JPH08149292A - Dot area decision device - Google Patents

Abstract

PURPOSE: To surely detect a peak/dip picture element regardless of high or low gradation in the dot area by deciding the dot area while taking notice of the peak picture element or the dip picture element depending on the degree of gradation of an image area. CONSTITUTION: A three-primary color signal is subjected to correction processing by an input processing circuit 2 and stored in an FIFO memory 3 and only information by a prescribed line selected for deciding the kind of source document is transferred to a character/dot/photograph decision circuit 4. The circuit 4 averages the density of a noted picture element and the density of surrounding picture elements and compares the average value with a prescribed threshold value to detect a peak/dip picture element. Which of the peak or dip picture element is adopted for the decision is decided based on the result of decision of high gradation, low gradation and intermediate tone. Then the peak picture element is taken notice and an area is discriminated to be a dot area when the number of peaks in a block is larger than the prescribed one.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a halftone dot area judging device for reading a manuscript image and judging whether or not the read manuscript image is a halftone dot area manuscript image.

[0002]

2. Description of the Related Art In a color digital copying machine, a color original is read by a scanner composed of a CCD (charge coupled device) or the like and converted into red (R), green (G) and blue (B) electric signals. , The original area is the character / line drawing area, halftone dot
It has a function of determining whether it is a (screen) area or a photographic area and performing image processing according to the type of the area.

Then, in the case of a character / line drawing area, edge emphasis or black character emphasis is performed, and in the case of a halftone dot area, smoothing or blurring processing is performed. The halftone dot area is determined as follows. Attention is paid to each pixel in the digital image information which is read from the document and temporarily stored, and the pixel of interest is surrounded by a fixed number of pixels (for example, 3 pixels × 3 pixels) that are continuous around the pixel of interest. It is determined whether the density is higher than that of the surroundings or lower than the surroundings. If the density is higher than the surroundings, this pixel is set as a peak pixel, and if the density is lower, this pixel is set as a dip pixel.

Then, the peak /
By checking whether the intervals between the dips are appropriate, or whether the appearance pitches of the peak pair and the dip pair are constant, it is determined whether or not it is a halftone dot area.

[0005]

By the way, there are various shades of color (gradation) in the halftone dot area, from high gradation to intermediate gradation and low gradation. In the high-tone halftone dot area, it is more convenient to detect dip pixels in which the halftone dot dot points are valley-shaped, and in the low-tone halftone dot area, the halftone dot candidate points are It is more convenient to detect a peak pixel having a sharp mountain shape. In the halftone halftone dot area, the dot candidate point of the halftone dot may be detected by either the peak pixel or the dip pixel.

If this characteristic is ignored and detection is performed by fixing either the peak pixel or the dip pixel as in the conventional case, accurate detection may not be possible in some cases. Therefore,
The present invention pays attention to the gradation of the halftone dot area, and can reliably detect the peak / dip pixel regardless of the gradation of the halftone dot area, and thus accurately determine the halftone dot area. It is an object of the present invention to provide a halftone dot area determination device that can achieve the following.

[0007]

A halftone dot area judging apparatus of the present invention for achieving the above object judges the gradation of an original image area in three steps based on the read original image data, and determines the higher order. When it is judged to be a tonal area, dip pixels are detected, when it is judged to be a low gradation area, peak pixels are detected, and when it is judged to be an intermediate gradation area, both peak pixels and dip pixels are detected. The halftone dot area is determined based on the detected density of peak pixels or dip pixels.

[0008]

In the low gradation region, the halftone dots are made up of small dots, so when attention is paid to the density, the ridges of the dots are sharp and the bottoms of the dots are gentle. In the high gradation region, halftone dots are composed of large dots, so the density valleys are narrow and deep, and the mountains are wide and gentle. In the halftone region, both the peak and the dip have the same sharpness.

Therefore, according to the present invention, attention is paid to either or both of the peak pixel and the dip pixel according to the degree of gradation of the image area, and the density of the detected peak pixel or the density of the dip pixel or both is detected. Based on this, the halftone dot area is determined.

[0010]

Embodiments will be described in detail below with reference to the accompanying drawings showing embodiments. FIG. 1 is a block diagram showing an electrical configuration of a main part of a color digital copying machine. In this color digital copying machine, a color original is read in one direction (referred to as a main scanning direction) by a scanner 1 composed of a CCD (charge coupled device) or the like, and a yellow (Y), a magenta (M), and a cyan (C) are used. Convert to primary color signal. The resolution of the scanner 1 is about 400 pixels per inch.

The Y, M, and C primary color signals are subjected to correction processing in accordance with the characteristics of toner in the input processing circuit 2,
Only the information of a fixed line (for example, 3 lines) selected for the document type determination is temporarily stored in the FIFO memory 3 and transferred to the character / dot / photo determination circuit 4 and stored in the line memory 41. In, it is determined whether the document area is a character / line drawing area, a halftone dot screen area, or a photograph area.

On the other hand, the Y, M, and C primary color signals are supplied to a black generation circuit 5, where black (BK) data for compensating for the insufficient density of a high-density original is created. Further, the four signals Y, M, C, and BK are taken out by the color selector circuit 6 one by one, and converted by the zoom / movement circuit 7 into signal contents corresponding to the set magnification.

Then, in the filter circuit 8, smoothing or edge sharpening is performed depending on the type of the document whose area is determined. Finally, halftone display (dithering) is performed by the gradation processing circuit 9 and output to the laser printer through the output control circuit 10. The outline of the signal flow in the color digital copying machine has been described above. The details will be described below centering on the determination function of the dot area in the character / dot / photograph determination circuit 4.

