 OpenCV  3.4.19-dev Open Source Computer Vision
Camera Motion Estimation

## Goal

In this tutorial you will learn how to use the reconstruction api for camera motion estimation:

• Load a file with the tracked 2d points and build the container over all the frames.
• Run libmv reconstruction pipeline.
• Show obtained results using Viz.

## Code

#include <opencv2/core.hpp>
#include <opencv2/sfm.hpp>
#include <opencv2/viz.hpp>
#include <iostream>
#include <fstream>
using namespace std;
using namespace cv;
using namespace cv::sfm;
static void help() {
cout
<< "\n------------------------------------------------------------------\n"
<< " This program shows the camera trajectory reconstruction capabilities\n"
<< " in the OpenCV Structure From Motion (SFM) module.\n"
<< " \n"
<< " Usage:\n"
<< " example_sfm_trajectory_reconstruction <path_to_tracks_file> <f> <cx> <cy>\n"
<< " where: is the tracks file absolute path into your system. \n"
<< " \n"
<< " The file must have the following format: \n"
<< " row1 : x1 y1 x2 y2 ... x36 y36 for track 1\n"
<< " row2 : x1 y1 x2 y2 ... x36 y36 for track 2\n"
<< " etc\n"
<< " \n"
<< " i.e. a row gives the 2D measured position of a point as it is tracked\n"
<< " through frames 1 to 36. If there is no match found in a view then x\n"
<< " and y are -1.\n"
<< " \n"
<< " Each row corresponds to a different point.\n"
<< " \n"
<< " f is the focal length in pixels. \n"
<< " cx is the image principal point x coordinates in pixels. \n"
<< " cy is the image principal point y coordinates in pixels. \n"
<< "------------------------------------------------------------------\n\n"
<< endl;
}
/* Build the following structure data
*
* frame1 frame2 frameN
* track1 | (x11,y11) | -> | (x12,y12) | -> | (x1N,y1N) |
* track2 | (x21,y11) | -> | (x22,y22) | -> | (x2N,y2N) |
* trackN | (xN1,yN1) | -> | (xN2,yN2) | -> | (xNN,yNN) |
*
*
* In case a marker (x,y) does not appear in a frame its
* values will be (-1,-1).
*/
static void parser_2D_tracks(const String &_filename, std::vector<Mat> &points2d )
{
ifstream myfile(_filename.c_str());
if (!myfile.is_open())
{
cout << "Unable to read file: " << _filename << endl;
exit(0);
} else {
double x, y;
string line_str;
int n_frames = 0, n_tracks = 0;
// extract data from text file
vector<vector<Vec2d> > tracks;
for ( ; getline(myfile,line_str); ++n_tracks)
{
istringstream line(line_str);
vector<Vec2d> track;
for ( n_frames = 0; line >> x >> y; ++n_frames)
{
if ( x > 0 && y > 0)
track.push_back(Vec2d(x,y));
else
track.push_back(Vec2d(-1));
}
tracks.push_back(track);
}
// embed data in reconstruction api format
for (int i = 0; i < n_frames; ++i)
{
Mat_<double> frame(2, n_tracks);
for (int j = 0; j < n_tracks; ++j)
{
frame(0,j) = tracks[j][i];
frame(1,j) = tracks[j][i];
}
points2d.push_back(Mat(frame));
}
myfile.close();
}
}
/* Keyboard callback to control 3D visualization
*/
bool camera_pov = false;
static void keyboard_callback(const viz::KeyboardEvent &event, void* cookie)
{
if ( event.action == 0 &&!event.symbol.compare("s") )
camera_pov = !camera_pov;
}
/* Sample main code
*/
int main(int argc, char** argv)
{
if ( argc != 5 )
{
help();
exit(0);
}
// Read 2D points from text file
std::vector<Mat> points2d;
parser_2D_tracks( argv, points2d );
// Set the camera calibration matrix
const double f = atof(argv),
cx = atof(argv), cy = atof(argv);
Matx33d K = Matx33d( f, 0, cx,
0, f, cy,
0, 0, 1);
bool is_projective = true;
vector<Mat> Rs_est, ts_est, points3d_estimated;
reconstruct(points2d, Rs_est, ts_est, K, points3d_estimated, is_projective);
// Print output
cout << "\n----------------------------\n" << endl;
cout << "Reconstruction: " << endl;
cout << "============================" << endl;
cout << "Estimated 3D points: " << points3d_estimated.size() << endl;
cout << "Estimated cameras: " << Rs_est.size() << endl;
cout << "Refined intrinsics: " << endl << K << endl << endl;
cout << "3D Visualization: " << endl;
cout << "============================" << endl;
viz::Viz3d window_est("Estimation Coordinate Frame");
window_est.setBackgroundColor(); // black by default
window_est.registerKeyboardCallback(&keyboard_callback);
// Create the pointcloud
cout << "Recovering points ... ";
// recover estimated points3d
vector<Vec3f> point_cloud_est;
for (int i = 0; i < points3d_estimated.size(); ++i)
point_cloud_est.push_back(Vec3f(points3d_estimated[i]));
cout << "[DONE]" << endl;
cout << "Recovering cameras ... ";
vector<Affine3d> path_est;
for (size_t i = 0; i < Rs_est.size(); ++i)
path_est.push_back(Affine3d(Rs_est[i],ts_est[i]));
cout << "[DONE]" << endl;
cout << "Rendering Trajectory ... ";
cout << endl << "Press: " << endl;
cout << " 's' to switch the camera pov" << endl;
cout << " 'q' to close the windows " << endl;
if ( path_est.size() > 0 )
{
// animated trajectory
int idx = 0, forw = -1, n = static_cast<int>(path_est.size());
while(!window_est.wasStopped())
{
for (size_t i = 0; i < point_cloud_est.size(); ++i)
{
Vec3d point = point_cloud_est[i];
Affine3d point_pose(Mat::eye(3,3,CV_64F), point);
char buffer;
sprintf (buffer, "%d", static_cast<int>(i));
viz::WCube cube_widget(Point3f(0.1,0.1,0.0), Point3f(0.0,0.0,-0.1), true, viz::Color::blue());
cube_widget.setRenderingProperty(viz::LINE_WIDTH, 2.0);
window_est.showWidget("Cube"+String(buffer), cube_widget, point_pose);
}
Affine3d cam_pose = path_est[idx];
viz::WCameraPosition cpw(0.25); // Coordinate axes
viz::WCameraPosition cpw_frustum(K, 0.3, viz::Color::yellow()); // Camera frustum
if ( camera_pov )
window_est.setViewerPose(cam_pose);
else
{
// render complete trajectory
window_est.showWidget("cameras_frames_and_lines_est", viz::WTrajectory(path_est, viz::WTrajectory::PATH, 1.0, viz::Color::green()));
window_est.showWidget("CPW", cpw, cam_pose);
window_est.showWidget("CPW_FRUSTUM", cpw_frustum, cam_pose);
}
// update trajectory index (spring effect)
forw *= (idx==n || idx==0) ? -1: 1; idx += forw;
// frame rate 1s
window_est.spinOnce(1, true);
window_est.removeAllWidgets();
}
}
return 0;
}

