JP2000261666A - Image processing unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a binary image desired by a user, independently of the background level and black level of an original. SOLUTION: A binary processing threshold calculation section 3 calculates a binary processing threshold object of a target pixel on the basis of multi-value image data of the target pixel and pixels around the target pixel, a threshold upper lower limit calculation section 7 calculates the upper limit and a lower limit of a binary processing threshold on the basis of a density histogram of multi-value image data corresponding to one page. A binary processing threshold limit section 4 limits the binary processing threshold value object calculated by the binary processing threshold calculation section 3 within a range of the upper limit and the lower limit calculated by a threshold upper lower limit value calculation section 7 to decide the binary processing threshold, and the binary processing section applies binary processing to the multi-value image data of the target pixel on the basis of this threshold.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus for binarizing multi-valued image data with a threshold, and more particularly to a process for determining a threshold for binarization based on a density histogram of the multi-valued image data. .

[0002]

2. Description of the Related Art A conventional example of determining a threshold for binarization based on a density histogram is disclosed in, for example, Japanese Patent Laid-Open No. 5-32.
As shown in Japanese Patent No. 8133, an image signal is divided into a plurality of partial images, a histogram based on brightness is created from the partial images, and a parameter smaller than the number of dimensions of the histogram is extracted from the histogram as a feature amount of the histogram. A method of outputting a threshold for binarization from the following has been proposed. Further, as another conventional example, a method of calculating a plurality of appropriate thresholds corresponding to the shape of a density histogram has been proposed as disclosed in, for example, Japanese Patent Application Laid-Open No. 8-96123.

As another conventional example, as shown in Japanese Patent Laid-Open No. 5-284356, for example, an image signal is M × N
A method has been proposed in which a pixel is divided into a plurality of small areas, a threshold value is calculated for each of the small areas, and binarization is performed. As another conventional example, for example, as disclosed in Japanese Patent Application Laid-Open No. 5-258107, a method has been proposed in which a threshold value is changed in association with the density distribution of one line of pixels near a binarized pixel.
Another conventional example is disclosed in, for example, JP-A-4-2076.
No. 68, JP-A-63-215262 and the like.

[0004]

Here, when binarizing multi-valued image data with a threshold value, an original having a low density level such as a pencil original or a carbon-copied character and a partial color on the original are considered. The lower limit of the threshold value is determined so that noise data on the background is not extracted more than necessary for characters written above, and the upper limit value of the threshold value is set to prevent white spots in the central part of the character. The value must be determined. However, the background level and black level of a document vary depending on the document, and there is a problem that if a certain upper limit value and lower limit value are set, a binary image desired by a user is not obtained.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide an image processing apparatus which can obtain a binary image desired by a user regardless of the background level or black level of a document.

[0006]

According to a first aspect of the present invention, there is provided an image processing apparatus for binarizing multi-valued image data of a pixel of interest using a threshold for binarization. Means for calculating a threshold value candidate for binarization of the pixel of interest based on the multivalued image data of the pixel; and setting the upper limit value and the lower limit value of the threshold value for binarization based on the density histogram of the multivalued image data for one page. Means for calculating, and if the binarization threshold candidate exceeds the upper limit, determine the upper limit as the binarization threshold of the pixel of interest, and the binarization threshold candidate falls below the lower limit. In this case, the lower limit value is determined as the threshold value for binarization of the pixel of interest. If the threshold value candidate for binarization falls within the range between the upper limit value and the lower limit value, the threshold value candidate for binarization is determined as the pixel of interest. Means for determining the threshold value for binarization of To.

[0007] The second means is to achieve the above object.
In an image processing apparatus for binarizing multi-valued image data of a pixel of interest using a threshold for binarization, means for calculating a candidate for a binarization threshold of the pixel of interest based on the multi-valued image data of the pixel of interest and neighboring pixels Means for calculating an upper limit value and a lower limit value of a threshold for binarization based on a density histogram of multi-valued image data for a plurality of lines including a line of a pixel of interest; When the threshold value exceeds the threshold value, the upper limit value is determined as the threshold value for binarization of the pixel of interest. When the candidate threshold value for binarization is lower than the lower limit value, the lower limit value is set as the threshold value for binarization of the pixel of interest. Means for determining, if the binarization threshold candidate is within the range of the upper limit value and the lower limit value, determining the binarization threshold candidate as the binarization threshold value of the pixel of interest. Features.

