A fisheye lens projection model calibration tool and sky view factor calculation method

Abstract

The present invention discloses a fisheye lens projection model calibration tool and a method for calculating a sky view factor. The calibration tool is formed by assembling N single modules with the same structure. The overall angle formed by the calibration tool is greater than the actual field of view angle φ of the lens being calibrated. The calibration tool is combined with an equal incident angle difference algorithm to enable fisheye lenses of various projection models to measure the sky view factor. The present invention adopts an equal incident angle difference algorithm. The calibration tool can be used to easily and quickly carry out fisheye lens field of view angle measurement, eccentricity error inspection, arbitrary integer incident angle image point position calibration of fisheye photos, and mathematical expression fitting of the actual projection model of fisheye photos, thereby quickly determining whether the fisheye lens can be used for sky view factor measurement. This makes sky view factor measurement no longer limited to the equidistant projection model fisheye lens, making the selection of fisheye lenses for measurement more flexible, lower in cost, and higher in accuracy of sky view factor measurement results.

Description

Technical Field

[0001] The present invention relates to the technical field of sky view factor measurement, and in particular to a fisheye lens projection model calibration tool and a sky view factor calculation method. Background Art

[0002] The Sky View Factor (SVF) is a dimensionless physical quantity that describes the geometry of three-dimensional space. It represents the ratio of the radiation received by a plane from the sky to the total radiation emitted by the entire hemisphere. It can be intuitively understood as the proportion of sky visible at a point in an urban street canyon. The SVF value ranges from 0 to 1. A larger SVF value indicates a smaller proportion of the sky radiation received by the measuring point plane is blocked by surrounding buildings (from the literature: Watson, ID and GT Johnson, Graphical estimation of sky view-factors in urban environments. Journal of Climatology, 1987. 7(2): p. 193-197.). SVF is a key factor affecting urban surface heat balance, microscale air circulation, and the diffusion of atmospheric pollutants. It is widely used in research on urban heat island effects and other aspects.

[0003] The fisheye photo method is the most commonly used and most direct SVF measurement method. In 1980, Steyn proposed an equidistant annular algorithm for calculating SVF using equidistant projection fisheye photos (from the literature: Steyn, DG, The Calculation of View Factors From Fisheye-lens Photographs. Atmosphere-ocean, 1980. 18(3): p. 254-258.). This algorithm is only applicable to circular fisheye photos with a field of view equal to 180° and generated based on an equidistant projection model. Many outdoor thermal comfort analysis software (such as Rayman, FIPS, etc.) have the Steyn algorithm written into the program. Researchers only need to input circular fisheye photos to obtain SVF calculation values. Because the software is too easy to use, most researchers are no longer familiar with the basic principles of the Steyn algorithm, and have not verified whether the provided circular fisheye photos meet the two necessary conditions of a field of view equal to 180° and an equidistant projection model. Therefore, there are often large errors in measuring SVF using the fisheye photo method. The reasons are as follows:

[0004] (1) The field of view of fisheye photos is not equal to 180°. The field of view of fisheye lenses on the market is mostly between 150° and 240°. Lenses with a field of view less than 180° cannot be used for SVF calculation. Lenses with a field of view greater than 180° should first capture the effective portion within the incident angle θ = 90° before performing SVF calculation.

[0005] (2) Fisheye photos that are not based on an equidistant projection model. There are only two known monocular equidistant projection model fisheye lens products, and both are out of production. In fact, it is difficult for researchers to provide standard equidistant projection model fisheye photos. Since fisheye photos are mainly used in entertainment, art, and monitoring, and are not scientific research tools for measurement purposes, there is currently no unified projection model product specification, nor is there a standard fisheye lens product dedicated to the sky view factor. There are as many as 6 commonly used classic projection models; in addition, various fisheye lens products are subject to the constraints of processing technology and supporting imaging modules, which may produce various non-standard projection models.

