JP2972172B2 - Halftone area detection method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a dot area detecting method for automatically determining and separating a dot area and a line drawing area in an image.

[0002]

2. Description of the Related Art When reproducing an image in which a halftone image such as a photograph or a picture and a line drawing such as a character are mixed in a copying machine or a facsimile, the quality of the reproduced image is improved. It is desirable to perform a moiré removal process on a halftone dot region such as a halftone photograph and a sharpening process on a line drawing region such as a character.

[0003] Further, even in the case of transmitting such an image in which a halftone image and a line image are mixed, it is necessary to perform processing according to the characteristics of each image area in order to improve the data compression ratio. It is desirable to perform an encoding process or the like. Japanese Patent Publication No. 5-33869 discloses a method for separating a dot region from an image in which a dot image and a line image are mixed.

In this method, as shown in the flow of the processing in FIG. 7, image data of an original image is extracted as a digital multi-tone signal by raster scan, and the density difference between light and dark adjacent pixels on the raster is determined. 8 (A) and 8 (B), a pixel satisfying any of the following conditions (i) to (iii) is defined as an extreme point indicating a peak or a valley of the density change from the difference signal. To be detected.

(I) The pixel (FIG. 8A) when the sign of the difference changes before and after is taken as the extreme point. (Ii) The pixel at which the sign of the difference value changes before and after the difference is zero (FIG. 8B) is defined as an extreme point. (Iii) A pixel (not shown) when the interval from the previous extreme point has reached a predetermined threshold value L _th0 is defined as an extreme point.

When the following conditions (iv) and (v) are simultaneously satisfied, it is determined as a halftone dot region based on the pole information obtained as described above. (Iv) The section length L (i) between the poles is a predetermined threshold L _th1
And L _th2 , that is, L _th1 <L (i) <L _th2 .

(V) The difference between the section length L (i) of the current position and the preceding section length L (i-1) is a predetermined threshold L _th3.
When L is within the range, that is, | L (i) −L (i−1) | ≦ L _th3 . Then, based on the determination result, the output of each pixel is converted into a signal corresponding to a line drawing or a signal corresponding to a halftone dot.

[0008]

In the above-mentioned conventional method, halftone dots are separated from an image on the assumption that peaks of peaks and valleys of density levels appear regularly in a halftone dot region. Since there are many extreme points in a character portion other than a halftone dot portion and in a continuous tone photographic portion, there is a problem that a sufficiently high separation rate cannot be expected with the above method.

Further, since the detection is performed by one-dimensional comparison of the pixels before and after the pixels arranged on the raster scan line, the halftone portion having a low halftone ratio, or the halftone portion having a high halftone ratio, and the original When the screen angle is shifted from the horizontal direction due to the rotation of the dot, the section length L between the extreme points
There is also a problem that (i) becomes long and it becomes difficult to separate from the character part.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a halftone dot area detecting method capable of accurately detecting a halftone dot area.

[0011]

In order to solve the above-mentioned problems, a matrix consisting of M × M pixels is sequentially applied to a digital multi-tone input image signal obtained from an input image, and The gray level has the maximum or minimum gray level in the matrix, and the average value of the gray level of the central pixel and the other two pixels symmetrically positioned with respect to the central pixel. It is detected from the difference whether or not the center pixel is an extreme pixel, and the pixels detected as the extreme pixels are counted in units of a two-dimensional area composed of N × N pixels larger than M, and From the relationship between the number of extreme pixels in the two-dimensional area and the number of extreme pixels in the surrounding two-dimensional area, the central pixel or all pixels of the two-dimensional area of interest are detected as halftone dots.

[0012]

The present invention detects a pixel extreme using a local two-dimensional extreme detection pattern composed of a matrix of M × M pixels,
Furthermore, it is detected whether or not a predetermined pixel in the block of interest belongs to a halftone portion by using a local two-dimensional halftone dot detection pattern composed of a block of N × N pixels. The halftone dots can be separated more accurately than the conventional method in which halftone dots are separated only by one-dimensional comparison of pixels.

[0013]

FIG. 2 shows an embodiment of a halftone dot area separating apparatus constructed by applying the method of the present invention. For the sake of simplicity, a case where a monochrome image is used as a processed image will be described as an example. If it is desired to apply the method to a color image, the color original image is decomposed into three primary colors such as RGB or YMC according to the display format of the reproduced image, such as CRT display or print display, and the dot separation processing according to the present invention is performed for each color. Good.

