NAME
	  glOrtho - multiply the current matrix	with an	orthographic
	  matrix


     C SPECIFICATION
	  void glOrtho(	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 nearer and farther
		      depth clipping planes.  These values are
		      negative if the plane is to be behind the
		      viewer.

     DESCRIPTION
	  glOrtho describes a transformation that produces a parallel
	  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:
		   (					   )
		   |__________	    0		0	t  |
		   |right-left				 x |
		   |		__________		   |
		   |	0	top-bottom	0	ty |
		   |					   |
		   |	0	    0	    __________	t  |
		   |			    zFar-zNear	 z |
		   |	0	    0		0	1  |
		   (					   )

	  where
				t  = -__________
				 x    right-left
				t  = -__________
				 y    top-bottom
				t  = -__________
				 z    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,
	  respectively,	assuming that the eye is located at (0,	0, 0).
	  -zFar	specifies the location of the far clipping plane.
	  Both zNear and zFar can be either positive or	negative.

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

     ERRORS
	  GL_INVALID_OPERATION is generated if glOrtho 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
	  glFrustum, glMatrixMode, glMultMatrix, glPushMatrix,
	  glViewport