JP3748366B2 - Binarization and binarization threshold determination method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for binarizing image data such as characters and images printed on a document by a scanner or the like.
[0002]
[Prior art]
FIG. 4 is a diagram showing an example of the configuration of a scanner which is this type of image reading apparatus. In the figure, 10 is an image sensor in which a plurality of reading pixels are arranged in the main scanning direction of the figure, and 11 is an image sensor 10. An A / D converter that converts an output signal into a digital signal, 12 is a line buffer that sequentially holds the output of the image sensor 10 as data for each line in the main scanning direction, and 2 is a threshold value calculated from the data of the line buffer 12 A threshold calculation unit 3 is a binarization processing unit that outputs binary data of the pixel of interest from the data of the threshold calculation unit 2 and the line buffer 12, 4 is a memory, 5 is a processing circuit, and 6 is a document.
[0003]
In this type of image reading apparatus, when the read data from the image sensor 10 is quantized into black and white binary values, the read data (IMGD) is compared with a predetermined threshold (Th), and the image at the pixel of interest is compared. Is configured to determine whether the data is black data or white data.
If the threshold value used as a reference for binarization is set to a certain level, for example, when the density of the background of the original 6, that is, the density of a blank portion where no image is formed, an image to be originally read is extracted. The problem that it becomes impossible to occur occurs.
In view of this, there is known a method of satisfactorily extracting an image to be read by making the threshold for binarization constant and determining each time according to the background density (dynamic threshold). As this method, for example, as shown in FIG. 5, in the threshold value calculation unit 2, a peripheral pixel (3 × 3) including the line data before and after the pixel of interest (X in the figure) to be read from the line buffer 12 is the center. (9 pixels on the matrix) is read out, the average value of the obtained data is obtained and this is used as the threshold value (Th), and the threshold value Th calculated in the binarization processing unit 3 and the read data IMGD at the target pixel are obtained. In comparison, it is considered to determine monochrome binary data in the target pixel.
[0004]
In the method in which the average value is obtained from the data around the pixel of interest described above and this is used as a threshold value, when the density difference between the background density and the extracted image is small, or as shown in FIG. When there is an image with high density such as a thick line near the image, the average value of the data calculated as the threshold value approximates or exceeds the density of the image to be extracted. There was a problem that it was impossible to extract the thin line).
[0005]
In order to solve the above-described problems, the present applicant has calculated a threshold value that follows the background density of a manuscript or the like by removing the image data portion to be extracted, thereby enabling accurate binarization. Proposed the following binarization threshold value determination method in Japanese Patent Application No. 11-172145.
That is, the binarization threshold value determining method related to the earlier application calculates an average density value in the target pixel (N) from the image data in the target pixel (N) to be binarized and its surrounding pixels, and this average density value And the difference between the threshold values of the pixel (N−1) immediately before the target pixel (N) to be binarized is used as a change value, and the follow-up indicating the degree of change in the threshold value of the target pixel (N) based on the change value. By calculating the value and adding the follow-up value and the average density, the threshold value for the pixel of interest (N) is obtained, and the change amount of the threshold value increases due to the presence of the image data (black data). Image data can be extracted satisfactorily by making the change in threshold value follow only the change in the background density.
[0006]
Describing the threshold value determination method of the previous application in more detail, FIG. 7 is a diagram showing the configuration of the threshold value calculation unit 2 for performing this threshold value determination method, and FIG. 8 is a flowchart diagram showing this threshold value determination method. In the threshold value determination method, image data IMGD (N) of the pixel of interest (N) and image data of a plurality of peripheral pixels centered on the pixel of interest (N) are used as data for each line in the main scanning direction. Read out from the line buffer storing the data of the previous and subsequent lines in the scanning direction, and calculate the average density value Mn (N) at the pixel of interest (N) from these read out n pieces of image data. (Formula 1)
Mn (N) = Σ (IMGD) / n Expression 1
Subsequently, the difference between the threshold value Th (N-1) of the immediately preceding pixel (N-1) in the main scanning direction of the target pixel (N) and the calculated average density Mn (N) of the target pixel (N) is calculated. (Formula 2), which is defined as a change value ΔTMn (N).
