JP2001298613A - Image processor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image processor which stably discriminates a character area regardless of change of the variable magnification of an image or contents of density correction. SOLUTION: A color image processor 1 is comprised of an A/D conversion part 11, a shading correction part 12, an input gradation correction part 13, a color correction part 14, an ink generating under-color removal part 15, a zoom processing part 16, a space filter part 17, a halftone generation part 18, and a character edge discrimination part 20, and the character edge discrimination part 20 is comprised of a line memory 21, edge detection parts 22C, 22M, and 22Y, character discrimination parts 23C, 23M, and 23Y, and a threshold setting part 26. And the threshold setting part 26 linearly calculates threshold Thmax, Thmin, and Thdif on the basis of a prescribed reference value in accordance with change of a variable magnification Z in the zoom processing part 16.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus which is applied to a digital color copying machine, a facsimile, and the like, and detects a character area of a text-photograph mixed document.

[0002]

2. Description of the Related Art In recent years, digitalization of OA equipment has been progressing,
In addition, demand for color image output is increasing, and digital color copying machines and color printers are becoming widespread. In these image forming apparatuses, image processing technology occupies an important position in realizing higher image quality and higher functionality. For example, in a digital color copier, an area separation process is performed on a document containing text and photographs, and based on the result of the separation, the character region is edge-emphasized and the photographic region is smoothed to improve the output image. I do.

As a method of detecting the edge of a character area,
There is known a method of calculating a feature amount around an edge pixel and performing a threshold process on the result to determine whether the pixel belongs to a character area. In this case, the threshold used in the threshold processing is a preset fixed value.

[0004]

An image obtained by scanning a document is often input to the image processing unit at the same magnification. However, when the document is scanned while changing the moving speed of the line sensor, the image is scanned in the sub-scanning direction. To obtain a scaled image. If such a scaled image is input to the image processing unit and threshold processing of the same-size image is executed as it is, an appropriate determination result cannot be obtained.

As a countermeasure against this, for example, Japanese Patent Application Laid-Open No. H11-5
In No. 8875, a maximum density difference is calculated as a feature amount around an image, and whether or not the area is a character area is determined by performing threshold processing on the maximum density difference. At that time, the threshold is switched according to the magnification of the image. I have. As the threshold value switching method, the scaling ratio is set to 50 to 70%, 70 to 100%, 1: 1, and 100 to 14.
It is divided into five stages of 0% and 140% or more, one threshold is set corresponding to each stage, and a threshold according to the magnification is selected.

In such a method, a large number of thresholds must be prepared in accordance with the number of variable magnifications, and the thresholds may change stepwise only by a 1% difference in the magnification. The judgment result fluctuates significantly according to the magnitude of the magnification.

On the other hand, a scanned image of a document may be input to an image processing unit after some density correction.
The density correction converts an image density value using a density correction curve or the like, and performs, for example, background removal, exposure adjustment, image quality adjustment, and the like. If threshold processing using a fixed threshold is performed on the image after such density correction as it is, an appropriate determination result cannot be obtained.

As a countermeasure against this, Japanese Patent Laid-Open Publication No.
No. 875, when threshold processing is performed on the maximum density difference, the threshold value is switched according to the equal magnification of the image and the plate making density setting.
As a threshold switching method, one fixed threshold is set for each of a plurality of types of input density correction.

In such a method, a large number of thresholds must be prepared in accordance with the number of types of density correction. If the types of density correction are grouped and one group is represented by one threshold, the boundary of the density correction group is determined. , The threshold value may change in a stepwise manner, and the determination result of the character area may fluctuate significantly depending on the type of density correction.

An object of the present invention is to provide an image processing apparatus capable of performing stable character area determination even when the magnification of an image changes.

It is another object of the present invention to provide an image processing apparatus capable of performing stable character area determination even when the content of density correction changes.

[0012]

SUMMARY OF THE INVENTION The present invention provides a reading means for scanning a document to read an image, a scaling means for scaling the read image at a predetermined magnification, and a feature amount in a predetermined area of the image. A feature value calculating means for calculating, a threshold value calculating means for calculating a threshold value according to the scaling factor, and an edge extracting means for comparing the calculated feature value with the calculated threshold value and extracting a character edge portion. It is an image processing apparatus characterized by the following.

