JP6901895B2 - Bandwidth limiting device and program - Google Patents

Description

The present invention relates to a band limiting device and a program for band limiting an input image.

Conventionally, the band limitation of an image is performed in order to suppress folding distortion in image reduction, extract a blurred image or an edge, compress image data, and the like. As a general band limiting method, low-pass type (LPF: Low-Pass Filter), band-pass type (BPF: Band-Pass Filter), and high-pass type (HPF: High-Pass Filter) linear filter processing Can be given.

On the other hand, a method of removing noise by wavelet degeneracy using wavelet transform (wavelet decomposition) has been proposed (see, for example, Non-Patent Document 1).

David L. Donoho, Iain M. Johnstone, Gerard Kerkyacharian and Dominique Picard, “Wavelet Shrinkage: Asymptopia?”, Journal of the Royal Statistical Society. Series B (Methodological), Vol. 57, No. 2, pp. 301-369 , 1995.

However, although the above-mentioned linear filter processing such as LPF can be easily incorporated into an apparatus, it is difficult to perform detailed control for each frequency band.

Further, the method disclosed in Non-Patent Document 1 removes noise by coring a component below the noise level among the wavelet-converted frequency decomposition coefficients, and does not limit the band. ..

An object of the present invention made in view of such circumstances is to provide a band limiting device and a program capable of performing detailed control for each frequency band and performing band limiting processing with high accuracy and high image quality. It is in.

In order to solve the above problems, the band limiting device according to the present invention is a band limiting device that limits the band of the input image, and frequency decomposition processing is performed on the input image to generate a frequency decomposition coefficient. generating a part, said frequency decomposition coefficients, the degenerate coefficients were strongly degenerate than the frequency decomposition coefficients of non-limiting the frequency band in which the frequency bandwidth can not exceed the limit threshold in the limiting frequency band in which the frequency band exceeds the limit threshold A band limitation that indicates the ratio of the frequency resolution coefficient to be reduced based on the reduction processing unit, the frequency reconstruction unit that performs frequency reconstruction processing on the reduction coefficient to generate a band limiting image, and the limitation threshold. The band limitation is performed by determining the amount and using a weighting coefficient obtained by normalizing the ratio of the sum of the absolute values of the frequency decomposition coefficients in the non-restricted frequency band to the sum of the absolute values of the frequency decomposition coefficients of the input image. A band limiting amount determining unit for adjusting the amount is provided, and the reduction processing unit determines a power threshold based on the band limiting amount adjusted by the band limiting amount determining unit, and the absolute value is equal to or less than the power threshold. The frequency decomposition coefficient is characterized in that the absolute value is strongly reduced than the frequency decomposition coefficient exceeding the power threshold value.

Further, in the band limiting device according to the present invention, the degeneracy processing unit uses the band limiting amount to determine a degenerate function representing an output value with respect to an input value of the frequency decomposition coefficient, and uses the degenerate function to determine the frequency. It is characterized by degenerating the decomposition coefficient.

Further, in the band limiting device according to the present invention, the frequency decomposition unit performs wavelet decomposition processing for octave-decomposing the input image to the low-frequency band side, or octave-decomposes the input image into the low-frequency band side and the high-frequency band side. It is characterized in that a wavelet packet decomposition process is performed and a wavelet filter having a tap length equal to or longer than a threshold value and having linear phase is used.

Further, in order to solve the above problems, the program according to the present invention is characterized in that the computer functions as the band limiting device.

According to the present invention, band limitation control can be performed in detail for each frequency band, and band limitation processing with high accuracy and high image quality can be performed.

It is a block diagram which shows the structural example of the band limiting apparatus which concerns on 1st Embodiment of this invention. It is a figure which shows the state of the frequency decomposition at the time of the 2nd floor wavelet packet decomposition of an input image. It is a figure which shows the limited frequency band and the non-limited frequency band used in the band limiting apparatus which concerns on 1st Embodiment of this invention. It is a figure which shows the example of the degeneracy function used in the band limiting apparatus which concerns on 1st Embodiment of this invention. It is a block diagram which shows the structural example of the band limiting apparatus which concerns on 2nd Embodiment of this invention. It is a figure which shows the example of the histogram used in the band limiting apparatus which concerns on 2nd Embodiment of this invention. It is a block diagram which shows the structural example of the band limiting apparatus which concerns on 3rd Embodiment of this invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(First Embodiment)
The band limiting device according to the first embodiment of the present invention will be described below. FIG. 1 shows a configuration example of the band limiting device according to the first embodiment of the present invention. The band limiting device 1 shown in FIG. 1 includes a frequency decomposition unit 11, a degeneration processing unit 12, and a frequency reconstruction unit 13.

The band limiting device 1 is a device that generates a band limiting image in which the band of the input image is limited and outputs the band limiting image to the outside.

The frequency decomposition unit 11 performs frequency decomposition processing on the input image to generate a frequency decomposition coefficient (frequency decomposition component), and outputs the frequency decomposition coefficient (frequency decomposition component) to the degeneration processing unit 12. It is assumed that the Parseval's equation holds between the frequency decomposition coefficients of each floor in the frequency decomposition.

As the frequency decomposition process, the frequency decomposition unit 11 performs a wavelet decomposition process for octave-decomposing the input image to the low-frequency band side or a wavelet packet decomposition process for octave-decomposing the input image to the low-frequency band side and the high-frequency band side. Suitable. Further, it is preferable that the wavelet filter used for these processes has a long tap length (greater than or equal to the threshold value) and has linear phase. CDF (Cohen-Daubechies-Feauveau) 5/3 and CDF9 / 7 are known as wavelet filters having a tap length of 3 or more and having linear phase.

As a band limiting method capable of controlling each frequency band and each phase position, a reduction process using a Fourier transform using a window function can be considered. In this process, although it depends on the characteristics of the window function, Gibbs phenomenon may occur by class approximation of discontinuous function with sinusoidal function and cosine wave function. The discrete cosine transform of the block partition region used in video coding also has the same problem. Further, when the number of pixels is N, there is a problem that the amount of calculation increases in the order of N log N. In that respect, according to the frequency decomposition process using the wavelet filter, these problems do not occur.

FIG. 2 is a diagram illustrating wavelet packet decomposition by the frequency decomposition unit 11. When an input image having a resolution of horizontal 8K and vertical 4K is decomposed into second-order wavelet packets, the frequency band of the input image is divided into 16 as shown in FIG. Is output. In the following description, the spatial frequency of an image will be represented by a resolution corresponding to the frequency.

The reduction processing unit 12 inputs band limitation information including a limitation threshold value which is a target value of the frequency band of the band limitation image, and reduces the frequency decomposition coefficient generated by the frequency decomposition unit 11 based on the band limitation information. A reduction coefficient is generated and output to the frequency reconstruction unit 13. The reduction processing unit 12 reduces the frequency decomposition coefficient of the frequency band exceeding the limit threshold (limit frequency band) more strongly than the frequency decomposition coefficient of the frequency band below the limit threshold (non-restriction frequency band).

FIG. 3 shows a limited frequency band and a non-restricted frequency band when the limiting threshold is set to horizontal 6K and vertical 3K for an input image having a resolution of horizontal 8K and vertical 4K. The band with diagonal lines is the non-restricted frequency band, and the band without diagonal lines is the limited frequency band.

It is known that white Gaussian thermal noise is included in a substantially constant proportion in the entire frequency band, and a frequency resolution coefficient having a small absolute value has a high possibility of thermal noise. Therefore, the degeneracy processing unit 12 may degenerate more strongly as the frequency decomposition coefficient has a smaller absolute value. For example, the degeneracy processing unit 12 degenerates a frequency decomposition coefficient whose absolute value is equal to or less than the power threshold value more strongly than a frequency decomposition coefficient whose absolute value exceeds the power threshold value.

When degenerating the frequency decomposition coefficient, the degeneracy processing unit 12 may prepare a plurality of types of degeneracy functions representing output values with respect to the input value of the frequency decomposition coefficient in advance, and degenerate the frequency decomposition coefficient by using the degeneracy function. An example in which three types of degenerate functions are used will be described with reference to FIG.

FIG. 4 is a diagram showing an example of a degenerate function. The first degenerate function shown in FIG. 4A is a linear function having a slope coefficient m = 1, and outputs the output value as it is with respect to the input value. The second reduction function shown in FIG. 4B is a linear function having a slope coefficient m = 0.1, and attenuates the frequency decomposition coefficient value in the frequency band by 20 [dB]. The third reduction function shown in FIG. 4C is a linear function having a slope coefficient m = 0.01, and attenuates the frequency decomposition coefficient value in the frequency band by 40 [dB].

The degeneracy processing unit 12 applies the first degeneracy function to a frequency decomposition coefficient having a high spectral power (exceeding a power threshold value) in an unrestricted frequency band. However, since the slope coefficient m = 1, there is no degeneracy. Further, the second degeneracy function is applied to the frequency decomposition coefficient having a high spectral power (exceeding the power threshold value) in the limited frequency band. The third degeneracy function is applied to the other frequency decomposition coefficients, that is, the frequency decomposition coefficients having low spectral power (not exceeding the power threshold).

The frequency reconstruction unit 13 performs frequency reconstruction processing on the degeneracy coefficient generated by the degeneration processing unit 12, generates a band-limited image, and outputs the band-restricted image to the outside. When the frequency decomposition unit 11 has performed the nth-order wavelet packet decomposition processing, the frequency reconstruction unit 13 performs the nth-order wavelet packet decomposition processing.

A computer can be preferably used to function as the band limiting device 1 described above, and such a computer stores a program describing processing contents for realizing each function of the band limiting device 1 in the storage unit of the computer. It can be realized by storing it in the computer and reading and executing this program by the CPU of the computer.

As described above, according to the invention according to the first embodiment, the frequency decomposition coefficient in the limited frequency band is reduced more strongly than the frequency decomposition coefficient in the non-restricted frequency band, so that the band is limited for each frequency band. Can be controlled in detail. Further, by using the power threshold value, it is possible to control the band limitation for each phase position. Further, by using the wavelet decomposition process or the wavelet packet decomposition process as the frequency decomposition process, problems such as a Gibbs phenomenon and an increase in the amount of calculation do not occur. In this way, it is possible to perform highly accurate and high-quality band limiting processing with a small amount of calculation.

(Second embodiment)
Next, the band limiting device according to the second embodiment of the present invention will be described. FIG. 5 is a block diagram showing a configuration example of the band limiting device according to the second embodiment. The band limiting device 2 shown in FIG. 5 includes a frequency decomposition unit 11, a degeneration processing unit 12', a frequency reconstruction unit 13, and a band limiting amount determining unit 14. The same configuration as that of the first embodiment is designated by the same reference number, and the description thereof will be omitted as appropriate.

Similar to the first embodiment, the frequency decomposition unit 11 performs frequency decomposition processing on the input image to generate a frequency decomposition coefficient, and outputs the frequency decomposition coefficient to the degeneration processing unit 12'.

The band limiting amount determination unit 14 inputs band limiting information including a limiting threshold value which is a target value of the frequency band of the band limiting image, and based on the band limiting information, the band limiting amount α indicates the ratio of the frequency decomposition coefficient to be reduced. Is determined and output to the reduction processing unit 12'. For example, when the resolution is horizontal 6K and vertical 3K for an input image with a resolution of 8K horizontal and 4K vertical, the frequency decomposition coefficient within the limited frequency band with respect to the total number of frequency decomposition coefficients of the input image. Since the ratio is 7/16, the band limit amount α is determined to be 7/16. The limit threshold value is output to the degeneracy processing unit 12'as it is.

The degeneracy processing unit 12'determines the power threshold value based on the band limit amount α. Specifically, the degeneracy processing unit 12'generates a histogram of the absolute value of the frequency decomposition coefficient. FIG. 6 is a diagram showing an example of a histogram generated by the degeneracy processing unit 12', in which the horizontal axis is the absolute value of the frequency decomposition coefficient and the vertical axis is the frequency.

The reduction processing unit 12'accumulates the frequencies in ascending order of the absolute value of the frequency partition coefficient using the histogram, and when the ratio of the cumulative number to the total number of frequency partition coefficients of the input image exceeds the band limit α. The absolute value of the frequency decomposition coefficient of is used as the power threshold.

Then, the degeneracy processing unit 12'reduces the frequency decomposition coefficient by using the limit threshold value and the determined power threshold value. Specifically, as in the first embodiment, the frequency decomposition coefficient in the limited frequency band is degenerated more strongly than the frequency decomposition coefficient in the non-restricted frequency band. Further, the frequency decomposition coefficient whose absolute value of the frequency decomposition coefficient is equal to or less than the power threshold is degenerated more strongly than the frequency decomposition coefficient exceeding the power threshold.

Similar to the first embodiment, the frequency reconstruction unit 13 performs frequency reconstruction processing on the degeneracy coefficient generated by the degeneration processing unit 12'to generate a band-limited image, and outputs the band-limited image to the outside.

A computer can be preferably used to function as the band limiting device 2 described above, and such a computer stores a program describing processing contents for realizing each function of the band limiting device 2 in the storage unit of the computer. It can be realized by storing it in the computer and reading and executing this program by the CPU of the computer.

As described above, according to the invention according to the second embodiment, an appropriate power threshold value can be determined based on the band limiting amount α. Therefore, it is possible to perform band limiting processing with higher accuracy and higher image quality than the invention according to the first embodiment.

(Third Embodiment)
Next, the band limiting device according to the third embodiment of the present invention will be described. FIG. 7 is a block diagram showing a configuration example of the band limiting device according to the third embodiment. The band limiting device 3 shown in FIG. 7 includes a frequency decomposition unit 11, a degeneration processing unit 12', a frequency reconstruction unit 13, and a band limiting amount determining unit 14'. The same configuration as that of the second embodiment is designated by the same reference number, and the description thereof will be omitted as appropriate.

The frequency decomposition unit 11 performs frequency decomposition processing on the input image to generate a frequency decomposition coefficient, and outputs the frequency division coefficient to the band limiting amount determination unit 14'. Only the output destination is different from the first embodiment.

The band limiting amount determination unit 14'inputs the frequency decomposition coefficient from the frequency decomposition unit 11 and calculates the ratio of the total absolute value of the frequency decomposition coefficient in the non-restricted frequency band to the total absolute value of the frequency decomposition coefficient of the input image. Determine the normalized weighting factor β. Further, as in the second embodiment, the band limiting amount α indicating the ratio of the frequency decomposition coefficient to be degenerated is determined based on the band limiting information. Then, the band limiting amount determining unit 14'adjusts the band limiting amount α using the weighting coefficient β, and outputs the adjusted band limiting amount α'to the degenerate processing unit 12'. Specifically, when the weighting coefficient β is larger than the threshold value, the band limiting amount α'is adjusted to be smaller than the original band limiting amount α.

Similar to the second embodiment, the degeneration processing unit 12'generates a histogram of the absolute value of the frequency partition coefficient, and sets the power threshold value based on the band limiting amount α'input from the band limiting amount determining unit 14'. decide. Then, the degeneracy processing unit 12'reduces the frequency decomposition coefficient by using the limiting threshold value and the determined power threshold value as in the second embodiment.

