Double Buffering

• glFlush
  • We have been using this at the end of our display function
  • This tells openGL to flush any buffers where it might be storing commands
  • This could be due to conditions like network buffers, or hardware buffers.
  • This tells opengl to completely render whatever we have in the pipeline
  • This is something we do when we want the scene to be "complete"
  • glFlush will return before the drawing is complete.
  • If we want to wait for the scene to be finished before continuing we should call glFinish which does the same thing but is blocking.
• When producing animation, (which is what we have been doing)
• It is usefull to use double buffering
• This is a technique where (at least) two frame buffers are employed
• one for drawing
• one for displaying.
• If the scene is too complex to be drawn in a single refresh cycle, we will see parts of the scene drawn in different frames.
• This can lead to strange, undesired effects
• or flickering
• To avoid this draw in one (frame) buffer
• Display from another (frame) buffer
• The display buffer may be used multiple times for refresh while the scene is being drawn.
• When the scene is complete swap bufferes, (probably just a pointer change in the display call).
• The glut man page says that the swap takes place only after the refresh is done, (or durring the vertical retrace) if possible
• It also says that while gl commands can be issued, they are not executed until the buffer swap is complete.
• Thus a partial scene is never drawn.
• Glut needs two bits of information to do double buffering
  • glutInitDisplayMode(GLUT_DOUBLE);
  • glutSwapBuffers();
• glutSwapBuffers calls a glFlush on all buffers.
• Some code (run it -d to turn double buffering off)
• Notice, double buffering does not speed up the computation, it just makes it look better.
• This is a standard animation technique.
• And a classic trade off, in this case memory for quality.
• Notice, we introduced the idle function glutIdleFunc
  • This calls a void func(void) when there is nothing else to do.
  • This means there are no other events.
  • In reasonable programs, this should not do very much (otherwise the system will "stall" while it completes its work.
  • This is the way to link programs where both want the main loop.
  • He suggests a glutPostRedisplay() instead of calling display();
  • setting the function to Null will disable the use of this function.
• glutPostRedisplay()
  • Creates a redisplay event
  • But if there are multiple redisplay calls only one is issued.
  • This allows events to be properly processed.