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GLFRUSTUM()                                                        GLFRUSTUM()

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:

       (((((2 zNear)/(right - left)) 0 0 0) (0 ((2 zNear)/(top - bottom)) 0 0)
       (A B C(-1)) (0 0 D 0)))

       (A = (right + left)/(right - left))

       (B = (top + bottom)/(top - bottom))

       (C = -((zFar + zNear)/(zFar - zNear)))

       (D = -((2 zFar zNear)/(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 = zFar/zNear

       roughly log_2 (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

SEE ALSO
       glOrtho, glMatrixMode, glMultMatrix, glPushMatrix, glViewport

                                                                   GLFRUSTUM()