[0006] The projection model of a fisheye lens refers to the correspondence between the incident angle of an object point and the position of its image point in a fisheye photograph. Theoretically, four of the six classic projection models of fisheye lenses have the potential to be applied to SVF measurement. The equidistant projection model is the simplest of the classic fisheye lens projection models. Its incident angle and image point position have a simple linear relationship, which is very easy to calculate. Therefore, since the introduction of the Steyn algorithm in 1980, SVF measurement has been highly dependent on fisheye lenses with equidistant projection models. Although there are currently few monocular equidistant projection model fisheye lenses on the market, there are many fisheye lens products based on other classic projection models or non-standard projection models. If a new universal algorithm can be established, it will break the monopoly of equidistant projection model fisheye lenses in sky view factor measurement applications and contribute to the development of urban thermal environment research.

[0007] In summary, the circular fisheye images currently available to researchers often suffer from inaccurate field of view (FOV) and deviations from the projection model. Calculating the true SVF value requires calibrating the FOV and projection model. However, most manufacturers do not provide detailed information on lens angles and projection models (especially the projection model), and there are currently no simple fisheye lens calibration tools available to researchers. Consequently, SVF measurements during outdoor thermal environment research remain inaccurate. Summary of the Invention

[0008] In order to overcome the technical problems in the background technology, the present invention provides a fisheye lens projection model calibration tool and a sky view factor calculation method to solve the above problems.

[0009] In order to solve the above technical problems, the present invention provides a fisheye lens projection model calibration tool, comprising N single modules with the same structure, wherein the single modules are spliced ​​with each other in pairs, and the overall angle formed by the N single modules is greater than the actual field of view angle of the lens being calibrated. The number of the single modules N≥1, the angle ζ of the single modules is an integer multiple of 5°, the shape of the single module is a fan-shaped plate structure, and the single module is provided with a positioning hole, a marking pin can be inserted into the positioning hole, and the positioning hole is connected to the scale indicator line for positioning.

[0010] Preferably, the single module has 6 positioning holes within every 5° range, and the 6 positioning holes are respectively located on concentric arcs AF of different radii. The angles of the 6 positioning holes relative to the center of the circle increase by 1°, and the radii of the concentric arcs corresponding to the 6 positioning holes satisfy:

[0011] (R B -R A )>(R c -R B )>(R D -R c )>(R E -R D )>(R F -R E ).

[0012] Preferably, the radius R of the arc A is A ≥15cm, radius R of arc F F ≥35cm.

[0013] Preferably, the single module is provided with snap-in positions and slots for splicing on both sides, the positioning holes are fan-shaped and relatively evenly distributed on the body of the single module, a reference line is also provided on the single module, the number of the single modules N≥5, and the angle ζ of the single module is recommended to be set to 30°.

[0014] Preferably, the marking pins include near pins, far pins and corner pins. The near pins are inserted into the positioning holes corresponding to arc A, the far pins are inserted into the positioning holes corresponding to arc F, and the corner pins are inserted near the maximum visible incident angles on the left and right sides of the lens or at a specific incident angle that needs to be calibrated.

[0015] The present invention provides a method for calculating a sky view factor, comprising the following steps:

[0016] S1. Assemble the calibration tool from individual modules, complete assembly and leveling of the projection model calibration tool, and adjust the fisheye lens system.

[0017] S2. Measure the field of view of the fisheye lens: Insert a corner pin near the maximum visible incident angle on the left and right sides of the lens and take a photo. Keeping all the arrangements unchanged, move the corner pin on each side, increasing the incident angle by 1°, and take another photo. Repeat the above steps until the corner pin is moved to a certain incident angle position where it is no longer visible in the field of view. The maximum incident angle on that side minus 1° is the incident angle on that side. The maximum incident angle on the left side is θ. L , the maximum incident angle on the right is θ R , then the real viewing angle of the fisheye lens is

[0018] S3. Checking the eccentricity error: Using the fisheye photo used for calibration, obtain the position of the image point of the mark at a 0° incident angle in the fisheye photo. If the position of the mark point does not coincide with the center of the circular fisheye photo, it proves that the lens has an eccentricity error. At this time, θ is obtained by measurement. L ≠θ R The position of the image point of the mark coincides with the center of the circular fisheye photo, proving that the lens does not have an eccentric error. At this time, θ is obtained by measurement. L =θ R ;

