Get most lowend framebuffer by OpenGL ES

[cotton5415]

Hi,
I'm new to use OpenGL ES and would like to know if it's possible to get the current screen images by OpenGL ES. I've ever get the current screen by using OpenGL on Max OS. But it seams no possible to porting the same method onto iPhone OS.
I read documents about OpenGL ES and OpenGL, there's no getBuffer() API in OpenGL ES, does it mean OpenGL ES can only RENDER to some buffers but can't GET data from certain/main screen buffer?

If it is true, Can I get current screen image by apply any Core Animation api?

Thank you all.