JPH104497A - Picture processor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To more correctly judge the dot cycle of a picture by permitting an area where the dot period cannot be correctly detected to be out of period detecting object. SOLUTION: A picture input device 1 outputs rgb data being almost linear as against a reflection rate. Then, a Log converting circuit 2 executes logarithm conversion and linear density cmy data is generated. A filter circuit 3 is constitued of a circuit where a flatening filter and an edge emphasizing filter are serially connected so as to change-over the coefficient of the flatening filter in accordance with a judgement result from a period property judging circuit 8. Since the period property of an original appears mainly in an intermediate part, an intermediate area detecting circuit 10 adopts a block as the valid one only when the average of in-block picture element values is within a certain range. A period property detecting circuit 9 adds DCT coefficients so as to finally obtain the average. That is, the highlight part and the dark part of the picture are not adopted as the object for period detection.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus having a function of determining a dot period of an image.

[0002]

2. Description of the Related Art Conventionally, various devices have been proposed which switch the processing such as filtering by detecting the period of a document, particularly a halftone dot. For example, the technique described in JP-A-1-133470 is an example of an image processing method for detecting a dot period by pattern matching and switching a filter coefficient. Further, the method described in Japanese Patent Application Laid-Open No. Hei 3-62355 uses pair length information (the number of pairs of white bit length and black bit length that are continuously developed adjacently) as a feature amount of halftone dot period detection. Then, the cycle is detected.

Further, the present applicant has previously proposed a color image processing apparatus (Japanese Patent Application No. 8-64538) for performing a filtering process suitable for an output image by an error diffusion method. Image processing methods (Japanese Patent Application No. 8-84185) for changing the criterion for image area separation have been proposed, but all of these methods use DCT (Discrete Cosine Transform) to detect periodicity.

[0004]

All of the above-mentioned techniques utilize the characteristics of the halftone portion of the halftone dot, but have the following problems. That is, (1) erroneous determination of a highlight part and a dark part; the period of a halftone dot is not correctly detected in a highlight part or a dark part by using any of the above-described feature amounts. This is because, in the case of a highlight portion, a desired gradation image is created due to missing dots in some places. That is, in the example in which the cycle is detected by pattern matching, an image including many highlight portions may be determined to have a lower number of lines, that is, a lower number of lines depending on a prepared pattern.

(2) Misjudgment of a halftone dot portion of a secondary color or more; A signal for making a judgment is generally a luminance signal for a monochrome reproducing device, and a signal for a color reproducing device.
Min (R, G, B) (when the maximum density is 0) is used to positively detect a green signal or a yellow dot. A halftone dot is used to create an image by superimposing dots of four colors. For example, if a portion made of a secondary color of cyan and magenta is subjected to pattern matching using a green signal, it will not be accurately matched. Or a higher number of lines, that is, a higher number of lines. This is because the characteristic of the green signal has a response in both cyan and magenta, and conversely, it is not a filter that can select only magenta.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to reduce the dot period of an image by excluding an area where the dot period cannot be accurately detected from the object of period detection. It is an object of the present invention to provide an image processing device for more accurately determining.

[0007]

In order to achieve the above object, according to the first aspect of the present invention, an original is read out digitally, a halftone period is detected from the original, and processing is performed in accordance with the detection result. An image processing apparatus for performing the following, comprising: means for determining a color of a pixel area of a predetermined size; and means for detecting a period of a halftone dot from the pixel area of the predetermined size when the color is not a specific color. It is characterized by:

[0008] In the invention described in claim 2, the specific color is:
It is characterized by highlight or gray color.

[0009] In the invention according to claim 3, the specific color is:
It is characterized by being a secondary color or more.

[0010]

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 denotes a periodicity judging circuit which operates at the time of pre-scanning. The periodicity detecting circuit 9 and the intermediate area detecting circuit 10
Consists of Reference numeral 11 denotes a coefficient switching circuit that switches a filter coefficient according to the result of the determination by the periodicity determination circuit.

First, the outline of FIG. 1 will be described. An image input apparatus 1 (resolution: 400 dpi) such as a scanner outputs rgb data (8 bits each) that is substantially linear with respect to 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 a smoothing filter is provided in accordance with a judgment result from a periodicity judgment circuit 8 described later. Switch coefficient.

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 subtracts the CMY signal based on the following equation. K ′ = 0.6 × K C ′ (M ′, Y ′) = C (M, Y) −K ′ The dither circuit 6 performs data conversion by using a dither table (dither matrix), and performs image conversion. The data is sent to the output device 7.

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

DCT is performed in blocks of 8 pixels × 8 pixels. For example, if a general A4 size is read at 400 dpi and the number of pixels is 4752 × 3360 pixels,
Processing of 594 × 420 blocks is performed. Since the periodicity of the document appears mainly in the intermediate portion, the intermediate region detection circuit 10 determines that the effective block is an effective block only when the average of the pixel values in the block is within a certain range (th1, th2). , The periodicity detection circuit 9
The CT coefficients are added, and the average is finally obtained. That is, for the above-described reason, the highlight portion and the dark portion of the image are not subjected to the period detection.

FIG. 4 is a processing flowchart of the average DCT coefficient calculator. FIG. 5 is a diagram showing how the DCT coefficients are averaged. The image data for eight lines is set (step 101), and the average value Ave of the pixel values in the block is calculated (step 102). That is, 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).
A range where the average of the pixel values in the block is (th1, th
Only in the case of (2), is an effective block (step 10).
3) Calculate a two-dimensional DCT coefficient (step 10)
4). The addition is performed for each coefficient (step 105). For example, as shown in FIG.
CT coefficients A00, B00, and C00 are added.

