JPS6180970A - Picture signal processor - Google Patents

Abstract

PURPOSE:To discriminate exactly a character image part and a halftone image part without losing the sharpness of the character image part by eliminating a halftone dot existing in a picture information source, and thereafter, discriminating the character image part and the halftone image part. CONSTITUTION:An analog picture signal 24 from an image sensor 22 is A/D- converted and applied to a series circuit of line memories 26, 28 and a series circuit of shift registers 271, 272. Also, outputs of the line memories 26, 28 are provided to a series circuit of shift registers 273, 274 and a series circuit of the same 275-276 are provided to a filter operating circuit 21, and nine kinds of image signals which have been inputted in said circuit are multiplied by a filter coefficient, and synthesized and calculated. As a result, an operating signal 29 is sent to a character and halftone deciding circuit 31, and in accordance with a signal which has been decided in this circuit, one output of a multivalue dither circuit 47 or a simple multivalue circuit 48 is selected and fetched by a multiplexer 46.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image signal processing device for performing appropriate image processing on an image information source containing a mixture of character images and halftone images. .

``Prior art'' Facsimile machines and some types of digital copying machines use image sensors to read image information on a document, quantize it according to the shading of the image, and then use it to display it on a recording device or display. The image is sent to a device and reproduced.

In such an image signal processing device, when a document contains text image parts such as characters and line drawings, and halftone image parts such as photographs and halftone printing, it is necessary to distinguish between the two parts. Processing images. This is because multi-value processing for each pixel only reproduces the character image part well, but there is a limit to the reproduction of the gradation of the halftone part, while pseudo halftone processing such as dithering This is because while the tone image portion is well reproduced, the outline of the character image portion becomes blurred and its reproducibility 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 pixel with the maximum density level and the pixel with the minimum density level are detected within each block, and the difference in density level between them is determined. If this difference is larger than a predetermined value, it is determined that the area is a binary image area or a character image area; otherwise, it is determined that it is a grayscale image area or a halftone image area.

"Problems to be Solved by the Invention" In general printed matter, halftones are often expressed by halftone images. Here, the halftone image refers to the area where halftone printing has been performed or the area where so-called "hatching has been performed" by the printer. In the halftone image area, the dots colored black are usually The gradation (density) is expressed by the percentage of area occupied within the area. Therefore, microscopically, the part of the halftone image is the part where so-called black and white binary expression is performed, and it is different from the part of the character image. In other words, the image signals obtained by reading these two image parts both contain high-frequency components, which makes it difficult to distinguish between the character image part and the halftone image part including the halftone image. It was considered a culprit.

In view of the above circumstances, an object of the present invention is to provide an image signal processing device that can accurately discriminate character images and halftone images from an image information source including halftone images and perform signal processing. shall be.

"Means for Solving the Problems" In the present invention, as shown in principle in FIG. 1, an image signal smoothing means 1 that performs smoothing for removing halftone dots on a quantized image signal; a character/halftone discriminating means 2 for discriminating between a character image portion consisting of characters or line drawings and a halftone image portion other than the character image portion based on the smoothed image signal;
The image signal processing device is equipped with a signal processing means 3 that performs different signal processing appropriate for recording or displaying the quantized image signal according to the determination result by the halftone determination means 2.

Here, as the image signal smoothing means 1, it is effective to use a spatial frequency filter having filter coefficients for smoothing the halftone image. In addition, as the signal processing means 3, a simple multi-value circuit is used in the character image part to express gradation for each pixel, and in the half-tone image part, a multi-value dither circuit is used to express a plurality of pixel groups as a unit. It is effective to perform gradation expression. Of course, the signal processing method is not limited to these methods.

According to the present invention, since halftone dots present in the image information source are removed before character image portions and halftone image portions are discriminated, accurate discrimination is possible.

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

FIG. 2 shows a document 11 as an image information source and a 3×3 pixel group P used for image judgment. -P6. In this embodiment, a window 12 for collecting a 3×3 pixel group is set on the document 11. Pixel P located at the center of window 12. is the pixel of interest that is the target of image processing. Pixel of interest P. As the original 11 is scanned in a plane, its position is successively changed.

Pixel of interest P. The eight pixels Pl-P8 surrounding the pixel P are the pixel P of interest. They are also peripheral pixels for determining the image state or smoothing the image signal. These pixels P explained above. - the respective concentration d of P8.

-d8 represents the photoelectric change detection power of the reading element as a signal level after A/D conversion. In this embodiment, the densities do'''''d6 are each expressed as 8-bit (2561st floor) digital data.

In this embodiment, a spatial frequency filter is first applied to the window 12 to smooth the output thereof. Before this explanation, the principle of the spatial frequency filter will be explained using FIGS. 3 and 4. First, in the spatial frequency filter of FIG. 3, the density d of the pixel of interest. is multiplied by 9 by the filter coefficient '9', and the densities d, ~d8 of the eight surrounding pixels are multiplied by -1 by the respective filter coefficients.The sum of these becomes the filter output.In this way, this space In the frequency filter, the filter coefficient corresponding to the pixel of interest P is set to be much larger than those of the surrounding pixels, so even if there is a slight change in density in the pixel of interest P, this will be magnified. That is, this spatial frequency filter is used to sharpen the image of the pixel of interest P.

Next, in the spatial frequency filter shown in FIG. 4, the density of each pixel is multiplied by 1×9 and added, and the density of the entire window is determined. Therefore, even if the densities are not uniform among the pixels within the window 12, these densities are smoothed (averaged) and output. That is, this spatial frequency filter is used to smooth the image of the pixel of interest Pa.

By applying the well-known principles of such spatial frequency filters, it is possible to create spatial frequency filters for smoothing halftone images. FIG. 5 shows a spatial frequency filter used in this embodiment for a halftone image having a halftone dot number (screen number) of 100 lines/inch or more.

FIG. 6 shows the image signal processing device of this embodiment. The spatial frequency filter shown in FIG. 5 is used in the filter calculation circuit 21 of this device. Now, the image sensor 22 of this device scans the original 11 (FIG. 2), and the A/D converter 23 quantizes the manually input analog image signal 24 pixel by pixel. The digital image signal 25 output from the A/D converter 23 as 8-bit parallel data is supplied to the line memory 26, and
The 8-bit parallel data is supplied to the serial circuits of the first and second shift registers 271 and 272, and is shifted by one pixel in synchronization with a video clock (not shown).

The line memory 26 is a type of delay memory that stores this 8-bit parallel data for one main scanning line and outputs it in order.The output is supplied to the line memory 28 having the same configuration, and also to the third and third lines. 4-7 foot register 27.
．． 27. The signal is supplied to a series circuit consisting of , and similarly shifted by one pixel. The output of the line memory 28 is supplied to a series circuit consisting of fifth and sixth shift registers 273 and 276, and similarly shifted by one pixel.

Digital image signal 25 output from A/D converter 23
The outputs of the line memories 26 and 28, the outputs of the shift registers 271 to 276, and the filter arithmetic circuit 21 are supplied.

The filter calculation circuit 21 multiplies nine types of manually inputted image signals by the filter coefficients shown in FIG. 5, and calculates their sum. Specifically, the pixel of interest P. The output of the third shift register 273 corresponding to the coefficient "1/
5" is used, and the other eight image signals have a coefficient of "-1".
/10" is used. The calculation output 29 is manually input to the character/halftone determination circuit 31.

FIG. 7 specifically shows the character/halftone determination circuit. The character/halftone determination circuit 31 also includes two line memories 33 and 34 and six shift registers 351 to 356, and samples densities corresponding to 3×3 pixel groups in the same manner as described with reference to FIG. These density data are supplied to a maximum value/minimum value detection circuit 38.

The maximum value/minimum value detection circuit 38 is composed of, for example, a plurality of digital comparators, and compares the magnitudes of the densities d, ~d, in the pixels of the 3×3 Mar-+J Fuchs composition as shown in FIG. , the maximum value M A X and the minimum value M I N.

The subtracter 41 calculates the difference between the maximum value MAX and the minimum value MIN, and outputs this as a density difference signal 42 in synchronization with a video clock (not shown). The concentration difference signal 42 is input to a comparator 43 and compared with a predetermined threshold level 44 .

