CN112050752B - Projector calibration method based on secondary projection - Google Patents
Projector calibration method based on secondary projection Download PDFInfo
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- CN112050752B CN112050752B CN202010907187.9A CN202010907187A CN112050752B CN 112050752 B CN112050752 B CN 112050752B CN 202010907187 A CN202010907187 A CN 202010907187A CN 112050752 B CN112050752 B CN 112050752B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/24—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures
- G01B11/25—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures by projecting a pattern, e.g. one or more lines, moiré fringes on the object
- G01B11/254—Projection of a pattern, viewing through a pattern, e.g. moiré
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B21/00—Measuring arrangements or details thereof, where the measuring technique is not covered by the other groups of this subclass, unspecified or not relevant
- G01B21/02—Measuring arrangements or details thereof, where the measuring technique is not covered by the other groups of this subclass, unspecified or not relevant for measuring length, width, or thickness
- G01B21/04—Measuring arrangements or details thereof, where the measuring technique is not covered by the other groups of this subclass, unspecified or not relevant for measuring length, width, or thickness by measuring coordinates of points
- G01B21/042—Calibration or calibration artifacts
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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
Abstract
The invention relates to a projector calibration method based on secondary projection, which comprises the following steps: projecting the primary projection pattern with the mark onto a calibration plate with a characteristic point, and obtaining a homography matrix of a projector plane and a calibration plate plane according to the coordinate of the center of the characteristic point in a world coordinate system, the pixel coordinate of the center of the characteristic point in a camera imaging plane, the pixel coordinate of the center of the mark in the camera imaging plane and the coordinate of the center of the mark in the projector plane; and generating a secondary projection pattern according to a plurality of position point arrays with known world coordinates, projecting the secondary projection pattern onto a calibration plate, acquiring the image to obtain the projection pattern without interference and the calibration plate pattern, and completing projector calibration. The method is effective and accurate in extracting the corresponding characteristic points.
Description
Technical Field
The invention belongs to the technical field of grating projection three-dimensional measurement, and particularly relates to a projector calibration method based on secondary projection, which is used in a digital grating projection three-dimensional measurement system.
Background
In the field of three-dimensional measurement, the digital grating projection three-dimensional measurement technology is taken as a main non-contact measurement method, and has the advantages of high precision, high resolution, low cost, high speed and the like, so that the digital grating projection three-dimensional measurement technology is widely applied to the fields of industrial detection, cultural relic reconstruction, reverse engineering and the like. The grating projection three-dimensional measurement technology is to project a sinusoidal grating pattern into a scene by a projector and acquire a corresponding deformed grating pattern, and the process is to link depth information in the scene with the phase of an image. And then processing the obtained modulation grating image through a grating analysis algorithm to extract phase distribution, and further reconstructing the surface of the measured object in a three-dimensional space by using the calibrated geometric relationship.
In the grating projection three-dimensional measurement, the calibration parameter precision of the system is the key for ensuring the measurement precision. The calibration technology of the camera has been developed, and the projector cannot find the exact relationship between the pixel point and the world coordinate like the camera, so that the calibration technology of the camera is always the focus of research. In order to solve the problem, researchers provide a plurality of effective methods, and the projector calibration method based on coordinate transformation comprises the steps of projecting a generated pattern with characteristic points onto a calibration plate, collecting an image with the characteristics of the calibration plate and the projection characteristic points, extracting coordinates of the projection characteristic points in the image, and obtaining the positions of the projection characteristic points on the calibration plate by using calibrated camera parameters. Compared with a calibration method based on coordinate mapping, the calibration method based on coordinate transformation has the advantages of simple operation, easy calculation and wide application because a large number of patterns are not required to be projected.
The projection calibration method based on coordinate transformation has a problem to be solved, and the pattern projected on the calibration plate can interfere with the existing calibration plate pattern, thereby influencing the extraction of the projected characteristic points and the characteristic points on the plate. To obtain accurate projector calibration results, the corresponding relationship between the projector pixels and the world coordinate positions of the calibration plate must be obtained accurately, and the method is convenient to calculate and easy to operate.
