CN104240262A - Calibration device and calibration method for outer parameters of camera for photogrammetry - Google Patents

Abstract

The invention discloses an outer parameter calibration device and a calibration methodof a camera for photogrammetry. The calibration device is composed of target mark post, a centering rod, a leveling mechanism and a datum mark. The calibration method comprises the following steps: firstly, adjusting a camera to point a target to be measured, embedding four datum points in a camera pointing direction; measuring a coordinate with high precision in a reference coordinate system; adjusting the centering rod of the calibration device to be vertical in accordance with the designed size of the calibration device; when the leveling mechanism is horizontally placed, and deriving the coordinate of a mark point in the reference coordinate system; secondly, shooting a landmark image on a calibration device through a camera, extracting a mark point coordinate by adopting an elliptic fitting algorithm, and resolving the outer parameters of the camera and the reference coordinate system to complete calibration by combining the parameter calibration results of the coordinate and the camera in the reference coordinate system. Compared with the prior art, the scheme is applicable to a three-dimensional calibration occasion of a plurality of cameras in a wide measurement scope, and has the characteristics of being convenient to operate and high in calibration precision.

Description

A kind of for photogrammetric Camera extrinsic number caliberating device and scaling method

Technical field

The present invention relates to the technical field of the demarcation for photogrammetric camera, be specifically related to a kind of for photogrammetric Camera extrinsic number caliberating device and scaling method.

Background technology

Photogrammetricly be widely used in the fields such as reconstructing three-dimensional model, body surface dimensional measurement, target location and attitude measurement, space articulation, medical diagnosis and exercise recovery.In order to obtain target three-dimensional information, the camera of photogrammetric general needs two or more carries out intersection measurement thus forms Stereo Vision Measurement System, therefore, multiple camera just becomes a measuring system measuring accuracy key influence factor in the position of the unified frame of reference and attitude is demarcated.In the present invention, the outer parameter of camera and camera are relative to the rotation matrix of the frame of reference and translation vector.Traditional camera parameter marked ratio more typically has zhang to propose based on coplanar calibrated reference method, the method allows coplanar reference object to move freely in visual field, by obtaining several high-quality uncalibrated images, the intrinsic parameter carrying out camera is demarcated, the a certain width of establishing shot, can calibration for cameras relative to the outer parameter of now fixing calibrated reference coordinate system, the method calibration process is simple, low to the requirement of target, but the method can not realize the unification of multiple plane coordinate system and the frame of reference, be only applicable to the camera calibration occasion that measurement range is less; In addition, also have a kind of use to assist camera to carry out the demarcation of full filed method, require that auxiliary camera collects the image demarcating reference frame reference mark and benchmark system reference mark simultaneously, this needs auxiliary camera to have larger field range, thus its imaging resolution can be lower, the precision that impact is demarcated; Auxiliary camera in above-mentioned scaling method can use laser tracker (Laser Tracker) to replace, laser tracker can high-acruracy survey spatial point, for the situation that the scope of " full filed " is very large, replace auxiliary camera to carry out the demarcation of full filed method with laser tracker to be conducive to improving stated accuracy, but laser tracker cost is higher, and demarcation flow process is comparatively loaded down with trivial details.

From above-mentioned analysis, the Camera extrinsic number in Stereo Vision Measurement System is demarcated should meet accuracy requirement, also makes to demarcate target easy to use, with low cost, applied widely as far as possible, can repeatedly use.From this angle, how by above-mentioned two kinds of typical scenarios, abandon its dross of selecting the essence, designs the external parameters calibration method and apparatus in a set of stereo visual system, and becoming one has key issue to be solved.

Summary of the invention

Fundamental purpose of the present invention is to overcome the deficiencies in the prior art, there is provided a set of external parameters calibration apparatus and method, this measurement device scope is wide, is convenient to processing and uses, measuring accuracy is high, for polyphaser stereo calibration in full filed on a large scale provides good solution.

