JP2003198839A - Character edge detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a character edge detector at a low cost that can detect a feature quantity with high accuracy by which the image quality can be enhanced even for a character on a dot or a color background. <P>SOLUTION: The character edge detector includes: an edge detection circuit 1 that detects a pixel area equivalent to a character edge being a first feature quantity from a received image signal with a characteristic nearly equal to a luminance signal characteristic or detects a character edge quantity representing a probability of a character edge area; a gamma conversion circuit that detects a second feature quantity different from the first feature quantity from the image signal; and a correction means (adder 5) for correcting the first feature quantity on the basis of the second feature quantity. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a character edge detecting device for reading a document with a scanner or the like, and detecting a character edge region in the document with high accuracy by a so-called image area separation technique. More precisely, the present invention relates to a character edge detecting device for detecting not only the character edge on the white background but also the halftone dot or the character edge area on the color background. Also, the control system is 0
The present invention relates to a character edge detection device capable of multi-valued output such as a detection result as a character portion edge amount when multi-step control is possible instead of / 1 switching. This character edge detecting device is a technique that can be used in image reproducing devices such as copying machines and facsimiles regardless of whether it is a monochrome machine or a color machine. Alternatively, it is also a technique that can be used in an image processing apparatus such as a PC or a workstation of a network compatible type (a system in which input / output devices are connected to a network) such as that which has been performed recently.

[0002]

2. Description of the Related Art As a conventional technique for detecting the amount of character edges in image data, for example, the following (1) Japanese Patent Laid-Open No. 10-29200 is available.
-42152, (2) JP-A-7-212583, and (3) JP-A-7-212611.

These are devices that control the spatial filter based on the detected edge amount. As a signal for calculating the edge amount, a "luminance signal" whose response is easily matched visually is positively used. The luminance signal referred to here is colorimetrically "CIE1976 (L * a * b *)".
L * signal of “color space” and “CIE1976 (L * u * v
*) An L * signal in the color space, and further includes a signal that is determined in a simple manner and has substantially such characteristics.

In addition to the luminance signal, the edge amount may be detected using the "g signal" as disclosed in Japanese Patent Laid-Open No. 11-8775 for the purpose of simplifying the circuit.
Further, as a device for detecting an edge amount by a monochrome signal instead of a color signal, there is a device as described in Japanese Patent Publication No. 6-5885, but, although not explicitly stated, in the device described above, As the input image signal, a signal having characteristics close to the luminance signal is often input.

When the edge amount is calculated using this luminance signal and the spatial frequency correction is performed based on the calculated edge amount, not only black characters on a white background but also a pattern such as a manuscript represented by photographic paper is used. It is possible to retain the resolution of the original image with a natural feeling even in the inner edge portion.

However, on the other hand, in a manuscript containing a lot of characters on the color halftone dot or characters on the solid color ground as represented by a "map", the background and the characters may be different from each other depending on the relationship between the background and the characters. The brightness values may be very close to each other, and as a result, the resolution of those areas is not compensated, and the characters in the reproduced image are difficult to read.

[0007]

When an image is reproduced by the device as enumerated in the prior art, for example, black characters whose background is a white background and edge portions in a pattern are preferably reproduced. On the other hand, in the case of a character whose background is a halftone dot or a color background, the contrast enhancement is insufficient depending on the relationship between the background color and the character color. This is a problem that occurs when the luminance values of the color of the background and the color of the character are close to each other because the characteristic of the image signal for calculating the feature amount is almost linear in the luminance.

Therefore, the present invention makes it possible to inexpensively provide a character edge detecting device which can maintain the conventional dot as it is, and can accurately detect a feature amount capable of improving the image quality even on a halftone dot or a color ground character. The purpose is to provide.

[0009]

In order to achieve the above object, the invention according to claim 1 is characterized in that a character edge, which is a first feature amount, is added to an input image signal having substantially a luminance signal characteristic. An edge detection circuit that detects a corresponding pixel region or a character edge amount that indicates the probability of being an edge region of a character, and a second feature amount that is different from the first feature amount from an image signal γ A character edge detection device including a conversion circuit and a correction unit that corrects the first feature amount based on the second feature amount is the most main feature.

According to a second aspect of the present invention, an image signal having substantially a luminance signal characteristic is calculated from the input color image signal, and a pixel corresponding to a character edge, which is the first feature amount, is calculated for the image signal. An edge detection circuit that detects a region or a character edge amount that indicates the probability of being an edge region of a character, and a γ conversion circuit that detects a second feature amount different from the first feature amount from an image signal. A character edge detection device provided with a correction unit that corrects the first feature amount based on the second feature amount is the most main feature.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of an example of a filtering device using an edge detector, and each processing block except the edge detector is the same as the processing block described in Japanese Patent Publication No. 6-5885. That is, the edge enhancer uses, for example, a mask of 5 × 5 pixels (coefficients are not specified)
The smoothing unit is a filtering process block using a mask of 5 × 5 pixels (coefficients are not explicitly shown).

