JPH07212611A - Method and unit for color picture processing - Google Patents

Abstract

PURPOSE:To provide the method and the unit for color picture processing in which increase in a picture noise by edge emphasis processing is suppressed and a visually natural edge emphasis is provided. CONSTITUTION:The unit is provided with an edge detection means 102 obtaining an edge signal from a luminance signal in a luminance/chrominance separate signal, an edge emphasis means 103b processing emphasizing the edge of a picture represented by a luminance signal, a smoothing means 103a smoothing the luminance signal, a saturation detection means 104 detecting a saturation signal of the picture from a chrominance signal in the luminance/chrominance separate signal, means 105,106 implementing luminance conversion by mixing an output of the edge emphasis signal and an output of the smoothing means based on the edge quantity signal detected by the edge detection means, and means 107, 111, 110a, 110b obtaining the chrominance corresponding to compression or expansion of saturation by using an output of the edge detection means and an output of the saturation detection means.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing method used in a digital full-color copying machine, a color facsimile, an image file system, etc., for reading a color original, performing image processing, and reproducing the original image on a recording medium. And equipment.

[0002]

2. Description of the Related Art In recent years, digital full-color copiers and the like are required to have higher functions, and the edge of an image read by an image scanner is appropriately corrected, or the edge emphasis amount is increased in the case of a map original. Mold edge enhancement processing is being performed. Also, color originals used in digital full-color copiers often have a mixture of black characters and color halftone photographs, and black characters are reproduced with sharp edges without coloring, and halftones are smooth with good color reproducibility. There is a demand for reproduction with various gradation characteristics, and there is a need for area identification processing that detects black character areas without misidentification or omission of identification and performs appropriate edge processing.

The structure and operation of an example of conventional edge enhancement processing in a digital full-color copying machine will be described below with reference to FIG. In FIG. 6, R,
G and B are color signals read by scanning a color original. The color signals R, G, and B are input in parallel to the halftone image filter processing circuit 601, the character image filter processing circuit 602, and the area identification circuit 609. The halftone image filter processing circuit 601 is a two-dimensional filter that performs band enhancement processing on the assumption that the pixel area of interest is a halftone image area. The frequency characteristic of this filter is set so as to remove the halftone dot component of the document and increase the sharpness of the image. The character image filter processing circuit 602 performs edge component emphasis processing on the assumption that the pixel area of interest is a character image area.

The halftone image filter processing circuit 601 and the character image filter processing circuit 60 obtained as described above.
The output of No. 2 and the output of No. 2 are switched by the selection circuit 603 according to the determination signal from the area identification circuit 609 described below, and output to the color processing circuit in the subsequent stage. The area identification circuit 609 includes a hue identification circuit 604, a threshold value storage ROM 607 that stores a threshold value for area determination, a signal synthesis circuit 605, an edge signal generation circuit 606, and a comparator 608. The signal synthesizing circuit 605 generates a luminance signal from the color signals R, G, B. The edge signal generation circuit 606 receives the luminance signal as an input, calculates the difference between the maximum value and the minimum value in an N × N pixel window centered on the pixel of interest, and outputs it as an edge signal. The comparator 608 compares the edge signal with a certain threshold value and outputs 1 to the selection circuit 603 as a character image area if the threshold value is equal to or more than the threshold value and 0 as a halftone image area if the edge signal is less than or equal to the threshold value. The hue identifying circuit 604 identifies the hue of the pixel of interest into seven hues of yellow, magenta, cyan, black, red, green, and blue, and outputs a hue signal. The threshold value storage ROM 607 uses the hue signal as an address, and a determination threshold value for area identification according to the hue is output to the comparator 608. The comparator 608 compares the threshold value for each hue with the edge signal. Through the above steps, it is determined whether the image is a halftone image or a character image, and edge enhancement processing suitable for each image is appropriately switched and executed.
However, in the configuration of the edge enhancement processing as described above, although there is an effect of suppressing the noise component in the image signal due to the edge enhancement, there is discontinuity in the processing for the halftone image and the processing for the character, so that the reproduced image An unnatural defect appears. Further, in the configuration of the area identification processing described above, when the threshold is set so that a character area having a slightly small edge component can be identified, an area having a slightly large edge component in the halftone portion is erroneously determined as a character area, and a halftone image is obtained. Cannot be reproduced smoothly. Also, in order to eliminate this misjudgment, only the character area with a sufficiently large edge component can be identified,
The reproducibility of characters will be poor. Further, there is a concern that the color of the image signal becomes achromatic because the signal after the edge enhancement exceeds the dynamic range of the image signal.

