CN115018921A - Camera calibration board and calibration method thereof - Google Patents

Camera calibration board and calibration method thereof Download PDF

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Publication number
CN115018921A
CN115018921A CN202210429763.2A CN202210429763A CN115018921A CN 115018921 A CN115018921 A CN 115018921A CN 202210429763 A CN202210429763 A CN 202210429763A CN 115018921 A CN115018921 A CN 115018921A
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mark point
mark points
points
calibration
camera
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范晓鹏
郝颖明
魏景阳
付双飞
吴清潇
朱枫
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Shenyang Institute of Automation of CAS
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Shenyang Institute of Automation of CAS
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    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T7/00—Image analysis
    • G06T7/80—Analysis of captured images to determine intrinsic or extrinsic camera parameters, i.e. camera calibration
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T5/00—Image enhancement or restoration
    • G06T5/80—Geometric correction

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Abstract

The invention belongs to the field of computer vision, and particularly relates to a camera calibration board and a calibration method thereof. The method comprises the following steps: the device comprises a plane board, and dot mark points and ring mark points which are distributed on the plane board in an array manner; the dot mark points and the ring mark points are coplanar points; the camera calibration board is divided into a core area and an extension area according to the mark point distribution area; the core area is an array formed by dot mark points and circular ring mark points and is used for being arranged in a camera view field range in the calibration process; the extension area is an array formed by dot mark points and used for covering a camera view field range in the calibration process, and when a camera calibration image is collected, part of the mark points in the extension area are allowed to exceed the camera view field range. The calibration board provided by the invention can enable the mark points to fill the field range of the camera as far as possible in the camera calibration process, and improve the camera calibration precision.

Description

Camera calibration board and calibration method thereof
Technical Field
The invention belongs to the field of computer vision, and particularly relates to a camera calibration board and a calibration method thereof.
Background
The calibration plate is used for calibrating internal parameters and external parameters of the camera in the applications of machine vision measurement, three-dimensional reconstruction and the like. When calibrating a camera, the larger the area proportion of envelopes formed by all mark points (or angular points) observed by the camera in an imaging range is, the higher the calibration accuracy of the obtained camera is, and currently, the more commonly used calibration boards include a checkerboard calibration board, a HALCON calibration board, a Charuco calibration board and the like. The chessboard pattern calibration board is a calibration board mainly used in an OpenCV open source library, and an imaging rotation angle is positioned by depending on the odd and even numbers of line and row corner points, so that the whole calibration board is required to be positioned within a camera view field range in the calibration process, a considerable part of area in the camera view field range is not covered by the calibration board, and the calibration effect is poor; on the other hand, the corner detection used by the checkerboard calibration plate recognition algorithm has high requirements on imaging quality, and corner recognition failure or detection accuracy reduction can be caused when the ambient light is bright or the exposure time of the camera is long; the HALCON calibration board is a calibration board formed by 7-by-7 dot mark point array distribution, the imaging rotation angle is positioned by means of the unfilled corner of the inner frame of the upper left corner, the whole calibration board is required to be positioned within the field of view of the camera in the calibration process like the chessboard grid calibration board, and the problem of excessive blank in the field of view cannot be avoided; the Charuco calibration board is a calibration board which is newly appeared in recent years, a chessboard grid and a two-dimensional code are combined, a local coding function is achieved, and in theory, part of angular points can be correctly identified when exceeding the presenting range of a camera, so that the field range of the camera can be covered in an area as large as possible, and an OpenCV open source library is also supported. However, the pattern of the Charucuo calibration plate is relatively complex, which is not beneficial to the realization of machining, and in addition, the problem of higher requirement on image quality exists as the chessboard grid calibration plate.
Disclosure of Invention
The invention aims to provide a plane calibration plate consisting of dot mark points and ring mark points, which overcomes the defect that the conventional calibration plate cannot improve the calibration precision of the camera internal reference and distortion coefficient.
The technical scheme adopted by the invention for realizing the purpose is as follows: a camera calibration plate, comprising: the device comprises a plane board, and dot mark points and ring mark points which are distributed on the plane board in an array manner;
the dot mark points and the ring mark points are coplanar points;
the camera calibration board is divided into a core area and an extension area according to the mark point distribution area;
the core area is an array formed by dot mark points and circular ring mark points and is used for being arranged in a camera view field range in the calibration process;
the extension area is an array formed by dot mark points and used for covering a camera view field range in the calibration process, and when a camera calibration image is collected, part of mark points in the extension area are allowed to exceed the camera view field range.
The core area is an array distributed by 4 rows by 5 columns; the 1 st row of the core area sequentially comprises a ring mark point, a dot mark point, a ring mark point, a dot mark point and a ring mark point from left to right, and the three rings from left to right sequentially comprise a No. 1 ring mark point, a No. 2 ring mark point and a No. 3 ring mark point;
two ends of the last row in the core area are provided with circular ring mark points, namely a No. 4 circular ring mark point and a No. 5 circular ring mark point, and a circular dot mark point is arranged between the two circular ring mark points;
and the mark points in the rest rows of the core area are all dot mark points.
The diameter of the dot mark point is equal to the outer diameter of the ring mark point, and the inner diameter of the ring mark point is equal to half of the outer diameter of the ring mark point; the inner circle and the outer circle of the circular ring mark point are concentric circles;
the center line spacing and the center column spacing of the dot mark points and the ring mark points are 2 times of the diameter of the dot mark points.
The dot mark points and the ring mark points are used as target areas for image processing, and the rest parts in the calibration plate and the inner circle areas of the ring mark points are used as background areas.
