JPH0512440A - Edge detector - Google Patents

Abstract

PURPOSE:To provide the device, which can surely detect only the edge part of an objective image from an image including background regardless lighting conditions, concerning the edge detector to detect the edge from the image. CONSTITUTION:An image memory 1 stores image data and stores image data prepared in advance. A space filter circuit 3 calculates luminance difference in a local according to image data, and a control circuit part 7 synchronously inputs the data at the same picture element position (address) to the input of a luminance ratio calculating means 5 according to the output of the space filter 3 and the data in the image memory 1. The luminance ratio calculating means 5 calculates a luminance ratio between the luminance difference in the local area and luminance of the image in the area and by comparing the luminance ratio with a threshold value, an edge judging means 6 judges whether the area is the edge or not.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an edge detecting device for detecting edges from an image.

[0002]

2. Description of the Related Art Edge detection is one of the most basic processes in image processing such as image feature extraction. The edge here is a boundary portion between the target object and the background, and generally means a portion where the brightness of the image largely changes.

FIG. 6 shows a conventional edge detecting device. Figure 6
In the above, by passing the image data stored in the image memory 1 through the spatial filter 2, only the luminance of the pixel at the location where the luminance of the image largely changes is emphasized. A difference type operator is often used for this spatial filter.
This is because the image data I (1,1) to I (m, n) and the weighting coefficient w (1,1) to within the local region (m × n pixels).
Sum of products operation with w (m, n)

[0004]

[Equation 1]

Is a method for obtaining the brightness difference in the local area. This matrix of weighting factors is called an operation mask. For example, the image data in the local area is shown in FIG.
In the case of (a), when weighting is performed using a 3 × 3 operation mask as shown in FIG. 7 (b), the luminance difference ΔI in the pixel I (2,2) in the local region is

[0006]

[Equation 2]

[0007] A spatial filter using such an operation mask is called a Laplacian filter.
Further, when different operation masks are used in the horizontal direction and the vertical direction as shown in FIG. 8, the brightness difference in the horizontal direction and the vertical direction is

[0008]

[Equation 3]

[0009] Then, the pixel I (2,2
The luminance difference ΔI in 2) is obtained by the following equation.

[0010]

[Equation 4]

A spatial filter using such a mask is called a Sobel filter. The spatial filtering processing by these difference type operators is described in, for example, 28th edition of Image Processing Handbook First Edition, 3rd edition (March, 1991).
It is described on pages 0 to 283.

The brightness difference of the pixels thus obtained is compared with a threshold value (fixed value 4) by the comparator 3 in FIG. 6, and if it is larger than the threshold value, the comparator 3 outputs "1", and if it is smaller, the comparison is made. Bowl 3
Outputs "0" to determine whether or not the pixel of interest is an edge, and as shown in FIG. 9, only the edge portion of the image is output and detected.

Now, the principle of detecting an edge in an image by using the brightness difference will be described. The difference between the target object and the background generally appears in the difference in surface reflectance. Now, when the incident light is i, the reflected light of the target object is I, the reflected light of the background is I ′, the surface reflectance of the target object is r, and the surface reflectance of the background is r ′,

[0014]

[Equation 5]

And the difference Δ in the reflected light between the target object and the background.
I is

[0016]

[Equation 6]

Therefore, the difference in surface reflectance appears as a difference in reflected light. The reflected light is the brightness of the image data, and the difference ΔI in the reflected light is the brightness difference between the target object and the background. If the difference in reflectance between the target object and the background is large, the brightness difference ΔI becomes large, and the edge can be detected.

[0018]

However, in the above-mentioned conventional apparatus, as is clear from (Equation 6), the brightness difference ΔI depends on the incident light i, and depends on the illumination condition at the time of image input. I understand that That is, in the case of an image input under dark illumination, even if the difference in reflectance between the target object and the background is large, the brightness difference ΔI becomes small when the incident light i is small, and it becomes difficult to detect it as an edge. . On the contrary, when the illumination at the time of image input is very bright, even if the difference in reflectance between the portion other than the target object and the background is small, the incident light i
Is very large, the luminance difference ΔI becomes large, and there is a risk of false detection as an edge.