According to the present invention, an appropriate character edge extraction result can be obtained even at the time of image scaling by calculating the threshold value in accordance with the scaling ratio of the image. Further, unlike the related art, it is not necessary to prepare a plurality of fixed thresholds according to the division of the magnification ratio, and the character extraction processing is simplified.

Further, the present invention is characterized in that the characteristic amount calculating means calculates a maximum density, a minimum density, and a maximum density difference in a predetermined area as a characteristic amount.

According to the present invention, a more appropriate character edge extraction result can be obtained by considering not only the maximum density difference but also the maximum density and the minimum density as feature amounts.

Further, the present invention is characterized in that the threshold value calculating means linearly calculates a threshold value according to a magnification based on a predetermined reference value.

According to the present invention, since the threshold value changes continuously according to the magnification of the image, a stable character edge extraction result can be obtained regardless of the magnitude of the magnification. Therefore, it is possible to prevent a problem that the threshold value changes in a stepwise manner even when the magnification changes slightly as in the related art, and the character edge extraction result fluctuates significantly.

Further, according to the present invention, the predetermined reference value is such that the scaling factor is one.
Is a threshold value when

According to the present invention, one threshold value is set for the same-size image that is frequently used, and when the scaling ratio is other than 1, a threshold value that is linearly calculated based on this threshold value is used to reduce the Various thresholds can be generated from the data.

Further, in the present invention, the threshold value calculating means includes a threshold value THmax for a maximum density, a threshold value THmin for a minimum density,
Calculate the threshold THdif for the maximum density difference, and set each threshold to THd
It is characterized by satisfying the relationship of if ≧ (THmax−THmin).

According to the present invention, character edge extraction can be performed by effectively utilizing the characteristic amount of the maximum density difference at any magnification.

Also, in the present invention, the threshold value calculating means includes a threshold value THmax for a maximum density, a threshold value THmin for a minimum density,
The threshold value THdif for the maximum density difference is calculated by a linear function that changes according to the magnification, and the change rate C of the threshold value THdif is the change rate A of the threshold value THmax and the change rate of the threshold value THmin.
It is within the range determined based on B.

According to the present invention, when the range in which the magnification changes is limited, character edge extraction can be performed by effectively utilizing the characteristic amount of the maximum density difference.

According to the present invention, there is provided a reading means for scanning a document to read an image, a density correcting means for correcting a density value of the read image, a feature calculating means for calculating a feature in a predetermined area of the image, An image processing apparatus comprising: a threshold calculating unit that calculates a threshold according to a density correction; and an edge extracting unit that compares a calculated feature amount with a calculated threshold to extract a character edge portion. .

According to the present invention, by calculating the threshold value in accordance with the density correction of the image, an appropriate character edge extraction result can be obtained for the image whose density has been corrected. Further, unlike the related art, it is not necessary to prepare a plurality of fixed threshold values corresponding to a plurality of types of density correction, and the character extraction processing can be simplified.

Further, the present invention is characterized in that the feature amount calculating means calculates a maximum density, a minimum density, and a maximum density difference in a predetermined area as a feature amount.

According to the present invention, a more appropriate character edge extraction result can be obtained by considering not only the maximum density difference but also the maximum density and the minimum density as feature amounts.

Further, the invention is characterized in that the threshold value calculating means calculates a value obtained by performing density correction on a predetermined reference value as a threshold value.

According to the present invention, a threshold value can be calculated by performing processing equivalent to the density correction performed on an image to a predetermined reference value, so that a stable character edge extraction result can be obtained. Therefore, it is possible to prevent a problem that the threshold value changes stepwise even if the type of the density correction slightly changes as in the related art, and the character edge extraction result fluctuates significantly.

Further, the present invention is characterized in that the predetermined reference value is a threshold value when the density correction is linear.

According to the present invention, one threshold value is set for linear density correction which is frequently used, and in the case of non-linear density correction, a threshold value calculated based on this threshold value is used, so that various data can be obtained with a small amount of data. Threshold can be generated.

