Calibration method for camera

Abstract

To provide a calibration method for a camera, capable of improving accuracy of calibration of a camera.SOLUTION: A calibration method for a camera 12 includes: an image acquisition step of capturing reflection light of illumination light irradiated from a light source to a calibration material MR including a reflector B having a reflection performance of reflecting and returning illumination light in an incident direction, by a camera arranged at a location optically conjugate to the light source to acquire a calibration image; a pattern information setting step of setting pattern information regarding the calibration material; a feature point detection step of detecting, as a feature point, a condensing position of the reflection light from the a calibration image; and a parameter calculation step of calculating a camera parameter, according to a location of the feature point and the pattern information.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to a method for calibrating a camera.

Background Art

[0002] Generally, when using a camera for object detection, calibration of the camera is performed. In camera calibration, a planar calibration pattern having a plurality of feature points aligned two-dimensionally is prepared, and the calibration pattern is photographed with the camera from a plurality of directions. Then, feature points are detected from a plurality of images photographed by the camera, and camera parameters are estimated using the coordinates of the detected feature points. The camera parameters include internal parameters (focal length, optical center) or distortion parameters (distortion coefficients) of the camera.

[0003] For example, Patent Document 1 discloses a method for calibrating a camera using a dot pattern in which a plurality of dots (black circles) are arranged in a grid pattern as a calibration pattern. In this method, the position of the center of gravity of the dots included in the dot pattern is detected as a feature point on the image (calibration image) photographed by the camera, and the camera parameters are obtained.

[0004] Further, Patent Document 1 also discloses a grid pattern and a checker pattern as calibration patterns other than the dot pattern. When these calibration patterns are used, the intersection (corner) of straight lines is detected as a feature point.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] When the grid pattern shown in XIA of FIG. 11 or the checker pattern shown in XIB of FIG. 11 among the calibration patterns disclosed in Patent Document 1 is used, the intersection (corner) H1 or H2 of the straight lines of each pattern on the calibration image is detected as a feature point (that is, edge detection). In such edge detection, it is necessary to use the luminance gradient (differential value) of the edge. Therefore, the detection position (coordinates) of the intersection H1 or H2 is easily affected by luminance noise, and there is a problem that the detection accuracy of the feature point is not good compared with the dot pattern.

[0007] On the other hand, when the dot pattern shown in XIC of FIG. 11 is used, since the position of the center of gravity of the dot D of the dot pattern on the calibration image is detected as a feature point, it is less affected by luminance noise compared with the grid pattern or the checker pattern, but there are problems as described below.

[0008] FIGS. 12 and 13 show an example of photographing the dot D on the dot pattern by the camera CAM. In FIG. 12, O c indicates the origin (camera origin) of the camera coordinate system with respect to the camera CAM. Also, in FIGS. 12 and 13, the U-axis and the V-axis are coordinate axes that constitute an image coordinate system (two-dimensional orthogonal coordinate system) defined on the image plane M of the camera CAM.

[0009] In an image captured using a camera having a normal lens (non - telecentric optical system), there is a characteristic that the magnification changes depending on the distance from the camera. That is, the farther the distance from the camera, the smaller the resulting image, and the closer the distance, the larger the resulting image. Therefore, as shown in FIG. 12, when the dot D on the dot pattern is photographed by the camera CAM from an oblique direction, as shown in FIG. 13, the image of the dot D (hereinafter referred to as the "dot image") DI on the image captured by the camera CAM becomes an oval shape distorted in a specific direction according to the positional relationship with the camera CAM. That is, in the dot image DI on the image captured by the camera CAM, the portion on the front side (the side where the distance from the camera CAM is short, the +V side) is relatively smaller than the portion on the back side (the side where the distance from the camera CAM is long, the -V side). Therefore, when trying to obtain the two - dimensional dot centroid position from the image captured by the camera CAM, due to the influence of the non - telecentric optical system, the dot centroid position may be obtained at a position deviated from the original position.

[0010] FIGS. 14 and 15 are diagrams for explaining the influence of the inclination of the camera CAM with respect to the dot pattern (dot D) on the detection result of the dot centroid position.

