Creating Widgets {#tutorial_creating_widgets}
================

Goal
----

In this tutorial you will learn how to

-   Create your own widgets using WidgetAccessor and VTK.
-   Show your widget in the visualization window.

Code
----

You can download the code from [here ](samples/cpp/tutorial_code/viz/creating_widgets.cpp).
@code{.cpp}
#include <opencv2/viz.hpp>
#include <opencv2/viz/widget_accessor.hpp>
#include <iostream>

#include <vtkPoints.h>
#include <vtkTriangle.h>
#include <vtkCellArray.h>
#include <vtkPolyData.h>
#include <vtkPolyDataMapper.h>
#include <vtkIdList.h>
#include <vtkActor.h>
#include <vtkProp.h>

using namespace cv;
using namespace std;

/*
 * @class WTriangle
 * @brief Defining our own 3D Triangle widget
 */
class WTriangle : public viz::Widget3D
{
    public:
        WTriangle(const Point3f &pt1, const Point3f &pt2, const Point3f &pt3, const viz::Color & color = viz::Color::white());
};

/*
 * @function WTriangle::WTriangle
 */
WTriangle::WTriangle(const Point3f &pt1, const Point3f &pt2, const Point3f &pt3, const viz::Color & color)
{
    // Create a triangle
    vtkSmartPointer<vtkPoints> points = vtkSmartPointer<vtkPoints>::New();
    points->InsertNextPoint(pt1.x, pt1.y, pt1.z);
    points->InsertNextPoint(pt2.x, pt2.y, pt2.z);
    points->InsertNextPoint(pt3.x, pt3.y, pt3.z);

    vtkSmartPointer<vtkTriangle> triangle = vtkSmartPointer<vtkTriangle>::New();
    triangle->GetPointIds()->SetId(0,0);
    triangle->GetPointIds()->SetId(1,1);
    triangle->GetPointIds()->SetId(2,2);

    vtkSmartPointer<vtkCellArray> cells = vtkSmartPointer<vtkCellArray>::New();
    cells->InsertNextCell(triangle);

    // Create a polydata object
    vtkSmartPointer<vtkPolyData> polyData = vtkSmartPointer<vtkPolyData>::New();

    // Add the geometry and topology to the polydata
    polyData->SetPoints(points);
    polyData->SetPolys(cells);

    // Create mapper and actor
    vtkSmartPointer<vtkPolyDataMapper> mapper = vtkSmartPointer<vtkPolyDataMapper>::New();
#if VTK_MAJOR_VERSION <= 5
    mapper->SetInput(polyData);
#else
    mapper->SetInputData(polyData);
#endif

    vtkSmartPointer<vtkActor> actor = vtkSmartPointer<vtkActor>::New();
    actor->SetMapper(mapper);

    // Store this actor in the widget in order that visualizer can access it
    viz::WidgetAccessor::setProp(*this, actor);

    // Set the color of the widget. This has to be called after WidgetAccessor.
    setColor(color);
}

/*
 * @function main
 */
int main()
{
    /// Create a window
    viz::Viz3d myWindow("Creating Widgets");

    /// Create a triangle widget
    WTriangle tw(Point3f(0.0,0.0,0.0), Point3f(1.0,1.0,1.0), Point3f(0.0,1.0,0.0), viz::Color::red());

    /// Show widget in the visualizer window
    myWindow.showWidget("TRIANGLE", tw);

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

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

Here is the general structure of the program:

-   Extend Widget3D class to create a new 3D widget.
@code{.cpp}
class WTriangle : public viz::Widget3D
{
    public:
        WTriangle(const Point3f &pt1, const Point3f &pt2, const Point3f &pt3, const viz::Color & color = viz::Color::white());
};
@endcode
-   Assign a VTK actor to the widget.
@code{.cpp}
// Store this actor in the widget in order that visualizer can access it
viz::WidgetAccessor::setProp(*this, actor);
@endcode
-   Set color of the widget.
@code{.cpp}
// Set the color of the widget. This has to be called after WidgetAccessor.
setColor(color);
@endcode
-   Construct a triangle widget and display it in the window.
@code{.cpp}
/// Create a triangle widget
WTriangle tw(Point3f(0.0,0.0,0.0), Point3f(1.0,1.0,1.0), Point3f(0.0,1.0,0.0), viz::Color::red());

/// Show widget in the visualizer window
myWindow.showWidget("TRIANGLE", tw);
@endcode
Results
-------

Here is the result of the program.

![image](images/red_triangle.png)