title: How to draw line in OpenGL?
body: <p>I want to draw a line in OpenGL.</p>
  <pre><code> glBegin(GL_LINES);
      glVertex2f(.25,0.25);
      glVertex2f(.75,.75);
  glEnd();
  </code></pre>
  <p>This code draws the line but if I want to draw a line from coordinate(10,10) to coordinate(20,20) what should I do?</p>
  <p>What does it mean (.25,.25) and (.75, .75)?</p>