On the other hand, in Japanese Patent Publication No. 5-56068, the output of the character image filter and the halftone image filter is used as the output signal of the edge detecting means in order to suppress the moire of the halftone dot original and guarantee the reproduction of the character. A method of mixing accordingly is disclosed. Therefore, the suitability of applying this conventional method to color image processing will be considered. When the above method is applied to the color image processing method, it is applied to the recording color signals of yellow, magenta, cyan, and black.
It can be applied to the input image signals red, green, and blue, and to be applied to the luminance signal after converting the input color signal into the luminance / chromaticity separation signal. When applied to a recording color signal, not only the main color but also the unnecessary color is edge-emphasized according to the edge degree, so that color mixing and turbidity in a color character and an increase in signals other than black in a black character occur. Therefore, in this case, it is necessary to use the hue determination together, and the problem caused by the erroneous determination cannot be avoided. When applied to red, green, and blue, the configuration disclosed in Japanese Examined Patent Publication No. 5-56068 is required in parallel for three systems, which has the drawback of increasing the cost. Also, if misregistration cannot be ignored between the input color signal signals, the peaks of the red, green, and blue processed signals of black characters, etc. will shift, and coloring will occur at the edges of reproduced black characters in the output image. After converting to luminance / chromaticity separation signal,
When applied to a luminance signal, there is an advantage that a latitude between suppression of moire removal of a halftone dot original and reproducibility of characters can be widely taken because edge detection can be performed according to human sense. However, according to this method alone, it is impossible to reproduce black ink in black, and turbidity occurs in color characters. The reason will be described with reference to FIG. In FIG. 7, the luminance of the luminance / chromaticity separation signal is plotted on the vertical axis, and the saturation calculated from the chromaticity is plotted on the horizontal axis, and the thick line represents the color gamut of the recording system. Generally, an input image signal of a black character portion or a color character portion in a white background is compared with the original color coordinates of the document (in FIG. 7, black characters are indicated by ■ and color characters are indicated by ●).
Colored characters are positioned on a straight line connecting the white background and the color coordinates of the original, and black characters are shifted from the straight line connecting the white background and the color coordinates of the original in a slightly saturated direction (in FIG. 7, black characters are □,
Color letters are indicated by ○). This characteristic is the MTF of the image input device.
Depends on characteristics and pixel shift performance. Generally, in a digital full-color copying machine, a contact type or a reduction type CCD image input device is used, and four scans are performed every recording color cycle. Vibration of each scan and R of MTF characteristics
Due to the difference in GB balance, the black character portion, which should originally be read in black, has some saturation. Further, the absolute value of MTF is usually 50 to 70% at 4 lp / mm, and as a result,
The black character and color character input signals move to interpolation points on a straight line connecting the white background and the color coordinates of the original as shown in FIG. This effect is particularly remarkable for characters of about 8 points or less, and in the image processing apparatus, the color coordinates of the input signal (see FIG.
It is preferable to predictively restore the middle and black characters by □ and the color characters by ◯ to the original color coordinates of the original document (in FIG. 7, black characters by □ and color characters by ●). However, Japanese Patent Publication No. 5-56068
When the method disclosed in the publication is applied to the luminance signal, the input signal is emphasized only in the luminance direction (the arrow direction in FIG. 7). At that time, the black character input signal has low luminance without the saturation error being corrected, and the color character input signal also has low luminance without saturation enhancement. As a result, for black characters, a large amount of cyan, magenta,
Since the color material of yellow is mixed in, it becomes impossible to reproduce the black ink in one color, and the unnecessary color material for the color characters increases to cause turbidity.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the problems of the prior art discussed above. That is, an object of the present invention is to provide a color image processing method and apparatus capable of suppressing an increase in image noise due to edge enhancement processing and providing visually natural edge enhancement. Another object of the present invention is to provide a color image processing method and apparatus that can easily set the edge enhancement amount. An object of the present invention is to enhance edge and brightness of a character signal within a predetermined range so that a character signal after edge enhancement is reproduced in an achromatic color for black characters and is reproduced in an original saturation for color characters. Control the quantity,
A color image processing method and apparatus are provided.