[0019] S4. Mark the imaging position at any incident angle: Assume that the incident angle of the target you want to mark is equal to θ S , on one side (or both sides) of the calibration tool θ S Insert a corner nail at the corresponding position and take a fisheye photo. The intersection of the corner nail and the reference line in the fisheye photo is obtained by reading the image. This point is the imaging position of the target incident angle;

[0020] S5. Verify the real imaging projection model of the fisheye lens: Use the fisheye photo used for verification to obtain the intersection points of all near-nail and baseline in the fisheye photo, measure and record the corresponding incident angle θ and their distance r from the center of the circle; organize the data and obtain the fitting relationship or The fitting relationship is the mathematical expression of the real projection model of the fisheye lens, where r 90 is the imaging radius corresponding to a 90° incident angle;

[0021] S6. After confirming that the lens does not have decentering error, calibrate the image point position r corresponding to the 90° incident angle. 90 Finally, the mathematical expression of the real projection model of the fisheye lens is fitted:

[0022]

[0023]

[0024] According to the equal incidence angle difference algorithm, the sky view factor is obtained as follows:

[0025]

[0026] Among them, θ i is the median incident angle of the ith ring, r 90 is the distance between the image point position corresponding to the 90° incident angle of the fisheye photo and the center of the photo circle; r i is θ i The distance between the corresponding image point position and the center of the circle, r i =r 90 g(θ i );Δr i is the width of each ring, Δr i =r 90 g(iΔθ)-r 90 g[(i-1)Δθ]Δr1=r 90 g(Δθ);α i is the total sky viewing angle of the ith ring.

[0027] Preferably, in step S3, if the distance between the imaging position and the center of the photo is a, and there is no eccentricity error, each point on the circle with a radius a from the center satisfies the angle of incidence equal to θ S , the incident angle in the inner area of ​​the circle is less than θ S , the incident angle of the outer area of ​​the circle (if any) is greater than θ S .

[0028] Preferably, in step S6, the portion of the fisheye photo within 90° of the incident angle is divided into n concentric rings, which are counted as the 1st to nth rings from the center outward, and the incident angle difference corresponding to the width of each ring is equal.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] 1. The technical solution of the present invention adopts a single-module assembly design. The number of single modules to be used can be selected according to the actual field of view angle range of the lens, thereby avoiding waste caused by idle single modules and saving storage space.

[0031] 2. By adopting the equal incidence angle difference algorithm that is universally applicable to various fisheye lens projection models, it is possible to quickly determine whether the fisheye lens can be used for sky view factor measurement, breaking the monopoly of the equidistant projection model fisheye lens in the application of sky view factor measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 a is a schematic structural diagram of a single module in an embodiment of the present invention;

[0033] Figure 1b is a distal side view (top) of a single module in an embodiment of the present invention;

[0034] Figure 1 c is a proximal side view (bottom) of a single module in an embodiment of the present invention;

[0035] Figure 2 Schematic diagram of the structure of a marking nail in an embodiment of the present invention;

[0036] Figure 3 This is a schematic diagram of the overall structure of a calibration tool formed by splicing single modules in an embodiment of the present invention;

[0037] Figure 4 Schematic diagram of a projection model of a fisheye lens in an embodiment of the present invention;

[0038] Figure 5 This is a flowchart of the process of measuring the sky view factor in an embodiment of the present invention;

[0039] Figure 6 These are circular fisheye photos taken outdoors at the same location using two commercially available fisheye lenses, A and B.

[0040] Figure 7 This is the calibration photo of fisheye lens A;

[0041] Figure 8 This is a photo to verify the projection model of fisheye lens B;

[0042] Figure 9 The following is an experimental diagram comparing the projection model of fisheye lens B with four classic projection models. DETAILED DESCRIPTION

[0043] The following will clearly and completely describe the technical solutions in this embodiment with reference to the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0044] It should be noted that all directional indications in this embodiment (such as up, down, left, right, front, back, etc.) are only used to explain the relative position relationship, movement status, etc. between the various components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0045] In addition, the terms "first," "second," and so on, used in this disclosure are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referenced. Thus, a feature specified as "first" or "second" may explicitly or implicitly include at least one such feature. In the description of this disclosure, "plurality" means at least two, such as two or three, unless otherwise specifically defined.