The above processing is performed in the main scanning direction (step 107). When 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.

In the case where the document has a specific period, a peak appears in the DCT coefficient, and the period calculating unit 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,
According to the position where the peak position exists, the determination is made in four stages of approximately 50 lines, approximately 100 lines, approximately 150 lines, and approximately 200 lines. FIG. 7 is a diagram illustrating such a determination of the 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.

Although the periodicity detection using the DCT has been described above, the present invention is not limited to this. For example, as described in the above-mentioned Japanese Patent Application Laid-Open No. 1-133470, pattern matching is used. It is also possible to provide a means for detecting a portion having a local cycle, and determine the most correlated one of the plurality of detected cycles as the cycle of the document. However, in this case,
For example, when matching is performed on a block-by-block basis, the average of a target block (for example, an 8 × 8 mask) is calculated, and only when the average value is within a certain range [thp1, thp2], the target block is determined as an effective area for period detection. There is a need to.

The operation of the first embodiment will be described below. (At the time of prescan) In the configuration of the first embodiment in FIG. 1, the processing unit (periodicity determination circuit) that operates at the time of 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 pre-scan is completed and before the main scan is started, the original document has no periodicity / 50 halftone dots / 100 halftone dots / 150 halftone dots / 2
It is already known which of the 00-dots.

(During main scan) The one-drum type color copying machine performs a total of four scans 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.

Now, the periodicity of the document has been determined by the pre-scan described above. The result of the determination is input to the coefficient switching circuit 11, and the coefficient switching circuit 11 sets a coefficient corresponding to the cycle in the filter circuit 3. Switching of the coefficient
For example, this is performed as described in FIG. 7 of the above-mentioned JP-A-1-133470 (basically, the higher the number of lines, the greater the degree of smoothness is to use a filter coefficient to remove the moire of the halftone dot image. ).

The result of the periodicity judgment of the present embodiment can be applied to the parameter control of the image area separation of the earlier application (Japanese Patent Application No. 8-84185). Also, when it is determined that the number of lines is low and achromatic by using the chromatic / achromatic determination of an image in combination with other information, for example, the processing is assumed to be a black-and-white newspaper original, and the background is actively removed. It is also possible to do.

<Embodiment 2> FIG. 8 shows the structure of Embodiment 2 of the present invention. The image reproducing system (that is, from the image input device to the image output device) is the same as that in the first embodiment, and the description is omitted. The periodicity determination circuit according to the second embodiment includes a primary color area detection circuit 12 and a periodicity detection circuit 9.

The periodicity determination circuit according to the second embodiment will be described with reference to an example in which a DCT process is performed on a green signal to determine a period. Generally, halftone originals are yellow, magenta,
It is composed of four color inks of cyan and black. However, in an area where two or more colors overlap, there is a possibility that the original period of the original document to be obtained cannot be detected. This is because the bandwidth of the green filter is wide and the response is not limited to magenta,
This is because other colors are imported. Such a problem may occur similarly even if the period is detected by, for example, pattern matching as described in JP-A-1-133470.

Therefore, a primary color area circuit 12 for detecting a primary color area (an area composed of only yellow, magenta, and cyan inks) from a document is provided in the periodicity determination circuit.
Only when the block of interest is a primary color area, it is regarded as an effective block, DCT coefficients are added, and the average is finally obtained.
That is, for the above-mentioned reason, the region of the image having the secondary color or more is not subjected to the period detection. The conditions are, for example, when the following colors are established with respect to the average value of RGB (when the maximum density is 0) in the block.

Yellow block; Rave> TH YR &Gave> TH YG &
Bave <TH YB magenta block; Rave> TH YR & Gave <TH YG &
Bave> TH YB cyan block; Rave <TH YR &Gave> TH YG &
Bave> TH YB FIG. 9 is a processing flowchart of the average DCT coefficient calculation unit according to the second embodiment. In FIG. 9, step 202
Then, the average value Rave, Ga of R, G, B in the block
ve and Bave are obtained. In step 203, it is determined whether or not the block is a primary color block (that is, a block in which the above condition is not satisfied). Subsequent processing is the same as in the first embodiment, and a description thereof will be omitted.

[0030]

As described above, according to the first aspect of the present invention, a color region whose period information is ambiguous is detected only by adding a simple circuit, and the region is excluded from the period detection region. Therefore, it is possible to accurately detect the period of the dot document.

According to the second aspect of the present invention, the highlight area or the dark area where the cycle information is ambiguous is detected by a simple circuit, and the area is excluded from the cycle detection area. Can be accurately detected.

According to the third aspect of the present invention, a simple circuit detects an area of a secondary color or more in which halftone dots whose cycle information is ambiguous overlap and excludes this area from the cycle detection area. , It is possible to accurately detect the period of the dot document.

[Brief description of the 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 according to the first embodiment.

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 the determination of the number of lines.

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

FIG. 9 is a processing flowchart of an average DCT coefficient calculation unit according to the second embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Image input device 2 Log conversion circuit 3 Filter circuit 4 Color correction circuit 5 UCR circuit 6 Dither circuit 7 Image output device 8 Periodicity judgment circuit 9 Periodicity detection circuit 10 Intermediate area detection circuit 11 Coefficient switching circuit

Claims (3)

[Claims] 1. An image processing apparatus for digitally reading an original, detecting a halftone period from the original, and performing processing in accordance with the detection result, wherein a means for determining a color of a pixel area of a predetermined size is provided. And a means for detecting a period of a halftone dot from the pixel area of the predetermined size when the color is not a specific color. 2. The image processing apparatus according to claim 1, wherein the specific color is a highlight or a gray color. 3. The image processing apparatus according to claim 1, wherein the specific color is a secondary color or more.