Composite polarization state underwater calibration method and system based on equal focal length refraction model

Abstract

The invention relates to a composite polarization state underwater calibration method and system based on an equal-focal-length refraction model, and the method comprises the steps: carrying out the approximate processing of a light incident angle under a small angle condition through a simultaneous geometric optical imaging equation and the Snell refraction law, obtaining a conversion relation between the coordinates of an underwater imaging point and the coordinates of a theoretical imaging point in the air, and carrying out the calculation of the conversion relation; establishing an equal focal length refraction mathematical model of the underwater camera and the projector; designing a composite polarization state coding sine projection calibration pattern based on the model; encoding horizontal polarized light of a green channel of the calibration pattern to cosine fringes in the horizontal direction and encoding vertical polarized light of a blue channel to sine fringes in the vertical direction by using the projection characteristic of polarized light; and the cosine fringes and the sine fringes are superposed to generate two groups of cross-polarization state fringe patterns, and phase calculation and re-projection error optimization are performed on the cross-polarization state fringe patterns based on an equal-focal-length refraction mathematical model to realize parameter calibration. According to the method, the calibration precision and efficiency of underwater vision measurement can be remarkably improved.

Description

Technical Field

[0001] The present disclosure relates to the field of computer vision, and in particular to a composite polarization state underwater calibration method and system based on an equal-focal-length refraction model. Background Art

[0002] High-precision underwater three-dimensional topography measurement is irreplaceable in key areas such as marine resource exploration, underwater engineering construction, archaeological excavation, and biological research. Optical three-dimensional measurement technology, with its non-contact, high-precision, and high-resolution advantages, has achieved high maturity in air. However, when applied underwater, the difference in refractive index between water and air causes unpredictable path deflections due to Snell's law, leading to optical path distortion, image position offset, and equivalent system parameter drift, rendering traditional air-based calibration models completely ineffective. Furthermore, multiple scattering caused by suspended particles in the water significantly reduces fringe contrast, blurs image details, and introduces phase errors, severely degrading the signal-to-noise ratio and reconstruction accuracy. Existing methods for addressing refraction include: direct underwater calibration, refraction compensation models, "virtual pinhole camera" models, and iterative optimization methods based on ray tracing. Limitations of these methods include difficulty balancing model complexity and accuracy, strong sensitivity to window parameters (thickness, refractive index, and pose), and a general lack of effective mechanisms to suppress scattering noise, resulting in significant noise interference in calibration feature point extraction.

[0003] In this context, the "equal-focal-length refraction model" provides a practical path to solving the refraction problem. Research has found that when the camera optical axis is strictly perpendicular to the plane window, underwater imaging can be equivalent to a "virtual camera" located on the air side, whose effective focal length satisfies the original air focal length, and the principal point remains basically unchanged. However, its practical application is limited by the installation error of the optical axis not being strictly perpendicular, the non-idealities of the window (such as uneven thickness and unevenness), and the interference of scattering noise on calibration feature extraction. In order to break through the bottleneck of scattering noise, polarization technology is introduced into the stripe projection system. However, there is a lack of a unified mathematical model that couples the equal-focal-length refraction model with composite polarization-coded depth, making it difficult to guide system optimization; there is a lack of a polarization encoding and decoding system architecture designed specifically for underwater stripe projection, which makes it impossible to maximize the scattering suppression and feature enhancement effects; and an efficient and robust underwater calibration process based on polarization-enhanced images has not yet been established, making it impossible to systematically solve the problem of accurate calibration of intrinsic / extrinsic parameters and interface parameters.

[0004] Therefore, developing a composite polarization-coded fringe projection underwater calibration method and system based on the equifocal-length refraction model, and deeply integrating the geometric optical simplified model with polarization physical information processing, is the inevitable direction to break through the dual constraints of refraction and scattering and achieve high-precision, high-efficiency, and high-robustness underwater three-dimensional measurement. Summary of the Invention

[0005] To address the issues of missing 3D object point cloud information due to phase unwrapping calculation errors, non-sinusoidal distribution errors, and phase saturation errors caused by HDR object surfaces in underwater fringe patterns, this paper proposes a composite polarization state underwater calibration method based on an equifocal refraction model to address the above issues.

[0006] According to one aspect of the present disclosure, a method for underwater calibration of composite polarization states based on an equal-focal-length refraction model is provided, comprising: S10. By combining the geometric optical imaging equation and Snell's refraction law, an approximate treatment is performed on the incident angle of the light under small angle conditions to obtain a conversion relationship between the coordinates of the underwater imaging point and the coordinates of the theoretical imaging point in air. Based on the conversion relationship, a mathematical model of equal focal length refraction of the underwater camera and projector is established; S20. Designing a composite polarization-encoded sinusoidal projection calibration pattern based on the equal-focal-length refraction mathematical model; S30, utilizing the polarization light projection characteristics, encoding the horizontal polarization light of the green channel of the composite polarization-state-encoded sinusoidal projection calibration pattern into cosine stripes in the horizontal direction, and encoding the vertical polarization light of the blue channel of the composite polarization-state-encoded sinusoidal projection calibration pattern into sinusoidal stripes in the vertical direction; S40, superimposing the horizontal cosine stripes and the vertical sine stripes to generate two sets of orthogonal polarization state stripe patterns; S50. Based on the equal-focal-length refraction mathematical model, phase calculation and reprojection error optimization are performed on two groups of orthogonal polarization fringe patterns to achieve parameter calibration.

[0007] Preferably, the geometric optical imaging equation and Snell's refraction law are combined to approximate the incident angle of the light under small angle conditions, which can be expressed as: , Where, is the coordinate of the underwater imaging point, It is expressed as the distance between the medium interface and the outer focal point of the lens, Expressed as focal length, Indicates space X Axis coordinates, In spatial coordinates Z Axis coordinates, and are the refractive indices of water and air, respectively. and are the angles between the imaging light and the normal of the water and air interfaces, respectively.

[0008] Preferably, the conversion relationship between the underwater imaging point coordinates and the theoretical imaging point coordinates in the air is: , Where, is the coordinate of the underwater imaging point, are the coordinates of the theoretical imaging point in the air.

[0009] Preferably, the horizontal polarized light of the green channel of the composite polarization state coded sinusoidal projection calibration pattern is encoded into cosine stripes in the horizontal direction, which is expressed as: , , , Where, is the horizontal axis coordinate of the projector coordinate system, For the cycle, A green light source, is the total intensity of green light at a row of pixels, is the intensity distribution of the horizontal polarization state of the pixels in this row.

[0010] Preferably, the vertically polarized light of the blue channel of the composite polarization state coded sinusoidal projection calibration pattern is encoded into sinusoidal stripes in the vertical direction, which is expressed as: , , , Where, is the horizontal axis coordinate of the projector coordinate system, For the cycle, is a blue light source, is the total intensity of blue light at a row of pixels, is the intensity distribution of the vertical polarization state of the pixels in this row.

[0011] Preferably, the cosine stripes in the horizontal direction and the sine stripes in the vertical direction are superimposed to generate two sets of orthogonal polarization state stripe patterns, which are expressed as: , Where, is the pixel coordinate, is the horizontal cosine fringe pattern recorded by the polarization camera when the front polarizer of the camera is 0°, This is the vertical sinusoidal fringe pattern recorded by the polarization camera when the front polarizer of the camera is at 90°.

[0012] Preferably, performing phase calculation and reprojection error optimization on two sets of orthogonal polarization state fringe patterns to achieve parameter calibration includes: The wrapped phase is extracted from two sets of orthogonal polarization fringe patterns by phase shifting method, and the two sets of wrapped phase are decoded by complementary Gray code to obtain the absolute phase distribution. Calculating the three-dimensional coordinates of the characteristic points of the calibration plate based on the two sets of orthogonal polarization fringe patterns; According to the conversion relationship between the coordinates of the underwater imaging point and the coordinates of the theoretical imaging point in the air, a reprojection error function is constructed and the internal and external parameters of the camera are optimized until the error converges.

[0013] According to one aspect of the present disclosure, a composite polarization state underwater calibration system based on an equal-focal-length refraction model is provided, comprising: The equal-focal-length refraction mathematical model construction module, which approximates the incident angle of light at small angles by combining the geometric optical imaging equation and Snell's refraction law, obtains the conversion relationship between the coordinates of the underwater imaging point and the coordinates of the theoretical imaging point in air, and establishes the equal-focal-length refraction mathematical model of the underwater camera and projector based on this conversion relationship; A composite polarization-coded sinusoidal projection calibration pattern design module is used to design a composite polarization-coded sinusoidal projection calibration pattern based on the equal-focal-length refraction mathematical model; The polarization encoding module uses the polarization projection characteristics to encode the horizontal polarization of the green channel of the composite polarization-encoded sinusoidal projection calibration pattern into horizontal cosine stripes, and encodes the vertical polarization of the blue channel of the composite polarization-encoded sinusoidal projection calibration pattern into vertical sinusoidal stripes. an orthogonal polarization state fringe pattern generation module, which superimposes the horizontal cosine stripes and the vertical sine stripes to generate two sets of orthogonal polarization state fringe patterns; The parameter calibration module performs phase calculation and reprojection error optimization on two sets of orthogonal polarization fringe patterns based on the equal-focal-length refraction mathematical model to achieve parameter calibration.

[0014] According to one aspect of the present disclosure, an electronic device is provided, comprising: a processor; and a memory for storing processor-executable instructions; wherein the processor is configured to: execute the above-mentioned composite polarization state underwater calibration method based on the equal-focal-length refraction model.

[0015] According to one aspect of the present disclosure, a computer-readable storage medium is provided, on which computer program instructions are stored. When the computer program instructions are executed by a processor, the above-mentioned composite polarization state underwater calibration method based on the equal focal length refraction model is implemented.

[0016] Compared with the prior art, the beneficial effects of the present disclosure are: 1) This paper uses an equal-focal-length refraction model to linearly magnify the underwater image into an image taken in air to complete the calibration of system parameters, effectively solving the impact of light refraction on underwater imaging quality.

[0017] 2) The present disclosure can obtain a set of phase-shifted fringe patterns with mutually perpendicular polarization states by calculating the composite polarization state projected fringe pattern captured by the camera, thereby reducing the number of required projected fringe patterns.

[0018] 3) This method can significantly improve the accuracy of underwater calibration and has better equipment parameter calibration performance than the traditional grayscale stripe method.

[0019] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosure.

[0020] Further features and aspects of the present disclosure will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings herein are incorporated into and constitute a part of the specification. These drawings illustrate embodiments consistent with the present disclosure and, together with the specification, are used to explain the technical solutions of the present disclosure.

[0022] Figure 1 A flow chart of a composite polarization state underwater calibration method based on an equal-focal-length refraction model is shown; Figure 2 The flow chart of composite polarization-coded fringe generation is shown; Figure 3 A schematic diagram of the light propagation of underwater camera imaging is shown; Figure 4 The equivalent imaging plane stereogram of the underwater camera is shown; Figure 5 Shows the schematic diagram of the design of the composite polarization-encoded sinusoidal projection calibration pattern; Figure 6 The figure shows the pose diagram of the underwater calibration plate; Figure 7 The figure shows the pose diagram of the calibration plate in the air; Figure 8 A schematic diagram showing the positional relationship between the calibration plate, camera, and projector in the air; Figure 9 A schematic diagram showing the positional relationship between the underwater calibration plate, camera, and projector; Figure 10 A schematic diagram of the camera feature point reprojection error distribution is shown; Figure 11 A schematic diagram of the projector feature point reprojection error distribution is shown; Figure 12 A structural block diagram of a composite polarization state underwater calibration system based on an equal-focal-length refraction model in an embodiment of the present disclosure is shown. DETAILED DESCRIPTION

[0023] Various exemplary embodiments, features, and aspects of the present disclosure will be described in detail below with reference to the accompanying drawings. The same reference numerals in the accompanying drawings represent elements with the same or similar functions. Although various aspects of the embodiments are shown in the accompanying drawings, the drawings are not necessarily drawn to scale unless otherwise indicated.

[0024] The word “exemplary” is used exclusively herein to mean “serving as an example, example, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments.

[0025] The term "and / or" herein simply describes an association relationship between associated objects, indicating that three relationships can exist. For example, "A and / or B" can represent the existence of three situations: A alone, A and B simultaneously, and B alone. Furthermore, the term "at least one" herein refers to any combination of at least two of any one or more of a plurality of items. For example, "at least one of A, B, and C" can represent any one or more elements selected from the set consisting of A, B, and C.

[0026] In addition, numerous specific details are provided in the following detailed description to better illustrate the present disclosure. Those skilled in the art will appreciate that the present disclosure can be practiced without certain specific details. In some instances, methods, means, components, and circuits well known to those skilled in the art are not described in detail in order to highlight the main points of the present disclosure.

[0027] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0028] Example 1 Based on the above ideas, the present invention proposes a composite polarization state underwater calibration method based on an equal-focal-length refraction model. Figure 1 A flow chart of a method for underwater calibration of composite polarization states based on an equal-focal-length refraction model is shown. The method comprises: S10. By combining the geometric optical imaging equation and Snell's refraction law, an approximate treatment is performed on the incident angle of the light under small angle conditions to obtain a conversion relationship between the coordinates of the underwater imaging point and the coordinates of the theoretical imaging point in air. Based on the conversion relationship, a mathematical model of equal focal length refraction of the underwater camera and projector is established; S20. Designing a composite polarization-encoded sinusoidal projection calibration pattern based on the equal-focal-length refraction mathematical model; S30, utilizing the polarization light projection characteristics, encoding the horizontal polarization light of the green channel of the composite polarization-state-encoded sinusoidal projection calibration pattern into cosine stripes in the horizontal direction, and encoding the vertical polarization light of the blue channel of the composite polarization-state-encoded sinusoidal projection calibration pattern into sinusoidal stripes in the vertical direction; S40, superimposing the horizontal cosine stripes and the vertical sine stripes to generate two sets of orthogonal polarization state stripe patterns; S50. Based on the equal-focal-length refraction mathematical model, phase calculation and reprojection error optimization are performed on two groups of orthogonal polarization fringe patterns to achieve parameter calibration.

[0029] The disclosed embodiments provide a composite polarization state underwater calibration method based on an equifocal refraction model. This method deeply integrates a simplified geometric optical model with polarization physics information processing. This method is the inevitable direction for overcoming the dual constraints of refraction and scattering and achieving high-precision, high-efficiency, and high-robustness underwater three-dimensional measurement. This requires not only innovative polarization encoding and decoding architectures at the optical system design level, but also collaborative research in model construction (unified refraction-polarization model), information processing (multimodal data fusion), and algorithm development (noise-resistant calibration process). Specifically, the method includes the following steps: S10. By combining the geometric optical imaging equation and Snell's refraction law, the incident angle of the light is approximated under small angle conditions to obtain the conversion relationship between the coordinates of the underwater imaging point and the coordinates of the theoretical imaging point in the air. Based on the conversion relationship, a mathematical model of equal focal length refraction of the underwater camera and projector is established.

[0030] In this example, an underwater measurement and calibration system was constructed. The system primarily consists of a 3LCD projector (EPSON CB-FH52, 1920×1080 pixels), a polarization-based monochrome CMOS camera (FLIR BFS-U3-51S5P-C, 2448×2048 pixels), a precision-machined black-background white-dot calibration plate, and a customized transparent water tank. The calibration plate features an 11×9 black-and-white circular array with optimized contrast. It contains five large white dots with a diameter of 5 mm, used to identify the calibration plate during calibration. Small white dots have a diameter of 2.5 mm, and the center-to-center distance between each dot is 10 mm. The experimental water tank is constructed of 60 cm × 30 cm × 30 cm PMMA with a thickness of 5 mm.

[0031] Figure 2 The overall process from the projector emitting composite polarization-coded stripes to the polarization camera capturing the image is shown. Represents four vertical and horizontal sinusoidal grayscale stripes with different phase shifts, Represents four different phase-shifted horizontal polarization state coded stripes, Represents four different phase-shifted vertical polarization coded stripes, This image shows four different phase-shifted composite polarization-state coded stripes. The projector emits vertical sinusoidal stripes (green channel, encoded as 0° horizontal polarization) and horizontal sinusoidal stripes (blue channel, encoded as 90° vertical polarization). The green channel light is modulated to a horizontal polarization state (0°), and the blue channel light is modulated to a vertical polarization state (90°). The resulting composite polarization-state coded stripes contain information about both orthogonal polarization states.

[0032] Furthermore, a mathematical model of equal-focal-length refraction of the underwater camera and projector is established. By combining the geometric optical imaging equation with Snell's refraction law, the following quantitative relationship can be established: , Where, is the coordinate of the underwater imaging point, It is expressed as the distance between the medium interface and the outer focal point of the lens, Expressed as focal length, Indicates space X Axis coordinates, In spatial coordinates Z Axis coordinates, and are the refractive indices of water and air, respectively. and are the angles between the imaging light and the normal of the water and air interfaces, respectively.

[0033] According to the above formula, the relationship between the object and the imaging position during underwater imaging cannot be directly applied to the optical imaging principle based on air medium: , Schematic diagram of underwater camera imaging light propagation, such as Figure 3 As shown. CCD means camera, represents an underwater spatial point, represents the imaging point, It is expressed as the distance between the medium interface and the outer focal point of the lens, Represented as the optical center, Expressed as focal length, and are the refractive indices of water and air, respectively. and =The angle between the imaging light and the normal of the water-air interface. Light path: Light emitted from underwater objects will be refracted (light deflected) when passing through water and the camera protective window, and finally enter the camera for imaging. Since the refractive index of water is higher than that of air, underwater imaging will be "magnified" and the actual focal length will be longer than that in air (about 1.33 times). When the object is far away from the camera, underwater imaging can be approximated as a simple magnification of the imaging in air, which is convenient for calibration calculation. The equivalent imaging plane stereogram of the underwater camera is as follows: Figure 4 As shown in the figure, it is intuitively shown that underwater camera imaging can be equivalent to the effect of "shooting with a longer focal length in the air". It can be seen that the imaging difference of the same object underwater and in the air is that the underwater imaging will appear larger and closer. Since underwater images cannot provide object distance The three-dimensional spatial information of the imaging point in the water medium depends only on the intrinsic parameters of the camera. Compared with the theoretical imaging point in air It is difficult to establish a deterministic mapping relationship between them. Due to the limitation of two-dimensional projection characteristics, it is impossible to invert the corresponding air medium imaging result through a single underwater image. and When the incident angle is small, the incident angle can be used to approximately replace the sine and tangent of the angle, and the above formula is equivalent to: , Arrange and solve to get: , By comparing the underwater image and the air image, we can see the changes.

[0034] , Further conclusion, point P Distance from the camera and When , it can be deduced that: , The conversion relationship between the coordinates of the underwater imaging point and the coordinates of the theoretical imaging point in the air is: , Where, is the coordinate of the underwater imaging point, are the coordinates of the theoretical imaging point in the air.

[0035] From the above formula, we can see that the imaging result in underwater environment can be simplified to the linear amplification process of imaging in air. Its magnification is determined by the ratio of the refractive index of water to that of air. Decide.

[0036] S20. Based on the equal-focal-length refraction mathematical model, design a composite polarization-state coded sinusoidal projection calibration pattern.

[0037] In this embodiment, the schematic diagram of the design of the composite polarization state coded sinusoidal projection calibration pattern is as follows: Figure 5 As shown. This pattern utilizes the polarization characteristics of the 3LCD projector to encode the green channel as sinusoidal stripes in the horizontal direction (0° polarization state) and the blue channel as sinusoidal stripes in the vertical direction (90° polarization state). The stripes of the two orthogonal polarization states are superimposed to form a composite coding pattern. The sinusoidal stripes shown in the figure have periodic light and dark variations. The horizontal and vertical stripes have a 90° phase difference and are distinguished by green and blue, corresponding to different polarization directions. This innovative design allows for the simultaneous acquisition of two orthogonal polarization information in a single projection. Compared to the traditional grayscale stripe method that requires multiple projections, it significantly improves the calibration efficiency and accuracy of the underwater 3D measurement system.

[0038] In terms of fringe coding strategy, the mathematical representation of the composite polarization state coded fringe system can be expressed as follows: Assuming that the green light source emitted by the projector is , the blue light source is The intensity function of the composite polarization state encoded sinusoidal stripes is periodic. T change.

[0039] S30. Utilizing the polarized light projection characteristics, encode the horizontal polarized light of the green channel of the composite polarization-encoded sinusoidal projection calibration pattern into cosine stripes in the horizontal direction, and encode the vertical polarized light of the blue channel of the composite polarization-encoded sinusoidal projection calibration pattern into sinusoidal stripes in the vertical direction.

[0040] In this embodiment, when the horizontal axis coordinate of the projector coordinate system is When is the independent variable, the horizontal polarization light of the green channel of the composite polarization state coded sinusoidal projection calibration pattern is encoded into the cosine stripes in the horizontal direction, which is expressed as: , , , Where, is the horizontal axis coordinate of the projector coordinate system, For the cycle, A green light source, is the total intensity of green light at a row of pixels, is the intensity distribution of the horizontal polarization state of the pixels in this row.

[0041] The vertically polarized light of the blue channel of the composite polarization-encoded sinusoidal projection calibration pattern is encoded into vertical sinusoidal stripes, which can be expressed as: , , , Where, is the horizontal axis coordinate of the projector coordinate system, For the cycle, is a blue light source, is the total intensity of blue light at a row of pixels, is the intensity distribution of the vertical polarization state of the pixels in this row.

[0042] S40 , superimposing the horizontal cosine stripes and the vertical sine stripes to generate two groups of orthogonal polarization state stripe patterns.

[0043] In this embodiment, the composite polarization-encoded phase-shift stripes have only two linear polarization states and are formed by superimposing two groups of mutually perpendicular polarization stripes. The two groups of stripes have different and , by calculating the two Stokes parameters of the fringe pattern captured by the polarization camera and , we can obtain a set of polarized sinusoidal structured light fringe patterns in the horizontal direction (0° polarization state) and another set of polarized sinusoidal structured light fringe patterns in the vertical direction (90° polarization state).

[0044] The horizontal cosine stripes and the vertical sine stripes are superimposed to generate two sets of orthogonal polarization fringe patterns, which are expressed as: , Where, is the pixel coordinate, is the horizontal cosine fringe pattern recorded by the polarization camera when the front polarizer of the camera is 0°, This is the vertical sinusoidal fringe pattern recorded by the polarization camera when the front polarizer of the camera is at 90°.

[0045] Polarization cameras can record polarization patterns at 0° and 90° simultaneously after a single exposure, so a single exposure can be used to obtain a single image. and a , that is, after calculation, a sinusoidal calibration pattern of vertical stripes (0° polarization state) and horizontal stripes (90° polarization state) can be obtained simultaneously after a single exposure for calibration of underwater cameras and projectors.

[0046] S50. Based on the equal-focal-length refraction mathematical model, phase calculation and reprojection error optimization are performed on two groups of orthogonal polarization fringe patterns to achieve parameter calibration.

[0047] In this embodiment, the corresponding wrapped phase is obtained by a phase shift method, and finally the complementary Gray code is used for decoding to obtain the absolute phase.

[0048] Phase solution and reprojection error optimization are performed on two sets of orthogonal polarization fringe patterns to achieve parameter calibration, including: extracting the wrapped phase from the two sets of orthogonal polarization fringe patterns through the phase shift method, decoding the two sets of wrapped phases using complementary Gray code to obtain the absolute phase distribution; calculating the three-dimensional coordinates of the characteristic points of the calibration plate based on the two sets of orthogonal polarization fringe patterns; constructing a reprojection error function and optimizing the internal and external parameters of the camera according to the conversion relationship between the coordinates of the underwater imaging point and the coordinates of the theoretical imaging point in the air until the error converges.

[0049] To evaluate the performance of the composite polarization-coded fringe calibration for an equal-focal-length refraction model, a polarization camera and projector were calibrated using a calibration plate in both air and water environments. During the calibration process, the polarization camera and projector remained fixed in position, while the calibration plate was moved and transformed multiple times. Figure 6 (a) and Figure 7 Figure (a) shows 12 sets of partial images of the calibration plate at different locations, captured by the polarization camera in air and water environments. This example shows the first image in each set. It's worth noting that in some of the underwater images of the calibration plate, bright spots appear at the top and right side of the pattern. This is due to light reflected from the water tank.

[0050] Figure 6 (b) and Figure 7 (b) The calibration plate center detection effect displayed by the polarization imaging system shows that the device can achieve complete feature point recognition in different media environments.

[0051] Figure 8 and Figure 9 Represent the spatial pose relationship of the camera-projector system in air and water environments, respectively, where Figure 8 1-12 in the figure represent the positional relationship between the 12 sets of calibration plates photographed by the polarization camera in the air and the camera and projector; Figure 8 (a) is the camera-centered external parameter visualization. Figure 8 (b) in the figure is the external parameter visualization centered on the projector; Figure 9 (a) is the camera-centered external parameter visualization. Figure 9 The 1-12 in the figure represent the positional relationship between the 12 sets of calibration plates, the camera, and the projector taken by the polarization camera in water. Figure 9 (b) is the visualization of external parameters centered on the projector. Table 1 compares the calibration parameters of the polarization imaging system in the dual medium environment of air and water.

[0052] Table 1 Comparison of calibration parameters in air and underwater

[0053] Comparing the calibration parameters in Table 1, the trends in focal length in underwater and air environments are consistent with the results derived from the mathematical model of equal-focal-length refraction. The experimentally measured underwater focal length is approximately 1.33 times the air focal length, a factor that perfectly matches the refractive index of water (approximately 1.33).

[0054] To verify the accuracy of the calibration method, the traditional gray-white stripe pattern and composite polarization state encoding methods were used to calibrate the parameters of the underwater camera and projector system, respectively. Figure 10 1-12 in the figure represent the reprojection error distribution of camera feature points of 12 sets of calibration plates taken by the polarization camera in the air. Figure 10 (a) and Figure 10 (b) in the figure shows the reprojection error distribution of the camera center coordinates calibrated by the method of this embodiment and the traditional gray-white stripes. Figure 11 1-12 in the figure represent the reprojection error distribution of camera feature points of 12 sets of calibration plates taken by the polarization camera in water. Figure 11 (a) and Figure 11 (b) in the figure shows the reprojection error distribution of the projector center coordinates. Figure 10 and Figure 11 The experimental figures presented here analyze the spatial distribution characteristics and error concentration. The experimental data show that compared to the diffuse reprojection error distribution of the traditional gray-white stripe method, the proposed method exhibits better spatial consistency and a more concentrated reprojection error distribution.

[0055] The disclosed embodiment proposes a composite polarization state underwater calibration method based on an equal-focal-length refraction model, constructs a projector calibration system based on an "inverse camera" model, and innovatively designs a sinusoidal stripe calibration pattern containing composite polarization state encoding by combining the polarized light projection characteristics of a 3LCD projector. By encoding the horizontally polarized light in the green channel of the projector onto sinusoidal stripes in the vertical direction, and encoding the vertically polarized light in the blue channel onto sinusoidal stripes in the horizontal direction. By calculating the composite polarization state projection stripe pattern captured by the camera, a set of phase-shifted stripe patterns with mutually perpendicular polarization states can be obtained. This method replaces the gray-white stripe pattern in the traditional camera-projector inverse calibration method, reducing the number of projected stripe patterns. At the same time, an underwater equal-focal-length refraction model is introduced to complete the calibration of system parameters. Compared with traditional solutions, it can significantly improve the calibration accuracy and measurement efficiency of underwater visual measurement systems.

[0056] Example 2 As another aspect of the embodiment of the present disclosure, a composite polarization state underwater calibration system 100 based on an equal focal length refraction model is also provided. Figure 12 Shown, including: Module 1: Build an equal-focal-length refraction mathematical model. By combining the geometric optical imaging equation and Snell's refraction law, the incident angle of light is approximated under small angle conditions to obtain the conversion relationship between the coordinates of the underwater imaging point and the coordinates of the theoretical imaging point in air. Based on this conversion relationship, the equal-focal-length refraction mathematical model of the underwater camera and projector is established. A composite polarization-encoded sinusoidal projection calibration pattern design module 2 is configured to design a composite polarization-encoded sinusoidal projection calibration pattern based on the equal-focal-length refraction mathematical model; Polarization encoding module 3, using the polarization projection characteristics, encodes the horizontal polarization of the green channel of the composite polarization state coded sinusoidal projection calibration pattern into horizontal cosine stripes, and encodes the vertical polarization of the blue channel of the composite polarization state coded sinusoidal projection calibration pattern into vertical sinusoidal stripes; An orthogonal polarization state fringe pattern generating module 4 is configured to superimpose the horizontal cosine fringe and the vertical sine fringe to generate two sets of orthogonal polarization state fringe patterns; The parameter calibration module 5 performs phase calculation and reprojection error optimization on two groups of orthogonal polarization fringe patterns based on the equal-focal-length refraction mathematical model to achieve parameter calibration.

[0057] In the absence of any contradiction, the above modules in the system of the embodiment of the present disclosure can implement any implementation of the above method.

[0058] Based on the description of the above embodiments, it can be seen that the embodiments of the present disclosure can achieve the following technical effects: 1) This paper uses an equal-focal-length refraction model to linearly magnify the underwater image into an image taken in air to complete the calibration of system parameters, effectively solving the impact of light refraction on underwater imaging quality.

[0059] 2) The present disclosure can obtain a set of phase-shifted fringe patterns with mutually perpendicular polarization states by calculating the composite polarization state projected fringe pattern captured by the camera, thereby reducing the number of required projected fringe patterns.

[0060] 3) This method can significantly improve the accuracy of underwater calibration and has better equipment parameter calibration performance than the traditional grayscale stripe method.

[0061] The present disclosure also provides an electronic device comprising: a processor; and a memory for storing instructions executable by the processor; wherein the processor is configured to implement the aforementioned method for underwater calibration of composite polarization states based on an equal-focal-length refraction model. The electronic device can be provided as a terminal, server, or other device.

[0062] The present disclosure also provides a computer-readable storage medium having computer program instructions stored thereon. When executed by a processor, the computer program instructions implement the aforementioned method for underwater calibration of composite polarization states based on the equal-focal-length refraction model. The computer-readable storage medium may be a non-volatile computer-readable storage medium.

[0063] Those skilled in the art will understand that in the above-mentioned composite polarization state underwater calibration method and system based on the equal-focal-length refraction model in the specific implementation manner, the writing order of each step does not mean a strict execution order and does not constitute any limitation on the implementation process. The specific execution order of each step should be determined by its function and possible internal logic.

[0064] The flow charts and block diagrams in the accompanying drawings show the possible architecture, functions and operations of the systems, methods and computer program products according to multiple embodiments of the present disclosure. In this regard, each box in the flow chart or block diagram can represent a part of a module, program segment or instruction, and the part of the module, program segment or instruction contains one or more executable instructions for realizing the prescribed logical function. In some alternative implementations, the functions marked in the box can also occur in a sequence different from that marked in the accompanying drawings. For example, two consecutive boxes can actually be executed substantially in parallel, and they can sometimes be executed in the opposite order, depending on the functions involved. It should also be noted that each box in the block diagram and / or flow chart, and the combination of the boxes in the block diagram and / or flow chart can be implemented by a dedicated hardware-based system that performs the prescribed function or action, or can be implemented by a combination of dedicated hardware and computer instructions.

[0065] While various embodiments of the present disclosure have been described above, the above descriptions are illustrative, non-exhaustive, and not intended to be limiting of the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is selected to best explain the principles of the embodiments, their practical applications, or technical improvements to existing technologies, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A composite polarization state underwater calibration method based on an equal-focal-length refraction model, characterized in that: The steps include: S10. By combining the geometric optical imaging equation and Snell's refraction law, an approximate treatment is performed on the incident angle of the light under small angle conditions to obtain a conversion relationship between the coordinates of the underwater imaging point and the coordinates of the theoretical imaging point in air. Based on the conversion relationship, a mathematical model of equal focal length refraction of the underwater camera and projector is established; S20. Designing a composite polarization-encoded sinusoidal projection calibration pattern based on the equal-focal-length refraction mathematical model; S30, utilizing the polarization light projection characteristics, encoding the horizontal polarization light of the green channel of the composite polarization-state-encoded sinusoidal projection calibration pattern into cosine stripes in the horizontal direction, and encoding the vertical polarization light of the blue channel of the composite polarization-state-encoded sinusoidal projection calibration pattern into sinusoidal stripes in the vertical direction; S40, superimposing the horizontal cosine stripes and the vertical sine stripes to generate two sets of orthogonal polarization state stripe patterns; S50. Based on the equal-focal-length refraction mathematical model, phase calculation and reprojection error optimization are performed on two groups of orthogonal polarization fringe patterns to achieve parameter calibration.

