JP2671637B2 - Method and apparatus for enlarging / reducing digital image - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for enlarging / reducing a digital image, and more particularly to a method newly designed as an interpolation filter for enlarging / reducing a digital image and an apparatus using the same. is there.

[0002]

2. Description of the Related Art Conventionally, the enlargement / reduction of an image has been performed with reference to FIGS.
It has been performed by using density interpolation using coordinate conversion of a digital image as shown in FIG. Generally, the coordinates of the enlarged / reduced image do not become the grid points in the coordinate system of the original image, and therefore the density of each pixel of the image after expansion / reduction is used by using the density at the surrounding grid points. Need to interpolate.

As conventional interpolation methods, the following three methods have been mainly used. As one of them, the nearest neighbor method is a method of using the density value of the grid point in the coordinate system of the original image having the shortest distance as the density value of the pixel to be interpolated. Second, the linear interpolation method is a method that uses linear interpolation of the density values of the four nearest grid points as the density values of the pixel to be interpolated. Thirdly, the cubic interpolation method is a method of performing the interpolation by the cubic expression by using the approximate expression of the sinc function that appears in the sampling theorem of the continuous signal and by using the density value at the grid point around the pixel to be interpolated. . As the interpolation formula used at this time, "cubic convolution" or the like has been mainly used. The equation is as shown in Equation 1.

Number 1

(Equation 1)

However, in such a conventional digital image enlarging / reducing method and apparatus thereof, a frequency which cannot be reproduced is included in the image signal because the sampling frequency becomes small at the time of reduction, and therefore aliasing error (alias) occurs. As a result, jaggies are conspicuous on the diagonal lines of the reduced pixels obtained by the phenomenon), which causes a problem that the quality is deteriorated. Furthermore, when the image is enlarged, the sampling interval becomes smaller, and even higher high frequency components can be reproduced, but the original image does not contain these high frequency components and the frequency components do not change. Therefore, there is a problem in that the image size becomes larger and the image looks blurred.

[0006]

SUMMARY OF THE INVENTION The present invention has the above-mentioned problem that occurs in a conventionally known method and apparatus for enlarging / reducing a digital image, that is, the diagonal line of an interpolated image at the time of reduction. This is done for the purpose of solving image deterioration due to jaggies and image blurring at the time of enlargement.

[0007]

As a result of intensive investigations to solve the above problems, the present inventors have designed a filter by the FIR digital filter method when enlarging / reducing a digital image, and the third order It has been found that the above drawbacks are solved by a method of adaptive digital image scaling, characterized by using an optimal interpolation filter made using spline interpolation, and a device using it.

That is, according to the present invention, in the case of reduction of a digital image, an optimum low pass filter designed by the FIR digital filter method is created by using cubic spline interpolation according to the reduction ratio. Interpolation is performed by the interpolation filter to generate a reduced image.

Further, in the case of enlarging a digital image,
An enlarged image is generated by interpolating an image signal using an optimum interpolation filter made by using cubic spline interpolation for a high-frequency emphasis type filter designed by the FIR digital filter method.

[0010]

When the image is “reduced”, the sampling frequency included in the image is reduced as a result. Therefore, the image signal contains a high frequency component that cannot be reproduced as it is. As a result, image quality deterioration such as folding error occurs. Therefore, in the digital image reduction method of the present invention, the original image signal is passed through an optimum low-pass filter according to the reduction ratio,
A high-frequency component that cannot be reproduced, as can be seen from the sampling theorem, is removed, and interpolation is performed using cubic spline interpolation corresponding to the impulse response of this image signal to generate a reduced image.

With respect to enlargement, more information can be reproduced because the sampling interval is smaller than that of the original image, but the image quality is blurred by simply enlarging the original image signal. Therefore, in the digital image enlarging method according to the present invention, the original image signal is emphasized with a high frequency component through a high-frequency emphasis type filter so that the original image signal is emphasized, and interpolation is performed by using cubic spline interpolation corresponding to the impulse response of the image signal. This is done to generate an enlarged image.

It should be noted that if the order of the interpolation filter is increased, the performance of the filter is improved, but the amount of calculation increases and the processing time becomes longer. Therefore, it is preferable to determine the order of the filter in consideration of the trade-off between the processing time and the performance of the filter that affects the image quality.

[0013]

EXAMPLES The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the examples.

A digital image enlargement / reduction method and apparatus according to the present invention will be described with reference to the drawings. 1 and 2 are block diagrams of the present invention.

In the case of image reduction, a low pass filter is first used as shown in FIG. Ideally, this low-pass filter is desired to have the characteristics as shown in FIG. 3, but this is practically impossible, so this is approximated by an FIR digital filter. The coefficient of the digital filter (impulse response) is standardized according to the required reduction ratio and so that the DC gain (amplitude value at frequency 0) becomes 1 so that the color tone of the original image does not change. Then, a cubic spline function for interpolating the impulse response of the filter obtained as a result is obtained, and a reduction interpolation filter as shown in FIGS. 4 and 5 is designed and generated using the obtained cubic spline function. After that, the image signal is filtered using this interpolation filter for reduction,
Generate the actual reduced image. Further, the image quality of the image is checked, the desired frequency characteristic is changed according to the image quality, and the optimum interpolation filter is obtained by feedback.