[0008]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of an image processing apparatus according to the present invention, FIG. 2 is an explanatory diagram showing a matrix of a binarization threshold value calculation unit in FIG. 1, and FIG. FIG. 7 is an explanatory diagram showing an example of a density histogram obtained by the above.

In FIG. 1, an image input apparatus 1 is, for example, a scanner, and an original image is, for example, 8 bits (= 0 to 25).
It is converted into multi-valued image data of 5). Image data holding unit 2
Holds multi-valued image data for one page from the image input device 1 and applies it to the binarization threshold value calculation unit 3, the threshold upper / lower limit value calculation unit 7, and the binarization processing unit 5.

[0010] The binarization threshold value calculation unit 3 is an image data holding unit 2
And the matrix a, b, cd, e, fg, h, i of 3 × 3 pixels a to i shown in FIG. Based on this, a threshold value candidate DynTh for binarization of the pixel of interest e is calculated.

DynTh = (a + b + c + d + e + f + g + h + i) / 9 (1) In parallel with this, the threshold upper / lower limit calculator 7 rasterizes one page of multi-valued image data held in the image data holding unit 2 Scan to create a density histogram. FIG. 3 shows an example of a density histogram in which the horizontal axis represents the density (0 to 255) and the vertical axis represents the frequency of the 256 types of density. Here, the frequency is expressed as h [density value], for example, the frequency of density value = 128 is expressed as h [128]. Next, the vertex (peak) of this density histogram is determined, for example, as follows.

H [10] <h [11] <h [12] <h
[13] <h [14] and h [14]> h [15]> h [16]> h [17]> h
If [18], the density value = 14 is the peak.

Since the density histogram may not be monotonically increasing or monotonically decreasing, it may be better to perform the processing in the following "2" units instead of the "1" units as described above.

H [6] <h [8] <h [10] <h [1
2] <h [14] and h [14]> h [16]> h [18]> h [20]> h
If [22], the density value = 14 is the peak.

The density value T at which the height of the frequency h [density value] determined as described above (the value on the vertical axis of the histogram) is the largest.
Hmax (the value on the horizontal axis of the histogram), the density value THmin having the next highest frequency, and the upper limit value THhig based on the following equation (2).
h and the lower limit value THlow are determined.

THhigh = THmax−offsetmax THlow = THmin + offsetmin where offsetmax and offsetmin are predetermined positive numbers. For example, as shown in FIG.
x is “243” and the next most frequent density value THmin
Is “37” and offsetmax = offsetmin = 50, TH
high = 193 and THlow = 87.

If the relationship of THlow <THhigh is not satisfied, or if (THhigh−THlow) <predetermined value, a calculation error is made and an error process such as using an initial value is performed.

The following binarization threshold limiter 4 defines the range of the binarization threshold DynTh calculated by the binarization threshold calculator 3 as:
Upper limit THhigh calculated by threshold upper / lower limit calculator 7
And the lower limit value THlow, the binarization threshold value TH is determined by restricting under the following determination conditions.

If DynTh> THhigh, then TH = THhigh if DynTh <THlow, then TH = THhigh, otherwise TH = DynTh, and the binarization processing unit 5 performs multi-valued processing on the target pixel held in the image data holding unit 2. Image data ImageData
Binarization is performed by the threshold value TH for binarization as follows.

If ImageData> TH, "1" (black) If ImageData≤TH, "0" (white) Next, a second embodiment will be described with reference to FIGS. Here, in the first embodiment, since the threshold value TH of all pixels is determined and binarized after creating the density histogram of the multi-valued image data for one page, the memory capacity of the image data holding unit 2 is also large. In addition, the processing time becomes longer.
Therefore, in the second embodiment, instead of the image data holding unit 2 that holds one page of image data, a line buffer 8 that holds several lines of image data is provided. The calculation unit 7a performs the following processing. Other configurations are the same as those of the first embodiment.

