JPH0832798A - Method and device for processing image - Google Patents

Abstract

PURPOSE:To obtain a line drawing image of high quality even when an image is enlarged. CONSTITUTION:An emphasis processing part 31 performs emphasis processing for density data on an aimed pixel and its peripheral pixels. The value obtained by the emphasis processing part 31 is multiplied by a coefficient M at a 1st multiplication processing part 321. For the density data, a smoothing processing part 33 performs smoothing processing and when an image selected with an image selection key 6 is the line drawing image, a threshold value processing part 34 performs threshold value processing for the value obtained by the smoothing processing part 33. A 2nd multiplication processing part 322 multiplies the value obtained by the threshold value processing part 34 by a coefficient N. When the image selected with the image selection key 6 is the line drawing image, M is set to N. Then an addition processing part 35 adds the output value of the 1st multiplication processing part 321 and the output value of the 2nd multiplication processing part 322 together and outputs the result as density data on the aimed pixel to an image formation part 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital image processing apparatus such as a copying machine or a printer having a magnifying function for magnifying and outputting a read original image, and more particularly, when a line drawing image such as a character is magnified. The present invention relates to an image processing method and an image processing device.

[0002]

2. Description of the Related Art An example of a conventional image processing method will be described with reference to FIG. In this image processing method, the light receiving elements of the image sensor, which is composed of a large number of light receiving elements arranged in one row, sequentially scan (main scan). Then, the image sensor moves in a direction orthogonal to the light receiving element array to perform sub-scanning, and obtains a light receiving signal of a level corresponding to the density of each pixel corresponding to each light receiving element over the entire surface of the original to obtain the original. The image can be read.

Next, with respect to the target pixel sequentially, the target pixel and pixels in the vicinity thereof, that is, the required number of pixels in the main scanning direction and the required number of lines in the sub-scanning direction,
These densities are extracted into a line memory, and the data of the extracted pixels are used to perform an emphasis process and a smoothing process. Then, a value obtained by multiplying the value obtained by the emphasis process by a predetermined coefficient M (M = 0 to 1) and a value obtained by the smoothing process by a predetermined coefficient N (N = 1-M) And are added. Then, this added value is output as the pixel density of the target pixel. The coefficients M and N are set to M = N when the input image is a line drawing image such as a character, and other images,
For example, in the case of a halftone dot image or a photographic image, appropriate values of M and N are set according to the input image.

On the other hand, an image processing apparatus has been proposed which is capable of obtaining a high-quality image without moire (Japanese Patent Publication No. 6-5885). In this image processing device, as shown in FIG.
As shown in, the detection process of detecting the edge of the image is performed. In this detection processing, the flat portion of the image and the halftone dot image are determined to be non-edge regions, the edge portion of the line drawing image such as characters is determined to be an edge region, and the coefficients M and N are controlled based on the determination result. Is. Then, the coefficient N is increased in the non-edge area, the coefficient M is increased in the edge area, and the coefficients M and N are increased at the boundary between the edge area and the non-edge area.
Is continuously changing.

[0005]

However, as in the image processing device described in Japanese Patent Publication No. 6-5885, the value obtained by the emphasis process and the value obtained by the smoothing process are used as the result of the detection process. In the method of adding at the ratio based on the above, an optimum image can be obtained by setting the coefficient M = N in the case of a line drawing image, but the jaggies (there is a backlash in the outline of the image) still remain in the outline of the line drawing image. As a result, high quality images cannot be obtained. This jaggy becomes more conspicuous as the enlargement ratio increases, and the image quality deteriorates. This is because this image processing device focuses on reproduction of the halftone dot image rather than the line drawing image.

The present invention solves the above problems, and an object of the present invention is to provide an image processing method and an image processing apparatus capable of obtaining a high-quality line drawing image even when the image is enlarged.

[0007]

In order to achieve the above object, the present invention performs emphasis processing on density data of a pixel of interest and pixels in the vicinity thereof and smoothing processing on the density data. Then, the line drawing image is enlarged in the image processing method in which the values obtained by both processes are multiplied by the first and second coefficients, respectively, and the value obtained by adding the multiplication results is output as the density data of the pixel of interest. In this case, using the first arithmetic expression set according to the enlargement ratio of the image, the threshold value processing is performed on the value obtained by performing the smoothing processing,
The value obtained by the threshold processing is multiplied by the second coefficient (claim 1).

