title: OpenGL "black screen" frustration
body: <p>I am still trying to figure out, why i see only typical "black screen". I render just one rectangle, but nothing happened.</p>
  
  <pre><code>#include "expwidget.h"
  #include &lt;iostream&gt;
  
  ExpWidget::ExpWidget(QObject *parent) :
      QGLWidget(QGLFormat(QGL::DoubleBuffer), (QWidget *) parent)
  {
  
      QGLFormat fmt = this-&gt;format();
      fmt.setDepth(true);
  
      this-&gt;setFormat(fmt);
  }
  
  
  
  void ExpWidget::initializeGL() {
  
      QGLWidget::initializeGL(); 
  
      std::cout &lt;&lt; "inicializace...\n";
  
      glClearColor(0.0f,0.0f,0.0f,0.0f);
  
      glEnable(GL_DEPTH_TEST);
      glEnable(GL_CULL_FACE);
      glDisable(GL_LIGHTING);
  
  }
  
  
  void ExpWidget::paintGL() {
  
      glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  
      glLoadIdentity();
  
      glColor3f(1, 0, .5);
      glBegin(GL_QUADS);
          glVertex3f(-1.0f,-1.0f,-5.0f);
          glVertex3f(1.0f,-1.0f,-5.0f);
          glVertex3f(1.0f,1.0f,-5.0f);
          glVertex3f(-1.0f,1.0f,-5.0f);
      glEnd();
  
      glFlush();
  
  
  }
  
  void ExpWidget::resizeGL(int w, int h) {
  
      glViewport(0, 0, w, h);
  
      glMatrixMode(GL_PROJECTION);
      glLoadIdentity();
  
      glFrustum(-2.0, 2.0, -2.0, 2.0, 0.0, -30.0);
  
      glMatrixMode(GL_MODELVIEW);
  
  
  }
  </code></pre>