Projection Transformations

The viewing volume determines how an object is projected onto the screen
(by using a perspective or an orthographic projection),
and it defines which objects or portions of objects are clipped out of the final image.

Viewing Volume Clipping

After the vertices of the objects in the scene have been transformed by the modelview and projection matrices, any primitives that lie outside the viewing volume are clipped. The six clipping planes used are those that define the sides and ends of the viewing volume. You can specify additional clipping planes and locate them wherever you choose.

Projection Matrix If commands should affect the projection matrix rather than the modelview matrix issue:

  glMatrixMode( GL_PROJECTION ); 
  glLoadIdentity();