CN1878319A - Video camera marking method based on plane homographic matrix characteristic line - Google Patents

Abstract

The invention relates to a camera demarcate method, based on the plane unit matrix character line. Wherein, shooting several images of one plane pane figure with known size at different altitudes to the camera; extracting the image coordinates of each pane angle point of each image; according to the relationship between said coordinate and other ordinates, calculating out the plane unit matrix; then based on the plane unit matrix, obtaining one character line that represents the track of main point of camera; dividing the character line into two groups, according to the positive slope or negative slope of character line, and calculating the intersection point of two groups of character lines as the initial value of main point; according to the relationship between the plane unit matrix, the main point and the main distance, calculating the initial value of main distance of camera, to complete the liner demarcation of camera; and using the variance of intersection point as target function, to use optimize method to optimize the initial value of liner estimation and calculating out the aberrance factor of camera lens, to complete the full demarcation of camera.

Description

Camera marking method based on plane homographic matrix characteristic line

Technical field

The present invention relates to a kind of camera marking method, be applicable to the camera calibration in the measurement of computer vision and close-up photography, also can be used for the geometric correction of image based on plane homographic matrix characteristic line.Belonging to advanced makes and automatic field.

Background technology

Camera calibration is the major issue in the photogrammetric and computer vision always, at present, and existing multiple camera marking method.Early stage camera marking method adopts 3 D stereo to demarcate thing, because the three-dimensional difficulty of thing on making of demarcating replaced by plane reference object at present gradually.Along with the development of digital camera machine technology, four intrinsic parameter camera models that adopt principal point and main distance to represent at present are enough to satisfy the general application in the computer vision.The existing scaling method of four intrinsic parameters in the plane reference object calibrating camera model that adopts generally comprises two steps, linear initial value is estimated and nonlinear parameter optimization (Z.Zhang, A Flexible NewTechnique for Camera Calibration, IEEE Trans.Pattern Analysis andMachine Intelligence, 22 (11): 1330-1334,2000.).Wherein, to adopt Cholesky to decompose in linear initial value estimation stages.Because noise in the image and camera lens distortion cause matrix not satisfy the condition that Cholesky decomposes sometimes, can not carry out Cholesky and decompose.In addition, the physical significance of existing linear initial value method of estimation is indeterminate.Therefore, this area needs a kind of linear initial value method of estimation that does not adopt Cholesky to decompose, and expects that this method has clear physical meaning.

Summary of the invention

The object of the invention is at the deficiencies in the prior art, a kind of camera marking method based on plane homographic matrix characteristic line is proposed, need not carry out Cholesky and decompose when linear initial value is estimated, the characteristic curve that characterizes video camera principal point track has clear physical meaning.

For realizing this purpose, technical scheme of the present invention is: the plane grid figure of taking a known dimensions with respect to video camera behind the multiple image of different attitudes, extract the image coordinate of each grid angle point in every width of cloth image, to every width of cloth image, according to grid angle point image coordinate of extracting and the corresponding relation between its world coordinates, obtain a plane homography matrix, again according to each plane homography matrix, the characteristic curve of the sign video camera principal point track that draws; According to the positive and negative of characteristic curve slope characteristic curve is divided into two groups, asks the intersection point of two stack features lines, with the centre coordinate of characteristic curve intersection point initial value as the video camera principal point; According to the relation between plane homography matrix and video camera principal point and the main distance, obtain the initial value of video camera master distance, the linear calibration who finishes video camera.Be target function further again, adopt optimal method that the initial value of Linear Estimation is optimized and obtains the camera lens distortion factor, finish the full demarcation of video camera with the variance of characteristic curve intersection point.Utilize full optimal value of demarcating lens distortion coefficient, principal point and the main distance of gained, can carry out geometric correction image.

Camera marking method of the present invention may further comprise the steps:

1, makes the plane grid figure of a known dimensions, on the grid figure of plane, set up a world coordinate system, take this plane grid figure from different orientation with video camera to be calibrated, obtain the multiple image of this plane grid figure with respect to the different attitudes of video camera.

2, extract the angle point of each grid in above-mentioned every width of cloth image, obtain the image coordinate of institute's each grid angle point that extracts in pixel coordinates is.

3,,, obtain a plane homography matrix according to the image coordinate of each grid angle point of extract and the corresponding relation between its world coordinates to every width of cloth image.

4, according to each plane homography matrix, the characteristic curve about the pixel coordinates variable that draws, this characteristic curve are the tracks that satisfies the video camera principal point of plane homography matrix.