Further, when the degeneracy function as shown in FIG. 4 is used, the slope coefficient m of the degeneracy function may be changed based on the band limiting amount α'. For example, if the initial value of the slope coefficient m of the degeneracy function is set as shown in FIG. 4, and the band limit α'is larger than the band limit α, the slope coefficient in FIGS. 4 (b) and 4 (c). The amount of degeneracy is increased by making m smaller than the initial value.

Similar to the second embodiment, the frequency reconstruction unit 13 performs frequency reconstruction processing on the degeneracy coefficient generated by the degeneration processing unit 12'to generate a band-limited image, and outputs the band-limited image to the outside.

A computer can be preferably used to function as the band limiting device 3 described above, and such a computer stores a program describing processing contents for realizing each function of the band limiting device 3 in the storage unit of the computer. It can be realized by storing it in the computer and reading and executing this program by the CPU of the computer.

As described above, according to the invention according to the third embodiment, the band limiting amount α can be adjusted by using the weighting coefficient β. Therefore, it is possible to perform band limiting processing with higher accuracy and higher image quality than the invention according to the second embodiment.

Although the above embodiments have been described as typical examples, it will be apparent to those skilled in the art that many modifications and substitutions can be made within the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited by the above-described embodiments, and various modifications and modifications can be made without departing from the scope of claims. For example, it is possible to combine a plurality of constituent blocks described in the configuration diagram of the embodiment into one, or to divide one constituent block.

Further, the program according to the present invention may be recorded on a computer-readable medium. It can be installed on a computer using a computer-readable medium. Here, the computer-readable medium on which the program is recorded may be a non-transient recording medium. The non-transient recording medium is not particularly limited, but may be, for example, a recording medium such as a CD-ROM or a DVD-ROM.

1,2,3 Band limiting device 11 Frequency decomposition section 12, 12'Degeneration processing section 13 Frequency reconstruction section 14, 14' Band limiting amount determination section

Claims (4)

It is a band limiting device that limits the band of the input image.
A frequency decomposition unit that generates a frequency decomposition coefficient by performing frequency decomposition processing on the input image, and
Degeneracy processing frequency band of the frequency decomposition coefficients within limits the frequency band that exceeds the limit threshold, the frequency band to generate the degenerate coefficients said was strongly degenerate than the frequency decomposition coefficients within the non-restricted frequency band that does not exceed the limit threshold Department and
A frequency reconstruction unit that generates a band-limited image by performing frequency reconstruction processing on the degeneracy coefficient.
Based on the limiting threshold, a band limiting amount indicating the ratio of the frequency decomposition coefficient to be reduced is determined, and the frequency decomposition coefficient in the non-restricted frequency band with respect to the sum of the absolute values of the frequency decomposition coefficient of the input image is determined. A band limit amount determination unit for adjusting the band limit amount using a weighting coefficient obtained by normalizing the ratio of the sum of absolute values is provided.
The reduction processing unit determines the power threshold based on the band limiting amount adjusted by the band limiting amount determining unit, and the absolute value exceeds the power threshold for the frequency decomposition coefficient whose absolute value is equal to or less than the power threshold. A band limiting device characterized in that the frequency is reduced more strongly than the frequency decomposition coefficient. The degeneracy processing unit uses the band limiting amount to determine a degeneration function representing an output value of the frequency decomposition coefficient with respect to an input value, and uses the degeneration function to degenerate the frequency decomposition coefficient. The band limiting device according to claim 1. The frequency decomposition unit performs wavelet decomposition processing for octave-decomposing the input image to the low-frequency band side, or wavelet packet decomposition processing for octave-decomposing the input image to the low-frequency band side and high-frequency band side, and the tap length is a threshold. The band limiting device according to claim 1 or 2 , wherein a wavelet filter having the above and linear phase is used. A program for operating a computer as a band limiting device according to any one of claims 1 to 3.

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