Objective quality evaluation method for holograms

Abstract

A method for objectively evaluating the quality of a hologram is provided. The quality evaluation method according to an embodiment of the present invention receives an original holographic fringe pattern and a target holographic fringe pattern to be compared, calculates the angular spectra of the input original holographic fringe pattern and the target holographic fringe pattern, and calculates PSNR as a quality value of the target holographic fringe pattern from the calculated angular spectra. Because the angular spectrum of a holographic fringe pattern has the same characteristics without changing with distance, the same PSNR value is obtained not only at the position of the holographic fringe pattern but also for diffracted wavefronts located at any distance, enabling objective quality evaluation of the holographic fringe pattern.

Description

[Technical Field]

[0001] The present invention relates to a quality evaluation method for a hologram, and more particularly to an objective quality evaluation method for a holographic fringe pattern or a numerically restored hologram image. [Background technology]

[0002] 1. Holographic fringe pattern generation and numerical recovery Figure 1 shows the hologram plane where the holographic fringe pattern (hologram) is located and its position relative to a three-dimensional object. The three-dimensional object can be considered as a collection of points (point-cloud) with unique amplitudes and unique phases, and light emitted from each point is propagated to the hologram plane in the (x, y) plane. In the hologram plane, the wavefronts propagating from all the points interfere with each other to form a holographic fringe pattern. The pth point in the point-cloud is located at a three-dimensional spatial position (x p , y p , z p ) with complex amplitude values ​​at p =a p exp(jφ p ), and a p is the characteristic amplitude, φ p represents the initial phase. The holographic fringe pattern H(x, y) in the hologram plane (x, y) can be generated numerically by scalar diffraction, and the Rayleigh-Sommerfeld integral, which is a representative method, can be expressed as follows:

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[0003] Due to this characteristic, a numerically reconstructed image at a specific distance d exhibits sharp reconstruction characteristics for objects located at distance d. However, points or objects farther from the plane at distance d tend to become increasingly blurred as the distance increases. This is due to the out-of-focus effect. An example of this is shown in Figure 2. Figure 2(a) shows a holographic fringe pattern generated by the Rayleigh-Sommerfeld integral, and Figures 2(b) through 2(e) show images numerically reconstructed from the holographic fringe pattern at different distances. Figure 2(b) shows a back-focus reconstructed image in which the rear portion is sharp, while the rest of the image is blurred due to defocus. Figure 2(c) shows a front-focus reconstructed image in which the front portion is in focus and sharp, but the rear portion is blurred due to being out of focus. It can be seen that at distances completely outside the range where the 3D object is located, the entire area is blurred, and the blur increases as the distance from the object increases. 2. Trends in quality evaluation of holographic fringe patterns PSNR (Peak Signal Noise Ratio) is the maximum signal-to-noise ratio, which indicates the power of noise relative to the maximum power that a signal can have, and the equations for this are as shown in equations (3) and (4).

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[0004] Here, I(i,j) is the original image, K(i,j) is the image containing noise, MAX is the maximum value that the image can have, and NM is the number of vertical x horizontal pixels of the image. According to the above formula, PSNR is the difference in pixel values ​​between two images expressed in terms of power. Because it can calculate the difference in pixel values, it is widely used in fields that calculate the difference in pixel values ​​that occurs depending on the degree of loss after lossy image compression and restoration. With the recent development of technology and the expansion of its use, there have been many attempts to compress holographic fringe patterns or change the hit rate of holographic fringe patterns. Because holographic fringe patterns contain a huge amount of data, various algorithms have been used to compress them for storage or transmission. Such compression / restoration processes always include a quality evaluation process. The comparison method between the original holographic fringe pattern and the fringe pattern lost through compression and restoration typically uses PSNR and SSIM (Structural Similarity Index), which are used for video or image quality evaluation. However, these methods were not proposed for comparing holographic fringe patterns, and therefore are not suitable for use as standards for evaluating holographic fringe patterns. Despite this, the reason for using PSNR and SSIM to compare holographic fringe patterns is that a suitable quality evaluation method dedicated to holographic fringe patterns has not yet been presented. Summary of the Invention [Problem to be solved by the invention]