The character / halftone dot / photograph determination circuit 4 first averages the densities of the pixel of interest and its surrounding pixels, and compares this average value with a set threshold value. For example, an average value A of the densities of all 54 elements is formed by forming a matrix of 9 dots × 6 lines (this area is defined as one block) as shown in FIG.
If VE is taken and AVE> TH _h (TH _h is a threshold value for high gradation detection), the block concerned has high gradation, AVE <TH.
_l (TH ₁ is a threshold for low gradation detection) is low gradation,
If TH ₁ <AVE <TH _h, it is determined to be halftone. Here, the threshold values TH _l and TH _h may be set to values that best divide each gradation. For example, if the level of image data is 256 gradations from 0 to 255, it is simply 3
It may be selected by dividing by, or may be set in consideration of the nonlinearity of the input characteristic of the scanner 1.

The information temporarily stored in the line memory 41 of the character / halftone dot / photograph determination circuit 4 is multiplied by a matrix of 3 dots × 3 lines (see FIG. 3) to detect peak / dip pixels. . The determination criterion of the peak / dip pixel detection is that the value obtained by subtracting the maximum value of the densities of the surrounding pixels A to H from the density of the pixel of interest X at the center is larger than the threshold value, and the pixel of interest X It is determined that the pixel is a pixel, and the pixel of interest X
When the value obtained by subtracting the density of is larger than the threshold value, the target pixel X is determined as a dip pixel.

Here, the threshold value is set to a value of 5 for all 256 gradations, for example. When the uniform gradation portion of the original is read, when comparing the photograph area and the halftone dot area, the gradation change rarely becomes 5 or more in the photograph area, and the gradation change becomes 5 or more in the halftone dot area. This is because a step naturally occurs. The above process is performed for each dot (pixel of interest) in the middle line temporarily stored in the line memory 41.

On the other hand, it is necessary to decide which of the peak pixel and the dip pixel determined by the above method should be adopted. This determination is performed according to the above-described determination result of high gradation, low gradation, and halftone. That is, if the pixel is in the low gradation block, the halftone dots are made up of small dots, so that the peaks are sharp and the valley bottom is gentle as shown in FIG. The exact position of cannot be detected. Therefore, focusing exclusively on the peak pixel, it is determined as a halftone dot area when the number of peaks in the block is larger than a predetermined number.

For pixels in a high gradation block, halftone dots are made up of large dots, so the valleys are deep and the mountains are gentle as shown in FIG. The exact position of cannot be detected. Therefore, focusing exclusively on the dip pixels, it is determined as a halftone dot area when the number of dips in the block is larger than a predetermined number. In the halftone block, both the peak and the dip have the same sharpness, and it is impossible to determine which of the peak pixel and the dip pixel is suitable. Therefore, attention is paid to both the peak pixel and the dip pixel, and the halftone dot area is determined by the logical sum of the number of peaks in the block being larger than a predetermined number or the number of dips in the block being larger than the predetermined number.

When the above processing is completed for the line temporarily stored in the line memory 41, it is similarly executed for the next line. As described above, according to the average gradation of the area, the detection by the peak pixel and the detection by the dip pixel are combined to determine whether the area is a halftone dot area. Therefore, the halftone dot area can be determined more reliably. be able to.

The present invention is not limited to the above embodiment. In the above embodiment, the block (see FIG. 2) made up of 6 × 9 = 54 pixels is assumed as the region for determining the gradation of the image, but the present invention is not limited to this.
Any size block may be envisioned. Further, in order to detect the peak / dip pixel, a matrix of 3 dots × 3 lines (see FIG. 3) is used, and the density of the target pixel X is compared with the density of the surrounding pixels A to H. However, as another method, for example, the peak / dip pixel can be detected by checking the density relationship between the target pixel and the adjacent pixel based on the pixel information of only one line.

[0021]

As described above, according to the halftone dot area determining apparatus of the present invention, one or both of the peak pixel and the dip pixel are focused on depending on the gradation level of the image area. Since the determination is performed, the peak / dip pixel can be surely detected regardless of the gradation level of the halftone dot area, and the determination of the halftone dot area can be made accurate.

[Brief description of drawings]

FIG. 1 is a block diagram showing an electrical configuration of a main part of a color digital copying machine.

FIG. 2 is a diagram showing a 9 × 6 matrix area set to determine a density gradation of an image area.

FIG. 3 is a diagram showing a 3 × 3 matrix region for detecting whether or not a pixel of interest is a peak / dip pixel.

FIG. 4 is a diagram showing a distribution of peaks forming halftone dots in a low gradation region.

FIG. 5 is a diagram showing a distribution of dips forming halftone dots in a high gradation area.

[Explanation of symbols]

 1 Scanner 2 Input processing circuit 4 Character / dot / photo judgment circuit 41 Line memory

Claims (1)

[Claims] 1. A conversion unit for reading an original image and converting it into image data corresponding to its density, a setting unit for setting a detection region of a fixed number of pixels centering on a pixel of interest, and a detection set by the setting unit. A halftone dot area determination device having a determination means for detecting a peak / dip pixel based on density information of pixel data in the area and determining whether or not the read original image is an original image of a halftone dot area. Further, it further comprises gradation determining means for determining the gradation of the original image area based on the read original image data, and the determining means determines a dip pixel when the gradation determining means determines a high gradation area. If it is determined to be a low gradation area, the peak pixel is detected, and if it is determined to be an intermediate gradation area, both the peak pixel and the dip pixel are detected. Halftone dot area determination apparatus, characterized in that for determining the halftone dot region based on the density of the click pixel or dip pixel.