JPH08116462A - Image processing unit - Google Patents

Abstract

PURPOSE: To discriminate clearly a black character part of an input color image from a half tone part without causing color blotting in the black character part or mis-discrimination of a color character of low saturation to be the black character or void around the black character in a light color background. CONSTITUTION: A 1st black character discrimination section 50 discriminates a black character of an input color image based on a lightness signal L and a saturation signal Cr. A 2nd black character discrimination section 60 discriminates again the black character of the input color image based on a discrimination output D1 of the 1st black character discrimination section 50, a saturation equivalent signal Ct and a black signal K.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention determines a black character portion of an input color image such as an image on a document or an image generated by a computer in a digital color image output system such as a digital color copying machine or a digital color printer. The present invention relates to an image processing device.

[0002]

2. Description of the Related Art Generally, in a digital color copying machine, a black character portion of an input color image, which is an image on the original, is discriminated and determined from an input color image signal obtained by optically reading the original, and a color image is obtained. With respect to the signal, in the black character portion of the input color image, processing such as enhancing edges and increasing contrast is performed, and in the halftone portion, processing such as smoothing edges and sufficiently reproducing halftones is performed. An output color image is formed on the sheet based on the processed color image signal.

In this case, the determination of the black character portion of the input color image in the image processing apparatus (image processing section) has been conventionally performed.
This is done by some single color representation signal for the input color image.

FIG. 10 shows an example of such a conventional digital color copying machine. The copying machine as a whole has an image reading device (image input device) 100, an image output device (image recording device) 200, and between them. Image processing device.

In the image output device 200, four color toner images of yellow, magenta, cyan and black (black) are sequentially transferred to form an output color image on one sheet of paper, and therefore the image reading device 100 is formed. Then, the document is scanned four times by the optical scanner, and the image reading device 10
From 0, input color image signals composed of red, green, and blue color signals R, G, and B each consisting of 8-bit digital data are repeatedly obtained for four planes.

The red, green, and blue color signals R, G, B from the image reading device 100 are supplied to a shading correction circuit 11, and CCDs for red, green, and blue color light in the image reading device 100 are supplied. Correction for uneven illuminance in the image sensor, variation in sensitivity between pixels inside the sensor, and the like are performed.

After the correction, the red, green, and blue color signals R, G,
The lightness scale conversion circuit 12 converts B into a lightness scale signal Lr, Lg, Lb from the reflectance signal on the original.

The signals Lr, Lg, Lb of the brightness scale
Is converted by the color conversion circuit 13 into a lightness signal L and a chromaticity signal a,
b. The lightness signal L and the chromaticity signals a and b are
L ^* a ^* b ^* It is a color expression signal on a uniform color space, but * is omitted for convenience. The chromaticity signals a and b are converted by the hue / saturation conversion circuit 14 into the hue signal Hu and the saturation signal Cr.
Is converted to.

Then, according to the lightness signal L from the conversion circuit 13 and the saturation signal Cr from the hue / saturation conversion circuit 14,
The black character determination unit 20 determines the black character portion of the input color image as described later.

The lightness signal L from the color conversion circuit 13 and the hue signal Hu and the saturation signal Cr from the hue / saturation conversion circuit 14
Is supplied to the color adjustment circuit 15 and subjected to processing such as color adjustment. Of the processed lightness signal L ′, hue signal Hu ′, and saturation signal Cr ′, the hue signal Hu ′ and the saturation signal Cr ′ are the chromaticity from the color conversion circuit 13 by the chromaticity conversion circuit 16. Similar to the signals a and b, they are converted into chromaticity signals a ′ and b ′ expressed on the a ^* and b ^* axes.

The lightness signal L'from the color adjusting circuit 15 and the chromaticity signals a'and b'from the chromaticity converting circuit 16 are converted by the color converting circuit 17 into yellow, magenta and cyan color signals Y'and M '. , C ′. The yellow, magenta, and cyan color signals Y ′, M ′, and C ′ are supplied to the lower color removal circuit 18 to generate the black signal K from the color signals Y ′, M ′, and C ′, and Color-removed color signals Y, M, and C of yellow, magenta, and cyan are obtained.