Also, in the present invention, the threshold value calculating means includes a threshold value THmax for a maximum density, a threshold value THmin for a minimum density,
Calculate the threshold THdif for the maximum density difference, and set each threshold to THd
It is characterized by satisfying the relationship of if ≧ (THmax−THmin).

According to the present invention, the threshold value THmax for the maximum density and the threshold value T for the minimum density correspond to the density correction.
Hmin can be varied. In addition, character edge extraction can be performed by effectively utilizing the characteristic amount of the maximum density difference in any density correction.

[0034]

FIG. 1 is a block diagram showing an embodiment of the present invention. The color image processing apparatus 1 has an A /
D (analog-digital) conversion unit 11, shading correction unit 12, input gradation correction unit 13, color correction unit
And a black generation and under color removal unit 15, a zoom processing unit 16, a spatial filter unit 17, a halftone generation unit 18, a character edge determination unit 20, and the like.

A color image input device 2 is connected to a stage preceding the color image processing device 1, and a color image output device 3 is connected to a stage subsequent thereto, so that a digital color copying machine or the like can be constituted as a whole.

The color image input device 2 is composed of, for example, a scanner equipped with a color CCD (Charge Coupled Device) line sensor, and converts the light reflected from the original into RG.
The signal is converted into an analog signal that has been color-separated into B (R: red, G: green, B: blue).

The A / D converter 11 converts an analog signal input from the color image input device 2 into an RGB digital signal. The shading correction unit 12 removes various types of distortion generated in the illumination system, the imaging system, the imaging system, and the like of the color image input device 2.

The input tone correction section 13 converts the RGB signals into density signals suitable for image processing, further performs density correction processing by tone conversion for each color, and outputs CMY (C: cyan, M:
Magenta, Y: yellow) signals. In the density correction processing, a gamma curve or a gamma table indicating the relationship between the density values before and after the correction is used. A plurality of gamma curves or gamma tables are set in advance, and when a desired density correction process is selected, a gamma curve or a gamma table according to the process is selected.

When the spectral characteristics of the CMY color materials used in the color image output device 3 have unnecessary absorption components, the color correction unit 14 performs processing for removing color turbidity and improving the color reproducibility of the CMY signals. The black generation and under color removal unit 15 calculates the C
A K (black) signal is generated from the MY signal and the original C
A new CMY signal is generated by subtracting the K signal from the MY signal.

The character edge judging section 20 extracts a character edge portion included in the color-corrected image, divides it into a character edge area and other areas, and outputs it as a character edge judging signal to the subsequent spatial filter section 17. I do.

The zoom processing section 16 scales an input image having M pixels in the main scanning direction and N pixels in the sub-scanning direction at a predetermined scaling factor Z to generate an image of MZ pixels and NZ pixels. In the case where the original is scanned, the zoom processing unit changes the speed in the sub-scanning direction by scanning while changing the moving speed of the line sensor.

The spatial filter unit 17 determines whether or not the character edge region is to be detected based on the character edge determination signal from the character edge determination unit 20 in order to enhance the reproducibility of the character when the document includes both characters and photographs. Enhancement processing is performed, and smoothing processing is performed on other areas.

The halftone generator 18 performs a tone reproduction process for each CMYK signal. The color image output device 3 is configured by a color printer such as an electrophotographic process or an ink jet, and performs color recording on paper based on a CMYK signal from the halftone generation unit 18.

FIG. 2 is a block diagram showing the structure of the character edge judgment section 20. The character edge determination unit 20 includes a line memory 21 and edge detection units 22C, 22M, and 22Y.
And character determination units 23C, 23M, and 23Y, AND circuits 24C, 24M, and 24Y, an OR circuit 25, and a threshold setting unit 26.

The line memory 21 stores the input tone correction unit 13
3 are stored for each of a plurality of lines, and as shown in FIG. 3, a 7 × 7 pixel mask centering on the target pixel is formed, and the edge detecting units 22C, 22M, 22Y and the character determining units 23C, 23M are formed. , 23Y. Here, an example in which a 7 × 7 pixel mask is used will be described, but the present invention can be applied to a mask other than 7 × 7.

