ing. If the weighting coefficient k _ij (i and j are arbitrary integers) is “0”, the coordinate position (i, j
The density value of the pixel corresponding to j) has no effect on the processing of the image filter. The weighting coefficient other than “0” of each component of the image filter is an effective weighting coefficient. Therefore, as shown in FIG. 6, image filters having different effective sizes are specified by the reference value ID. That is, when the reference value ID is “1”, the image filter having the largest effective size is specified. When the reference value ID is “2”, the image filter having the middle effective size is specified. Is "3", the image filter having the smallest effective size of substantially 3 × 3 is specified.

The reference value ID is small in the high density range or the low density range as shown in FIG. 5, and has the maximum value "3" in the intermediate density range. The image filter is an image filter having a large effective size when the target pixel is in the high density area or the low density area of the original image, and is an image filter having a small effective size when the pixel of interest is in the intermediate density area.

The image filter shown in FIG. 6 shows a case where the filter size is 5 × 5 as an example, but the present invention is not limited to this. However, in order to favorably remove the deterioration of image quality and the occurrence of moiré due to the influence of halftone dots, an image filter having the largest effective size (see FIG. 6A)
Is preferably about twice or more the halftone dot size of the original image.

The filter operation unit 40 performs the operation shown in Equation 1 using the weighting coefficients of the image filters obtained from the weighting coefficient set selection unit 30 to generate the filtered signal S ′. This filtered signal S '
Is an output signal of the image processing device 250.

Here, the data output from the weighting coefficient set selecting section 30 to the filter calculating section 40 may be a weighting coefficient of each component, but is not limited to this.

When a plurality of weighting coefficients are stored in advance in a storage member such as a RAM in the filter operation section 40,
The weighting coefficient set selection unit 30 can be omitted. That is, the ID reference table 20 stores the average density value D
If the offset value of the address for accessing the RAM of the filter operation unit 40 is output as the reference value ID according to
If the filter calculation unit 40 accesses the RAM based on the offset value indicated by the reference value ID, it can directly obtain each weighting coefficient corresponding to the average density value D.

As described above, the image processing apparatus of this embodiment performs image processing using an image filter having a small effective size in an intermediate density region in which moire is not conspicuous, and performs image processing in a high density region in which moiré and image quality degradation are conspicuous. In the low-density area, image processing is performed by an image filter with a large effective size. Image quality can be prevented from deteriorating.

The effective size of the image filter applied in the high-density region or the low-density region is smoothed by performing a weighted average of the density in an area larger than the halftone dot size of the original image. The frequency component of the dot pattern can be removed, and the influence of the dot size can be removed. Therefore, occurrence of moiré and deterioration of image quality do not occur in the filtered image.

<4. Second Embodiment> Next, an image processing apparatus according to a second embodiment of the present invention will be described. FIG. 7 is a schematic configuration diagram showing an image processing device 250 according to an embodiment of the present invention. The image processing device 250 according to this embodiment is configured to be able to change the size of the image filter to an appropriate size when the halftone dot size of the original image is changed. Therefore, the difference from the image processing apparatus of FIG. 4 described in the first embodiment is that image processing is performed based on halftone information HD.

As a method of detecting the halftone dot size of the original image, a method of measuring and detecting the halftone dot size when the image input unit 100 reads the original image, or a method in which the operator inputs the original image to the image input unit 100. There is a method in which the operator himself / herself sets and inputs the dot size from the operation input unit 201 when setting. Information on the dot size obtained in this manner is called dot information, and is sent to the image processing device 250 of this embodiment.

Then, in the image processing device 250 shown in FIG. 7, the input dot information HD is input to the dot information storage unit 50, where it is stored and held. And the dot information HD
Is sent to the weighting coefficient set selection unit 30. If it is not necessary to hold the dot information HD, the dot information HD may be directly input to the weighting coefficient set selection unit 30 without providing the dot information storage unit 50.