ΔTMn (N) = Th (N−1) −Mn (N) Equation 2
The calculated ΔTMn (N) is multiplied by a tracking coefficient fk, which is an arbitrary coefficient, to calculate a tracking value F (N). (Formula 3)
F (N) = ΔTMn (N) × fk Equation 3
Further, the calculated follow-up value F (N) and the average density value Mn (N) at the target pixel (N) are added to calculate the threshold Th (N) at the target pixel (N). (Formula 4)
Th (N) = F (N) + Mn (N) Expression 4
According to the binarization threshold value determination method of the previous application, when determining the threshold value of the target pixel, the average density value of the image data of the target pixel and its surrounding pixels, the threshold value of the immediately preceding pixel adjacent to the target pixel, and Difference is calculated as a change value, so that it is possible to reduce the influence that the amount of change in the threshold increases due to the presence of image data (black data). It can be done well.
That is, even in the portion that moves from the background portion to the image region, the density change value is obtained by calculating the difference between the average density value around the pixel of interest and the threshold value in the immediately preceding pixel (that is, the background portion), and further to this change value. By setting and multiplying the tracking coefficient, which is an arbitrary coefficient, to an appropriate value, the amount of change in the threshold can be reduced.
The follow-up coefficient fk is an arbitrary value of 0 <fk ≦ 1, and the follow-up coefficient fk is set to a value close to 1 to increase the follow-up value F (N), thereby reducing the amount of change in the threshold Th. Even if the change in the average density Mn increases due to the appearance of the image data, the amount of change in the threshold Th can be suppressed, and the threshold can be made to follow only the background density of the document.
Further, by appropriately setting the follow-up coefficient fk, the threshold value can be made to follow the change of the average value density Mn, so that, for example, even in an image data region where fine lines are dense, binarization can be favorably performed. .
That is, when the follow-up coefficient fk is set to a value close to 0, the follow-up value F (N) becomes small, and the calculated threshold value Th (N) becomes a value close to the average value concentration Mn, thereby following the change. It becomes. Accordingly, when there are a large number of white and black data in a document in a narrow range, the tracking coefficient is set to a value close to 1, and vice versa, the image data is set to a value close to 0. The amount of change in the threshold value Th can be set as appropriate.
[0007]
[Problems to be solved by the invention]
In the binarization threshold value determination method related to this earlier application, for example, when a document in which the background density of the document is close to the density of the image to be extracted and only the image portion is extracted by removing the background portion The tracking coefficient fk in Equation 3 is set to a value close to 0 so that the threshold value approaches the background density, that is, the average density value of surrounding pixels. The follow-up value F (N) calculated in this way is reduced, the threshold Th (N) at the pixel of interest (N) is set to a value close to the average density value Mn, the background portion is removed, and only the image portion is extracted. ing.
At this time, when there are thick lines or filled areas extending in the main scanning direction and extending over a plurality of lines in the sub-scanning direction, the following problems have occurred.
That is, in such a region, since the average density value Mn is the density of the image to be extracted, the calculated threshold value of each pixel of the sensor gradually approaches the extracted image density in the main scanning direction.
FIGS. 9 and 10 schematically show these, and sensors S1, S2, S3 in which 0, 1, 2,..., N−1, N and reading pixels are sequentially arranged in the main scanning direction are shown. .. Shows the result of binarization using the output (input image) obtained by reading the painted area on the document and the threshold value calculated by setting the tracking coefficient fk to a value close to 0. Yes. When calculating the threshold value for each pixel in this binarization threshold value determining method, the threshold value is calculated with reference to the threshold value of the immediately preceding pixel. Therefore, as shown in the upper diagram of FIG. The threshold value in the vicinity of the pixel 0) of the sensor S1 shown in FIG. 5 is close to the initial setting value, so that the extracted image can be binarized as black data, but the downstream side in the main scanning direction (that is, the vicinity of the pixel N) As the value approaches, the threshold value becomes an average density value, and in this region, the value becomes closer to the output of the input image. FIG. 10 shows binarized data of l, (l + 1), (l + 2), and (l + 3) lines in the main scanning direction corresponding to the read pixel of the sensor S1, and a threshold value in the vicinity of the downstream side in the main scanning direction. Therefore, a portion exceeding the density of the input image data is generated, and white data (0) is output. Therefore, as shown in the lower diagram of FIG. 10, the binarized output result is obtained such that the result becomes gradually lighter in the main scanning direction.
Further, in a scanner in which a plurality of sensors are arranged in a staggered manner in the main scanning direction as shown in FIG. 9 and the reading range is enlarged, an output that is apparent at the joint portion of each sensor as shown in FIG. As a result, there is a problem in that good output cannot be obtained.