[0011] FIG. 14 shows a state where the camera CAM is facing the dot pattern (dot D) directly (the optical axis AX of the camera CAM is orthogonal to the dot D, that is, the state where the camera CAM is not inclined with respect to the dot pattern). In this case, since the dot image DI on the image plane M of the camera CAM and the dot D before projection are in a similar shape, the dot centroid position G1 on the image plane M (the centroid position obtained two - dimensionally from the shape of the dot image DI on the image plane M) coincides with the actual dot centroid position G0 projected on the image plane M (the projection point obtained by projecting the centroid position g of the dot D before projection onto the image plane M), and the dot centroid position can be detected without error.

[0012] FIG. 15 shows a state in which the camera CAM is tilted with respect to the dot pattern (dots D). In this case, due to the influence of the non-telecentric optical system, the dot image DI on the image plane M has an asymmetric shape with one side (the left side in FIG. 15) and the other side (the right side in FIG. 15) sandwiching the actual dot centroid position G0 projected onto the image plane M (the projection point of the centroid position g of the dot D before projection onto the image plane M). Therefore, the dot centroid position G1 on the image plane M does not coincide with the actual dot centroid position G0 projected onto the image plane M, resulting in a detection error in the dot centroid position.

[0013] Further, when using a planar pattern such as a dot pattern, a grid pattern, and a checkerboard pattern as a calibration object, it may not be possible to accurately grasp the position error and the like of the feature points on each pattern. Therefore, it is difficult to perform calibration on the calibration object, which becomes a factor causing a decrease in the calibration accuracy of the camera.

[0014] The present invention has been made in view of such circumstances, and an object thereof is to provide a camera calibration method capable of improving the accuracy of camera calibration.

Means for Solving the Problems

[0015] The present invention comprises the following aspects in order to achieve the above object.

[0016] The camera calibration method according to the first aspect of the present invention includes an image acquisition step of irradiating a calibration object including a reflector having a property of reflecting incident light back in the incident direction with illumination light from a light source, and photographing the reflected light of the illumination light with a camera arranged at a position optically conjugate to the light source to obtain a calibration image; a pattern information setting step of setting pattern information regarding the calibration object; a feature point detection step of detecting a light condensing position of the reflected light as a feature point from the calibration image; and a parameter calculation step of calculating camera parameters based on the pattern information and the position of the feature point.

[0017] The calibration method of the camera according to the second aspect of the present invention is as follows. In the first aspect, in the image acquisition step, the camera captures the reflected light from at least two reflectors of the calibration device.

[0018] The calibration method of the camera according to the third aspect of the present invention is as follows. In the first aspect or the second aspect, the pattern information includes information regarding the arrangement and interval of the reflectors on the calibration device.

[0019] The calibration method of the camera according to the fourth aspect of the present invention is as follows. In any one of the first aspect to the third aspect, the reflectors are spherical and arranged in an array on the calibration device.

[0020] The calibration method of the camera according to the fifth aspect of the present invention is as follows. In the fourth aspect, the surface of the reflector is a mirror surface, or a rough surface from which specularly reflected light that can be distinguished from the diffusely reflected light incident non-perpendicularly to the surface of the reflector can be obtained.

[0021] The calibration method of the camera according to the sixth aspect of the present invention is as follows. In the fourth aspect, the reflectors are made of glass or sapphire.

Effect of the Invention

[0022] According to the present invention, the calibration accuracy of the camera can be improved.

Brief Description of the Drawings

[0023]

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Mode for Carrying Out the Invention

[0024] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0025] 〔Projection Model of Camera〕 First, the projection model of the camera in the present embodiment will be described. FIG. 1 is an explanatory diagram for explaining the projection model of the camera in the present embodiment.

[0026] As shown in FIG. 1, the world coordinate system (world coordinate system) is a coordinate system representing positions in a three-dimensional space (real space), with the origin at O w and mutually orthogonal X w axis, Y w axis, Z wIt is a three-dimensional orthogonal coordinate system with the axis as the coordinate axis. Note that for the world coordinate system, any coordinate system may be used as long as it can specify the position (three-dimensional position) in the three-dimensional space. The camera coordinate system has the center O of the optical axis of the camera c as the origin, and from the origin O c the right direction is the X c axis, the downward direction is the Y c axis, and the optical axis direction is the Z c axis, which is a three-dimensional orthogonal coordinate system. The image coordinate system has the upper left of the image plane IP, which is at a focal length f away from the origin O of the camera coordinate system in the Z c direction, as the origin, and has a two-dimensional orthogonal coordinate system (pixel coordinate system) with the U axis and the V axis in the directions parallel to the X c axis and the Y c axis respectively. c axis respectively.