The weighting coefficient set selecting section 30 sets the size of the image filter used in the filter calculating section 40 based on the halftone dot information HD, and sets the number of weighting coefficients corresponding to the size to the filter calculating section. Send to section 30.

When the halftone dot size of the original image increases, it is necessary to increase the size of the image filter based on the halftone information HD. This is because the size of the image filter having the largest effective size needs to be about twice as large as halftone dots in order to prevent moiré and deterioration of image quality.
Conversely, when the halftone dot size of the original image is reduced, it is necessary to reduce the size of the image filter based on the halftone information HD. This is because applying an image filter having an unnecessarily large size only blurs the original image, and reduces the reproducibility of the original image.

For example, in the weighting coefficient set selection unit 30, before the halftone dot size was changed, the effective size of the 5 × 5 size image filter was changed.
When the halftone dot size is changed and becomes large, an image filter of a size such as 7 × 7 or 9 × 9 is applied, and the effective size of the image filter of that size is changed based on the reference value ID. Conversely, when the halftone dot size is changed and becomes smaller, an image filter of a size such as 3 × 3 is applied, and the reference value I for the image filter of that size is applied.
The effective size is changed based on D.

Further, the halftone dot information HD input to the image processing device 250 may be sent to the average density value extracting section 10.
In the average density value extraction unit 10, a region formed by a pixel of interest for calculating the average density value D and its neighboring pixels is:
It is preferable to set an area larger than the dot size of the original image so as not to detect the dot shape. However, when the area for extracting the average density value D is fixed and the halftone dot size increases, the area may become smaller than the halftone dot size. In order to prevent such a situation, the halftone dot information HD is input to the average density value extraction unit 10, and the area for extracting the average density value D is changed based on the halftone dot size indicated by the halftone dot information HD. Then, it is possible to always extract the average density value D which is not affected by the appropriate halftone dot.

The other processing flows are the same as those described in the first embodiment. Therefore, detailed processing of each unit will be omitted, and the outline will be described. The main signal S for the target pixel is guided to the average density value extraction unit 10 and the filter calculation unit 40. Then, the average density value extraction unit 10 extracts the average density value D in an area corresponding to the halftone dot size and sends it to the ID reference table 20. In the ID reference table 20, as shown in FIG. 5, a reference value ID corresponding to the average density value D is output. Therefore, when the average density value D is in the high density range or the low density range, the value of the reference value ID is small, and when the average density value D is in the intermediate density range, the value of the reference value ID is large. Then, as described above, the weighting coefficient set selection unit 30 sets the size of the image filter according to the dot information HD, and selects a set of weighting coefficients based on the reference value ID for the image filter of that size. Determine the effective size of the image filter. At this time, when the reference value ID is large, a set of weighting coefficients for reducing the effective size of the image filter is selected, and when the reference value ID is small, a set of weighting coefficients for increasing the effective size of the image filter is selected. Then, the filter operation section 40 performs the operation shown in Expression 1 based on the size of the image filter obtained by the weighting coefficient set selection section 30 and each weighting coefficient to generate a filtered signal S ′.

As described above, also in the image processing apparatus of this embodiment, the image processing is performed by the image filter having a small effective size in the intermediate density region where the moire is not conspicuous, and the moire and the image quality are remarkably deteriorated. Alternatively, since image processing is performed by an image filter having a large effective size in a low-density area, a high-density area or a low-density area is not deteriorated without deteriorating an edge portion of an image which is relatively widely distributed in an intermediate density area in an original image. Image quality can be prevented from deteriorating.

The effective size of the image filter applied in the high-density area or the low-density area is determined by performing a smoothing process by performing a weighted average of the density in an area larger than the halftone dot size of the original image. The frequency component of the dot pattern can be removed, and the influence of the dot size can be removed. Therefore, occurrence of moiré and deterioration of image quality do not occur in the filtered image.

