JPS63199569A - Image processor - Google Patents

Abstract

PURPOSE:To reproduce the color half tone of a colored original and a color character at a high quality through the aid of a simple processing circuit constitution by provid ing a circuit which generates one kind of a signal, being common to the respective color signals of red, green and blue, and the circuit which detects a value, correspond ing to the density gradient of the colored original, and a data conversion circuit which converts an input density value adaptively. CONSTITUTION:The density information of the colored original is read as the density signal of 8-bits through three kinds of color separation filters of red, green and blue. In a picture processing device 8, the respective color signals R,G,B of red, green and blue are supplied to the adaptive data conversion circuit 5, and at the same time, the density gradient value of one kind of the density signal is detected by a density gradient detection circuit 1a, and the operation of the adaptive data conversion circuit 5 is controlled according to this density gradient value. Namely, the respective color signals R, G, B are converted adaptively according to the density gradient value of one kind of the density signal. Thus, by simply dither-processing a converted density data by a dither-processing circuit 3, the part of the character is binarized, the part of the half tone is dither-processed, and an area where both are mixed, is adaptively processed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a document reading device used in a digital copying machine, a facsimile machine, etc. The present invention relates to an image processing apparatus that identifies both areas after reading a color document containing a mixture of areas, and performs appropriate processing on each area.

[Conventional technology and its problems]

Many common documents contain a mixture of binary images such as characters and halftone images such as photographs and prints. If such a document is simply converted into a binary format and recorded, the quality of the text will be good, but the quality of the photo will deteriorate; conversely, if the document is converted into a binary format using halftone generation methods such as dithering, the quality of the photo will be good. However, the quality of the characters deteriorates. In order to obtain good quality for each text and photo, two different types of areas should be identified. For example, text should be divided into two simple
Then, the photo needs to be dithered.

As a method for distinguishing between the character area and the halftone area, there is a method that detects the density gradient in the document. One of them is the absolute value of the difference between the focus value, which is the gray value data of the pixel of interest, and the defocus value, which is the average value of the pixels surrounding the pixel of interest, as shown in Japanese Patent Application Laid-Open No. 58-220563. There is a method of determining that it is a character area when it exceeds a certain threshold. Also, JP-A-58-3374
As shown in the above publication, there is a method in which each area is determined based on the difference between the maximum and minimum gray values of pixels within a block.

As circuits for faithfully reproducing color characters and halftone images using these methods, there are circuits shown in FIGS. 7 and 8.

The circuit shown in Figure 7 uses three types of input signals R, G, and
For B, each separation processing circuit IR, IG.

Characters using the concentration gradient detection method described above in IB.

Separates halftones, performs binarization processing on three types of signals R, G, and B using binarization circuits 2R, 2G, and 2B, and independently performs dither processing using dither processing circuits 3R, 3G, and 3B. By doing this, the output color signals r, g, b
This is what you get. This method of separating regions independently for each color has a high accuracy, but has the problem of increasing the hardware scale.

On the other hand, in the circuit shown in FIG. 8, a luminance signal Y is formed from one specific type of signal, for example, three types of signals R, G, and B by a matrix circuit (not shown), and this signal is processed in the separation processing circuit 1. Characters and halftones are separated using only the luminance signal Y, and the result is red.

It is designed to be divided into three types: green and blue. According to the circuit shown in FIG. 8, the separation process can be reduced, but
In order to increase the accuracy for color characters, new character determination processing is required.

That is, it is necessary to provide a character color determining circuit 4 for determining the character color based on the three types of signals R, G, and B, red, green, and blue, and converting it into seven colors. In FIG. 8, the binarization circuit 2 and dither processing circuit 3 for each color are represented by a common block.

In addition, in both the circuits shown in FIGS. 7 and 8, the halftone area and the character area are absolutely separated depending on whether the density gradient value exceeds a certain threshold value, so the sharpness that exists in the halftone area is In some cases, a change in density may be mistakenly identified as a character and output as a colored edge.

The present invention was devised to solve the above-mentioned problems, and an object of the present invention is to reproduce color halftones and color characters of a color original with high image quality using a simple processing circuit configuration.

[Means for solving problems]

In order to achieve the above object, the present invention provides an apparatus for reading and recording a color original in which a character area and a halftone area are mixed.
! l. a circuit that generates one type of signal common to each blue color signal; and a circuit that detects a value corresponding to the density gradient of the color original from the one type of signal;
The present invention is characterized in that it further includes a data conversion circuit that adaptively converts the input gray value of each of the color signals according to a value corresponding to the density gradient.