[0027] First, the coordinates (x w , y w , z w ) of a point P (object point) in the three-dimensional space in the world coordinate system can be converted into the coordinates (x, y, z) in the camera coordinate system as shown in the following formula (1) by using the rotation matrix R and the translation vector t of the camera.

[0028]

Equation

[0029] Next, when the coordinates (pixel coordinates) of the projection point Q where the point P at (x, y, z) is projected onto the image plane IP as seen from the camera coordinate system are (u, v), the relational expressions shown in the following formulas (2) to (7) hold.

[0030]

Number

[0031]

Number

[0032]

Number

[0033]

Number

[0034]

Number

[0035]

Number

[0036] Here, (x´, y´) represents the coordinates of the projection point obtained by projecting the point P at (x, y, z) onto the normalized image plane (z = 1) as seen from the camera coordinate system. Also, (x´´, y´´) represents the coordinates of the projection point (distorted point) obtained by projecting the point P onto the normalized image plane when considering the lens distortion of the camera.

[0037] Also, f x and f y represent the focal lengths in the x - direction and y - direction expressed in pixel units. Also, c x and c y represent the optical center in the image coordinate system (the position where the optical axis of the camera intersects the image plane IP, the optical center in pixel units). Also, k1, k2, k3 are the radial distortion coefficients, and p1, p2 are the tangential distortion coefficients. In this specification, the focal lengths f x and f y and the optical center c x, c y is called the internal parameter of the camera, and the distortion coefficients k1, k2, k3, p1, and p2 are called the distortion parameters of the camera.

[0038] 〔Calibration Device〕 FIG. 2 is a block diagram showing an example of the schematic configuration of the calibration device 10 of the present embodiment. As shown in FIG. 2, the calibration device 10 includes an arithmetic control unit 20 and a storage unit 22. Further, a camera 12, an operation unit 14, and an output unit 16 are connected to the calibration device 10.

[0039] The camera 12 generates an image IM (calibration image) obtained by photographing a calibration object MR (see FIGS. 3 and 4) from different directions and outputs it to the calibration device 10. The camera 12 used in the present embodiment has a non - telecentric optical system (an optical system having non - telecentricity on the subject side), and photographing is performed by an imaging element via a lens group as the non - telecentric optical system.

[0040] The operation unit 14 includes operation members such as a keyboard and a mouse, and receives inputs of various operations by an operator.

[0041] The output unit 16 is a device for outputting the calculation result and the like by the arithmetic control unit 20. The output unit 16 includes, for example, an operation UI (User Interface) and a monitor (for example, a liquid crystal display, etc.) for displaying the calculation result. Further, in addition to or instead of the monitor, the output unit 16 may include a printer or a speaker, etc.

[0042] The arithmetic control unit 20 controls the operation of the calibration device 10. The arithmetic control unit 20 is composed of an arithmetic device such as a personal computer, and includes an arithmetic circuit composed of various processors and memories. The various processors include a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), an ASIC (Application Specific Integrated Circuit), and programmable logic devices [such as SPLD (Simple Programmable Logic Devices), CPLD (Complex Programmable Logic Device), and FPGA (Field Programmable Gate Arrays)]. Note that the various functions of the calibration device 10 may be realized by one processor or by a plurality of processors of the same type or different types.

[0043] The storage unit 22 stores a control program and various data. The storage unit 22 is composed of, for example, a hard disk drive (HDD) or a semiconductor storage device (SSD). The storage unit 22 may include a temporary storage element composed of a RAM (Random Access Memory) such as a DRAM (Dynamic Random Access Memory) or an SRAM (Static Random Access Memory), and may function as a work area of the arithmetic control unit 20.

[0044] A plurality of calibration images IM captured by the camera 12 are temporarily stored in the storage unit 22. Also, the camera parameters CP obtained by the arithmetic control unit 20 are stored in the storage unit 22. The camera parameters CP include the internal parameters (focal length, optical center) of the camera 12 and distortion parameters (radial and tangential distortion coefficients).

[0045] The arithmetic control unit 20 functions as an image acquisition unit 30, a pattern information setting unit 32, a feature point detection unit 34, and a parameter calculation unit 36 by reading and executing a control program stored in the storage unit 22. The functions of each unit constituting the arithmetic control unit 20 will be described later.

[0046] FIG. 3 is a perspective view showing the calibration object MR, and FIG. 4 is a sectional view taken along line IV-IV of FIG. 3.

