Confused about Frustum Culling

Question

I'm new to OpenGL. I'm using it with JOGL.

I'm reading about frustum culling:

http://www.lighthouse3d.com/opengl/viewfrustum/

http://www.crownandcutlass.com/features/technicaldetails/frustum.html

I'm not sure exactly what it's supposed to do. Doesn't OpenGL cull off-screen objects automatically? (Is this culling significantly slower than just not sending the objects in the first place?)

From what I'm reading, it doesn't look like this will handle avoiding drawing objects that are obscured behind another but are within the viewing frustum. Does that mean that the only benefit is avoid sending off-screen objects to OpenGL?

Answer 1

Yes, you are basically right. Frustum culling cuts off objects that are outside the camera pyramid. OpenGL, when rendering a scene, of course does this as well but on a per vertex basis. Frustum culling works per object and its performance boost potential is therefore much higher.
One of the bottlenecks is transferring the data between CPU and GPU. If you need to transfer for example only 1/4 of all the objects/vertices in an outdoor scene, frustum culling can yield a nice performance boost.