A calibration method of a camera calibration board comprises the following steps:
1) camera calibration, comprising: calibrating a calibration plate coordinate system, calibrating a calibration plate plane coordinate system, calibrating an image coordinate system and calibrating a camera coordinate system;
in the camera calibration image acquisition process, a calibration plate is placed in front of a camera lens as required;
2) collecting not less than 4 calibration plate images, carrying out binarization segmentation on the images for the collected calibration plate images by calibration plate identification, carrying out region growth on a mark point imaging region, judging the region after the growth is finished, and identifying a dot mark point and a ring mark point in the images;
3) traversing and searching all the identified circular ring mark points, searching other two circular ring mark points which are closest to the circular ring mark point in an Euclidean distance aiming at each searched circular ring mark point, further identifying other circular ring mark points and circular dot mark points in a core area according to whether the combination condition between the three circular ring mark points meets the set combination condition, and judging whether the core area of the calibration plate is successfully identified according to the identification condition;
4) if the core area of the calibration plate is successfully identified, matching the identified dot mark points in the image by using the mapping relation of the homography and the two-dimensional coordinates of dot mark points of the extension area in the plane coordinate system of the calibration plate, using the successfully matched dot mark points including the dot mark points and the ring mark points in the core area and the dot mark points of the extension area, calculating the homography of the plane coordinate system of the calibration plate relative to the image coordinate system of the camera again, trying to continuously match the identified unmatched dot mark points in the image until more dot mark points cannot be matched, and finishing the identification of the extension area of the calibration plate;
5) the identification of the calibration plate in each image is finished, the identification result of the calibration plate is a two-dimensional coordinate and an imaging coordinate of a group of mark points under a plane coordinate system of the calibration plate, and each image corresponds to a group of rotation-translation variation relation (R) i |t i ) All will beAnd (5) performing optimization solution by using the recognition result as an input variable of nonlinear optimization to finally obtain camera calibration parameters, namely completing camera calibration.
The step 1) specifically comprises the following steps:
(1) defining mode of the coordinate system of the calibration plate: the geometric center position of the surface of the calibration plate is defined as the origin of a coordinate system of the calibration plate; defining the direction from the No. 1 circular ring mark point to the No. 3 circular ring mark point as the positive direction of the X axis of the coordinate system of the calibration plate; defining the direction from the No. 1 circular ring mark point to the No. 4 circular ring mark point as the positive direction of the Y axis of the coordinate system of the calibration plate; the Z-axis direction, the X-axis direction and the Y-axis direction of the calibration plate coordinate system meet the right-hand rule;
(2) defining a plane coordinate system of the calibration plate: the origin of the calibration plate plane coordinate system is the origin of the calibration plate coordinate system, and the X axis and the Y axis of the calibration plate plane coordinate system are the same as the X axis and the Y axis of the calibration plate coordinate system;
(3) definition mode of image coordinate system: the origin of the image coordinate system is positioned at the upper left corner of the image, the positive direction of the u axis is horizontally towards the right, and the positive direction of the v axis is vertically downwards;
(4) definition mode of camera coordinate system: the origin of a camera coordinate system is defined as the optical center position of a lens, the positive direction of an X axis is in the same direction with the positive direction of a u axis of an image coordinate system, the positive direction of a Y axis is in the same direction with the positive direction of a v axis of the image coordinate system, and the direction of a Z axis, the direction of the X axis and the direction of the Y axis meet the right-hand rule;
after each coordinate system is determined, an included angle between the positive direction of the Z axis of the coordinate system of the calibration board and the positive direction of the Z axis of the coordinate system of the camera in the image acquisition process is set to be not less than 20 degrees and not more than 40 degrees, and the imaging of each calibration board ensures the integrity of a core area, namely 20 marking points in 4 rows and 5 columns of the core area completely appear in an image and cannot be intersected with the boundary of the image.
In step 2), the area growth is performed on the mark point imaging area, the area after the growth becomes an undetermined area, and the area to be determined is determined, specifically:
if the length-width ratio of the external rectangle of the region to be determined is smaller than a set threshold value, the ratio of the area of the region to be determined to the area of the external rectangle of the region to be determined is larger than the set threshold value, and meanwhile, a hole with the size larger than the area of the set threshold value is arranged at the central position of the region to be determined, the image region is judged to be a region where the circular ring mark point is located;
if the length-width ratio of the circumscribed rectangle of the undetermined region is smaller than a set threshold, the ratio of the area of the undetermined region to the area of the circumscribed rectangle of the undetermined region is larger than the set threshold, and meanwhile, the undetermined region is not the region where the circular ring mark points are located, the undetermined region is judged to be the region where the circular dot mark points are located;
and the undetermined area which does not meet the imaging characteristic constraint of the circular mark points or the imaging characteristic constraint of the circular mark points is not used for the next step of target identification.
In step 3), the set combination condition includes:
case 1: 1 collinear dot mark point exists between the No. 1 circular ring mark point and the No. 2 circular ring mark point, 2 collinear dot mark points exist between the No. 1 circular ring mark point and the No. 4 circular ring mark point, the No. 1 circular ring mark point reaches the No. 2 circular ring mark point, and then the No. 4 mark point meets the clockwise sequencing, in a triangle formed by the No. 1 circular ring mark point, the No. 2 circular ring mark point and the No. 4 circular ring mark point on the calibration plate, the No. 1 circular ring mark point corresponds to a right-angle vertex;
case 2: 1 collinear dot mark point exists between the No. 2 circular ring mark point and the No. 3 circular ring mark point, 2 collinear dot mark points exist between the No. 3 circular ring mark point and the No. 5 circular ring mark point, the No. 3 circular ring mark point reaches the No. 2 circular ring mark point and then reaches the No. 5 mark point to meet the anticlockwise sorting, and the No. 3 circular ring mark point corresponds to a right-angle vertex in a triangle formed by the No. 2 circular ring mark point, the No. 3 circular ring mark point and the No. 5 circular ring mark point on the calibration plate;
case 3: 2 collinear dot mark points exist between the No. 1 circular ring mark point and the No. 4 circular ring mark point, 3 collinear dot mark points exist between the No. 4 circular ring mark point and the No. 5 circular ring mark point, the No. 4 circular ring mark point reaches the No. 1 circular ring mark point and then reaches the No. 5 mark point to meet the clockwise sequencing, and the No. 4 circular ring mark point corresponds to a right-angle vertex in a triangle formed by the No. 1 circular ring mark point, the No. 4 circular ring mark point and the No. 5 circular ring mark point on the calibration plate;
case 4: there are 2 collineation dot mark points between No. 3 ring mark points and No. 5 ring mark points, there are 3 collineation dot mark points between No. 4 ring mark points and No. 5 ring mark points, and No. 5 ring mark points to No. 3 ring mark points, reach No. 4 mark points again and satisfy anticlockwise sequencing, in the triangle that No. 3 ring mark points, No. 4 ring mark points and No. 5 ring mark points constitute on the calibration board, No. 5 ring mark points correspond right angle summit.
