JPH0368081A - Method and device for setting thershold of binarization - Google Patents

Abstract

PURPOSE:To reduce the measurement error due to a measurer and to improve the reliability of the measured value of a dot area rate by digitally converting light quantity information as the density to set a threshold for binarization by arithmetic processing. CONSTITUTION:The photoelectric signal as density information is converted to a digital density value and is stored in a memory 9 with respect to each picture element,and it is discriminated whether the digital density value of each picture element is within a preliminarily determined variance allowable range or not by the change rate of density values of picture elements successively extracted with an arbitrary sampling rate. The digital density value of a shadow part 2a and that of a highlight part 2b are obtained, and the threshold for binarization is set by the opertion in an arithmetic processing part 10 based on these digital density values. Thus, the measurement error due to the measurer is reduced to obtain the accurate and high-reliable measured value of the dot area rate.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention provides, for example, a highly accurate threshold value for binarization of a halftone area ratio measuring device that accurately evaluates and manages the halftone quality of printed matter. The present invention relates to a @ value setting method for binarization of a halftone area ratio measuring device and a threshold value setting device thereof.

<Prior art> Conventionally, a light source is used to illuminate a printed matter, the amount of reflected light or transmitted light from the printing halftone dots is received as density information by a light receiving element such as a CCD sensor, and the light is charged and converted by the light receiving element. The halftone area is calculated by dividing the number of pixels having the binarized photoelectric signal by the total number of pixels required for measurement as the halftone area ratio. Rate measuring devices are known.

<Problems to be Solved by the Invention> However, in the conventional halftone area ratio measuring device, the measurer himself has to adjust the knob for setting the threshold value for binarization according to the printed matter being measured. Because methods such as setting thresholds are used, measurements may vary depending on the individual measurer, and even measurements by the same person may vary each time, making it difficult to obtain an accurate and reliable network. There is a problem in that it is difficult to obtain the point area ratio as a measured value.

Furthermore, when the light-receiving element is a CCD sensor or the like, a measurement error also occurs when the pixel straddles the boundary between a shadow area (image area) and a highlight area (non-image area).

Therefore, as a threshold setting method for setting a threshold for binarization from the distribution state of the measured concentration, as shown in Fig. It has also been proposed to set a threshold for binarization in the valley. In FIG. 3, the horizontal axis is the density, and the vertical axis is the frequency of pixels where the density appears (number of pixels per unit area).

However, the threshold setting method that sets the threshold for binarization by setting the threshold in the valley between the two peaks of this bimodal distribution is For example, if the ratio of the peaks is large and a distribution state with unclear bimodality is obtained, as shown in Figure 4, the valley between the peaks cannot be detected. There is a difficulty in that the threshold value for binarization cannot be automatically set with high precision.

Furthermore, there is also a dot area ratio measuring device that calculates the dot area ratio from the density according to the Yule-Nielsen principle, but this is difficult to measure because measurement conditions such as illumination must be kept strictly constant.

The present invention has been made in view of the problems of the above-mentioned prior art, and its purpose is to provide a threshold value setting method for binarization that can set a threshold value for binarization with high precision. An object of the present invention is to provide a threshold value setting device.

(Means for solving !III) Threshold value setting method for binarization according to claim 1 of the present invention 1: converting a photoelectric signal as density information into a digital density value for each pixel. The digital density value of each pixel is stored, and it is determined whether the digital density value of each pixel is within a predetermined variation tolerance range based on the rate of change in the density value of each pixel extracted sequentially at an arbitrary sampling rate. The method is characterized in that values are determined for each of the shadow and highlight areas, and a calculation is performed based on the digital density value of the shadow area and the digital density value of the highlight area to set a threshold value for binarization.

The threshold setting device according to claim 2 of the present invention includes: a light receiving unit that converts light amount information including density information into a photoelectric signal for each pixel; and converting the photoelectric signal output from the light receiving unit into a digital density signal. an analog-to-digital conversion means for storing the digital density signal as a digital density value for each pixel; and a storage means for storing the digital density signal as a digital density value for each pixel; This is determined by the rate of change in the density value of each pixel sequentially extracted at an arbitrary sampling rate, the digital density value is calculated for each of the shadow part and the highlight part, and the digital density value of the shadow part and the digital density value of the shadow part are determined. The present invention is characterized in that it has the following: arithmetic processing means for calculating a threshold value for binarization based on the digital density value of the highlight portion.