[0007]

A color image processing method according to the present invention comprises a step of obtaining an edge amount signal from a luminance signal of a luminance / chromaticity separation signal, a luminance signal and an edge portion obtained by smoothing an image from the luminance signal. Is obtained, a step of obtaining a chroma signal of an image from the chromaticity signal of the luminance / chromaticity separation signal, a step of obtaining a luminance signal obtained by smoothing the image on the basis of the edge amount signal, and an edge portion. The method is characterized by including a step of mixing the emphasized luminance signals and a step of obtaining a chromaticity signal corresponding to compression or expansion of saturation using the edge amount signal and the saturation signal. Further, the color image processing apparatus of the present invention for carrying out the above method is represented by the edge detection means (102 in FIG. 1) for obtaining an edge amount signal from the luminance signal of the luminance / chromaticity separation signal and the luminance signal. Edge enhancement means (103b) for enhancing the edge portion in the image, a smoothing means (103a) for smoothing the luminance signal, and the luminance /
Saturation detecting means (104) for detecting the saturation signal of the image from the chromaticity signal of the chromaticity separation signal, and the output of the edge enhancing means and the smoothing means of the smoothing means based on the edge amount signal detected by the edge detecting means. A color for obtaining a chromaticity signal corresponding to compression or expansion of saturation using means (105, 106) for performing luminance conversion by mixing the outputs and the output of the edge detection means and the output of the saturation detection means. It is characterized by having a basic configuration including means (107, 111, 110a, 110b) for performing degree conversion.

Another feature of the present invention is that, in the above basic structure, a first non-linear conversion means 105 for non-linearly converting the output of the edge detection means is provided. The first non-linear conversion means has a conversion characteristic (FIG. 9) in which the conversion output signal fe is standardized between 0 and 1 and controlled by a plurality of parameters. The plurality of parameters are a first parameter (e _{0 in} FIG. 9) that controls the upper limit value of the converted output signal that sets fe = 0, and a second parameter that controls the lower limit value of the converted output signal that sets fe = 1. Parameter (e _{1 in} FIG. 9). In addition, adjusting means can be provided to adjust those parameters.

Another feature of the present invention is that, in the basic configuration, a second non-linear conversion means 107 for non-linearly converting the output of the saturation detection means is provided. The second non-linear conversion means has a conversion characteristic (FIG. 5) in which the conversion output signal fc is standardized between 1 and −1 and controlled by a plurality of parameters. The plurality of parameters are a first parameter for controlling the upper limit value of the converted output signal where fc = −1, a second parameter for controlling the lower limit value of the converted output signal where fc = 0, and fc = 0. A third parameter for controlling the upper limit of the converted output signal
and a fourth parameter for controlling the lower limit value of the converted output signal with c = 1. In addition, adjusting means can be provided to adjust those parameters.

Another feature of the present invention is that, in the basic configuration, the means for performing the luminance conversion outputs the output L * p of the smoothing means and the output L * c of the edge enhancing means as the first There is a weighted averaging means for obtaining the weighted average output L * ′ by the calculation of the following equation, using the output fe of the conversion means as the weight value. L * '= fe * L * c + (1-fe) * L * p

Another feature of the present invention is that the means for performing the luminance conversion in the basic configuration has a limiting means for limiting the maximum value of the luminance conversion output based on the edge weight amount and the saturation conversion signal. It is in.