In the step 3), judging whether the core area of the calibration plate is successfully identified specifically includes:
for the found three circular ring mark points, firstly calculating the size of a vertex angle corresponding to each circular ring mark point in the formed triangle, marking the circular ring mark point with the largest vertex angle as V, and then judging the number of collinear circular point mark points between the V and the other two circular ring mark points;
if the number of the collinear dot mark points is 1 and 2 respectively, the condition 1 or the condition 2 is preliminarily judged, wherein the ring mark point with the number of the collinear dot mark points being 1 between V is marked as S, and the other ring mark point is marked as L; if the number of the collinear dot mark points is 2 and 3 respectively, the condition is preliminarily judged as 3 or 4, wherein the ring mark point with the number of the collinear dot mark points being 2 between V is marked as S, and the other ring mark point is marked as L;
if the VSL is sorted clockwise, the situation 1 is judged preliminarily or the situation 2 is judged, and further, if the VSL is sorted clockwise, the situation 1 is judged; if the VSL is sorted in the anticlockwise direction, the situation is judged to be 2;
if the VSL is in clockwise sequencing, the situation 3 is judged preliminarily or the situation 4 is judged, and further, if the VSL is in clockwise sequencing, the situation 3 is judged; if the VSL is sorted in the counterclockwise direction, the situation is judged to be 4;
if the found three ring mark points meet any combination condition, calculating a homography matrix of the plane coordinate system of the calibration plate relative to the image coordinate system of the camera by using the imaging coordinates of the identified ring mark points and collinear dot mark points and two-dimensional coordinates of the ring mark points and collinear dot mark points under the plane coordinate system of the calibration plate, matching other ring mark points and dot mark points identified by the image by using the mapping relation of the homography matrix and the two-dimensional coordinates of the ring mark points and dot mark points of the core area under the plane coordinate system of the calibration plate, if the number of the matched mark points is more than a set threshold value, successfully identifying the core area, and if not, continuously searching other three ring mark point combinations.
And 5), calibrating the camera through the pinhole model and the lens distortion model:
the camera acquires images of the calibration plate at each relative position and corresponds to a rotational-translational change relation (R | t), and each imaged mark point on the calibration plate in each image corresponds to two nonlinear constraint equations, namely:
u ij ＝f(R i ,t i ,fu,fv,u 0 ,v 0 ,k 1 ,k 2 ,k 3 ,p 1 ,p 2 ,X ij ,Y ij ,Z ij )
v ij ＝g(R i ,t i ,fu,fv,u 0 ,v 0 ,k 1 ,k 2 ,k 3 ,p 1 ,p 2 ,X ij ,Y ij ,Z ij )
wherein u is ij And v ij Representing an imaging coordinate corresponding to a jth control point in an ith image acquired in the camera calibration process; x ij 、Y ij And Z ij Representing the three-dimensional coordinates of the jth control point in the ith image acquired in the camera calibration process under the calibration plate coordinate system, wherein Z ij Set to 0; (R) i |t i ) Representing the rotation and translation change relation of the coordinate system of the calibration plate of the ith image acquired in the calibration process of the camera relative to the coordinate system of the camera; fu and fv are the focal ratio in pixels, u 0 And v 0 Is a reference point, fu, fv, u 0 、v 0 Is the camera internal reference, k 1 、k 2 、 k 3 As radial distortion coefficient, p 1 、p 2 Is a tangential distortion coefficient;
and completing the calibration of the camera according to the optimal solution of the camera internal parameter, the radial distortion coefficient and the tangential distortion coefficient.
The invention has the following beneficial effects and advantages:
1. the calibration board provided by the invention can enable the mark points to fill the camera view field range as much as possible in the camera calibration process, and improve the camera calibration precision.
2. The invention meets the calibration requirement of the relative pose relationship of the binocular (or multi-view) camera, namely, the core area on the calibration plate is ensured to be positioned in the public view field range of the binocular (or multi-view) camera, and the area of the extension area is increased according to the actual requirement so as to ensure that enough mark points are arranged in the view field range of each camera, thereby improving the calibration precision of the binocular (or multi-view) camera.
Drawings
FIG. 1 is a schematic structural view of the present invention;
wherein, 1 is number 1 ring mark point, 2 is number 2 ring mark point, 3 is number 3 ring mark point, 4 is number 4 mark point, 5 is number 5 mark point.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and examples.
As shown in fig. 1, the present invention provides a planar calibration plate consisting of circular dot marks and circular ring marks, wherein the marks are coplanar points, and the marks are distributed on the calibration plate in an array. According to different distribution areas of the mark points, a core area and an extension area are divided on the calibration plate. The core area is mainly used for identifying and positioning the calibration plate; the extension area mainly provides a larger imaging area, and the number of parameter solution constraint equations calibrated by the camera is increased. In the calibration process, it is required to ensure that the core area on the calibration plate is located within the field of view of the camera, and the extension area is filled with the field of view of the camera as much as possible. The extension area can have part of the mark points beyond the field of view.
The calibration board can be divided into a target area and a background area according to the identification content, wherein the dot mark points and the ring mark points are the target area, the rest areas are the background area, and the area where the inner circle of the ring mark points is also belongs to the background area. The target area is set to black and the background area is set to white.
The circular ring mark points are formed by concentric circles;
the diameter of the inner circle is equal to half of the diameter of the outer circle;
the outer diameter of the circular ring mark point is equal to the diameter of the circular dot mark point;
the diameter of the dot mark point is equal to the outer diameter of the ring mark point, and the inner diameter of the ring mark point is equal to half of the outer diameter of the ring mark point; the inner circle and the outer circle of the circular ring mark point are concentric circles;
the center line spacing and the center column spacing of the dot mark points and the ring mark points are 2 times of the diameter of the dot mark points.