The technical solution used in the present invention is: a kind of for photogrammetric Camera extrinsic number caliberating device, whole Camera extrinsic number caliberating device is made up of target bar, centering rod, levelling gear and datum mark four parts.Target bar is above demarcation linkage, and be provided with the circular index point of 10 one row above target bar, the monumented point center of circle is aligned in target plane.The below of mechanism is centering rod, for aiming at datum mark, ensure the positional precision of demarcating erection, and monumented point line and centering rod axis are on the same line, and distance bottom each monumented point center of circle to centering rod is known, namely below relative Calibration bar, the position of reference point accurately obtains, if make centering rod perpendicular to surface level, reference point coordinate (Y-axis of the frame of reference is perpendicular to surface level) then by demarcating in advance, namely can obtain the coordinate of center under the frame of reference of each circular index point.In order to make centering rod perpendicular to surface level, so require that the levelling gear place plane vertical with centering rod ensures level, therefore, this device has two support bars after centering rod, there is levelling bolt near ground location bottom support bar, making target bar perpendicular to surface level for regulating; During leveling, first slant angle sensor is fixed on objective table, then by rotating levelling bolt, sensor reading is reached and is worth closest to 0.In addition, after centering rod lower end is connected with reference point position, and levelling gear adjustment level, whole caliberating device can carry out 360 ° of rotations around reference point vertical direction, and target bar can be observed by multiple camera, thus can realize multiple stage camera and demarcate simultaneously.

Based on said apparatus, the invention provides a kind of Camera extrinsic number scaling method, the method flow that Camera extrinsic number is demarcated is as follows:

Step 1), now intrinsic parameter demarcation is carried out to the camera of external parameters calibration, intrinsic parameter comprises: the photocentre coordinate of camera, equivalent focal length and distortion factor;

Step 2), be embedded with four datum marks in demarcation bar riding position, the coordinate under the frame of reference of each datum mark has been measured in advance and has been obtained;

Step 3), first the centering rod below target is placed on a position 1, two levelling gears that then adjustment is demarcated bottom shaft bar make whole target bar perpendicular to surface level, as shown in Figure 5.Installation flat board bottom target bar there is tilt quantity sensor, current location whether leveling can be judged according to its reading;

Step 4), then use camera take current target bar image and record.Then target bar is moved to a position 2, repeat the process of above-mentioned aligning and leveling, then take the target bar image at a position 2 place.Until the image taking of four some positions is complete;

Step 5), according to demarcating the coordinate of four reference points under the frame of reference in advance, can after caliberating device adjustment level, the coordinate of ten monumented points under the frame of reference on its surface plate;

Step 6), gather four width circle marker dot image by external parameters calibration device, as shown in Figure 6, utilize the method for ellipse fitting to extract monumented point centre coordinate, by splicing, be combined into 40 monumented point imaging datas;

Step 7), the pixel coordinate that extracts of the coordinate of input monumented point under the frame of reference and imaging, and camera intrinsic parameter, calculate Camera extrinsic number, namely camera is tied to the rotation matrix of the frame of reference relative to the translation vector (photocentre position) of the frame of reference and camera coordinates.

Further, for multiple camera, caliberating device can rotate centered by reference point, makes monumented point plane facing to the camera needing to demarcate, repeats step 5) to step 7), the demarcation of all cameras can be completed.

Compared with prior art, beneficial effect of the present invention is as follows:

1), this invention by the reference point coordinate of calibration for cameras all under measuring basis coordinate system, just can realize multiple stage camera stereo calibration, and measurement range is wide;

2), by 40 monumented points set up the three-dimensional coordinate known point of calibrated reference and the corresponding relation of picture point, adopt nonlinear method to calculate Camera extrinsic number, higher measuring accuracy can be obtained;

3), this caliberating device processing cost is low, simple and easy to use, can repeatedly use.