[0046] In the present invention, unless otherwise specified or limited, the terms "connection" and "fixation" should be understood in a broad sense. For example, "fixation" can mean fixed connection, detachable connection, or integration; mechanical connection or electrical connection; direct connection or indirect connection through an intermediate medium; internal communication between two elements or interaction between two elements, unless otherwise specified. Those skilled in the art will be able to understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0047] In addition, the technical solutions of the various embodiments of the present invention may be combined with each other, but this must be based on the premise that they can be implemented by a person of ordinary skill in the art. If the combination of technical solutions is inconsistent or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention. It should be understood that the specific embodiments described herein are only used to illustrate the present invention and are not intended to limit the present invention.

[0048] Combine Figure 1-3 As shown, Embodiment 1 of the present invention specifically discloses a fisheye lens projection model calibration tool, comprising N single modules 1 of identical structure, wherein the single modules 1 are spliced ​​together in pairs, and the overall angle formed by the N single modules 1 is greater than the true field of view angle φ of the lens being calibrated. The number of single modules 1, N, is ≥ 1, and the angle ζ of the single modules 1 is an integer multiple of 5°. The single module 1 is shaped like a fan-shaped plate structure, and is provided with a positioning hole 4, into which a marking pin can be inserted. The positioning hole 4 is connected to a scale indicator line for positioning. The technical solution of the present invention adopts a single module 1 assembly design, and the number of single modules 1 to be used can be selected according to the true field of view angle range of the lens, thereby avoiding waste caused by idle single modules 1, saving storage space, and reducing measurement costs.

[0049] Furthermore, both sides of the single module 1 are provided with card positions 2 and slots 3 for splicing, and the positioning holes 4 are fan-shaped and relatively evenly distributed on the body of the single module 1. A reference line 5 is also provided on the single module 1. There are 6 positioning holes 4 within every 5° range of the single module 1, and the 6 positioning holes 4 are respectively located on concentric arcs AF of different radii. The positioning hole 4 on the arc F is slightly larger than the other positioning holes 4. The size of the positioning hole 4 should match the marking pin so that the marking pin remains fixed and vertically upward after insertion without loosening or tilting. Based on this goal, the single module board should have a certain thickness. The angles of the 6 positioning holes 4 relative to the center of the circle increase by 1°. In order to make the positioning holes 4 fully dispersed for easy observation, the radius of the above-mentioned concentric arcs meets the following requirements: (R B -R A )>(R C -R B )>(R D -R C )>(R E -R D )>(R F -R E ). Recommended radius R of arc A A ≥15cm, radius R of arc F F ≥35cm to ensure clear imaging and easy observation.

[0050] Furthermore, the number N of the single modules 1 is ≥ 5, and is usually 6 to 8. The angle ζ of the single modules 1 is usually set to 30°.

[0051] The marking pins include a near pin 6, a far pin 8, and a corner pin 7. Near pin 6 is inserted into the positioning hole 4 corresponding to arc A, and far pin 8 is inserted into the positioning hole 4 corresponding to arc F. Corner pins 7 are inserted near the maximum visible angle of incidence on the left and right sides of the lens. Several of each type of marking pin are provided, the number determined based on actual needs. Given that images appear larger near the lens and smaller far away, far pin 8 should be slightly thicker and longer than near pin 6 to facilitate alignment within the fisheye image. Corner pin 7 should be slightly longer than near pin 6 and have the brightest color to easily distinguish it from the numerous marking pins in the verification photo.

[0052] Assembly and setting of fisheye lens projection model verification tool:

[0053] Assemble the calibration tool: Insert the latch 2 of a single module 1 into the slot 3 of another single module 1. Connect multiple modules 1 in this manner, forming a semicircular sector. Secure the slots with marking pins through the joints to form the fisheye lens projection model calibration tool. At this point, ensure that the reference lines 5 on each module 1 are connected to form an arc. The total number of modules 1 can be determined based on actual usage requirements, but the calibration tool should cover a minimum of 180° of incident angle. To maintain flatness, the entire calibration tool must be placed flat on a larger surface during use, ensuring that each module 1 is on the same level and the tool as a whole does not bend.