As a result, high-frequency components that cannot be reproduced and which can be seen from the sampling theorem due to sampling at a sampling interval longer than the original image are removed. Image quality deterioration can be improved.

In the case of enlarging an image, as shown in FIG. 2, first, a high-frequency emphasis type filter is used. Ideally, this high-frequency emphasis type filter is desired to have the characteristics shown in FIG. 6, but since it is practically impossible, this is approximated by an FIR digital filter. Using the required enlargement ratio as a guide, standardize the coefficient (impulse response) of the digital filter so that the DC gain (amplitude value at frequency 0) becomes 1 so that the color tone of the original image does not change. A cubic spline function for interpolating the impulse response of the filter obtained as a result is obtained, and an interpolation filter for enlargement as shown in FIGS. 7 and 8 is designed and generated using the obtained cubic spline function. After that, the image signal is filtered using this enlargement interpolation filter to generate an actual enlarged image. Further, as in the case of the reduced image, the image quality of the image is checked, the desired frequency characteristic is changed according to the image quality, and the optimum interpolation filter is obtained by feedback.

As a result, the high frequency components of the image signal are emphasized and sharpened, so that the blur of the image quality can be eliminated.

The low-pass filter shown in FIG. 5 is designed as a 9th-order FIR digital filter, and the high-pass filter shown in FIG. 8 is designed as a 3rd-order filter. Equations 2 and 3 are examples of interpolation equations used here.

In the case of "reduction" (50% reduction, low pass
(Use filter) The interpolation function in this case is as shown in Equation 2.
It is used in place of C (x) in the above-mentioned formula 1.

Equation 2

(Equation 2)

In the case of "enlargement" (any one of the following filters can be used at any enlargement ratio.) The interpolation function in this case is as shown in Formula 3.

Equation 3

(Equation 3)

[0024]

As described above, according to the present invention, it is possible to provide a very preferable digital reduced image in which image deterioration such as jaggies due to a folding error (aliasing phenomenon) is prevented when the image is reduced. When the image is enlarged, the blur of the image quality can be eliminated and a high-quality digital enlarged image can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a digital image reduction apparatus according to the present invention.

FIG. 2 is a block diagram showing a configuration of a digital image enlarging device according to the present invention.

FIG. 3 is a characteristic diagram showing characteristics of an ideal low-pass filter for reducing a digital image.

FIG. 4 is a characteristic diagram showing a kernel of a reduction interpolation filter of a digital image reduction method according to the present invention.

FIG. 5 is a characteristic diagram showing amplitude response characteristics of a reduction interpolation filter of a digital image reduction method according to the present invention.

FIG. 6 is a characteristic diagram showing characteristics of a high-frequency emphasis type filter for enlarging an ideal digital image.

FIG. 7 is a characteristic diagram showing a kernel of an interpolation filter for enlargement in a method of enlarging a digital image according to the present invention.

FIG. 8 is a characteristic diagram showing an amplitude response characteristic of a magnification interpolation filter of a digital image magnification method according to the present invention.

FIG. 9 is a schematic diagram showing coordinate conversion of an image in enlarging a digital image.

FIG. 10 is a schematic diagram showing coordinate conversion of an image in reducing a digital image.

Claims (6)

(57) [Claims] 1. A method of enlarging / reducing a digital image, characterized by using an optimum interpolation filter produced by using cubic spline interpolation for a filter designed by the FIR digital filter method. Method for enlarging / reducing various digital images. 2. A method of enlarging a digital image, comprising:
An adaptive digital image enlarging method characterized by using an optimum enlarging interpolation filter created by using cubic spline interpolation for a high-frequency emphasis type filter designed by the IR digital filter method. 3. A method of reducing a digital image, comprising: F
An adaptive digital image reduction method characterized by using an optimal reduction interpolation filter made by using cubic spline interpolation for a low-pass filter designed by the IR digital filter method. 4. An apparatus for enlarging a digital image, comprising:
An adaptive digital image enlarging device comprising an optimum enlarging interpolation filter made by using cubic spline interpolation for a high-frequency emphasis type filter designed by the IR digital filter method. 5. A digital image reducing apparatus, comprising:
An adaptive digital image reduction apparatus comprising an optimum reduction interpolation filter made by using cubic spline interpolation for a low-pass filter designed by the IR digital filter method. 6. A digital image enlarging / reducing apparatus, an optimum reducing interpolation filter made by using cubic spline interpolation for a low-pass filter designed by the FIR digital filter method, and an FIR digital filter. 3 for the high-frequency emphasis type filter designed by the method
An adaptive digital image enlarging / reducing device, which is provided with an optimum enlarging interpolation filter created by using a second spline interpolation.