```     NAME
glFrustum - multiply the current matrix by a perspective
matrix

C SPECIFICATION
void glFrustum( GLdouble left,
GLdouble right,
GLdouble bottom,
GLdouble top,
GLdouble zNear,
GLdouble zFar	)

PARAMETERS
left,	right Specify the coordinates for the left and right
vertical clipping	planes.

bottom, top Specify the coordinates for the bottom and top
horizontal clipping planes.

zNear, zFar Specify the distances to the near	and far	depth
clipping planes.	Both distances must be
positive.

DESCRIPTION
glFrustum describes a	perspective matrix that	produces a
perspective projection.  The current matrix (see
glMatrixMode)	is multiplied by this matrix and the result
replaces the current matrix, as if glMultMatrix were called
with the following matrix as its argument:

(					   )
|  __________			   |
|  right-left       0       A       0   |
|					   |
|	  0	  __________   B       0   |
|		  top-bottom		   |
|	  0	       0       C       D   |
|					   |
|	  0	       0       -1      0   |
(					   )

__________
A = right-left
__________
B = top-bottom
__________
C = -zFar-zNear
____________
D = - zFar-zNear
Typically, the matrix	mode is	GL_PROJECTION, and (left,
bottom, -zNear) and (right, top,  -zNear) specify the	points
on the near clipping plane that are mapped to	the lower left
and upper right corners of the window, assuming that the eye
is located at	(0, 0, 0).  -zFar specifies the	location of
the far clipping plane.  Both	zNear and zFar must be
positive.

Use glPushMatrix and glPopMatrix to save and restore the
current matrix stack.

NOTES
Depth	buffer precision is affected by	the values specified
for zNear and	zFar.  The greater the ratio of	zFar to	zNear
is, the less effective the depth buffer will be at
distinguishing between surfaces that are near	each other.
If

r = _____
zNear
roughly log (r) bits of depth	buffer precision are lost.
Because r approaches infinity	as zNear approaches 0, zNear
must never be	set to 0.

ERRORS
GL_INVALID_VALUE is generated	if zNear or zFar is not
positive.

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

ASSOCIATED	GETS
glGet	with argument GL_MATRIX_MODE
glGet	with argument GL_MODELVIEW_MATRIX
glGet	with argument GL_PROJECTION_MATRIX
glGet	with argument GL_TEXTURE_MATRIX

glOrtho, glMatrixMode, glMultMatrix, glPushMatrix,
glViewport

```