Further, in the first arithmetic expression, the value obtained by the smoothing process is S, the value obtained by the threshold process is S ′,
S ′ = (2n−1) S−, where n is the magnification of the image.
(N-1) (claim 2).

Further, in the image processing method according to the first aspect, when the density change of the line drawing image occurs in less than 2 pixels, the threshold processing is the second arithmetic expression set according to the enlargement ratio of the image. (Claim 3).

Further, in the second arithmetic expression, the value obtained by the smoothing process is S, the value obtained by the threshold process is S ',
If the magnification of the image is n, then S '= {(2n-1) S
-(X-1)} / (2n-2X + 1), and
X is 1 when the density change of the line drawing image occurs for each pixel, and n when the density change occurs for 2 pixels or more, and between 1 and n when the density change occurs between 1 pixel and 2 pixels. Is changed linearly (claim 4).

The range of pixels in the vicinity of the target pixel is set according to the enlargement ratio (claim 5).

Further, an emphasis processing means for emphasizing the density data of the pixel of interest and pixels in the vicinity thereof, a smoothing processing means for smoothing the density data, and an output value of the emphasizing processing means. The first multiplication means for multiplying the first coefficient, the second multiplication means for multiplying the output value of the smoothing processing means by the second coefficient, and the output values of the first and second multiplication means are added. In an image processing apparatus including an addition processing unit that outputs a value as the density data of the pixel of interest, threshold value processing is performed using a first arithmetic expression that is set for the output value of the smoothing unit according to the enlargement ratio of the image. The second multiplication means is provided with a threshold processing means for performing a calculation, and the second multiplication means multiplies the value obtained by the threshold processing calculation by a second coefficient (claim 6).

Further, in the image processing apparatus according to the present invention, a fine line detecting means for detecting the number of pixels in which the density change of the line drawing image occurs is provided, and the threshold processing means is set in accordance with the magnification of the image. The threshold value calculation is performed using the second arithmetic expression (claim 7).

The range of pixels in the vicinity of the pixel of interest is set according to the enlargement ratio (claim 8).

[0015]

According to the first and sixth aspects of the invention, when the line drawing image is enlarged, the density data of the pixel of interest and the pixels in the vicinity thereof are first emphasized, and the density data is converted into the density data. On the other hand, smoothing processing is performed. Next, threshold processing is performed on the value obtained by performing the smoothing processing using the first arithmetic expression. Appropriate threshold processing according to the enlargement ratio of the image is performed by this first arithmetic expression. Then, the value obtained by the threshold processing is multiplied by the second coefficient, while the value obtained by the emphasis processing is multiplied by the first coefficient.
Further, the value obtained by adding these multiplication results is output as the density data of the target pixel.

According to the invention of claim 2, the first
By the threshold processing using the arithmetic expression S ′ = (2n−1) S− (n−1), the range of the pixels having the density data of the intermediate gradation is reduced. Therefore, even when enlarging the image,
The contour of the enlarged line drawing image becomes clear, and a high quality line drawing image can be obtained.

According to the third and seventh aspects of the invention,
When the density change of the line drawing image occurs in 2 pixels or less, an appropriate threshold value processing is performed according to the enlargement ratio of the image by the second arithmetic expression.

According to the invention described in claim 4, the threshold value processing is performed by the second arithmetic expression S '= {(2n-1) S- (X-
1)} / (2n-2X + 1). In this case, when the density change of the line drawing image occurs for each pixel, X = 1, the second arithmetic expression becomes S ′ = S, and the threshold processing is not performed. by this,
A minute line drawing image can be reproduced without being crushed. Also,
When the density change of the line drawing image occurs in 2 pixels, X = n
And the second arithmetic expression becomes equal to the first arithmetic expression,
The same threshold value processing as in the second aspect of the invention is performed. Further, when the density change of the line drawing image occurs between 1 pixel and 2 pixels, X changes linearly between 1 and n, and the threshold value is changed by the second arithmetic expression according to the degree of density change. Processing will be performed.

