| OpenCV 4.3.0 Open Source Computer Vision

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;
static 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;
}
static 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());
Vec3f 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.