The black signal K and the color signals Y, M and C are
The color selection circuit 19 sequentially selects one by one in accordance with the scanning cycle of the image reading apparatus 100 and the development cycle of the image output apparatus 200.

In the switching circuit 31, the black output K0 or the color signals Y, M, C from the color selection circuit 19 is output by the determination output D0 of the black character determination unit 20 in the area which is not determined as the black character portion of the input color image. On the other hand, in the area determined to be the black character portion of the input color image, the lightness signal L ′ from the color adjusting circuit 15 is extracted as the black signal K in that area. This is because, in the black thin line portion of the input color image, the image reading apparatus 100 has a pixel shift between the CCD image sensors for red, green and blue color lights in the image reading apparatus 100.
This is a measure against the case where only the low level black signal K may be generated in the lower color removal circuit 18 due to the color shift at the time of reading the original document.

The black signal K or the color signals Y, M, C from the switching circuit 31 is determined in the filter circuit 32 based on the determination output D0 of the black character determination unit 20 as the area determined to be the black character portion of the input color image. Image sharpness is increased, and the image is smoothed in a region that is not determined to be a black character portion.

Further, the black signal K from the filter circuit 32
Alternatively, the color signals Y, M, and C are output by the gradation reproduction control circuit 33 based on the determination output D0 of the black character determination unit 20.
The contrast of the image is increased in the area determined to be the black character portion of the input color image, and the halftone is sufficiently processed in the area not determined to be the black character portion. The tone reproduction control circuit 33 also performs color balance adjustment and the like, and the black signal K or the color signals Y, M, and C from the tone reproduction control circuit 33 are output to the image output device 200.

Although not shown in the figure, the black character determination unit 20 determines the character portion of the input color image from the lightness signal L by pattern matching in a window of 5 × 5 pixels, and further determines that character portion. A portion of the determined area where the saturation signal Cr is equal to or lower than a predetermined threshold value is determined to be a black character portion.

In this case, the input color image shown in FIG.
As shown in (A), when the black character portion 1k exists in the background 1w of itself and others, due to the color shift in the image reading apparatus 100 described above, the input color image signal from the image reading apparatus 100 is shown in FIG. ) (The figure shows saturation,
The same side 1 of the achromatic gray signal 2k that has lower saturation toward the upper side
A chromatic color signal 2c such as a red or blue color signal is generated in a portion about the pixel width.

Even in such a case, the white background 1w
The black character determination unit 20 outputs the saturation signal C so that the black character portion 1k in the inside is clearly output in black without causing color fringing.
The threshold value Co for r is set to a relatively high saturation side as shown in FIG. 11B, and the portion of the chromatic color signal 2c is also determined to be a black character portion.

Alternatively, by setting the threshold value Co to the low saturation side, it is determined that the portion of the chromatic color signal 2c is not a black character portion, and the background 1w on a white background becomes clear without any residual color. As for the output of white, a portion of the gray signal 2k which is determined to be a black character portion and has a width of about 1 pixel around the gray signal 2k is processed to be white when the saturation of the portion is equal to or lower than a predetermined level.

[0020]

However, since the above-mentioned conventional image processing apparatus discriminates and determines the black character portion of the input color image by the single color expression signal for the output color image, the color blurring in the black character portion occurs. The black character part is identified as the halftone part without causing any inconvenience such as the occurrence or misjudgment of the low saturation color character part as the black character part, or the surrounding part of the black character part in the light background is outlined. However, there is a drawback that it is difficult to make a clear decision.

That is, in the conventional image processing apparatus, the image reading apparatus 100 causes a color shift as shown in FIG.
The portion of the chromatic color signal 2c as shown in FIG. 11 is also determined as the black character portion, and the black character portion 1k in the white background 1w as shown in FIG. 11A is output clearly in black without causing color bleeding. As described above, the saturation signal Cr in the black character determination unit 20
When the threshold Co for is set to a relatively high saturation side, when the low saturation color character portion 1c exists in the white background 1w in the input color image, as shown in FIG. As shown in FIG. 11D (the same figure also shows saturation, the lower the saturation is, the chromatic signal 3c in the color character portion 1c exceeds the threshold Co toward the low saturation side. There is a possibility that the low-saturation color character portion 1c is determined to be a black character portion.