The edge detectors 22C, 22M, 22Y
Determine whether the target pixel is an edge of the image by color,
If it is an edge, an edge detection signal EDGE is output.

The character determination units 23C, 23M, and 23Y calculate the maximum density, the minimum density, and the maximum density difference for each of the 49 pixels in the mask as feature amounts, and supply these feature amounts and the threshold value setting unit 26. Threshold for maximum concentration
The threshold value THmax for the minimum density, the threshold value THdif for the maximum density difference, and the threshold value THdif for the maximum density difference are compared to determine whether the pixel of interest is a character area. If the pixel of interest is a character area, a character determination signal TEXT is output.

The AND circuits 24C, 24M, and 24Y calculate the logical product of the edge detection signal EDGE and the character determination signal TEXT for each color, and output the determination signal TEDGE. The OR circuit 25 calculates the logical sum of the color-based determination signals TEDGE output from the AND circuits 24C, 24M, and 24Y, and outputs a final character edge determination signal to the spatial filter unit 17.

The threshold value setting unit 26 sets a threshold value THma according to a change in the magnification ratio Z in the zoom processing unit 16 based on a predetermined reference value.
x, THmin, THdif are calculated linearly.

The edge detectors 22C, 22M, 22
The processing of Y, character determination units 23C, 23M, 23Y and threshold value setting unit 26 will be described in detail.

FIG. 4 shows the edge detectors 22C, 22M, 2
It is a block diagram which shows the structure of 2Y. Line memory 21
As shown in FIG. 5, a 3 × 3 pixel mask centering on the pixel of interest is extracted from the 7 × 7 pixel mask read out from the main scanning sobel filter unit 31m and the sub-scanning sobel filter unit. 31s.

The main scanning sobel filter section 31m performs a convolution operation on the extracted 3 × 3 pixel mask using, for example, a 3 × 3 main scanning sobel filter as shown in FIG. Similarly, the sub-scanning Sobel filter unit 31s applies the extracted 3 × 3 pixel mask to, for example, FIG.
A convolution operation using a 3 × 3 sub-scanning Sobel filter as shown in FIG.

The results of the convolution operation on the target pixel become absolute values by the absolute value calculation units 32m and 32s, respectively, and are further added by the adder 33. Here, the reason for taking the absolute value is that a negative coefficient exists in each Sobel filter, and the reason for adding the values is to consider the results in both the main scanning direction and the sub-scanning direction.

The addition result is compared with a predetermined threshold value edgeTH by the comparator 34. If the addition result is equal to or more than the threshold value edgeTH, the edge detection signal EDGE is set to "1", otherwise, the edge detection signal EDGE is set to "0". I do.

FIG. 8 shows the character determination units 23C, 23M, 23
FIG. 3 is a block diagram showing a configuration of Y. Character determination unit 23C,
Each of 23M and 23Y includes a feature amount calculation unit 40 and a determination circuit 44. In general, the character area includes a case where a high density character exists on a low density background and a case where a low density character exists on a high density background. It has the characteristic of being low. Further, the character region has a feature that the maximum density difference is high because the contrast between the character and the background is high. Therefore, the maximum density, the minimum density, and the maximum density difference around the pixel of interest are calculated as feature amounts, and threshold processing is performed to determine whether the pixel of interest belongs to the character area.

In the feature value calculating section 40, the maximum value detecting section 41 detects the maximum density, the minimum value detecting section 42 detects the minimum density, and the differentiator for the 7 × 7 pixel mask read from the line memory 21. 43 calculates the maximum density difference by subtracting the minimum density from the maximum density.

In the next judgment circuit 44, the comparator 45 compares the maximum density with the threshold value THmax and determines that the maximum density is equal to the threshold value THmax.
When the minimum density is smaller than the threshold value THmin, the comparator 46 outputs “1” when the minimum density is smaller than the threshold value THmin, and the comparator 47 outputs “1” when the minimum density is smaller than the threshold value THmin.
Compared with THdif, if the maximum density difference is greater than the threshold value THdif, "1" is output. These outputs are ANDed by the AND circuit 48, and when "1" is satisfied for all the feature amounts, a character determination signal is output. TEXT is "1".