5, according to the positive and negative of characteristic curve slope characteristic curve is divided into two groups, to be positive one group and slope be negative one group to slope, and asking slope is every characteristic curve and the intersection point of slope for every characteristic curve in bearing a group in positive a group.

6, ask the centre coordinate of characteristic curve intersection point, with the initial value of this centre coordinate as the video camera principal point; According to the relation between plane homography matrix and video camera principal point and the main distance, obtain the initial value of the main distance of video camera; Finish the linear calibration of video camera.

Method of the present invention is after finishing the linear calibration of video camera, can further carry out the full demarcation of video camera, be specially: with 0 is the initial value of camera lens distortion factor, result with the linear calibration is the initial value of video camera principal point and main distance, variance with the characteristic curve intersection point is a target function, adopt optimal method that camera lens distortion factor, principal point and main distance are optimized, draw the optimal value of camera lens distortion factor, principal point and main distance, finish the full demarcation of video camera.

After the full demarcation of finishing video camera, can utilize video camera to demarcate the optimal value of lens distortion coefficient, principal point and the main distance of gained entirely, image is carried out geometric correction.

Compared with prior art, method of the present invention does not need to carry out Cholesky and decomposes, and does not therefore exist because noise in the image and camera lens distortion cause the problem that the Cholesky decomposition can not be carried out sometimes in the prior art.In addition, the characteristic curve among the present invention has clear physical meaning, and the track of the video camera principal point of a given plane homography matrix is satisfied in its expression.That is to say that satisfy all video cameras of a given plane homography matrix, their principal point all is positioned on same the characteristic curve.

Description of drawings

Fig. 1 is a multiple image captured in the one embodiment of the invention.

Fig. 2 is the grid angle point that piece image extracts from Fig. 1.

Fig. 3 is the characteristic curve that every width of cloth image draws from Fig. 1.

Fig. 4 is the result after the characteristic curve among Fig. 3 passes through parameter optimization.

Embodiment

In order to understand technical scheme of the present invention better, be described in further detail below in conjunction with drawings and Examples.

Camera marking method of the present invention may further comprise the steps:

1. photographic images: the plane grid figure of on a rigid plane, making a known dimensions, take this plane grid figure from a plurality of different orientation with video camera to be calibrated, obtain the multiple image of this plane grid figure with respect to the different attitudes of video camera.Fig. 1 is the multiple image of captured plane grid figure among the embodiment.Set up coordinate system XOY on the grid figure of plane, this coordinate system is called world coordinate system.If the length of side of each grid is a, then the world coordinates of each grid angle point can be expressed as P _Ij(ia, ja, 0), wherein i and j are natural number, i.e. X _Ij=ia, Y _Ij=ja, Z _Ij=0.

2. extraction angle point: can adopt any Angular Point Extracting Method known in the art to extract the angle point of each grid in every width of cloth image, preferably adopt the Harris Angular Point Extracting Method.Fig. 2 is the grid angle point that piece image extracts from Fig. 1, and establishing the institute's image coordinate of each angle point that extracts in pixel coordinates is is I _Ij(u _Ij, v _Ij).

3. ask the plane homography matrix: with the world coordinates (X of each angle point _Ij, Y _Ij) and image coordinate (u _Ij, v _Ij) substitution (1) formula, can obtain plane homography matrix H, this is that those skilled in the art is known.Its midplane homography matrix H can be represented by (2) formula.

$s\left[\begin{array}{c}{u}_{\mathrm{ij}}\\ v_{\mathrm{ij}}\\ 1\end{array}\right]=H\left[\begin{array}{c}{X}_{\mathrm{ij}}\\ Y_{\mathrm{ij}}\\ 1\end{array}\right]---\left(1\right)$

$H=\left[\begin{array}{ccc}{h}_{11}& h_{12}& h_{13}\\ h_{21}& h_{22}& h_{23}\\ h_{31}& h_{32}& 1\end{array}\right]---\left(2\right)$

4. picture characteristic curve: for each plane homography matrix H, can be by (3) formula the straight line that draws about pixel coordinates variable u and v.This straight line is the track that satisfies the video camera principal point of plane homography matrix H, and it has represented the key character of plane homography matrix H, in this article this straight line is called characteristic curve.

v＝Eu+F (3)

In the formula:

$E=k^2\frac{h_{11}{h}_{32}-h_{31}{h}_{12}}{h_{31}{h}_{22}-h_{21}{h}_{31}}---\left(4\right)$

$F=\frac{h_{21}{h}_{31}+h_{22}{h}_{32}}{h_{31}^2+h_{32}^2}-\frac{h_{11}{h}_{31}+h_{12}{h}_{32}}{h_{31}^2+h_{32}^2}E---\left(5\right)$

K is the length-width ratio of pixel unit.Can prove for the pinhole camera modeling of being represented by four intrinsic parameters, no matter what value k gets, all characteristic curves all intersect at the principal point of video camera.So any value between the k desirable 0.5 to 1 preferably gets 1.The characteristic curve that Fig. 3 draws for the plane homography matrix according to every width of cloth image correspondence among Fig. 1.

