JP2006179079A - Hologram device and its encoding/decoding method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To raise a hologram reproduction quality level by performing encoding appropriately. <P>SOLUTION: A hologram device performs hologram recording by entering a coherent recording reference beam and a coherent data beam corresponding to a 2-dimensional gray level image pattern established in a spatial light modulator to a hologram recording medium to form a interference pattern. The hologram device has an encoding processing section in which; a modulation code composing a central bit and a peripheral bit of the central bit concerned and a feature quantity code composing a plurality of bits expressing the feature quantity extracted according to a bit pattern of the modulation code concerned are made corresponding in advance; bit-string data of a recording object is divided at each of the plurality of bits; the encoding of every divided plurality of bits is carried out to the modulation codes matched with the feature quantity code which is expressed by the plurality of bits concerned; and the modulation image data are created in order to set the 2-dimensional gray level image pattern in the spatial light modulator based on each modulation code to which encoding is carried out. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a hologram apparatus and an encoding / decoding method thereof.

  As a hologram recording medium on which digital data is recorded as a hologram, for example, there is a medium provided by sealing a photosensitive resin (for example, photopolymer) with a glass substrate. When recording digital data as a hologram on a hologram recording medium, first, a coherent laser beam from a laser device is separated into two laser beams by a PBS (Polarization Beam Splitter). Then, one laser beam (hereinafter referred to as “reference beam”) and an SLM (Spatial Light Modulator) in which digital data is formed as a two-dimensional gray image pattern are irradiated with the other laser beam. By irradiating the hologram recording medium with a predetermined angle with a laser beam (hereinafter referred to as “data beam”) reflecting the information of the image pattern, digital data is recorded on the hologram recording medium. It will be.

  More specifically, the photosensitive resin constituting the hologram recording medium has a finite number of monomers and is irradiated with a laser beam (hereinafter abbreviated as a laser beam) composed of a reference beam and a data beam. As a result, the monomer changes from polymer to polymer depending on the energy of the laser beam and the irradiation time. And when this monomer changes to a polymer, an interference fringe made of a polymer corresponding to the energy of the laser beam is formed. Therefore, when the interference fringes are formed in the hologram recording medium, digital data is recorded as a hologram. Thereafter, the remaining monomer moves (diffuses) to the place where the monomer has been consumed, and further changes to a polymer by being irradiated with a laser beam. FIG. 15 schematically shows a state where the monomer changes into a polymer in accordance with the energy of the laser beam in the hologram recording medium.

  Further, when a large amount of digital data is recorded on the hologram recording medium, it is possible to perform so-called angle multiplex recording in which a large number of holograms are formed by changing the incident angle of the reference beam to the hologram recording medium. For example, one hologram formed on a hologram recording medium is referred to as a page, and a multiplexed hologram composed of a large number of pages is referred to as a book. FIG. 16 is a diagram schematically showing a book and a page in angle multiplex recording. As shown in FIG. 16, in angle multiplexing recording, for example, a 10-page hologram is formed by changing the incident angle of the reference beam for one book. Thus, a large amount of digital data can be recorded by angle multiplexing recording.

  When reproducing digital data from a hologram recording medium, the interference fringes indicating the digital data are irradiated with a reference beam at the same incident angle as when the interference fringes are formed, and are diffracted by the interference fringes. A reference beam (hereinafter referred to as a reproduction beam) is received by an image sensor or the like. The reproduction beam received by the image sensor or the like becomes a two-dimensional gray image pattern indicating the digital data described above. The digital data can be reproduced by decoding the digital data from the two-dimensional grayscale image pattern with a decoder or the like.

  By the way, in the case of hologram recording, digital data to be recorded is encoded as a two-dimensional bit array on a hologram recording medium. For example, in a two-dimensional bit array, the brightness of the “bright spot (deep)” for causing interference with the hologram recording medium is expressed by “1”, so that interference with the hologram recording medium is not caused. The brightness of “dark spot (light)” is expressed by “0”. In addition, when encoding digital data, redundant bits as error correction codes are generally added. Then, a two-dimensional gray image pattern formed in the SLM based on the two-dimensional bit array is recorded as a hologram.

  On the other hand, in the case of hologram reproduction, a two-dimensional grayscale image pattern recorded on a hologram recording medium is captured by an image sensor such as a CCD sensor or a CMOS sensor. This captured image pattern is a reproduction of a two-dimensional gray image pattern at the time of hologram recording, and generally has the same image size as at the time of hologram recording. Further, the captured image pattern is decoded in the order of the two-dimensional bit array and the digital data.

As described above, a hologram recording / reproducing apparatus for recording / reproducing holograms on / from a hologram recording medium is disclosed, for example, in FIG.
JP 2004-177958 A

  By the way, the luminance (shading) of the pixels of the captured image pattern at the time of reproducing the hologram is originally expressed in two gradations of monochrome at the bright point or the dark point, but when the bright point appears dark or the dark point is bright. May appear. That is, the brightness of the pixels of the captured image pattern is expressed in monochrome multi-tone. Here, FIG. 17 shows a histogram display of the luminance of the pixel of the captured image pattern versus its appearance frequency.

  In FIG. 17, area A is a distribution of pixels that should be dark spots, and area B is a distribution of pixels that should be bright spots. Note that the reason why the region B is more dispersed than the region A is that the brightness of the pixel that should be a bright spot changes more greatly due to the influence of diffraction than the pixel that should be a dark spot. Also, the problem here is the pixels distributed in the region C. Pixels distributed in the region C are pixels that are difficult to separate into brightness of bright points or dark points. Therefore, the luminance of the pixels distributed in the region C may be erroneously identified from the bright point to the dark point, or from the dark point to the bright point.

As described above, examples of the phenomenon in which the separability between the bright spot and the dark spot deteriorates include the following factors.
First, the first factor is aberration in the optical system. Aberrations in the optical system appear due to, for example, lens aberrations (spherical aberration, coma aberration, astigmatism, wavefront aberration, etc.) and displacement of the optical system arrangement position.

  The second factor is vibration. In the case of hologram recording, the data beam and the reference beam are required to have high incidence angles, and basically, vibration is not allowed while these beams are incident on the hologram recording medium. If vibration occurs, interference fringes are not accurately formed on the hologram recording medium.

  The third factor is the sensing capability of the optical cell of the image sensor. Wiring for connecting the optical cells constituting the image sensor is indispensable. In addition, not all the optical cells of the image sensor have an appropriate sensing capability, and if the laser beam is received in the wiring area between the optical cells, the amount of light received by the optical cell is naturally Get smaller.

  The fourth factor is a laser beam diffraction phenomenon. When a light shielding region and a light transmission region are included in a laser beam irradiation target, the laser beam is always diffracted. Therefore, when the laser beam is irradiated away from the edge (boundary) of the light shielding area and the light transmitting area, the diffraction becomes small. On the other hand, when the laser beam is irradiated at a position close to the edge, the diffraction becomes large. A so-called light avalanche effect occurs. Therefore, in the SLM or the image sensor, since the influence of the edge is small in the region where the bright points are continuous, the amount of diffraction becomes small, resulting in brightening. On the other hand, the region where the bright points are discontinuous has a large influence of the edge, so that the amount of diffraction becomes large, resulting in darkness.

  The fifth factor is electrical noise generated by an image sensor or the like, and noise due to stray light components.

  Therefore, by taking any one of the above-described first to fifth factors, the separability between the bright spot and the dark spot is improved. However, the problems of the aberration (first factor) in the optical system and the sensing capability (third factor) of the light cell of the image sensor are phenomena caused by manufacturing variations in the manufacturing process, and vibration (second factor). The problems of laser beam diffraction (fourth factor) and noise (fifth factor) are phenomena that occur accidentally. For this reason, it is often difficult to take a general-purpose countermeasure in advance for the first to fifth factors.

  Therefore, in order to improve the separation of bright and dark points at the time of hologram reproduction in advance and for general use, as described above, encoding that provides redundant bits as error correction codes is performed during hologram recording. Generally done. However, since there are a finite number of monomers consumed in the hologram recording medium, it is necessary to suppress the recording of redundant bits such as error correction codes as much as possible, and there is a limit to the improvement of error correction coding. Further, the error correction code itself has poor separability between bright and dark spots due to the above-described first to fifth factors, and there is a possibility that error correction will not be performed properly.

  Thus, it is important how to record a two-dimensional gray image pattern on a hologram recording medium during hologram recording, that is, how to properly encode data to be recorded on the hologram recording medium. It was a difficult task.

  A main aspect of the present invention for solving the above-described problems is that a coherent recording reference beam and a coherent data beam corresponding to a two-dimensional gray image pattern set by a spatial light modulator are used as a hologram recording medium. In a hologram apparatus that performs hologram recording to form an interference fringe by being incident on a modulation code, a modulation code composed of a central bit and peripheral bits of the central bit, and a feature amount extracted according to the bit pattern of the modulation code A plurality of feature codes represented by a plurality of bits are associated in advance, the bit string data to be recorded is divided for each of the plurality of bits, and each of the divided plurality of bits is referred to as the feature code represented by the plurality of bits. Encoding to the modulated modulation code is made, and based on each of the encoded modulation code, the spatial light modulator Forming a modulated image data for setting the two-dimensional gray image pattern Te, and have a coding processing unit.

  In another aspect of the present invention for solving the above-described problem, a coherent recording reference beam and a coherent data beam corresponding to a two-dimensional gray image pattern set by a spatial light modulator are holograms. In a hologram apparatus that performs hologram recording so as to form an interference fringe by being incident on a recording medium, bit string data to be recorded is arranged into two-dimensional data, and attention bits of the arranged two-dimensional data and the surroundings of the attention bits The feature amount of the target image data composed of bits is extracted, the extracted feature amount is encoded into a modulation code of a plurality of bits previously associated with the feature amount, and the encoding is performed. An encoding processing unit configured to form modulation image data for setting the two-dimensional gray image pattern in the spatial light modulator based on each modulation code; And the.

