The older a man gets, the farther he had to walk
to school as a boy!

Concept 1.13: Animation 
=======================

Idea:
=====
Click on the rendering screen with the left mouse
button to start the animation.  The code screen
has certain function calls highlighted.  Scroll
down to the main() function.

Double Buffering:
=================
OpenGL provides double-buffering.  One frame is 
displayed while the other is drawn.  When the drawing
of a frame is complete, the two buffers are swapped,
so the one that was being viewed is now used for 
drawing, and vice versa.

We initialize double buffering by a call to
glutInitDisplayMode(GLUT_DOUBLE......);  in the main
function.  We swap buffers to create an animation by
calling glutSwapBuffers(); in our display() routine.

To see how the animation would look like without double
buffering, click on "Single Buffer" option and then 
execute the program as before.  See a difference!

Drawing a new frame:
====================
Scroll up to the spinDisplay() routine.  Here we change
the angle of rotation by updating spin.  Once this is
done we call glutPostRedisplay(); which calls the display()
function and hence swaps buffers with the new spin value.
This is why at each step the square appear slightly 
rotated (actually exactly rotated by spin degrees).

Bring in the mice:
==================
This is the easiest part.  To enable swapping of buffers
and starting the animation we use the void mouse() 
function.  This function check to see which button the
user has clicked and posts a call to glutIdleFunc()
In our case the argument for glutIdleFunc() is either
spinDisplay (if the user clicks with the left mouse
button) to start spinning the square or NULL if the
middle or right button were used (i.e do nothing and
hence no animation).

As simple as that!. Now you are ready to tackle some
of the more involved examples I have which use translation
and rotation.