Transformations

Goal

In this tutorial you will learn how to

• How to use makeTransformToGlobal to compute pose
• How to use makeCameraPose and Viz3d::setViewerPose
• How to visualize camera position by axes and by viewing frustum

Code

You can download the code from here.

#include <opencv2/viz.hpp>
#include <iostream>
#include <fstream>

using namespace cv;
using namespace std;

void help()
{
 cout
 << "--------------------------------------------------------------------------" << endl
 << "This program shows how to use makeTransformToGlobal() to compute required pose,"
 << "how to use makeCameraPose and Viz3d::setViewerPose. You can observe the scene "
 << "from camera point of view (C) or global point of view (G)" << endl
 << "Usage:" << endl
 << "./transformations [ G | C ]" << endl
 << endl;
}

Mat cvcloud_load()
{
 Mat cloud(1, 1889, CV_32FC3);
 ifstream ifs("bunny.ply");

 string str;
 for(size_t i = 0; i < 12; ++i)
 getline(ifs, str);

 Point3f* data = cloud.ptr<cv::Point3f>();
 float dummy1, dummy2;
 for(size_t i = 0; i < 1889; ++i)
 ifs >> data[i].x >> data[i].y >> data[i].z >> dummy1 >> dummy2;

 cloud *= 5.0f;
 return cloud;
}

int main(int argn, char **argv)
{
 help();

 if (argn < 2)
 {
 cout << "Missing arguments." << endl;
 return 1;
 }

 bool camera_pov = (argv[1][0] == 'C');

 viz::Viz3d myWindow("Coordinate Frame");

 myWindow.showWidget("Coordinate Widget", viz::WCoordinateSystem());

 Point3f cam_pos(3.0f,3.0f,3.0f), cam_focal_point(3.0f,3.0f,2.0f), cam_y_dir(-1.0f,0.0f,0.0f);

 Affine3f cam_pose = viz::makeCameraPose(cam_pos, cam_focal_point, cam_y_dir);

 Affine3f transform = viz::makeTransformToGlobal(Vec3f(0.0f,-1.0f,0.0f), Vec3f(-1.0f,0.0f,0.0f), Vec3f(0.0f,0.0f,-1.0f), cam_pos);

 Mat bunny_cloud = cvcloud_load();
 viz::WCloud cloud_widget(bunny_cloud, viz::Color::green());

 Affine3f cloud_pose = Affine3f().translate(Vec3f(0.0f,0.0f,3.0f));
 Affine3f cloud_pose_global = transform * cloud_pose;

 if (!camera_pov)
 {
 viz::WCameraPosition cpw(0.5); // Coordinate axes
 viz::WCameraPosition cpw_frustum(Vec2f(0.889484, 0.523599)); // Camera frustum
 myWindow.showWidget("CPW", cpw, cam_pose);
 myWindow.showWidget("CPW_FRUSTUM", cpw_frustum, cam_pose);
 }

 myWindow.showWidget("bunny", cloud_widget, cloud_pose_global);

 if (camera_pov)
 myWindow.setViewerPose(cam_pose);

 myWindow.spin();

 return 0;
}

Explanation

Here is the general structure of the program:

• Create a visualization window.

  viz::Viz3d myWindow("Transformations");

• Get camera pose from camera position, camera focal point and y direction.

  Point3f cam_pos(3.0f,3.0f,3.0f), cam_focal_point(3.0f,3.0f,2.0f), cam_y_dir(-1.0f,0.0f,0.0f);
  
  Affine3f cam_pose = viz::makeCameraPose(cam_pos, cam_focal_point, cam_y_dir);

• Obtain transform matrix knowing the axes of camera coordinate system.

  Affine3f transform = viz::makeTransformToGlobal(Vec3f(0.0f,-1.0f,0.0f), Vec3f(-1.0f,0.0f,0.0f), Vec3f(0.0f,0.0f,-1.0f), cam_pos);

• Create a cloud widget from bunny.ply file

  Mat bunny_cloud = cvcloud_load();
  viz::WCloud cloud_widget(bunny_cloud, viz::Color::green());

• Given the pose in camera coordinate system, estimate the global pose.

  Affine3f cloud_pose = Affine3f().translate(Vec3f(0.0f,0.0f,3.0f));
  Affine3f cloud_pose_global = transform * cloud_pose;

• If the view point is set to be global, visualize camera coordinate frame and viewing frustum.

  if (!camera_pov)
  {
   viz::WCameraPosition cpw(0.5); // Coordinate axes
   viz::WCameraPosition cpw_frustum(Vec2f(0.889484, 0.523599)); // Camera frustum
   myWindow.showWidget("CPW", cpw, cam_pose);
   myWindow.showWidget("CPW_FRUSTUM", cpw_frustum, cam_pose);
  }

• Visualize the cloud widget with the estimated global pose

  myWindow.showWidget("bunny", cloud_widget, cloud_pose_global);

• If the view point is set to be camera's, set viewer pose to cam_pose.

  if (camera_pov)
   myWindow.setViewerPose(cam_pose);

Results

1. Here is the result from the camera point of view.

   

2. Here is the result from global point of view.