  ADVANTAGE OF THE INVENTION According to this invention, the hologram apparatus which improved the hologram reproduction quality by performing encoding appropriately, and its encoding method can be provided.

<Overall configuration of hologram device>
Based on FIG. 1, a configuration of a hologram apparatus according to an embodiment of the present invention will be described.

  The hologram device includes a CPU 1, a memory 2, an interface 3, a connection terminal 4, a buffer 5, a reproduction / recording determination unit 6, an encoder 7, an SLM 9, a laser device 10, a first shutter 11, a first shutter control unit 12, a PBS 13, a first 2 shutter 14, second shutter controller 15, galvo mirror 16, galvo mirror controller 17, dichroic mirror 18, servo laser device 19, scanner lens 20, Fourier transform lenses 21, 26, detector 23, disk controller 24, A disk drive unit 25, an image sensor 27, an image sensor control unit 28, a filter 29, a decoder 30, and a half-wave plate 31 are included.

  The interface 3 exchanges data between a host apparatus (personal computer, workstation, etc.) connected via the connection terminal 4 and the hologram apparatus.

  The buffer 5 stores reproduction instruction data from the host device for reproducing the hologram recorded on the hologram recording medium 22. The buffer 5 stores recording instruction data for recording the bit string data from the host device on the hologram recording medium 22. Further, the buffer 5 stores bit string data to be recorded on the hologram recording medium 22.

  The reproduction / recording determination unit 6 determines whether or not a reproduction instruction signal or a recording instruction signal is recorded in the buffer 5 at a predetermined timing. When it is determined that the reproduction instruction data is stored in the buffer 5, the reproduction / recording determination unit 6 transmits an instruction signal for executing reproduction processing in the hologram device to the CPU 1. When the reproduction / recording discriminating unit 6 discriminates that the recording instruction data is stored in the buffer 5, the reproducing / recording discriminating unit 6 transmits an instruction signal for executing the recording process in the hologram device to the CPU 1 to record the hologram from the host device. Data to be stored in the working medium 22 is transmitted to the encoder 7. Further, the reproduction / recording determination unit 6 transmits information on the amount of data to be stored in the hologram recording medium 22 to the CPU 1.

  The encoder 7 is an embodiment of an “encoding processing unit” according to the present invention. That is, the encoder 7 first stores the bit string data transferred from the buffer 5 in a memory 71 such as SRAM or DRAM. In the memory 71, the bit string data is arranged into two-dimensional data. The encoder 7 performs an encoding process according to the present invention on the two-dimensional data stored in the memory 71. Then, the two-dimensional data subjected to the encoding processing according to the present invention is finally converted into modulated image data (for example, vertical 1280 bits × horizontal 1280 bits≈1.6 megabits), which is a modulation code array pattern. And supplied to the SLM 9. A detailed description of the encoding process according to the present invention will be described later.

  The SLM 9 forms a two-dimensional gray image pattern based on the modulated image data supplied from the encoder 7. The two-dimensional gray image pattern is, for example, one bit value (for example, “1”) of each bit constituting the modulated image data as a bright point (dark) and the other bit value (for example, “0”). The dark spot (light) is set. Here, the SLM 9 expresses a dot pattern of 1280 vertical pixels × 1280 horizontal pixels of the modulated image data of about 1.6 megabits. Further, the SLM 9 reflects to the Fourier transform lens 21 when a laser beam from the laser device 10 is incident, as will be described later. The reflected laser beam becomes a laser beam (hereinafter referred to as “data beam”) in which information of the two-dimensional gray image pattern formed by the SLM 9 is reflected. In addition, as shown in FIG. 1, it is not restricted to the case where the other laser beam from PBS13 injects into SLM9 directly. For example, a PBS (not shown) may be provided on the optical path between the second shutter 14 and the SLM 9 and a laser beam separated from the PBS may be incident on the SLM 9.

  The laser device 10 emits a coherent laser beam excellent in temporal coherence and spatial coherence to the first shutter 11. As the laser device 10, for example, a helium neon laser, an argon neon laser, a helium cadmium laser, a semiconductor laser, a dye laser, a ruby laser, or the like is used to form a hologram on the hologram recording medium 22.

  The CPU 1 performs overall control of the hologram reproducing device. When the CPU 1 receives the instruction signal based on the recording instruction data from the reproduction / recording determination unit 6, the CPU 1 reads the address information from the pits already formed on the hologram recording medium 22 from the memory 2. The CPU 1 then applies a laser beam from the servo laser device 19 (hereinafter referred to as “servo laser beam”) to a pit provided in the hologram recording medium 22 indicating the next address information. An instruction signal for rotating the disk 22 is transmitted to the disk controller 24. Further, the CPU 1 transmits an instruction signal to the galvo mirror control unit 17 in order to cause the galvo mirror control unit 17 to adjust the angle of the galvo mirror 16.

  Further, the CPU 1 calculates the number of holograms (that is, the number of pages) formed on the hologram recording medium 22 based on the data amount information from the reproduction / recording determination unit 6. Further, the CPU 1 transmits an instruction signal for opening the first shutter 11 and the second shutter 14 to the first shutter control unit 12 and the second shutter control unit 15, respectively. As a result, hologram recording on the hologram recording medium 22 is started. Then, when the recording process based on the recording instruction data is completed, the CPU 1 closes the first shutter 11 and the second shutter 14 with respect to the first shutter control unit 12 and the second shutter control unit 15. The instruction signal is transmitted. As a result, hologram recording on the hologram recording medium 22 is completed.

  On the other hand, when the CPU 1 receives the instruction signal based on the reproduction instruction data from the reproduction / recording discriminating unit 6, the CPU 1 servos the pit indicating the address information corresponding to the reproduction instruction signal formed on the hologram recording medium 22. In order to irradiate the servo laser beam from the laser device 19, an instruction signal for rotating the hologram recording medium 22 is transmitted to the disk control unit 24. Further, when receiving an instruction signal based on the reproduction instruction data, the CPU 1 transmits an instruction signal for opening the first shutter 11 to the first shutter control unit 12 and also transmits a second signal to the second shutter control unit 15. An instruction signal for closing the shutter 14 is transmitted. Further, the CPU 1 transmits an instruction signal to the galvo mirror control unit 17 in order to cause the galvo mirror control unit 17 to adjust the angle of the galvo mirror 16. As a result, hologram reproduction from the hologram recording medium 22 is started. When the CPU 1 determines that a predetermined time has elapsed in the reproduction process based on the reproduction instruction data, the CPU 1 transmits an instruction signal for closing the first shutter 11 to the first shutter control unit 12. As a result, the hologram reproduction from the hologram recording medium 22 is completed. Note that the CPU 1 may end the reproduction process from a signal based on the determination result from the DSP 28.

  The first shutter control unit 12 performs control for opening the first shutter 11 or closing the first shutter 11 based on an instruction signal from the CPU 1. Further, the first shutter control unit 12 performs control for closing the first shutter 11 based on the instruction signal from the image sensor control unit 28. The first shutter control unit 12 transmits an open state instruction signal to the first shutter 11 when opening the first shutter 11. The first shutter control unit 12 transmits a closed state instruction signal to the first shutter 11 when the first shutter 11 is closed.

  The first shutter 11 enters an open state based on an open state instruction signal from the first shutter control unit 12. Alternatively, the first shutter 11 enters a closed state based on a closed state instruction signal from the first shutter control unit 12. When the first shutter 11 is closed, the laser beam from the laser device 10 is blocked from entering the half-wave plate 31.

The half-wave plate 31 is provided with a predetermined inclination in order to determine the angle at which the laser beam from the laser device 10 is incident on the PBS 13. The predetermined inclination of the half-wave plate 31 is determined so that the ratio of separation into two laser beams in the PBS 13 is a desired ratio.
The PBS 13 separates the laser beam from the half-wave plate 31 into two laser beams. One laser beam separated by the PBS 13 is incident on the second shutter 14. The other laser beam (hereinafter referred to as “reference beam”) is incident on the galvo mirror 16.

The galvo mirror 16 reflects the reference beam from the PBS 13 to the dichroic mirror 18.
The galvo mirror control unit 17 adjusts the angle at which the reference beam reflected by the galvo mirror 16 is incident on the hologram recording medium 22 via the dichroic mirror 18 and the scanner lens 20 based on the instruction signal from the CPU 1. Therefore, the angle of the galvo mirror 16 is controlled. At the time of recording on the hologram recording medium 22, the angle adjustment of the galvo mirror 16 by the galvo mirror control unit 17 is performed in order to cause the hologram recording medium 22 to record information of a two-dimensional grayscale image pattern as a hologram.

  More specifically, a data beam and a reference beam interfere with each other in the hologram recording medium 22 to form a three-dimensional interference fringe (hologram). That is, by forming a hologram on the hologram recording medium 22, the information of the two-dimensional gray image pattern set by the SLM 9 is recorded. Further, the carbo mirror control unit 17 enables angle multiplex recording by adjusting the angle of the carbo mirror 16, that is, by changing the incident angle of the reference beam to the hologram recording medium 22. Hereinafter, one hologram formed on the hologram recording medium 22 is referred to as a page, and a multiplex recording hologram in which a large number of pages are overlapped by angle multiplex recording is referred to as a book.