[0005] Therefore, the present invention has been made in consideration of the above problems, and an object of the present invention is to provide a method for objectively evaluating the quality of holographic fringe patterns and restored images using angular spectrum and PSNR (Peak Signal Noise Ratio). [Means for solving the problem]

[0006] To achieve the above object, one embodiment of an objective quality assessment method includes the steps of inputting an original holographic fringe pattern, inputting a target holographic fringe pattern to be compared, calculating the angular spectrum of the input original holographic fringe pattern and the target holographic fringe pattern, and calculating a quality value of the target holographic fringe pattern from the calculated angular spectrum. The target holographic fringe pattern may be a holographic fringe pattern in which loss has occurred from the original holographic fringe pattern by adjusting the number of bits or otherwise. The target holographic fringe pattern may be a lossy holographic fringe pattern that is restored after compressing the original holographic fringe pattern. The quality value of the target holographic fringe pattern may show the same value regardless of the position in the direction of the restored image of the numerical value from the holographic fringe pattern. The objective quality assessment method of the present invention may further include a step of converting the input holographic fringe pattern into a holographic fringe pattern composed of complex amplitudes if the input holographic fringe pattern is not a holographic fringe pattern composed of complex amplitudes having amplitude and phase. The case where the holographic fringe pattern is not a holographic fringe pattern composed of complex amplitudes may be a case where the holographic fringe pattern is composed of amplitudes only or a case where the holographic fringe pattern is composed of phases only. The calculating step may calculate a PSNR value by taking the absolute value of each of the calculated angular spectra, and calculate a quality value of the target holographic fringe pattern. The calculation step is The angular spectrum is calculated using the following formula:

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[0007] As described above, according to the present invention, the angular spectrum of a holographic fringe pattern is calculated and the PSNR is calculated based on the calculated angular spectrum. However, since the angular spectrum does not change with distance and has the same characteristics, the same PSNR value will be obtained not only at the position of the holographic fringe pattern but also for diffracted wavefronts located at any distance, making it possible to objectively evaluate the quality of the holographic fringe pattern. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 2 is a diagram illustrating the relationship between a hologram plane and an object plane. [Figure 2] FIG. 10 shows a holographic fringe pattern and a restored image of distance-specific values ​​restored by Fresnel approximation. [Figure 3] This figure shows the position of the holographic fringe pattern and (a) the propagation image (numerical restored image) of the diffracted wavefront at each position at propagation distances d1 and d2 from the holographic fringe pattern, (b) the Fourier spectrum at each distance, and (c) the angular spectrum at each position. [Figure 4] 1 is a flowchart illustrating a conventional objective quality assessment method for holographic fringe patterns. [Figure 5] 1 is a flowchart illustrating an objective quality evaluation method for a holographic fringe pattern using an angular spectrum according to an embodiment of the present invention. [Figure 6] 10 is a flowchart illustrating a method for objective quality evaluation of a holographic fringe pattern using an angular spectrum according to another embodiment of the present invention. [Figure 7] This figure shows the holographic fringe pattern, numerical reconstruction, and PSNR for the angular spectrum. [Figure 8] FIG. 10 is a block diagram showing an objective quality evaluation system for a holographic fringe pattern using an angular spectrum according to yet another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0009] The invention will now be explained in more detail with reference to the drawings. In an embodiment of the present invention, a method for objectively evaluating the quality of a hologram is presented, which is a technology for objectively evaluating the quality of a holographic fringe pattern or a restored image of a hologram that has been numerically reconstructed. 1. Requirements for objective quality assessment of holographic fringe pattern and numerical reconstruction images To perform an objective quality assessment of a holographic fringe pattern, the following requirements must be met: (1) Objective quality assessment of the holographic fringe pattern or the restored image of the numerical value must be performed regardless of the holographic display. Because the holographic fringe pattern contains specific parameters of the holographic display system (wavelength, pixel interval, etc.), compatibility of holographic content between holographic displays with different system parameters is low. (2) The quality assessment method for the holographic fringe pattern and the restored numerical image must be the same, and the same results must be obtained. Because the holographic fringe pattern and the restored numerical image contain the same information about the 3D object, the quality assessment method for the holographic fringe pattern and the restored numerical image must be the same, and the quality assessment results must be the same. (3) The results of the objective quality evaluation must be consistent regardless of the position from which the evaluation is performed, up to an infinite distance, in the direction of the numerically restored image from the holographic fringe pattern. (4) It must be possible to evaluate the quality of any form of holographic fringe pattern, including complex amplitude holograms, amplitude holograms, and phase holograms.