Accompanying drawing explanation

Fig. 1 is the composition structural drawing of Camera extrinsic number caliberating device;

Fig. 2 is Camera extrinsic number scaling method flow process;

Fig. 3 is calibration of camera internal parameters image;

Fig. 4 is that calibration of camera internal parameters logos point extracts schematic diagram;

Fig. 5 is the image that Camera extrinsic number timing signal gathers, (a) uncalibrated image for gathering when caliberating device is placed in reference point position 1, (b) uncalibrated image for gathering when caliberating device is placed in reference point position 2, (c) uncalibrated image for gathering when caliberating device is placed in reference point position 3, (d) uncalibrated image for gathering when caliberating device is placed in reference point position 4;

Fig. 6 is that Camera extrinsic number uncalibrated image monumented point extracts schematic diagram.

Embodiment

The present invention is further illustrated below in conjunction with accompanying drawing and specific embodiment.

Step 1), intrinsic parameter demarcation is carried out to every platform camera, position, 2 meters, camera front is placed on by intrinsic parameter scaling board, as shown in Figure 3, the orientation of change scaling board and attitude, gather 20 width scaling board images, extract the monumented point coordinate of every width image, as shown in Figure 4, carry out intrinsic parameter demarcation, and preserve calibration result;

Step 2), according to the field range of camera, be embedded with four datum marks in demarcation bar riding position, the coordinate under the frame of reference of each datum mark has been measured in advance and has been obtained, and i-th reference point coordinate is designated as:

$P_i^G={(X_i^G,Y_i^G,Z_i^G)}^T,\{i=\mathrm{1,2,3,4}\}$

Wherein, i is reference point sequence number, be i-th reference point three-dimensional coordinate under the frame of reference, be the three-dimensional coordinate X component of i-th reference point under the frame of reference, be the three-dimensional coordinate Y-component of i-th reference point under the frame of reference, be the three-dimensional coordinate Z component of i-th reference point under the frame of reference.

Step 3), first the centering rod below target is placed on a position 1, as as shown in Fig. 5 image (a), then two levelling gears that adjustment is demarcated bottom shaft bar make gusseted planar horizontal, whole target bar is perpendicular to surface level, installation flat board bottom target bar there is tilt quantity sensor, current location whether leveling can be judged according to its reading;

Step 4), then use camera take current target bar image and record.Then target bar is moved to a position 2, repeat the process of above-mentioned aligning and leveling, then take the target bar image at a position 2 place, until the image taking of four some positions is complete;

Step 5), when caliberating device levelling gear regulate level, after centering rod Uprightness adjusting, according to demarcating witness marker point design attitude and size on the coordinate of four reference points under the frame of reference and caliberating device in advance, the coordinate of ten monumented points under the frame of reference on target bar can be calculated, from top to bottom, on i-th reference point position, a jth monumented point three-dimensional coordinate is designated as monumented point:

$P_{\mathrm{ij}}^G={(X_{\mathrm{ij}}^G,Y_{\mathrm{ij}}^G,Z_{\mathrm{ij}}^G)}^T,\{i=\mathrm{1,2,3,4};j=\mathrm{0,1},...,9\}$

Wherein, i is reference point sequence number, and j is monumented point sequence number, be jth monumented point three-dimensional coordinate under the frame of reference on i-th reference point position, for this three-dimensional coordinate X component, for this three-dimensional coordinate Y-component, for this three-dimensional coordinate Z component.Known each monumented point, to the vertical range of reference point position stiff end, is designated as L _j, then:

$P_{\mathrm{ij}}^G=P_i^G+\left[\begin{array}{ccc}0& 1& 0\end{array}\right]L_j,\{i=\mathrm{1,2,3,4};j=\mathrm{0,1},...,9\}$

Wherein, i is reference point sequence number, and j is monumented point sequence number, for jth monumented point three-dimensional coordinate under the frame of reference on i reference point position, for i reference point three-dimensional coordinate under the frame of reference, L _jfor a jth monumented point is to the vertical range of reference point position stiff end.