5. ask the characteristic curve intersection point: according to the positive and negative of characteristic curve slope characteristic curve is divided into two groups, to be positive one group and slope be negative one group to slope, and to ask slope be every characteristic curve and the intersection point of slope for each bar characteristic curve in bearing a group in positive a group.The method of asking two straight-line intersections is that people are known.If slope is that m bar characteristic curve is arranged in positive one group, and slope has n bar characteristic curve in negative one group, then can obtain m * n intersection point.In the embodiment shown in fig. 3, have 12 characteristic curves, wherein slope is positive 6 and represents with solid line, and slope is represented by dotted lines for negative 6.Obtain slope and be each bar characteristic curve and slope in positive one group for totally 36 at the intersection point of each bar characteristic curve in negative a group, with little box indicating.

6. ask the initial value of principal point: ask the centre coordinate of above-mentioned m * n characteristic curve intersection point,, be expressed as (u with the initial estimate of this centre coordinate as the video camera principal point ₀, v ₀).The method of asking a point set center is well-known to those skilled in the art.In the embodiment shown in fig. 3, the center of 36 intersection points is represented with asterisk.

Ask the initial value of main distance then: the initial value (u that obtains principal point ₀, v ₀) after, can obtain the initial value of video camera master as follows, the linear calibration who finishes video camera apart from α and β.

$a=h_{11}{h}_{12}-u_0(h_{31}{h}_{12}+h_{11}{h}_{32})+u_0^2{h}_3{h}_{32}---\left(6\right)$

$b=h_{21}{h}_{22}-u_0(h_{31}{h}_{22}+h_{21}{h}_{32})+v_0^2{h}_{31}{h}_{32}---\left(7\right)$

c＝(h ₁₁-u ₀h ₃₁) ²-(h ₁₂-u ₀h ₃₂) ² (8)

d＝(h ₂₁-v ₀h ₃₁) ²-(h ₂₂-v ₀h ₃₂) ² (9)

$\mathrm{\α}=\sqrt{\frac{\mathrm{bc}-\mathrm{ad}}{b(h_{32}^2-h_{31}^2)+d\left(h_{31}{h}_{32}\right)}}---\left(10\right)$

$\mathrm{\β}=\sqrt{\frac{\mathrm{bc}-\mathrm{ad}}{a(h_{32}^2-h_{31}^2)+c\left(h_{31}{h}_{32}\right)}}---\left(11\right)$

Parameter optimization: in theory, for the pinhole camera modeling of being represented by four intrinsic parameters, all characteristic curves all intersect at the principal point of video camera.In fact, because The noise in camera lens distortion and the image, characteristic curve does not intersect at a point, but is dispersed in a zone, as shown in Figure 3.Therefore, above-mentioned linear calibration's result is an approximate estimated value, and obtain more also needing the initial value that obtains among the above-mentioned linear calibration is optimized near the calibration result of true value.When optimizing process began, the distortion factor of establishing camera lens was 0.Describe with the embodiment that only considers camera lens single order coefficient of radial distortion k1 below.

At first use (12) and (13) formula that the image coordinate of each angle point is carried out geometric distortion correction,

$U_{\mathrm{ij}}=\frac{u_{\mathrm{ij}}-u_0}{1+k_1[{\left(\frac{u_{\mathrm{ij}}-u_0}{\mathrm{\α}}\right)}^2+{\left(\frac{v_{\mathrm{ij}}-v_0}{\mathrm{\β}}\right)}^2]}+u_0---\left(12\right)$

$V_{\mathrm{ij}}=\frac{v_{\mathrm{ij}}-v_0}{1+k_1[{\left(\frac{u_{\mathrm{ij}}-u_0}{\mathrm{\α}}\right)}^2+{\left(\frac{v_{\mathrm{ij}}-v_0}{\mathrm{\β}}\right)}^2]}+v_0---\left(13\right)$