[0047] The calibration object MR is a structure in which a plurality of balls B are arranged in a housing F (a ball target). The intervals and diameters of the balls B are assumed to be known. Information regarding the calibration object MR (for example, information regarding the type of the calibration object MR, the arrangement, intervals, and diameters of the balls B. Hereinafter, referred to as pattern information.) is stored in the storage unit 22 of the calibration device 10.

[0048] The housing F is a member having a planar shape (plate shape) formed of a material with high rigidity and stability over time (for example, metal or ceramics, etc.). A plurality of recesses H are formed in the housing F in an array.

[0049] The ball B is a spherical member formed of a material with high rigidity and stability over time (for example, metal, etc.). The ball B is fitted into the recess H and fixed to the housing F. The ball B is an example of a reflector and has a reflection performance of reflecting (specular reflection) the illumination light incident perpendicularly to its surface back in the incident direction. The sphericity of the ball B (see, for example, Japanese Industrial Standard JIS B 1501: 2009) is high, and the surface of the ball B is mirror-finished. Here, the surface roughness (arithmetic mean roughness) Ra (Japanese Industrial Standard JIS B0601: 2001) of the mirror surface is, for example, 0.2 μm or less.

[0050] Note that the surface of the ball B may be a rough surface instead of a mirror surface as long as the reflected light described later can be detected. Here, a rough surface is a surface that is rougher than a mirror surface (for example, Ra > 0.2 μm in one example), but is a surface that satisfies at least one of the following conditions. (A) A surface capable of reflecting specular reflection light (L21 and L22) with a strength equal to or greater than a specified value. (B) A surface (a surface where the difference between the intensity of the reflected light of the illumination light incident perpendicularly to the surface of the ball B and the intensity of the diffusely reflected light diffusely reflected around the perpendicular incidence position becomes equal to or greater than a threshold value) from which specular reflection light (L21 and L22) distinguishable from the diffusely reflected light diffusely reflected after being incident non-perpendicularly on the surface of the ball B from the light source 50 (for example, stronger than the diffusely reflected light) can be obtained.

[0051] Further, the ball B may have, for example, retroreflective performance (refer to Japanese Industrial Standard JIS Z8713: 1995) formed of glass or sapphire.

[0052] In the examples shown in FIGS. 3 and 4, the balls B are arranged at equal intervals, but it is not limited thereto. For example, as long as the intervals d1 and d2 between the balls B are known, the intervals d1 and d2 between the balls B may be different or may be non-uniform. Also, the diameters of the balls B may be different from each other. Further, the calibration device MR may be, for example, a one-dimensional calibrator (ball-bar type calibrator) in which a plurality of balls B are arranged in a linear housing (bar), or may be one in which a plurality of balls B are arranged in an L-shaped housing (bar).

[0053] According to the calibration device MR configured as described above, it becomes possible to measure the positions (three-dimensional coordinates) of the respective balls B arranged in the housing F with a highly accurate measuring device (traceable three-dimensional coordinate measuring device). Therefore, it becomes possible to perform traceable calibration for the actually used calibration device MR. Thereby, calibration of the camera 12 can be performed with high accuracy based on highly accurate pattern information (including information regarding the arrangement, interval, and diameter of the balls B).

[0054] In this embodiment, the ball B is shown as an example of the reflector. However, the present invention is not limited to this, and for example, a calibration device provided with a retroreflector (retroreflector, see Japanese Industrial Standard JIS Z8713: 1995. For example, a cube corner or the like) may be used. In the case of a retroreflector, the range of the tilt angle at which the retroreflective performance can be obtained with respect to the incident light may be narrower than that of the ball B. However, when an imaging system that allows the tilt angle conditions is used, the retroreflector can be used.

[0055] Hereinafter, for the sake of simplicity of explanation, an example will be described in which the balls B are arranged at equal intervals along two directions orthogonal to each other, and the intervals d1 and d2 between adjacent balls B along these two directions are equal.

[0056] FIG. 5 is a diagram for explaining an example of calibration using the calibration device MR. In the example shown in FIG. 5, for the sake of simplicity of illustration, the light source 50 is arranged on the perpendicular bisector of the line segment connecting the centers of the balls B1 and B2, but the present invention is not limited to this.

