I was trying to understand the gluOrtho2D function. I have drawn 4 lines originating from the center reaching up to 4 corners of the screen.

You can follow the below code. osize is a variable which is used to set the parameters of gluOrtho2D. It will create a window of size 2*osize.

If works fine when osize is 1. Lines reach the corners. But as I increase the value of osize, the length of the lines decreases (cross becomes smaller and does not cover the whole screen). But I think it should reach the corner.

void display()
    glClear( GL_COLOR_BUFFER_BIT );

    //glViewport(0, 0, 100, 100);

    glMatrixMode (GL_PROJECTION);
    float osize = 1.2;
    //glOrtho(-osize*1.0, osize*1.0, osize*1.0, -osize*1.0, -1.0, 1.0);
    gluOrtho2D(-osize*1.0, osize*1.0, osize*1.0, -osize*1.0);
    glMatrixMode (GL_MODELVIEW);

       glColor3f(0.0, 0.0, 1.0);
       glVertex2f(0.0, 0.0);
       glVertex2f(-osize*1.0, -osize*1.0);
       glVertex2f(0.0, 0.0);
       glVertex2f(-osize*1.0, osize*1.0);       
       glVertex2f(0.0, 0.0);
       glVertex2f(osize*1.0, -osize*1.0);
       glVertex2f(0.0, 0.0);
       glVertex2f(osize*1.0, osize*1.0);

    glutSwapBuffers();  //includes glFlush();

What is the problem?

  • 4
    \$\begingroup\$ Are you resetting the projection matrix to the identity anywhere? If not, all the gluOrtho2D() calls will stack on top of each other... \$\endgroup\$ Commented Aug 20, 2013 at 20:52
  • \$\begingroup\$ Ok got it. After setting the Projection matrix to Identity, it works as expected. Thank you. \$\endgroup\$
    – Shashwat
    Commented Aug 21, 2013 at 18:56

1 Answer 1


Maybe you inverted bottom with top? Ortho matrix will scale Y by 2 / (top - bottom), in your case a negative number.

  • \$\begingroup\$ In fact it only alters the order which lines will be drawn, as we normally think of Y increasing up. \$\endgroup\$
    – felipe
    Commented Aug 21, 2013 at 19:04
  • \$\begingroup\$ The difference between the both is just that it will invert the y-axis. \$\endgroup\$
    – Shashwat
    Commented Aug 21, 2013 at 19:17

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