not able to draw line using QGLWiget - qt

Question

New to qt opengl. Am trying to draw a line in my opengl window it is not displaying.
Code is executing without error. Please help me. what is wrong in the below mentioned code.
//glwidget.cpp
#include "glwidget.h"
#include <QDebug>
#include <QGLWidget>
#include <QPainter>
GLWidget::GLWidget(QWidget *parent) :
QGLWidget(parent)
{
}
void GLWidget::initializeGL()/*initialize*/
{
qDebug()<<"iniialize";
glClearColor(0.0, 0.0, 102.0/255.0, 0.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);
}
void GLWidget::resizeGL(int width, int height)
{
qDebug()<<"resizeGL";
glViewport(0, 0, (GLint)width, (GLint)height);
}
void GLWidget::paintGL()
{
qDebug()<<"paintGL";
glBegin(GL_LINES);
glVertex3f( 0.0f, 0.0f, 0.0f);
glVertex3f(-1.0f,-1.0f, 0.0f);
glEnd();
}
#ifndef GLWIDGET_H
#define GLWIDGET_H
#include <QWidget>
#include <QGLWidget>
#include <QPainter>
class GLWidget : public QGLWidget
{
Q_OBJECT
public:
explicit GLWidget(QWidget *parent = 0);
void initializeGL();
void resizeGL(int width, int height);
void paintGL();
};
main.cpp
#include "mainwindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;/*Mainwindow*/
w.show();
return a.exec();
}

Answer 1

Well, you should think about what you told OpengGL to do here:
With your
glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);
call, you set up your projection matrix to map the world in the area 0 to 1 (in X- and Y-direction) to your screen.
Then you draw a line from 0 to -1 (in X- and Y-direction). That line lies outside your screen now.
Try some better coordinates in your GL_LINES call.
Plus, you might want to start your paintGL() routine with clearing the screen:
glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);