JP2968276B2 - Halftone area separation method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for separating a halftone dot region of an image represented by a digital multi-tone image signal.

[Conventional technology]

When reproducing an image in which a picture expressed by a halftone dot and a line drawing such as a character are mixed, it is preferable to perform a moire removal process on the halftone portion and a sharpening process on the line drawing in order to improve the image quality. When transmitting an image, it is preferable to perform a process of replacing a halftone dot with a certain value in order to improve the compression ratio.

In order to perform such processing, it is necessary to separate a halftone dot area of an image from a non-halftone dot area as preprocessing.

Oki Electric Research and Development,
No. 132, Vol. 53, No. 4 (October 1986), pp. 71-76, "Method of Reproducing Halftone Photograph by Dot Printer" is known. In this method, the sign of the difference between the one-dimensional image signals is obtained, the pixel position at which the sign of the difference changes is examined, and each of the locations satisfying the following conditions is defined as a pole.

(A) When the sign of the difference changes.

(B) When the sign of the difference value changes before and after the difference is zero.

(C) When the distance from the previous pole reaches a predetermined threshold value.

 Then, a region between the poles satisfying the following condition is defined as a dot region.

(B) Section length L (i) between extreme points, predetermined threshold TH1, TH
2 satisfies the relationship TH1 <L (i) <TH2.

(B) The current section length L (i) and the preceding section length L (i−
1), the relationship | L (i) −L (i−1) | ≦ TH is established for the predetermined threshold TH3.

[Problems to be solved by the invention]

However, according to this conventional algorithm, a high separation rate cannot be expected because many extreme points are detected also in a character portion and a photograph portion. In particular, since the processing is performed in one dimension, a halftone dot portion having a low halftone dot ratio, a halftone dot portion having a high halftone dot ratio, or a halftone dot portion whose screen angle is slightly shifted from the horizontal direction due to rotation of a document, Since the section length between the poles is long, it is impossible to separate the character portion from the character portion.

Accordingly, an object of the present invention is to provide a new halftone dot region separation method that can solve the above-described problems.

[Means for solving the problem]

The halftone dot separation method of the present invention is a method of separating a halftone dot region by detecting a peak or a valley peak (pole) of a halftone dot from a digital multi-tone image signal. In the detection method.

That is, according to the present invention, in the two-dimensional gradation pattern in the M × M matrix represented by the digital multi-tone image signal, (a) the density level of the central pixel is higher than the density levels of all other pixels. (B) the height of the peak of a halftone dot obtained by a specific calculation from the density levels of a plurality of sets of pixels and the density level of the center pixel which are point-symmetrical with respect to the center pixel; When the depth of the valley peak is greater than or equal to a certain threshold value, the center pixel is extracted as a peak or valley peak pixel of a halftone dot. (C) The threshold value is determined according to the magnitude of the density level of the center pixel. Change. That is, when the density level of the central pixel is high, the density is changed to high, and when the density level is low, the density is changed to low.

(Operation)

According to the present invention, in order to extract a peak pixel of a peak or a valley of a halftone dot by examining a local two-dimensional pattern, compared with a conventional method of examining a one-dimensional pattern, the dot ratio is low, Alternatively, the extraction accuracy of the peak pixel of the halftone dot is increased even in a high halftone dot region, a halftone dot region with a tilted screen angle, and the like.

Further, when the peak portion of a halftone dot is digitized, the higher the peak at the center of the halftone dot (the lower the peak if it is a valley peak), the greater the difference in density level between the center and its vicinity. According to the present invention, by focusing on this property, the threshold value for determining the peak pixel is appropriately changed according to the magnitude of the density level of the candidate pixel of the peak pixel. False extraction of pixels can be reduced.

As described above, according to the present invention, a peak pixel of a halftone dot can be extracted with high accuracy, and thus the separation rate of a halftone dot region is greatly improved.

〔Example〕

FIG. 1 is a block diagram showing one embodiment of the present invention.
In FIG. 1, reference numeral 1 denotes a portion for extracting a halftone dot peak pixel, and 2 denotes a portion for determining a halftone dot region from the extracted peak pixel.

In the peak pixel extracting unit 1, reference numeral 3 denotes a buffer memory for temporarily storing a digital multi-tone image signal input from the outside, and reference numeral 4 denotes a density of a central pixel in a 3 × 3 (generally M × M) matrix. A maximum / minimum check unit for checking whether the level is maximum or minimum, an arithmetic unit for calculating the height or depth of a peak, and a threshold value setting unit for setting a threshold value th1 for peak pixel determination. , 7 are judgment units for judging the peak pixel of a peak or a valley by comparing the operation value of the operation unit 5 with the threshold value set by the threshold value setting unit 6.

The algorithm for determining the peak pixel is as follows. The maximum / minimum check unit 4 checks whether the central pixel in the 3 × 3 matrix on the image stored in the buffer memory 3 has the maximum density level or the minimum density level as compared with the surrounding pixels. Here, if the density level of each pixel in the 3 × 3 matrix is represented as shown in FIG. 2, Lc> L1 and Lc> L2 and Lc> L3 and Lc> L4 and Lc> L5 and Lc> L6 and Lc> When L7 and Lc> L8, the central pixel is set to the maximum density level, and when Lc <L1, Lc <L2, Lc <L3, Lc <L4, Lc <L5, Lc <L6, Lc <L7, and Lc <L8 Then, the central pixel is set to the minimum density level.

The maximum / minimum check unit 4 outputs the determination result to the threshold value setting unit 6 and the determination unit 7, and outputs the density level Lc of the target pixel, that is, the center pixel, to the threshold value setting unit 6.