According to the fifth and eighth aspects of the invention,
The enhancement process and the smoothing process are performed using the density data of the range of the pixel of interest set in accordance with the magnification of the image and the pixels in the vicinity thereof. Therefore, when the enlargement ratio increases,
Since the range of pixels in the vicinity of the pixel of interest becomes large, the emphasis process and the smoothing process are performed more appropriately. Further, since the threshold value processing is performed for the smoothing processing, the range of pixels having the density data of the intermediate gradation is reduced to one pixel. Therefore, even when the image is enlarged, the contour of the enlarged line drawing image becomes clear, and a high quality line drawing image can be obtained.

[0020]

Embodiments of an image processing apparatus to which an image processing method according to the present invention is applied will be described below with reference to the drawings. FIG. 1 is a block diagram showing the main part of the configuration of the first embodiment.

This image processing apparatus is provided with an image reading section 1, an enlargement processing section 2, an image processing section 3 and the like, and a magnification setting key 5, an image selection key 6 and the like are provided at appropriate places on the surface of the apparatus body. Has been done. The magnification setting key 5 is used to set the enlargement ratio for the original document, and is, for example, 100% (normal size) to 4
It can be set arbitrarily within the range of 00% (4 times).
The image selection key 6 includes a selection key for selecting a line drawing image such as a character, a halftone dot image, and a photographic image, and the operator selects the key according to the type of the original image.

The image reading section 1 is provided with a light receiving section, an image memory and the like, and reads and stores an image signal having the pixel density of the original image. The light receiving section is configured by arranging a large number of light receiving elements, such as CCDs, arranged in one row. Then, while each light receiving element scans the document (main scanning), it moves in the direction orthogonal to the light receiving element row and performs sub scanning, so that the light receiving signal of a level corresponding to the density of each pixel corresponding to each light receiving element However, it can be obtained over the entire surface of the original.

The image memory stores the light receiving signal level of each pixel obtained by the light receiving section, and has a capacity capable of storing the image signal for all pixels of the original image. In addition,
When the light-receiving signal level is 0, it is white, and when it is 1, it is black, and the interval is read with 256 (8-bit) gradations, for example.

The enlargement processing section 2 enlarges the original image at the enlargement magnification set by the magnification setting key 5. The obtained image signal is sequentially sent to the line memory 4. The line memory 4 is a buffer for storing the image signals to be processed by the image processing unit 3, that is, the pixel density of the pixel of interest and its vicinity and sending it to the image processing unit 3. In this embodiment, as the line memory 4, a line memory capable of storing image signals of three lines in parallel in the sub-scanning direction is adopted.

The image processing unit 3 is composed of an emphasis processing unit 31, a multiplication processing unit 32, a smoothing processing unit 33, a threshold value processing unit 34, an addition processing unit 35, etc., and each processing unit is a shift register,
It is composed of logic circuits such as an adder, a multiplier, and a divider. This image processing unit 3 is to obtain a high-quality image,
The image processing is performed on the image signal stored in the line memory 4.

Image signals of three lines centering on the line of the pixel of interest are sequentially input from the line memory 4 to the emphasis processing unit 31 in accordance with a clock signal generated by a clock generation unit (not shown). ing. Similarly, the image signals of the line of the pixel of interest are sequentially input to the smoothing processing unit 33.

The emphasis processing section 31 is 3 × centered on the pixel of interest.
The pixel density in the range of 3 is emphasized by Laplacian or the like. Laplacian is, for example, the number 1 or the number 2
Is used, and by convolving the matrix and the image signal of the original,
The emphasis process is performed.

[0028]

[Equation 1]

[0029]

[Equation 2]

The smoothing processing unit 33 makes 1 × around the pixel of interest.
The pixel density in the range of 3 is subjected to smoothing processing, and for example, a matrix as shown in Expression 3 is used. When Equation 3 is used, the smoothing process uses the average value of the pixel densities in the range of 1 × 3 centering on the target pixel as the target pixel density.

[0031]

(Equation 3)

The threshold processing unit 34 performs threshold processing on the value obtained by the smoothing processing by using the equation (4). However,
In Expression 4, the value obtained by the smoothing process is S, the value obtained by this threshold process is S ′, and the enlargement ratio of the image is n.

[0033]

[Equation 4]

This threshold value processing is S '= when S'≤0.
When 0 (white pixel) and 1 ≦ S ′, S ′ = 1 (black pixel) is set, and in the range of 0 <S ′ <1 between them, the density of the intermediate gradation is linearly interpolated.

