solvePnP returns wrong result

asked 2013-01-25 07:51:01 -0500

jbseano gravatar image

Hi everyone,

I'm using the function solvePnP to estimate the pose of my robot by visual markers. Some times I get wrong results in two consecutives frames. In the file problem.cpp you can see one of these results.

The points sets correspond to the same marker in two consecutives frames. The variation between them is very small, and the result of the solvePnP is very different, but only in the rotation vector. Translation vector is ok.

This happens approximely one time each 30 frames. I have tested CV_ITERATIVE and CV_P3P method with the same data, and they return the same result.

This is a example of the issue:

#include <opencv2/calib3d/calib3d.hpp>
#include <opencv2/core/core.hpp>
#include <iostream>
using namespace std;
using namespace cv;
int main(){
vector<Point2f> points1, points2;
//First point's set
points1.push_back(Point2f(384.3331f,  162.23618f));
points1.push_back(Point2f(385.27521f, 135.21503f));
points1.push_back(Point2f(409.36746f, 139.30315f));
points1.push_back(Point2f(407.43854f, 165.64435f));

//Second point's set
points2.push_back(Point2f(427.64938f, 158.77661f));
points2.push_back(Point2f(428.79471f, 131.60953f));
points2.push_back(Point2f(454.04532f, 134.97353f));
points2.push_back(Point2f(452.23096f, 162.13156f));

//Real object point's set
vector<Point3f> object;
object.push_back(Point3f(-88.0f,88.0f,0));
object.push_back(Point3f(-88.0f,-88.0f,0));
object.push_back(Point3f(88.0f,-88.0f,0));
object.push_back(Point3f(88.0f,88.0f,0));

//Camera matrix
Mat cam_matrix = Mat(3,3,CV_32FC1,Scalar::all(0));
cam_matrix.at<float>(0,0) = 519.0f;
cam_matrix.at<float>(0,2) = 320.0f;
cam_matrix.at<float>(1,1) = 522.0f;
cam_matrix.at<float>(1,2) = 240.0f;
cam_matrix.at<float>(2,2) = 1.0f;

//PnP
Mat rvec1i,rvec2i,tvec1i,tvec2i;
Mat rvec1p,rvec2p,tvec1p,tvec2p;
solvePnP(Mat(object),Mat(points1),cam_matrix,Mat(),rvec1i,tvec1i,false,CV_ITERATIVE);
solvePnP(Mat(object),Mat(points2),cam_matrix,Mat(),rvec2i,tvec2i,false,CV_ITERATIVE);
solvePnP(Mat(object),Mat(points1),cam_matrix,Mat(),rvec1p,tvec1p,false,CV_P3P);
solvePnP(Mat(object),Mat(points2),cam_matrix,Mat(),rvec2p,tvec2p,false,CV_P3P);

//Print result
cout<<"Iterative: "<<endl;
cout<<" rvec1 "<<endl<<" "<<rvec1i<<endl<<endl;
cout<<" rvec2 "<<endl<<" "<<rvec2i<<endl<<endl;
cout<<" tvec1 "<<endl<<" "<<tvec1i<<endl<<endl;
cout<<" tvec1 "<<endl<<" "<<tvec2i<<endl<<endl;

cout<<"P3P: "<<endl;
cout<<" rvec1 "<<endl<<" "<<rvec1p<<endl<<endl;
cout<<" rvec2 "<<endl<<" "<<rvec2p<<endl<<endl;
cout<<" tvec1 "<<endl<<" "<<tvec1p<<endl<<endl;
cout<<" tvec1 "<<endl<<" "<<tvec2p<<endl<<endl;

return 0;

}

And this is the result:

Iterative: 
rvec1 
[-0.04097605099283788; -0.3679435501353919; 0.07086072250132323]
rvec2 
[0.4135950235376482; 0.6834759799439329; 0.1049879790744613]
tvec1 
[502.4723979671957; -582.21069174737; 3399.430492848247]
tvec2 
[774.9623278021523; -594.8332356366083; 3338.42153723169]
P3P: 
rvec1 
[-0.08738607323881876; -0.363959462471951; 0.06617591006606272]
 rvec2 
[0.4239629869157338; 0.7210136877984544; 0.1133539043199323]
tvec1 
[497.3941378807597; -574.3015171812298; 3354.522829883918]
tvec1 
[760.2641421675842; -582.2718972605966; 3275.390313948845]

Thanks.

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Comments

Have you tried to reproject the image points to see what is happening visually? Maybe there is an angle "wrapping" somewhere. For example, if you have a vehicle that is heading approximately 0 degrees (North), you might have a few readings at 359, 0, 1 degrees.

I also know that the rotation vector (rvec) is expressed as "compact" representation of Rodrigues parameters, which are not easy to visualize. You will want to use cv::Rodrigues function to convert your rvec to easier to visualize Euler angles (roll, pitch, heading).

The iterative methods used for pose estimation require an initial guess. It is possible that the method will converge to an incorrect pose if given a bad initial guess. I would recommend using the EPnP algorithm which is non-iterative.

FndrPlayer39 gravatar imageFndrPlayer39 ( 2013-09-04 17:55:38 -0500 )edit

I am also facing the same problem.@jbseano did you find out the solution for your problem.

ak1 gravatar imageak1 ( 2016-10-12 22:52:45 -0500 )edit