Another feature of the present invention is that the means for performing the color conversion in the basic configuration calculates a coefficient k from the edge weight amount fe and the saturation conversion signal fc by calculating k = 1 + fe · fc. That is, it has a coefficient determining means and a coefficient operating means for multiplying the chromaticity signal by the coefficient k calculated by the coefficient determining means.

[0013]

In the present invention, the edge detecting means detects the edge amount indicating the degree of the edge attribute of the edge portion in the image from the luminance signal of the luminance / chromaticity separation signal. Brightness /
By calculating the difference between the pixel of interest and the peripheral pixel using the pixel of interest in the luminance signal of the chromaticity separation signal and a plurality of peripheral pixels within a predetermined distance range from the pixel of interest, The edge amount e is detected. The edge amount is converted as a continuous amount fe normalized between 0 and 1 by using a non-linear function that can be adjusted from the outside. This edge signal fe is a continuous quantity that quantifies the information on the frequency characteristic of the image and the contrast of its density.

The saturation detecting means detects the saturation C * around the pixel of interest or the pixel of interest from the chromaticity signal of the luminance / chromaticity separation signal.

The smoothing means smoothes the luminance signal. This smoothing means has a filter characteristic set in advance for halftone image processing, and performs processing suitable for a halftone image portion. The edge emphasizing unit has a filter characteristic set for character image processing, emphasizes an edge portion of the image represented by the luminance signal, and performs a process suitable for the character image.

The means for performing brightness conversion mixes the output of the edge enhancing means and the output of the smoothing means based on the edge amount signal. Specifically, the output of the smoothing means, that is, the filter output P preset for halftone image processing and the output of the edge emphasizing means, that is, the character image are output by using the edge weight signal fe obtained by standardizing the edge amount signal as the weight. The filter output C set for processing is mixed by the following equation. T = fe · C + (1-fe) · P (1)

Luminance signal limiting means (108, 109)
Is limited to the luminance signal obtained by the equation (1) by the following equation (2) according to the limiting value L (fe, C *) determined by the saturation C * and the edge signal fe: IF T> L ( fe, C *) THEN T = L (fe, C *) (2) The limit value L (fe, C *) is the edge signal fe and the saturation signal C.
The larger the value of *, the brighter the value. That is, when the edge signal is small, or in the case of a photographic image, the limit value L (fe, C *) has no effect. Even if the edge signal is large, the limit value L (fe, C *) does not work when the saturation signal C * is small and the character is black. When the edge signal is large and the saturation signal C * is also large,
In the case of a color character, the limit value L (fe, C *) is most effective and limits the luminance signal so as not to fall below a certain value. This prevents the unwanted color from being mixed into the color character.

On the other hand, the chromaticity conversion means (107, 110A, 110b, 111) for the chromaticity signal performs a non-linear conversion of the saturation signal C * and normalizes it between -1 and 1. Using the signal fc and the edge signal fe, the coefficient k is calculated by k = 1 + fe · fc (3), and this is calculated as the chromaticity signal Qi (i = 1,
The chromaticity signal Qi 'is obtained by multiplying 2). Qi ′ = k · Qi (4) The saturation conversion signal fc monotonously changes from −1 to 1 with respect to the saturation signal C *. That is, fc is in the vicinity of -1 in the low saturation part, and changes to 1 as the saturation becomes higher. If the edge signal is small, or if it looks like a photographic image, fe ◆
Since it becomes 0, k ◆ 1 regardless of the saturation conversion signal fc, and in this case, the chromaticity signal does not change before and after the processing. When the edge signal is large and the saturation signal C * is also large, that is, in the case of a color character, fc is positive and k> 1. As a result, the processed chromaticity signal Qi 'is enhanced in saturation. When the edge signal is large and the saturation signal C * is small, that is, when a black character appears, fc becomes negative and 0 ≦ k <1.
Becomes As a result, the chroma of the processed chromaticity signal Qi 'is compressed and attracted to an achromatic color. Therefore, the input color coordinates (∘ in FIG. 7) of the color characters in FIG. 7 are emphasized in the saturation direction while being emphasized within the range where the brightness is limited, and the original color coordinates of the original document (in FIG. 7) are emphasized. ●) Further, the input color coordinates of black characters in FIG. 7 (□ in FIG. 7) are compressed in the saturation direction while the luminance is emphasized, and become the original color coordinates of the original document (■ in FIG. 7). The mixing and limitation of the luminance signals and the degree of compression and enhancement in the saturation direction at this time are controlled by the parameters of the non-linear conversion means of the edge detection means and the saturation detection means, and the conversion parameters are set to be changeable from the outside. It