As described above, the above-mentioned conventional techniques have a problem that the edge of the target portion to be detected cannot be detected or the non-target portion is detected as an edge depending on the illumination condition at the time of image input.

Therefore, the present invention has been made to solve the above problems, and can reliably detect only the edge portion of the target image from the image including the background without depending on the illumination condition. The purpose is to obtain the device.

[0021]

In order to achieve the above object, the present invention provides a spatial filter means for obtaining a luminance difference in a local area from image data, a luminance difference obtained by the spatial filter means and the local area. Brightness ratio calculating means for calculating the brightness ratio with the brightness of the image in the inside, and edge determining means for determining whether or not the image data is an edge by comparing the output of the brightness ratio calculating means with a threshold value. It is characterized by having.

[0022]

The present invention has the following actions due to the above-mentioned configuration. By passing the image data through the spatial filter means, the brightness difference ΔI in the local area is obtained. Then, the brightness ratio ΔI / I between the brightness difference ΔI and the brightness I of the image data in the local area is calculated by the brightness ratio calculating means. Since the brightness ratio ΔI / I has a property that it does not depend on the illumination condition, if the edge determining means determines whether it is an edge by comparing the brightness ratio ΔI / I with a threshold value, the illumination condition is determined. It is possible to reliably detect only the edge portion of the target image from the image including the background without depending.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, a first embodiment of the present invention will be described with reference to FIG. In FIG. 1, an image memory 1 is a memory for storing image data, and image data prepared in advance is stored therein. The delay circuit 2 is a circuit that divides the image data sent from the image memory 1 into three horizontal line data and sends them to the spatial filter 3. The spatial filter 3 is a spatial filter circuit that obtains the brightness difference in the local area, and is shown in FIG.
3 × 3 Laplacian mask is used. Control unit 7
Is a circuit for controlling the output of the spatial filter 3 stored in the buffer 4 and the image memory 1 so that the data of the same pixel position (address) is synchronously input to the input of the luminance ratio calculating means 5. The brightness ratio calculating means 5 is a circuit for obtaining the brightness ratio between the brightness difference in the local area and the image data in the area, and the edge determining means 6 compares the brightness ratio with a threshold value to determine whether it is an edge. It is a circuit to do.

First, the control unit 7 controls the output of the image memory 1 so that only the data of three consecutive vertical lines of the horizontal lines of the image data stored in the image memory 1 is transmitted through the delay circuit 2 to the space. Send to filter 3. In the spatial filter 3, the operation mask is sequentially shifted horizontally by one pixel from the sent data for three lines, and the brightness difference at the mask center in the local area for one line (within the mask) is obtained and stored in the buffer 4. send. Here, since the operation of the spatial filter 3 is the same as that of the conventional device shown in FIG. 6, detailed description thereof will be omitted. The obtained brightness difference for one line is stored in the buffer 4. Next, the control unit 7 first sends the data for one pixel from the buffer 4 to the luminance ratio calculation means 5, and immediately thereafter, the control unit 7 controls the image memory 1 to obtain the same data as the data output from the buffer 4. The image data of the address is sent to the brightness ratio calculating means 5. This is repeated while shifting the entire line one line at a time. As a result, the brightness difference in the local area and the brightness of the local area center pixel are sent to the brightness ratio calculating means 5 one after another in synchronization. In this way, the brightness ratio between the brightness difference in the local area and the brightness of the central pixel of the local area is obtained from the two data sent to the brightness ratio calculating means 5. The obtained brightness ratio and the threshold value are compared by the edge determining means 6, and if the threshold value is larger than the threshold value, the edge determining means 6 outputs "1", and if smaller, outputs "0". The pixel is determined to be an edge and detected.

The characteristics of the brightness ratio will be described below. Now, if the luminance of the pixel of interest in the local region is replaced with the luminance I of the target object, and the average luminance within the local region is replaced with the background luminance I ′, then the local data obtained by passing the image data through the spatial filter The brightness difference at
Since it is approximately equal to the brightness difference ΔI between the target object and the background, the brightness ratio is calculated from (Equation 5) and (Equation 6).