[0008]
In order to prevent the output density from occurring at the joint portion of this sensor, it is conceivable that the threshold value calculated in the upstream sensor in the main scanning direction is reflected in the threshold value in the adjacent downstream sensor. In order to prevent an unreadable region from being generated by arranging a plurality of sensors in the main scanning direction, adjacent sensors are arranged at a predetermined interval in the sub-scanning direction. The output (threshold value) of the sensor must be held by the offset amount, and there has been a new problem that adding a circuit for this causes an increase in cost.
[0009]
The present invention has been made to solve these problems, and prevents the occurrence of light and shade of the output generated at the joint portion of the sensor with a simple configuration.
[0010]
[Means for Solving the Problems]
In order to solve the above-described problem, in the binarization method and the binarization threshold value determination method of the present invention, the data reading direction in the main scanning direction in each line data alternates between the forward direction and the reverse direction for each line. It was configured to switch to.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the binarization and binarization threshold value determination method of the present invention will be described in detail with reference to the drawings.
Since the configuration of the image reading apparatus to which the binarization threshold value determining method of the present invention is applied may be the same as the configuration of the conventional apparatus shown in FIG. 4, the configuration of the apparatus is based on FIG. 1 with reference to FIG. The binarization and binarization threshold value determination method of the present invention will be described.
[0012]
In the image reading apparatus to which the binarization method of the present invention is applied, the threshold value calculation unit 2 first sets the reading direction of the image data IMGD read from the line buffer 12 as the forward direction in the main scanning direction. That is, the sensor S1 shown in FIG. 9 is set to read in the order of read pixels 0, 1, 2, 3,..., N−1, N.
[0013]
In the binarization operation, pixel data stored as data for each line in the line buffer 12 are sequentially read in the forward direction in the main scanning direction, and a unit pixel (target pixel) to be quantized to binary and its The threshold value calculation unit 2 calculates the threshold value Th from the image data of the peripheral pixels, and the binarization processing unit 3 compares the calculated threshold value Th with the image data IMGD of the target pixel to create binary data.
When the threshold value Th is calculated in the threshold value calculation unit 2, the average value calculation unit 21 sets the image data IMGD (N) of the target pixel (N) and the target pixel (N) in the same manner as in the above-described conventional device. Image data at a plurality of peripheral pixels at the center is read from the line buffer 12. (STEP1)
[0014]
Next, an average density value Mn (N) at the target pixel (N) is calculated from the n pieces of image data read out by the following Equation 1. (STEP2)
Mn (N) = Σ (IMGD) / n Expression 1
Subsequently, in the change value calculation unit 22, the threshold value Th (N−1) of the immediately preceding pixel (N−1) in the main scanning direction of the target pixel (N) and the average density value Mn (N) in the target pixel (N) calculated in STEP 2. N) is calculated (Equation 2 below), and this is defined as a change value ΔTMn (N). (STEP3)
ΔTMn (N) = Th (N−1) −Mn (N) Equation 2
The calculated change value ΔTMn (N) is multiplied by a tracking coefficient fk, which is an arbitrary coefficient, to calculate a tracking value F (N) (Formula 3 below). (STEP4)
F (N) = ΔTMn (N) × fk Equation 3
Next, the follow-up value F (N) calculated in STEP 4 and the average density value Mn (N) in the target pixel (N) calculated in the average value calculation unit 21 are added to obtain a threshold Th (N) in the target pixel (N). ) Is calculated (Formula 4 below). (STEP5)
Th (N) = F (N) + Mn (N) Expression 4
The calculated threshold Th (N) is output to the binarization processing unit 3 and used for the binarization processing of the image data in the pixel of interest (N). At the same time, the threshold value Th (N) is stored in the threshold value holding unit 25 of the threshold value calculation unit 2 and used for calculating the threshold value in the subsequent pixel of interest (N + 1).
[0015]
When the calculated threshold value Th (N) is output to the binarization processing unit 3, it is determined whether or not there is a subsequent pixel of interest, that is, an image to be read in STEP 6, and if there is a subsequent image, the number of the pixel of interest is counted up. (N = N + 1) and return to STEP 1 to continue the threshold determination process. If there is no subsequent image, the threshold determination process is terminated.
[0016]
In STEP 6, when it is detected that the reading of image data for one line is completed, the process proceeds to STEP 7 and it is determined whether or not there is subsequent line data.
If it is determined in STEP 7 that there is subsequent line data, the threshold value calculation unit 2 reverses the setting of the reading direction.
That is, since the image data IMGD is read in the forward direction in the main scanning direction in the immediately preceding line, the reading pixels N, N− in the reverse direction in the main scanning direction, that is, in the sensor S1 shown in FIG. Set to read in the order of 1,..., 3, 2, 1, 0 and return to STEP1.