Further, by setting the threshold value Co to the low saturation side, it is determined that the portion of the chromatic color signal 2c is not a black character portion, and the white background 1w is clearly white without any color residue. In the case where the peripheral portion of the gray signal 2k, which is determined to be a black character portion, has a width of about 1 pixel, the saturation of the portion is equal to or lower than a predetermined level At FIG.
As shown in FIG. 12A, when the black character portion 4k exists in the light-colored background 4c, as shown in FIG. 12B in the input color image signal from the image reading apparatus 100 (the same figure also shows saturation, As shown in FIG. 12C in the output color image signal, a portion of the chromatic color signal 5c having a pixel width around the gray signal 5k in the chromatic color signal 5c (which also shows the saturation, It is a white signal 6w with lower saturation toward the upper side,
That is, in the output color image signal, the white signal 6w is generated between the gray signal 6k and the chromatic color signal 6c, and in the output color image, the peripheral portion of the black character portion in the light color background may be blank.

Therefore, according to the present invention, a color blur occurs in a black character portion, or a low saturation color character portion is erroneously determined to be a black character portion, or a peripheral portion of the black character portion in a light-colored background becomes blank. It is possible to distinguish the black character portion of the input color image from the halftone portion and to make a clear determination without causing the above disadvantage.

[0024]

According to the present invention, there is provided a first black character determining unit for determining a black character portion of an input color image from a first color expression signal for the input color image, and the first black character determining unit for the input color image. Second color signal different from
And a second black character determination unit that re-determines the black character portion of the input color image from the color expression signal and the determination output of the first black character determination unit.

[0025]

In the image processing apparatus of the present invention configured as described above, for example, the first color expression signal is a signal expressed in the Lab color space which is one of the color expression signals of the lightness / chromaticity system. , The black character portion of the input color image is determined by the first black character determining portion from the lightness signal L and the saturation signal Cr obtained from the chromaticity signals a and b, and the second color expression signal is non-lightness. A signal expressed in the YMC color space, which is one of the color expression signals of the chromaticity system, and a saturation equivalent signal Ct and black generated from the yellow, magenta, and cyan color signals Y ', M', and C '. When the black character portion of the input color image is re-determined by the second black character determination unit from the signal K and the determination output of the first black character determination unit, the threshold value Co for the saturation signal Cr in the first black character determination unit is It can be set to a relatively high saturation side.

By setting the threshold value Co for the saturation signal Cr in the first black character determining section to a relatively high saturation side, the first black character determining section shows the input color image as shown in FIG. 11 (A). When the black character portion 1k exists in the white background 1w, the chromatic color signal 2c portion is also determined to be the black character portion. However, in the input color image, as shown in FIG. When the low-saturation color character portion 1c exists, the low-saturation color character portion 1c may also be determined as a black character determination portion.

However, when the first black character determination unit determines that the black character portion, the second black character determination unit determines that the black signal K is less than or equal to the predetermined threshold value and the saturation equivalent signal Ct is greater than the predetermined threshold value. Only when the black character portion is determined again, the second black character determination portion determines that the portion of the chromatic color signal 2c is still the black character portion, and the black character portion 1k in the white background 1w is The color is clearly output in black without causing color bleeding, and the low-saturation color character portion 1c in the white background 1w is correctly determined again as not a black character portion.

Further, since the threshold value Co for the saturation signal Cr in the first black character judging section can be set to a relatively high saturation side, the peripheral portion of the gray signal 5k as shown in FIG. 12B is processed to be white. It is not necessary, and the peripheral portion of the black character portion 4k in the light-colored background 4c will not be output as white.

Conversely, the same applies when the first color expression signal is a non-lightness chromaticity color expression signal and the second color expression signal is a lightness chromaticity color expression signal.