Therefore, for example, when n = 2, the equation 4 is S ′ = 3S−1, so that S ′ = 1/3, S ′ =
In the case of 0, 2/3 ≦ S, S ′ = 1, and in the case of 1/3 <S <2/3, linear interpolation is performed in the range of 0 <S ′ <1 and the intermediate gradation is displayed.

Further, for example, when n = 4, the equation 4 is S '.
= 7S−3, S ′ = 0,4 when S ≦ 3/7.
/ 7 ≦ S, S ′ = 1, and 3/7 <S <4/7, 0.
Interpolation is linearly performed in the range of <S '<1, and the gray scale is represented.

The threshold processing unit 34 uses the image selection key 6
If the image selected in step 1 is not a line drawing image, n = 1. In this case, Equation 4 is S ′ = S, and threshold processing is not performed. That is, in the case of a halftone dot image or a photographic image, by using the output value of the smoothing processing unit 33, a high quality image can be obtained.

The first multiplication processing unit 321 includes the emphasis processing unit 3.
The value obtained in 1 is multiplied by the coefficient M. The second multiplication processing unit 322 multiplies the value obtained by the threshold processing unit 34 by the coefficient N. The values of the coefficients M and N are determined by the type of image selected by the image selection key 6, and in the case of a line drawing image, M = N, and M = N.
= 0.6, the best line drawing image is obtained. In the case of photographic images and halftone images, reduce M,
Increasing N gives good images.

The addition processing unit 35 is the first multiplication processing unit 32.
1 and the values obtained by the second multiplication processing unit 322 are added,
This added value is output to the image forming unit 7 as the image density of the pixel of interest. The image forming unit 7 is composed of a laser emitting unit, a photoconductor, and the like, and forms an image based on image density data.

As described above, when the threshold value processing is performed on the smoothed value, the range of pixels represented by the intermediate gradation is shortened. Therefore, even when the image is enlarged, a high-quality image having a sharp contour with no jaggies is obtained. Can be obtained. Also,
The edges of lines such as letters can be reproduced neatly. In the case of a photographic image other than a line drawing image or a halftone dot image, high-quality images can be obtained by not performing threshold processing.

The same effect can be obtained even if the matrix size for the emphasis processing is set to 5 × 5, but a sufficiently high-quality image is obtained with 3 × 3. The same effect can be obtained even when the matrix size for performing the smoothing process is set to 1 × 3 or 3 × 5.

Next, a second embodiment of the image processing apparatus to which the image processing method according to the present invention is applied will be described. Second
In the embodiment, the range of the pixel of interest and the pixels in the vicinity thereof, that is, the matrix size for performing the emphasis process and the smoothing process is set according to the enlargement ratio.

The emphasis process is performed 3 × with the pixel of interest at the center.
This is performed for the range of (2n + 1). Laplacian is 1
In the case of <n ≦ 2, for example, a matrix as shown in Expressions 5 and 6, and in the case of <n ≦ 4, a matrix as shown in Expressions 7 and 8 is used.

[0044]

(Equation 5)

[0045]

(Equation 6)

[0046]

(Equation 7)

[0047]

(Equation 8)

Further, the smoothing process is carried out in the range of 1 × (2n−1) centering on the pixel of interest. In the case of 2 <n ≦ 3, for example, the matrix shown in Expression 10 is used in the case of 3 <n ≦ 4.

[0049]

[Equation 9]

[0050]

[Equation 10]

The threshold value processing when the enlargement ratio n = 4 will be qualitatively described with reference to FIG. In the original image, as shown in FIG. 2, at the black-and-white boundary of the line drawing image, that is, at the edge portion of the character or line, one pixel is represented by an intermediate gradation in order to improve the image quality. . At this time, when the image is magnified 4 times, as shown in FIG. 2, since the range of the intermediate gradation is 4 pixels, the outline of the character or line is blurred. On the other hand, when the smoothing process is performed on the range of 1 × (2n−1), that is, 1 × 7 pixels, the range of the intermediate gradation becomes 7 pixels as shown in FIG. The outline of the line will be more blurred.

Therefore, when threshold processing is performed, S '= 0 when S≤3 / 7, S' = 1,3 / 7 <S when S / 4 = 0.
In the case of <4/7, the interpolation is performed linearly in the range of 0 <S ′ <1 and is represented by the intermediate gradation, so that the range of the intermediate gradation is returned to one pixel as shown in FIG. Therefore, it is possible to obtain a high-quality image in which the outlines of characters and lines are not blurred.