[0010] 2. Characteristics of the Angular Spectrum The Fourier spectrum shows that the frequency spectrum of a two-dimensional plane with an arbitrary pattern can be expressed as a combination of various harmonic waves with specific amplitudes and frequencies. Similarly, the angular spectrum shows that a two-dimensional plane with an arbitrary pattern can be expressed as a combination of various plane waves with a single wavelength, specific amplitudes, and incident angles. A typical optoelectronic holographic display system displays a holographic fringe pattern on a spatial light modulator and inputs a reference wave to the spatial light modulator. The input reference wave is diffracted by the fringe pattern to form a three-dimensional object in free space. In this case, the angular spectrum A(α / λ, β / λ; 0) for the holographic fringe pattern H(x, y; 0) displayed on the spatial light modulator can be expressed as Equation 5.

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[0011] 3. Objective quality evaluation method for holograms A holographic fringe pattern is composed of a pattern for diffracting light and is generated using scalar diffraction such as the Rayleigh-Sommerfeld integral so that the diffracted wavefront can form a 3D object at any distance. When a reference wave is incident on a holographic fringe pattern generated or acquired by a camera, the reference wave is diffracted by the holographic fringe pattern, and the 3D object used at the time of generation is optically or numerically reconstructed at a certain distance (e.g., distance d2), as shown in Figure 3(a). However, at a different distance (e.g., distance d1) from a certain position on the 3D object, a defocused image is reconstructed. The Fourier spectrum of the holographic fringe pattern and the restored image of the numerical value at distances 0, d1, and d2 is shown in Figure 3(b). The Fourier spectrum indicates the spatial frequency distribution of various harmonic waves for the image, so the Fourier spectrum of the holographic fringe pattern and the restored image will have different distributions depending on the distance. Finally, the angular spectrum shown in Figure 3(c) represents the distribution of various plane wave incident angles relative to the image. Because the angular spectrum is linear and a distance-invariant system, the angular spectrum of the holographic fringe pattern and the restored image at each distance have the same distribution. This is because the absolute value of the angular spectrum at distance 0 is the same as the absolute value of the angular spectrum at distance d, as shown in Equation 9 below.

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[0012] FIG. 4 is a flowchart showing a conventional objective quality assessment method using a holographic fringe pattern or its corresponding numerically restored image. Conventionally, PSNR has been used to assess the degree of quality degradation after compressing and restoring a holographic fringe pattern. The holographic fringe pattern or the numerically restored image has been independently dualized and used as the target for calculating PSNR. However, although these two images contain the same information, they have different focal points depending on the restoration distance, resulting in different PSNR results. For the same reason, using the Fourier spectrum of the holographic fringe pattern or the numerically restored image as the target for objective quality assessment may also be unsuitable. However, because the angular spectrum has distance invariant properties, it can be considered suitable for objective quality evaluation of holographic fringe patterns and numerically restored images. FIG. 5 is a flowchart illustrating an objective quality evaluation method for a holographic fringe pattern using an angular spectrum according to one embodiment of the present invention. As shown in the figure, first, the original holographic fringe pattern H ref (x,y) and the target holographic fringe pattern H, where the information or pattern is lost and is used for comparison. tar (x, y) is input (S110, S115). The fringe pattern may be composed of amplitude-only, phase-only, or complex amplitude, and if it is amplitude-only or phase-only, it is converted into a complex amplitude having amplitude and phase (S120, S125). For the converted complex amplitude, the angular spectrum A is calculated using the following formula 10. ref (f X ,f Y ) and A tar (f X ,f Y ) is calculated (S130, S135).