In the second embodiment, instead of performing a binarization after creating a density histogram of image data for one page and determining thresholds for all pixels as in the first embodiment, a threshold value is set. As shown in FIG. 5, for example, as shown in FIG. 5, the upper / lower limit calculation unit 7a creates a density histogram for six lines with the line of the target pixel e as the fifth line,
Performs the binarization up to the pixel e before the pixel of interest e as shown in FIG.

Here, the histogram of the pixels on the first line of the document may not be accurate. In this case, the upper limit value THhigh and the lower limit value T
Hlow is wrong. Therefore, the threshold upper / lower limit calculation unit 7a
Determines the upper limit THhigh and the lower limit THlow after a certain number of lines have elapsed. Also, a threshold upper / lower limit value calculation unit 7a
It is better to set the upper limit value THhigh and the lower limit value THlow for the binarization threshold limit unit 4 each time one line is completed, instead of setting the upper limit value THhigh and the lower limit value THlow for each pixel.

[0023]

As described above, according to the first aspect of the present invention, a binarization threshold candidate for a target pixel is calculated based on the multi-valued image data of the target pixel and neighboring pixels, and one page. The upper and lower limits of the binarization threshold are calculated based on the density histogram of the multi-valued image data of
Limiting the thresholding threshold candidates to the range between the upper limit and the lower limit, 2
Since the binarization threshold is determined, a binary image desired by the user can be obtained regardless of the background level or black level of the document.

According to the second aspect of the present invention, based on the multi-valued image data of the pixel of interest and neighboring pixels, the 2
A threshold value candidate for binarization is calculated, and an upper limit value and a lower limit value of a threshold value for binarization are calculated based on a density histogram of multi-valued image data for a plurality of lines including a line of a pixel of interest. Since the threshold value candidates are limited to the range between the upper limit value and the lower limit value to determine the threshold value for binarization, a binary image desired by the user can be obtained regardless of the background level or black level of the document.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an embodiment of an image processing apparatus according to the present invention.

FIG. 2 is an explanatory diagram showing a matrix of a binarization threshold value calculation unit in FIG. 1;

FIG. 3 is an explanatory diagram showing an example of a density histogram obtained by a threshold upper / lower limit value calculator in FIG. 1;

FIG. 4 is a block diagram illustrating an image processing apparatus according to a second embodiment.

FIG. 5 is an explanatory diagram showing the number of lines for which a threshold upper / lower limit value calculation unit in FIG. 4 obtains a density histogram;

FIG. 6 is an explanatory diagram illustrating a binarization process according to the second embodiment.

[Explanation of symbols]

 2 image data holding unit 3 binarization threshold calculation unit 4 binarization threshold limit unit 5 binarization processing unit 7, 7a threshold upper / lower limit calculation unit 8 line buffer

Claims (2)

[Claims] An image processing apparatus for binarizing multi-valued image data of a pixel of interest using a threshold for binarization, wherein the threshold for binarization of the pixel of interest is determined based on the multi-valued image data of the pixel of interest and neighboring pixels. Means for calculating a candidate; means for calculating an upper limit value and a lower limit value of a threshold for binarization based on a density histogram of multi-valued image data for one page; If the threshold value is exceeded, the upper limit value is determined as the binarization threshold value of the target pixel. If the binarization threshold candidate is smaller than the lower limit value, the lower limit value is determined as the binarization threshold value of the target pixel. Means for determining the binarization threshold candidate as the binarization threshold of the pixel of interest when the binarization threshold candidate is within the range of the upper limit value and the lower limit value. Characteristic image processing device. 2. An image processing apparatus for binarizing multi-valued image data of a pixel of interest using a threshold for binarization, comprising: a threshold for binarizing a pixel of interest based on the multi-valued image data of the pixel of interest and neighboring pixels; Means for calculating a candidate; means for calculating an upper limit value and a lower limit value of a threshold for binarization based on a density histogram of multi-valued image data for a plurality of lines including a line of a pixel of interest; If the candidate exceeds the upper limit, the upper limit is determined as the threshold for binarization of the pixel of interest. If the candidate for threshold for binarization is lower than the lower limit, the lower limit is set to the threshold of the pixel of interest. Means for determining as a threshold for binarization, and determining the candidate for binarization as a threshold for binarization of a pixel of interest when the binarization threshold candidate is within the range of the upper limit and the lower limit. An image processing apparatus comprising: