JPH09275489A - Image processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the degradation of picture quality caused by erroneous separation as conventionally generated at mesh points in a little lines by detecting the cycle of an original and changing the discrimination reference of image area separation corresponding to the detected cycle when separating the image area of a character edge area from a digital image signal. SOLUTION: The original is read out as the digital image signal, and the cycle of the original is detected from the digital image signal by a cyclicity detection circuit 12. Then, when separating the image area of the character edge area from the digital image signal, an edge separator circuit 10 separates the edge while switching a parameter corresponding to the result. A discrimination circuit 11 discriminates a black character when the edge is not colored, discriminates a color character when the edge is colored and discriminates a picture pattern in the other case. Thus, a filter circuit 3, UCR circuit 5 and dither circuit 6 are switched. Namely, the cycle contained in the original is specified by prescan and based on the result, the discrimination reference of edge separation to be performed at the time of main scan is changed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing method in which a criterion for image area separation performed in main scanning is changed based on original information detected in prescan.

[0002]

2. Description of the Related Art At present, in an image processing apparatus such as a color copying machine, when processing and reproducing an original in which a character area and a picture area are mixed, the area of the image is recognized by a so-called image area separation technique. However, the processing is switched between the character portion and the pattern portion in one document. As a conventional color copying machine using such an image area separating technique, for example, there is one described in Japanese Patent Publication No. 7-22330.

The above-mentioned conventional apparatus performs image area separation during so-called pre-scanning without image forming processing to perform area determination (character area / halftone area), and processing is performed in main scanning in accordance with the determined result. The parameter is switched. Further, in the area identification signal processing circuit described in Japanese Patent Laid-Open No. 5-145751, the image area separation result at the time of prescan is stored in the storage device, and the processing is performed by using the result stored in the storage device at the main scan. Switch.

[0004]

As described above, in the above-mentioned conventional apparatus, the image area separation processing is premised on the so-called local processing, and the processing always involves an erroneous determination. Specifically, in many cases, characters are determined based on edge information. Therefore, when trying to determine character edges with high accuracy, patterns such as photographic paper edges and halftone dot images are also detected. The number of locations that are erroneously determined as character edges increases. In particular, the tendency is strong for a low number of lines (50 lines to 85 lines) in the halftone dot image. Here, the low-frequency halftone image is
Specific manuscripts include halftone images by general printing and dither output manuscripts by an inkjet or a fusion printer.

The present applicant has previously proposed an image processing method in which halftone dot separation, which is one of image area separations during prescan, is activated, and image area separation during main scan is controlled based on the information ( (See Japanese Patent Laid-Open No. 4-180350). In this method, it is determined whether or not a halftone image is included in the prescan, and when it is determined that the halftone image is not included, the halftone dot determination in the main scan is ignored (turned off). As a result, it is possible to prevent deterioration of image quality due to a local erroneous halftone dot determination for a document that does not include a halftone dot region. However, when it is determined that the halftone dot area is included in the pre-scan, the halftone dot separation is activated even in the main scan, and the above-mentioned problem, that is, the erroneous separation in the low frequency halftone dot is likely to occur. The problem still exists.

The present invention has been made in consideration of the above circumstances, and an object of the present invention is to detect the number of halftone dots included in a document during prescanning and locally determine the area during main scanning. At this time, an object of the present invention is to provide an image processing method for determining an area by changing parameters according to the detection result.

Another object of the present invention is to determine whether or not a manuscript contains a character, and when it is judged that the manuscript does not contain a character, the detected period information is ignored and a parameter for halftone dot separation is used. An object of the present invention is to provide an image processing method for determining a region by using the method.

[0008]

In order to achieve the above object, according to the invention of claim 1, an original is read as a digital image signal, the cycle of the original is detected from the digital image signal, and the digital image signal is detected. Is characterized in that when a predetermined area is separated into image areas, the determination criterion for image area separation is changed according to the detected cycle.