The data conversion circuit compares a value corresponding to the concentration gradient with a high threshold value and a low threshold value, and performs binarization processing when it is higher than the high threshold value, dither processing when it is lower than the low threshold value, and performs both. When the value is between the dark values, it is possible to use a method that performs a predetermined conversion process based on the manual grayscale signal and a value corresponding to the density gradient.

[Effect]

In the present invention, one type of signal, for example, a luminance signal, common to each color signal is generated from red, green, and blue color signals obtained by color separation of a color original, and from this one type of signal, the color original is The value corresponding to the concentration gradient of is detected. and,
The manual grayscale signal of the color signal is adaptively converted in accordance with a value corresponding to this density gradient. For example, when the value corresponding to the density gradient is higher than a high threshold, the area is treated as a character area and binarization processing is performed, and when it is lower than the low threshold, it is regarded as a halftone area and dither processing is performed. Further, when the value corresponding to the density gradient is between the two thresholds, a predetermined conversion process is performed based on the human-powered gradation signal and the density gradient value, and the error is corrected in a pseudo manner.

〔Example〕

DETAILED DESCRIPTION OF THE INVENTION Hereinafter, features of the present invention will be specifically described based on examples with reference to the drawings.

FIG. 2 shows a schematic block diagram of the entire image processing device.

In the figure, reference numeral 6 is a color image human-powered device using, for example, a CCD (charge-coupled device) image sensor, which converts the grayscale information of a color document into an 8-pit image through three types of color separation filters: red, green, and blue. It is read as a grayscale signal. The resolution is approximately 160 dots/cm (400 dots/inch). The gray level signals of the read color original are stored in an image memory 7 of 8 bits per pixel. The image memory 7 has a capacity to store red, a, and blue color signals of a color document for one page of A4 size, and its output is supplied to an image processing device 8.

As shown in FIG. 1, the image processing device 8 includes a density gradient detection circuit 1a that detects the density gradient of a color document, and an adaptive circuit that adaptively converts the human input signals R9G and B according to the detected density gradient value. The data conversion circuit 5 is composed of a dither processing circuit 3 that allows the image signal converted by 5° to be outputted to a color image output device 10 such as a color printer. Note that details of the image processing device 8 will be described later.
Further, signal control of the entire device is performed by a control device 9. The color image output bag W10 is for recording a binary image at a resolution of approximately 320 dots/cm (800 dots/inch). Although an electrophotographic full-color laser beam printer is taken as an example here, a color thermal printer or a color inkjet printer may also be used.

Next, the image processing device 8 will be explained.

In the image processing device 8, red, green, and blue color signals R,
C, , B are supplied to the adaptive data conversion circuit 5, and the density gradient detection circuit 1a detects the density gradient value of one type of density signal representing the density of the color document. control the behavior of That is, 1
The reproducibility of character parts is improved by adaptively converting each color signal R, G, and B obtained by three color filters of red, green, and blue according to the density gradient value of each type of gray signal.

The density signal used in this density gradient detection circuit 1a is as follows:
It is considered best to use a luminance signal that corresponds to human visual characteristics.

The luminance signal Y is obtained, for example, using equation (1).

y=OJR+0.59G+O,lIB ・ ・ ・
・(0) The R, G, and B coefficients in equation (1) are known from televisions, etc., but the density signal Y is expressed in a general equation as shown in equation (2), and the spectral characteristics of the input device are Depending on the difference, the values of coefficients a, b, and c in equation (2) may be set arbitrarily.

Yd= a-R+ b-c+ c-B...-(
2) In this embodiment, the density gradient detection circuit 1a detects the density gradient in a color original using the luminance signal Y obtained from each color signal R, G, and B by matrix calculation based on equation (1). Detected. As the concentration gradient detection circuit 1a, a circuit that makes a determination based on the output of a bandpass filter for extracting character components, which is described in Japanese Patent Application No. 197665/1988, which was previously proposed by the present inventor, is used here. The coefficients of this bandpass filter are shown in FIG. 3, and the frequency characteristics are shown in FIG. 4.

As can be seen from Figure 4, the frequency characteristics of the bandpass filter are:
133 lines (5,23fl p/mm) ~175 lines (6
, 91p/mm) is set to remove the halftone dot components of halftone dot printing, and the high frequency components in the document excluding the halftone dot components, that is, the character components, are extracted. Note that lp is line pairs (li]e pair
s), and 133 lines and 175 lines each have a dot density of 1334! p/inch, meaning 175βp/inch.

FIG. 5 shows the separation error of one character part in the halftone part with respect to the density gradient value g obtained by this process.

As can be seen from this figure, the separation threshold is r in the figure for the concentration gradient value g. If set to high (as shown by It can be seen that there is a reciprocal relationship between the two departments.