[0057] As shown in FIG. 5, when photographing the calibration device MR, illumination light is irradiated coaxially with the lens 120 of the camera 12 with respect to the calibration device MR (coaxial illumination). The imaging system of the calibration device MR includes a light source 50, a half mirror 52, and a camera 12. The camera 12 includes a lens (objective lens) 120 and an image sensor (for example, including a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), etc.) 122. The arithmetic control unit 20 (see FIG. 2) of the calibration device 10 performs light emission control of the light source 50, imaging control of the camera 12, and the like.

[0058] The light source 50 is a device that irradiates the calibration device MR with visible light, and includes, for example, a light emitting diode or the like. The light source 50 is arranged at a position optically conjugate to the camera origin (object-side principal point) O c via the half mirror 52. Here, the camera origin O cis located, for example, at the center (optical axis center) of the lens 120 of the camera 12.

[0059] Among the illumination light from the light source 50, the components L11 and L12 that are perpendicularly incident on the surfaces of the balls B1 and B2 are reflected (retroreflected) toward the light source 50 along their respective incident optical paths. Then, the reflected lights L21 and L22 reflected from the surfaces of the balls B1 and B2 are reflected by the half mirror 52 and imaged on the image sensor 122 through the lens 120. Thereby, images of the reflected lights L21 and L22 retroreflected by the balls B1 and B2 are obtained.

[0060] The reference numerals P1 and P2 in FIG. 5 indicate the positions (condensing positions) where the reflected lights L21 and L22 from the balls B1 and B2 are imaged on the image sensor 122, respectively, and the reference numeral P0 indicates the conjugate image of the light source 50.

[0061] The image acquisition unit 30 acquires a calibration image IM including the images P1 and P2 of the reflected lights L21 and L22 photographed by the image sensor 122.

[0062] The pattern information setting unit 32 acquires pattern information regarding the calibration object MR from the storage unit 22.

[0063] The feature point detection unit 34 detects the images P1 and P2 (bright field observation images) of the reflected lights L21 and L22 and detects the centroid positions of the respective images P1 and P2.

[0064] The parameter calculation unit 36 calculates the camera parameter CP based on the positions of the feature points (centroids of the images P1 and P2 of the reflected lights L21 and L22) detected by the feature point detection unit 34 and the pattern information (for example, information regarding the arrangement and interval of the balls B1 and B2 in the calibration object MR) set by the pattern information setting unit 32.

[0065] As described above, in the present embodiment, the light source 50 is the camera origin O via the mirror 52 cIt is installed at a position conjugate to the (object-side principal point). Therefore, the imaging center (center of gravity) of the light source 50 reflected by the calibration device MR lies on the line connecting the viewpoint (camera origin O c ) and the center of the calibration device MR.

[0066] In this embodiment, the reflected lights L21 and L22 reflected perpendicularly to the surfaces of the balls B1 and B2 are observed. Therefore, according to this embodiment, distortion and pixel error due to the inclination of the pattern do not occur, and the detection pixel position accuracy of the calibration device MR is not affected by the posture of the calibration device MR. Thereby, the calibration of the camera 12 can be carried out with high precision. Also, a traceable calibration device can be provided.

[0067] In the example shown in FIG. 5, the reflected lights L21 and L22 from the two balls B1 and B2 are photographed. However, the number of balls used for calibration is not limited to two, and three or more balls may be used.

[0068] Also, in the example shown in FIG. 5, the illumination light transmitted through the half mirror 52 reaches the balls B1 and B2, and the reflected light reflected by the half mirror 52 reaches the camera 12. However, the arrangement of the optical system in the imaging system is not limited to this. For example, the illumination light reflected by the half mirror 52 may reach the balls B1 and B2, and the reflected light transmitted through the half mirror 52 may reach the camera 12.

[0069] Hereinafter, the processing procedure of the calibration process (an example of the calibration method) executed by the calibration device 10 of this embodiment will be described. FIG. 6 is a flowchart showing the overall process of the calibration process executed by the calibration device 10 of this embodiment. It is assumed that various initial setting processes such as operation confirmation of each part of the calibration device 10 are performed at the start of the flowchart shown in FIG. 6.

[0070] (Step S10: Image acquisition step) First, the image acquisition unit 30 acquires a plurality of calibration images IM captured by the camera 12 and stores them in the storage unit 22.

[0071] The image acquisition unit 30 may acquire each calibration image IM from the camera 12 through a cable or a recording medium. Also, each calibration image IM may be stored in an external storage device such as an external server installed outside the calibration device 10, and the image acquisition unit 30 may acquire each calibration image IM from the external storage device via a wired or wireless network.