  Further, at the time of reproduction from the hologram recording medium 22, the carbo mirror control unit 17 controls the angle of the galvo mirror 16 so that the reference beam is incident on the hologram formed on the hologram recording medium 22. The angle adjustment of the carbo mirror 16 at the time of reproduction by the carbo mirror control unit 17 is performed with respect to a hologram formed based on the data to be reproduced, and the reference beam when the data to be reproduced is formed as a hologram. The reference beam is executed so as to be incident on the hologram at the same angle as the incident angle.

  The servo laser device 19 detects the position of the hologram formed on the hologram recording medium 22 based on the address information indicated by the pit by irradiating the pit provided on the hologram recording medium 22. A servo laser beam is emitted to the dichroic mirror 18. The servo laser beam emitted from the servo laser device 19 is a beam having a predetermined wavelength that does not affect the hologram formed on the hologram recording medium 22. In the present embodiment, a blue laser beam is used as a laser beam emitted from the laser device 10, and a red laser beam having a longer wavelength than the blue laser beam is used as a servo laser beam.

  The servo laser beam is emitted from the servo laser device 19 when, for example, the hologram device starts to start and the servo laser beam starts to be emitted, and the servo laser beam is continuously emitted while the hologram device is started. is there. However, although the servo laser device 19 continues to emit the servo laser beam, it is not limited to this. For example, when data is recorded on the hologram recording medium 22 by the hologram apparatus, the hologram recording medium 22 is stopped. Therefore, the irradiation of the servo laser beam by the servo laser device 19 may be stopped during a period when the irradiation of the servo laser beam to the pit is not necessarily required. As a result, the load related to the emission of the servo laser beam of the servo laser device 19 can be reduced.

  The dichroic mirror 18 transmits the reference beam reflected by the galvo mirror 16 and makes it incident on the scanner lens 20. The dichroic mirror 18 reflects the servo laser beam emitted from the servo laser device 19 so as to enter the scanner lens 20.

  The scanner lens 20 irradiates the reference beam from the galvo mirror 16, which is incident on the dichroic mirror 18 and whose angle is adjusted by the galvo mirror control unit 17, to the hologram recording medium 22 without fail. Refract. Further, the scanner lens 20 causes the servo laser beam from the servo laser device 19 reflected by the dichroic mirror 18 to enter the hologram recording medium 22.

  The second shutter control unit 15 performs control for opening or closing the second shutter 14 based on an instruction signal from the CPU 1. The second shutter control unit 15 transmits an open state instruction signal to the second shutter 14 when the second shutter 14 is opened. The second shutter control unit 15 transmits a closed state instruction signal to the second shutter 14 when the second shutter 14 is closed.

  The second shutter 14 is in an open state based on the open state instruction signal from the second shutter control unit 15. Alternatively, the second shutter 14 enters a closed state based on a closed state instruction signal from the second shutter control unit 15. When the second shutter 14 is closed, the incidence of one laser beam separated by the PBS 13 on the SLM 9 is blocked. The second shutter 14 may be provided on the optical path of the data beam incident on the hologram recording medium 22 from the SLM 9 via the Fourier transform lens 21.

  The Fourier transform lens 21 condenses the data beam from the SLM 9 and applies it to the hologram recording medium 22 after performing a Fourier transform process on the data beam.

  The hologram storage medium 22 uses a photosensitive resin (for example, photopolymer, silver salt emulsion, dichromated gelatin, photoresist, etc.) that can store data as a hologram, and the photosensitive resin is sealed with a glass substrate. Configured. On the hologram recording medium 22, a hologram is formed by interference between the Fourier-transformed data beam indicating the two-dimensional gray image pattern from the Fourier transform lens 21 and the reference beam from the scanner lens 20. In the hologram recording medium 22, the angle of the galvo mirror 16 is adjusted by the galvo mirror control unit 17, and a hologram is formed again by the interference between the reference beam and the data beam from the galvo mirror 16 after the angle adjustment. Thus, angle multiplex recording is performed and a book is formed.

  Further, for example, a wobble is previously formed on the glass substrate constituting the hologram storage medium 22, and address information for determining the position of the hologram formed on the hologram storage medium 22 is used as the pit. Is formed in advance. Then, a servo laser beam from the servo laser device 19 incident from the scanner lens 20 is irradiated to a pit indicating address information. The servo laser beam after irradiating the pit indicating the address information is incident on the detector 23.

  The Fourier transform lens 26 is a beam diffracted by a hologram recorded on the hologram recording medium 22 (hereinafter referred to as a reproduction beam) when a reference beam is incident on the hologram recording medium 22 during hologram reproduction. Is incident. The incident angle of the reference beam at the time of reproducing the hologram is required to be the same as the incident angle of the reference beam at the time of recording the hologram to be reproduced. The Fourier transform lens 26 emits the reproduction beam subjected to the inverse Fourier transform to the image sensor 27.

  The image sensor 27 receives a reproduction beam that has been subjected to inverse Fourier transform from the Fourier transform lens 26. The image sensor 27 is composed of, for example, a CCD sensor or a CMOS sensor, and reproduces a two-dimensional grayscale image pattern set in the SLM 9 from the reproduction beam. Hereinafter, the reproduced two-dimensional gray image pattern is referred to as a captured image pattern. Based on the instruction signal from the image sensor control unit 28, the image sensor 27 converts the light / dark (light / dark) level of the captured image pattern into an electric signal strength level. Then, modulated image data of an analog amount corresponding to the light intensity of the captured image pattern is supplied to the filter 29. In this embodiment, when the image sensor control unit 28 determines that the image sensor 27 has been irradiated with a reproduction beam having a predetermined light amount or more, the image sensor control unit 28 causes the first shutter control unit 12 to An instruction signal for closing the shutter 11 is transmitted.

  In the present embodiment, the SLM 9 and the image sensor 27 can form patterns having the same image size (for example, 1280 pixels × 1280 pixels). In the present embodiment, the SLM 9 and the image sensor 27 can form patterns having the same image size, but the present invention is not limited to this. For example, the image size of the image sensor 27 may be set larger than the image size of the SLM 9. By setting the image size of the image sensor 27 to be larger than the image size of the SLM 9, the reproduction beam from the Fourier transform lens 26 is reliably irradiated to the image sensor 27, and the two-dimensional grayscale image pattern set in the SLM 9. Can be reliably reproduced. In addition, it is possible to reduce the requirement of the image sensor control unit 28 to move the image sensor 27 to a predetermined position with high accuracy.

  The filter 29 performs a filtering process on the analog image modulation image data supplied from the image sensor 27 in order to improve the separability of the binarization process by the decoder 30. In other words, the captured image pattern captured by the image sensor 27 has a separability between bright and dark spots compared to the two-dimensional gray image pattern set in the SLM 9 due to noise received by the data beam, reproduction beam, and the like. May get worse. In this case, the decoder 30 cannot appropriately determine whether the analog amount of the modulated image data is at a level indicating a bright point or a level indicating a dark point, and binarization processing is not appropriately performed. It becomes. Therefore, the filter 29 performs level correction of the modulated image data of an analog amount as its filter processing.

  In the present embodiment, a binarization processing unit (not shown) is provided between the filter 29 and the decoder 30, and binarization processing of the modulated image data subjected to the filter processing in the filter 29 is performed. Then, it is assumed that the digitized digital amount of modulated image data is supplied to the decoder 30.

  The decoder 30 is an embodiment of a “decoding processor” according to the present invention. The decoder 30 performs a decoding process according to the present invention on the modulated image data from the filter 29. A detailed description of the decoding process according to the present invention will be described later.

  The detector 23 receives the servo laser beam after the servo laser beam from the servo laser device 19 irradiates a pit indicating address information formed on the hologram recording medium 22. The detector 23 is composed of, for example, a four-divided photodiode, and transmits the light amount information of the servo laser beam detected by the four-divided photodiode to the disk control unit 24. In addition, the detector 23 transmits the address information to the CPU 1 based on the servo laser beam irradiated with the pits indicating the address information.

  The disk control unit 24 servo-controls the disk drive unit 25 based on the light amount information of the servo laser beam from the detector 23. In addition, the disk control unit 24 performs hologram recording for reproducing (or recording) a servo laser beam to irradiate a pit indicating desired address information of the hologram recording medium 22 based on an instruction signal from the CPU 1. An instruction signal for rotating the medium 22 is transmitted to the disk drive unit 25. Further, the disk control unit 24 rotates the hologram recording medium 22 to form a hologram at another position of the hologram recording medium 22 when the book is formed on the hologram recording medium 22. An instruction signal is transmitted to the disk drive unit 25.

  The memory 2 stores in advance program data for the CPU 1 to perform the processing described above. The memory 2 stores address information from pits formed on the hologram recording medium 22 from the CPU 1 described above. The memory 2 is composed of a nonvolatile memory element that can repeatedly write and read data by electrically erasing the data.

<First Embodiment>
<< Encoding >>
=== Overview ===
The flow of encoding according to the first embodiment of the present invention will be described based on the flowchart shown in FIG.

  First, the encoder 7 has a feature of a plurality of bits (for example, 3 bits or 4 bits) expressing a modulation code composed of a central bit and its peripheral bits and a feature quantity extracted according to the bit pattern of the modulation code. The quantity code is associated in advance (S200). Note that table information indicating this association is stored in the memory 71 in advance.

  The central bit is a bit used as a reference for extracting a feature value from the modulation code, and is normally set to 1 bit, but is not limited thereto. The peripheral bits of the central bit are usually set to 8 bits close to the central bit of 1 bit. However, the present invention is not limited to this, and it may be set to a bit located near the central bit. Good.

