JPH01292477A - Image processor - Google Patents

Abstract

PURPOSE:To obtain a binarized image of high quality corresponding to respective sorts by finding out maximum and minimum density values from image information within a prescribed range of an objective image element, deciding the sort of an image based upon the ratio of the values and binarizing a threshold corresponding to the decided sort. CONSTITUTION:Image information is inputted to line buffers 1 and the maximum and minimum density values in a local area are found out from the image information by a maximum/minimum detecting circuit 20. The ratio of the two values is calculated from the found values by a dividing circuit 21. The found ratio is compared with a prescribed threshold Th by a comparator 22 and the sort of the image is discriminated and decided. The 1st threshold memory 3 formed by a moving threshold calculating circuit 8 for a character part or the 2nd threshold memory 4 shown by a dither pattern for a photographing part is selected by a selector 5 on the basis of the decided result. Then image data delayed by a delay 7 is binarized by a binarizing circuit 6 on the basis of the value stored in the selected threshold memory.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Field of Application) The present invention is capable of processing a document image in which a text portion and a photo portion are mixed, with high resolution of the text portion and high gradation of the photo portion. The present invention relates to an image processing device that can maintain and binarize images.

(Prior Art) In an image processing device (document image processing device) that can handle not only code information but also image information, high-contrast image parts such as characters and line drawings ( The text part) is binarized using a fixed threshold set in advance, and the image part (photo part) with gradation, such as a photograph, is binarized using a pseudo gradation method such as the dither method. There is.

Incidentally, there is a problem in that when a character part is binarized using the dither method, its resolution deteriorates, and when a photographic part is binarized using a fixed threshold value, its gradation is impaired. In order to avoid such problems, conventional methods separate image regions according to their image characteristics and binarize them by adaptively using the above-mentioned fixed threshold or threshold determined by the dither method for each region. It is supposed to be done.

Conventionally, however, the maximum density difference ΔD l1ax of image densities in a local area of the image to be processed is detected, and this maximum density difference ΔD wax is compared with a predetermined threshold Th to determine whether the local area is a text area or not. part 2.
It is used to switch the form of value processing.

However, in such a processing method, since the value of the maximum density difference ΔD IIax detected here is small for low-contrast character parts such as so-called blurred characters, they are incorrectly determined to be photographic parts. Sometimes. As a result, a problem arose in that the resolution of text portions with low contrast was significantly impaired.

For example, as shown in FIG. 6, a document P that has been read and input has a character part A1 with high contrast, a photograph part B having gradation,
When the image information is composed of character portions C with low contrast such as faded characters, typical signal levels of image information in each area A, B, and C of the document P are as shown in FIG. Assuming that the dynamic range of the input image density is given by 8 bits [0 to FP]h, for example, (
4X4) Maximum density difference ΔD wa in local area of pixel
x is [DD-FP]h in area A and [DD-FP]h in area B.
10-40]h, and in region C, it is about [10-40]h. For this reason, [801h is used as the judgment threshold Th, ΔDmax>Th...Character part ΔD IIaX
≦Th...When determining the type of image of a local area as a photographic portion, a problem arises in that not only the area B but also the area C is erroneously determined as a photographic portion. Also, if [301h is used as the determination threshold Th, then the area B
A problem has arisen in that there is a risk that the text will be determined as a character part.

In this way, in the conventional device that determines the maximum density difference ΔD 1 lax using a predetermined threshold value and identifies the type of area,
For document images that include text with low contrast,
There are problems in that it is very difficult to binarize the text portion with good resolution and to binarize the photographic portion with good gradation. - (Problem to be solved by the invention) In this way, conventional devices perform binary processing on image information containing a mixture of text and photographic areas without simultaneously satisfying the resolution for the text and the gradation for the photographic areas. However, there are problems in that it is extremely difficult to adaptively control the format of the binarization process, especially when text portions with low contrast are included.

The present invention has been made in consideration of these circumstances, and its purpose is to provide high resolution for the text portion and high resolution for the photographic portion, even if the image information includes text portions with low contrast. An object of the present invention is to provide an image processing device that can perform binarization with good gradation.

[Configuration of the Invention (Means for Solving the Problems) An image processing apparatus according to the present invention calculates a maximum density value and a minimum density value from image information within a predetermined range including a pixel of interest in an image to be processed. The type of image formed by the pixel of interest is determined by discriminating the ratio between the maximum density value and the minimum density value as characteristic information of the pixel of interest using a predetermined threshold, and the type of image formed by the pixel of interest is determined according to the determination result. The present invention is characterized in that a threshold value for binarizing the image information of is set.