[0030]

FIG. 1 shows an example of a digital color copying machine using an example of the image processing apparatus of the present invention. The portion between the image reading apparatus 100 and the image output apparatus 200 is the image processing apparatus of the present invention. This is an example.

The image processing apparatus of this example is basically the same as the conventional image processing apparatus shown in FIG. 10 except for the portion related to black character determination. However, an undercolor removal chroma generation circuit 40 is provided in place of the undercolor removal circuit 18 of the conventional image processing apparatus shown in FIG.
At 0, the black signal K is generated from the yellow, magenta, and cyan color signals Y ′, M ′, and C ′ from the color conversion circuit 17, and the undercolor removed yellow, magenta, and cyan color signals Y and M are generated. , C are obtained and a saturation equivalent signal Ct is generated.

FIG. 2 shows an example of the undercolor removal saturation generation circuit 40. Here, it is assumed that the brightness of the output image becomes higher as the value of the signal is closer to zero.

In the maximum value detection circuit 41 and the minimum value detection circuit 42, the maximum and minimum values of the yellow, magenta and cyan color signals Y ', M', C'from the color conversion circuit 17 are calculated, respectively, In the degree-equivalent amount calculation circuit 43, the difference between the maximum value and the minimum value is the color signals Y ′, M ′,
It is calculated as a saturation equivalent amount for C ′.

The saturation equivalent signal from the saturation equivalent amount calculation circuit 43 is smoothed by the smoothing circuit 44 including a spatial filter having a pixel size of 3 × 3 pixels having a line buffer, and smoothed. The saturation equivalent signal Cs is supplied to the adjustment circuit 45, and the adjusted saturation equivalent signal Ct is output from the adjustment circuit 45.

In the adjusting circuit 45, Ct = d · Cs + e
As shown by (where d and e are adjustment coefficients), the larger the input saturation-equivalent signal Cs, the smaller the color signals Y ′, M ′, and C ′ for black amount generation, as described later. The input saturation-equivalent signal Cs and the output saturation-equivalent signal Ct may have a linear relationship so that the output saturation-equivalent signal Ct that is subtracted from the value increases, or the input may be performed by a lookup table or the like. The saturation-equivalent signal Cs and the output saturation-equivalent signal Ct may have a non-linear relationship.

Further, in the black amount generation adjusting circuit 46,
Delay buffer 47k obtained from the minimum value detection circuit 42
The chroma equivalent signal Ct from the adjusting circuit 45 is subtracted from the minimum value signal delayed by the delay time in the smoothing circuit 44 to generate the black signal Ko which is the basis of the undercolor removal amount calculation. A black signal K derived from the black signal Ko as the output of the undercolor removal saturation generation circuit 40 is obtained.

Ko = min (Y ', M', C ')-Ct
The black signal Ko, which is the basis for calculating the undercolor removal amount, represented by
And the black signal K derived as the output of the undercolor removal / saturation generation circuit 40 have a linear relationship as represented by K = f · Ko + g (where f and g are adjustment coefficients). Alternatively, a non-linear relationship may be established by a lookup table or the like.

The black signal Ko is supplied to the undercolor removal amount calculation circuit 48, and the undercolor removal amount for the color signals Y ', M', C'is calculated. The undercolor removal amount signal Ucr and the black signal Ko
Is expressed by Ucr = h · Ko + j (where h and j are adjustment coefficients), the linear relationship is such that the undercolor removal amount signal Ucr increases as the black signal Ko increases. Alternatively, a non-linear relationship may be established by a lookup table or the like. For example, in the case of 100% undercolor removal, by setting h = 1.0 and j = 0, the undercolor removal is executed from the white level. Further, the undercolor removal amount may be set separately for the color signals Y ′, M ′, C ′ or may be set to be the same.

The operation circuits 49y, 49m, 49c
In the above, from the color signals Y ′, M ′, C ′ delayed by the delay time in the smoothing circuit 44 by the delay buffers 47 y, 47 m, 47 c, the separate or common lower color removal amount signal Ucr is obtained. Yellow, magenta, and cyan color signals Y, M, and C which have been subtracted and whose undercolor has been removed are obtained.