  Also, the modulation code is, for example, a 3 bit × 3 bit bit array composed of 1 central bit and 8 peripheral bits. The characteristic amount of the modulation code is a characteristic of the modulation code, for example, the distribution degree of “0/1”, the total number of “0/1”, the boundary (edge) of “0/1”, etc. using a plurality of bits. Quantified. For example, when the feature amount code is expressed in 3 bits, the feature amount extracted from the modulation code can be expressed as 2 to the third power (= 8). In the following, in order to quantify the characteristic amount of such a modulation code, a differential amount indicating the degree of state change of peripheral bits with respect to the central bit is employed.

  The encoder 7 stores the serial bit string data (for example, m × n bits) to be recorded transferred from the buffer 5 in the memory 71. The encoder 7 first performs an existing encoding process such as an error correction code assignment or a scramble process on the bit string data stored in the memory 71.

  Next, the encoder 7 divides the bit string data stored in the memory 71 into a plurality of bits and performs reading (S201). Then, the encoder 7 encodes each of the divided plurality of bits into a modulation code associated in advance with the feature code represented by each of the bits (S202). As a result, the encoder 7 forms modulated image data for setting a two-dimensional grayscale image pattern in the SLM 9 based on each of the encoded modulation codes (S203).

  The modulated image data is data in which modulation codes are arranged two-dimensionally. In addition, this arrangement order is, for example, an order having randomness such as so-called interleaving processing. As a result, a fixed recorded image pattern can be prevented from being formed, and the hologram recording / reproducing quality can be improved.

  As described above, in the encoding according to the first embodiment of the present invention, each of a plurality of bits divided from the bit string data to be recorded is finally encoded into a modulation code for setting a two-dimensional grayscale image pattern. Will be. Here, for example, in the case of a 3-bit modulation code composed of a central bit of 1 bit and peripheral bits of 2 bits adjacent thereto, the bright point of the pixel in the two-dimensional gray image pattern is expressed by “1”. Consider a case where a dark spot of a pixel in a two-dimensional gray image pattern is expressed as “0”.

  In this case, there may occur a case where adjacent pixels in the two-dimensional gray image pattern continuously indicate a bright point or a dark point, such as “110”, “011”, “001”, and “100”. As a result, in the diffracted light (reproduced beam) at the time of reproducing the hologram, there is a high possibility that the area of the light beam surface of the reproduced beam indicating the bright spot and the dark spot is increased. In other words, the state quantity of the bright and dark points of the captured image pattern changes during hologram reproduction due to factors such as light diffraction, but there is a high possibility that the amount of change (degree of luminance change) can be reduced. . Therefore, in the captured image pattern at the time of hologram reproduction, the separability between the bright spot and the dark spot becomes high, and the hologram reproduction quality can be improved.

  Further, in the encoding according to the first embodiment of the present invention, a plurality of bits divided from the bit string data to be recorded are associated with the feature code represented by the plurality of bits, and consequently the modulation code. Here, the feature quantities extracted according to the bit pattern of the modulation code are easily distinguishable from each other due to their nature. For this reason, even if the bright point of one pixel of the captured image pattern becomes dark due to factors such as light diffraction, the influence of the state change of the one pixel on the whole can be reduced. Therefore, the hologram reproduction quality can be improved.

=== Features of Modulation Code ===
The feature amount of the modulation code according to the first embodiment of the present invention will be described in detail. Here, as described above, as the characteristic amount of the modulation code according to the first embodiment of the present invention, a differential amount indicating the degree of state change of the peripheral bits with respect to the central bit is employed. This differential quantity is expressed as a scalar quantity or a vector quantity. For example, a differential amount expressed as a scalar amount (hereinafter simply referred to as a scalar amount) is obtained by a horizontal or vertical primary spatial differential filter, a horizontal or vertical Sobel filter, a 4-neighbor or 8-neighbor Laplacian filter. It is to be calculated. On the other hand, the differential amount expressed as a vector amount (hereinafter simply referred to as a vector amount) is, for example, a horizontal calculated by a horizontal spatial and vertical primary spatial differential filter or a horizontal and vertical Sobel filter. It is calculated by the differential amount of each of the direction component and the vertical direction component. The horizontal direction and the vertical direction described above are the two coordinate axis directions with the central bit in the orthogonal coordinate system as the origin.

  Here, it can be said that the vector amount appropriately represents the degree of state change over the entire modulation code, that is, the characteristics of the bit pattern of the modulation code, as compared with the scalar differential amount. Therefore, as the differential amount according to the first embodiment of the present invention, a vector amount represented by the differential amount in each of the two coordinate axis directions with the central bit in the orthogonal coordinate system as the origin is adopted. As a result, it is possible to further reduce the influence of the change in the state of one pixel of the captured image pattern on the whole during hologram reproduction. Therefore, the hologram reproduction quality can be improved.

  A vector amount extraction method will be described with reference to FIGS.

  FIG. 4 is a diagram for explaining a vector amount extraction method using horizontal and vertical Sobel filter coefficients according to the present invention.

  In the orthogonal coordinate system defined by the X-axis and the Y-axis, a central grid of 3 × 3 grids (modulation codes) represents one central bit. Here, when the coordinate of the central grid is (x, y), the central bit is expressed as f (x, y). In addition, eight peripheral grids close to the central grid represent 8-bit peripheral bits. Similarly, the 8-bit peripheral bits are f (x-1, y-1), f (x, y-1), f (x + 1, y-1), f (x-1, y), f ( x + 1, y), f (x-1, y + 1), f (x, y + 1), and f (x + 1, y + 1), respectively.

  Here, each bit value of the central bit and the peripheral bit is multiplied by a differential filter coefficient (weight), and a linear sum obtained by adding the multiplication results is calculated. That is, this linear sum is a vector quantity v (x, y) indicating the degree of state change of the peripheral bits with respect to the central bit. In the example shown in FIG. 4, an example is shown in which horizontal and vertical Sobel filter coefficients are used as the above-described differential filter coefficients.

First, the horizontal direction Sobel filter coefficient calculates the degree of change in the horizontal direction (X-axis direction) of the peripheral bits with respect to the central bit f (x, y), that is, the X-axis direction component fx of the vector quantity v (x, y). 3 × 3 differential filter coefficients for The horizontal direction Sobel filter coefficients expressed by the 3 × 3 differential filter coefficients correspond to the central bit and the peripheral bit of the 3 × 3 lattice, respectively. In the horizontal direction Sobel filter coefficient, the differential filter coefficients of the peripheral bits f (x−1, y) and f (x + 1, y) having the same Y coordinate as the central bit f (x, y) are emphasized. It becomes. Here, the X-axis direction component fx of the vector quantity v (x, y) using the horizontal direction Sobel filter coefficient is obtained by, for example, the following (Expression 1).
fx = 1 × f (x + 1, y + 1) + 2 × f (x + 1, y) + 1 × f (x + 1, y−1)
−1 × f (x−1, y + 1) −2 × f (x−1, y) −1 × f (x−1, y−1)
... (Formula 1)

On the other hand, the vertical direction Sobel filter coefficient determines the degree of change in the vertical direction (Y-axis direction) of the peripheral bits with respect to the central bit f (x, y), that is, the Y-axis direction component fy of the vector quantity v (x, y). 3 × 3 differential filter coefficients for The horizontal direction Sobel filter coefficients expressed by the 3 × 3 differential filter coefficients correspond to the central bit and the peripheral bit of the 3 × 3 lattice, respectively. In the horizontal direction Sobel filter coefficient, the differential filter coefficients of the peripheral bits f (x, y + 1) and f (x, y-1) having the same X coordinate as the central bit f (x, y) are emphasized. It becomes. Here, the Y-axis direction component fy of the vector quantity v (x, y) using the vertical direction Sobel filter coefficient is obtained by, for example, the following (Expression 2).
fy = 1 × f (x−1, y + 1) + 2 × f (x, y + 1) + 1 × f (x + 1, y + 1)
−1 × f (x−1, y−1) −2 × f (x, y−1) −1 × f (x + 1, y−1)
... (Formula 2)

Here, using the X-axis direction component fx and the Y-axis direction component fy of the vector quantity v (x, y), the magnitude component | v (x, y) | Calculated by the theorem. Further, the direction component θ of the vector quantity v (x, y) is obtained by an arctan function. The magnitude component | v (x, y) | of the vector quantity v (x, y) is obtained by the following (formula 3), and the direction component θ of the vector quantity v (x, y) is given by the following (formula 4). I want.
| V (x, y) | = √ (fx ^ 2 + fy ^ 2) (Formula 3)
θ = arctan (fy ÷ fx) (Formula 4)

  Thus, the vector quantity v (x, y) is extracted from the modulation code using the horizontal and vertical Sobel filter coefficients. The vector quantity v (x, y) may be extracted from the target image data using the horizontal and vertical primary spatial differential filter coefficients shown in FIG. In this case, the X-axis direction component fx and the Y-axis direction component fy of the vector quantity v (x, y) are obtained by, for example, the following (Expression 5) and (Expression 6).

fx = 1 × f (x + 1, y + 1) + 1 × f (x + 1, y) + 1 × f (x + 1, y−1)
−1 × f (x−1, y + 1) −1 × f (x−1, y) −1 × f (x−1, y−1)
... (Formula 5)
fy = 1 × f (x−1, y + 1) + 1 × f (x, y + 1) + 1 × f (x + 1, y + 1)
−1 × f (x−1, y−1) −1 × f (x, y−1) −1 × f (x + 1, y−1)
... (Formula 6)

  FIG. 6 shows a specific example of the vector amount extracted from the modulation code. In FIG. 6, the horizontal and vertical Sobel filter coefficients shown in FIG. 4 are used. Further, in FIG. 6, although the details will be described later, since the modulation code is designed to express only the directional component θ of the vector quantity, the magnitude component of the vector quantity is not obtained.