Further, even when the dot size of the original image is changed, the image processing apparatus of this embodiment inputs dot information indicating the changed dot size,
Since the size of the image filter can be automatically changed based on the halftone dot information, the influence of the halftone dot size can be always removed, and the occurrence of moire and the deterioration of the image quality can be favorably prevented.

<5. Modification> In the ID reference table 20 in the image processing device 250 of the above embodiment, FIG.
It has been described that the reference value ID is output based on the relationship between the average density value D and the reference value ID as shown in FIG. However, looking at the relationship in FIG. 5, when the average density value D continuously changes from the low density range to the high density range, the output reference value ID changes stepwise. That is, referring to the relationship of FIG.
D cannot take an intermediate value such as "1.5" or "2.3".

However, when the average density value D continuously changes from a low density range to a high density range in order to increase the accuracy of image processing using an image filter, the output reference value ID is continuously changed to an intermediate value. It may be set so as to change while taking the values. In this case, when the reference value ID indicates an intermediate value such as “1.5” in the weighting coefficient set selection unit 30, the weighting coefficient when the reference value ID shown in FIG. And the weighted coefficient when the reference value ID shown in FIG. 6B is “2”, and the intermediate value of each component is derived, and the obtained value is used as each weighted coefficient.

In the embodiment, as an example, when the average density value D is in the high density range or the low density range, the reference value I
The value of D becomes smaller, and the reference value I
The case where the value of D is large has been described. However, the present invention is not limited to such a relationship. As a result, when the average density value D indicates a high density area or a low density area, the image filter becomes effective. The size may be increased and the effective size of the image filter may be reduced when indicating the intermediate density range.

Further, by changing the relationship between the average density value D and the reference value ID in the ID reference table 20 to another relationship, the effective size of the image filter can be increased or decreased for an arbitrary density range. Can be.
Therefore, if the image processing apparatus of this embodiment is applied, it is possible to remove the deterioration of the image quality and the occurrence of moire in an arbitrary density range.

The image filter applied in the image processing apparatus described above can be generally expressed as m × n
(M, n are integers of 2 or more) image filters, and the target pixel is located at the center of the image filters.

[0056]

As described above, according to the first aspect of the present invention, an average density value is obtained from a pixel group including a pixel of interest and its neighboring pixels, and an image is formed according to the obtained average density value. Since the effective size of the filter is specified and the main signal of the pixel of interest is subjected to the filter operation using the image filter of the effective size to generate an output signal, moire and image quality degradation occurring in an arbitrary density range are reduced. In addition to this, the original image is not degraded in a density region where processing such as moiré removal is not required.

According to the second aspect of the present invention, the effective size of the image filter specified according to the average density value is larger than the size of each halftone dot constituting the halftone original. The effect of size can be eliminated.
Therefore, occurrence of moiré and deterioration of image quality do not occur in the filtered image.

According to the third aspect of the invention, when the average density value is in the high density range or the low density range, the effective size of the image filter is set large, and when the average density value is in the intermediate density range. In, the effective size of the image filter is set small, so that moiré that stands out in the high density area or low density area can be removed well, and the edge part of the image reproduced with high resolution, which is often distributed in the middle density area, deteriorates I will not let you.

According to the fourth aspect of the present invention, an average density value is obtained from a pixel group including a target pixel and its neighboring pixels.
The effective size of the image filter is specified according to the obtained average density value, and an output signal is generated by performing a filter operation on the main signal of the pixel of interest using the image filter of the effective size. It is possible to suppress moiré and deterioration of image quality occurring in the region, and does not cause deterioration of the original image in a density region where processing such as moiré removal is not required.

According to the fifth aspect of the present invention, the effective size of the image filter specified in accordance with the average density value is larger than the size of each halftone dot constituting the halftone original, and The effect of size can be eliminated.
Therefore, occurrence of moiré and deterioration of image quality do not occur in the filtered image.