(Function) The threshold value setting method for binarization and the r74 value setting device according to the present invention digitally converts the light amount information as density and sets the threshold value for binarization with high precision through arithmetic processing. Therefore, the measurement error caused by the measurer can be minimized, and the measurement of the dot area ratio becomes even more reliable.

(Example) Hereinafter, an example in which a threshold value setting method for binarization and a threshold value setting apparatus thereof according to the present invention are applied to a halftone area ratio measuring apparatus will be described with reference to the drawings.

FIG. 2 is a schematic configuration diagram of a threshold value setting device for binarization used in the dot area ratio measuring device according to the present invention.
In the figure, 1 is a light source, 2 is a transparent original as an object to be measured, 3 is an imaging lens system, 4 is a filter, and 5 is a COD sensor as a light receiving means. A light source 1 illuminates a transparent original 2. As shown in FIG. 1, the transparent original 2 has a large number of shadow parts 2a and highlight parts 2b. Here, the shadow area 2a is the drawing area where ink is applied, and the highlight area 2a is the drawing area where ink is applied.
b is a non-image area to which ink is not applied, and 2C is a boundary between the shadow area 2a and the highlight area 2b. The illumination light transmitted through the transparent original 2 passes through an imaging lens system 3 and a filter 4 and is imaged on a CCD sensor 5. The CCD sensor 5 outputs a photoelectric signal as density information corresponding to the amount of transmitted light to the analog amplifier circuit 6. The analog amplification circuit 6 amplifies the photoelectric signal to an operating level of an A/D converter 7 as an analog-to-digital conversion means, which will be described later.

The A/D converter 7 digitizes the photoelectric signal and outputs a digital density signal to the digital arithmetic circuit 8 .

The digital arithmetic circuit 8 is composed of a memory 9, an arithmetic processing section IO, and an output section 1. The memory 9 functions as a storage means for storing digital density signals as digital density values for each pixel. The arithmetic processing unit IO functions as an arithmetic processing means that reads the digital density value of the extracted pixel and performs the calculation described below.

As shown in FIG. 1, the permissible variation value (slope) ΔD of the digital density value is determined in advance through experiments or the like. The arithmetic processing unit IO calculates, for example, the rate of change, that is, the slope, of the digital density value of each pixel sequentially extracted from the memory 9 at an arbitrary sampling rate for the shadow portion 2a.

Specifically, if the pixels extracted at an arbitrary sampling rate are a, b, c..., then the rate of change of a and b,
Next, the rate of change in the digital density value of each pixel that is sequentially extracted is calculated, such as the rate of change in b and C. When the rate of change in the digital density value of each pixel sequentially extracted at the arbitrary sampling rate is within the range of variation tolerance ΔD, it is considered to be a continuous part of the image area or non-image area, and the digital Find the average concentration value.

The arithmetic processing unit 10 performs the process of calculating the average of the digital density values for each pixel, and calculates the average digital density value DS. That is, calculation processing for calculating the average of this average digital density value and the digital density value of the next pixel is executed one after another.

Note that the average value of the digital density values can also be found by dividing the sum of the digital density values of pixels within the range of the permissible variation value ΔD by the total number of pixels used to calculate the sum.

In this way, when performing arithmetic processing to obtain the average digital density (IDS), when the boundary area 2C is reached, the rate of change between the digital density value of a certain pixel i and the digital density value of a certain pixel j is determined to be permissible. The value ΔD is exceeded.The arithmetic processing unit 10 stops the calculation for calculating the average when the range of this fluctuation tolerance value ΔD is exceeded.In this way, the final average density value DS for the shadow portion 2a is determined. In the boundary portion 2C, the rate of change in digital density values between pixels is so large that it falls outside the permissible variation range, and the calculation processing unit 10 continues to suspend calculation.