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The features of the present invention will be specifically described below with reference to the drawings based on the embodiments. FIG. 1 shows an example of a configuration for implementing the color image processing apparatus of the present invention. A luminance / chromaticity separation signal is used as the input signal. YIQ, Yxy, YES, L * u * v *, L * a *
A color signal such as b * corresponds to this, and in this embodiment, L
* A * b * will be described as a typical example.

The luminance signal L * is input in parallel to the edge detector 102, the smoothing circuit 103a, and the edge enhancer 103b. On the other hand, the chromaticity signals a * b * are the smoothing circuits 101a,
It is input in parallel to b. The edge detector 102 is composed of two one-dimensional digital filters and outputs the edge amount e of the luminance signal L *. The edge amount e is calculated by the non-linear converter 105.
The edge weight amount fe, which is input to the input terminal 1 and is normalized between 0 and 1, is output.

On the other hand, the smoothed chromaticity signals a '* b' * output from the smoothing circuits 101a and 101b are the saturation generation circuit 10
4, the saturation signal C * is generated by the conversion corresponding to the definitional expression C * = {a ′ * ² + b ′ * ² } (5). The saturation generation circuit 104 may be composed of a multiplier and an adder, or may be composed of a lookup table. In this way, the edge weight amount fe and the saturation C * of the pixel of interest of the input signal L * a * b * are calculated, and the luminance / chromaticity signal is controlled by both of these signals.

FIG. 8 shows an example of the configuration of the edge detector 102. The edge detection unit has two digital filters 102-1 and 102- having detection sensitivity in the main scanning direction and the sub scanning direction.
2 and the filter coefficient of the digital filter 102-1 is “−0.25 0 0.5 0 −0.25 −0.25 0 0.5 0 −0.25 −0.25 0 0.5 0. -0.25 -0.25 0 0.5 0 -0.25 -0.25 0 0.5 0 -0.25 ", and the filter coefficient of the digital filter 102-2 is" -0.25-. 0.25 -0.25 -0.25 -0.25 0 0 0 0 0 0 0.50 0.50 0.50 0.50 0.50 0 0 0 0 0 0 -0.25 -0.25 -0 0.25-0.25-0.25 ".

Both digital filter outputs e _fs and e _ss
After passing through the absolute value conversion circuit 102-3, the comparator 102-
4 is input, and the larger one is output as the edge amount e. At this time, the digital filter coefficient is 400d.
It is designed to have a maximum detection sensitivity of 4 lp / mm for pi data, and particularly attaches great importance to the detection of edges of characters of 8 points or less. Further, since the digital filters each have a maximum detection sensitivity of 4 lp / mm in one direction and have an averaging effect in the other direction, the effect of suppressing the edge amount of a halftone dot printed image in which edges are two-dimensionally distributed Has,
Further, the maximum value selection in the comparator 102-4 expands the edge amount detection latitude of the fine character and halftone dot printed image.

FIG. 9 shows the conversion characteristic of the nonlinear converter 105 for the edge amount e. The conversion characteristic of the non-linear converter 105 is determined by parameters e ₀ and e ₁ which can be set from the outside. In this embodiment, e ₀ is set to 0 and e ₁ is set to 100. In order to prevent an unnatural defect in the output image, it is desirable to increase the distance between e ₀ and e ₁ as much as possible, and the setting depends on the configuration of the edge detector 102 and the setting parameters. In the present embodiment, by configuring the edge detector 102 as described above, the distance between e ₀ and e ₁ can be expanded to a range that does not cause a problem.