In other words, by effectively utilizing the characteristic of an image that the minimum density value itself is lower for text parts compared to photographic parts, we can determine what value the maximum density value can take compared to this minimum density value. This method is characterized in that the type of image to be processed is identified by checking the .

(Operation) According to the present invention, the maximum density value and the minimum density value within a predetermined range as a local region including the pixel of interest in the image to be processed are determined, and the ratio of these density values is calculated as the feature information of the image. Since the type of the image is identified and judged as such, even if the image has low contrast, it will not be mistakenly identified as a photographic portion depending on whether the maximum density value is higher or lower than the minimum density value. As a result, even if a document image contains various types of text and photo areas, it is possible to accurately identify the text and photo areas and adaptively set thresholds for binarization. Therefore, it is possible to perform binarization processing on text portions with good resolution and on photographic portions with good gradation.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic configuration diagram of an image processing apparatus according to an embodiment. Image information read and input by a reading device such as an image scanner is input to this image processing device at a rate of, for example, 8 pixels per pixel.
Input as bit digital data. The line buffer l temporarily stores such image information and provides it to the image processing described below, and is realized as, for example, a 4-line buffer.

The identification means 2 inputs the image information outputted from the line buffer 1 in synchronization with a predetermined clock, and calculates the maximum density value and minimum density value in a local area such as (4×4) pixels from the image information, respectively. , the ratio between the maximum density value and the minimum density value is obtained as feature information of the image, and the type of the image is identified and determined from this feature information.

The selection means (selector) 5 is controlled to switch according to the result of this discrimination, and the first threshold value given from the first threshold value generation means (memory) 3 or the dither value given from the second threshold value generation means (memory) 4 is controlled. A second threshold value such as matrix is selectively extracted as a threshold value for performing binary processing on the image information.

Then, the line buffer 1 is read out, and the delay means 7
The image information that is delayed by a predetermined timing and guided to the comparison means (binarization circuit) 6 is binarized using the threshold extracted by the selection means 5 and output.

Here, the first threshold value is determined by the dynamic threshold calculation means 8 according to the input image information, specifically, according to the maximum density value and minimum density value in the local area determined by the identification means 2. This is something that is dynamically required.

Now, the identification means 2 that executes the image type identification process is as follows:
A maximum/minimum density value detection circuit 20 which respectively calculates the maximum density value and minimum density value in a local region of (4×4) pixels including the pixel of interest from the image information read out from the 4-line buffer 1, and the maximum/minimum density value detection circuit 20. A division circuit 21 that divides the maximum density value Dff1aX obtained by the circuit 20 by its minimum density value Dmin to obtain the ratio.
and a comparison circuit 22 for identifying and determining the type of image information by comparing the ratio (DIlax/Dmin) between the maximum density value and the minimum density value obtained by the division circuit 21 as a predetermined threshold Th. It consists of As a result of this comparison by the comparison circuit 22, a selection control signal for selecting a threshold value for binarizing the image information is generated and output.

FIG. 2 shows an example of the configuration of the maximum/minimum value detection circuit 20 in the identification means 2. As shown in FIG. The maximum/minimum value detection circuit 20 detects the maximum and minimum values of the density within a (4×4) pixel area including the pixel of interest, as shown in FIG. 3, for the pixel of interest in the image to be processed. It is something to seek.

The maximum/minimum value detection circuit 20 receives, for example, input signals (8 bits/pixel) from the line buffer 1 sequentially in units of 4 pixels in the column direction in synchronization with the clock CLK, as shown in the operation timing shown in FIG. Image information selector 2
0a to comparators 20b, 20c.

It is sequentially distributed to 20d and 20e. Incidentally, the distribution of the image information input column by column to the comparators 20b, 20c, 20d, and 20e by the selector 20a is controlled by selection signals SEL and SE2 from a 2-bit counter 20h that operates in response to the clock CLK. be carried out.

These comparators 20b, 20c, 20d, and 20e compare image information in units of four pixels in the column direction,
The maximum density and minimum density in that column are determined respectively.

The comparators 20f and 20g at the next stage are the comparators 20b and 20g.
e.

The signals from 20d and 20e are input at the timing of EIHI;1, and the maximum and minimum values obtained in the column direction are compared in the row direction, and the maximum and minimum values among them are determined, respectively. be. Through the above comparison process, the maximum value D11aX and the minimum value D+gin of the density within the (4×4) pixel area shown in FIG. 3 are respectively determined and output at the timing of EDG2.