Step 6), gather four width circle marker dot image by external parameters calibration device, as shown in Figure 6, utilize the method for ellipse fitting to extract monumented point centre coordinate, by four width image zooming-out point coordinate, be combined into 40 monumented point imaging datas;

Step 7), according to perspective projection imaging photogrammetry principles, the pixel coordinate that the coordinate of input monumented point under the frame of reference and imaging are extracted, has following relation:

$k\·\left[\begin{array}{c}{X}_{\mathrm{ij}}\\ Y_{\mathrm{ij}}\\ 1\end{array}\right]=\left[\begin{array}{cccc}{F}_x& 0& C_x& 0\\ 0& F_y& C_y& 0\\ 0& 0& 1& 0\end{array}\right]\left[\begin{array}{cc}{R}_{g2x}& T_{g2x}\\ 0^T& 1\end{array}\right]\left[\begin{array}{c}{X}_{\mathrm{ij}}^G\\ Y_{\mathrm{ij}}^G\\ Z_{\mathrm{ij}}^G\\ 1\end{array}\right]$

Wherein, (X _ij, Y _ij), { i=1,2,3,4; J=0,1 ..., 9} is the pixel coordinate that a jth monumented point of i-th reference point position correspondence extracts, and k is the Z component of monumented point three-dimensional coordinate under camera coordinates system, (C _x, C _y) be figure principal point, be the image coordinate of optical axis and image planes intersection point, (F _x, F _y) be transverse and longitudinal equivalent focal length, be the horizontal stroke of actual optical focal length with pixel, the ratio of longitudinal size, R _g2xfor reference coordinate is tied to the rotation matrix of camera coordinates system, T _g2xfor reference coordinate is tied to the translation vector of camera coordinates system.Principal point and equivalent focal length are the intrinsic parameters of camera, describe the characteristic of camera itself; And rotation matrix and translation vector describe is relative position relation between the frame of reference and camera coordinates system and attitude relation, by this rotation matrix and translation vector, reverse can go out rotation matrix and the translation vector that camera coordinates is tied to the frame of reference, i.e. the outer parameter of camera.R _g2xand T _g2xbe defined as follows:

$R_{g2x}=\left[\begin{array}{ccc}{r}_0& r_1& r_2\\ r_3& r_4& r_5\\ r_6& r_7& r_8\end{array}\right]$

T _g2x＝(T _x,T _y,T _z) ^T

Step 8), definition a projection matrix M:

$M=\left[\begin{array}{cccc}{F}_x& 0& C_x& 0\\ 0& F_y& C_y& 0\\ 0& 0& 1& 0\end{array}\right]\left[\begin{array}{cc}{R}_{g2x}& T_{g2x}\\ 0^T& 1\end{array}\right]$

The expansion of M:

$M=\left[\begin{array}{cccc}{m}_0& m_1& m_2& m_3\\ m_4& m_5& m_6& m_7\\ m_8& m_9& m_{10}& m_{11}\end{array}\right]=\left[\begin{array}{cccc}{F}_x{r}_0+C_x{r}_6& F_x{r}_1+C_x{r}_7& F_x{r}_2+C_x{r}_8& F_x{T}_x+C_x{T}_z\\ F_y{r}_3+C_y{r}_6& F_y{r}_4+C_y{r}_7& F_y{r}_5+C_y{r}_8& F_y{T}_y+C_y{T}_z\\ r_6& r_7& r_8& T_z\end{array}\right]$

Then perspective projection imaging relation can be described as with projection matrix:

$k\left[\begin{array}{c}{X}_{\mathrm{ij}}\\ Y_{\mathrm{ij}}\\ 1\end{array}\right]=M\left[\begin{array}{c}{X}_{\mathrm{ij}}^G\\ Y_{\mathrm{ij}}^G\\ Z_{\mathrm{ij}}^G\\ 1\end{array}\right]$

Above formula is launched into scale form and cancellation k, just obtains the collinearity equation of the object point in description perspective projection imaging, picture point, photocentre three point on a straight line relation:

$\left\{\begin{array}{c}\frac{X_{\mathrm{ij}}-C_x}{F_x}=\frac{r_0{X}_{\mathrm{ij}}^G+r_1{Y}_{\mathrm{ij}}^G+r_2{Z}_{\mathrm{ij}}^G+T_x}{r_6{X}_{\mathrm{ij}}^G+r_7{Y}_{\mathrm{ij}}^G+r_8{Z}_{\mathrm{ij}}^G+T_z}\\ \frac{Y_{\mathrm{ij}}-C_y}{F_y}=\frac{r_3{X}_{\mathrm{ij}}^G+r_4{Y}_{\mathrm{ij}}^G+r_5{Z}_{\mathrm{ij}}^G+T_y}{r_6{X}_{\mathrm{ij}}^G+r_7{Y}_{\mathrm{ij}}^G+r_8{Z}_{\mathrm{ij}}^G+T_z}\end{array}\right.$

When intrinsic parameter is known, then by transposition, above formula is converted into about T _x, T _y, T _zand r ₀~ r ₉system of homogeneous linear equations:

$\left\{\begin{array}{c}{r}_0{X}_{\mathrm{ij}}^G+r_1{Y}_{\mathrm{ij}}^G+r_2{Z}_{\mathrm{ij}}^G+T_x-\frac{X_{\mathrm{ij}}-C_x}{F_x}(r_6{X}_{\mathrm{ij}}^G+r_7{Y}_{\mathrm{ij}}^G+r_8{Z}_{\mathrm{ij}}^G+T_z)=0\\ r_3{X}_{\mathrm{ij}}^G+r_4{Y}_{\mathrm{ij}}^G+r_5{Z}_{\mathrm{ij}}^G+T_y-\frac{Y_{\mathrm{ij}}-C_y}{F_y}(r_6{X}_{\mathrm{ij}}^G+r_7{Y}_{\mathrm{ij}}^G+r_8{Z}_{\mathrm{ij}}^G+T_z)=0\end{array}\right.$

Due to T _z> 0, can make t _i=r _i/ T _z, i=0,1 ..., 8, t ₉=T _x/ T _z, t ₁₀=T _y/ T _z, above formula can be converted into about t ₀~ t ₁₀system of linear equations:

$\left\{\begin{array}{c}{X}_{\mathrm{ij}}^G{t}_0+Y_{\mathrm{ij}}^G{t}_1+Z_{\mathrm{ij}}^G{t}_2+t_9-\frac{X_{\mathrm{ij}}-C_x}{F_x}(X_{\mathrm{ij}}^G{t}_6+Y_{\mathrm{ij}}^G{t}_7+Z_{\mathrm{ij}}^G{t}_8)=\frac{X_{\mathrm{ij}}-C_x}{F_x}\\ X_{\mathrm{ij}}^G{t}_3+Y_{\mathrm{ij}}^G{t}_4+Z_{\mathrm{ij}}^G{t}_5+t_{10}-\frac{Y_{\mathrm{ij}}-C_y}{F_y}(X_{\mathrm{ij}}^G{t}_6+Y_{\mathrm{ij}}^G{t}_7+Z_{\mathrm{ij}}^G{t}_8)=\frac{Y_{\mathrm{ij}}-C_y}{F_y}\end{array}\right.$

Wherein unknown number 11, and two equations can be listed from each monumented point, when the three-dimensional coordinate of monumented point under the frame of reference { i=1,2,3,4; J=0,1 ..., 9} and monumented point extract coordinate (X _ij, Y _ij), { i=1,2,3,4; J=0,1 ..., 9} is known, by the system of equations that 40 monumented points are corresponding, calculates t ₀~ t ₁₀.Again according to r ₆ ²+ r ₇ ²+ r ₈ ²=1, have $t_6^2+t_7^2+t_8^2=\frac{1}{T_z^2},$ Then:

$T_z=\frac{1}{\sqrt{t_6^2+t_7^2+t_8^2}}$

And then obtain two other component r of rotation matrix and translation vector _i=t _it _z, (i=0,1 ..., 8) and T _x=t ₉t _z, T _y=t ₁₀t _z, thus calculate the rotation matrix R that reference coordinate is tied to camera coordinates system _g2xwith translation vector T _g2x, and then release the rotation matrix that camera coordinates is tied to the frame of reference and be and translation vector complete the external parameters calibration of this camera.

