JPH07226842A - Method/device for controlling base in color image-forming device - Google Patents

Abstract

PURPOSE:To reproduce the black character of high quality by discriminating whether a character is halftone or not and whether a color is black or not in a noticed image element, detecting base quantity and substituting the signal of the image element judged to be the black character for a signal corresponding to base quantity. CONSTITUTION:YMC conversion circuits 8 and 15 convert system values L* a* and b* into recording signals YMC, and ground color removal processing circuits 9 and 18 generate the black signals K of proper quantity so that the muddiness of the colors do not occur. A base recognition circuit 14 detects a base level from the signals of L*, a* and b*. A selector 10 is switched based on a character/halftone discrimination result and a color/black discrimination result in a character/halftone in a color/black recognition circuit 16. Namely, the selector 10 is switched to a circuit 15-side in an area judged to be the black character, and it is switched to a circuit 9-side in areas except the area. Thus, the black character in the white base can be reproduced without color remaining, and the base remains when the black characters are written on the low density base/on the color base.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus, and more particularly to a background control device in a color image forming apparatus.

[0002]

2. Description of the Related Art A conventional digital color image forming apparatus uses image data that is color-separated and read, and the image data is a character, an image, or a chromatic color or an achromatic color. It is common to have a mode in which it is determined and control is performed so as to select the optimum post-processing for each area (for example, JP-A-2-295354).
(See the official gazette).

For example, for characters, control is performed by using a filter with strong edge emphasis, high gradation of contrast, and output means with high resolution.
On the other hand, for the halftone region, control is performed so that processing that emphasizes gradation is performed. Further, with respect to the black character area, the black amount and the background removal amount are controlled so that the black character region can be reproduced in a single color, and the control of the remaining color in the periphery is performed.

[0004]

However, in an input image, it is very difficult to make a decision in an image that does not fall within the category of the above decision result or in a narrow area seen by the separation circuit.
There is, so to speak, an image located in the boundary area of the judgment.

For example, 1) black characters in a manuscript with a dark background such as a newspaper or color paper, 2) a region in which the background is covered with a pattern on the back due to back movement in a magazine, or a background colored in a light color The black letters above, etc.

Since these images are on the boundary of character / halftone determination, there is no problem in that they are uniformly processed as a black character area or a halftone area, but some characters and backgrounds are in halftone. If there is a mixture of cases where the characters are determined or the characters are determined to be black characters, there is an inconvenience that the quality of characters and the quality of the background appear to be extremely deteriorated.

Therefore, an object of the present invention is to reproduce high-quality black characters even when it is difficult to determine the type of image.

[0008]

In order to solve the above problems, the present invention sequentially pays attention to adjacent pixels of input image data, determines whether the pixel of interest is a character or halftone, and determines the color. It is characterized in that it is determined whether it is black or not, the peripheral amount around the pixel of interest is referred to detect the background amount, and the signal of the pixel determined to be a black character is replaced with a signal corresponding to the background amount.

[0009]

When the pixel of interest is a black character, the level of pixels around the black character is replaced with a signal according to the background amount, so that the density around the black character and the background density are equal.
The deterioration of the quality of the background around the black characters is prevented.

[0010]

Embodiments of the present invention will be described below with reference to the drawings.

First, the configuration of the entire system of the color image forming apparatus according to the present invention will be described. 1 and 2
FIG. 1 is a diagram for explaining an example of a schematic configuration of a system of a color image forming apparatus. The color image forming apparatus includes blue, green and red color signals BGR from the color image reading apparatus.
Is inputted and converted into respective color signals YMCK of yellow, magenta, cyan and black.

In FIGS. 1 and 2, a shading correction circuit 1 scans an image of a color original in a color image reading device, and separates the color image with a color CCD (charge coupled device) sensor to read the BGR pixel. The correction of misalignment, variation between sensor chips, variation between pixels in the chip, unevenness of light amount, etc. is performed for each pixel.

The L ^* conversion circuit 2 converts the reflectance signal read by the CCD sensor into a lightness scale signal L ^* _bgr , and the filter circuit 3 receives the input B signal.
MTF (modulation) of each GR signal
The transfer function characteristic is corrected.

The L ^* a ^* b ^* conversion circuit 4 converts the lightness scale signal L ^* _bgr from the standard system value signal L ^* a.
It is converted into ^* b ^* . Here, the system value (L ^* a ^* b ^* ) signal represents the lightness on the L ^* axis, and the saturation and hue on the two-dimensional plane of the a ^* axis and the b ^* axis orthogonal to the L ^* axis ( (See FIG. 4C).