The smoothing by the smoothing circuit 44 is not always necessary. In that case, the delay buffer 47 is used.
k, 47y, 47m and 47c are also unnecessary.

As shown in FIG. 1, the black signal K and the color signal Y obtained from the undercolor removal saturation generation circuit 40 are
M and C are black signals K and color signals Y, M, obtained from the lower color removal circuit 18 of the conventional image processing apparatus shown in FIG.
Similar to C, the color reading circuit 19 causes the image reading device 10
One by one is sequentially selected according to the scanning cycle of 0 and the developing cycle of the image output device 200.

In the example of FIG. 1, the input color image is input to the first black character determining section 50 by the lightness signal L from the color conversion circuit 13 and the saturation signal Cr from the hue / saturation conversion circuit 14 as described later. The black character part of is determined,
Further, the determination output D1 of the first black character determination unit 50 and the saturation equivalent signal Ct from the undercolor removal saturation generation circuit 40 described above.
And the black signal K, the second black character determination unit 60 re-determines the black character portion of the input color image as described later, and the determination output D2 of the second black character determination unit 60
In the switching circuit 31, the black signal K or the color signals Y, M and C from the color selecting circuit 19 and the lightness signal L ′ from the color adjusting circuit 15 are switched, the characteristics of the filter circuit 32 are switched, and gradation reproduction control is performed. The characteristics of the circuit 33 are switched.

FIG. 3 shows the first black character judging section 50 of the example of FIG.
An example is shown below. In this example, in the character determination circuit 51, the lightness signal L is binarized, and a window of, for example, 5 × 5 pixels is formed for the binarized lightness signal, and by pattern matching in the window,
Pixels that look like characters are extracted and determined, and at the same time, the dot determination circuit 52 extracts and determines pixels that look like dots by pattern matching within a window of, for example, 5 × 5 pixels for the similarly binarized lightness signal. It

Further, in the black judging circuit 53, it is judged whether or not the pixel of interest of the input color image is black (gray) depending on whether or not the saturation signal Cr becomes less than or equal to the predetermined threshold value Co. Basically, if the saturation is zero, it can be determined to be black, but the portion of the chromatic color signal 2c as shown in FIG. 11B due to the color shift in the image reading apparatus 100 described above is also determined to be black. As shown in FIG. 11B, the threshold value Co for the saturation signal Cr is set to a relatively high saturation side so that the black character portion having a certain saturation depending on the color of the original document is also determined to be black. Is set.

Since it is desirable that the saturation range determined as a black character portion also varies depending on the character density, in this example, the threshold value Co is changed within a certain range according to the lightness signal L. To be

In the black character determination circuit 54, among the portions determined by the character determination circuit 51 as pixels likely to be characters, the portions other than the portions determined by the halftone dot determination circuit 52 as pixels likely to be a halftone dot are excluded. A portion determined to be black by the black determination circuit 53 is determined to be a black character portion. At the same time, the black character determination circuit 54 removes noise from the determination result and corrects it by pattern matching.

Furthermore, in order to prevent the occurrence of color fringing around the pixels determined by the black character determination circuit 54 to be the black character portion, the determination result of the black character determination circuit 54 in the determination area expansion circuit 55 is The pixel is expanded into a region of several pixels around the pixel, and the expansion result is taken out as the judgment output D1 of the first black character judgment unit 50.

FIG. 4 shows the second black character judging section 60 of the example of FIG.
An example is shown below. In this example, the comparison circuit 61 compares the saturation equivalent signal Ct from the undercolor removal saturation generation circuit 40 with a predetermined threshold value Cth, and the comparison circuit 61 outputs the saturation equivalent signal Ct from the threshold value Cth. A determination signal Sc that is on (high level) when it is large and is off (low level) when it is less than or equal to the threshold value Cth is obtained, and the black signal K from the undercolor removal saturation generation circuit 40 is obtained in the comparison circuit 62. Is compared with a predetermined threshold value Kth, and the comparison circuit 62
From this, it is possible to obtain the determination signal Sk that is turned on when the black signal K is equal to or less than the threshold value Kth and is turned off when it is greater than the threshold value Kth.