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[0013] FIG. 6 is a flowchart illustrating a method for objectively evaluating the quality of a holographic fringe pattern using an angular spectrum according to another embodiment of the present invention. The presented objective quality assessment method is based on the original holographic fringe pattern H ref After compressing (x, y), the restored holographic fringe pattern (S213) is used as the target holographic fringe pattern Htar This is the case when it is input as (x,y). Thereafter, steps S210 to S250 are substantially similar to steps S110 to S150 shown in FIG. 5, and therefore detailed description of these steps will be omitted.

[0014] 4. Experimental Results FIG. 7 shows the results of an experiment conducted based on the objective quality evaluation method according to an embodiment of the present invention. Three 3D objects were used: a single point, five points, and the 1,453 points that make up the airplane shown in FIG. 2. The reference holographic fringe pattern was generated using the Rayleigh-Sommerfeld integral shown in Equation 1 and converted into an 8-bit grayscale image. The target holographic fringe pattern had its number of bits adjusted to 4 bits, 3 bits, 2 bits, and 1 bit to artificially add loss. For example, a 1-bit holographic fringe pattern would be a binary pattern. Through this process, five holographic fringe patterns, including the reference, were prepared for each object. The generated holographic fringe patterns were numerically restored using Equation 8. The absolute value of the angular spectrum for each generated holographic fringe pattern was calculated using Equations 10 and 11. The PSNR is calculated using the target image and reference image created through this process. As shown in Figure 7, the holographic fringe pattern and the numerically restored image contain the same information, but the calculated PSNR distributions are different. The PSNR of the holographic fringe pattern is relatively lower than that of the numerically restored image. The numerically restored image shows a relatively high value, and it can be seen that the PSNR decreases as the number of points increases. However, the PSNR for the absolute value of the angular spectrum shows a relatively appropriate PSNR distribution. The PSNR value decreases as the number of bits decreases, and the distribution remains similar even when the number of points increases. Furthermore, since the absolute value of the angular spectrum for the holographic fringe pattern and the numerically restored image is the same, it is not necessary to calculate the PSNR independently. This experiment demonstrated that methods for calculating PSNR for holographic fringe patterns and numerically restored images show different distributions depending on various factors such as the number of points, making stable quality assessment difficult. However, the method according to an embodiment of the present invention can calculate stable quality assessment results regardless of changes in various factors such as the number of points.

[0015] 5. Objective quality evaluation system for holograms 8 is a diagram showing the configuration of an objective quality assessment system for a holographic fringe pattern using angular spectrum according to yet another embodiment of the present invention. As shown, the quality assessment system according to this embodiment of the present invention can be realized by a computing system including a communication unit 310, an output unit 320, a processor 330, an input unit 340, and a storage unit 350. The communication unit 310 is a communication means for connecting to external devices and accessing external networks. The output unit 320 outputs the results of the calculations performed by the processor 330, and the input unit 340 transfers user instructions to the processor 330. The storage unit 350 provides storage space necessary for the calculations performed by the processor 330. The processor 330 performs an objective quality assessment of the holographic fringe pattern using the angular spectrum, as shown in Figures 5 and 6 above, and outputs the results of the assessment via the output unit 320 or transfers them via the communication unit 310.