First, attention is paid to the 3 × 3 bit modulation code A having the central bit A (“1”). Here, by substituting each bit value of the modulation code A into the above-described equations (1), (2), and (4), the X-axis direction component fx, the Y-axis direction component fy, and the direction component of the vector quantity Each θ is obtained as follows.
fx = 1 + 2 + 1−0−0−0 = 4
fy = 0 + 0 + 1−0−0−1 = 0
θ = arctan (0 ÷ 4) = 0 (rad)

When attention is paid to the 3 × 3 bit modulation code B having the central bit B (“0”), as in the case of the modulation code A, each bit value of the target image data B is expressed by the above-described formula ( By substituting into 1), Expression (2), and Expression (4), the X-axis direction component fx, the Y-axis direction component fy, and the direction component θ of the vector quantity are obtained as follows.
fx = 1 + 2 + 1-1-0-0 = 3
fy = 1 + 2 + 1-0-0-1 = 3
θ = arctan (3 ÷ 3) = π / 4 (rad)

=== Feature code ===
A feature amount code representing the feature amount of the modulation code according to the first embodiment of the present invention will be described. As described above, a vector amount is employed as the feature amount of the modulation code. Here, the magnitude component of the vector quantity represents the ratio of “1” and “0” in the modulation code, that is, the density ratio in the two-dimensional density image pattern. On the other hand, the direction component θ of the vector quantity represents the distribution of “1” and “0” in the modulation code, that is, the distribution tendency of light and shade in the two-dimensional light and shade image pattern. Therefore, if the change in the pixel state of the captured image pattern during hologram reproduction increases due to factors such as light diffraction, the magnitude component of the vector quantity is significantly affected compared to the direction component of the vector quantity. Become. That is, it does not make much sense to express the magnitude component of the vector quantity as the modulation code according to the present invention. Therefore, only the direction component of the vector quantity is expressed as the modulation code. As a result, the redundancy of the modulation code, that is, the bit use efficiency is improved.

FIG. 7 shows an example of the correspondence between the direction component θ of the vector quantity and the feature quantity code.
The example shown in FIG. 7 is a case where the direction component θ of the vector quantity obtained by the above-described calculation is associated with a 3-bit feature quantity code. In other words, the direction component θ of the vector quantity obtained by the above-described calculation is limited to 2 3 (8) representations. For this reason, the direction component θ of the vector quantity obtained by the above-described calculation has a range of “0 to 2π (rad)”, but “0, π / 4, π / 2, 3π / 4, π, It belongs to any one of 8 regions partitioned by 5π / 4, 3π / 2, and 7π / 4 (rad) ". Here, for the sake of convenience, any one of the codes “0 to 7” is assigned to the angle for dividing the direction component θ of the vector amount obtained by the above-described calculation. For example, when “π / 8” is obtained as the directional component θ of the vector quantity by the above-described calculation, “π / 8” belongs to the region of “0 to π / 4 (rad)” and a code of “0” is given. Is done.

  As shown in FIG. 7, the direction component θ of the vector quantity to which the code of “0 to 7” is assigned is respectively the feature quantity code of “000, 001, 010, 011, 100, 101, 110, 111”. It shall be uniquely associated with the order. For example, in the example shown in FIG. 6, the direction component θ of the vector quantity extracted from the modulation code A is “0 (rad)”, and a code of “0” is given. Therefore, the code “0” is associated with the feature code “000”. As a result, when the bit string data to be recorded is divided every 3 bits, when these 3 bits are “000”, this “000” is finally encoded into the modulation code A shown in FIG. Will be.

  On the other hand, in the example shown in FIG. 6, the direction component θ of the vector quantity extracted from the modulation code B is “π / 4 (rad)”, and a code “1” is given. Therefore, the code “1” is associated with the feature code “001”. As a result, when the bit string data to be recorded is divided every 3 bits, when these 3 bits are “001”, this “001” is finally encoded into the modulation code B shown in FIG. Will be.

  Of course, the correspondence between the direction component θ of the vector quantity and the feature quantity code is not necessarily unique. For example, the feature quantity codes “000, 111, 001, 110, 010, 101, 010, 100” are sequentially associated with the direction component θ of the vector quantity to which the codes “0 to 7” are assigned. Will be. Further, the feature amount code does not necessarily have to be a 3-bit expression. If the feature amount code is expressed in more bits than 3 bits, the direction component θ of the calculated vector amount can be expressed with high accuracy.

=== Modulation code ===
FIG. 8 shows an example of the modulation code.
In the example shown in FIG. 8, the modulation code is represented as a two-dimensional bit array (where n is a natural number) of (2n + 1) rows × (2n + 1) columns. This two-dimensional bit array has one bit value (“1”) for setting a pixel of the two-dimensional gray image pattern as a bright point and the other bit value (“1”) for setting the pixel as a dark point ( "0"). Furthermore, the modulation code represents 8n direction components θ of 8n bits located at the outer edge of the two-dimensional bit array and vector quantities. In the example shown in FIG. 8, “n = 1” is used, and the modulation code is a 3D × 3D two-dimensional bit array, and the vector amount is 8 bits excluding the central bit of the two-dimensional bit array. Eight directional components θ are expressed.

  Here, as shown in FIG. 4, the center bit of the 3D × 3 column two-dimensional bit array shown in FIG. 8 is expressed as f (x, y), and its peripheral bits are expressed as f (x−1). , Y-1), f (x, y-1), f (x + 1, y-1), f (x-1, y), f (x + 1, y), f (x-1, y + 1), f These are expressed as (x, y + 1) and f (x + 1, y + 1), respectively.

  For example, the direction component θ of the vector quantity to which the code of “0” is given is based on the central bit f (x, y) in the modulation code as a reference from the central bit f (x, y) to the peripheral bit f (x + 1, This represents the direction to y), that is, the right direction in FIG. Therefore, the direction component θ of the vector quantity to which the code “0” is assigned is the center bit f (x, y) in the 3 rows × 3 columns two-dimensional bit array (modulation code) in the right direction in FIG. ) To the peripheral bit f (x + 1, y). Therefore, the central bit f (x, y) and the peripheral bit f (x + 1, y) are set to “1” indicating the bright point of the two-dimensional gray image pattern.

  Similarly, the direction component θ of the vector amount to which the code “1” is assigned expresses the upper right direction in FIG. 4 with reference to the center bit f (x, y) in the modulation code. . Therefore, the central bit f (x, y) and the peripheral bit f (x + 1, y + 1) in the modulation code are set to “1” indicating the bright point of the two-dimensional grayscale image pattern.

  Similarly, the direction component θ of the vector quantity to which the code “2” is assigned expresses the upward direction in FIG. 4 with reference to the center bit f (x, y) in the modulation code. Therefore, the central bit f (x, y) and the peripheral bit f (x, y + 1) in the modulation code are set to “1” indicating the bright point of the two-dimensional grayscale image pattern.

  Similarly, the direction component θ of the vector quantity to which the code “3” is assigned represents the upper left direction in FIG. 4 with reference to the center bit f (x, y) in the modulation code. . Therefore, the central bit f (x, y) and the peripheral bit f (x−1, y + 1) in the modulation code are set to “1” indicating the bright point of the two-dimensional grayscale image pattern.

  Similarly, the direction component θ of the vector amount to which the code “4” is assigned represents the left direction in FIG. 4 with reference to the center bit f (x, y) in the modulation code. Therefore, the central bit f (x, y) and the peripheral bit f (x−1, y) in the modulation code are set to “1” indicating the bright point of the two-dimensional grayscale image pattern.

  Similarly, the directional component θ of the vector quantity to which the code “5” is assigned represents the diagonally lower left direction in FIG. 4 with reference to the center bit f (x, y) in the modulation code. . For this reason, the central bit f (x, y) and the peripheral bit f (x-1, y-1) in the modulation code are set to “1” indicating the bright point of the two-dimensional grayscale image pattern.

  Similarly, the direction component θ of the vector quantity to which the code “6” is assigned represents the downward direction in FIG. 4 with the central bit f (x, y) in the modulation code as a reference. Therefore, the central bit m (x, y) and the peripheral bit m (x, y−1) in the modulation code are set to “1” indicating the bright point of the two-dimensional grayscale image pattern.

  Similarly, the direction component θ of the vector amount to which the code “7” is assigned represents the right direction in FIG. 4 with reference to the center bit f (x, y) in the modulation code. Therefore, the central bit m (x, y) and the peripheral bit m (x + 1, y) in the modulation code are set to “1” indicating the bright point of the two-dimensional grayscale image pattern.

  As described above, the modulation code is expressed by a two-dimensional bit array of (2n + 1) rows × (2n + 1) columns. Further, the modulation code uses 8n bits located at the outer edge of the two-dimensional bit array to express the direction component θ of 8n vector quantities. Here, the greater the number of bits of the modulation code, the higher the degree of freedom of pattern configuration, and the direction component θ of the vector quantity obtained by the above-described calculation can be expressed with high accuracy.

  In addition, since the central bit of the modulation code and its peripheral bits are set to “1” at the same time, the range of the bright point of the captured image pattern at the time of hologram reproduction is increased, and the separation of the bright point and the dark point is improved. It will be. Further, it is possible to arrange bright and dark spots in a well-balanced manner as a two-dimensional gray image pattern, and there is a possibility that uniform characteristics can be obtained in hologram recording that consumes a finite monomer.