FIG. 9 is a block diagram showing the configuration of the threshold value setting section 26. The scaling factor Z used in the zoom processing unit 16 is input to each of the subtractors 51a to 51c, and the difference from the scaling factor 1 is calculated. Each difference result is calculated by multipliers 52a to 52
c, and performs a linear operation by multiplying by a predetermined rate of change A, B, C. Each multiplication result is input to each of the adders 53a to 53c, and predetermined reference values Max, Min, and Dif are added, and finally, a threshold value THmax (Z) regarding the maximum density as a linear function that changes according to the scaling factor Z , A threshold THmin (Z) for the minimum density and a threshold THdif (Z) for the maximum density difference. Here, the arithmetic expression of each threshold value can be expressed as the following expressions (1) to (3). THmax (Z) = Max + A · (Z-1) ... (1) THmin (Z) = Min + B · (Z-1) ... (2) THdif (Z) = Dif + C · (Z-1) ... (3)

Here, the scaling factor Z is a scaling factor of the input image, and becomes 1 when the magnification is 100%. Further, the change rates A, B, and C may be equal values or different values, and each threshold value THmax
When (Z), THmin (Z), and THdif (Z) are to be changed in the same manner according to the scaling factor Z, they are set to the same value, and when it is desired to provide different characteristics according to the scaling factor Z, they are set to different values. Further, the threshold value is reduced at the time of reduction (Z <1) and at the time of enlargement (Z>
If the threshold is increased in 1), the rate of change is a positive rate. Conversely, if the threshold is increased during reduction and the threshold is reduced during enlargement, the rate of change is negative.

Further, if each threshold value satisfies the relationship of the following equation (4), character edge extraction can be carried out by effectively utilizing the feature amount of the maximum density difference at any magnification. THdif ≧ (THmax−THmin)… (4)

FIG. 10 is a graph showing the relationship between each threshold value and the scaling factor Z. When the magnification Z is Zp ≦ Z ≦ Zq (Zp <
1, when Zq> 1), the condition for always satisfying the relationship of equation (4) is examined. First, the scaling factors Zp and Zq
If the expression (4) is satisfied in the above, the following expressions (5) and (6) are obtained. THdif (Zp) ≧ (THmax (Zp) −THmin (Zp))… (5) THdif (Zq) ≧ (THmax (Zq) −THmin (Zq))… (6)

Next, when the equations (1) to (3) are substituted into the equations (5) and (6), the following equations (7) and (8) are obtained. (Dif−C · | Zp−1 |) ≧ (Max−A · | Zp−1 |) − (Min−B · | Zp−1 |) C ≦ A−B− (Max−Min−Dif) / | Zp−1 |… (7) (Dif + C · | Zp−1 |) ≧ (Max + A · | Zp−1 |) − (Min + B · | Zp−1 |) C ≦ A−B + (Max−Min−Dif) / | Zp−1 |… (8)

Therefore, the rate of change A of the threshold THmax and the threshold THmin are set so that the relations of the equations (7) and (8) always hold.
It is preferable to select the change rate C of the threshold value THdif within a range determined based on the change rate B of the threshold value. Even if the scaling factor Z changes, the threshold value THdif can be used effectively, and the feature amount of the maximum density difference is effective. The character edge extraction utilized in the method can be performed.

As described above, the threshold value setting unit 26 changes the threshold values THmax, THmin, and THdif in accordance with the scaling factor Z in the zoom processing unit 16, thereby realizing appropriate threshold processing.

Next, the threshold setting unit 26 sets the input tone correction unit 1
Threshold values THmax, THmin, THdif in accordance with the density correction processing in step 3.
An example in which is changed will be described.

FIG. 11 is a block diagram showing a configuration of the threshold value setting unit 26 according to the second embodiment of the present invention. In the present embodiment, the color image processing apparatus 1 of FIG. 1, the character edge determination unit 20 of FIG. 2, and the edge detection units 22C, 22M, and 2 of FIG.
2Y, the configuration is the same as that of the character determination units 23C, 23M, and 23Y in FIG.