According to a second aspect of the invention, the predetermined area is a character edge area.

According to a third aspect of the invention, the predetermined area is a halftone dot area.

According to a fourth aspect of the present invention, the original is read as a digital image signal, the period of the original is detected from the digital image signal, and it is determined whether or not the original contains characters. When the predetermined area is separated from the image signal in the image area, the determination result is prioritized over the detection result and the determination criterion of the image area separation is changed.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be specifically described below with reference to the drawings. <Embodiment 1> FIG. 1 shows the structure of Embodiment 1 of the present invention.
In the figure, 1 is an image input device such as a scanner, and 2 is a Log that converts a linear reflectance signal into a linear density signal.
A conversion circuit, a filter circuit including a smoothing filter and an edge emphasis filter, a color correction circuit for converting a cmy signal into YMC of a complementary color, a UCR circuit for performing processing for subtracting a K signal from each color material signal, Reference numeral 6 denotes a dither circuit for expressing halftones using dither for characters and pictures, and reference numeral 7 denotes an image output device such as a printer.

Reference numeral 8 is a color separation circuit for separating color regions from the original, 9 is a parameter storage unit for storing parameters according to the detected period, and 10 is an edge separation circuit for separating character edges using the parameters. Reference numeral 11 is a determination circuit that determines the area based on the separation result of the separation circuit, and 12 is a periodicity detection circuit that operates during prescan.

First, the outline of FIG. 1 will be described. The image input device 1 (resolution is 400 dpi) such as a scanner outputs rgb data (8 bits each) which is almost linear with respect to the reflectance. Next, Lo is pre-processed for performing color correction at a later stage.
The logarithmic conversion is performed in the g conversion circuit 2, and the density linear c
Generate my data.

The filter circuit 3 is composed of a circuit in which a smoothing filter and an edge emphasizing filter as shown in FIG. 2 are connected in series, and when a signal from a judging circuit 11 described later is a black character or a color character. In the above, the smoothing filter is made to be through and only the edge enhancement processing is performed.

The color correction circuit 4 converts the cmy signal into a complementary color YMC signal in consideration of the characteristics of the image input device 1 and the image output device 7. As a color correction method, linear approximation, so-called masking method, tetrahedral interpolation method, triangular prism interpolation method, and the like have been proposed, and color correction is performed using these methods. At this time, the K signal is simultaneously calculated from min (C, M, Y).

The UCR circuit 5 switches the processing as follows based on the area determination signal from the determination circuit 11. That is, for the black character area, K is made to be through and C = M
= Y = 0. For the color character area, K is through, C is through, M is through, and Y is through, and other than the above, that is, K '= 0.6 × K C' (M ', Y') = C (M, Y) ) -K '.

Then, the dither circuit 6 prepares two kinds of dither tables (dither matrices) having different sizes (not shown), and when the signal from the judging circuit 11 is a black character or a color character, the size is changed. Of the dither table (eg, 1 × 1), and in other cases, the dither table of larger size (eg, 2 × 2) is used to send the data to the image output device 7.

FIG. 3 shows the configuration of the periodicity detection circuit. Here, the DCT (Discrete Cosine Transform) is used to detect the periodicity, and the average DCT coefficient calculation unit 21
And a period calculation unit 22. The details of DCT are described in, for example, Japanese Patent Application Laid-Open No. H4-221265.

The DCT is performed in block units of 8 pixels × 8 pixels. For example, if the number of pixels when reading a general A4 size at 400 dpi (4752 × 3360 pixels), processing of 594 × 420 blocks is performed. It will be. FIG. 4 is a processing flowchart of the average DCT coefficient calculation unit. FIG. 5 is a diagram showing how the DCT coefficients are averaged.