[0054] Inserting marking pins: When in use, insert the near pin 6 into the positioning hole 4 of arc A, and insert the far pin 8 into the positioning hole 4 of arc F. The image distortion is most obvious at the edge of the fisheye photo, so it is not necessary to insert marking pins in every hole of arc A and arc F. When the angle of incidence is small, the marking pins can be sparser, while the marking pins need to be denser at locations with large angles of incidence. It is recommended to insert marking pins in the following situations: Figure 3 The near pin 6 and the far pin 8 are inserted at the incident angle position shown.

[0055] Benchmark alignment: Place the fisheye lens system 9 (including the lens and imaging module) at the center of the projection model calibration tool. Adjust the position, height, and angle of the system so that the calibration tool plate is not visible in the fisheye photo it captures. Only the benchmark line 5 forms a horizontal line passing through the center of the photo. The near pin 6 and the far pin 8, which are at the same angle of incidence, are aligned, with the near pin 6 directly obscuring the far pin 8. At this point, benchmark alignment is complete, and fisheye lens calibration can begin.

[0056] Example 2:

[0057] A method for calculating a sky view factor comprises the following steps:

[0058] S1. Assemble the calibration tool into a single module 1, complete the assembly and leveling of the projection model calibration tool, and adjust the fisheye lens shooting system;

[0059] S2. Measuring the fisheye lens field of view (calibration 1): Insert a corner pin 7 near the maximum visible angle of incidence on both the left and right sides of the lens and take a photo. Keeping all the arrangements unchanged, move the corner pin 7 on each side, increasing the angle of incidence by 1°, and take another photo. Repeat this process until the corner pin 7 is no longer visible in the field of view. The maximum angle of incidence on that side is the angle minus 1°. The maximum angle of incidence on the left side is θ. L , the maximum incident angle on the right is θ R , then the real viewing angle of the fisheye lens is The present invention can measure the true field angle φ of a fisheye lens with an accuracy of ±1°. The "field angle" refers to the maximum visual range of a fisheye lens.

[0060] S3. Check for eccentricity (check 2): This means that the optical center of the lens is not in the same position as the geometric center of the image. If the fisheye lens has eccentricity, it cannot be used for sky viewing factor measurement. Take a fisheye photo and read the image to obtain the position of the image point of the 0° incident angle in the fisheye photo. If the position of the image point of the 0° incident angle does not coincide with the center of the circular fisheye photo, it proves that the lens has eccentricity. At this time, the θ L ≠θ R ; The position of the image point of the 0° incident angle mark coincides with the center of the circular fisheye photo, proving that the lens has no decentering error. At this time, θ is obtained by measurement. L =θ R The tool of the present invention can intuitively and quickly check whether a fisheye lens has eccentricity error.

[0061] S4. Mark the imaging position of any integer incident angle (verification 3): Assume that the target incident angle you want to mark is equal to θ S , on one side (or both sides) of the calibration tool θ S Insert a corner pin 7 at the corresponding position and take a fisheye photo. The intersection of the corner pin 7 and the reference line 5 in the fisheye photo is obtained by reading the image. This point is the imaging position of the target incident angle. If the distance between the imaging position and the center of the photo is a, in the absence of eccentricity error, all points on the circle with a radius a from the center of the circle satisfy the incident angle equal to θ. S , the incident angle in the inner area of ​​the circle is less than θ S , the incident angle of the outer area of ​​the circle (if any) is greater than θ S .

[0062] S5. Verify the fisheye lens real imaging projection model (verification 4): Figure 4 A schematic diagram of the fisheye lens projection model is disclosed in . The projection model of the fisheye lens refers to the correspondence between the incident angle of the object point and the position of its image point on the fisheye photo. Figure 4 In the figure: O is the object point, θ is the angle of incidence, I is the image point, r t is the radius of the fisheye photo. Verification 4 is to obtain the intersection points of all the near nails 6 and the reference line 5 in the fisheye photo used for verification, and measure and record the incident angle θ and the distance r from the center of the circle corresponding to these intersection points in turn; sort out the data and obtain the fitting relationship or The fitting relationship is the mathematical expression of the real projection model of the fisheye lens, where r 90 is the imaging radius corresponding to a 90° incident angle.

