JPS6180969A - Picture signal processor - Google Patents

Abstract

PURPOSE:To secure the sharpness with respect to all of a character image, a photograph image, and a halftone dot image by discriminating a net halftone dot image and other character image part from a photographic image part in which a halftone is represented, and executing an optimum signal processing with regard to each of them. CONSTITUTION:An analog picture signal 32 from an image sensor 31 is converted to a digital signal 34 of 6 bits by an A/D converter 33, and supplied to a halftone dot image processing part 36, a photographic image processing part 37 and a character image processing part 38, respectively. Also, the digital picture signal 34 is supplied to a deciding circuit 35, and in this circuit, at every prescribed signal processing area, to which said signal belongs, (1) a character image, (2) a photographic image, or (3) a halftone dot image is decided. A result of this decision is sent to a multiplexer 47, by which one output of the halftone dot image processing part 36, the photographic image processing part 37, and the character image processing part 38 is selected and outputted.

Description

Detailed Description of the Invention "Field of Industrial Application" The present invention also applies to image information sources in which three types of images, character images, photographic images, and halftone dot images, coexist. The present invention relates to an image signal processing device for performing.

``Prior Art'' Image processing devices that read a document using an image sensor and record or display it using quantized image signals have a method of distinguishing between each part when the document contains text and photographs. Image processing is being performed. This is because pixel-by-pixel binarization processing (simple binarization processing) only reproduces the character image part well, making it impossible to reproduce the halftone parts of photographs, etc., while pseudo-binarization processing such as dither processing This is because while halftone processing can satisfactorily reproduce the halftones of photographs, etc., the reproducibility of line drawings such as characters deteriorates.

As a conventional technique for distinguishing between a character image portion and a halftone image portion, a device disclosed in Japanese Patent Application Laid-Open No. 58-3374 is a typical example. This device (method) divides the document surface (screen) into blocks each consisting of multiple pixels.
The difference in density level is found between the pixel with the maximum density level and the pixel with the minimum density level within each block. If this difference is larger than a predetermined value, it is determined that it is a binary image area or a character image area, and in other cases, it is determined that it is a grayscale image area or a bright and dark image area.

By the way, some manuscripts use halftone dots to reproduce halftones or emphasize specific parts of text. If a document part using such halftone dots (halftone image) is read as is and subjected to image processing, moiré will occur due to the relationship between the halftone dot repetition period and the sub-scanning period, making it extremely difficult to reproduce the image. deteriorate. Therefore, conventional techniques have been used to detect the halftone dot image portion existing on the document and to change the image processing between this portion and other portions. A typical halftone dot detection device for this purpose is the technique described in Japanese Patent Laid-Open No. 58-218271. This device compares a binary image signal obtained by scanning a document with a detection signal consisting of a black and white code string corresponding to the positions and sizes of white and black dots on the document. By determining whether or not these signals match, the halftone image area is compared to other non-halftone images (
D Distinguish from B.

In this way, conventional techniques have been used to distinguish between ■character image parts and halftone image parts, and ■distinguish between halftone image parts and non-halftone image parts. It is useful for sex.

However, a document contains a mixture of three areas: a character image, a halftone image, and a photograph. Here, the character image portion is a portion composed of line drawings such as characters. In addition, the halftone image part is the part where intermediate tones are expressed, and includes the image part where gradation is expressed by halftone printing or the image part where so-called "shading" is performed by the printer. say. Furthermore, the photographic portion is a portion in which halftones are similarly expressed, and refers to image portions other than character images and halftone dot image portions.

Photographs using a photosensitive substance such as silver chloride can be cited as a representative example of this photographic part.

"Problem to be Solved by the Invention" Now, with regard to manuscripts in which three types of image parts coexist: a character image part, a halftone image part, and a photograph part, all three types are considered to be good. There was no way to reproduce it. That is, (2) in an image signal processing device that processes character images and halftone images separately, moiré occurs in halftone dot areas. In order to prevent the occurrence of moire, the halftone image portion is smoothed and then pseudo halftone processing is performed using a dither method or the like. However, when such smoothing is performed, a new problem arises in that the sharpness of the photographic portion decreases.

On the other hand, in the image signal processing method that processes halftone images and non-halftone images separately, the text and photograph parts are included in the non-halftone areas, so the reproducibility of text is lower than the reproducibility of halftones. There is a problem in that if one of these is attempted to be performed satisfactorily, the reproducibility of the other becomes insufficient.

