Transformations {#tutorial_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 ](samples/cpp/tutorial_code/viz/transformations.cpp).
@code{.cpp}
#include <opencv2/viz.hpp>
#include <iostream>
#include <fstream>

using namespace cv;
using namespace std;

/*
 * @function cvcloud_load
 * @brief load bunny.ply
 */
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;
}

/*
 * @function main
 */
int main(int argn, char **argv)
{
    if (argn < 2)
    {
        cout << "Usage: " << endl << "./transformations [ G | C ]" << endl;
        return 1;
    }

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

    /// Create a window
    viz::Viz3d myWindow("Coordinate Frame");

    /// Add coordinate axes
    myWindow.showWidget("Coordinate Widget", viz::WCoordinateSystem());

    /// Let's assume camera has the following properties
    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);

    /// We can get the pose of the cam using makeCameraPose
    Affine3f cam_pose = viz::makeCameraPose(cam_pos, cam_focal_point, cam_y_dir);

    /// We can get the transformation matrix from camera coordinate system to global using
    /// - makeTransformToGlobal. We need the axes of the camera
    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.
    Mat bunny_cloud = cvcloud_load();
    viz::WCloud cloud_widget(bunny_cloud, viz::Color::green());

    /// Pose of the widget in camera frame
    Affine3f cloud_pose = Affine3f().translate(Vec3f(0.0f,0.0f,3.0f));
    /// Pose of the widget in global frame
    Affine3f cloud_pose_global = transform * cloud_pose;

    /// Visualize camera frame
    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 widget
    myWindow.showWidget("bunny", cloud_widget, cloud_pose_global);

    /// Set the viewer pose to that of camera
    if (camera_pov)
        myWindow.setViewerPose(cam_pose);

    /// Start event loop.
    myWindow.spin();

    return 0;
}
@endcode
Explanation
-----------

Here is the general structure of the program:

-   Create a visualization window.
@code{.cpp}
/// Create a window
viz::Viz3d myWindow("Transformations");
@endcode
-   Get camera pose from camera position, camera focal point and y direction.
@code{.cpp}
/// Let's assume camera has the following properties
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);

/// We can get the pose of the cam using makeCameraPose
Affine3f cam_pose = viz::makeCameraPose(cam_pos, cam_focal_point, cam_y_dir);
@endcode
-   Obtain transform matrix knowing the axes of camera coordinate system.
@code{.cpp}
/// We can get the transformation matrix from camera coordinate system to global using
/// - makeTransformToGlobal. We need the axes of the camera
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);
@endcode
-   Create a cloud widget from bunny.ply file
@code{.cpp}
/// Create a cloud widget.
Mat bunny_cloud = cvcloud_load();
viz::WCloud cloud_widget(bunny_cloud, viz::Color::green());
@endcode
-   Given the pose in camera coordinate system, estimate the global pose.
@code{.cpp}
/// Pose of the widget in camera frame
Affine3f cloud_pose = Affine3f().translate(Vec3f(0.0f,0.0f,3.0f));
/// Pose of the widget in global frame
Affine3f cloud_pose_global = transform * cloud_pose;
@endcode
-   If the view point is set to be global, visualize camera coordinate frame and viewing frustum.
@code{.cpp}
/// Visualize camera frame
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);
}
@endcode
-   Visualize the cloud widget with the estimated global pose
@code{.cpp}
/// Visualize widget
myWindow.showWidget("bunny", cloud_widget, cloud_pose_global);
@endcode
-   If the view point is set to be camera's, set viewer pose to **cam_pose**.
@code{.cpp}
/// Set the viewer pose to that of camera
if (camera_pov)
    myWindow.setViewerPose(cam_pose);
@endcode
Results
-------

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

    ![image](images/camera_view_point.png)

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

    ![image](images/global_view_point.png)