In FIG. 2, an input image signal section 1 raster scans an original image in which a halftone image such as a halftone photograph and a line image such as a character are mixed, and performs a digital multi-gradation comprising a luminance signal corresponding to a density level. It is converted into an input image signal and at least scan lines necessary for the subsequent separation processing, for example, N × 3 scan lines (N is the pixel size of a unit block composed of N × N pixels for halftone dot detection described later). This is a circuit for storing data in a line memory or the like.

The pole detecting section 2 sequentially applies a matrix of predetermined M × M pixels to each pixel of the digital multi-tone input image sent from the input image signal section 1, and This circuit detects whether or not the center pixel is an extreme point indicating a peak or a valley of a density change based on a density relationship with surrounding pixels.

As the extreme point detection conditions in the extreme point detection section 2, the following (Ia) and (Ib) are adopted as AND conditions, and detection is performed by a local two-dimensional extreme point detection pattern. Pole detection condition (Ia) As shown in FIGS. 3A and 3B, the density level of the center pixel m ₀ of the matrix of M × M pixels is equal to all other pixels m ₁ in the M × M pixel area. When the density level is larger or smaller than the density level of ~ m _i, the central pixel m ₀ is detected as a pole. That, m _0> m ₁ ~m _i or m ₀ <m ₁ ~m _i.

Pole detection condition (Ib) As shown in FIGS. 3A and 3B, the density level of the central pixel m ₀ of the matrix composed of M × M pixels is symmetrically located with respect to the central pixel. When the absolute value of the difference from the average value of the density levels of the other two pixels is larger than the fixed threshold value Δm _TH , the center pixel m ₀ is detected as an extreme point giving a peak or a valley of a density change. That is, when in the in center pixel m ₀ the other two pixels at the positions symmetrical m _a, and _{m b, | 2m 0 -m a} -m b | ≧ Δm TH.

As a specific example, in the case of FIG. 3A, | 2m ₀ −m ₁ −m ₈ | ≧ Δm _TH | 2m ₀ −m ₂ −m ₇ | ≧ Δm _TH | 2m ₀ −m ₃₋ m ₆ | ≧ Δm _TH | 2m ₀ −m ₄ −m ₅ | ≧ Δm _TH , and in the case of FIG. 3B, | 2m ₀ −m ₃ −m ₂₂ | ≧ Δm _TH | 2m ₀ − _{m 8 -m 17 | ≧ Δm TH} | 2m 0 -m 1 -m 24 | ≧ Δm TH | 2m 0 -m 7 -m 18 | is ≧ Delta] m _TH.

The halftone dot area detecting section 3 has N × N pixels (however,
N> M), for example, the image is divided in units of a block B having a 9 × 9 pixel size (N = 9) as shown in FIG. Are counted, and the number of extreme pixels on the side with the larger count value is determined as the extreme pixel of the block. Then, the number of extreme pixels P ₀ of the target block B ₀ shown in FIG. Left and right diagonal surrounding blocks B ₁
The center pixel n ₀ (see FIG. 4) of the block of interest B ₀ or all the pixels n _{0 to} n ₈₀ in the block B ₀ belong to the halftone dot region based on the relationship with the number P of the pole pixels of B ₈ to B ₈ . It is a circuit for determining whether or not the above is true.

The halftone dot detection condition in the halftone dot area detection unit 4 is to execute the processing one pixel at a time or
The following condition (II
Any of a) to (IIb) is adopted, and detection is performed using a local two-dimensional halftone dot detection pattern.

When dot separation processing is executed while moving block B of N × N pixels one pixel at a time, dot detection condition (IIa) As shown in FIG. 5, the block of interest B ₀ and the surrounding blocks B ₁ to dot at B _8, when the block B number pole pixel P is equal to or greater than a predetermined threshold value P _TH is present above a predetermined threshold B _TH, the center pixel n ₀ of the block of interest B ₀ (see FIG. 4) Detected as part. That is, [the number of blocks of P> P _TH .SIGMA.B]> B _TH.