Thus, by setting the range of the pixel of interest and the pixels in the vicinity thereof, that is, the matrix size for performing the emphasizing process and the smoothing process, in accordance with the enlargement ratio,
It is possible to obtain a high-quality line drawing image with a sharper contour regardless of the enlargement ratio n of the image.

The line memory 4 may have a capacity of 3 × 9 if the maximum enlargement ratio of the image processing apparatus is n = 4. Therefore, a high-quality line drawing image can be obtained with a small number of lines.

Next, a third embodiment of the image processing apparatus to which the image processing method according to the present invention is applied will be described. FIG.
FIG. 9 is a block diagram showing a main part of the configuration of the third embodiment. The same parts as those in the first embodiment are designated by the same reference numerals.

The minute line detecting section 36 detects how many pixels the density change of the line drawing image of the document occurs. For the density of the pixel of interest of the original image, for example, a matrix as shown in Expressions 11 and 12 is used, or Expression 13 is used.
Detecting is performed by convolving the target pixel of the document image with the pixel density of the range in the vicinity thereof using a matrix as shown in FIG. Thus, the density of each pixel is detected, and the densities of adjacent pixels are compared to detect how many pixels the density change occurs.

[0057]

[Equation 11]

[0058]

(Equation 12)

[0059]

(Equation 13)

In this case, the line memory 4 is 5 × 5.
It suffices if it has a capacity of. However, similarly to the first embodiment, the emphasis processing is 3 × 3 and the smoothing processing is 1 × 3.

The threshold processing unit 37 performs threshold processing on the value obtained by the smoothing processing by using the equation (14). However, in Expression 14, the value obtained by the smoothing process is S, the value obtained by this threshold process is S ′, and the enlargement ratio of the image is n.

[0062]

[Equation 14]

This threshold value processing is the same as in the first embodiment.
When S ′ ≦ 0, S ′ = 0 (white pixel), when 1 ≦ S ′, S ′ = 1 (black pixel), and in the range of 0 <S ′ <1 between them, linear interpolation is performed. This is the density of the gradation.

The X in the equation 14 is 1 if the minute line detection unit 36 detects that the density change occurs for each pixel, and it is n if it is 2 pixels or more, and the density change is 1
If it occurs in the range of pixels to 2 pixels, it is linearly changed between 1 and n.

In Equation 14, if X = 1, then S '=
It becomes S. Therefore, when the density change occurs for each pixel, the threshold processing is not performed. On the other hand, when X = n in Expression 14, Expression 4 is obtained. Therefore, if the density change is 2 pixels or more, the threshold processing similar to that in the first embodiment is performed.

If the density change is every 1.5 pixels and the enlargement ratio n = 2, X = 1.5. On the other hand, if the density change is every 1.5 pixels and the enlargement ratio n = 4,
Let X = 2.5.

As described above, according to the third embodiment, since the minute line detecting section 36 is provided and the threshold processing section 37 performs the threshold processing according to the number of pixels in which the density change occurs,
It is possible to reproduce a line drawing image without breaking the space between lines. Therefore, a high-quality image can be obtained.

In the third embodiment as well, as in the second embodiment, the matrix size for carrying out the emphasizing process and the smoothing process may be set in accordance with the enlargement magnification. That is, the emphasis processing is performed with 3 × (2n
The +1) range, and the smoothing process is performed for the 1 × (2n−1) range centered on the pixel of interest. Further, regarding the matrix size of the line memory 4, the maximum enlargement ratio is n =
If it is 4, the capacity of 5 × 9 may be provided.

In this case, similarly to the second embodiment, a higher quality line drawing image can be obtained regardless of the enlargement ratio n of the image.

The matrix size used in the minute line detector 36 is not limited to 5 × 5, and may be 3 × 3 or 7 × 7. Further, the capacity of the line memory 4 may be set according to its matrix size.