[0072] (Step S12: Pattern information setting step) Next, the pattern information setting unit 32 sets pattern information regarding the calibration object MR. The pattern information includes, for example, information regarding the arrangement and interval of the balls B1 and B2 in the calibration object MR. The pattern information is information used to define points (object points) in three-dimensional space corresponding to feature points (image points) in the image coordinate system.

[0073] The pattern information setting unit 32 may acquire, for example, pattern information input by the operator via the operation unit 14. Also, the pattern information may be stored in the storage unit 22 in advance, and the pattern information setting unit 32 may acquire it from the storage unit 22. Note that the pattern information setting step only needs to be performed at least before the parameter calculation step described later. For example, it may be performed before the image acquisition step or after the feature point detection step.

[0074] (Step S14: Feature point detection step) Next, the feature point detection unit 34 executes a feature point detection process for detecting a plurality of feature points for each of the plurality of calibration images IM captured by the camera 12. Specifically, the feature point detection unit 34 sequentially reads the plurality of calibration images IM stored in the storage unit 22. Then, for each of the read calibration images IM, the feature point detection unit 34 performs predetermined image processing (for example, binarization processing or grayscale conversion) on the calibration image IM, and then detects each feature point (image point) from the calibration image IM, and obtains the coordinates (pixel coordinates) of each feature point in the image coordinate system. In the present embodiment, for example, images P1 and P2 of the reflected lights L21 and L22 are detected from the bright field observation images of the balls B1 and B2 shown in FIG. 5, and the centroid positions of the images P1 and P2 are detected as feature points. The coordinates of the feature points detected by the feature point detection unit 34 are temporarily stored in the storage unit 22.

[0075] If there is a calibration image IM in which the detection of feature points fails among the plurality of calibration images IM, the feature point detection unit 34 performs an exclusion process of excluding the calibration image IM in which the detection of feature points fails from the target of the parameter calculation process described later. As a result, in the parameter calculation process, the camera parameter CP can be calculated based on the calibration images IM after the exclusion process (that is, the calibration images IM in which the detection of feature points is successful) among the plurality of calibration images IM.

[0076] (Step S16: Parameter calculation step) Next, the parameter calculation unit 36 executes the calculation process of the camera parameters CP. Specifically, the parameter calculation unit 36 calculates the camera parameters CP based on the positions of the feature points on each calibration image IM detected by the feature point detection unit 34 and the pattern information set by the pattern information setting unit 32. The camera parameters CP can be calculated using a known method (for example, Zhang's method). Zhang's method is a method of optimizing parameters so that the position of the feature points (image points) on the captured image (calibration image IM) and the position of the points (object points, known) in the three-dimensional space corresponding to the feature points have a correct correspondence relationship (Z. Zhang, "A flexible new technique for camera calibration", IEEE Transactions on Pattern Analysis and Machine Intelligence, Vol.22, No.11, pp.1330-1334, 2000.).

[0077] The camera parameters CP calculated by the parameter calculation unit 36 are the internal parameters of the camera 12 (focal length f x , f y , optical center c x , c y ) and distortion parameters (distortion coefficients k1, k2, k3, p1, p2). The parameter calculation unit 36 stores the calculated camera parameters CP in the storage unit 22.

[0078] (Step S18: Output step) Next, the parameter calculation unit 36 outputs the calculation result of the camera parameters CP to the output unit 16. As a result, the operator can obtain the camera parameters CP, and thus it becomes possible to perform image correction and the like on the images captured by the camera 12 based on the camera parameters CP.

[0079] Thus, the flowchart of the overall calibration process executed by the calibration device 10 ends.

[0080] 〔Example〕 FIG. 7 is a graph showing the tilt resistance of the calibration method. The horizontal axis of FIG. 7 indicates the tilt angle (degree) of the camera with respect to the calibration object MR or the calibration pattern, and the vertical axis indicates the pixel error (pixel).

[0081] The example shows an example in which calibration is performed using a calibration object MR (sphere target). Comparative Example 1 shows an example in which calibration is performed using the dot pattern illustrated in XIC of FIG. 11, and Comparative Example 2 shows an example in which calibration is performed using the checker pattern illustrated in XIB of FIG. 11.