As shown in dot detection condition (IIb) FIG. 5, the pole between the target block B ₀ and the surrounding blocks B ₁ .about.B _8, the block of interest B ₀ and the surrounding blocks B ₁ .about.B ₈ When the sum 絶 対 | ΔP | of the absolute values of the differences ΔP in the number of pixels is equal to or smaller than a predetermined threshold value ΔP _TH , the center pixel n ₀ in the target block B _{0 is} defined as a halftone dot portion.
That is, Σ | ΔP | <ΔP _TH . When the dot separation processing is executed while moving the block B of L × L pixels one block at a time, the dot detection condition (IIc) As shown in FIG. 5, the block of interest B ₀ and the surrounding blocks B _{1 to} B ₈ , When there is a block B in which the number of extreme pixels P is equal to or more than a predetermined threshold value P _{TH and} equal to or more than a predetermined threshold value B _TH , all the pixels n _{0 to} n ₈₀ in the target block B ₀ are used.
Is detected as a halftone dot.

The halftone detection condition (IId) as shown in FIG. 5, the pole between the target block B ₀ and the surrounding blocks B ₁ .about.B _8, the block of interest B ₀ and the surrounding blocks B ₁ .about.B ₈ When the sum 絶 対 | ΔP | of the absolute values of the differences ΔP in the number of pixels is equal to or smaller than a predetermined threshold value ΔP _TH , all the pixels n _{0 to} n ₈₀ in the target block B ₀ are detected as halftone dots.

The area determination signal output section 4 is a circuit for outputting a determination signal as to whether each pixel is a halftone area or a line drawing area based on the detection result of the halftone area detection section 3.
The operation of the embodiment having the above configuration will now be described with reference to the flowchart of FIG. Note that the above (IIa) is used as a condition for detecting a halftone dot region in the halftone dot region detection unit 3. The input image is composed of a halftone portion and a line drawing portion, and does not include a continuous tone portion such as a continuous tone photograph.

The input image signal unit 1 raster-scans the original image and converts it into a digital multi-gradation input image signal composed of luminance signals, and converts this image signal into N signals necessary for the subsequent halftone dot separation processing.
X3 scan lines are stored (step [1]). For example, as shown in FIG. 4, when N = 9 is adopted as the pixel size of the block B of N × N pixels, N × 3 = 9
× 3 = 27 scan lines of image data are stored.

The pole detection section 2 sequentially applies, for example, a matrix composed of 3 × 3 pixels shown in FIG. 3A to each pixel of the image data stored in the input image signal section 1 to obtain the above-mentioned pole detection condition (Ia ) And (IIb), it is determined whether or not the center pixel m _{0 of the} matrix is the extreme point of the density change for all the pixels on the 27 scan lines (step [2]).

When the extreme point detection is completed for all the pixels in step [2], steps [3] to [3] are performed.
In [8], a region determination as to whether or not each pixel is a halftone portion is performed based on the halftone detection condition (IIa). That is, first, in step [3], the above-examined image is divided into 9 × 9 pixel-size blocks B as shown in FIG. The number is counted, and the number of extreme pixels on the side with the larger count value is set as the number of extreme pixels P of the block.

[0028] Next, in step [4], the block surrounding the block of interest B ₀ shown in FIG. 5 B ₁ ~B ₈
The number ΔB of blocks whose pole pixel number P is equal to or greater than a predetermined threshold value P _TH is determined, and it is determined in step [5] whether or not the block number ΔB is equal to or greater than a predetermined threshold value B _TH .

In step [5], if the number of blocks ΣB is larger than the threshold B _TH,
Central pixel n ₀ of ₀ (see FIG. 4) is determined to be a halftone unit, the process proceeds to step [6]. On the other hand, the number of blocksΣB
There is smaller than the threshold B _TH is the center pixel n ₀ of the block of interest B ₀ is determined not to be a halftone unit, a step [7]
Move to.

The area determination signal output section 4 receiving the above detection result from the halftone area detection section 3 outputs a determination signal as to whether or not the center pixel n ₀ is a halftone area (step [6]).
[7]). As described above, the halftone dot separation process is repeatedly executed for every N × 3 scan lines for all pixels of the input image (step [8]).

[9]) For all the pixels of the input image, a region is divided into a halftone dot portion and a line drawing portion.

In the above embodiment, the case where the original image is composed of a halftone portion such as a halftone dot photograph and a line drawing portion such as a character has been described. In some cases, continuous tone portions such as pictures are mixed. In such a case, the present invention may be applied after first differentiating the input image signal and removing the continuous tone portion from the edge density (see, for example, JP-A-58-115975). By doing so, it is also possible to separate the three parts, the halftone part, the character part, and the continuous tone part.

In the above embodiment, a matrix of 3 × 3 pixels is used as a matrix of M × M pixels for providing a pole detection pattern, and an N × N matrix for providing a halftone detection pattern is used.
Although a block of pixels having a size of 9 × 9 pixels is used, it is a matter of course that the values of the pixel sizes M and N can be arbitrarily adopted while maintaining the relationship of N> M.