Then, the AND gate 63 causes the judgment output D1 of the first black character judgment unit 50 and the comparison circuits 61 and 6 to operate.
The signal D2 of the logical product of the determination signals Sc and Sk from 2 is obtained, and the signal D2 is used as the determination output of the second black character determination unit 60.

Therefore, in the second black character determining section 60,
The saturation equivalent signal Ct becomes larger than the threshold value Cth in the portion where the first black character determination unit 50 determines the black character portion and the determination output D1 of the first black character determination unit 50 is turned on, and the black signal K Only the portion where is less than or equal to the threshold value Kth is finally determined as the black character portion.

In the first black character judging section 50 of the example of FIG. 3, the threshold Co for the saturation signal Cr is set to a relatively high saturation side, so that the input color image is as shown in FIG. 11 (A). When the black character portion 1k exists in the white background 1w, the chromatic signal 2c as shown in FIG. 11B in the input color image signal from the image reading apparatus 100 due to the color shift in the image reading apparatus 100. Is also determined to be a black character portion, but in the input color image shown in FIG.
When the low-saturation color character portion 1c is present in the white background 1w as shown in (C), the low-saturation color character portion 1c is shown in FIG. 11D in the input color image signal. The portion of the chromatic color signal 3c as shown may be determined to be a black character portion.

However, in the portion of the chromatic color signal 3c,
In the second black character determination unit 60, the determination signal Sc from the comparison circuit 61 is turned off when the saturation equivalent signal Ct becomes less than or equal to the threshold value Cth, or the black signal K becomes greater than the threshold value Kth for comparison. Since the determination signal Sk from the circuit 62 is turned off, the portion of the chromatic color signal 3c, that is, the low-saturation color character portion 1c is not determined to be a black character portion by the second black character determination portion 60.

In the portion of the colored signal 2c which is determined to be the black character portion by the first black character determining portion 50, the saturation equivalent signal Ct is set to the threshold value C in the second black character determining portion 60.
When it becomes larger than th, the judgment signal Sc from the comparison circuit 61 is turned on, and the black signal K becomes the threshold value Kth.
The judgment signal Sk from the comparison circuit 62 becomes as follows.
Is turned on, the portion of the chromatic color signal 2c is
The black character determination unit 60 also determines that it is a black character portion. Therefore, the black character portion 1k in the white background 1w is clearly output in black without causing color fringing.

Further, in the first black character determining unit 50 of the example of FIG. 3, when the black character portion 1k is present in the white background 1w as shown in FIG. 11A, the white background 1w causes the remaining color to remain. A gray signal 2 as shown in FIG. 11B, which is determined as a black character portion so that it is output in clear white without being generated.
Since a portion of about 1 pixel width around k is not processed to white, when a black character portion 4k exists in the light background 4c as shown in FIG. 12A, it is shown in FIG. 12C. As described above, the white signal 6w is not inserted between the gray signal 6k and the chromatic color signal 6c, and the black character portion 4 in the light color background 4c is
The peripheral part of k is not output in white.

FIG. 5 shows the second black character judging section 6 of the example of FIG.
Another example of 0 is shown. When the color misregistration in the image reading apparatus 100 described above is large, the input color image in FIG.
As shown in (A), when the black thin line portion 7k exists in the white background 7w, the first black character determination unit 50 in the example of FIG.
FIG. 6 in the input color image signal from the image reading apparatus 100.
The portion of the chromatic color signal 8c around the gray signal 8k as shown in (B) is also determined to be a black character portion.
In the black character determination unit 60, the saturation equivalent signal Ct becomes less than or equal to the threshold value Cth in the portion of the chromatic color signal 8c, the determination signal Sc from the comparison circuit 61 is turned off, and the portion of the chromatic color signal 8c. May not be determined to be a black character part.