[0063] S6. Before performing SVF measurement, the fisheye lens must be calibrated according to the above calibration methods 2, 3, and 4. After confirming that there is no eccentricity error in the lens, calibrate the image point position r corresponding to the 90° incident angle.90 Finally, the mathematical expression of the real projection model of the fisheye lens is fitted:

[0064]

[0065]

[0066] where r 90 is the distance between the image point on the fisheye photograph and the center of the photograph when the incident angle θ = 90°. The mathematical expression g(x) can be obtained by fitting, or by directly calculating the inverse function of f(x).

[0067] According to the geometric knowledge of radiation, the integral formula of the sky view factor can be obtained (Formula 3):

[0068]

[0069] Where θ is the angle of incidence, and α is the sum of the sky viewing angles on the circle corresponding to a certain angle of incidence.

[0070] Substituting the true projection model expression into formula (3) can calculate the SVF numerical solution in the fisheye photo of any projection model.

[0071] To facilitate programming calculations, the present invention further provides an equal incidence angle difference algorithm (Formula 4) based on Formula (3). This method divides the portion of the fisheye photo within a 90° incident angle into n concentric rings, which are counted from the center to the outside as the 1st to nth rings. The incident angle difference corresponding to the width of each ring is equal. Equal incidence angle difference algorithm:

[0072]

[0073] Among them, θ i is the median incident angle of the ith ring, r 90 is the distance between the image point position corresponding to the 90° incident angle of the fisheye photo and the center of the photo circle; r i is θ i The distance between the corresponding image point position and the center of the circle, r i =r 90 g(θ i );Δr i is the width of each ring, Δr i =r 90 g(iΔθ)-r 90 g[(i-1)Δθ]Δr1=r 90 g(Δθ);α i is the total sky viewing angle of the ith ring.

[0074] The greater the number of concentric rings, the more accurate the sky view factor calculation result. It is recommended that n ≥ 40 to make the calculation error ε ≤ 0.5%.

[0075] Using the above-mentioned Verification 1, Verification 2, Verification 3, Verification 4 and the sky view factor, the incident angle difference algorithm formula (4) is used to take photos outdoors through a fisheye lens and calculate the sky view factor. The workflow diagram is as follows: Figure 5 shown.

[0076] Example 3:

[0077] Figure 6 These are circular fisheye images taken outdoors at the same location using two commercially available fisheye lenses, A and B. These images were taken outdoors at the same location using the same lens under identical conditions: (a) taken with fisheye lens A, (b) taken with fisheye lens B. The difference in the images is significant, and it's impossible to tell with the naked eye which projection model each uses, nor can it be determined whether they can be used to calculate the sky view factor.

[0078] The tool and method of the present invention are used to calibrate lens A and lens B. The calibration is performed to: (1) determine whether the fisheye photo taken by the lens can be used for sky viewing factor calculation, that is, whether it meets the two conditions of field of view angle ≥ 180° and no decentering error; (2) if it can be used for calculation, fit the mathematical expression of its imaging projection model, and substitute the equal incidence angle difference algorithm formula (4) provided by the present invention to calculate the sky viewing factor.

[0079] The specific steps are:

[0080] First, calibrate the fisheye lens A. Complete the assembly, leveling, and setting of the fisheye lens projection model calibration tool indoors, and adjust the fisheye lens shooting system 9. Use the fisheye lens A to take a fisheye photo and extract the image portion of the marking nail in the middle of the photo, such as Figure 7 According to the method of calibrating the eccentricity error, the imaging position of the 0° incident angle marker can be found. The center of the fisheye photo is Figure 7 The cross circle symbol is marked in the middle. It can be observed that the projection position of the 0° incident angle mark pin does not coincide with it, indicating that there is an eccentricity error in the fisheye lens A. According to the method of measuring the field of view of the fisheye lens, the maximum incident angle on the left side of the fisheye lens A is measured to be θ L =88°, the maximum incident angle on the right is θ R=84°, and the true field of view angle φ of fisheye lens A is 172°. The calibration tool provided by the present invention confirms that fisheye lens A has decentration error and its field of view angle is less than 180°. Therefore, lens A does not meet the sky view factor measurement requirements, and its projection model formula is not fitted. The calibration of fisheye lens A illustrates that the calibration tool provided by the present invention can quickly eliminate fisheye lenses that are unsuitable for sky view factor measurement.

[0081] Then, calibrate the fisheye lens B. Complete the assembly, leveling, and setting of the fisheye lens projection model calibration tool indoors, and adjust the fisheye lens shooting system 9. Use the fisheye lens B to take a fisheye photo. Extract the image portion of the marking nail in the middle of the photo, such as Figure 8 According to the method of calibrating the eccentricity error, the imaging position of the 0° incident angle marker can be found. Figure 8 The center of the circle in the fisheye photo is marked with a cross circle symbol. It can be seen that the projection position of the 0° incident angle mark coincides well, indicating that there is no decentering error in fisheye lens B. According to the method for measuring the field of view of the fisheye lens, the maximum incident angle on the left side of fisheye lens B is measured to be θ L =108°, the maximum incident angle on the right is θ R =108°. The true field of view of fisheye lens B

[0082] Fisheye lens B has no decentering error and its field of view exceeds 180°, so lens B can be used for sky viewing factor measurement and continue to fit its projection model formula.

[0083] According to the marking method of the imaging position at any incident angle, insert a corner nail 7 at the position where the incident angle θ = 90°, take a fisheye photo, and find the imaging position of the 90° corner nail 7 on this calibration photo and mark it. The distance between this position and the center of the photo is r 90 , the location corresponds to the outdoor real-life photo Figure 6 (b) The location of the inner circle, that is, Figure 6 Only the imagery within this circle is used when calculating the sky view factor.

[0084] According to the fisheye lens real imaging projection model verification method, the projection model of the fisheye lens B is fitted into the mathematical expressions of formula (5) and formula (6):

[0085]

[0086]

[0087] like Figure 9 As shown, it can be found that the real projection model of fisheye lens B does not belong to any of the four classic projection models.

[0088] Because fisheye lens B is not based on an equidistant projection model, the existing Steyn formula cannot be used to calculate the sky viewing factor. However, the equal incidence angle difference algorithm provided by the present invention can still accurately calculate the sky viewing factor.

[0089] Substitute formula (5) and (6) into the equal incidence angle difference algorithm formula (4) provided by the present invention, and automatically accumulate them through programming. Figure 6 (b) The calculated sky view factor for this location is 0.198. This result is consistent with measurements using an equidistant projection model fisheye lens. However, Lens B's market price is only 1 / 10 to 1 / 8 of the price of an equidistant projection model fisheye lens. Using the calibration tool and universal formula provided by this invention allows for more flexible selection of fisheye lenses with various projection models, thereby reducing the cost of sky view factor measurements.

[0090] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

[0091] Although the above describes the specific embodiments of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without any creative work are still within the scope of protection of the present invention.

Claims

1. A fisheye lens projection model calibration tool, characterized in that: It includes N single modules with the same structure, and the single modules are spliced ​​with each other in pairs. The overall angle formed by the N single modules is larger than the real field of view angle of the lens being tested. The number of the single modules N is greater than or equal to 1, the angle ζ of the single modules is an integer multiple of 5°, the shape of the single module is a fan-shaped plate structure, and the single module is provided with a positioning hole, a marking pin is inserted into the positioning hole, and the positioning hole is connected to the scale indicator line for positioning; The single module has 6 positioning holes within every 5° range. The 6 positioning holes are respectively located on concentric arcs AF of different radii. The angles of the 6 positioning holes relative to the center of the circle increase by 1°. The radii of the concentric arcs corresponding to the 6 positioning holes meet the following requirements: (R B -R A )>(R C -R B )>(R D - C )>(R E -R D )>(R F -R E )。 2. A fisheye lens projection model verification tool according to claim 1, characterized in that: Radius R of arc A A ≥15cm, radius R of arc F F ≥35cm.