The threshold value setting section 26 includes a function processing section 61 using the density correction function γ of the input tone correction section 13, and the density correction function γ is constituted by a gamma curve or a gamma table. When the predetermined reference thresholds Max and Min are input, the function processing unit 61 converts them into γ (Max) and γ (Min), and outputs them as a threshold THmax for the maximum density and a threshold THmin for the minimum density, respectively.

A differentiator 62 calculates a difference between γ (Max) and γ (Min), a differentiator 63 calculates a difference between Max and Min, and
Reference numeral 4 divides the difference result of the differentiator 62 by the difference result of the differentiator 63, and the multiplier 65 multiplies the result by a predetermined reference threshold value Dif and outputs the result as a threshold value THdif relating to the maximum density difference. Here, the arithmetic expression of each threshold can be expressed as the following expressions (11) to (13). THmax = γ (Max)… (11) THmin = γ (Min)… (12) THdif = Dif ｛((γ (Max) −γ (Min)) / (Max−Min)｝… (13)

Here, the reference thresholds Max, Min, and Dif are thresholds set in advance when there is no density correction, that is, when the density correction function γ is linear.

FIG. 12 is a graph showing an example of the density correction function γ. The broken line graph shows the linear characteristic without density correction, and the solid line graph shows the non-linear characteristic for density correction. For example, when the density value before correction is x, the density value after correction is γ (x).

FIG. 13 is a graph showing a method of calculating the threshold values THmax, THmin, and THdif. In the density correction function γ shown in FIG.
1) Output threshold values THmax and THmin according to (12).
Also, according to equation (13), the difference between γ (Max) and γ (Min) and M
ax, by multiplying the ratio with the difference of Min to the reference threshold Dif,
Output the threshold value THdif.

At this time, the threshold values satisfy the following equations (14) and (15), so that character edge extraction can be performed effectively utilizing the characteristic amount of the maximum density difference regardless of the density correction. it can. THmax ≧ THmin… (14) THdif ≧ (THmax−THmin)… (15)

In each of the above embodiments, the method of changing the threshold values THmax, THmin, and THdif according to the scaling factor Z and the method of changing the threshold values THmax, THmin, and THdif according to the density correction function γ have been described. A method of changing the threshold values THmax, THmin, and THdif according to both the scaling factor Z and the density correction function γ is also possible.

[0074]

As described above in detail, according to the present invention, an appropriate character edge extraction result can be obtained even at the time of image scaling by calculating the threshold value according to the image scaling ratio. Further, unlike the related art, it is not necessary to prepare a plurality of fixed thresholds according to the division of the magnification ratio, and the character extraction processing is simplified.

Further, by calculating the threshold value in accordance with the density correction of the image, an appropriate character edge extraction result can be obtained for the image whose density has been corrected. Further, unlike the related art, it is not necessary to prepare a plurality of fixed threshold values corresponding to a plurality of types of density correction, and the character extraction processing can be simplified.

Further, by considering not only the maximum density difference but also the maximum density and the minimum density as the feature amount, a more appropriate character edge extraction result can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a configuration of a character edge determination unit 20.

FIG. 3 is an explanatory diagram showing an example of a 7 × 7 pixel mask centering on a target pixel.

FIG. 4 is a block diagram illustrating a configuration of edge detection units 22C, 22M, and 22Y.

FIG. 5 is an explanatory diagram showing an example of a 3 × 3 pixel mask centered on a target pixel.

FIG. 6 is an explanatory diagram showing an example of a 3 × 3 main scanning Sobel filter.

FIG. 7 is an explanatory diagram illustrating an example of a 3 × 3 sub-scanning sobel filter.

FIG. 8 is a block diagram illustrating a configuration of character determination units 23C, 23M, and 23Y.

FIG. 9 is a block diagram showing a configuration of a threshold setting unit 26.

FIG. 10 is a graph showing a relationship between each threshold value and a scaling factor Z.

FIG. 11 shows a threshold setting unit 2 according to the second embodiment of the present invention.
6 is a block diagram showing a configuration of FIG.

