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fglFrustum(3G)		       OpenGL Reference			fglFrustum(3G)

NAME
     fglFrustum - multiply the current matrix by a perspective matrix

FORTRAN SPECIFICATION
     SUBROUTINE fglFrustum( REAL*4 left,
			    REAL*4 right,
			    REAL*4 bottom,
			    REAL*4 top,
			    REAL*4 near,
			    REAL*4 far )

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.

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

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

		  |					       |

		  |					       |

		  |					       |

		  |   ____________			       |
					  0	   A	   0
		  |   right - left			       |

		  |					       |
				    ____________
		  |	   0			   B	   0   |
				    top - bottom
		  |					       |

		  |	   0		  0	   C	   D   |

		  |					       |
			   0		  0	   -1	    0

				     ____________
				 A =
				     right - left

				     ____________
				 B =
				     top - bottom

				       __________
				  C = -
				       far - near

				       __________
				  D = -
				       far - near

     Typically, the matrix mode is GL_PROJECTION, and (left, bottom, -near)
     and (right, top,  -near) specify the points on the near clipping plane
     that are mapped to the lower left and upper right corners of the window,

									Page 1

fglFrustum(3G)		       OpenGL Reference			fglFrustum(3G)

     assuming that the eye is located at (0, 0, 0).  -far specifies the
     location of the far clipping plane.  Both near and far must be positive.

     Use fglPushMatrix and fglPopMatrix to save and restore the current matrix
     stack.

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

					 ____
				     r =
					 near

     roughly log (r) bits of depth buffer precision are lost.  Because r
		2
     approaches infinity as near approaches 0, near must never be set to 0.

ERRORS
     GL_INVALID_VALUE is generated if near or far is not positive, or if left
     = right, or bottom = top.

     GL_INVALID_OPERATION is generated if fglFrustum is executed between the
     execution of fglBegin and the corresponding execution of fglEnd.

ASSOCIATED GETS
     fglGet with argument GL_MATRIX_MODE
     fglGet with argument GL_MODELVIEW_MATRIX
     fglGet with argument GL_PROJECTION_MATRIX
     fglGet with argument GL_TEXTURE_MATRIX
     fglGet with argument GL_COLOR_MATRIX

SEE ALSO
     fglOrtho, fglMatrixMode, fglMultMatrix, fglPushMatrix, fglViewport

									Page 2

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