Further, if demarcate multiple stage camera simultaneously, when target bar is in a certain reference point position, after centering rod lower end is connected with reference point position, and during levelling gear adjustment level, caliberating device can be made to rotate around reference point vertical direction, target monumented point can be observed by next camera, and take target image, thus complete multiple stage camera calibration.

Claims (3)

1., for a photogrammetric Camera extrinsic number caliberating device, it is characterized in that being made up of target bar, centering rod, levelling gear and datum mark four parts, target bar is above demarcation linkage, and target bar is rectangular parallelepiped, and be provided with the circular index point of 10 one row above target, the monumented point center of circle is aligned in target plane, be centering rod below target bar, for aiming at datum mark, ensure the positional precision of demarcating erection, and monumented point line and centering rod axis are on the same line, and distance bottom each monumented point center of circle to centering rod is known, if the position coordinates of reference point is known below demarcation bar, when centering rod is perpendicular to surface level, reference point coordinate then by demarcating in advance, namely the coordinate of center under the frame of reference of each circular index point can be obtained, wherein the Y-axis of the frame of reference is perpendicular to surface level, levelling gear is specific as follows: after centering rod, have two support bars, the plane orthogonal that centering rod and two support bars form, levelling bolt is had near ground location bottom support bar, for regulating level, slant angle sensor is utilized to be fixed on objective table, then by rotating levelling bolt, sensor reading is reached be worth closest to 0, in addition, after centering rod lower end is connected with reference point position, and levelling gear adjustment level, whole caliberating device can carry out 360 ° of rotations around reference point vertical direction, and target bar can be observed by multiple camera, thus can realize multiple stage camera and demarcate simultaneously.

2., for a photogrammetric Camera extrinsic number scaling method, utilize caliberating device described in claim 1, it is characterized in that, external parameters calibration method flow is as follows:

Step 1), be embedded with 4 datum marks in demarcation bar riding position, its three-dimensional coordinate under the frame of reference is surveyed in advance;

Step 2), first the centering rod below target is placed on a position 1, the levelling gear that then adjustment is demarcated bottom shaft bar makes whole target bar perpendicular to surface level, as shown in Figure 5; Installation flat board bottom target bar there is tilt quantity sensor, current location whether leveling can be judged according to its reading;

Step 3), then use camera take current target bar image and record; Then target bar is moved to a position 2, repeat the process of above-mentioned aligning and leveling, then take the target bar image at a position 2 place; Until the image taking of 4 some positions is complete, then the external parameters calibration work of this camera is completed;

Step 4) if demarcate multiple stage camera simultaneously, when target bar is in a certain reference point position, caliberating device can be made to rotate around reference point vertical direction, target monumented point can be observed by next camera, and take target image, thus simplify multiple stage camera calibration flow process, complete multiple stage camera calibration;

Step 5), according to demarcating the coordinate of four reference points under the frame of reference in advance, can after caliberating device adjustment level, the coordinate of ten monumented points under the frame of reference on its surface plate;

Step 6), gather four width circle marker dot image by external parameters calibration device, utilize the method for ellipse fitting to extract monumented point centre coordinate, by splicing, be combined into 40 monumented point imaging datas;

Step 7), the input monumented point pixel coordinate that coordinate and imaging are extracted under the frame of reference, and camera intrinsic parameter, calculate Camera extrinsic number, namely camera is tied to the rotation matrix of the frame of reference in the photocentre position of the frame of reference and camera coordinates.

3. Camera extrinsic number scaling method according to claim 2, is characterized in that, for multiple camera, caliberating device can rotate centered by reference point, make monumented point plane facing to the camera needing to demarcate, repeat step 5)-step 7), the demarcation of all cameras can be completed.