Image data for 8 lines is set (step 101), and the average value Ave of the pixel values in the block is calculated (step 102). That is, since the periodicity of the document appears mainly in the middle part (that is, all the pixels in the block are not black pixels or white pixels), the average of the pixel values in the block is within a certain range (th1, th2). Only if the block is a valid block (step 103), 2D D
The CT coefficient is calculated (step 104). The addition is performed for each coefficient (step 105). For example, as shown in FIG. 5, DCT coefficients A00 and B at the same position in each block
00 and C00 are added.

When the above processing is performed in the main scanning direction (step 107) and the processing in the main scanning direction is completed (step 106), the same processing is performed in the sub scanning direction (step 109) and the processing in the sub scanning direction is completed. Then (step 108), the average DCT is calculated (step 11).
0). In the example of FIG. 5, the average DC for three blocks
T, F00 = (A00 + B00 + C00) /
3,. . . , F77.

Since the peak appears in the DCT coefficient when the original has a specific cycle, the cycle calculating section 22 detects the peak position using a 3 × 3 mask. Specifically, a mask as shown in FIG. 6 is applied to the entire 8 × 8 average DCT coefficient, and 8 × 8
M is calculated at each of the positions 8. Then, the period of the target document is determined from the peak position having the maximum M value. here,
In FIG. 7, when the peak position is in the shaded area, it is determined that the original has a low number of lines. If no peak is detected or the maximum M is smaller than a predetermined value, it is determined that there is no periodicity.

Next, the image area separating section will be described in detail below. The image area separation unit according to the first embodiment includes an edge separation circuit 10 and
The color separation circuit 8 is provided, and the halftone dot separation circuit 13 is provided in the second embodiment described later. Among them, the edge separation circuit 10 and the halftone dot separation circuit 13 are described in, for example, the paper “Image Area Separation Method for Character / Picture (Halftone, Photo) Mixed Image” (Journal of the Institute of Electronics, Information and Communication Engineers Vol. J75-DI1 No. 1). pp39-47
The method of "4.2 Edge region detection" described in January 1992) is used.

The edge separation circuit 10 is a circuit for detecting an edge area of a character in a document. In this embodiment, for example, the method described in the above-mentioned paper is used as the character edge detection method. According to this method, edge enhancement is applied to input image data of 64 gradations, then ternarization is performed using two types of fixed thresholds, and the continuity of black pixels and white pixels after ternarization is detected by pattern matching. If at least one black continuous pixel and one white continuous pixel are present in the block of 5 pixels, the block of interest is determined to be an edge region, and if not, it is determined to be a non-edge region.

Here, the ternarization parameter in the edge area detection is switched between when the original is determined to have a low number of lines and when it is determined to be other than that (that is, no periodicity and high number of lines). To That is, at low-frequency halftone dots, since there are many places where the pattern is erroneously separated as a character, a parameter that is more difficult to determine as a character than a pattern is used.

When it is determined in the prescan that the original has a low number of lines, a parameter that makes it more difficult to detect edges is set. As a result, the inside of a character is less likely to be detected as an edge, but the area that is erroneously detected as a character in a low-frequency halftone dot image is reduced. The reason for doing this is that considering the deterioration of the entire image, it is better that a part of the character is subjected to the pattern processing (this processing does not make the character unreadable) rather than the deterioration of the pattern portion remarkably. , Is based on the idea.

As an example of the parameters to be prepared, the following parameters are set, for example. For low lines: White pixels <32/255, black pixels ≧ 192/255 For default (high lines, no periodicity); White pixels <64/255, black pixels ≧ 128/255 Color separation circuit 8 is the original This is a circuit for detecting a chromatic portion from the inside. As this color separation method, for example, JP-A-3-260
The chromatic / achromatic color region separation method described in Japanese Patent No. 878 is used. In this method, the difference between the R, G, and B digital signals is obtained, and when the maximum difference is equal to or larger than a predetermined threshold value, it is determined that the color is chromatic.