Disclosure of Invention
The invention aims to provide a projector calibration method based on secondary projection, which can solve the problem that a pattern projected on a calibration plate in the projector calibration process interferes with the existing calibration plate pattern in a digital grating projection three-dimensional measurement system.
In order to solve the technical problem, the projector calibration method based on the secondary projection of the invention comprises the following steps:
projecting a primary projection pattern with a mark onto a calibration plate with a feature point by using a projector, acquiring a primary calibration plate image with the mark and the feature point by using a camera, and acquiring an A homography matrix of a camera imaging plane and a calibration plate plane according to a coordinate of a feature point center in a world coordinate system of the calibration plate plane and a pixel coordinate in the camera imaging plane
Obtaining a B homography matrix of the camera imaging plane and the projector plane according to the pixel coordinates of the mark center on the camera imaging plane and the coordinates on the projector plane
According to the formula
C homography matrix for calculating plane of projector and plane of calibration plate
Setting a plurality of position point arrays with known world coordinates, and calculating the world coordinates of each position point and the C homography matrix
Obtaining the coordinate point set P of the mapping points of all the position points on the plane of the projectorP(ii) a Generating a circular feature mapping point with a set radius by taking the mapping point coordinate as a center, and further obtaining a secondary projection pattern with a black background and a white circular feature mapping point array; projecting the secondary projection pattern onto a calibration plate by a projector, and collecting an image of the secondary calibration plate by a camera; according to the pixel coordinates of each mapping point in the camera imaging plane and the A homography matrix
Obtaining a world coordinate point set P of each mapping pointW(ii) a Point set P corresponding to different calibration plate positionsPAnd set of points PWAnd calibrating the projector by a Zhang Zhen you camera calibration method to obtain the internal parameters and the distortion coefficient of the projector.
The marks in the primary projection patterns are in a cross shape, and the characteristic points of the calibration plate are black circular characteristic points.
The marks in the primary projection pattern are white dots, and the characteristic points of the calibration plate are black and white grid matrixes.
The marks in the primary projection pattern are black and white lattice matrixes, and the characteristic points of the calibration plate are circular ring characteristic points.
The calibration board is provided with a plurality of characteristic points which are arranged in an array form, and the centers of the characteristic points in the first row and the first column are used as the origin of a world coordinate system.
The A homography matrix
The obtaining method comprises the following steps: for any feature point c on the calibration board, the pixel coordinate P of the center of the feature point c in the camera imaging plane is obtained by using the function in opencvC1Let the world coordinate of the center of the feature point c be PWAccording to the formula
Calculating to obtain A homography matrix of camera imaging plane and calibration plate plane
The A homography matrix
The obtaining method comprises the following steps: aiming at any characteristic point c on the calibration plate, a circle center fitting mode after edge extraction is used for obtaining a pixel coordinate P of the center of the characteristic point c in the camera imaging planeC1Let the world coordinate of the center of the feature point c be PWAccording to the formula
Calculating to obtain A homography matrix of camera imaging plane and calibration plate plane
The B homography matrix
The obtaining method comprises the following steps: extracting the pixel coordinates of the cross mark on the camera imaging plane by adopting a threshold segmentation method, and solving the pixel coordinate P of the center of the cross mark on the camera imaging planeC2(ii) a Let the coordinate of the center of the cross mark on the plane of the projector be PPAccording to the formula
B homography matrix of camera imaging plane and projector plane is obtained through calculation
According to the secondary calibration plate image, obtaining the pixel coordinates of each circular feature mapping point on the camera imaging plane by using a function in opencv; let the pixel coordinate of any circular feature mapping point in the camera imaging plane be PC3According to the formula
Obtaining the coordinate P of the circular feature mapping point in the world coordinate systemW3Then obtaining the world coordinate point set P of all the circular feature mapping points in the calibration plate planeW。
According to the secondary calibration plate image, obtaining the pixel coordinates of each circular feature mapping point on the camera imaging plane by using a circle center fitting mode after edge extraction; let the pixel coordinate of any circular feature mapping point in the camera imaging plane be PC3According to the formula
Obtaining the coordinate P of the circular feature mapping point in the world coordinate systemW3Then obtaining the world coordinate point set P of all the circular feature mapping points in the calibration plate planeW。
Advantages and advantageous effects of the invention
The invention provides a secondary projection method, which comprises the steps of firstly obtaining a homography matrix of a projector plane and a calibration plate plane through projecting cross marks, then generating a secondary projection pattern according to a plurality of position point arrays with known world coordinates, projecting the secondary projection pattern onto the calibration plate, acquiring an image to obtain a projection pattern and a calibration plate pattern without interference, acquiring the image to obtain the projection pattern and the calibration plate pattern without interference, and completing projector calibration. The method is simple to operate and high in space utilization rate, one image can contain the feature points of two modes, extraction of the corresponding feature points is more effective and accurate, and the method has good application value.