  Further, as shown in FIG. 8, the modulation code is expressed by a two-dimensional bit array of 3 rows × 3 columns, and 8 direction components θ of the vector quantity are expressed using 8 bits excluding the central bit. I will do it. As a result, the trade-off relationship between redundancy and separability is established with a good balance.

  In the modulation code shown in FIG. 8, when expressing each direction component θ of the vector quantity, one bit indicating each direction component θ and one or a plurality of peripheral bits represent pixels of the two-dimensional grayscale image pattern. It is assumed that one bit value (“1”) for setting a bright point is set. That is, in the modulation code, when expressing the direction components of 8n vector amounts, a plurality of bit values (for example, “1”) indicating a bright point can be made close to each other.

  For example, in the direction component θ of the vector quantity to which the code “0” is assigned, the central bit f (x, y) and the peripheral bit f (x + 1, 2) in the 3 × 3 two-dimensional bit array (modulation code). In addition to y), the peripheral bits f (x + 1, y + 1) and f (x + 1, y−1) are also set to “1” indicating the bright point of the two-dimensional grayscale image pattern. That is, in the 3D × 3D two-dimensional bit array, the bits indicating “1” are the central bit f (x, y), the peripheral bits f (x + 1, y), f (x + 1, y + 1), f It is arranged close to (x + 1, y-1).

  As a result, in the diffracted light (reproduced beam) at the time of hologram reproduction, there is a high possibility that the area of the light beam surface of the reproduced beam indicating the bright point will increase, and the state change amount of the bright point due to factors such as diffraction can be reduced. The possibility increases. Therefore, in the captured image pattern at the time of hologram reproduction, the separability between the bright spot and the dark spot becomes high, and the hologram reproduction quality can be improved.

  In the modulation code shown in FIG. 8, the central bit f (x, y) is set to one bit value (“1”) for setting the pixel of the two-dimensional grayscale image pattern as a bright point. However, the present invention is not limited to this. That is, the central bit f (x, y) is irrelevant in the expression of the modulation code related to the direction component θ of the vector quantity described above.

  Therefore, as shown in FIG. 9, the central bit f (x, y) of the modulation code is set to one bit value (“1”) or the other bit value (“0”), thereby obtaining a vector quantity. 16n direction components can be expressed. That is, the central bit m (x, y) of the modulation code is set to one or the other bit value, so that the direction component θ of the vector quantity is expressed by being multiplexed from 8n to 16n. Thus, the bit use efficiency is improved. In the example shown in FIG. 9, eight directional components of the vector quantity shown in FIG. 8 are multiplexed into 16 directional components.

=== Specific Example of Encoding ===
A specific example of encoding (including decoding) according to the first embodiment of the present invention is shown in FIG. In the specific example shown in FIG. 10, the feature amount code is expressed in 3 bits, the modulation code is expressed as a two-dimensional bit array of 3 rows × 3 columns, and the direction component of the vector amount is used as the feature amount of the modulation code. This is a case where θ is adopted.

  As shown in FIG. 10, the bit string data to be recorded is divided every 3 bits so as to be surrounded by a dotted line. This bit string data has already been subjected to existing encoding processing such as error correction code assignment and scramble processing. Here, each of the divided three bits represents a direction component θ of a vector amount set in advance. For example, when the divided 3 bits are “000”, the divided 3 bits “000” represents “000” of the feature amount code. Further, “000” of the feature amount code is associated with “0” of the direction component θ of the vector amount. Therefore, the divided 3 bits “000” are encoded into a modulation code of 3 rows × 3 columns associated with “0” of the direction component θ of the vector quantity.

  This series of processing is performed for all the divided 3 bits in the recording bit string data. Then, modulated image data that is an arrangement pattern of modulation codes as shown in FIG. 10 is formed. In addition, a two-dimensional gray image pattern corresponding to “0” and “1” of the modulated image data is set in the SLM 9.

<< Decryption >>
=== Overview ===
The flow of decoding according to the first embodiment of the present invention will be described based on the flowchart shown in FIG.

  The decoding according to the first embodiment of the present invention corresponds to the above-described encoding according to the first embodiment of the present invention. That is, a modulation code composed of a central bit and peripheral bits of the central bit and a multi-bit feature amount code expressing a feature amount extracted according to the bit pattern of the modulation code are associated in advance. The bit string data to be recorded is divided into a plurality of bits, and each divided plurality of bits is encoded into a modulation code associated with the feature amount code represented by the plurality of bits, and the encoding is performed. It is assumed that a two-dimensional gray image pattern set in the SLM 9 is recorded on the hologram recording medium 22 based on each modulation code.

  First, the image sensor 27 receives diffracted light obtained as a result of making a coherent reproduction reference beam incident on the hologram recording medium 22 via the Fourier transform lens 26. As a result, the image sensor 27 captures the two-dimensional gray image pattern recorded on the hologram recording medium 22 (S300). The analog amount of the two-dimensional gray image pattern captured by the image sensor 27 is converted into digital amount of modulated image data through filter processing and binarization processing in the filter 20 (S301).

  Based on the digital amount of modulated image data, the decoder 30 obtains a feature amount code associated with each modulation code constituting the modulated image data (S302). Then, the decoder 30 decodes the bit string data to be reproduced with the obtained feature amount code (S303).

=== Example of Decoding ===
A specific example of decoding (including encoding) according to the first embodiment of the present invention is shown in FIG.

  As shown in FIG. 10, the image sensor 27 reproduces a two-dimensional gray image pattern at the time of hologram recording. Further, the reproduced two-dimensional gray image pattern is converted into an array pattern of modulation codes of 3 rows × 3 columns, that is, modulated image data, through filter processing and binarization processing. Next, the direction component θ of the vector quantity corresponding to the modulation code surrounded by the dotted line in the modulated image data is obtained. The direction component θ of the vector quantity is associated with a 3-bit feature quantity code, that is, the original 3 bits divided at the time of encoding. Therefore, the original bit string data at the time of encoding is decoded by the feature amount code corresponding to the direction component θ of the obtained vector amount.

  Thus, the decoder 30 is provided assuming that the hologram apparatus according to the first embodiment of the present invention reproduces the encoded two-dimensional grayscale image pattern according to the first embodiment of the present invention. It was. That is, in this case, the hologram apparatus according to the first embodiment of the present invention can cope with hologram recording and reproduction corresponding to the encoding according to the first embodiment of the present invention. Note that it may be assumed that the hologram that has been encoded according to the first embodiment of the present invention and recorded on the hologram recording medium 22 is reproduced by another reproduction-only hologram device. Therefore, the reproduction-only hologram apparatus according to the first embodiment of the present invention is provided with a decoder 30 that performs decoding according to the first embodiment of the present invention.

Second Embodiment
<< Encoding >>
=== Overview ===
An encoding flow according to the second embodiment of the present invention will be described based on the flowchart shown in FIG.

  First, the encoder 7 arranges and stores the recording bit string data (for example, m × n bits) transferred from the buffer 5 in the memory 71 into two-dimensional data (for example, m bits × n bits) (S1100). Then, the encoder 7 extracts the feature amount of the target image data composed of the target bit of the two-dimensional data arranged in the memory 71 and the peripheral bits of the target bit (S1101).

  Note that the bit of interest is a bit of interest for extracting a feature amount from two-dimensional data, and is usually set to 1 bit, but is not limited thereto. This attention bit is finally converted into a modulation code to be described later in the process of encoding.

  The peripheral bits of the target bit are usually set to 8 bits that are close to the target bit of 1 bit. However, the present invention is not limited to this, and it may be set to a bit located in the vicinity of the target bit. Good.

  The attention image data is set to extract a feature amount for the attention bit. The attention image data has, for example, a 3 bit × 3 bit bit arrangement composed of one attention bit and eight peripheral bits.

  Further, the feature amount of the target image data is obtained by quantifying the features of the target image data, for example, the distribution degree of 0/1, the total number of 0/1, the boundary (edge) of 0/1, and the like. In the following, in order to quantify the feature amount of such attention image data, a differential amount indicating the degree of state change of surrounding bits with respect to the attention bit is adopted.

  The encoder 7 encodes the feature amount extracted from the target image data into a multi-bit modulation code that is associated with the feature amount in advance (S1102). Then, the encoder 7 forms modulated image data for setting a two-dimensional grayscale image pattern in the SLM 9 based on each of the coded modulation codes (S1103). That is, the modulated image data is data in which modulation codes are two-dimensionally arranged.

  Thus, in the second embodiment according to the present invention, the bit of interest of the two-dimensional data is finally encoded into a multi-bit modulation code for setting a two-dimensional gray image pattern; Become. Here, for example, when a 3-bit modulation code is used, a bright point of a pixel in a two-dimensional gray image pattern is represented by “1”, and a dark point of a pixel in a two-dimensional gray image pattern is represented by “0”. think of.

  In this case, there may occur a case where adjacent pixels in the two-dimensional gray image pattern continuously indicate a bright point or a dark point, such as “110”, “011”, “001”, and “100”. As a result, in the diffracted light (reproduced beam) at the time of reproducing the hologram, there is a high possibility that the area of the light beam surface of the reproduced beam indicating the bright spot and the dark spot is increased. In other words, the state quantity of the bright and dark points of the captured image pattern changes during hologram reproduction due to factors such as light diffraction, but there is a high possibility that the amount of change (degree of luminance change) can be reduced. . Therefore, in the captured image pattern at the time of hologram reproduction, the separability between the bright spot and the dark spot becomes high, and the hologram reproduction quality can be improved.