The operation of the first embodiment will be described below. (Prescan) In the configuration of the embodiment of FIG. 1, the processing unit (periodicity detection circuit) that operates during prescan is surrounded by a dotted line. The pre-scan here means not only the case where the color scanner actually scans the original, but also the case of the first use when the input image is stored in the memory and is used a plurality of times. Before the prescan is completed and the main scan is started, it is known whether the original is a low frequency halftone dot.

(At the time of main scan) The 1-drum type color copying machine scans a total of 4 times for each CMYK plate,
The four-drum type performs one scan to reproduce a color image. However, as described above, when all images are stored in the memory and used multiple times, the scanner does not operate.

By the above-described prescan, it is known whether the original is a low frequency halftone dot. Edge separation circuit 10
Switches the parameters described above according to the result and separates the edges. Then, the determination circuit 11 determines that the pixel of interest or the block of interest is a black character if the edge is achromatic, a color character if the edge is chromatic, and a pattern otherwise, and as described above, the local character is used. Image reproduction processing (filter circuit, UCR circuit, dither circuit)
Switches.

<Second Embodiment> FIG. 8 shows the structure of a second embodiment of the present invention. Image reproduction system (that is,
Since the steps from the image input device to the image output device) are the same as those in the first embodiment, the description thereof will be omitted.

The image area separation unit of the second embodiment is the halftone dot separation circuit 1.
3, an edge separation circuit 10, and a color separation circuit 8,
By comprehensively judging these separation results, the black character, the color character, and the other, that is, the pattern are judged.

That is, when a halftone dot separation circuit (OFF) & edge separation circuit (ON) & color separation circuit (OFF); black character halftone dot separation circuit (OFF) & edge separation circuit (ON) & color separation circuit (ON) ); Color letters When combinations other than the above are specified; it is determined to be a pattern.

Further, in the second embodiment, for the sake of simplification of explanation, the parameter of the edge separation is fixed regardless of the determination of the number of lines, but the same as in the first embodiment described above. It is also possible to change the setting.

The halftone dot separation circuit 13 is a circuit for detecting a halftone dot (print) area in the original. This detection method also uses the “4.1 halftone dot area detection” method described in the above-mentioned paper. This method focuses on the fact that the density change between the color halftone dot area and the black and white halftone dot area is significantly different from the density change in the character area, and performs peak pixel detection, halftone dot area detection, and halftone dot area correction. This is to separate the dot areas.

The peak pixel is detected by, for example, in the block of 3 × 3 pixels, the density level L of the central pixel is higher or lower than that of all the surrounding pixels, and
The density levels a and b of the diagonal pixels existing diagonally across L and the central pixel are | 2 × L−a−b |> TH for all four pairs.
When it is (fixed threshold), the central pixel is set as the peak pixel. The detection of the halftone dot area is performed, for example, in four blocks each including 4 × 4 pixels, if there are two or more blocks including the peak pixel, the target block is set as the halftone dot candidate area, and the other blocks are non-halftone dots. Determined as a candidate area. After determining the halftone dot / non-halftone dot candidate region, if four or more of the nine blocks centering on the target block are halftone dot candidate regions, the target block is set as the halftone dot region, and otherwise the target block is selected. It is a non-dot area.

Here, the parameter TH for peak pixel detection in halftone dot area detection is determined when the original is determined to have a low number of lines and when it is determined to be other than that (that is, no periodicity and a high number of lines). Try to switch. That is, in the low-frequency halftone dot, there are many places where the pattern is erroneously separated as a character, so a parameter that can detect the halftone dot well is used.

When the original is determined to have a low number of lines by the prescan, a parameter is set so that the halftone dot is detected more as a halftone dot. This makes it easier for the character to be determined as a design. The reason for doing this is that considering the deterioration of the entire image, it is better that a part of the character is subjected to the pattern processing (this processing does not make the character unreadable) rather than the deterioration of the pattern portion remarkably. , Is based on the idea.