The core area is a pattern which is formed by 5 circular mark points and 15 circular mark points and has a layout of 4 rows by 5 columns, wherein:
the 1 st row sequentially comprises circular ring mark points, circular point mark points, circular ring mark points, circular point mark points and circular ring mark points from left to right, and the three circular rings from left to right are sequentially marked as a No. 1 circular ring, a No. 2 circular ring and a No. 3 circular ring;
line 2 is 5 dots feature points;
line 3 is 5 dots feature points;
the 4 th row is sequentially provided with a ring mark point, a dot mark point and a ring mark point from left to right, and the two rings are sequentially marked as a No. 4 ring and a No. 5 ring from left to right;
the extension area consists of dot mark points distributed in an array;
to further illustrate the identification method of the calibration plate image, a coordinate system associated with the camera calibration process is first defined.
A calibration method of a camera calibration board comprises the following steps:
1) camera calibration, comprising: calibrating a calibration plate coordinate system, calibrating a calibration plate plane coordinate system, calibrating an image coordinate system and calibrating a camera coordinate system;
in the camera calibration image acquisition process, a calibration plate is placed in front of a camera lens as required;
2) collecting at least 4 calibration plate images, identifying the calibration plate by firstly carrying out binarization segmentation on the images for the collected calibration plate images, carrying out region growth aiming at a mark point imaging region, judging the region after the growth is finished, and identifying a dot mark point and a ring mark point in the images;
3) traversing and searching all the identified circular ring mark points, searching other two circular ring mark points which are closest to the circular ring mark point in an Euclidean distance aiming at each searched circular ring mark point, further identifying other circular ring mark points and circular dot mark points in a core area according to whether the combination condition between the three circular ring mark points meets the set combination condition, and judging whether the core area of the calibration plate is successfully identified according to the identification condition;
4) if the core area of the calibration plate is successfully identified, matching the identified dot mark points in the image by using the mapping relation of the homography and the two-dimensional coordinates of dot mark points of the extension area in the plane coordinate system of the calibration plate, using the successfully matched dot mark points including the dot mark points and the ring mark points in the core area and the dot mark points of the extension area, calculating the homography of the plane coordinate system of the calibration plate relative to the image coordinate system of the camera again, trying to continuously match the identified unmatched dot mark points in the image until more dot mark points cannot be matched, and finishing the identification of the extension area of the calibration plate;
5) and finishing the identification of the calibration plate in each image, wherein the identification result of the calibration plate is a group of two-dimensional coordinates and imaging coordinates of the mark points under a plane coordinate system of the calibration plate, each image corresponds to a group of rotation and translation change relations (Ri | ti), all the identification results are used as nonlinear optimization input variables to carry out optimization solution, and finally, camera calibration parameters are obtained, namely, the camera calibration is finished.
In step 1), calibrating a plate coordinate system: the geometric center position of the surface of the calibration plate is defined as the origin of a coordinate system of the calibration plate; defining the direction from the No. 1 circular ring mark point to the No. 3 circular ring mark point as the positive direction of the X axis of the coordinate system of the calibration plate; defining the direction from the No. 1 circular ring mark point to the No. 4 circular ring mark point as the positive direction of the Y axis of the coordinate system of the calibration plate; the Z-axis direction, the X-axis direction and the Y-axis direction of the calibration plate coordinate system meet the right-hand rule;
calibrating a plate plane coordinate system: the origin of the coordinate system is the origin of the coordinate system of the calibration plate, and the X axis and the Y axis of the coordinate system are the same as the X axis and the Y axis of the coordinate system of the calibration plate;
image coordinate system: the origin is positioned at the upper left corner of the image, the positive direction of the u axis is horizontally towards the right, and the positive direction of the v axis is vertically downwards;
camera coordinate system: the origin is defined as the position of the optical center of the lens, the positive direction of an X axis is in the same direction with the positive direction of a u axis of an image coordinate system, the positive direction of a Y axis is in the same direction with the positive direction of a v axis of the image coordinate system, and the direction of a Z axis, the direction of the X axis and the direction of the Y axis meet the right-hand rule;
calibrating a pinhole model and a lens distortion model by using a camera;
in formula 2, (X, Y, Z) is the coordinate of the mark point in the calibration plate coordinate system, and since the invention uses the planar calibration plate, the Z component of the mark point in the calibration plate coordinate system can be set to 0; (R | t) is a rotational translation matrix of the calibration plate relative to the camera coordinate system; (X, Y, Z) is converted into coordinates (X, Y, Z) under a camera coordinate system through (R | t) rotation translation transformation, wherein Z of the mark point under the camera coordinate system is not equal to 0;
in equation 3, fu and fv are the focal ratio in pixels, u 0 And v 0 Is a reference point, fu, fv, u 0 、 v 0 Collectively referred to as camera reference; k is a radical of 1 ,k 2 And k 3 Is the radial distortion coefficient, p, of the camera lens 1 And p 2 Is a tangential distortion coefficient, k, of the camera lens 1 、k 2 、k 3 And p 1 、p 2 Collectively referred to as distortion coefficients; u and v are imaging coordinates (containing distortion) corresponding to the landmark points;
Figure RE-RE-GDA0003775644910000091
x′＝x/z
y′＝y/z
r 2 ＝x′ 2 +y′ 2
x″＝x′·(1+k 1 ·r 2 +k 2 ·r 4 +k 3 ·r 6 )+2·p 1 ·x′·y′+p 2 ·(r 2 +2·x′ 2 )
y″＝y′·(1+k 1 ·r 2 +k 2 ·r 4 +k 3 ·r 6 )+p 1 ·(r 2 +2·y′ 2 )+2·p 2 ·x′·y′
u＝fu·x″+u 0
v＝fv·y″+v 0 (3)
the camera acquires images of the calibration plate at each relative position, corresponding to a rotation and translation change relation (R | t), and each imaged marking point on the calibration plate in each image corresponds to two constraint equations;
the following uses equation 4 as a more general description of equations 2 and 3, where u ij And v ij Representing an imaging coordinate corresponding to a jth control point in an ith image acquired in the camera calibration process;
X ij 、Y ij and Z ij Representing the three-dimensional coordinates of the jth control point in the ith image acquired in the camera calibration process under the calibration plate coordinate system, wherein Z ij May be set to 0;
u ij ＝f(R i ,t i ,fu,fv,u 0 ,v 0 ,k 1 ,k 2 ,k 3 ,p 1 ,p 2 ,X ij ,Y ij ,Z ij )
v ij ＝g(R i ,t i ,fu,fv,u 0 ,v 0 ,k 1 ,k 2 ,k 3 ,p 1 ,p 2 ,X ij ,Y ij ,Z ij ) (4)
(R i |t i ) Representing the rotation and translation change relation of the coordinate system of the calibration plate of the ith image acquired in the calibration process of the camera relative to the coordinate system of the camera;
fu、fv、u 0 、v 0 for camera reference and k 1 、k 2 、k 3 、p 1 、p 2 Is a distortion coefficient;