"Means for Solving Problems" In the present invention, as shown in principle in FIG.
1 is used to sequentially determine to which part of a character image, a photographic image, or a halftone dot image a signal processing area to be subjected to image processing belongs over the entire area of the processing target. Then, optimal processing is performed within the image processing section 12 according to the discrimination result performed within the image discriminating section 11.

The part 14 in the image processing unit 12 that processes character images is M
It may be provided with a JF correction means I5 and a multivalue conversion means 16 for converting the image signal corrected by the JF correction means I5 into multivalue. Further, a halftone dot image processing section 17 in the image processing unit 12 includes a halftone dot removal half-immersion 18 that smooths and removes halftone dots, and an intermediate halftone dot removal section 18 that performs pseudo-halftone processing on the image signal after halftone dot removal. The adjustment processing means 19 may also be provided.

Furthermore, the section 21 in the image processing section that processes photographic images is
It may also include a halftone processing means 22 that performs multivalued pseudo halftone processing.

"Example" The present invention will be described in detail below with reference to Examples.

FIG. 2 shows an image signal processing device used in a document reading device as an embodiment of the present invention. An image sensor 31 of this device scans a document (not shown) and outputs an analog image signal 32. A/D converter 3
quantizes this pixel by pixel and outputs it as a parallel digital image signal 34 of 6 bits (density of 64i levels). The digital image signal 34 is supplied to a determination circuit 35 to determine the image state, and is also supplied to a halftone image processing section 36, a photographic image processing section 37, and a character image processing section 38 for image processing. It has become.

For each predetermined signal processing area, the determination circuit 35 determines whether
It is determined whether the image belongs to a character image, a photographic image, or a halftone dot image. In this example, such a determination is made as follows.
This is done by analyzing the constituent frequency components of an image within a window called the language processing area. For example, FIGS. 3 and 4 show the same image expressed by a photograph and halftone dots, respectively, for one scanning line. As can be understood from these figures, both exhibit smooth density changes when viewed macroscopically. However, when viewed microscopically, a photographic image still exhibits smooth density changes, whereas a halftone image reproduces the black and white dots that make up the halftone dots, causing considerable vibrational variation in density. .

Therefore, if Fourier transform is performed for each predetermined signal processing region as a frequency analysis, in the case of a photographic image, a peak will occur only in a low frequency portion corresponding to the overall density change. On the other hand, in the case of halftone dots □images, one peak occurs in this low frequency part, the vibration part (
Peaks also occur in higher frequency components corresponding to halftone dots). FIGS. 5 to 7 show typical examples of Fourier transform outputs after quaternary conversion for halftone dots, photographs, and character images, respectively.

FIG. 8 specifically shows a determination circuit that performs determination based on the principle described above. The digital image signal 34 is input to a shift register 41 of this circuit. The shift register 41 has a 33-stage configuration, with 166 stages on each side, including the pixel to be image processed (pixel of interest).
Accumulates image signals of pixels. The area of the image signals stored in this shift register 41 corresponds to the signal processing area in the present invention.

The image signal of this signal processing area is inputted in parallel to the Fourier transform circuit 42 and subjected to Fourier transform. The conversion output is manually input to the 4-value conversion circuit 43 for each spatial frequency component, and 4
Valued. The 2-bit 4-valued data output from each 4-valued circuit 43 is used as address information for determination R.
The signal is supplied to an OM (read only memory) 44. The pattern data shown in FIGS. 5 to 7 is input to the judgment ROM 44, and depending on the contents of the address information, one of the three types of images is read out as a judgment result signal. Output as 45. The determination result signal 45 is supplied as a 2-bit signal to a multiplexer 47 shown in FIG.

In the image signal processing apparatus shown in FIG. 2, the halftone image processing section 36, the photographic image processing section 37, and the character image processing section 38 each process the digital image signal 34 in parallel. In the halftone image processing section 36, the digital image signal 34 is first smoothed by a halftone dot removal circuit 48, and small fluctuations in density peculiar to halftone images are removed. The image signal 49 after halftone removal is input to a pseudo halftone processing circuit 51 . The pseudo halftone processing circuit 51 sets a threshold value for each pixel using, for example, a dither method, and performs pseudo halftone processing. At this time, each dot constituting the dither matrix may be expressed in binary terms of white and black, or
Three-value or more multi-value processing may be performed. The image signal processed by the pseudo halftone processing circuit 51 is delayed by a predetermined amount in a delay circuit (not shown), and is supplied to the input terminal of the multiplexer 47 as the image signal 52 of the pixel of interest.

Next, in the photographic image processing section 37, the digital image signal 34 is directly input to the pseudo halftone processing circuit 53, and the same processing as the pseudo halftone processing circuit 51 is performed.