2. The method according to claim 1, characterized in that Combining the geometric optical imaging equation and Snell's refraction law, the incident angle of light is approximated under small angle conditions and expressed as: ， Where, is the coordinate of the underwater imaging point, It is expressed as the distance between the medium interface and the outer focal point of the lens, Expressed as focal length, Indicates space X Axis coordinates, In spatial coordinates Z Axis coordinates, and are the refractive indices of water and air, respectively. and are the angles between the imaging light and the normal of the water and air interfaces, respectively.

3. The method according to claim 2, characterized in that The conversion relationship between the coordinates of the underwater imaging point and the coordinates of the theoretical imaging point in the air is: ， Where, is the coordinate of the underwater imaging point, are the coordinates of the theoretical imaging point in the air.

4. The method according to claim 1, wherein The horizontal polarization light of the green channel of the composite polarization state coded sinusoidal projection calibration pattern is encoded into the cosine stripes in the horizontal direction, which is expressed as: ， ， ， Where, is the horizontal axis coordinate of the projector coordinate system, For the cycle, A green light source, is the total intensity of green light at a row of pixels, is the intensity distribution of the horizontal polarization state of the pixels in this row.

5. The method according to claim 1, wherein The vertically polarized light of the blue channel of the composite polarization-encoded sinusoidal projection calibration pattern is encoded into vertical sinusoidal stripes, which can be expressed as: ， ， ， Where, is the horizontal axis coordinate of the projector coordinate system, For the cycle, is a blue light source, is the total intensity of blue light at a row of pixels, is the intensity distribution of the vertical polarization state of the pixels in this row.

6. The method according to claim 1, characterized in that The horizontal cosine stripes and the vertical sine stripes are superimposed to generate two sets of orthogonal polarization fringe patterns, which are expressed as: ， Where, is the pixel coordinate, is the horizontal cosine fringe pattern recorded by the polarization camera when the front polarizer of the camera is 0°, This is the vertical sinusoidal fringe pattern recorded by the polarization camera when the front polarizer of the camera is at 90°.

7. The method according to claim 1, characterized in that Phase calculation and reprojection error optimization are performed on two sets of orthogonal polarization fringe patterns to achieve parameter calibration, including: The wrapped phase is extracted from two sets of orthogonal polarization fringe patterns by phase shifting method, and the two sets of wrapped phase are decoded by complementary Gray code to obtain the absolute phase distribution. Calculating the three-dimensional coordinates of the characteristic points of the calibration plate based on the two sets of orthogonal polarization fringe patterns; According to the conversion relationship between the coordinates of the underwater imaging point and the coordinates of the theoretical imaging point in the air, a reprojection error function is constructed and the internal and external parameters of the camera are optimized until the error converges.

8. A composite polarization state underwater calibration system based on an equal-focal-length refraction model, characterized in that: include: The equal-focal-length refraction mathematical model construction module, which approximates the incident angle of light at small angles by combining the geometric optical imaging equation and Snell's refraction law, obtains the conversion relationship between the coordinates of the underwater imaging point and the coordinates of the theoretical imaging point in air, and establishes the equal-focal-length refraction mathematical model of the underwater camera and projector based on this conversion relationship; A composite polarization-coded sinusoidal projection calibration pattern design module is used to design a composite polarization-coded sinusoidal projection calibration pattern based on the equal-focal-length refraction mathematical model; The polarization encoding module uses the polarization projection characteristics to encode the horizontal polarization of the green channel of the composite polarization-encoded sinusoidal projection calibration pattern into horizontal cosine stripes, and encodes the vertical polarization of the blue channel of the composite polarization-encoded sinusoidal projection calibration pattern into vertical sinusoidal stripes. an orthogonal polarization state fringe pattern generation module, which superimposes the horizontal cosine stripes and the vertical sine stripes to generate two sets of orthogonal polarization state fringe patterns; The parameter calibration module performs phase calculation and reprojection error optimization on two sets of orthogonal polarization fringe patterns based on the equal-focal-length refraction mathematical model to achieve parameter calibration.

9. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein: When the processor executes the computer program, the composite polarization state underwater calibration method based on the equal-focal-length refraction model according to any one of claims 1 to 7 is implemented.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the composite polarization state underwater calibration method based on the equal-focal-length refraction model according to any one of claims 1 to 7 is implemented.

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