Therefore, in the example of FIG. 5, the chromatic color signal 8c portion is also determined to be a black character portion by the second black character determining unit 60 from the edge component of the saturation equivalent signal Ct in the chromatic color signal 8c portion. This is the case.

That is, in the example of FIG. 5, the saturation equivalent signal Ct from the undercolor removal saturation generation circuit 40 is supplied to the edge detection circuit 64 including a line buffer and a spatial filter having a pixel size such as 5 × 5 pixels. Then, the edge component Ec of the saturation equivalent signal Ct is detected from the edge detection circuit 64, the edge component Ec is compared with the predetermined threshold value Eth in the comparison circuit 65, and the edge component Ec is detected from the comparison circuit 65. A determination signal Se that is turned on when the threshold value is equal to or lower than Eth and is turned off when the threshold value is greater than Eth.
Is obtained.

The AND gate 63 obtains a logical product signal D2 of the judgment output D1 of the first black character judgment unit 50 and the judgment signals Sc, Sk and Se from the comparison circuits 61, 62 and 65, and the signal D2 thereof is obtained. Is the determination output of the second black character determination unit 60. However, the saturation equivalent signal Ct,
Black signal K and determination output D1 of the first black character determination unit 50
Is delayed by the delay buffers 66, 67 and 68 by the delay time in the edge detection circuit 64 and supplied to the comparison circuits 61 and 62 and the AND gate 63, respectively.

In the portion of the chromatic color signal 8c as shown in FIG. 6B as a color blur in the black thin line portion 7k as shown in FIG. 6A, the saturation equivalent signal Ct has noise of about one pixel width. 6C is obtained as the edge component Ec from the edge detection circuit 64, and the signal 8e becomes equal to or lower than the predetermined threshold Eth.

Therefore, in the second black character determining section 60 of the example of FIG. 5, the comparison circuit 65 is used in the portion of the chromatic color signal 8c.
The determination signal Se from is turned on, the portion of the chromatic color signal 8c is also determined to be a black character portion, and the black fine line portion 7k in the white background 7w is also clearly output in black without causing color bleeding.

When the color misregistration in the image reading apparatus 100 is small, the second black character determining section 60 may be constructed as shown in FIG.

In the image processing apparatus of the example shown in FIG.
As schematically shown in FIG. 4, a portion 9a that is determined to be a black character portion by the first black character determination unit 50 as in the example of FIG. 3 and comparison circuits 61 and 62 of the second black character determination unit 60 as in the example of FIG. Is determined to be a black character portion by the logical product of the determination signals Sc and Sk from the above, or the logical product of the determination signals Sc, Sk and Se from the comparison circuits 61, 62 and 65 of the second black character determination portion 60 as in the example of FIG. Finally, only the portion 9c where the overlapped portion 9b overlaps is determined to be a black character portion, which causes color fringing in the black character portion, or a low-saturation color character portion is erroneously determined to be a black character portion, or The black character portion 9d of the input color image is clearly identified by being identified as the halftone portion without causing inconvenience such as a white portion around the black character portion in the background.

FIG. 8 shows another example of a digital color copying machine using another example of the image processing apparatus of the present invention.

In the image processing apparatus of this example, in the undercolor removal saturation generation circuit 71, the brightness scale conversion circuit 1
Red, green, and blue color signals Lr, L on the lightness scale from 2
The black signal K is generated from g and Lb, the red, green, and blue color signals Lr ′, Lg ′, and Lb ′ from which the undercolor is removed are obtained, and the saturation equivalent signal Ct is generated. The undercolor removal / saturation generation circuit 71 in the image processing apparatus of the example of FIG.
The configuration can be the same as that of the undercolor removal saturation generation circuit 40 shown as an example in FIG.

The black signal K obtained from the undercolor removal chroma generation circuit 71 and the red, green, and blue color signals L from which the undercolors have been removed.
The color conversion circuit 72 converts r ′, Lg ′, and Lb ′ into the black signal K and the color signals Y, M, and C of yellow, magenta, and cyan with the undercolor removed, and the black signal K and the color signal Y. , M, C are sequentially selected one by one in accordance with the scanning cycle in the image reading apparatus 100 and the developing cycle in the image output apparatus 200 by the color selection circuit 19.