Drawings
FIG. 1 is a flow chart of the present invention.
Fig. 2a is a picture of a cross sign projection pattern, and fig. 2b is a schematic diagram of a sign board.
FIG. 3 is a first projection pattern acquired at one of the calibration plate fixation locations;
FIG. 4 is a cross-hair marker pattern obtained from a threshold segmentation;
fig. 5 is a schematic diagram of a desired location point.
Fig. 6 is a projection pattern of the projection position point on the projector plane.
FIG. 7 is a second projection pattern acquired by the camera;
FIG. 8 is a schematic diagram of the back projection error calculated after calibration according to the present invention.
Detailed Description
The invention is further described with reference to the following drawings and detailed description.
The invention mainly calibrates the projector in the digital grating projection three-dimensional measurement system through secondary projection, and well solves the problem that the pattern projected on the calibration plate in the prior calibration process interferes with the pattern of the existing calibration plate.
The invention is described in detail below by taking the cross-shaped sign, the white background as the calibration plate and the black circular characteristic dot matrix as an example.
As shown in fig. 1, the projector calibration method based on the reprojection of the present invention includes the following steps:
inputting a primary projection pattern with cross marks into a projector and projecting the primary projection pattern onto a calibration plate to ensure that all the cross marks are projected onto the calibration plate and fully displayed, wherein the cross mark projection pattern is a black background and a white cross and is used for conveniently extracting the center of the cross, and reference is made to fig. 2 a; the calibration plate is a white background with an array of black circular feature points thereon, see fig. 2 b;
secondly, acquiring an image by aligning the calibration plate with a camera, and ensuring that the whole calibration plate is in the acquired image to obtain a primary calibration plate image with a cross mark and a black circular feature point array;
step three, taking the center of the black circular feature point in the first row and the first column as the origin of a world coordinate system, and referring to fig. 2 b; setting the circle center distance of adjacent black circular feature points to be 20mm, sequentially setting the world coordinates of the circle centers of the first row of black circular feature points on the calibration plate to be (0,0) and (0,20) … … from left to right and from top to bottom, sequentially setting the world coordinates of the circle centers of the second row of black circular feature points to be (20,0) and (20,20) … …, and so on; the corresponding relation between the plane of the projector and the plane of the calibration plate is obtained, and the method comprises the following steps:
(1): utilizing a processor to obtain the pixel coordinates of the circle center of the circular feature point in the camera imaging plane by using an image processing mode; for any circular feature point c, the pixel coordinate P of the circle center of the circular feature point c in the camera imaging plane can be obtained by using the function in opencvC1(Can also use the edge extraction after the center of a circle fitting method to solve PC1) Setting the world coordinate of the center of the circle of the circular characteristic point c as PWAccording to the formula
The homography matrix of the camera imaging plane and the calibration plate plane can be obtained
Refer to fig. 5; by utilizing a processor, firstly, a threshold segmentation method is adopted to extract the pixel coordinate of the cross mark on the camera imaging plane, and then the pixel coordinate P of the center of the cross mark is obtained by linear fittingC2(ii) a Let the coordinate of the center of the cross mark on the plane of the projector be PPAccording to the formula