A smoothing circuit 10 for the chromaticity signals a * b * is also provided.
Although 1a and 1b are not always necessary, they are necessary when the input device knows the positional deviation for each scan and the RGB balance of MTF. In this embodiment, a smoothing filter that is isotropic in the main scanning direction and the sub-scanning direction is used, and the filter coefficient is set to "0 0.125 0 0.125 0.5 0.125 0 0.125 0".

The edge weight fe and the saturation C * will be described below.
The conversion mode of the luminance signal L * performed by using is described. The luminance signal L * is input to the smoothing circuit 103a and the edge enhancer 103b in parallel with the input to the edge detector 102. The smoothing circuit 103a and the edge enhancer 103b are both configured by a phase-preserving two-dimensional digital filter. The smoothing circuit 103a has a spatial frequency characteristic of approximately 2 l so that moire does not occur even when a halftone image is input.
It has a peak at p / mm and is designed so that the gain is sufficiently reduced at 4 lp / mm or more, and a size of 7 × 5 was used in the examples. The edge intensifier 103b is designed to have a characteristic having a peak at 4 lp / mm so that characters mainly having 8 points or less can be sufficiently emphasized. In the embodiment, a size of 5 × 5 is used. FIG. 2 shows an example of the spatial frequency characteristic of the filter. By performing two kinds of spatial frequency conversion as shown in FIG. 2, an output (L * p) suitable for reproducing a halftone image is obtained from the smoothing circuit 103a, and the edge enhancer 103 is provided.
An output (L * c) suitable for reproducing the character image is obtained from b. The output signals of both of them and the edge weight amount fe are input to the weight averaging circuit 106, the output signals of both of them are mixed by the edge weight amount fe, and the mixed signal L * ′ L * ′ = fe · L * c + ( 1-fe) .L * p (6) is output. As is clear from the equation (6), in the case of a halftone image in which the edge weight fe is small, the mixed signal L * ′ becomes almost equal to L * p, and the optimum signal for the halftone image is output. . Further, in the case of a character image having a large edge weight fe, the mixed signal L * ′ is almost equal to L * c, and the optimum signal for the character image is output. Further, since the edge weight amount fe is a continuous amount, the output does not switch discontinuously with respect to the transition of the edge amount, and an unnatural defect does not appear in the reproduced image unlike the conventional example.

Next, the mixed signal L * 'is the edge weight fe.
And the limit value L * max determined from the saturation signal C *. The course will be described with reference to FIGS. 3 and 4. FIG. 3 is a detailed configuration diagram of the limit value determination circuit 108 of FIG. According to FIG. 3, the saturation signal C * is input to the maximum limit value determination circuit 108-1 and outputs the maximum limit value L * a. As shown in FIG. 4, the relationship between the maximum limit value L * a and the saturation signal C * is such that the maximum limit value L * a linearly changes to a bright value as the saturation increases, and exceeds the constant value C * ₁ . And L * ₁
It will be fixed to. At this time, (C * ₁ , L * ₁ ) is set to be changeable from the outside, and (50, 100) is used in this embodiment. The maximum limit value determination circuit 108-1 can be realized by a combination of a multiplier and a limiter or a lookup table.

Next, the maximum limit value L * a and the edge weight amount f
e is input to the limit value mixing circuit 108-2, and the limit value L
* Max is calculated by L * max = fe * L * a + (1-fe) * 255 (7). The fixed value 255 is the maximum luminance value that can be taken without any limitation. Limit value mixing circuit 10
8-2 can be realized by a combination of a multiplier and a limiter or a lookup table. From the above, the limit value determination circuit 108 causes the edge weight amount f
A limit value L * max determined from e and the saturation signal C * is output. At this time, in the case of a halftone image with a small edge weight fe, L * max is 255, and the limit value does not work. In the case of a character image having a large edge weight fe, the limit value L * max changes according to the saturation signal C * of the pixel. In the setting of FIG. 4, the limit value L * max linearly changes to a bright value as the saturation increases. That is, this action limits the enhancement of the luminance to the color character having a large edge weight fe and a high saturation. Next, the mixed signal L * ′ and the limit value L * m
ax is input to the maximum value limiter 109, and the limit value L * ma
The luminance conversion output L * o limited by x is IF L * '> L * max THEN L * o = L * max ELSEIF L *' ≦ L * max THEN L * o = L * '(8) The output is limited.

Next, the conversion form of the chromaticity signals a * b * will be described. The saturation signal C * is input to the non-linear converter 107 and converted into a saturation conversion signal fc normalized between -1 and 1. FIG. 5 shows an example of the conversion form of the signal fc. The conversion form is controlled by _four parameters C * _{1 to} C * ₄ ,
C * ₁ is the upper limit of C * where fc = −1, and C * ₂ is fc =
0 is the lower limit of C *, C * ₃ is the upper limit of C * where fc = 0, and C * ₄ is the lower limit of C * where fc = 1,
These are set to be changeable from the outside, and are set to (10, 15, 20, 50) in this embodiment. Such a non-linear converter 107 can be realized by a lookup table. The edge weight amount fe and the saturation conversion signal fc are input to the coefficient determining circuit 111, and the coefficient k is calculated by k = 1 + fe · fc (9). The coefficient determination circuit 111 can be realized by a combination of a multiplier and an adder. The coefficient k has a value of 1 in the case of a halftone image having a small edge weight fe, a value of 0 in the case of a large edge weight fe and a black character having low saturation, and a large edge weight fe. And, in the case of a color character with high saturation, it becomes a void 2. The coefficient k is the chromaticity signal a of the pixel of interest in the coefficient operating circuits 110a and 110b.
It acts on * and b *, and is converted into a * o and b * o as in a * o = a * · k, b * o = b * · k (10). Therefore, the input chromaticity signals a * and b * are output without conversion in the case of a halftone image, and are saturated in the case of a black character to become achromatic. Further, in the case of a color character, saturation emphasis having a different degree depending on the original saturation is applied, and the maximum is doubled.

Through the above steps, in the present embodiment, the increase of noise in the halftone image due to the edge enhancement processing is suppressed, visually natural edge enhancement is provided, and an unnatural image defect does not occur. Further, the edge emphasis amount can be set in a simple form. Further, the brightness and saturation are controlled so that the character signal after edge enhancement is reproduced in an achromatic color for black characters and the original saturation for color characters, and the input chromaticity in the case of FIG. It is possible to approximate the chromaticity of the original document.

[0031]

As described above, according to the present invention,
Since the edge enhancement and smoothing process changes continuously according to the edge strength of the input signal, visually natural edge enhancement can be performed, and at the same time, the coloring of black characters on the input device and the decrease of the saturation of color characters can be prevented. It can be adaptively corrected.
Further, the parameters for that purpose are set with two simple parameters for the edge emphasis amount and at most four for the saturation emphasis amount, so that the adjustment can be easily performed.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of an embodiment of a color image processing apparatus of the present invention.

FIG. 2 is a diagram showing an example of spatial frequency characteristics of a luminance signal smoothing circuit 103a and a luminance signal edge enhancer 103b.

FIG. 3 is a diagram showing a configuration example of a luminance signal limit value determination circuit 108.

FIG. 4 is an explanatory diagram illustrating an example of characteristics of a maximum limit value determination circuit 108-1 (FIG. 3) that is a part of the brightness signal limit value determination circuit 108.

FIG. 5 is an explanatory diagram showing an example of conversion characteristics of a saturation signal nonlinear converter 107.

FIG. 6 is a configuration diagram of a conventional edge enhancement processing method.

FIG. 7 is an explanatory diagram illustrating a problem in black character / color character processing in a conventional edge enhancement processing method.

8 is an example of a configuration of a luminance signal edge detector 102. FIG.

9 is an explanatory diagram illustrating an example of conversion characteristics of a non-linear converter 105 for a luminance signal edge amount. FIG.