Next, the digital density value corresponding to the highlight section 2b is read out from the memory 9 and input to the arithmetic processing section 10. When the rate of change in digital density values for each pixel sequentially extracted at an arbitrary sampling rate for the highlight portion 2b falls within the variation tolerance range, the arithmetic processing unit 10, for example, The change rate calculation process for the digital density values of pixel m and pixel n sequentially extracted in step 1 is started. When the rate of change in the digital density values of pixels sequentially extracted at an arbitrary sampling rate is within the variation tolerance value ΔD, the process of calculating the average of the digital density values is repeated. Then, the arithmetic processing unit 10 obtains the final average density value DH for the highlight portion 2b. Then, using the average digital density value DS for the shadow portion 2a and the average digital density (l DH) for the highlight portion 2b, a calculation is performed to obtain a threshold value DX for the boundary portion 2C for binarization using the following formula. I do.

DX-DH・α+DS・(1-α) Here, α is a parameter, which is determined in advance through experiments or the like.

Alternatively, the density values of the two pixels T and T, which form the boundary, may be determined and calculated using the formula: DX=-Eyo'2'Yo.

Here, a plurality of this threshold (llIDx) are obtained for each boundary 2C between the shadow part 2a and the highlight part 2b, and this threshold D
The average of X is used as the final threshold DX.

The threshold value DX obtained in this manner is outputted from the output section 11, and each density value is binarized using this threshold value DX. Further, each pixel may be binarized at the time when each threshold value is determined, and by this binarization, the halftone area ratio is determined by the same arithmetic processing as in the conventional method. That is, the halftone dot area ratio is calculated by dividing the number of pixels that are binarized and determined to be image areas by the total number of measured pixels.

In the above embodiments, an embodiment in which the present invention is applied to the measurement of the dot area ratio of a transparent original 2 has been described, but the present invention can also be applied to a reflective original.

(Effect of the invention) The threshold value setting method for binarization and the threshold value setting device according to the present invention can set the threshold value for binarization of the halftone area ratio measuring device with high accuracy, so that it is easy for the measurer to set the threshold value for binarization. The resulting measurement errors can be reduced, more accurate and reliable measurement values can be obtained, and the halftone dot quality can be accurately evaluated and managed.

[Brief explanation of drawings]

FIG. 1 is a waveform diagram used to explain the threshold setting method according to the present invention, and FIG. 2 is a block diagram showing a schematic configuration of a threshold setting device for binarization used in a halftone area ratio measuring device implementing the present invention. FIG. 3 is a diagram used to explain a conventional threshold value setting method for binarization, and FIG. 4 is a diagram used to explain a problem with the conventional threshold value setting method for binarization. 2...Transparent original 2a...Shadow area 2b...Highlight area 2c...Boundary area 5...CCD sensor (light receiving means) 7...A/D converter 8...Digital calculation Circuit 9...Memory (storage means) 10...Arithmetic processing section (arithmetic processing means) Fig. 1

Claims (2)

[Claims] (1) Convert the photoelectric signal as density information into a digital density value and store it for each pixel, and check whether the digital density value of each pixel is within a predetermined permissible variation range at any sampling rate. The digital density values are determined based on the rate of change in the density values of each pixel sequentially extracted by , and the digital density values are calculated for each of the shadow and highlight areas, based on the digital density values of the shadow area and the highlight area. A threshold setting method for binarization that performs calculations and sets a threshold for binarization. (2) light receiving means for converting light amount information including density information into a photoelectric signal for each pixel; and analog-to-digital conversion means for converting the photoelectric signal output from the light receiving means into a digital density signal; converting the digital density signal into a digital signal; A storage means for storing each pixel as a density value, and a digital density value for each pixel stored in the storage means for sequentially extracting at an arbitrary sampling rate whether or not the digital density value for each pixel is within a predetermined variation tolerance range. The density value of each pixel is discriminated based on the rate of change, the digital density value is determined for each shadow area and highlight area, and binarization is performed based on the digital density value of the shadow area and the digital density value of the highlight area. A threshold setting device for binarization, comprising: arithmetic processing means for calculating a threshold for;