Then, in the example of FIG. 8, the saturation equivalent signal Ct and the black signal K from the undercolor removal saturation generation circuit 71.
Thus, the first black character determination unit 80 determines the black character portion of the input color image, and further, the determination output D3 of the first black character determination unit 80 and the lightness signal L from the color conversion circuit 13 are determined.
And the saturation signal Cr from the hue / saturation conversion circuit 14, the black character portion of the input color image is redetermined in the second black character determination portion 90, and the determination output D4 of the second black character determination portion 90 causes the switching circuit 31 to determine. Color selection circuit 19
Black signal K or color signals Y, M, C from the color adjustment circuit 1
The lightness signal L'from 5 is switched, and the filter circuit 3
The characteristics of No. 2 are switched, and the characteristics of the gradation reproduction control circuit 33 are switched.

The first black character determining section 80 and the second black character determining section 80 in this example
The black character determination unit 90 may basically have a configuration opposite to that of the first black character determination unit 50 and the second black character determination unit 60 in the example of FIG.

FIG. 9 shows an example of the first black character judging section 80 and the second black character judging section 90, and the first black character judging section 80 is shown in FIG. 5 of the second black character judging section 60 of the example of FIG. In the example, the second black character determination unit 90 is configured by a portion excluding the portion related to the determination output D1, and the first black character determination unit 50 of the example of FIG. An AND gate 56 for obtaining the logical product of the determination signal D1 from the determination area expansion circuit 55 and the determination output D3 of the first black character determination unit 80 is added. Image reading device 1 described above
If the color shift at 00 is small, the first black character determination unit 8
0 may be configured by a portion excluding the portion related to the determination output D1 in the example of the second black character determination unit 60 of the example of FIG. 1 shown in FIG.

Also in the example of FIG. 8, the same effect as that of the example of FIG. 1 can be obtained.

The present invention can also be applied to the case of determining a black character portion of an input color image such as an image generated by a computer in a digital color printer or the like.

[0071]

As described above, according to the present invention, color bleeding occurs in a black character portion, or a color character portion with low saturation is erroneously determined as a black character portion, or a black character portion in a light-colored background is surrounded. The black character portion of the input color image can be discriminated as the halftone portion and can be clearly determined without causing inconvenience such as a white portion.

[Brief description of drawings]

FIG. 1 is a block diagram showing an example of a digital color copying machine using an example of an image processing apparatus of the present invention.

FIG. 2 is a block diagram showing an example of the undercolor removal saturation generation circuit.

FIG. 3 is a block diagram showing an example of a first black character determination unit.

FIG. 4 is a block diagram showing an example of a second black character determination unit.

FIG. 5 is a block diagram showing another example of the second black character determination unit.

FIG. 6 is a diagram for explaining color fringing in a black thin line portion.

FIG. 7 is a diagram schematically showing a relationship between a determination made by a first black character determination unit and a determination made by a second black character determination unit.

FIG. 8 is a block diagram showing another example of a digital color copying machine using another example of the image processing apparatus of the present invention.

FIG. 9 is a block diagram showing an example of a first black character determination unit and a second black character determination unit.

FIG. 10 is a diagram showing an example of a digital color copying machine using an example of a conventional image processing apparatus.

FIG. 11 is a diagram for explaining black character determination.

FIG. 12 is a diagram for explaining black character determination.

[Explanation of symbols]

 50,80 1st black character determination part 60,90 2nd black character determination part

Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display area H04N 1/48 H04N 1/46 A

Claims (1)

[Claims] 1. A first black character determination unit for determining a black character portion of an input color image from a first color expression signal for an input color image, and a first black character determination signal for the input color image, which is different from the first color expression signal. An image processing apparatus comprising: a second black character determination unit that re-determines a black character portion of an input color image based on the second color expression signal and the determination output of the first black character determination unit.