So as to obtain the B homography matrix of the camera imaging plane and the projector plane
(2) According to the formula
C homography matrix of projector plane and calibration plate plane is obtained through calculation
Step four, calculating the mapping point coordinates of the calibration plate on the plane of the projector according to the position points on the calibration plate to be projected, and referring to the figure 5; setting world coordinates of the position points in the first row in the plane of the calibration plate as (10,10), (10,30) … …, setting world coordinates of the position points in the second row as (30,10), (30,30) … …, and so on; for any one of the position points W1, the world coordinate is set as PW1According to the formula
The coordinate P of the projection point of the position point on the plane of the projector can be obtainedP1Further obtain the coordinate point set P of the mapping points of all the position points on the plane of the projectorP；
Step five: generating a secondary projection pattern according to the coordinates of all position points on the calibration plate at the projection points on the plane of the projector: referring to fig. 6, circular feature mapping points with a radius of 20 pixels are generated with the mapping point coordinates as the center, and an array is formed by the circular feature mapping points, so as to obtain a secondary projection pattern with a black background and a white circular feature mapping point array; inputting the secondary projection pattern into a projector and projecting the secondary projection pattern onto a calibration plate, and defining a point formed by projecting the circular feature mapping point onto the calibration plate as a circular feature projection point;
and step six, collecting the image of the secondary calibration plate by using a camera, and referring to the figure 7. The processor 7 is used for obtaining the pixel coordinates of each circular feature projection point on the camera imaging plane by using the function in opencv (the P can be obtained by using the circle center fitting mode after the edge is extractedC3) (ii) a Let the pixel coordinate of any circular feature projection point on the camera imaging plane be PC3According to the formula
The world coordinate P of the projection point of the circular feature in the plane of the calibration plate can be obtainedW3And then obtaining a world coordinate point set P of all the projection points of the circular feature in the plane of the calibration plateW；
Step seven, the point set P obtained in the step four is usedPAnd the point set P obtained in the step sixWStoring the point sets in a computer, adjusting the calibration plate to different positions, and repeating the fourth step to the sixth step to obtain point sets P corresponding to different positionsPAnd set of points PWAnd calibrating the projector according to the corresponding point sets by a mature Zhang-Yong camera calibration method to obtain internal parameters and distortion coefficients of the projector.
Fig. 8 is a schematic diagram of the back projection error of the projector after calibration is completed, and the result shows that the calibration precision is very accurate.
The marks in the primary projection pattern are not limited to cross marks, and the characteristic points of the calibration plate are not limited to black circular characteristic points. The mark in the primary projection pattern can also be a white dot, and the characteristic point of the calibration plate is a black-white grid matrix. The mark in the primary projection pattern can also be a black and white lattice matrix, and the characteristic points of the calibration plate are circular ring characteristic points.
It should be understood that the above examples are only for clarity of illustration and are not intended to limit the embodiments. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. And obvious variations or modifications therefrom are within the scope of the invention.