the aim of camera calibration in the invention is to determine the internal parameters fu, fv and u of the camera 0 、v 0 And distortion coefficient k 1 、k 2 、k 3 、p 1 、p 2 ；
One correctly identified mark point in the formula 4 corresponds to two constraint equations; assuming that a total of N images (N is not less than 4) are acquired in a certain calibration process, the number of mark points which can be correctly identified in each image is M i (subscript i corresponds to the picture number), a total of
Figure RE-RE-GDA0003775644910000101
A non-linear constraint equation of which X ij 、Y ij And Z ij And u ij And v ij Input variables for non-linear optimization, (R) i |t i )、fu、fv、u 0 、 v 0 And k 1 、k 2 、k 3 、p 1 、p 2 A target variable for non-linear optimization;
the invention uses LM algorithm to complete nonlinear optimization solution, and the identification method of calibration plate imaging is explained by aiming at the input variable of the nonlinear optimization process;
when an image is collected in the camera calibration process, a calibration plate is placed in front of a camera lens, the included angle between the positive direction of a Z axis of a calibration plate coordinate system and the positive direction of the Z axis of the camera coordinate system is required to be not less than 20 degrees and not more than 40 degrees, and the imaging of each calibration plate is required to ensure the integrity of a core area, namely 20 marking points in 4 rows and 5 columns of the core area are required to be completely appeared in the image and cannot be intersected with the image boundary;
collecting not less than 4 calibration plate images, for the collected calibration plate images, firstly identifying dot mark points and ring mark points in the images by the calibration plate, then identifying a core area of the calibration plate according to the combination condition of the dot mark points and the ring mark points, and finally further searching imaging mark points of an epitaxial area according to the identification result of the core area;
in order to identify the mark point, firstly, the image is divided into two values, then the region growing is carried out on the mark point imaging region, and the region of the growing end is judged:
if the length-width ratio of the imaging external rectangle is smaller than the set threshold value, the ratio of the imaging area of the mark point to the area of the area external rectangle is larger than the set threshold value, and a hole with the area larger than the set threshold value is arranged at the imaging center position of the mark point, the image area is judged to be the area where the annular mark point is located;
if the length-width ratio of the imaging circumscribed rectangle is smaller than the set threshold, the ratio of the imaging area of the mark point to the area circumscribed rectangle is larger than the set threshold, and the mark point is not a circular mark point, the image area is judged to be the area where the dot mark point is located;
the region which does not satisfy the imaging characteristic constraint of the ring mark points or the imaging characteristic constraint of the dot mark points is not used as a candidate region for next target identification;
the found circular ring mark points and the found circular dot mark points are respectively placed in a queue array, all the identified circular ring mark points are searched in a traversing way aiming at the circular ring mark point queue array, then, other two circular ring mark points with the shortest Euclidean distance away from the circular ring mark point are searched for aiming at each found circular ring mark point, and then, whether the three circular ring mark points meet one of the following 4 combination conditions is judged:
case 1. the number 1 ring index point, the number 2 ring index point and the number 4 ring index point according to claim 3, wherein 1 collinear dot index point exists between the number 1 ring index point and the number 2 ring index point, 2 collinear dot index points exist between the number 1 ring index point and the number 4 ring index point, and the number 1 ring index point to the number 2 ring index point and then to the number 4 index point satisfy clockwise sorting;
case 2. the number 2 ring mark point, the number 3 ring mark point and the number 5 ring mark point according to claim 3, wherein 1 collinear dot mark point exists between the number 2 ring mark point and the number 3 ring mark point, 2 collinear dot mark points exist between the number 3 ring mark point and the number 5 ring mark point, and the number 2 ring mark point to the number 3 ring mark point and then to the number 5 mark point satisfy clockwise sorting;
case 3. the number 1 ring mark point, the number 4 ring mark point and the number 5 ring mark point according to claim 3, wherein 2 collinear dot mark points exist between the number 1 ring mark point and the number 4 ring mark point, 3 collinear dot mark points exist between the number 4 ring mark point and the number 5 ring mark point, and the number 1 ring mark point to the number 4 ring mark point and then to the number 5 mark point satisfy the counterclockwise ordering;
case 4. the number 3 ring mark point, the number 4 ring mark point and the number 5 ring mark point according to claim 3, wherein 2 collinear dot mark points exist between the number 3 ring mark point and the number 5 ring mark point, 3 collinear dot mark points exist between the number 4 ring mark point and the number 5 ring mark point, and the number 3 ring mark point to the number 4 ring mark point and then to the number 5 mark point satisfy the counterclockwise ordering;
judging whether the core area of the calibration plate is successfully identified, specifically:
for the found three circular ring mark points, firstly calculating the size of a vertex angle corresponding to each circular ring mark point in the formed triangle, marking the circular ring mark point with the largest vertex angle as V, and then judging the number of collinear circular point mark points between the V and the other two circular ring mark points;
if the number of the collinear dot mark points is 1 and 2 respectively, the condition 1 or the condition 2 is preliminarily judged, wherein the ring mark point with the number of the collinear dot mark points being 1 between V is marked as S, and the other ring mark point is marked as L; if the number of the collinear dot mark points is 2 and 3 respectively, the condition is preliminarily judged to be 3 or 4, wherein the ring mark point with the number of the collinear dot mark points between V being 2 is marked as S, and the other ring mark point is marked as L;
if the VSL is sorted clockwise, the situation 1 is judged preliminarily or the situation 2 is judged, and further, if the VSL is sorted clockwise, the situation 1 is judged; if the VSL is sorted in the anticlockwise direction, judging that the VSL is in a case 2;
if the VSL is in clockwise sequencing, the situation 3 is judged preliminarily or the situation 4 is judged, and further, if the VSL is in clockwise sequencing, the situation 3 is judged; if the VSL is sorted in the counterclockwise direction, the situation is judged to be 4;
if the found three ring mark points meet any combination condition, calculating a homography matrix of the plane coordinate system of the calibration plate relative to the image coordinate system of the camera by using the imaging coordinates of the identified ring mark points and collinear dot mark points and two-dimensional coordinates of the ring mark points and collinear dot mark points under the plane coordinate system of the calibration plate, matching other ring mark points and dot mark points identified by the image by using the mapping relation of the homography matrix and the two-dimensional coordinates of the ring mark points and dot mark points of the core area under the plane coordinate system of the calibration plate, if the number of the matched mark points is more than a set threshold value, successfully identifying the core area, and if not, continuously searching other three ring mark point combinations.
