OpenCV  4.10.0
Open Source Computer Vision
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Transformations

Prev Tutorial: Pose of a widget
Next Tutorial: Creating Widgets

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;
}
Affine3 translate(const Vec3 &t) const
a.translate(t) is equivalent to Affine(E, t) * a, where E is an identity matrix
n-dimensional dense array class
Definition mat.hpp:812
Template class for 3D points specified by its coordinates x, y and z.
Definition types.hpp:255
The Viz3d class represents a 3D visualizer window. This class is implicitly shared.
Definition viz3d.hpp:68
This 3D Widget represents camera position in a scene by its axes or viewing frustum....
Definition widgets.hpp:544
Clouds.
Definition widgets.hpp:681
Compound widgets.
Definition widgets.hpp:514
#define CV_32FC3
Definition interface.h:120
int main(int argc, char *argv[])
Definition highgui_qt.cpp:3
"black box" representation of the file storage associated with a file on disk.
Definition core.hpp:102
STL namespace.

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