[Explanation of sign]

101a, 101b ... Smoothing circuit, 102 ... Edge detector, 103a ... Smoothing circuit, 103b ... Edge enhancer,
104 ... Saturation generation circuit, 105 ... Non-linear converter, 106
... Weighted averaging circuit, 107 ... Non-linear converter, 108 ... Limit value determination circuit, 109 ... Maximum value limiter, 110a, 11
0b ... Coefficient acting circuit, 111 ... Coefficient determining circuit.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical indication location H04N 1/46 H04N 1/40 101 C 101 D 1/46 Z (72) Inventor Satoshi Suzuki Kanagawa 2274 Hongo, Ebina City Inside Fuji Xerox Co., Ltd.

Claims (11)

[Claims] 1. An edge detecting unit for obtaining an edge amount signal from a luminance signal of a luminance / chromaticity separation signal, an edge enhancing unit for enhancing processing of an edge portion in an image represented by the luminance signal, and the luminance signal. Smoothing means for smoothing, a saturation detecting means for detecting a saturation signal of an image from the chromaticity signal of the luminance / chromaticity separation signal, and the edge enhancement based on the edge amount signal detected by the edge detecting means. Chromaticity corresponding to compression or expansion of saturation using means for performing luminance conversion by mixing the output of the means and the output of the smoothing means, and the output of the edge detection means and the output of the saturation detection means A color image processing apparatus comprising: a means for performing chromaticity conversion to obtain a signal. 2. The color image processing apparatus according to claim 1, further comprising a first non-linear conversion means 105 for performing non-linear conversion of the output of the edge detection means. 3. The first non-linear conversion means has a conversion characteristic in which a conversion output signal fe is standardized between 0 and 1 and has conversion characteristics controlled by a plurality of parameters, and the plurality of parameters are fe = 3. The color according to claim 2, further comprising a first parameter for controlling an upper limit value of the converted output signal which becomes 0 and a second parameter which controls a lower limit value of the converted output signal for which fe = 1. Image processing device. 4. The color image processing apparatus according to claim 2, further comprising adjusting means for adjusting a parameter of a conversion characteristic in the first converting means. 5. The color image processing apparatus according to claim 1, further comprising second non-linear conversion means for performing non-linear conversion of the output of the saturation detection means. 6. The second non-linear conversion means has a conversion characteristic in which a conversion output signal fc is standardized between 1 and −1 and has conversion characteristics controlled by a plurality of parameters, and the plurality of parameters are fc. A first parameter for controlling the upper limit value of the converted output signal with −1, a second parameter for controlling the lower limit value of the converted output signal with fc = 0, and an upper limit of the converted output signal with fc = 0. The color image processing apparatus according to claim 5, further comprising a third parameter for controlling a value and a fourth parameter for controlling a lower limit value of the converted output signal with fc = 1. 7. The color image processing apparatus according to claim 6, further comprising adjusting means for adjusting parameters of conversion characteristics in the second non-linear converting means. 8. The means for performing the luminance conversion is an output L * p of the smoothing means and an output L * c of the edge enhancing means.
3. The color image processing apparatus according to claim 2, further comprising a weighted averaging unit that obtains a weighted average output L * ′ by the calculation of the following equation using the output fe of the first conversion unit as a weight value. L * '= fe * L * c + (1-fe) * L * p 9. The color image processing according to claim 1, wherein the means for performing the luminance conversion has a limiting means for limiting the maximum value of the luminance conversion output based on the edge weight amount and the saturation conversion signal. apparatus. 10. The means for performing the color conversion calculates a coefficient k from the edge weight amount fe and the saturation conversion signal fc as k = 1 + fe.
The color image processing apparatus according to claim 1, further comprising: coefficient determining means for calculating by the calculation of fc, and coefficient operating means for multiplying the chromaticity signal by the coefficient k calculated by the coefficient determining means. 11. A step of obtaining an edge amount signal from a luminance signal of a luminance / chromaticity separation signal, a step of obtaining a luminance signal obtained by smoothing an image and a luminance signal enhancing an edge part from the luminance signal, Obtaining a chroma signal of an image from a chromaticity signal of a chromaticity separation signal; mixing a luminance signal smoothing the image based on the edge amount signal and a luminance signal emphasizing an edge portion; the edge amount A color image processing method using a signal and the saturation signal to obtain a chromaticity signal corresponding to compression or expansion of the saturation.