How do you plot points in QT?

Question

I am writing an application in C++ with QT where you have n points and compute the convex hull of this. However, once this is computed I have no idea how to plot the points and

Answer 1

Graphics View, addEllipse

QGraphicsView does 2D plotting very well and gives you many options for how to display it. It isn't as tailored for plotting scientific data as much as qwt, but just for showing a bunch of points, or geometry or animations and lots of other things it works very well. See Qt's Graphics View Framework documentation and examples.

Here is how you plot a bunch of points in a QGraphicsScene and show it in a QGraphicsView.

#include 

#include 
#include 
#include 
#include 

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QVector  points;

    // Fill in points with n number of points
    for(int i = 0; i< 100; i++)
       points.append(QPointF(i*5, i*5));

    // Create a view, put a scene in it and add tiny circles
    // in the scene
    QGraphicsView * view = new QGraphicsView();
    QGraphicsScene * scene = new QGraphicsScene();
    view->setScene(scene);

    for(int i = 0; i< points.size(); i++)
        scene->addEllipse(points[i].x(), points[i].y(), 1, 1);

    // Show the view
    view->show();

    // or add the view to the layout inside another widget

    return a.exec();
}

Note: You will probably want to call setSceneRect on your view, otherwise the scene will just auto-center it. Read the descriptions for QGraphicsScene and QGraphicsView in the Qt Documentation. You can scale the view to show more or less of the scene and it can put scroll bars in. I answered a related question where I show more about what you can do with a QGraphicsView that you may want to look at also.