The threshold value setting unit 6 sets the threshold value th1 for peak pixel determination to A when the density level Lc is equal to or greater than the threshold value th2 when the target pixel is the peak, and sets th1 to B when Lc is smaller than th2.
(Where A> B), and when the pixel of interest has a valley peak, th1 is set to C.

The reason why the threshold value is set in this manner is to increase the accuracy of peak pixel extraction by focusing on the property that the higher the peak, the greater the difference in density level from the surroundings, as described above.

Note that the lower the valley peak, the greater the density level difference from the surroundings.Therefore, the determination threshold value for the target pixel determined to be the minimum density level is also controlled to increase as the density level of the target pixel decreases. , The accuracy of peak pixel extraction is further improved.

The arithmetic unit 5 calculates the density levels La and Lb of the four sets of peripheral pixels and the density level Lc of the target pixel which are in point symmetry with respect to the target pixel.
Respectively, Zi = | 2 × Lc−La−Lb | is calculated.

That is, Z ₁ when La = L1, Lb = L8, La = L2, Lb
= Z ₂ when the L7, La = L3, Lb = L6 and then Z ₃ when the, L
Z ₄ and calculates each when the a = L4, Lb = L5.

Determination unit 7, the pixel of interest by the maximum / minimum check unit 4 is determined to be a pixel of maximum density level or minimum density level, and when the Z ₁ value of Z ₄ is not less than th1 all, of the maximum density level It is determined that the target pixel is a valley peak pixel and that the minimum density level target pixel is a valley peak pixel.

Similar peak pixel extraction is performed while moving the pixel of interest one pixel at a time.

In the halftone dot area separation determination unit 2, reference numeral 8 denotes a buffer memory for temporarily storing the result of peak pixel extraction, and reference numeral 9 denotes N × N
The counting unit counts the peak peak pixels and the valley peak pixels separately for each block having a size of (N> M) and outputs the larger park pixel number. Reference numeral 10 denotes a determination unit that determines a halftone dot portion based on the number of peak pixels in a block. This determination is performed for each pixel or each block.

In the case of the determination for each pixel, in the 3 × 3 blocks B0 to B8 as shown in FIG. 3, when the peak pixel number is equal to or greater than the threshold value th3 in P or more blocks, the central target block B0
Is determined to be a pixel in the halftone dot area.

When the determination is performed on a block-by-block basis, if there are P or more blocks having a peak pixel number of th3 or more in a 3 × 3 block, all the pixels in the target block B0 are determined to be pixels in the halftone dot area. judge.

FIG. 4 is a flowchart showing the flow of the processing described above. However, the separation determination of the halftone dot area is performed in units of one pixel.

FIG. 5 shows an image signal processing system of a copying machine to which the present invention is applied. Numeral 12 denotes a halftone dot area separation circuit as shown in FIG. 1, which determines the separation between a halftone area and a non-halftone area from a digital multi-tone image signal input from the outside, and outputs an area discrimination signal. . The digital multi-tone image signal is a luminance signal in the case of monochrome reproduction, and is a color signal after color separation in the case of color reproduction.

The digital multi-tone image signal is sequentially processed by a sharpening processing circuit 13 and a dither processing circuit 14 for character area processing, and sequentially processed by a smoothing processing circuit 15 and a dither processing circuit 16 for halftone dot area processing. Is output to the image signal selection circuit 17. Dither processing circuit
14 uses a Bayer-type dither matrix that emphasizes resolution, and the other dither processing circuit 16 uses a spiral dither matrix that emphasizes gradation.

The image signal selection circuit 17 outputs the input image signal from the dither processing circuit 16 to the outside when the output of the dot area separation circuit 12 indicates a halftone area, and performs dither processing when the output indicates a non-halftone area. The input image signal from the circuit 14 is output to the outside.

In the case where a picture represented by a continuous photograph is mixed in the image to be processed, the photograph portion is removed by a method using edge density (for example, a method described in Japanese Patent Application Laid-Open No. 58-115975). Thereafter, by applying the present invention, it is possible to separate a halftone dot region, a photograph region, and a character (line drawing) region.

〔The invention's effect〕

As is apparent from the above description, according to the present invention, a halftone dot region can be separated with higher precision than in the past, so that high-quality reproduction of an image in which a halftone dot region and a character region are mixed can be efficiently performed. Transmission and the like become possible.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing a matrix for peak pixel extraction, FIG. 3 is a diagram showing an arrangement of blocks for halftone dot area separation determination,
FIG. 4 is a schematic flowchart of the process, and FIG. 5 is a block diagram of an image signal processing system of a copying machine to which the present invention is applied. 1... Peak pixel extraction unit, 2...
3 ... buffer memory, 4 ... maximum / minimum check unit,
5 ... Calculation unit, 6 ... Threshold setting unit, 7 ... Determination unit, 8 ...
... buffer memory, 9 ... counter, 10 ... determiner.

Claims (1)

(57) [Claims] 1. A method for separating a halftone dot region by detecting a peak or a valley peak pixel of a halftone dot from a digital multitone image signal, comprising the steps of: In each two-dimensional area unit, the density level of the central pixel is the maximum or minimum as compared with the density levels of all other pixels, and the density level of a plurality of sets of pixels that are point-symmetrical to the central pixel and the central pixel. When the value corresponding to the height of the peak or the depth of the valley peak of the halftone dot obtained by the specific calculation from the density level is equal to or greater than a certain threshold value, the center pixel is set to the peak or the valley peak of the halftone dot. A halftone dot area separation method, wherein the threshold value is extracted as a pixel, and the threshold value is changed to a large value when the density level of the central pixel is large and to a small value when the density level is small.