In a line where the reading direction is set to read the image data IMGD in the reverse direction with respect to the main scanning direction, the threshold value calculation unit 2 reads the image data from the line buffer 12 and the pixels N, N−1,. Reading in the order of 1, 0, threshold determination and binarization are performed. In this case, the change value ΔTMn (N) in STEP 3 is calculated by setting the number of the pixel to be read as R, the immediately preceding pixel (N−1) is (R + 1), and the threshold Th (N−1) is the pixel number (R + 1). ) Is used.
[0017]
Threshold value determination and binarization are sequentially performed on the line, and when it is detected in STEP 6 that the reading of the image data for one line has been completed, the process proceeds to STEP 7 to determine whether there is subsequent line data.
If it is determined in STEP 7 that there is subsequent line data, the threshold processing unit 2 reverses the setting of the reading direction and sets the main scanning direction to read in the forward direction.
[0018]
FIG. 2 is a diagram schematically illustrating a binarization result when the reading direction of the l-th line is set to the forward direction in the sensor S1.
That is, for the l-th line, image data is read in the forward direction in the main scanning direction and binarized, and the subsequent (l + 1) -th line reads image data in the reverse direction, and the (l + 2) -th line thereafter is the forward direction. The (l + 3) -th line is configured to perform binarization by reversing the reading direction bi-directionally for each line in the reverse direction.
[0019]
Also in the binarization and binarization threshold value determination method of the present invention, when the tracking coefficient fk in Equation 3 is set to a value close to 0, the threshold value of each pixel of the sensor calculated when the filled area is read. The white data (0) is output gradually approximating the extracted image density toward the downstream side in the reading direction, but the reading direction of the image data is reversed bi-directionally for each line. Therefore, as shown in the lower diagram of FIG. 2, white data does not appear concentrated on one side, and white data appears every other line, so that it gradually fades in one direction. An output is not obtained, and a good result is obtained.
Also, in the joint portion of each sensor, white data appears at the ends of both sensors, so that there is no problem that the shading is conspicuous.
[0020]
【The invention's effect】
As described above in detail, in the binarization method and binarization threshold value determination method of the present invention, the data reading direction in the main scanning direction in each line data is alternately switched between the forward direction and the reverse direction for each line. With this configuration, there is an effect that it is possible to prevent the occurrence of light and shade of the output generated at the joint portion of the sensor with a simple configuration without adding a dedicated circuit or the like.
[Brief description of the drawings]
FIG. 1 is a flowchart showing a threshold determination method according to the present invention.
FIG. 2 is a diagram illustrating a binarization result according to the present invention.
FIG. 3 is a diagram illustrating an output of a joint portion of a sensor in a binarization result according to the present invention.
FIG. 4 is a diagram illustrating a configuration of an image reading apparatus. FIG. 5 is a diagram illustrating a conventional threshold value determination method. FIG. 6 is a diagram illustrating threshold values obtained by a conventional threshold value determination method.
7 is a diagram illustrating a configuration of a threshold value calculation unit 2. FIG.
FIG. 8 is a flowchart showing a conventional threshold value determination method.
FIG. 9 is a diagram illustrating an image reading apparatus.
FIG. 10 is a diagram showing a binarization result obtained by the threshold value determination method of the previous application.
FIG. 11 is a diagram showing an output image obtained by the threshold value determination method of the previous application.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Image sensor 11 A / D converter 12 Line buffer 2 Threshold calculation part 3 Binarization processing part 4 Memory 5 Processing circuit 6 Original

Claims (1)

Line data is configured by arranging and storing a plurality of pixel data in the main scanning direction, and further storing image data two-dimensionally by storing a plurality of line data in the sub-scanning direction. In the method for determining a threshold value for binarizing the data of each pixel,
In the line data related to binarization, the average density of the target pixel is calculated from the data of the target pixel to be binarized and its peripheral pixels, and this average density value and the previous pixel in the main scanning direction of the target pixel to be binarized The difference between the threshold value and the change value is obtained as a change value. Based on the calculated change value, a follow-up value indicating the change degree of the threshold value in the target pixel is calculated, and the follow-up value and the average density value are added. A threshold value for the pixel of interest is obtained, and pixel data and immediately preceding pixel data are sequentially read out in the forward direction in the main scanning direction to calculate a binary threshold value for each pixel data for one line, and the line following the line data In the data, the pixel data and the immediately preceding pixel data in the direction opposite to the main scanning direction and the direction in which the pixel data is read in the immediately preceding line data Binarization threshold value determination method characterized by calculating the next read out binarization threshold.

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