If you try to completely separate something with such characteristics into two areas using a single threshold, areas that are text areas but have values slightly lower than the threshold will be dithered, while areas that have values slightly higher than the threshold will be dithered. The halftone area is completely binarized, and both result in a decrease in quality.

Therefore, in this embodiment, as in the case of Japanese Patent Application No. 61-197665, two threshold values '@all+rain', high and low, are provided as thresholds for the density gradient value g. Opening price r□8. For regions having values between r1°, a method is adopted in which the grayscale values are adaptively converted to reproduce the image without much deterioration in image quality.

In this embodiment, as shown in FIG. 1, in the density gradient detection circuit 1a, the luminance signal Y is used to detect the density gradient value g by the bandpass filter.

Then, each color signal R, G is determined based on the formula shown below.

The configuration is such that each of the grayscale values of the human image B is converted. Note that since the same processing is applied to each color signal R, G, and B, the gray value of each signal R1G and B is represented by d here.

Now, when the gray value of the pixel being scanned is dSa and the gradient value is g, g is r. A normalized concentration gradient value r normalized within the range of lz rain is obtained using equation (3).

roam rain Next, when the normalized concentration gradient value is greater than 1, r=l
and when r is smaller than 0, r=Q.

Changes in the normalized concentration gradient value r are shown on the horizontal axis of FIG. 5 along with changes in the concentration gradient value g.

The density value d is determined by the normalized density gradient value r obtained by equation (3).
is converted as shown in equation (4).

However, dc: gradation value after conversion d: gradation value before conversion th: Binarization threshold for text dmax: Maximum gradation value, that is, if the gradation value d before conversion is equal to or greater than the binarization threshold for text th If so, then dC=r(dmax ti) + d; otherwise, dc=rd+d.

In the adaptive data conversion circuit 5, Equations (3) and (4)
By calculating the gradation value dc after conversion,
Data suitable for each of the text portion and halftone portion is generated.

For example, the concentration gradient value g is now the high threshold value r3. If it is larger than H, that is, if it is a character area, from equation (3),
r>l, and is replaced by r=l.

Here, according to equation (4), the gray value d before conversion is 2 for characters.
When it is equal to or greater than the value threshold th, d C = d ma,,
When it is smaller than the threshold th, dc=Q, and it can be seen that the data is converted into completely binarized data.

Furthermore, if the concentration gradient value g is smaller than the low threshold value rmi□,
That is, in the case of a halftone area, r=0, and from equation (4), d and -d are obtained, and the grayscale value is preserved as is.

If the concentration gradient value g is high threshold r, 8 and low threshold r ++I
When the value is between n, data conversion is performed adaptively using the normalized concentration gradient value r and the pre-conversion gray value d based on equation (4), making it possible to correct errors in a pseudo manner. .

In this way, by simply dithering the converted grayscale data in the dither processing circuit 3, the character part can be divided into 2 parts.
The halftone part is dithered, and areas where both are mixed are adaptively processed.

Next, another example of data conversion in the adaptive data conversion circuit 5 will be described below.

Now, when the gray value of the pixel being scanned is d1 and the density gradient value is g, g is converted as follows.

r 11411 r m111 However, RC: Intensity of concentration gradient n: Intensity constant Next, when RC is larger than n, it is set as R6-n, and when Ro is smaller than 0, it is set as R620. Note that the reason for setting Rc=n when R1 is larger than n is to limit the maximum value of Rc, and this conversion is not necessarily necessary.

Further, the intensity R of the density gradient is set again as R=RC+1 (6), and the density value is converted as follows using the density gradient intensity R and the density value d.

dc = R (d - th) + th ・・・・・・
・(7) However, dc: gradation value after conversion th: density gradient value g higher than low threshold r1□ due to conversion equal to or higher than the binarization threshold for characters
If the value is higher than the binarization threshold th, the gray value d of a pixel having (d-
th).

Furthermore, at the gray value d of a pixel having a low threshold value r1 and a lower density gradient value g, Rc''O is obtained, so from equation (6), R
=1. Therefore, from equation (7), dc=d, and the gray value does not change.

In other words, as explained above, the gray value d is the low threshold rsl
In an area smaller than h, that is, in a halftone area, the grayscale data is saved as is, but for an image in a mixed area and one character area, the threshold value th for binarization is applied.

It is adaptively and appropriately converted depending on the magnitude of the grayscale value d and the intensity R of the density gradient.

Next, an example of processing for each character color of yellow, magenta, cyan, blue, green, red, and black is shown in FIG.

Now, in FIG. 6, the concentration gradient value g is the high threshold value r.