FIG. 6 shows an example of a copying machine constructed by using the halftone dot area separating apparatus of the above embodiment. In FIG. 6, the block indicated by reference numeral 5 is a halftone dot area separating apparatus shown in FIG. It is. In FIG. 6, reference numeral 6 denotes a sharpening processing circuit for character area processing,
7 is a Bay emphasizing the resolution for sharpening characters and the like.
er type dither processing circuit, 8 is a smoothing processing circuit for halftone area processing, 9 is a vortex type dither processing circuit that emphasizes gradation for producing halftones such as halftone pictures, and 10 is an area separation device. 5 receives the halftone dot determination signal from
Or an image signal selection circuit for selecting any one of the signals 9.

The input image signal is judged by the dot area separating device 5 as to whether or not each pixel belongs to a dot portion, and the judgment result is sent to the image signal selection circuit 10. The image signal selection circuit 10 selects and outputs an image signal from the dither processing circuit 7 when the determination signal is not a halftone area signal, and outputs the dither processing circuit when the determination signal is a halftone area signal. The image signal on the ninth side is selected and output.

As a result, in the line drawing area, the dither processing circuit 7
Image signals such as characters that have been sharply processed by
In the halftone dot region, an image signal such as a halftone photograph subjected to pseudo halftone processing by the dither processing circuit 9 is selectively output. Therefore, if the image signal output from the image signal selection circuit 10 is subjected to necessary processing and then copied and reproduced, the line drawing portion such as a character is sharpened, and the dot portion such as a halftone photograph becomes natural. A high quality binary image subjected to pseudo halftone processing is obtained.

[0036]

As described above, according to the present invention, the following effects can be obtained. A matrix consisting of M × M pixels is sequentially applied, and the gradation level of the center pixel of the matrix is maximum or minimum with respect to the other pixels, and is located at a symmetric position with respect to the center pixel and the center pixel. Whether or not the central pixel is an extreme pixel is detected from the difference between the average value of the gradation levels of the other two pixels and whether the central pixel is an extreme pixel or not. The central pixel or all the pixels of the two-dimensional area of interest are detected as halftone dots from the relationship between the number of pixels of the two-dimensional area of interest and the number of extreme pixels of the two-dimensional area around the two-dimensional area. As a result, a high separation rate can be achieved, and the screen angle shifts from the horizontal direction due to the low dot ratio or the high dot ratio, or the rotation of the document. Even halftone dots Can be separated.

In detecting an extreme point, not only is the density level of the target central pixel and the density level of each peripheral pixel individually compared, but also the density level of the central pixel and a symmetrical position with respect to the central pixel. The extreme points are detected on condition that the absolute value of the difference from the average value of the density levels of the other two pixels in the above is larger than a certain threshold value, so that the levels are individually compared with surrounding pixels. It is possible to perform pole detection that is more resistant to digital noise as compared with the case of using only the above.

[Brief description of the drawings]

FIG. 1 is a flowchart of the operation of an embodiment of the present invention.

FIG. 2 is a diagram showing an embodiment of the present invention.

FIG. 3 is a diagram showing an example of a matrix composed of M × M pixels used in the present invention.

FIG. 4 is a diagram showing an example of a block including N × N pixels.

FIG. 5 is a diagram showing a relationship between a block of interest and surrounding blocks.

FIG. 6 is a diagram illustrating an example of a copying machine configured by applying the present invention.

FIG. 7 is an explanatory diagram of a conventional system.

FIG. 8 is a diagram illustrating an example of pole detection by a conventional method.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H04N 1/40-1/409 H04N 1/46 H04N 1/60

Claims (1)

(57) [Claims] 1. A matrix comprising M × M pixels is sequentially applied to a digital multi-gradation input image signal obtained from an input image, wherein a gradation level of a center pixel of the matrix is a maximum or minimum level in the matrix. And the other two pixels having a gray level and being symmetrical with respect to the center pixel.
Detecting whether or not the center pixel is an extreme pixel from the difference between the average value of the gradation levels of the two pixels and the pixel detected as being the extreme pixel from the N × N pixels larger than the M; The center pixel or all pixels of the two-dimensional area of interest are determined as halftone dots based on the relationship between the number of extreme pixels of the two-dimensional area of interest and the number of extreme pixels of the surrounding two-dimensional area. A halftone dot area detection method characterized by detecting.