The hue / saturation conversion circuit 5 receives the hue (Hue) and the saturation (Ch) from the system value signals L ^* a ^* b ^*.
roma) signal is generated.

The color adjusting circuit 6 has H '= H + ΔH (hue conversion), L ^* ' = L ^* + ΔL ^* (lightness parallel movement), L ^* '.
= ΒL ^* + ΔL ^* (gamma correction of lightness and parallel movement),
C ′ = rC = r (√ (a ^{* 2} + b ^{* 2} ) (gamma conversion of saturation) for color adjustment, color recognition, and conversion processing. Also, a ^* b ^* conversion circuit. In step 7, the hue and saturation are inversely converted into a ^* b ^* .

The YMC conversion circuit 8 has a system value L
^* a ^* b ^* is the recording signal Y for yellow, magenta, and cyan
The color conversion processing circuit 9 converts the signal into MC, and the undercolor removal processing circuit 9 generates a proper amount of the black signal K so as not to cause color turbidity and reduces YMC according to the amount.

When the selector circuit 10 receives the discrimination result of the character / halftone / color / black recognition circuit 16 to be described later, and when it is discriminated in the black and character area, the K signal is converted into the L ^* signal, Further, the YMC signal is the background data (L ^* a) output from the recognition result of the background recognition circuit 14 described later.
^* b ^* ) is replaced with the background YMC signal obtained by conversion by the YMC conversion circuit. It should be noted that the K signal is replaced with the L ^* signal in the above-mentioned color image reading device when the black character or line drawing at the same place is read by the blue, green and red image sensors, and the delicate positions of blue, green and red are detected. This is to prevent the amount of the K signal resulting from the undercolor removal from decreasing due to the influence of the shift and the shift of the MTF. If the influence is not large, the K signal may be used as it is.

The filter circuit 11 adjusts the sharpness of yellow, magenta, cyan and black (L ^* ), and the gamma correction circuit 12 controls the gamma of the gradation suitable for the characteristics such as halftone / character. The gradation characteristic conversion circuit 13 performs conversion, and performs non-linear gradation conversion suitable for the recording characteristics of the image output apparatus to which the YMCK signal from the color image forming apparatus is supplied, and further performs color balance adjustment and the like. .

The background recognition circuit 14 knows the background level of the original from the L ^* a ^* b ^* signals. The YMC conversion circuit 15 and the undercolor removal processing circuit 18 include the YMC conversion circuit 8 and the undercolor removal processing circuit. It is similar to 9. The character / halftone / color / black recognition circuit 16 recognizes whether the pixel of interest is included in the character area or the halftone area, and whether it is included in the chromatic color area or the achromatic color area. , A region / mode signal generation circuit 1 for generating an identification signal
Reference numeral 7 is for generating a signal for switching the processing of the pixel of interest from a document mode designated by the user, a region signal set by a region designation means (not shown), and the like.

Note that FIG. 3 omits the undercolor removal processing circuit 18 shown in FIG. 2, and when the starting point of undercolor removal is set from a point of low lightness, the undercolor removal remains on the background. If applied when not involved, hardware costs can be saved.

In the block diagrams shown in FIGS. 1 and 2,
The signal system from the shading correction circuit 1 to the undercolor removal processing circuit 9 and the filter circuit 11 to the gradation characteristic conversion circuit 1
Since the circuits up to 3 have the same configuration as that of the conventional color image forming apparatus, a detailed description thereof will be omitted here, and a circuit of a portion directly related to the present invention will be described in detail below.

The operation of the background recognition circuit 14 will be described with reference to FIG. FIG. 10A shows that a 7 × 5 area around the target pixel is referenced. The value of each reference pixel is, as shown in FIG.
The distance S in the ^* a ^* b ^* space is stored. The distance S is represented by the following expression, for example. However, L ^* w, a ^*
w and b ^* w are L ^* a ^* b ^* values on a white background.

S = √ [(L ^* -L ^* w) ² + (a ^* -a
^{* W) 2 + (b *} -b * w) 2 ] used as the amount of the base to explain this distance S, below.

Further, FIG. 7B is a diagram showing a distribution of the background amount by cutting out one scan line in the 7 × 5 reference area. In this figure, the minimum value in the reference area is selected and this minimum value is set as the background amount. The pixel having the minimum value in the two-dimensional setting area is output as the background color component L ^* a ^* b ^* of the pixel of interest. The output background color component is converted from L ^* a ^* b ^* to YMC in the YMC conversion circuit 15 shown in FIGS.