Then, based on the multilevel signal from the edge detector, the signal from the edge enhancer and the output from the smoothing device are proportionally distributed to obtain output data. At this time, it goes without saying that the greater the output from the edge detector, the greater the proportion from the edge enhancer. Input image data is 6
It is assumed that the brightness is almost linear (hereinafter, a brightness signal) of 00 dpi and 8 bits, and the paper white side is “0” as a signal value for explanation.
And

FIG. 2 is a block diagram of the character edge detecting device (edge detector of FIG. 1) according to the first embodiment of the present invention. The edge detection circuit 1 is a part that performs a filtering process based on a first-order differential coefficient that is conventionally performed as shown in FIG. Here, a filter for detecting vertical and horizontal edges as shown in FIG. 3 is prepared, and one having a larger absolute output value is selected. Of course, those having a coefficient for detecting an oblique edge are added to them. May be added.

The halftone dot character detection circuit 2 comprises a block for detecting a halftone dot on the character background and a block for detecting a portion corresponding to the back of the character. The latter may be, for example, a filtering process based on a second derivative as shown in FIG.
Here, the method described in Japanese Patent Laid-Open No. 10-285394 is used because the target areas to be detected are the same.

FIG. 5 shows a block diagram of a halftone dot character detection circuit. Each component of FIG. 5 will be described. The halftone dot area detection circuit 11 uses the technique previously proposed by the applicant (see Japanese Patent Application Laid-Open No. 4-25282). The main point of the determination method described in the above publication is to detect peak peak pixels and valley peak pixels that are often present in the halftone dot portion, and if these pixels are present within a predetermined number, the block of interest is defined as a halftone dot area. It is a judgment.

Further, the ridge pixel detection circuit 12 detects pixels belonging to a linear portion by pattern matching, but uses a technique capable of detecting a character portion buried in a halftone dot (Japanese Patent Application Laid-Open No. 3-82269). ). That is, the ridge pixel pattern uses a local area of a predetermined size (for example, a pixel size of 5 × 5), and the level of the central pixel in the area is Lc,
Further, when the levels of the pixel pairs at arbitrary symmetrical positions with the central pixel Lc interposed therebetween are La and Lb, Lc-La ≧ TH and Lc-Lb ≧ TH (TH
If even one of the thresholds is satisfied, the central pixel Lc is extracted as a ridge pixel.

The portions having the above-mentioned two characteristics are the character pixels on the halftone dot. Therefore, after ANDing them by the AND circuit 13, the multivalued circuit 3 selects the halftone dot in the 5 × 5 pixel mask, for example. The character pixels on the dots are counted, and the count value is output as the central pixel value.

The γ conversion circuit 4 is a circuit for performing primary conversion such as table conversion. The shape of this table may be determined experimentally in consideration of device characteristics and preferences. In addition, a copying machine generally has a mode (character mode, map mode, pencil character mode, pattern mode, etc.) based on several document types, and a plurality of conversion tables are appropriately prepared based on this mode. However, switching depending on the mode is also effective in improving the image quality.

In the adder (correction means) 5, the output value from the edge detection circuit 1 is added to the output value from the γ conversion circuit 4, that is, the former characteristic amount is corrected to obtain output data. Although the adder 5 is used here, a correction method of multiplying the output value from the γ conversion circuit 4 is also conceivable when it is built. That is, the γ conversion circuit 4
Although there are several possible configurations for correcting the output value of the edge detection circuit 1 based on the output value from, the cost performance should be taken into consideration when performing the configuration.

In the above description, the edge amount to be calculated is described as a multi-valued signal. However, when the processing system has two switching such as 0/1, the output data is appropriately binarized. In some cases, the detection result of the edge detection circuit or the γ conversion result of the γ conversion circuit 4 is binarized to adder 5
The configuration may be such that the logical sum is taken.

FIG. 6 is a block diagram of a character edge detecting device (edge detector of FIG. 1) according to a second embodiment of the present invention. The difference from the first embodiment shown in FIG.
A flat ground character detection circuit 6 is provided in place of the halftone dot character detection circuit 2.