  Further, in the second embodiment according to the present invention, the feature amount (from the target image data including the target bit is not performed by encoding the target bit of the two-dimensional data, that is, the state amount of the target bit as it is into the modulation code. For example, after extracting the above-described vector amount), the feature amount is encoded. For this reason, even if the bright point of one pixel of the captured image pattern may become dark due to factors such as light diffraction, the captured image pattern is one of the captured image patterns as long as the encoding is performed based on the relationship between the bit of interest and the peripheral bits. It is possible to reduce the influence of the pixel state change on the whole. Therefore, the hologram reproduction quality can be improved.

=== Extraction of feature amount of attention image data ===
The feature amount extraction of the target image data according to the second embodiment of the present invention will be described in detail.
Here, the feature amount of the target image data according to the second embodiment of the present invention may be the same as the feature amount of the modulation code in the first embodiment of the present invention. Therefore, as the description of the feature amount of the target image data according to the second embodiment of the present invention, the above description of the feature amount of the modulation code is used instead. Similarly, since the modulation code according to the second embodiment of the present invention is exactly the same as the modulation code according to the first embodiment of the present invention, the modulation code according to the first embodiment of the present invention Substitute the description.

  Based on FIG. 13, a method of extracting a vector amount from the target image data will be described. In FIG. 13, the horizontal and vertical Sobel filter coefficients shown in FIG. 4 are used.

First, attention is focused on 3 × 3 bit attention image data A having attention bit A (“1”) as the attention bit in the bit string data to be recorded arranged as two-dimensional data. Here, by substituting each bit value of the target image data A into the above-described Expression (1), Expression (2), and Expression (4), the X-axis direction component fx, the Y-axis direction component fy, and the direction of the vector amount Each component θ is obtained as follows.
fx = 1 + 2 + 1−0−0−0 = 4
fy = 0 + 0 + 1−0−0−1 = 0
θ = arctan (0 ÷ 4) = 0 (rad)

When attention is paid to the 3 × 3 bit attention image data B with the attention bit B (“0”) as the attention bit, each bit value of the attention image data B is changed as in the case of the attention image data A. By substituting into the above-described equations (1), (2), and (4), the X-axis direction component fx, the Y-axis direction component fy, and the direction component θ of the vector quantity can be obtained as follows. Become.
fx = 1 + 2 + 1-1-0-0 = 3
fy = 1 + 2 + 1-0-0-1 = 3
θ = arctan (3 ÷ 3) = π / 4 (rad)

=== Specific Example of Encoding ===
A specific example of encoding (including decoding) according to the second embodiment of the present invention is shown in FIG. In the specific example shown in FIG. 14, the target image data and the modulation code are each expressed as a two-dimensional bit array of 3 rows × 3 columns, and the vector component direction component θ is used as the feature amount of the target image data. It is.

  As shown in FIG. 14, in the bit string data to be recorded arranged in the two-dimensional data, attention image data of 3 rows × 3 columns surrounded by a dotted line is noticed. Then, the direction component θ of the vector quantity indicating the degree of state change of the 8-bit peripheral bits with respect to the target bit in the target image data is calculated. Then, the bit of interest is encoded into a modulation code of 3 rows × 3 columns surrounded by a dotted line in the modulated image data by the direction component θ of the calculated vector quantity. This series of processing is performed for all the target bits in the recording bit string data. Then, modulation image data that is an arrangement pattern of modulation codes as shown in FIG. 14 is formed. As a result, a two-dimensional gray image pattern corresponding to “0” and “1” of the modulated image data is set in the SLM 9.

<< Decryption >>
=== Overview ===
The flow of decoding according to the second embodiment of the present invention will be described based on the flowchart shown in FIG.

  The decoding according to the present invention corresponds to the above-described encoding according to the present invention. That is, the bit string data to be recorded is arranged into two-dimensional data, the feature amount of the attention image data composed of the attention bit of the arranged two-dimensional data and the peripheral bits of the attention bit is extracted, and the extracted feature A two-dimensional grayscale image pattern set in the SLM 9 is encoded based on the modulation image data configured by the encoded modulation code by encoding into a modulation code of a plurality of bits that is associated with the amount in advance. It is assumed that the information is recorded on the hologram recording medium 22.

  First, the image sensor 27 receives diffracted light obtained by making a coherent reproduction reference beam incident on the hologram recording medium 22 via the Fourier transform lens 26. As a result, the image sensor 27 takes in the two-dimensional gray image pattern recorded on the hologram recording medium 22 (S1200). The analog amount of the two-dimensional gray image pattern captured by the image sensor 27 is converted into digital amount of modulated image data through filter processing and binarization processing in the filter 20 (S1201).

  Based on the digital amount of the modulated image data, the decoder 30 obtains a feature amount associated with the modulation code constituting the modulated image data. Further, the decoder 30 obtains attention image data from which the obtained feature amount can be extracted (S1202). Then, the decoder 30 decodes the bit string data to be reproduced by the attention bit in the obtained attention image data.

=== Example of Decoding ===
A specific example of decoding (including encoding) according to the present invention is shown in FIG. In the specific example shown in FIG. 14, the target image data and the modulation code are each expressed as a two-dimensional bit array of 3 rows × 3 columns, and the vector component direction component θ is used as the feature amount of the target image data. It is.

  As shown in FIG. 14, the image sensor 27 reproduces a two-dimensional gray image pattern at the time of hologram recording. Further, the reproduced two-dimensional gray image pattern is converted into an array pattern of modulation codes of 3 rows × 3 columns, that is, modulated image data, through filter processing and binarization processing. Next, the direction component θ of the vector quantity corresponding to the modulation code surrounded by the dotted line in the modulated image data is obtained. In addition, from the direction component θ of the vector quantity, attention image data of 3 rows × 3 columns surrounded by a dotted line is obtained. This series of processing is performed for each modulation code in the modulated image data. Finally, the target bit of each obtained target image data is arranged, and the bit string data to be reproduced, that is, the original recorded bit string data is decoded.

  Thus, the decoder 30 is provided assuming that the hologram apparatus according to the second embodiment of the present invention reproduces the encoded two-dimensional grayscale image pattern according to the second embodiment of the present invention. It was. That is, in this case, the hologram apparatus according to the second embodiment of the present invention can cope with hologram recording and reproduction corresponding to the encoding according to the second embodiment of the present invention. Note that it is also assumed that a hologram that has been encoded according to the second embodiment of the present invention and recorded on the hologram recording medium 22 is reproduced by another reproduction-only hologram device. Thus, as a reproduction-only hologram apparatus according to the second embodiment of the present invention, a decoder 30 that performs decoding according to the second embodiment of the present invention is provided.

  As mentioned above, although embodiment of this invention was described, embodiment mentioned above is for making an understanding of this invention easy, and is not for limiting and interpreting this invention. The present invention can be changed / improved without departing from the gist thereof, and the present invention includes equivalents thereof.

It is a figure which shows the structure of the hologram apparatus which concerns on this invention. It is a flowchart which shows the flow of the encoding process of the hologram apparatus which concerns on 1st Embodiment of this invention. It is a flowchart which shows the flow of the decoding process of the hologram apparatus which concerns on 1st Embodiment of this invention. It is a figure explaining the extraction method of the feature-value (vector quantity) using the Sobel filter coefficient based on this invention. It is a figure which shows the 1st spatial differential filter coefficient based on this invention. It is a figure which shows the specific example of the feature-value (vector amount) extracted from the modulation code which concerns on 1st Embodiment of this invention. It is a figure which shows matching with the feature-value (vector amount) and feature-value code | cord | chord which concern on 1st Embodiment of this invention. It is a figure which shows the example of the modulation code which concerns on this invention. It is a figure which shows the other example of the modulation code based on this invention. It is a figure which shows the specific example of the encoding / decoding which concerns on 1st Embodiment of this invention. It is a flowchart which shows the flow of the encoding process of the hologram apparatus which concerns on 2nd Embodiment of this invention. It is a flowchart which shows the flow of the decoding process of the hologram apparatus which concerns on 2nd Embodiment of this invention. It is a figure which shows the specific example of the feature-value (vector amount) extracted from the attention image data which concerns on 2nd Embodiment of this invention. It is a figure which shows the specific example of the encoding / decoding which concerns on 2nd Embodiment of this invention. It is the figure which showed typically a mode when a monomer changes to a polymer in the hologram recording medium. It is a figure explaining the recording format of the medium for hologram recording. It is the figure which displayed the brightness | luminance of the pixel of the conventional captured image data, and its appearance frequency as a histogram.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 CPU 2 Memory 3 Interface 4 Connection terminal 5 Buffer 6 Playback | recording discrimination | determination part 7 Encoder 8 Mapping process part 9 SLM 10 Laser apparatus 11 1st shutter 12 1st shutter control part 13 PBS 14 2nd shutter 15 2nd shutter control part 16 galvo mirror 17 galvo mirror control unit 18 dichroic mirror 19 servo laser device 20 scanner lens 21, 26 Fourier transform lens 22 hologram recording medium 23 detector 24 disk control unit 25 disk drive unit 27 image sensor 28 image sensor control unit 29 filter 30 Decoder

Claims (20)

