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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  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
        ------------       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 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 cor-ners corners
       ners 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  log2(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, or if left = right, or bottom =  top.

       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
       glGet with argument GL_COLOR_MATRIX

SEE ALSO
       glOrtho(3G), glMatrixMode(3G), glMultMatrix(3G), glPushMatrix(3G), glViewport(3G)




                                                                                               GLFRUSTUM(3G)