This threshold level 44 is defined as concentration −/J<64
When expressed in stages, it is set to a value between 20 and 30, for example.

The purpose of this comparison is to distinguish between the character image portion and the halftone image portion based on the density gradient. In other words, in a character image, there are generally two 7i13 degrees: the background color part of the original and the character part, so the density gradient is steep, and the density change is smooth (multi-step) in the intermediate sub-image part and They can be distinguished by comparing their concentration scents.

The comparison result 45 is supplied to a multiplexer 46 shown in FIG. A multi-value dither circuit 47 and a simple multi-value circuit 48 are connected to the multiplexer 46 . Multilevel dither circuit 4
7 and the simple multivalued circuit 48 are configured to sequentially input the pixel signal 51 of the pixel of interest delayed by the delay circuit 49, and process these signals in parallel in time.

Here, the multivalued dither circuit 47 is a circuit that performs image processing suitable for reproducing halftone images, and it variably sets a plurality of threshold levels for each dot making up the dither matrix, and uses this matrix as a unit. An image signal 53 that enables expression of many gradations is created as follows. On the other hand, the simple multi-value circuit 48 performs multi-value conversion for each pixel using a fixed threshold level, and creates an image signal 54 that can express several levels of gradation.

The multiplexer 46 selects the optimal one from these two types of image signals 53 and 54 according to the comparison result 45 of the determination circuit 31. That is, in the case of a halftone image in which the density difference signal 42 shown in FIG. 7 is smaller than the threshold level 44, the image signal 53 is selected, and in other cases, the image signal 54 is selected. The image signal 53 or 54 selected by the multiplexer 46 is the image signal 5
5 and is supplied to a recording section or a display section (not shown).

Note that in the embodiment described above, the image state was sequentially determined for each pixel and smoothing was performed as preprocessing for each pixel. It is also effective to extend and apply the discrimination result of the pixel group of , since it reduces noise by one degree and simplifies signal processing.

In addition, in the embodiment, a simple multi-value circuit is used for character images, and a multi-value dither circuit is used for halftone images to process the picture beak, but the process may be performed using circuits other than these. Of course. The circuit for distinguishing between a character image and a halftone image is also not limited to the circuit shown in the embodiment.

[Effects of the Invention] As described above, according to the present invention, since the smoothed image signal is used to determine the image state, the sharpness of the character image portion is not lost.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the basic configuration of the present invention, and FIGS. 2 to 7 are for explaining one embodiment of the present invention.
Of these, Figure 2 is an explanatory diagram showing the relationship between the original and the pixel group used for image processing, Figure 3 is a configuration diagram showing an example of a spatial frequency filter used for sharpening, and Figure 4 is a diagram for smoothing. 5 is a block diagram showing an example of a spatial frequency filter used for this device, FIG. 6 is a block diagram showing an outline of the image signal processing device, and FIG. 7 is a block diagram showing an outline of the image signal processing device. FIG. 2 is a block diagram of a character/halftone determination circuit. ■... Image signal smoothing means, 2... Character/halftone discrimination means, 3...
Signal processing means, 21″°°...° filter calculation circuit, 31... Character/halftone determination circuit, 47...
...Multi-value dither circuit, 48...Simple multi-single series circuit. Applicant: Fuji Xerox Co., Ltd. Representative
Person Patent Attorney Umeo Yamauchi Figure 1 Figure 2, -2111 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7

Claims (1)

[Claims] 1. Image signal smoothing means for smoothing a quantized image signal according to the shading of the image, and a character or line drawing based on the smoothed image signal. A character/halftone discrimination means for discriminating between a character image part and a halftone image part other than the character image part; 1. An image signal processing device comprising: signal processing means each performing different signal processing. 2. The image signal processing device according to claim 1, wherein the image signal is smoothed using a spatial frequency filter for removing halftone dots in the halftone image portion. 3. In the character image part, a simple multi-value circuit is used to express gradation for each pixel, and in the half-tone image part, a multi-value dither circuit is used to express gradation in units of multiple pixel groups. The image signal processing device according to claim 1, further comprising processing means.