FIG. 7 shows a block diagram of a flat ground character detection circuit. Each component of FIG. 7 will be described. The ridge pixel detection circuit 12 in FIG. 7 is similar to the above-described device in FIG. The flat ground detection circuit 14 makes a determination based on a plurality of intra-block average values separated by a predetermined distance from the target pixel or block as shown in FIG.

Here, in consideration of cost performance, the average value (Aave, Aave,
Bave) is calculated. Aave, Bav
When the value of e satisfies the following conditions, the pixel of interest is made a flat ground pixel and adaptive. min (Aave, Bave) ≥ Th_1 & | Aave-Bave | ≤ Th_2

FIG. 9 is a block diagram of a character edge detecting device (edge detector of FIG. 1) according to a third embodiment of the present invention. Here, as the input image data, RGB density signals or CMY density signals obtained by color-correcting the RGB density signals are used. Then, for example, the characteristic that the primary color such as "cyan character" or "magenta character" is often used as the color of the halftone dot character in the map manuscript is used after the γ conversion circuit 4 for those characters. With the aim of calculating the edge amount such that the response of the signal of (1) is made large, and the luminance signal responds to that of the general pattern edge (ie, the signal after the γ conversion circuit 4 is made as small as possible). There is.

The points added to the first embodiment shown in FIG. 2 are a specific signal selection circuit 7 and a luminance signal calculation circuit 8. These two parts will be described below. For the sake of explanation, the input signal is a CMY signal (white side is “0”)
And

In the specific signal selection circuit 7, here, C
Although a (cyan) signal is used, for example, an M (magenta) signal may be used, or a larger one of these two signals may be selected for each pixel of interest.

The luminance signal calculation circuit 8 calculates a signal having a response corresponding to the luminance signal based on the following linear linear equation. When more accuracy is required, a predetermined γ conversion is performed on each CMY signal before performing the following calculation,
For example, the following linear linear equation may be changed to a non-linear quadratic equation.

L = α × C + γ × M + β × Y;

FIG. 10 is a block diagram of a character edge detection device (edge detector of FIG. 1) according to a fourth embodiment of the present invention. This is also a device having a mechanism in which the output value of only "cyan characters" and "magenta characters" existing on a flat ground is increased by using a color image signal. The difference from the third embodiment shown in FIG. 9 is that the halftone dot character detection circuit 2 is replaced with a flat ground character detection circuit 6.

[0030]

As described above, according to the present invention,
In the edge detection based on the luminance signal, the edge amount for the second halftone dot character edge area, whose performance is insufficient, is calculated and corrected. In addition to the area, since the conventional detection of the edge amount is insufficient, it is possible to improve the image quality of the character edge on the halftone dot, which has a poor resolution.

[Brief description of drawings]

FIG. 1 is a block diagram of an example of a filtering device that uses an edge detector.

FIG. 2 is a block diagram of a character edge detection device according to the first embodiment of the present invention.

FIG. 3 is a diagram showing a filter.

FIG. 4 is a diagram showing a filter.

FIG. 5 is a block diagram of a halftone dot character detection circuit.

FIG. 6 is a block diagram of a character edge detection device according to a second embodiment of the present invention.

FIG. 7 is a block diagram of a flat ground character detection circuit.

FIG. 8 is a diagram showing an example of a detection filter used in a flat ground detection circuit.

FIG. 9 is a block diagram of a character edge detection device according to a third embodiment of the present invention.

FIG. 10 is a block diagram of a character edge detection device according to a fourth embodiment of the present invention.

[Explanation of symbols]

1 Edge detection circuit 2 Halftone dot character detection circuit 3 Multi-valued circuit 4 γ conversion circuit 5 adder

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 5C077 MP02 MP05 MP07 MP08 PP15                       PP27 PP33 PP36 PP46 PP47                 5L096 BA17 FA06 FA44

Claims (2)

[Claims] 1. A character edge which detects a pixel area corresponding to a character edge which is a first feature amount or which represents a probability of being an edge area of a character in an input image signal having substantially a luminance signal characteristic. An edge detection circuit that detects the amount,
A γ conversion circuit that detects a second feature amount different from the first feature amount from the image signal, and a correction unit that corrects the first feature amount based on the second feature amount are provided. Characterized character edge detection device. 2. An image signal having approximately a luminance signal characteristic is calculated from an input color image signal, and a pixel area corresponding to a character edge which is a first feature amount is detected in the image signal, or An edge detection circuit that detects a character edge amount that represents the probability of being an edge region of a character, a γ conversion circuit that detects a second feature amount different from the first feature amount from an image signal, and the second feature. A character edge detection device comprising: a correction unit that corrects the first characteristic amount based on the amount.