I have two questions as follows.
    1.  How to use the double buffer in the OpenGL ES?  We know that
in the windows,we can use the glutInitDisplayMode(GLUT_DOUBLE|
GLUT_RGB); but in the OpenGL ES, I just find the chooseConfig and the
eglSwapBuffers is used to swap the back buffer and the draw buffer.
Dose the OpenGl ES use  the double buffer in the default?
   2  When I draw a line rotate with 72degree/second, and the refresh
cycle is 1/60 second, I find there are 2 lines rotate.  How to resolve
it?


   thanks .