FIG. 12 is a graph showing an example of a density correction function γ.

FIG. 13 is a graph showing a method of calculating threshold values THmax, THmin, and THdif.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Color image processing apparatus 11 A / D conversion part 12 Shading correction part 13 Input gradation correction part 14 Color correction part 15 Black generation and under color removal part 16 Zoom processing part 17 Spatial filter part 18 Halftone generation part 20 Character edge judgment part 21 line memory 22C, 22M, 22Y edge detecting unit 23C, 23M, 23Y character determining unit 26 threshold setting unit

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G06T 7/60 250 B41J 3/00 M 5L096 H04N 1/393 A (72) Inventor Takuya Iwanami Osaka-shi, Osaka 22-22 Nagaikecho, Abeno-ku F-term in Sharp Corporation (reference) 2C262 AA26 AA27 AB07 AB11 AB13 BB03 BB41 BC03 BC07 BC09 BC11 DA03 DA13 DA15 EA07 2C362 CA18 CA24 CB24 CB25 CB29 CB30 CB34 CB37 CB62 5B030 A01 CB08 CC02 CD05 CE03 CE11 CE16 CH11 DA08 DB06 DC16 5C076 AA21 AA22 AA27 BA06 CA10 CB01 5C077 MM16 MP05 MP06 MP08 PP03 PP15 PP20 PP27 PP33 PP47 PP68 PQ12 PQ20 5L096 AA02 AA06 BA17 CA14 EA03 FA06 FA14 FA51 GA51

Claims (11)

[Claims] A scanning unit that scans a document to read an image; a scaling unit that scales the read image at a predetermined scaling factor; a feature amount calculation unit that calculates a feature amount in a predetermined area of the image; An image processing apparatus comprising: a threshold calculating unit that calculates a threshold according to a scaling factor; and an edge extracting unit that compares a calculated feature amount with the calculated threshold to extract a character edge portion. 2. The image processing apparatus according to claim 1, wherein said characteristic amount calculating means calculates a maximum density, a minimum density, and a maximum density difference in a predetermined area as a characteristic amount. 3. The image processing apparatus according to claim 1, wherein said threshold value calculating means linearly calculates a threshold value according to a magnification based on a predetermined reference value. 4. The image processing apparatus according to claim 3, wherein the predetermined reference value is a threshold value when the magnification is 1. 5. The threshold value calculating means calculates a threshold value THmax for a maximum density, a threshold value THmin for a minimum density value, and a threshold value THdif for a maximum density difference, wherein each threshold value satisfies a relationship of THdif ≧ (THmax−THmin). 3. The image processing apparatus according to claim 2, wherein: 6. The threshold value calculating means calculates a threshold value THmax for a maximum density, a threshold value THmin for a minimum density value, and a threshold value THdif for a maximum density difference by a linear function that changes in accordance with a scaling factor. C is the change rate A of the threshold THmax and the threshold
3. The image processing apparatus according to claim 2, wherein the value is within a range determined based on a change rate B of THmin. 7. A reading means for scanning a document to read an image, a density correcting means for correcting a density value of the read image, a feature value calculating means for calculating a feature value in a predetermined area of the image, An image processing apparatus comprising: a threshold calculating unit that calculates a threshold value in accordance with the threshold value; and an edge extracting unit that compares the calculated feature amount with the calculated threshold value to extract a character edge portion. 8. The image processing apparatus according to claim 7, wherein said characteristic amount calculating means calculates a maximum density, a minimum density, and a maximum density difference in a predetermined area as a characteristic amount. 9. The image processing apparatus according to claim 7, wherein said threshold value calculating means calculates a value obtained by performing density correction on a predetermined reference value as a threshold value. 10. The image processing apparatus according to claim 9, wherein the predetermined reference value is a threshold value when the density correction is linear. 11. The threshold calculating means calculates a threshold THmax for a maximum density, a threshold THmin for a minimum density, and a threshold THdif for a maximum density difference, wherein each threshold satisfies a relationship of THdif ≧ (THmax−THmin). 9. The image processing apparatus according to claim 8, wherein: