   automatic frustum culling
  I got a 2D game I'm working on in SDL and OpenGL.  So I might have a map that is multiple screens tall and wide.  If I walk towards the edge of the screen the map scrolls and stuff on the other side gets scrolled off of the screen.One think I noticed is if I spawn like a few thousand monsters everything gets laggy if they are all being drawn on the screen.  But if I run away and scroll them off the screen the lag goes away.  Anybody can tell me why this is?  I mean my code still tries to draw them all I thought it would be laggy no matter what.  I thought frustum culling or whatever it is was something you had to take care of yourself.  OpenGL or something seems to be magically doing it for me.   
  Screen space clipping is applied by OpenGL. So if the bottleneck is the number of drawn on-screen pixels and not the number of triangles, then your application will speed up if everything is out of view.   