The division circuit 21 divides the maximum density value Dl1aX obtained in this way by its minimum value Da+1n, and calculates ΔD - (Dma
x /Dmin), and this feature information ΔD is compared with a predetermined threshold Th in the comparison circuit 22.゛This comparison judgment is as follows: ΔD>Th... Pixel of character area ΔD≦Th
...Performed as a pixel in a non-character area. As described above, the type of image constituted by the pixel of interest within a local predetermined range is discriminated and determined, and the selection of the threshold value by the selector 5 is controlled according to the result of this determination.

Note that the first threshold value to be selected by the selector 5 is generated by the dynamic threshold value calculation means 8 according to the image information. Specifically, according to the maximum density value D IIaX and the minimum density value D win determined by the maximum/minimum value detection circuit 23, the threshold value Bh for binarizing the image information is determined, for example, by Bh − (Dwax + Dajn ) / It is dynamically determined as 2. On the other hand, the second threshold value for binarizing the photographic area is given as dither pattern information (dither matrix) as shown in FIG. 5, for example. Such threshold value Bh (first threshold value) or the threshold value indicated by the dither pattern (second threshold value) is selectively extracted via the selector 5 based on the above-described identification determination result,
Binary processing of the image information is performed.

According to this apparatus configured in this manner, the difference in density of image information is small, and even when it is unclear from this difference in density alone whether the image part is a low-contrast character image or a photograph part. However, if it is a character image part, the minimum density value itself is a low value, whereas if it is a photograph part, the image density is high overall. Character image parts can be reliably detected and identified. That is, as described above, the maximum density value D max and the minimum density value D si in the local area
If we calculate the ratio ΔD to n, we can see that the minimum density value itself is high in photographic areas where the overall image density is high, and the minimum density value is low in character areas with low contrast. Even if the difference in density between the photo portion and the low-contrast character portion is the same, the difference in the minimum density value causes a large difference in the ratio ΔD between the maximum density value D l1aX and the minimum density value Dmin. As a result, it is possible to reliably determine a low-contrast character image as a character part, and to set an appropriate threshold for binarizing the character part.

Therefore, it is possible to identify the image type with high reliability regardless of the contrast (density difference) in a local area, and to binarize text with high resolution even in document images that contain multiple types of information. In addition, the photographic area can be binarized with good gradation.

Note that the present invention is not limited to the embodiments described above. For example, the size of the local area is shown in the example (4×4)
is not limited to this area, and the apparatus can also be configured to set it variably depending on the image to be processed. In addition, the means for adaptively generating the threshold value can be modified in various ways.
The dither pattern used to binarize the photographic area is also not particularly limited.

Further, the size of the dither matrix is not limited, and the dither pattern can be arranged not only in a dot-distributed manner but also in a dot-concentrated manner. Furthermore, in the embodiment, the density information was directly obtained from the read and input image information, but it is also possible to obtain the feature amount using a value converted from this amount to image density (logarithm of the reciprocal of the reflectance). . In addition, the present invention can be implemented with various modifications without departing from the gist thereof.

[Effects of the Invention] As explained above, according to the present invention, the ratio of the maximum density value to the minimum density value in a local area of image information is used as characteristic information of the image information to identify the image type, and the image information is Since the threshold value for binarization processing is selected and set, text parts can be binarized with good resolution regardless of their contrast, and photographic parts can be binarized with good gradation. This has great practical effects, such as the ability to As a result, a high-quality binarized image, which is the basis for various image processing, can be obtained, and great effects such as being able to improve the processing efficiency can be achieved.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram of an image processing device according to an embodiment of the present invention, FIG. 2 is a diagram showing a specific configuration example of the identification means in the embodiment device, and FIG. 3 is a diagram showing the image processing in the identification device. FIG. 4 is a diagram showing the concept of a pixel area, FIG. 4 is a diagram showing the processing operation timing of the embodiment device, and FIG. 5 is a diagram showing an example of a dither pattern as a second threshold for binarizing a photographic part. It is. FIG. 6 is a diagram showing an example of a document image to be processed, and FIG. 7 is a diagram showing the density level of a typical image signal of the image to be processed shown in FIG. l...Line buffer, 2...Identification means, 3...
First threshold generation means/means (memory) 1.4... Second threshold generation means (memory), 5... Selector, 6... Comparator (binarization circuit), 8... Dynamic threshold calculation circuit, 20
... Maximum/minimum value detection circuit, 21... Division circuit, 2
2... Comparison circuit. 1st cause 4Q9 EDG2
JJφ1”. Figure 4 g5 Figure 6

Claims (1)

[Claims] means for respectively obtaining a maximum density value and a minimum density value from image information within a predetermined range including the pixel of interest in the image to be processed; An image processing apparatus comprising: means for determining the type of image to be formed; and means for setting a threshold value for binarizing the image information of the pixel of interest according to the determination result.