[0016] 6. Variations So far, a method for objectively evaluating the quality of a holographic fringe pattern and a restored image using the angular spectrum and PSNR has been described in detail with reference to a preferred embodiment. Conventional hologram quality assessment methods use PSNR to evaluate the quality of holographic fringe patterns or numerical values ​​in restored images. However, the PSNR values ​​of holographic fringe patterns and their corresponding restored numerical values ​​differ, and the focus-defocus regions vary depending on the restoration position, resulting in different PSNR values. These characteristics mean that PSNR, which has been used to objectively assess the quality of images, is not suitable for evaluating hologram quality. In an embodiment of the present invention, the angular spectrum for a holographic fringe pattern is calculated, and the PSNR is calculated based on the calculated angular spectrum. Because the angular spectrum uses complex amplitude as input, it can be applied to amplitude-only or phase-only holographic fringe patterns. Furthermore, because it is a distance-invariant system, it has the same characteristics regardless of distance. Therefore, the same PSNR value can be obtained not only at the position of the holographic fringe pattern, but also for diffracted wavefronts located at any distance. Conventionally, there has been no suitable method for objectively evaluating the quality of holographic fringe patterns. However, the present invention provides a method for performing objective quality evaluation, which is expected to be useful in a variety of hologram content application fields in the future. Meanwhile, in the above embodiment, the PSNR value is calculated based on the absolute value of the angular spectrum of the original holographic fringe pattern and the target holographic fringe pattern, i.e., the amplitude of the angular spectrum. Here, the amplitude of the angular spectrum may be replaced with the phase of the angular spectrum, and the technical idea of ​​the present invention may also be applied in this case. The technical concepts of the present invention may also be applied to a computer-readable recording medium incorporating a computer program that causes the functions of the apparatus and method according to the present embodiments to be performed. The technical concepts of various embodiments of the present invention may be realized in the form of computer-readable code recorded on a computer-readable recording medium. The computer-readable recording medium may be any data storage device that can be read by a computer and that can store data. For example, the computer-readable recording medium may be a ROM, RAM, CD-ROM, magnetic tape, floppy disk, optical disk, hard disk drive, etc. The computer-readable code or program stored on the computer-readable recording medium may be transmitted via a network connecting computers.

[0017] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to the above embodiments. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications or alterations within the scope of the technical spirit described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

Claims

1. inputting an original holographic fringe pattern; inputting a target holographic fringe pattern to be compared; Calculating angular spectra of the input original holographic fringe pattern and the target holographic fringe pattern, respectively; calculating a quality value of the target holographic fringe pattern from the calculated angular spectrum; An objective quality evaluation method comprising:

2. The target holographic fringe pattern is 2. The objective quality evaluation method according to claim 1, wherein the holographic fringe pattern is an original holographic fringe pattern with loss due to bit number adjustment.

3. The target holographic fringe pattern is 3. The objective quality evaluation method according to claim 2, wherein the original holographic fringe pattern is compressed and then restored to produce a lossy holographic fringe pattern.

4. The quality value of the target holographic fringe pattern is 3. The objective quality evaluation method according to claim 2, wherein the same value is obtained from any position in the direction of the image of the restored numerical value from the holographic fringe pattern.

5. 2. The objective quality evaluation method of claim 1, further comprising the step of converting the input holographic fringe pattern into a holographic fringe pattern composed of complex amplitudes if the input holographic fringe pattern is not a holographic fringe pattern composed of complex amplitudes having an amplitude and a phase.

6. The holographic fringe pattern is not a holographic fringe pattern composed of complex amplitudes when:

6. The objective quality evaluation method according to claim 5, wherein the holographic fringe pattern is composed of amplitude only or phase only.

7. The calculation steps are:

2. The objective quality evaluation method according to claim 1, wherein the PSNR value is calculated by taking the absolute value of each calculated angular spectrum, and the quality value of the target holographic fringe pattern is calculated.

8. The calculation step is The angular spectrum was calculated using the following formula: [Equation 15] A ｒｅｆ (f Ｘ ,f Ｙ ) is an original holographic fringe pattern angular spectrum, A ｔａｒ (f Ｘ ,f Ｙ ) is the angular spectrum of the target holographic fringe pattern, H ｒｅｆ (x, y) is the original holographic fringe pattern, H ｔａｒ (x,y) is the target holographic fringe pattern; f Ｘ and f Ｙ 8. The objective quality assessment method according to claim 7, wherein σ is a spatial frequency.

9. The calculation steps are: The PSNR value was calculated using the following formula: [0016] MAX is the maximum value of the holographic fringe pattern, The absolute value |A| for the complex amplitude A=α+jβ is [Equation 8] is calculated by 9. The objective quality evaluation method according to claim 8, wherein NM is the number of vertical x horizontal pixels.

10. a processor that receives an original holographic fringe pattern and a target holographic fringe pattern to be compared, calculates angular spectra of the input original holographic fringe pattern and the target holographic fringe pattern, and calculates a quality value of the target holographic fringe pattern from the calculated angular spectra; a storage unit that provides the necessary storage space for the processor; An objective quality evaluation system comprising:

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