No output for OpenGL

Question

I am a newbie in QT and working with its openGL in the Ubuntu Platform

I have created a template as in :

My objective :

On clicking of the "Compute" button, two things are supposed to happen :

1. A text will be printed in the text box
2. A line will be drawn on expOutput, which is an object of Experimental, inherited from QGLWidget.

I am creating two signal slots. The codes of the program are given below :

nearestneighbour.h

#ifndef NEARESTNEIGHBOUR_H
#define NEARESTNEIGHBOUR_H

#include <QWidget>
#include <QGLWidget>
#include "experimental.h"


namespace Ui {
class NearestNeighbour;
}

class NearestNeighbour : public QWidget
{
    Q_OBJECT

public:
    explicit NearestNeighbour(QWidget *parent = 0);
    ~NearestNeighbour();


signals:
    void clicked();

private slots:
    void on_Compute_clicked();

private:
    Ui::NearestNeighbour *ui;

};

#endif // NEARESTNEIGHBOUR_H

nearestneighbour.cpp

#include "nearestneighbour.h"
#include "ui_nearestneighbour.h"

NearestNeighbour::NearestNeighbour(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::NearestNeighbour)
{
    //expt = new Experimental(this);
    ui->setupUi(this);
    QObject::connect(ui->Compute, &QPushButton::clicked, ui->expOutput,&Experimental::draw);
    QObject::connect(ui->Compute, &QPushButton::clicked, this,&NearestNeighbour::on_Compute_clicked);
}

NearestNeighbour::~NearestNeighbour()
{
    delete ui;
}


void NearestNeighbour::on_Compute_clicked()
{
    ui->textEdit->setText("Hello World !!");
}

experimental.h

#ifndef EXPERIMENTAL_H
#define EXPERIMENTAL_H

#include <QWidget>
#include <QGLWidget>
#include <iostream>
#include "nearestneighbour.h"
// #include "ui_nearestneighbour.h"

class Experimental : public QGLWidget
{
    Q_OBJECT
public:
    explicit Experimental(QWidget *parent=NULL);


public slots:
    void draw(void);

protected:
    void initializeGL();
    void paintGL();
};

#endif // EXPERIMENTAL_H

experimental.cpp

    #include "experimental.h"

Experimental::Experimental(QWidget *parent): QGLWidget(QGLFormat(QGL::SampleBuffers), parent){}

void Experimental::initializeGL(){
    std::cout<<"In initializegl  !! ";
    qglClearColor(Qt::white);
    glViewport(0,0,600,300);
    glMatrixMode(GL_MODELVIEW);
    glOrtho(0,300,0,600,0,0);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
}

void Experimental::paintGL(){
    std::cout<<"In paintgl  !! ";
}

void Experimental::draw(void){
    std::cout<<"In draw !!";
    glLoadIdentity();
    qglColor(Qt::blue);
    glBegin(GL_LINE);
        glVertex2i(100,100);
        glVertex2i(200,200);
    glEnd();
    updateGL();
}

Problem :

Thanks to some great suggestions, I was able to remove the errors and the system compiled fine.

The output of the first screen is coming as

I am clicking on the "Compute" button, and the line should be drawn on the openGl widget. However, there are no outputs but after exiting the GUI mode, one can note that there are some relevant messages, which indicate that the functions have been visited.

Can somebody help me in solving the problem?

Answer 1

If you need to draw outside of paintGL() handler, you have to make context current by a call to makeCurrent() and Qt really doesn't take kindly to attempts to draw outside of paint event plus you would have an issue with order of output (a "painter" problem)

On Qt5+ you should not to use in new programs the old QGLWidget, which relies on fixed pipeline and left for compatibility with legacy code, QOpenGLWidget is your friend. There is a following example in Assistant:

  class MyGLWidget : public QOpenGLWidget
  {
  public:
      MyGLWidget(QWidget *parent) : QOpenGLWidget(parent) { }

  protected:
      void initializeGL()
      {
          // Set up the rendering context, load shaders and other resources, etc.:
          QOpenGLFunctions *f = QOpenGLContext::currentContext()->functions();
          f->glClearColor(1.0f, 1.0f, 1.0f, 1.0f);
          ...
      }

      void resizeGL(int w, int h)
      {
          // Update projection matrix and other size related settings:
          m_projection.setToIdentity();
          m_projection.perspective(45.0f, w / float(h), 0.01f, 100.0f);
          ...
      }

      void paintGL()
      {
          // Draw the scene:
          QOpenGLFunctions *f = QOpenGLContext::currentContext()->functions();
          f->glClear(GL_COLOR_BUFFER_BIT);
          ...
      }

  };

QGlWidget you're using got following virtual methods:

virtual void    glDraw ()
virtual void    glInit ()
virtual void    initializeGL ()
virtual void    initializeOverlayGL ()
virtual void    paintGL ()
virtual void    paintOverlayGL ()
virtual void    resizeGL ( int width, int height )
virtual void    resizeOverlayGL ( int width, int height )

You almost never need to override the first two and usually have to override others. glDraw calls glPaintGL after calling makeCurrent