Claims (9)
1. A projector calibration method based on secondary projection is characterized in that the method comprises the following steps:
projecting a primary projection pattern with a mark onto a calibration plate with characteristic points by using a projector, wherein the calibration plate is provided with a plurality of characteristic points arranged in an array form, and the centers of the characteristic points in a first row and a first column are used as the origin of a world coordinate system; acquiring a primary calibration plate image with marks and characteristic points by a camera, and determining the world of the calibration plate plane according to the centers of the characteristic pointsObtaining A homography matrix of camera imaging plane and calibration plate plane from coordinates in coordinate system and pixel coordinates in camera imaging plane
Obtaining a B homography matrix of the camera imaging plane and the projector plane according to the pixel coordinates of the mark center on the camera imaging plane and the coordinates on the projector plane
According to the formula
C homography matrix for calculating plane of projector and plane of calibration plate
Setting a plurality of position point arrays with known world coordinates, and calculating the world coordinates of each position point and the C homography matrix
Obtaining the coordinate point set P of the mapping points of all the position points on the plane of the projectorP(ii) a Generating a circular feature mapping point with a set radius by taking the mapping point coordinate as a center, and further obtaining a secondary projection pattern with a black background and a white circular feature mapping point array; projecting the secondary projection pattern onto a calibration plate by a projector, and collecting an image of the secondary calibration plate by a camera; according to the pixel coordinates of each mapping point in the camera imaging plane and the A homography matrix
Obtaining a world coordinate point set P of each mapping pointW(ii) a Point set P corresponding to different calibration plate positionsPAnd set of points PWAnd calibrating the projector by a Zhang Zhen you camera calibration method to obtain the internal parameters and the distortion coefficient of the projector.
2. The calibration method for a projector based on secondary projection as claimed in claim 1, wherein the marks in the primary projection pattern are cross-shaped, and the feature points of the calibration plate are black circular feature points.
3. The calibration method for a projector based on secondary projection as claimed in claim 1, wherein the marks in the primary projection pattern are white dots, and the characteristic points of the calibration plate are black and white grids.
4. The calibration method for a projector based on secondary projection as claimed in claim 1, wherein the marks in the primary projection pattern are black and white grids, and the feature points of the calibration plate are circular ring feature points.
5. The calibration method for projector based on secondary projection as claimed in claim 1, wherein said A homography matrix
The obtaining method comprises the following steps: for any feature point c on the calibration board, the pixel coordinate P of the center of the feature point c in the camera imaging plane is obtained by using the function in opencvC1Let the world coordinate of the center of the feature point c be PWAccording to the formula
Calculating to obtain A homography matrix of camera imaging plane and calibration plate plane
6. The calibration method for projector based on secondary projection as claimed in claim 1, wherein said A homography matrix
The obtaining method comprises the following steps: aiming at any characteristic point c on the calibration plate, a circle center fitting mode after edge extraction is used for obtaining a pixel coordinate P of the center of the characteristic point c in the camera imaging planeC1Let the world coordinate of the center of the feature point c be PWAccording to the formula
Calculating to obtain A homography matrix of camera imaging plane and calibration plate plane
7. The method for calibrating a projector based on reprojection of claim 1, wherein the B homography matrix
The obtaining method comprises the following steps: extracting the pixel coordinates of the cross mark on the camera imaging plane by adopting a threshold segmentation method, and solving the pixel coordinate P of the center of the cross mark on the camera imaging planeC2(ii) a Let the coordinate of the center of the cross mark on the plane of the projector be PPAccording to the formula
B homography matrix of camera imaging plane and projector plane is obtained through calculation
8. The calibration method for projector based on secondary projection as claimed in claim 1, wherein the pixel coordinates of each circular feature mapping point on the camera imaging plane are obtained from the secondary calibration plate image by using the function in opencv; let the pixel coordinate of any circular feature mapping point in the camera imaging plane be PC3According to the formula
Obtaining the coordinate P of the circular feature mapping point in the world coordinate systemW3Then obtaining the world coordinate point set P of all the circular feature mapping points in the calibration plate planeW。
9. The projector calibration method based on the quadratic projection of claim 1, characterized in that according to the quadratic calibration plate image, the pixel coordinates of each circular feature mapping point on the camera imaging plane are obtained by using the circle center fitting mode after the edge extraction; let the pixel coordinate of any circular feature mapping point in the camera imaging plane be PC3According to the formula
Obtaining the coordinate P of the circular feature mapping point in the world coordinate systemW3Then obtaining the world coordinate point set P of all the circular feature mapping points in the calibration plate planeW。
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| CN101476882B (en) * | 2009-01-08 | 2010-07-28 | 上海交通大学 | Structured light three-dimensional detection method based on homography matrix |
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