In the present embodiment, a description will be given using a 7 × 5 area, but it is clear that the area size may be changed according to the resolution of the system. It is also possible to use the area size as a parameter so that the area size can be adjusted according to the document type. Also, as the background amount,
When calculating the distance S between L ^* a ^* b ^* , the value that can be taken is small due to the characteristics of the background amount. Therefore, in order to reduce the hardware circuit scale, the above equation S = √ [( ^{L * -L * w) 2 +} (a * -a * w) 2 +
Instead of (b ^* -b ^* w) ² ], S = (L ^* -L ^* w) + (a ^* -a ^* w) + (b ^* -b
It is also possible to substitute an approximate expression such as ^* w). Further, in the case of black-and-white copying, or for the purpose of further reducing the hardware scale, it is possible to adopt a configuration in which S = (L ^* -L ^* w).

Furthermore, the L ^* a ^* b ^* value (L
^* w, a ^* w, b ^* w) may be simplified by assuming (0, 0, 0) from the beginning.

Next, the selector 10 will be described.

The selector 10 selects whether to perform normal processing or the processing according to the present invention. The selector 10 is switched on the basis of the character / halftone / color / black discrimination result and the color / black discrimination result in the character / halftone / color / blackness recognition circuit 16. That is, the selector 10 is switched to the YMC conversion circuit 15 side in the area determined to be a black character, and is switched to the undercolor removal processing circuit 9 side in the other areas. For example, when the original is a photographic original and the pictogram identification processing is not particularly required, the adaptive processing by the character / halftone identification may not be performed, and the background processing of the present invention may not be performed. .
It is possible to select whether or not to use this function when processing an office document mainly composed of characters and line drawings.

The state of the image when the background processing of the present invention is not performed and when it is performed will be described with reference to FIGS.
In the figure, for the sake of explanation, a cross-sectional view in the scan direction indicated by an arrow in FIG. 4A is used. Further, for the purpose of explanation, the concept of the present invention will be described with reference to the background black figure, but the color image that is actually output is printed in four colors according to the image processing described above.

When the background treatment of the present invention is not performed, and when there are black characters on the white background, according to the character / halftone discrimination result,
For example, when the process of cutting the signals other than the black signal is performed, the result of
In the case of an original having a small difference between the background level and the white background level as shown in (a), even if the background processing is performed to set the pixel level around the black character to 0, the result is as shown in FIG. Since the difference between the surrounding level and the background level is small, a natural black character can be reproduced. However, when there is a black character on a low density background or color background as shown in FIG. 6A, the pixel level around the black character is changed. When the background processing is set to 0, FIG.
The difference in level between the character area around the black line on the low-density background or the color background and the halftone area of the background becomes large as shown in, and black characters appear to float from the background and the image looks unnatural. Inconvenience occurs. Further, in a region where the character / halftone discrimination result is unstable, the processing frequently changes depending on the discrimination result, so that the reproduction image quality of the black character is further deteriorated.

On the other hand, when the background processing of this method is applied, as shown in FIGS. 7 (b) and 8 (b), the pixel level around the black character is not set to 0 level, but is set to the peripheral level. Since the background level of the pixel is replaced, the pixel level around the black character coincides with the surrounding background level, and the black character is reproduced naturally. Note that FIGS. 7A and 8A correspond to FIGS. 5A and 6A, respectively.

By replacing the pixel level around the black character with the background level of the peripheral pixels as in the above-described embodiment, the above-mentioned problems can be solved.

However, when there are black characters on the white background shown in FIG. 5 (a), it may be desired that there is no residual color as shown in FIG. 5 (b). In this case, by adding a background correction circuit as shown below to the background recognition circuit 14, it becomes possible to meet this requirement.

The control circuit will be described below with reference to FIGS. 9 and 10.

The first background correction circuit 18 uses the table shown in FIG. 9 for the detected background amount, for example.
The second background correction circuit 19 controls the correction ratio α of the output background amount. The correction amount α obtained from this table may be configured to reduce the distance of the output background color component L ^* a ^* b ^{* from} the white background, as shown in FIG. Inputs L ^* ₁ , a ^* ₁ , b ^* ₁ and outputs L ^* ₂ , a ^* ₂ , b ^* _{2 of} the second background correction circuit 19
The relationship _{^{between, L * 2 = f 1 (}} L * 1, α), a * 2 = f
₁ (a ^* ₁ , α) and b ^* ₂ = f ₁ (b ^* ₁ , α). Further, as shown in FIG. 10B, the ratio α is set to Y ₁ , M ₁ , and C ₁ after the output background color component L ^* a ^* b ^* is converted by the third background correction circuit 20. The function used is g, and the reduced output Y ₂ = g ₁ (Y ₁ , d), M
₂ = g ₁ (M ₁ , d) and C ₂ = g ₁ (C ₁ , d) may be obtained.