The mapping relation of the homography array and the mapping method of the homography array are as follows:
Figure RE-RE-GDA0003775644910000131
Figure RE-RE-GDA0003775644910000132
wherein the content of the first and second substances,
Figure RE-RE-GDA0003775644910000133
representing the homogeneous coordinate of the ith mark point identified in the k frame of the calibration plate collected image under the calibration plate plane coordinate system;
Figure RE-RE-GDA0003775644910000134
representing the homogeneous coordinate of the ith mark point identified in the k frame of calibration plate collected image in the image coordinate system; h represents a homography.
If the found three circular mark points meet one of the combination conditions in 4, calculating a homography of the plane coordinate system of the calibration board relative to the image coordinate system of the camera by using the two-dimensional coordinates of the corresponding mark points under the plane coordinate system of the calibration board and the corresponding imaging coordinates, matching the imaging identification results corresponding to other circular mark points and circular mark points in the core area of the calibration board in the image by using the mapping relation of the homography and the two-dimensional coordinates of other mark points under the plane coordinate system of the calibration board, if the number of the matched mark points is more than a set threshold value, successfully identifying the core area, and if not, continuously searching the combination of the other three circular mark points;
if the found three circular ring mark points do not meet any one of the 4 combination conditions, other three circular ring mark point combinations are continuously found;
if the core area is successfully identified, matching an imaging identification result corresponding to a dot mark point of the epitaxial area of the calibration plate in the image by using a homography mapping relation and two-dimensional coordinates of other mark points under a planar coordinate system of the calibration plate, calculating the homography of the planar coordinate system of the calibration plate relative to a camera image coordinate system again by using the mark points which are successfully matched, and then trying to continuously match the imaging identification result corresponding to the dot mark point of the epitaxial area of the calibration plate in the image until more mark points cannot be matched;
and when the identification of the calibration plate in each image is finished, the identification result of the calibration plate is the two-dimensional coordinates and the imaging coordinates of a group of mark points in the plane coordinate system of the calibration plate, and each image corresponds to a group of rotation translation change relations (R) i |t i ) And taking the identification results of the calibration plates in all the images to be identified as input variables in the nonlinear optimization solving process to complete optimization solving, and finally obtaining camera calibration results fu, fv and u 0 、v 0 And k 1 、k 2 、 k 3 、p 1 、p 2 ；
Example (b):
assuming that the field angle of the camera is 50 ° by 50 ° and the optimal working distance is about 700mm, the distance of 700mm can be calculated, and the field coverage of the camera is 652mm by 652 mm. Therefore, in order to calibrate the camera at a distance of 700mm and fill the camera view field range with the calibration plate as much as possible, the size of the calibration plate should be equivalent to the camera view field coverage range, so the specific design example of the calibration plate according to the scheme of the present invention is as follows:
the calibration plate consists of 20 rows by 23 columns of marking points;
1) the diameter of the dot mark point is 15mm, the outer diameter of the ring mark point is 15mm, and the inner diameter of the ring mark point is 7.5 mm;
2) the line spacing and the column spacing of the center of the mark point are respectively 30mm and 30 mm;
3) the 1 st mark point at the upper left corner of the core area is positioned at the 9 th row and the 10 th column of the whole calibration plate, and the area inside a dotted line frame in the drawing is the core area;
4) the core area consists of 4 rows by 5 columns of mark points, wherein the 1 st row comprises 3 circular mark points which are respectively positioned in the 1 st column, the 3 rd column and the 5 th column; the 4 th row comprises 2 circular mark points which are respectively positioned in the 1 st column and the 5 th column, and the rest mark points in the core area are circular mark points;
5) the extension area only consists of dot mark points;
6) in order to obtain higher imaging quality, the target area is black, and the background area is white;
7) when an image is collected in the camera calibration process, a calibration plate is placed in front of a camera lens, the included angle between the positive direction of a Z axis of a calibration plate coordinate system and the positive direction of the Z axis of the camera coordinate system is required to be not less than 20 degrees and not more than 40 degrees, and the imaging of each calibration plate is required to ensure the integrity of a core area, namely 20 marking points in 4 rows and 5 columns of the core area are required to be completely appeared in the image and cannot be intersected with the image boundary;
8) collecting not less than 4 calibration plate images, carrying out binarization segmentation on the images for the collected calibration plate images by the calibration plate identification, then carrying out region growing identification on dot mark points and ring mark points in the images, identifying a core region of the calibration plate according to the combination condition of the dot mark points and the ring mark points, and finally further searching imaging mark points of an epitaxial region according to the identification result of the core region;
9) taking the identification result of each calibration plate (the two-dimensional coordinates and the corresponding imaging coordinates of the mark point in the calibration plate plane coordinate system) as the input parameters of the nonlinear optimization, and then calculating the optimized target variables fu, fv, u by the LM nonlinear optimization algorithm 0 、v 0 And k 1 、k 2 、k 3 、p 1 、p 2 And finishing camera calibration.
While the invention has been described in connection with specific embodiments thereof, it will be understood that these should not be construed as limiting the scope of the invention, which is defined in the following claims, and any variations which fall within the scope of the claims are intended to be embraced thereby.

Claims (10)

1. A camera calibration plate, comprising: the device comprises a plane board, and dot mark points and ring mark points which are distributed on the plane board in an array manner;
the dot mark points and the ring mark points are coplanar points;
the camera calibration board is divided into a core area and an extension area according to the mark point distribution area;
the core area is an array formed by dot mark points and circular ring mark points and is used for being arranged in a camera view field range in the calibration process;
the extension area is an array formed by dot mark points and used for covering a camera view field range in the calibration process, and when a camera calibration image is collected, part of mark points in the extension area are allowed to exceed the camera view field range.