The processed image signal is similarly delayed by a delay circuit (not shown) and is supplied to the other input terminal of the multiplexer 47 as the image signal 54 of the pixel of interest.

Finally, in the character image processing section 38, the digital image signal 34 is input to the MTF correction circuit 55, where deterioration in resolution caused by the optical system is corrected. The corrected image signal 56 is manually input to a multi-value conversion circuit 57, where it is multi-valued pixel by pixel.

The multivalued image signal is delayed by a delay circuit (not shown),
It is supplied to the remaining input terminals of the multiplexer 47 as the image signal 58 of the pixel of interest.

In this way, the multiplexer 47 selects one of the three types of broiled images [No.
5 and outputs it as an image signal 59.

That is, for example, when the determination circuit 35 determines that the pixel of interest is a pixel of one halftone image, the image signal 52 that has been processed for the halftone image is selected and output as the image signal 59.

Similarly, when the pixel of interest is determined to be part of a photographic image, the image signal 54 is selected, and when the pixel of interest is determined to be part of a character image, the image signal 58 is selected.

As a result, the image signal 59 output from the multiplexer 47
This means that the image signal 59 is always subjected to optimal image processing, and the image is reproduced satisfactorily in a recording section or a display section (not shown) that uses this image signal 59.

Note that in the embodiment described above, the image state is determined for all pixels one by one, but the determination result of a specific pixel of interest is extended and applied as the determination result of a group of pixels in a predetermined range including this pixel of interest. This is also effective from the viewpoint of simplifying signal processing and removing noise. Further, in the embodiment, a pseudo halftone processing circuit is provided for each of the two processing sections, but a switching circuit or the like may be provided so that one pseudo halftone processing circuit is shared by these processing sections. Further, in the embodiment, dither processing is performed using a pseudo-halftone processing circuit, but it is also possible to express halftones using an area gradation method that allocates patterns with print ratios corresponding to gradations.

"Effects of the Invention" As described above, according to the present invention, the image area where halftones are expressed is divided into the halftone image area and the other areas, and optimal signal processing is performed for each area. Sufficient sharpness of images and photographic images can be ensured, and also halftone images can be well reproduced.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram for explaining the principle and deformability of the present invention, FIG. 2 is a block diagram of an image signal processing device in an embodiment of the present invention, and FIG. 3 is a diagram of reflectance characteristics of a photographic image. Figure 4 is a characteristic diagram showing an example of reflectance characteristics of a halftone image, and Figures 5 to 7 are characteristic diagrams of spatial frequency characteristics in each image area of halftone dots, photographs, and characters. FIG. 8 is a block diagram showing a specific example of the determination circuit shown in FIG. 2, which is a characteristic diagram showing a typical example. 11... Image discrimination unit, 12... Image processing unit, 15... MTF correction means, 16... Multi-value conversion means, 18... - Halftone removal means, 19.22...Halftone processing means, 35...
...Judgment circuit, 36... Halftone image processing section, 37
... Photographic image processing section, 38 ... Character image processing section, 48 ... Halftone removal circuit, 51.53 ... Pseudo halftone processing circuit, 53・・・
...MTF correction circuit, 57...Multi-value conversion circuit. Applicant: Fuji Xerox Co., Ltd. Representative
Person Patent Attorney Umeo Yamauchi Fig. 2 Fig. 3 Fig. 4 Fig. 4 The owner of the drawing is Takarizono! Figure 5 Figure 6 Figure 7

Claims (1)

[Claims] 1. The signal processing area to be subjected to image processing is sequentially shifted, and it is determined whether the signal processing area belongs to a character image, a photographic image, or a halftone image over the entire area to be processed. The image processing unit includes an image discriminating unit that performs sequential discrimination, and an image processing unit that performs image processing suitable for each image state on one or more image signals belonging to the signal processing area according to the discrimination results of the image discriminating unit. An image signal processing device characterized by: 2. The part of the image processing unit that processes character images is MTF (
ModulationTransferFunction
(n) The image signal processing apparatus according to claim 1, further comprising an MTF correction means for performing correction, and a multi-value converting means for converting the image signal corrected by this means into multi-value. 3. The part of the image processing unit that processes the halftone image includes halftone removal means for smoothing and removing halftone dots, and halftone processing means for performing pseudo halftone processing on the image signal after halftone dot removal. An image signal processing device according to claim 1, characterized in that the image signal processing device comprises: 4. The image signal processing device according to claim 1, wherein a portion of the image processing unit that processes photographic images is provided with halftone processing means for performing multi-value pseudo halftone processing.