As a result, in the case of a black character on a white background, it becomes possible to reproduce without any color residue as shown in FIG. 5 (b), and in the case of a black character on a low density background / color background, as shown in FIG. 8 (b). As shown, the base remains. When the background level is at an intermediate level, as shown in FIG. 11 (b), the intermediate level is set so that continuous changes occur.

In the above description, the input BGR signal is converted into the L ^* a ^* b ^* signal, but
The background may be recognized and processed by other luminance / color difference system signals of the L ^* a ^* b ^* signal, or the background may be recognized and processed by a similar method with the BGR or YMC signal. Needless to say.

[0039]

As described above, by applying the present invention, even when a character / halftone discrimination process is applied to an image which is difficult to discriminate, the reproduction density around the black character is equal to the surrounding background level. As a result, high quality black characters can be reproduced.

[Brief description of drawings]

FIG. 1 is a block diagram illustrating an example of a schematic configuration of a system of a color image forming apparatus.

FIG. 2 is a block diagram following FIG.

FIG. 3 is another block diagram following FIG. 1.

FIG. 4 is an explanatory diagram showing an L ^* a ^* b ^* space.

FIG. 5 is an explanatory diagram showing pixel levels before and after background processing when black characters are present on a white background.

FIG. 6 is an explanatory diagram showing pixel levels before and after background processing when black characters are present on a low-density background / color background.

FIG. 7 is an explanatory diagram showing pixel levels before and after improved background processing when black characters are present on a white background.

FIG. 8 is an explanatory diagram showing improved pixel levels before and after background processing when black characters are present on a low-density background / color background.

FIG. 9 is a graph showing characteristics of a table used in the background correction circuit.

FIG. 10 is a block diagram illustrating a configuration example of a background correction circuit.

FIG. 11 is an explanatory diagram showing pixel levels before and after background processing when the background level is an intermediate level.

[Explanation of symbols]

1 ... Shading correction circuit, 2 ... L ^* conversion circuit, 3 ...
Filter circuit, 4 ... L ^* a ^* b ^* conversion circuit, 5 ... Hue
Saturation conversion circuit, 6 ... Color adjustment circuit, 7 ... a ^* b ^* conversion circuit, 8 ... YMC conversion circuit, 9 ... Undercolor removal processing circuit, 10
... selector circuit, 11 ... filter circuit, 12 ... gamma correction circuit, 13 ... gradation characteristic conversion circuit, 14 ... background recognition circuit, 15 ... YMC conversion circuit, 16 ... character / halftone / color /
Black recognition circuit, 18 ... Undercolor removal processing circuit

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical display location H04N 1/46 H04N 1/46 Z

Claims (3)

[Claims] 1. The pixels adjacent to the input image data are sequentially focused, and it is determined whether the pixel of interest is a character or halftone and whether it is color or black. A background control method in a color image forming apparatus, wherein the background amount is referred to and the signal of a pixel determined to be a black character is replaced with a signal corresponding to the background amount. 2. A means for converting blue, green, and red signals obtained by sequentially focusing on adjacent pixels of input image data into color signals having a different expression format, and the target pixel being a character or an intermediate pixel. To determine whether the pixel of interest is a color or black, a method of detecting the background amount by referring to peripheral pixels centered on the pixel of interest, and a yellow color of the pixel determined to be a black character. , A unit for replacing magenta and cyan signals with yellow, magenta and cyan signals corresponding to the background amount, and a background control device in a color image forming apparatus. 3. A means for converting blue, green, and red signals obtained by sequentially focusing on adjacent pixels of input image data into color signals of a different expression format, and the pixel of interest is a character or an intermediate pixel. To determine whether the target pixel is a color or black, a unit that refers to peripheral pixels centering on the target pixel and detects the background amount, and the background amount to the background amount. A color image comprising means for performing a correction in accordance with the corrected color, and means for replacing the yellow, magenta, and cyan signals of the pixel determined to be a black character with the yellow, magenta, and cyan signals corresponding to the corrected background amount. Substrate control device in forming apparatus.