[0082] In the case of Comparative Example 1, as shown in FIG. 13, since an object closer to the camera is captured larger, the larger the tilt angle (absolute value) with respect to the dot pattern, the larger the pixel error (absolute value), and the tilt resistance is deteriorated.

[0083] In the example, regardless of the tilt angle (absolute value) of the camera 12 with respect to the calibration object MR, the pixel error (absolute value) is small. Also, in Comparative Example 2, regardless of the tilt angle (absolute value) of the camera with respect to the checker pattern, the pixel error (absolute value) is small.

[0084] FIGS. 8 to 10 are graphs showing the luminance noise resistance of the calibration method, corresponding to the example (sphere target), Comparative Example 1 (dot pattern), and Comparative Example 2 (checker pattern), respectively. The horizontal axis of FIGS. 8 to 10 indicates the sample number, and the vertical axis indicates the pixel error (pixel). Note that only the vertical axis of FIG. 8 has a narrowed range so that the graph is not collapsed.

[0085] In FIGS. 8 to 10, the luminance noise added during the shooting of the calibration object MR or the calibration pattern is increased in the order of Noise = 0.0, 4.0, 8.0.

[0086] In Comparative Example 2 (Fig. 10), since the luminance gradient (differential value) is used when detecting the edges of the checker pattern, the pixel error becomes significantly larger according to the luminance noise. Among the examples shown in Figs. 8 to 10, it can be seen that it is the most sensitive to luminance noise.

[0087] On the other hand, the dot pattern according to Comparative Example 1 (Fig. 9) has higher luminance noise resistance than the checker pattern.

[0088] Furthermore, in the Example (Fig. 8), as is clear from the range of the vertical axis, the luminance noise resistance is higher than in either of Comparative Examples 1 and 2.

[0089] Summarizing the above results in a table gives the following table. In the Example, both the tilt resistance and the luminance noise resistance are good.

[0090]

Table 1

[0091] 〔Effect〕 According to the present embodiment, for the calibration device MR including a reflector (ball B or retroreflector) having a reflection performance of reflecting and returning illumination light in the incident direction, illumination light (coaxial illumination light) is irradiated from a light source 50 arranged at a position conjugate with the camera origin O c and the reflected light from the reflector is photographed by the camera 12 to obtain a calibration image IM. Then, by detecting the position of the reflected light as a feature point from this calibration image IM, it is possible to realize highly accurate calibration with excellent tilt resistance and luminance noise resistance.

[0092] Further, according to the present embodiment, by using the calibration device MR as described above, it becomes possible to measure the position (three-dimensional coordinates) of the reflector (ball B or retroreflector) by a high-precision measuring device (traceable three-dimensional coordinate measuring device). As a result, it becomes possible to perform traceable calibration for the calibration device MR, and it becomes possible to improve the calibration accuracy.

Explanation of Signs

[0093] 10…Calibration device, 12…Camera, 14…Operation unit, 16…Output unit, 20…Arithmetic control unit, 22…Storage unit, 30…Image acquisition unit, 32…Pattern information setting unit, 34…Feature point detection unit, 36…Parameter calculation unit, 50…Light source, 52…Mirror, 120…Lens, 122…Image sensor, MR…Calibration device, B…Ball, F…Housing

Claims

1. An image acquisition step of photographing the reflected light of the illumination light irradiated from a light source with a camera disposed at a position optically conjugate with the light source with respect to a calibration object including a reflector having a reflection performance of reflecting the illumination light back in the incident direction, and acquiring a calibration image; A pattern information setting step of setting pattern information regarding the calibration object; A feature point detection step of using, as a feature point, the condensing position of the reflected light from the calibration image; A parameter calculation step of calculating camera parameters based on the pattern information and the position of the feature point; A calibration method for a camera comprising the above steps.

2. The calibration method according to Claim 1, wherein, in the image acquisition step, the reflected light from at least two reflectors of the calibration object is photographed by the camera.

3. The calibration method according to Claim 1, wherein the pattern information includes information regarding the arrangement and interval of the reflectors in the calibration object.

4. The calibration method according to any one of Claims 1 to 3, wherein the reflector has a spherical shape and is arranged in an array on the calibration object.

5. The calibration method according to Claim 4, wherein the surface of the reflector is a mirror surface or a rough surface capable of obtaining specular reflection light distinguishable from the diffused reflection light incident non-vertically on the surface.

6. The calibration method according to Claim 4, wherein the reflector is made of glass or sapphire.

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