[0026]

[Equation 7]

[0027] That is, it is understood that the brightness ratio ΔI / I is determined only by the values of the reflectances r and r ′ and is not dependent on the incident light i. Therefore, even if the illumination is dark and the difference (ΔI) between the luminance level of the target to be detected and the luminance level of the background is small as shown in FIG. 5A, the reflectance r of the target object and the reflectance of the background are small. If there is a large difference with r ′, the brightness ratio ΔI / I becomes relatively large and can be detected as an edge. Even if the illumination is bright and the luminance difference (ΔI) between the non-target luminance and the background is large as shown in FIG. 5B, the reflectance r of the non-target portion and the reflectance r ′ of the background are
If there is not a large difference between and, the luminance ratio ΔI / I becomes relatively small and is not erroneously detected as an edge.

Generally, in the human visual system, the minimum luminance difference required for perceiving the target luminance from the background luminance is
It is known that it is almost proportional to the background brightness. Therefore, if the luminance ratio ΔI / I ′ between the luminance difference ΔI between the target object and the background and the luminance I ′ of the background is calculated from (Equation 5) and (Equation 6),

[0029]

[Equation 8]

It can be seen that, as in the case of (Equation 7), it is not dependent on the incident light i. By utilizing this, the present invention can be implemented even with the configuration shown in FIG. The second embodiment of the present invention will be described below with reference to FIG.

In the configuration of FIG. 3, the image memory 1, the delay circuit 2, the spatial filter 3, the buffer 4, the brightness ratio calculating means 5, and the edge determining means 6 are the same as those of the configuration of FIG. The control unit 7 controls the output of the spatial filter 3 stored in the buffer 4 and the output of the smoothing filter 8 stored in the buffer 9 so that the same pixel position (address) is input to the luminance ratio calculation unit 5. It is a circuit for inputting the data of the above. The control section 7 also has a function of controlling the image memory 1 and sending image data to the delay circuit 2. The smoothing filter 8 is 3 × 3 as shown in FIG.
This is a smoothing filter circuit that obtains the average luminance in the local region by performing a product-sum operation of the image data in the mask and the weighting factor of the mask using the operation mask of. The buffer 9 has the same configuration as the buffer 4, and is for storing the output of the smoothing filter 8.

First, the control unit 7 operates in the same manner as in the first embodiment to send the image data stored in the image memory 1 to the delay circuit 2. The data sent to the delay circuit 2 is
The output of the spatial filter 3 is sent to the buffer 4, and the output of the smoothing filter 8 is sent to the buffer 9.
Next, the control unit 7 sends the data for one pixel from the buffer 4 to the brightness ratio calculating unit 5, and at the same time, sends the image data of the same address as the output of the buffer 4 from the buffer 9 to the brightness ratio calculating unit 5. This is repeated while shifting the entire line one line at a time. As a result, the brightness difference in the local area and the average brightness in the local area are sent to the brightness ratio calculating means 5 one after another in synchronization. Then, the brightness ratio calculation means 5 and the edge determination means 6 are
The edge can be detected from the image data by operating in the same manner as in the above embodiment.

In this way, if edge detection is performed using the luminance ratio ΔI / I 'between the luminance difference ΔI in the local area and the average luminance I'in the local area, a processing effect closer to that of the human visual system is expected. It is possible and practically extremely advantageous.

By the way, the luminance ratio ΔI in the first embodiment
Luminance ratio logarithmic value log (ΔI /
I) is

[0035]

[Equation 9]

Therefore, the present invention can be practiced even if the brightness ratio calculating means 5 in FIG. 1 is configured as shown in FIG. 2 by using the difference value after logarithmic conversion of ΔI and I. In FIG. 2, logarithmic conversion means 51 and logarithmic conversion means 52 are prepared to perform logarithmic conversion, and difference value calculation means 53 is used to obtain the difference value after logarithmic conversion. In this way, the output of the buffer 4 (luminance difference ΔI in the local region) and the output of the image memory 1 (luminance I at the center of the local region) in FIG. 1 are logarithmic conversion means 51 and logarithmic conversion means in FIG. 2, respectively. Logarithmically converted by 52, log (Δ
I) and log (I) are obtained. Next, the difference value calculation means 53 calculates the luminance ratio logarithmic value = log (ΔI) −log.
(I) is obtained, and the brightness ratio calculating means can be realized.
This has the advantage that it is not necessary to use a divider having a complicated structure. The brightness ratio calculating means 5
It goes without saying that the above-mentioned configuration is also effective in the second embodiment.

