GLFRUSTUM(3G)                                                    GLFRUSTUM(3G)



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 glatrixode) is multiplied by
       this  matrix  and  the  result  replaces  the  current  matrix,  as  if
       glultatrix were called with the following matrix as its argument:







          2 zNear
        ------------       0              A              0
        right - left

                        2 zNear
            0         ------------        B              0
                      top - bottom



            0              0              C              D


            0              0              -1             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 GLPROJECTION, 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 win-
       dow, 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 glPushatrix and glPopatrix 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  effec-
       tive  the  depth buffer will be at distinguishing between surfaces that
       are near each other.  If

                                  r = zFar / zNear

       roughly log2(r) bits of depth buffer precision  are  lost.   Because  r
       approaches  infinity  as zNear approaches 0, zNear must never be set to
       0.

ERORS
       GLINVALIDVALUE is generated if zNear or zFar is not positive,  or  if
       left = right, or bottom = top.

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

ASOCIATED GETS
       glGet with argument GLMATRIXMODE
       glGet with argument GLMODELVIEWMATRIX
       glGet with argument GLPROJECTIONMATRIX
       glGet with argument GLTEXTUREMATRIX
       glGet with argument GLCOLORMATRIX

SEE ALSO
       glOrtho, glatrixode, glultatrix, glPushatrix, glViewport



                                                                 GLFRUSTUM(3G)