3. The fisheye lens projection model verification tool according to claim 1, characterized in that: Both sides of the single module are provided with snap-in positions and slots for splicing. The positioning holes are fan-shaped and relatively evenly distributed on the body of the single module. A reference line is also provided on the single module. The number of the single modules N is greater than or equal to 5.

4. The fisheye lens projection model verification tool according to claim 1, characterized in that: The marking pins include near pins, far pins and corner pins. The near pins are inserted into the positioning holes corresponding to arc A, the far pins are inserted into the positioning holes corresponding to arc F, and the corner pins are inserted near the maximum visible incident angles on the left and right sides of the lens or at a specific incident angle that needs to be calibrated.

5. A method for calculating a sky view factor, characterized in that: The following steps are involved: S1. Assemble the calibration tool from individual modules, complete assembly and leveling of the projection model calibration tool, and adjust the fisheye lens system. S2. Measure the field of view of the fisheye lens: Insert a corner pin near the maximum visible incident angle on the left and right sides of the lens and take a photo. Keeping all the arrangements unchanged, move the corner pin on the left and right sides, increasing the incident angle by 1°, and take another photo. Repeat the above steps until the corner pin is moved to a certain incident angle position where it is no longer visible in the field of view. The incident angle minus 1° is the maximum incident angle on the left or right side. The maximum incident angle on the left is θ. L , the maximum incident angle on the right is θ R , then the real viewing angle of the fisheye lens is S3. Checking the eccentricity error: Using the fisheye photo used for calibration, obtain the position of the image point of the mark at a 0° incident angle in the fisheye photo. If the position of the mark point does not coincide with the center of the circular fisheye photo, it proves that the lens has an eccentricity error. At this time, θ is obtained by measurement. L ≠θ R The position of the image point of the mark coincides with the center of the circular fisheye photo, proving that the lens does not have an eccentric error. At this time, θ is obtained by measurement. L =θ R ; S4. Mark the imaging position at any incident angle: Assume that the incident angle of the target you want to mark is equal to θ S , on one or both sides of the calibration tool θ S Insert a corner nail at the corresponding position and take a fisheye photo. Obtain the intersection of the corner nail and the reference line in the fisheye photo by reading the image. This intersection is the imaging position of the target incident angle. S5. Verify the real imaging projection model of the fisheye lens: Use the fisheye photo used for verification to obtain the intersection points of all near-nail and baseline in the fisheye photo, measure and record the corresponding incident angle θ and their distance r from the center of the circle; organize the data and obtain the fitting relationship or The fitting relationship is the mathematical expression of the real projection model of the fisheye lens, where r 90 is the imaging radius corresponding to a 90° incident angle; S6. After confirming that the lens does not have decentering error, calibrate the image point position r corresponding to the 90° incident angle. 90 Finally, the mathematical expression of the real projection model of the fisheye lens is fitted: According to the equal incidence angle difference algorithm, the sky view factor is obtained as follows: Among them, θ i is the median incident angle of the ith ring, r 90 is the distance between the image point position corresponding to the 90° incident angle of the fisheye photo and the center of the photo circle; r i is θ i The distance between the corresponding image point position and the center of the circle, r i =r 90 g(θ i );Δr i is the width of each ring, Δr i =r 90 g(iΔθ)-r 90 g[(i-1)Δθ]Δr1=r 90 g(Δθ);α i is the total sky viewing angle of the ith ring.

6. A method for calculating a sky view factor according to claim 5, characterized in that: In step S3, if the distance between the imaging position and the center of the photo is a, and there is no eccentricity error, each point on the circle with a radius a from the center satisfies the incident angle equal to θ S , the incident angle in the inner area of ​​the circle is less than θ S , the incident angle in the outer area of ​​the circle is greater than θ S .

7. A method for calculating a sky view factor according to claim 5, characterized in that: In step S6, the portion of the fisheye photo within a 90° incident angle is divided into n concentric rings, which are counted as the 1st to nth rings from the center outward, and the field angle difference corresponding to the width of each ring is equal.

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