2. The camera calibration plate of claim 1, wherein the core region is an array of 4 rows by 5 columns; the 1 st row of the core area sequentially comprises a circular ring mark point, a circular dot mark point, a circular ring mark point, a circular dot mark point and a circular ring mark point from left to right, and the three circular rings sequentially comprise a No. 1 circular ring mark point, a No. 2 circular ring mark point and a No. 3 circular ring mark point from left to right;
two ends of the last row in the core area are provided with circular ring mark points, namely a No. 4 circular ring mark point and a No. 5 circular ring mark point, and a circular dot mark point is arranged between the two circular ring mark points;
and the mark points in the rest rows of the core area are all dot mark points.
3. The camera calibration plate of claim 1, wherein the diameter of the dot mark point is equal to the outer diameter of the ring mark point, and the inner diameter of the ring mark point is equal to half of the outer diameter of the ring mark point; the inner circle and the outer circle of the circular ring mark point are concentric circles;
the center line spacing and the center column spacing of the dot mark points and the ring mark points are 2 times of the diameter of the dot mark points.
4. The camera calibration plate of claim 1, wherein the dot mark points and the ring mark points are used as a target area for image processing, and the remaining portion of the calibration plate and the inner circle area of the ring mark points are used as a background area.
5. A calibration method of a camera calibration plate according to claim 1, comprising the steps of:
1) camera calibration, comprising: calibrating a calibration plate coordinate system, calibrating a calibration plate plane coordinate system, calibrating an image coordinate system and calibrating a camera coordinate system;
in the camera calibration image acquisition process, a calibration plate is placed in front of a camera lens as required;
2) collecting not less than 4 calibration plate images, carrying out binarization segmentation on the images for the collected calibration plate images by calibration plate identification, carrying out region growth on a mark point imaging region, judging the region after the growth is finished, and identifying a dot mark point and a ring mark point in the images;
3) traversing and searching all the identified circular ring mark points, searching other two circular ring mark points which are closest to the circular ring mark point in an Euclidean distance aiming at each searched circular ring mark point, further identifying other circular ring mark points and circular dot mark points in a core area according to whether the combination condition between the three circular ring mark points meets the set combination condition, and judging whether the core area of the calibration plate is successfully identified according to the identification condition;
4) if the core area of the calibration plate is successfully identified, matching the identified dot mark points in the image by using the mapping relation of the homography and the two-dimensional coordinates of dot mark points of the extension area in the plane coordinate system of the calibration plate, using the successfully matched dot mark points including the dot mark points and the ring mark points in the core area and the dot mark points of the extension area, calculating the homography of the plane coordinate system of the calibration plate relative to the image coordinate system of the camera again, trying to continuously match the identified unmatched dot mark points in the image until more dot mark points cannot be matched, and finishing the identification of the extension area of the calibration plate;
5) the identification of the calibration plate in each image is finished, the identification result of the calibration plate is a two-dimensional coordinate and an imaging coordinate of a group of mark points under a plane coordinate system of the calibration plate, and each image corresponds to a group of rotation translation change relations (R) i |t i ) And taking all the recognition results as input variables of nonlinear optimization to carry out optimization solution, and finally obtaining camera calibration parameters, namely completing camera calibration.
6. The calibration method of the camera calibration board according to claim 5, wherein the step 1) specifically comprises:
(1) defining mode of calibration plate coordinate system: the geometric center position of the surface of the calibration plate is defined as the origin of a coordinate system of the calibration plate; defining the direction from the No. 1 circular ring mark point to the No. 3 circular ring mark point as the positive direction of the X axis of the coordinate system of the calibration plate; defining the direction from the No. 1 circular ring mark point to the No. 4 circular ring mark point as the positive direction of the Y axis of the coordinate system of the calibration plate; the Z-axis direction, the X-axis direction and the Y-axis direction of the calibration plate coordinate system meet the right-hand rule;
(2) defining a plane coordinate system of the calibration plate: the origin of the calibration plate plane coordinate system is the origin of the calibration plate coordinate system, and the X axis and the Y axis of the calibration plate plane coordinate system are the same as the X axis and the Y axis of the calibration plate coordinate system;
(3) definition mode of image coordinate system: the origin of the image coordinate system is positioned at the upper left corner of the image, the positive direction of the u axis is horizontally towards the right, and the positive direction of the v axis is vertically downwards;
(4) definition mode of camera coordinate system: the origin of a camera coordinate system is defined as the optical center position of a lens, the positive direction of an X axis is in the same direction with the positive direction of a u axis of an image coordinate system, the positive direction of a Y axis is in the same direction with the positive direction of a v axis of the image coordinate system, and the direction of a Z axis, the direction of the X axis and the direction of the Y axis meet the right-hand rule;
after each coordinate system is determined, an included angle between the positive direction of the Z axis of the coordinate system of the calibration board and the positive direction of the Z axis of the coordinate system of the camera in the image acquisition process is set to be not less than 20 degrees and not more than 40 degrees, and the imaging of each calibration board ensures the integrity of a core area, namely 20 marking points in 4 rows and 5 columns of the core area completely appear in an image and cannot be intersected with the boundary of the image.
7. The calibration method of the camera calibration plate according to claim 5, wherein in step 2), the region growing is performed on the mark point imaging region, the region where the growing is completed becomes an undetermined region, and the region to be calibrated is determined, specifically:
if the length-width ratio of the circumscribed rectangle of the region to be determined is smaller than a set threshold, the ratio of the area of the region to be determined to the circumscribed rectangle of the region to be determined is larger than the set threshold, and a hole with the area larger than the set threshold is arranged at the center of the region to be determined, the image region is judged to be the region where the annular mark point is located;
if the length-width ratio of the circumscribed rectangle of the undetermined region is smaller than a set threshold, the ratio of the area of the undetermined region to the area of the circumscribed rectangle of the undetermined region is larger than the set threshold, and meanwhile, the undetermined region is not the region where the circular ring mark points are located, the undetermined region is judged to be the region where the circular dot mark points are located;
and the undetermined area which does not meet the imaging characteristic constraint of the circular mark points or the imaging characteristic constraint of the circular mark points is not used for the next step of target identification.