Hologram recording is performed so that an interference fringe reference beam and a coherent data beam corresponding to a two-dimensional gray image pattern set by a spatial light modulator are incident on a hologram recording medium to form interference fringes. In the hologram device to perform,
A modulation code configured by a central bit and peripheral bits of the central bit and a multi-bit feature amount code expressing a feature amount extracted according to the bit pattern of the modulation code are associated in advance,
Dividing bit string data to be recorded into the plurality of bits,
Each of the divided plurality of bits is encoded into the modulation code associated with the feature code represented by the plurality of bits,
Based on each of the encoded modulation codes, the spatial light modulator forms modulated image data for setting the two-dimensional gray image pattern,
A hologram apparatus comprising an encoding processing unit. An image sensor that receives diffracted light obtained by making a coherent reproduction reference beam incident on the hologram recording medium and captures the two-dimensional gray image pattern recorded on the hologram recording medium;
Based on the modulated image data corresponding to the captured two-dimensional gray image pattern, the feature amount code associated with each of the modulation codes constituting the modulated image data is obtained, and the feature amount code is used to determine the feature amount code. A decoding processing unit for decoding bit string data;
The hologram apparatus according to claim 1, comprising:   The hologram apparatus according to claim 1, wherein the feature amount is a differential amount indicating a degree of state change of the peripheral bits with respect to the central bit.   4. The hologram apparatus according to claim 3, wherein the feature amount is a vector amount expressed using the differential amount with respect to each of two coordinate axis directions with the central bit in the orthogonal coordinate system as an origin. . Hologram recording is performed so that an interference fringe reference beam and a coherent data beam corresponding to a two-dimensional gray image pattern set by a spatial light modulator are incident on a hologram recording medium to form interference fringes. In the hologram device to perform,
Arrange the bit string data to be recorded into two-dimensional data,
Extracting the feature amount of the target image data composed of the target bits of the arranged two-dimensional data and the peripheral bits of the target bits;
The extracted feature value is encoded into a modulation code of a plurality of bits previously associated with the feature value,
Forming modulated image data for setting the two-dimensional gray image pattern in the spatial light modulator based on the encoded modulation code;
A hologram apparatus comprising an encoding processing unit. An image sensor that receives diffracted light obtained by making a coherent reproduction reference beam incident on the hologram recording medium and captures the two-dimensional gray image pattern recorded on the hologram recording medium;
Based on the modulation image data corresponding to the captured two-dimensional grayscale image pattern, the feature amount associated with the modulation code constituting the modulation image data can be obtained, and the obtained feature amount can be extracted. A decoding processing unit that obtains the target image data, and decodes the bit string data by the target bits in the determined target image data;
The hologram apparatus according to claim 5, further comprising:   The hologram apparatus according to claim 5, wherein the feature amount is a differential amount indicating a degree of state change of the peripheral bit with respect to the bit of interest.   The hologram apparatus according to claim 7, wherein the feature amount is a vector amount expressed using the differential amount with respect to each of two coordinate axis directions with the target bit in the orthogonal coordinate system as an origin. .   The vector amount is a vector representation of two differential amounts of a horizontal component and a vertical component calculated by a horizontal and vertical primary spatial differential filter, or a horizontal and vertical Sobel filter, 9. The hologram apparatus according to claim 4 or 8, wherein:   10. The hologram apparatus according to claim 4, wherein the modulation code expresses only the direction component among the magnitude component and the direction component of the vector quantity.   The modulation code is a two-dimensional bit array (where n is a natural number) of (2n + 1) rows × (2n + 1) columns, and 8n direction components of the vector quantity are 8n bits located on the outer edge of the two-dimensional bit array. The hologram apparatus according to claim 10, wherein:   2. The modulation code is a two-dimensional bit array of 3 rows × 3 columns, and the eight direction components of the vector quantity are expressed by 8 bits excluding the central bit of the two-dimensional bit array. 11. The hologram device according to 11.   The modulation code is composed of one bit value for setting a pixel of the two-dimensional gray image pattern as a bright point and the other bit value for setting the pixel as a dark point, and the vector quantity 13. The hologram apparatus according to claim 11, wherein when expressing each of the directional components, one bit indicating each directional component and its peripheral bits are set as the one bit value.   14. The modulation code is configured to double a direction component represented by the vector quantity by setting a central bit of the two-dimensional bit array to one or the other bit value. The hologram device according to any one of the above. A hologram as an interference fringe formed by making a coherent recording reference beam and a coherent data beam corresponding to a two-dimensional gray image pattern set by a spatial light modulator enter a hologram recording medium. In the hologram apparatus for reproducing based on the diffracted light obtained by making a coherent reproducing reference beam incident on the hologram recording medium,
A modulation code configured by a central bit and peripheral bits of the central bit and a multi-bit feature amount code expressing a feature amount extracted according to the bit pattern of the modulation code are associated in advance,
Dividing bit string data to be recorded into the plurality of bits,
Each of the divided plurality of bits is encoded into the modulation code associated with the feature code represented by the plurality of bits,
When the two-dimensional gray image pattern set in the spatial light modulator is recorded on the hologram recording medium, based on each of the encoded modulation codes,
An image sensor that receives diffracted light obtained by making a coherent reproduction reference beam incident on the hologram recording medium and captures the two-dimensional gray image pattern recorded on the hologram recording medium;
Based on the modulated image data corresponding to the captured two-dimensional gray image pattern, the feature amount code associated with each of the modulation codes constituting the modulated image data is obtained, and the feature amount code is used to determine the feature amount code. A decoding processing unit for decoding bit string data;
A hologram device characterized by comprising: A hologram as an interference fringe formed by making a coherent recording reference beam and a coherent data beam corresponding to a two-dimensional gray image pattern set by a spatial light modulator enter a hologram recording medium. In the hologram apparatus for reproducing based on the diffracted light obtained by making a coherent reproducing reference beam incident on the hologram recording medium,
Arrange the bit string data to be recorded into two-dimensional data,
Extracting the feature amount of the target image data composed of the target bits of the arranged two-dimensional data and the peripheral bits of the target bits;
The extracted feature value is encoded into a modulation code of a plurality of bits previously associated with the feature value,
When the two-dimensional grayscale image pattern set in the spatial light modulator is recorded on the hologram recording medium, based on the modulated image data configured by the encoded modulation code,
An image sensor that receives the diffracted light and captures the two-dimensional gray image pattern recorded on the hologram recording medium;
Based on the modulation image data corresponding to the captured two-dimensional grayscale image pattern, the feature amount associated with the modulation code constituting the modulation image data can be obtained, and the obtained feature amount can be extracted. A decoding processing unit that obtains the target image data, and decodes the bit string data by the target bits in the determined target image data;
A hologram device characterized by comprising: Hologram recording is performed so that an interference fringe reference beam and a coherent data beam corresponding to a two-dimensional gray image pattern set by a spatial light modulator are incident on a hologram recording medium to form interference fringes. A method for encoding a hologram device, comprising:
A modulation code configured by a central bit and peripheral bits of the central bit and a multi-bit feature amount code expressing a feature amount extracted according to the bit pattern of the modulation code are associated in advance,
Dividing bit string data to be recorded into the plurality of bits,
Each of the divided plurality of bits is encoded into the modulation code associated with the feature code represented by the plurality of bits,
An encoding method comprising: forming modulated image data for setting the two-dimensional grayscale image pattern in the spatial light modulator based on each of the encoded modulation codes. Hologram recording is performed so that an interference fringe reference beam and a coherent data beam corresponding to a two-dimensional gray image pattern set by a spatial light modulator are incident on a hologram recording medium to form interference fringes. A method for encoding a hologram device, comprising:
Arrange the bit string data to be recorded into two-dimensional data,
Extracting the feature amount of the target image data composed of the target bits of the arranged two-dimensional data and the peripheral bits of the target bits;
The extracted feature value is encoded into a modulation code of a plurality of bits previously associated with the feature value,
An encoding method comprising: forming modulated image data for setting the two-dimensional grayscale image pattern in the spatial light modulator based on each of the encoded modulation codes. A hologram as an interference fringe formed by making a coherent recording reference beam and a coherent data beam corresponding to a two-dimensional gray image pattern set by a spatial light modulator enter a hologram recording medium. A method of decoding a hologram apparatus for performing reproduction based on diffracted light obtained by making a coherent reproduction reference beam incident on the hologram recording medium,
A modulation code configured by a central bit and peripheral bits of the central bit and a multi-bit feature amount code expressing a feature amount extracted according to the bit pattern of the modulation code are associated in advance,
Dividing bit string data to be recorded into the plurality of bits,
Each of the divided plurality of bits is encoded into the modulation code associated with the feature code represented by the plurality of bits,
When the two-dimensional gray image pattern set in the spatial light modulator is recorded on the hologram recording medium, based on each of the encoded modulation codes,
Receiving the diffracted light obtained by making a coherent reproduction reference beam incident on the hologram recording medium and capturing the two-dimensional gray image pattern recorded on the hologram recording medium;
Based on the modulated image data corresponding to the captured two-dimensional grayscale image pattern, the feature amount code associated with the modulation code constituting the modulated image data is obtained, and the bit string is obtained by the feature amount code. A decryption method characterized by decrypting data. A hologram as an interference fringe formed by making a coherent recording reference beam and a coherent data beam corresponding to a two-dimensional gray image pattern set by a spatial light modulator enter a hologram recording medium. A method of decoding a hologram apparatus for performing reproduction based on diffracted light obtained by making a coherent reproduction reference beam incident on the hologram recording medium,
Arrange the bit string data to be recorded into two-dimensional data,
Extracting the feature amount of the target image data composed of the target bits of the arranged two-dimensional data and the peripheral bits of the target bits;
The extracted feature value is encoded into a modulation code of a plurality of bits previously associated with the feature value,
When the two-dimensional grayscale image pattern set in the spatial light modulator is recorded on the hologram recording medium, based on the modulated image data configured by the encoded modulation code,
Receiving the diffracted light and capturing the two-dimensional gray image pattern recorded on the hologram recording medium;
Based on the modulated image data corresponding to the captured two-dimensional gray image pattern, the feature amount associated with the modulation code constituting the modulated image data is obtained,
Obtaining the attention image data from which the obtained feature amount can be extracted;
The decoding method, wherein the bit string data is decoded by the attention bit in the obtained attention image data.

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