According to the first and second embodiments described above, the luminance ratio between the luminance difference in the local area obtained by passing the image data through the spatial filter and the luminance of the image in the local area. Since the edge is detected by using, when the brightness difference between the target brightness level to be detected and the background brightness level is small depending on the lighting conditions, or when the brightness difference between the non-target brightness and the background brightness is large. However, it is possible to reliably detect only the edge portion of the target image from the image including the background.

In the first and second embodiments, the Laplacian mask is used as the spatial filter 3, but
Any mask may be used as long as it is a difference type operation mask. Further, in the second embodiment, the operation mask of the smoothing filter 8 may be any mask as long as it has a function of smoothing the inside of the local area, and is not limited to the mask of FIG. These operation masks are not limited to 3 × 3, but may be 4 × 4, 5 × 5, or other sizes.

The reciprocal of the luminance ratio in the first and second embodiments may be used. In this case, the edge determining means can be realized by outputting "1" when the reciprocal of the luminance ratio is smaller than the threshold value and outputting "0" when it is larger.

[0040]

As described above, according to the present invention,
Since the brightness ratio in the local area is used, when the brightness difference between the brightness level of the target to be detected and the brightness level of the background to be detected is small, or the brightness difference between the brightness outside the target and the brightness of the background is small. Even if it is large, only the edge portion of the target image can be reliably detected from the image including the background.

[Brief description of drawings]

FIG. 1 is a schematic block diagram of an edge detection device according to a first embodiment of the present invention.

FIG. 2 is a schematic block diagram of brightness ratio calculating means in the first and second embodiments of the present invention.

FIG. 3 is a schematic block diagram of an edge detection device according to a second embodiment of the present invention.

FIG. 4 is an operation mask configuration diagram of a smoothing filter according to a second embodiment of the present invention.

5A is an explanatory view of operating characteristics under dark illumination according to the first embodiment of the present invention, and FIG. 5B is an explanatory view of operating characteristics under bright illumination according to the first embodiment of the present invention.

FIG. 6 is a schematic block diagram of an edge detection device in a conventional example.

FIG. 7A is a layout diagram of image data in a local area used for explaining the spatial filter in the conventional example, and FIG. 7B is an operation of the spatial filter in the first and second embodiments of the present invention and the conventional example. Mask configuration diagram

FIG. 8A is a horizontal operation mask configuration diagram of a Sobel filter used to describe a conventional example, and FIG. 8B is a vertical operation mask configuration diagram of a Sobel filter used to describe a conventional example.

FIG. 9 is an explanatory diagram of operating characteristics in a conventional example.

[Explanation of sign]

1 image memory 2 delay circuit 3 spatial filters 4 buffers 5 Luminance ratio calculation means 6 Edge determination means 7 control unit 51 Logarithmic conversion means 52 Logarithmic conversion means 53 Difference value calculating means

Claims (2)

[Claims] 1. A spatial filter means for obtaining a luminance difference in a local area from image data, and a luminance ratio calculation for calculating a luminance ratio between the luminance difference obtained by the spatial filter means and the luminance of an image in the local area. An edge detecting device comprising: a means for determining the edge of the image data by comparing the output of the brightness ratio calculating means with a predetermined threshold value. 2. The brightness ratio calculating means calculates logarithmically the brightness difference in the local area and the brightness of the image in the local area calculated by the spatial filter means, and calculates the difference value of the output after the logarithmic conversion. The edge detection device according to claim 1, wherein the edge detection device comprises a difference value calculation means.