## Explanation

Firstly, we need to load the file containing the 2d points tracked over all the frames and construct the container to feed the reconstruction api. In this case the tracked 2d points will have the following structure, a vector of 2d points array, where each inner array represents a different frame. Every frame is composed by a list of 2d points which e.g. the first point in frame 1 is the same point in frame 2. If there is no point in a frame the assigned value will be (-1,-1):

/* Build the following structure data
*
* frame1 frame2 frameN
* track1 | (x11,y11) | -> | (x12,y12) | -> | (x1N,y1N) |
* track2 | (x21,y11) | -> | (x22,y22) | -> | (x2N,y2N) |
* trackN | (xN1,yN1) | -> | (xN2,yN2) | -> | (xNN,yNN) |
*
*
* In case a marker (x,y) does not appear in a frame its
* values will be (-1,-1).
*/
...
for (int i = 0; i < n_frames; ++i)
{
Mat_<double> frame(2, n_tracks);
for (int j = 0; j < n_tracks; ++j)
{
frame(0,j) = tracks[j][i];
frame(1,j) = tracks[j][i];
}
points2d.push_back(Mat(frame));
}

Secondly, the built container will be used to feed the reconstruction api. It is important outline that the estimated results must be stored in a vector<Mat>:

bool is_projective = true;
vector<Mat> Rs_est, ts_est, points3d_estimated;
reconstruct(points2d, Rs_est, ts_est, K, points3d_estimated, is_projective);
// Print output
cout << "\n----------------------------\n" << endl;
cout << "Reconstruction: " << endl;
cout << "============================" << endl;
cout << "Estimated 3D points: " << points3d_estimated.size() << endl;
cout << "Estimated cameras: " << Rs_est.size() << endl;
cout << "Refined intrinsics: " << endl << K << endl << endl;

Finally, the obtained results will be shown in Viz, in this case reproducing the camera with an oscillation effect.

## Usage and Results

In order to run this sample we need to specify the path to the tracked points file, the focal length of the camera in addition to the center projection coordinates (in pixels). You can find a sample file in samples/data/desktop_trakcks.txt

./example_sfm_trajectory_reconstruction desktop_tracks.txt 1914 640 360

The following picture shows the obtained camera motion obtained from the tracked 2d points: 