8. The calibration method of camera calibration board according to claim 5, wherein in step 3), the set combination comprises:
case 1: 1 collinear dot mark point exists between the No. 1 circular ring mark point and the No. 2 circular ring mark point, 2 collinear dot mark points exist between the No. 1 circular ring mark point and the No. 4 circular ring mark point, the No. 1 circular ring mark point reaches the No. 2 circular ring mark point, and then the No. 4 mark point meets the clockwise sequencing, in a triangle formed by the No. 1 circular ring mark point, the No. 2 circular ring mark point and the No. 4 circular ring mark point on the calibration plate, the No. 1 circular ring mark point corresponds to a right-angle vertex;
case 2: 1 collinear dot mark point exists between the No. 2 circular ring mark point and the No. 3 circular ring mark point, 2 collinear dot mark points exist between the No. 3 circular ring mark point and the No. 5 circular ring mark point, the No. 3 circular ring mark point reaches the No. 2 circular ring mark point and then reaches the No. 5 mark point to meet the anticlockwise sorting, and the No. 3 circular ring mark point corresponds to a right-angle vertex in a triangle formed by the No. 2 circular ring mark point, the No. 3 circular ring mark point and the No. 5 circular ring mark point on the calibration plate;
case 3: 2 collinear dot mark points exist between the No. 1 circular mark point and the No. 4 circular mark point, 3 collinear dot mark points exist between the No. 4 circular mark point and the No. 5 circular mark point, the No. 4 circular mark point reaches the No. 1 circular mark point and then reaches the No. 5 mark point to meet the clockwise sequencing, and in a triangle formed by the No. 1 circular mark point, the No. 4 circular mark point and the No. 5 circular mark point on the calibration plate, the No. 4 circular mark point corresponds to a right-angle vertex;
case 4: there are 2 collineation dot mark points between No. 3 ring mark points and No. 5 ring mark points, there are 3 collineation dot mark points between No. 4 ring mark points and No. 5 ring mark points, and No. 5 ring mark points to No. 3 ring mark points, reach No. 4 mark points again and satisfy anticlockwise sequencing, in the triangle that No. 3 ring mark points, No. 4 ring mark points and No. 5 ring mark points constitute on the calibration board, No. 5 ring mark points correspond right angle summit.
9. The calibration method of the camera calibration board according to claim 5, wherein in the step 3), it is determined whether the core area of the calibration board is successfully identified, specifically:
for the found three circular ring mark points, firstly calculating the size of a vertex angle corresponding to each circular ring mark point in the formed triangle, marking the circular ring mark point with the largest vertex angle as V, and then judging the number of collinear circular point mark points between the V and the other two circular ring mark points;
if the number of the collinear dot mark points is 1 and 2 respectively, the condition 1 or the condition 2 is preliminarily judged, wherein the ring mark point with the number of the collinear dot mark points being 1 between V is marked as S, and the other ring mark point is marked as L; if the number of the collinear dot mark points is 2 and 3 respectively, the condition is preliminarily judged to be 3 or 4, wherein the ring mark point with the number of the collinear dot mark points between V being 2 is marked as S, and the other ring mark point is marked as L;
if the VSL is sorted clockwise, the situation 1 is judged preliminarily or the situation 2 is judged, and further, if the VSL is sorted clockwise, the situation 1 is judged; if the VSL is sorted in the anticlockwise direction, judging that the VSL is in a case 2;
if the VSL is in clockwise sequencing, the situation 3 is judged preliminarily or the situation 4 is judged, and further, if the VSL is in clockwise sequencing, the situation 3 is judged; if the VSL is sorted in the counterclockwise direction, the situation is judged to be 4;
if the found three ring mark points meet any combination condition, calculating a homography matrix of the plane coordinate system of the calibration plate relative to the image coordinate system of the camera by using the imaging coordinates of the identified ring mark points and collinear dot mark points and two-dimensional coordinates of the ring mark points and collinear dot mark points under the plane coordinate system of the calibration plate, matching other ring mark points and dot mark points identified by the image by using the mapping relation of the homography matrix and the two-dimensional coordinates of the ring mark points and dot mark points of the core area under the plane coordinate system of the calibration plate, if the number of the matched mark points is more than a set threshold value, successfully identifying the core area, and if not, continuously searching other three ring mark point combinations.
10. The calibration method of the camera calibration board according to claim 5, wherein in the step 5), the calibration of the camera is performed by a pinhole model and a lens distortion model:
the camera acquires images of the calibration plate at each relative position and corresponds to a rotational-translational change relation (R | t), and each imaged mark point on the calibration plate in each image corresponds to two nonlinear constraint equations, namely:
u ij ＝f(R i ,t i ,fu,fv,u 0 ,v 0 ,k 1 ,k 2 ,k 3 ,p 1 ,p 2 ,X ij ,Y ij ,Z ij )
v ij ＝g(R i ,t i ,fu,fv,u 0 ,v 0 ,k 1 ,k 2 ,k 3 ,p 1 ,p 2 ,X ij ,Y ij ,Z ij )
wherein u is ij And v ij Representing an imaging coordinate corresponding to a jth control point in an ith image acquired in the camera calibration process; x ij 、Y ij And Z ij Representing the three-dimensional coordinates of the jth control point in the ith image acquired in the camera calibration process under the calibration plate coordinate system, wherein Z ij Set to 0; (R) i |t i ) Representing the rotation translation change relation of the coordinate system of the ith image calibration board acquired in the camera calibration process relative to the camera coordinate system; fu and fv are the focal ratio in pixels, u 0 And v 0 Is a reference point, fu, fv, u 0 、v 0 Is the camera internal reference, k 1 、k 2 、k 3 Is the radial distortion coefficient, p 1 、p 2 Is a tangential distortion coefficient;
and completing the calibration of the camera according to the optimal solution of the camera internal parameter, the radial distortion coefficient and the tangential distortion coefficient.
CN202210429763.2A 2022-04-22 2022-04-22 Camera calibration board and calibration method thereof Pending CN115018921A (en)

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