Assume that the above is the case. At this time, the document is determined to be a text area, and if the manual gradation value is larger than the binary darkness value th for text, it is raised to the maximum gradation value r255, and if it is smaller, it is lowered to the minimum gradation value ``0''. . At this time, 2
If the value conversion threshold th is set to be smaller than the gradation value of the main component colors constituting the color character, only the main component colors will be raised to the maximum value, and the other colors will be lowered to the minimum value. Therefore, the human-powered color characters are binarized into red, green, and blue, respectively, and the unprocessed signal shown by the broken line is converted into the signal shown by the solid line, so that the color character is faithfully reproduced.

Also, for the character part, the maximum value is "255" and the minimum value is "0".
'', the image does not deteriorate even if dither processing is performed by the dither processing circuit 3.

In this way, the adaptive data conversion circuit 5 shown in FIG. 1 adaptively converts data for the red, green, and blue human input signals R, G, and B, respectively. Note that data conversion can be easily performed using a look-up table or the like, and the adaptive data conversion circuit 5 can be realized with a very simple hardware configuration.

In addition, in this embodiment, since the data is directly converted, there is no need to use dither processing and binarization processing for output as in the conventional example, and it is possible to simply dither the converted data. In this way, it is possible to output an image corresponding to dither processing for the halftone region and binarization processing for the text region, and it is possible to appropriately reproduce the text and halftone regions with high image quality.

In the above-mentioned embodiment, the density gradient value was detected using a bandpass filter, but the method for detecting the density gradient value in the original document is as described in Japanese Patent Application Laid-open No. 58-3374.
The method that takes the difference between the maximum value and the minimum value within the window as shown in the Japanese Patent Publication No. 58-220563, and the method that takes the absolute value of the difference between the focus value and the defocus value as shown in Japanese Patent Application Laid-Open No. 58-220563 are adopted. You may.

In other words, any method may be used as long as it can detect a value corresponding to the density gradient of a color original, and it is not suitable for methods where errors in the character area and halftone area have a contradictory relationship as shown in Figure 5. Applicable to all.

〔Effect of the invention〕

As described above, in the present invention, a value corresponding to the density gradient of a color document is detected, and manual grayscale signals of red, green, and blue are adaptively converted according to this value. For example, if the darkness value for area identification is set to two levels and the density gradient value of the read image is at both thresholds, the data is adaptively converted to a predetermined value according to the a-light value and the density gradient value. do. This prevents sharp density changes in halftone areas from being mistaken for characters, and prevents unnaturally colored edges from occurring. Further, according to the present invention, since the value corresponding to the density gradient of a color original is detected from one type of human input signal, there is no need to detect the density gradient in each of the three systems of red, green, and blue. The circuit configuration becomes simple. Therefore, according to the invention, colored characters.

High-quality images can be provided with a simple circuit configuration even for color originals containing color halftone images.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an example of the configuration of an image processing device of the present invention, FIG. 2 is a schematic diagram showing the overall configuration of the image processing device, and FIG. 3 is an explanatory diagram showing coefficients of a bandpass filter for character component extraction. , Fig. 4 is a graph showing the spatial frequency characteristics of the same band filter, Fig. 5 is a graph showing changes in the separation error of one halftone character area with respect to the density gradient value, and Fig. 6 is for explaining the processing for text color. 7 and 8 are block diagrams of different conventional color image processing devices, respectively. l: Separation processing circuit la: Concentration gradient detection circuit 2
: Binarization circuit 3: Dither processing circuit 4: Character color determination circuit 5: Adaptive data conversion circuit 6: Color image input device 7: Image memory 8: Image processing device
9: Control device 10: Color image output device Patent applicant: Fuji Xerox Co., Ltd. Agent
Masu Kobori (and 2 others) Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. Spatial frequency [jlp/mm Koshin 5 U O,,,,,OO,250,50,75L,,,,,I
Normalized concentration gradient value r Fig. 6 RGB Fig. 7 Fig. 8

Claims (1)

[Claims] 1. In an apparatus for reading and recording a color original in which a character area and a halftone area are mixed, each color signal is obtained from red, green, and blue color signals obtained by color-separating the color original. a circuit that generates one type of signal common to the two, a circuit that detects a value corresponding to the density gradient of the color original from the one type of signal, and an input of each color signal according to the value corresponding to the density gradient. An image processing device comprising: a data conversion circuit that adaptively converts grayscale values. 2. The data conversion circuit compares a value corresponding to the concentration gradient with a high threshold value and a low threshold value, and performs binarization processing when the value is higher than the high threshold value, dither processing when it is lower than the low threshold value, and 2. The image processing apparatus according to claim 1, wherein when the value is between both threshold values, a predetermined conversion process is performed based on the input gradation signal and a value corresponding to the density gradient.