glViewport - set the viewport

glViewport
	glViewport(x, y, width, height) -> None

void glViewport( GLint x,
                 GLint y,
                 GLsizei width,
                 GLsizei height )

x, y Specify the lower left corner of	the viewport
     rectangle, in pixels. The initial value is (0,0).

width, height
     Specify the width and height of the viewport.  When a
     GL context is first attached to a window, width and
     height are set to the dimensions	of that	window.

glViewport specifies the affine transformation of x and y
from normalized device coordinates to	window coordinates.
Let (x  , y  ) be normalized device coordinates.  Then the
windowncoordinates (x	, y ) are computed as follows:
                     w   w
                  x  = (x  +1)(_____)	+ x
                   w	   nd	(  2  )
                  y  = (y  +1)(______) + y
                   w	   nd	(  2   )
Viewport width and height are	silently clamped to a range
that depends on the implementation.  To query	this range,
call glGet with argument GL_MAX_VIEWPORT_DIMS.

GL_INVALID_VALUE is generated	if either width	or height is
negative.

GL_INVALID_OPERATION is generated if glViewport is executed
between the execution	of glBegin and the corresponding
execution of glEnd.

glGet	with argument GL_VIEWPORT
glGet	with argument GL_MAX_VIEWPORT_DIMS

glDepthRange