[0071]

As described above, the first, second, and sixth aspects are provided.
According to the invention, when the line drawing image is enlarged, the smoothing process is performed on the density data of the target pixel and the pixels in the vicinity thereof by using the first arithmetic expression set according to the enlargement ratio of the image. Since the threshold value processing is performed on the obtained value and the value obtained by this threshold processing is multiplied by the second coefficient, it is possible to obtain a high-quality line drawing image with a clear contour without jaggies. You can

According to the inventions of claims 3, 4, and 7,
When the density change of the line drawing image occurs in less than 2 pixels, the threshold processing is performed by the second setting which is set according to the enlargement ratio of the image.
Since it is performed by using the arithmetic expression of, it is possible to reproduce a line drawing image of a minute line drawing image without being crushed between the lines. Therefore, a high-quality image can be obtained.

According to the fifth and eighth aspects of the invention, the range of pixels in the vicinity of the pixel of interest is set according to the enlargement ratio, so that the contour is more clearly defined regardless of the enlargement ratio of the image. A high-quality line drawing image can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing a main part of a configuration of a first embodiment of an image processing apparatus to which the present invention is applied.

FIG. 2 is an explanatory diagram of threshold processing.

FIG. 3 is a block diagram showing a main part of a configuration of a third embodiment of an image processing apparatus to which the present invention is applied.

FIG. 4 is a block diagram for explaining a conventional image processing method.

FIG. 5 is a block diagram for explaining a conventional image processing apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 image reading unit 2 enlargement processing unit 3 image processing unit 31 enhancement processing unit 321 first multiplication processing unit 322 second multiplication processing unit 33 smoothing processing unit 34, 37 threshold processing unit 35 addition processing unit 36 minute line detection unit 4 Line memory 5 Magnification setting key 6 Image selection key 7 Image forming section

Claims (8)

[Claims] 1. The enhancement data is applied to the density data of the pixel of interest and its neighboring pixels, and the smoothing process is applied to the density data, so that the values obtained by both of the processings are first and second values, respectively. In the image processing method of multiplying the coefficient of 2 and outputting the value obtained by adding the multiplication result as the density data of the pixel of interest, when the line drawing image is enlarged, the first set according to the enlargement ratio of the image. Using the arithmetic expression of, the threshold value processing is performed on the value obtained by performing the smoothing processing,
An image processing method, wherein the value obtained by the threshold processing is multiplied by the second coefficient. 2. The first arithmetic expression is S ′ = (2n−1), where S is a value obtained by the smoothing processing, S ′ is a value obtained by the threshold processing, and n is an image enlargement ratio. ) S- (n
The image processing method according to claim 1, wherein the image processing method is -1). 3. The image processing method according to claim 1, wherein
When the density change of the line drawing image occurs in less than 2 pixels, the threshold processing is performed by the second setting which is set according to the enlargement ratio of the image.
An image processing method characterized in that the image processing method is performed by using the arithmetic expression. 4. The second arithmetic expression is S ′ = {(2n−, where S is a value obtained by the smoothing process, S ′ is a value obtained by the threshold process, and n is an image enlargement ratio. 1) S-
(X-1)} / (2n-2X + 1) and X
Is 1 when the density change of the line drawing image occurs for each pixel, and n when the density change occurs for 1 pixel or more.
4. The image processing method according to claim 3, wherein when it occurs between pixels, it is linearly changed between 1 and n. 5. The image processing method according to claim 1, wherein a range of pixels in the vicinity of the pixel of interest is set according to the enlargement ratio. 6. An enhancement processing means for performing enhancement processing on density data of a pixel of interest and pixels in the vicinity thereof, a smoothing processing means for performing smoothing processing on the density data, and an output value of the enhancement processing means. The first multiplication means for multiplying the first coefficient, the second multiplication means for multiplying the output value of the smoothing processing means by the second coefficient, and the output values of the first and second multiplication means are added. In an image processing apparatus including an addition processing unit that outputs a value as the density data of the pixel of interest, threshold value processing is performed using a first arithmetic expression that is set for the output value of the smoothing unit according to the enlargement ratio of the image. An image processing apparatus comprising: a threshold processing unit that performs a calculation, wherein the second multiplication unit multiplies a value obtained by the threshold processing calculation by a second coefficient. 7. The image processing apparatus according to claim 6,
A minute line detecting means for detecting the number of pixels in which the density change of the line drawing image occurs is provided, and the threshold processing means performs the threshold processing operation using the second arithmetic expression set according to the enlargement ratio of the image. An image processing device characterized by being present. 8. The image processing apparatus according to claim 6, wherein a range of pixels in the vicinity of the target pixel is set according to the enlargement ratio.