According to the present invention, when the average density value is in the high density range or the low density range, the effective size of the image filter is set to be large, and when the average density value is in the intermediate density range. In, the effective size of the image filter is set small, so that moiré that stands out in the high density area or low density area can be removed well, and the edge part of the image reproduced with high resolution, which is often distributed in the middle density area, deteriorates I will not let you.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an overall configuration of an apparatus to which an embodiment of the present invention is applied.

FIG. 2 is an explanatory diagram illustrating an outline of image processing according to the embodiment of the present invention;

FIG. 3 is a diagram illustrating weighting coefficients of respective components of an image filter.

FIG. 4 is a schematic configuration diagram illustrating an image processing apparatus according to the first embodiment of the present invention.

FIG. 5 is a diagram illustrating a relationship between an average density value D and a reference value ID.

FIG. 6 is a diagram illustrating an example of an image filter applied in the image processing apparatus according to the embodiment of the present invention;

FIG. 7 is a schematic configuration diagram illustrating an image processing apparatus according to a second embodiment of the present invention.

FIG. 8 is a diagram showing a positional relationship between halftone dots and pixels to be read.

FIG. 9 is a schematic configuration diagram showing a conventional image processing apparatus.

FIG. 10 is a diagram illustrating an example of a conventional image filter.

FIG. 11 is an explanatory diagram showing a conventional method for generating one halftone dot.

FIG. 12 is a diagram showing an edge portion of a halftone dot image.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Average density value extraction part 20 ID reference table 30 Weighting coefficient set selection part 40 Filter calculation part 50 Halftone dot information storage part 100 Image input part 200 Image processing part 201 Operation input part 202 Information display part 250 Image processing device 300 Image output part S Main signal S 'Filtered signal D Average density value ID Reference value HD Halftone information

Claims (6)

[Claims] 1. A method of performing a predetermined process for each pixel on an original image, comprising: (a) obtaining an average density value from a pixel group including a pixel of interest and its neighboring pixels; A step of specifying an effective size of an image filter according to the average density value obtained in the step (a); and (c) the use of the image filter of the effective size specified in the step (b). Performing a filter operation on the main signal to generate an output signal. 2. A method for subjecting an original image obtained by reading a halftone original to predetermined processing for each pixel, comprising: (a) calculating an average density from a pixel group including a target pixel and its neighboring pixels; Calculating a value, (b) specifying an effective size of the image filter according to the average density value obtained in the step (a), and (c) an effective size specified in the step (b) Performing a filter operation on the main signal of the pixel of interest using an image filter to generate an output signal, the effective size of the image filter specified in the step (b), An image processing method characterized in that the size of each halftone dot document is larger than each halftone dot. 3. The method according to claim 1, wherein, in the step (b), when the average density value is in a high density range or a low density range, the effective size is set large, and An image processing method, wherein the effective size is set small when the density value is in the intermediate density range. 4. An apparatus for performing a predetermined process for each pixel on an original image, comprising: (a) means for calculating an average density value from a pixel group including a target pixel and its neighboring pixels; Means for specifying an effective size of the image filter according to the average density value obtained by the means (a); and (c) using the image filter of the effective size specified by the means (b) for the pixel of interest. Means for performing a filter operation on the main signal to generate an output signal. 5. An apparatus for performing a predetermined process for each pixel on an original image obtained by reading a halftone dot document, comprising: (a) calculating an average density from a pixel group including a target pixel and its neighboring pixels; Means for determining a value, (b) means for specifying an effective size of an image filter according to the average density value obtained in the means (a), and (c) effective size specified in the means (b) Means for performing a filter operation on the main signal of the pixel of interest using an image filter to generate an output signal, wherein the effective size of the image filter specified by the means (b) is An image processing apparatus characterized in that the size is larger than each halftone dot forming a dot document. 6. The apparatus according to claim 4, wherein the means (b) sets the effective size to be large when the average density value is in a high-density region or a low-density region. An image processing apparatus, wherein the effective size is set to be small when the density value is in an intermediate density range.