Qt OpenGL Minimal QGLWidget Implementation, Drawing Triangle

main.cpp

#include <QApplication>
#include <QGLWidget>

class GLWidget : public QGLWidget {
public:
	GLWidget(QWidget *parent = 0);
	~GLWidget();
	QSize sizeHint() const { return QSize(400, 400); }

protected:
	void initializeGL();
	void paintGL();
	void resizeGL(int width, int height);
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    GLWidget glWidget;
    glWidget.show();
    return app.exec();
}

GLWidget::GLWidget(QWidget *parent) : QGLWidget(parent) {}

GLWidget::~GLWidget() { }

void GLWidget::initializeGL() { }

void GLWidget::paintGL()
{
	QSize viewport_size = size();
	glViewport(0, 0, viewport_size.width(), viewport_size.height());

	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	glFrustum(-1, 1, -1, 1, 5, 7); // near and far match your triangle Z distance

	glMatrixMode(GL_MODELVIEW);

	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	glLoadIdentity();
	glTranslatef(-1.5f, 0.0f, -6.0f);

	glBegin(GL_TRIANGLES);
	glColor3f(1, 0, 0);
	glVertex3f(0.0f, 1.0f, 0.0f);
	glColor3f(0.0, 1, 0); 
	glVertex3f(-1.0f, -1.0f, 0.0f);
	glColor3f(0.0, 0.0, 1);
	glVertex3f(1.0f, -1.0f, 0.0f);
	glEnd();
}

void GLWidget::resizeGL(int w, int h)
{
	QGLWidget::resize(w, h);
}