(U in the formula _Ij, V _Ij) be the angle point image coordinate after proofreading and correct.Use (U then _Ij, V _Ij) replacement (u _Ij, v _Ij), as obtaining the plane homography matrix after the correction as described in the step 3.As described in step 4,5 and 6, obtain the principal point and the main distance of proofreading and correct the back video camera.With the variance of being asked two stack features line intersection points in the step 5 is target function, gets minimum value to the lens distortion coefficient k by making this target function ₁Be optimized with the principal point and the main distance of video camera, obtain the lens distortion coefficient k at last ₁With the principal point of video camera and the optimal value of main distance, finish the full demarcation of video camera.Any optimal method that makes target function get minimum value all is applicable to the present invention.Fig. 4 is the result after the characteristic curve among Fig. 3 passes through parameter optimization.

After obtaining video camera principal point, main distance and lens distortion coefficient, available these parameters are carried out geometric correction to image.Therefore, method of the present invention also can be used for the correction of piecture geometry fault.

Method of the present invention does not need to carry out Cholesky and decomposes.If represent relation between world coordinate system and the pixel coordinate system can prove that so characteristic curve slope and the intercept represented by (4) and (5) formula can further be expressed as with Eulerian angles:

$E=-k\frac{\sin \mathrm{\θ}}{\tan \mathrm{\ψ}}---\left(14\right)$

$F=k{u}_0\frac{\sin \mathrm{\θ}}{\tan \mathrm{\ψ}}+v_0---\left(15\right)$

In the formula, θ is the angle of pitch of world coordinate system with respect to pixel coordinate system, and ψ is the yaw angle of world coordinate system with respect to pixel coordinate system.By (14) and (15) formula as can be seen, characteristic curve and world coordinate system are irrelevant with respect to the translation vector of pixel coordinate system with respect to the roll angle and the world coordinate system of pixel coordinate system.Therefore, when taking uncalibrated image, control pitching angle theta and yaw angle ψ consciously, make the absolute value of characteristic curve slope be unlikely to very big, method so of the present invention always can be carried out.

In addition, the characteristic curve among the present invention has clear physical meaning, and the track of the video camera principal point of a given plane homography matrix is satisfied in its expression.That is to say that satisfy all video cameras of a given plane homography matrix, their principal point all is positioned on same the characteristic curve.So the characteristic curve that the plane homography matrix of being obtained by several plane graticule pictures of taking under same camera parameters draws must intersect at the principal point of this video camera.

Claims (3)

1, a kind of camera marking method based on plane homographic matrix characteristic line is characterized in that comprising following concrete steps:

1) the plane grid figure of a known dimensions of making, on the grid figure of plane, set up a world coordinate system, take this plane grid figure from different orientation with video camera to be calibrated, obtain the multiple image of this plane grid figure with respect to the different attitudes of video camera;

2) extract the angle point of each grid in above-mentioned every width of cloth image, obtain the image coordinate of institute's each grid angle point that extracts in pixel coordinates is;

3),, obtain a plane homography matrix according to the image coordinate of each grid angle point of extract and the corresponding relation between its world coordinates to every width of cloth image;

4) according to each plane homography matrix, the characteristic curve about the pixel coordinates variable that draws, this characteristic curve are the tracks that satisfies the video camera principal point of plane homography matrix;

5) according to the positive and negative of characteristic curve slope characteristic curve is divided into two groups, to be positive one group and slope be negative one group to slope, and asking slope is every characteristic curve and the intersection point of slope for every characteristic curve in bearing a group in positive a group;

6) ask the centre coordinate of characteristic curve intersection point, with the initial value of this centre coordinate as the video camera principal point; According to the relation between plane homography matrix and video camera principal point and the main distance, obtain the initial value of the main distance of video camera; Finish the linear calibration of video camera.

2, according to the camera marking method based on plane homographic matrix characteristic line of claim 1, it is characterized in that after finishing the linear calibration of video camera, with 0 is the initial value of camera lens distortion factor, result with the linear calibration is the initial value of video camera principal point and main distance, variance with the characteristic curve intersection point is a target function, adopt optimal method to camera lens distortion factor, principal point and main apart from being optimized, draw the optimal value of camera lens distortion factor, principal point and main distance, finish the full demarcation of video camera.

3, according to the camera marking method based on plane homographic matrix characteristic line of claim 2, it is characterized in that after the full demarcation of finishing video camera, utilize video camera to demarcate the optimal value of lens distortion coefficient, principal point and the main distance of gained entirely, image is carried out geometric correction.

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