The relationship between the prepared low line count parameters and the default (high line count, no periodicity) parameters is as follows. TH (for default)> TH (for low number of lines) Further, as a parameter to be switched in a cycle, (2) the number of blocks in the halftone dot area detection described on page 42 of the above-mentioned paper may be used. For example, the following switching is performed. For low line count; 1 or more blocks containing peak pixels Default (high line count, no periodicity); 2 or more blocks containing peak pixels

<Third Embodiment> FIG. 9 shows the structure of a third embodiment. A character presence / absence determining circuit 14 is further added to the second embodiment. The character presence / absence determining circuit 14 determines whether or not the document of interest includes characters, and this circuit operates during pre-scanning. That is, the circuit performs image area separation (edge separation, halftone dot separation) on the data whose resolution has been converted by thinning processing (for example, thinning for each pixel in both main scanning and sub-scanning), and characters are printed on the original. It is determined whether it is included.

In the third embodiment, at the end of the prescan, the character presence / absence determination circuit 14 obtains information as to whether or not the original includes a character area, and the periodicity detection circuit 12 determines the low number of cycles in the original. Information about whether or not the halftone dot area that the user has is obtained.

In this embodiment, if it is determined that the original does not include characters, the image area separation is turned off during the main scan, or the parameter for halftone dot separation is used for the pattern in most areas. It is set so that it is judged. As a result, the image area is separated from the original that does not include characters, and the image quality of the design is not deteriorated.

[0044]

As described above, according to the first and second aspects of the present invention, the period included in the original document is specified by the prescan, and the edge separation determination criterion performed at the main scan based on the result. Therefore, it is possible to eliminate the deterioration of the image quality due to the erroneous separation, which has conventionally occurred in the low frequency halftone dot.

According to the first and third aspects of the present invention, the period included in the original is specified by the prescan, and the criterion for the halftone dot separation performed during the main scan is changed based on the result. It is possible to eliminate the deterioration of the image quality due to the erroneous separation that has conventionally occurred in the halftone dots.

According to the fourth aspect of the present invention, the period included in the manuscript and whether the manuscript contains characters in the prescan are specified. If the manuscript does not contain characters, the result is prioritized over the period information. The image area separation determination standard used in the main scanning is changed, so that the image area is not deteriorated with respect to a document containing no characters.

[Brief description of drawings]

FIG. 1 shows a configuration of a first exemplary embodiment of the present invention.

FIG. 2 shows a specific example of a filter circuit.

FIG. 3 shows a configuration of a periodicity detection circuit.

FIG. 4 is a processing flowchart of an average DCT coefficient calculation unit.

FIG. 5 is a diagram showing how DCT coefficients are averaged.

FIG. 6 is a diagram illustrating calculation of a periodic parameter.

FIG. 7 is a diagram illustrating a low frequency area.

FIG. 8 shows a configuration of a second embodiment of the present invention.

FIG. 9 shows a configuration of a third embodiment of the present invention.

[Explanation of symbols]

 1 Image Input Device 2 Log Converter Circuit 3 Filter Circuit 4 Color Correction Circuit 5 UCR Circuit 6 Dither Circuit 7 Image Output Device 8 Color Separation Circuit 9 Parameter Storage 10 Edge Separation Circuit 11 Judgment Circuit 12 Periodicity Detection Circuit

Claims (4)

[Claims] 1. A document is read out as a digital image signal, the period of the document is detected from the digital image signal,
When separating a predetermined area from the digital image signal,
An image processing method, characterized in that a determination criterion for image area separation is changed according to the detected cycle. 2. The image processing method according to claim 1, wherein the predetermined area is a character edge area. 3. The image processing method according to claim 1, wherein the predetermined area is a halftone dot area. 4. A document is read as a digital image signal, the period of the document is detected from the digital image signal, it is determined whether or not the document contains characters, and a predetermined area is determined from the digital image signal. An image processing method, wherein when the image area separation is performed, the determination result is prioritized over the detection result and an image area separation determination criterion is changed.