gluPickMatrix	- define a picking region

	  void gluPickMatrix( GLdouble x,
			      GLdouble y,
			      GLdouble delX,
			      GLdouble delY,
			      GLint *viewport )

	  x, y Specify the center of a picking region in window

	  delX,	delY
	       Specify the width and height, respectively, of the
	       picking region in window	coordinates.

	       Specifies the current viewport (as from a glGetIntegerv

	  gluPickMatrix	creates	a projection matrix that can be	used
	  to restrict drawing to a small region	of the viewport.  This
	  is typically useful to determine what	objects	are being
	  drawn	near the cursor.  Use gluPickMatrix to restrict
	  drawing to a small region around the cursor.	Then, enter
	  selection mode (with glRenderMode) and rerender the scene.
	  All primitives that would have been drawn near the cursor
	  are identified and stored in the selection buffer.

	  The matrix created by	gluPickMatrix is multiplied by the
	  current matrix just as if glMultMatrix is called with	the
	  generated matrix.  To	effectively use	the generated pick
	  matrix for picking, first call glLoadIdentity	to load	an
	  identity matrix onto the perspective matrix stack.  Then
	  call gluPickMatrix, and finally, call	a command (such	as
	  gluPerspective) to multiply the perspective matrix by	the
	  pick matrix.

	  When using gluPickMatrix to pick NURBS, be careful to	turn
	  off the NURBS	property GLU_AUTO_LOAD_MATRIX.	If
	  GLU_AUTO_LOAD_MATRIX is not turned off, then any NURBS
	  surface rendered is subdivided differently with the pick
	  matrix than the way it was subdivided	without	the pick

	  When rendering a scene as follows:
	  glMatrixMode(GL_PROJECTION); glLoadIdentity();
	  gluPerspective(...); glMatrixMode(GL_MODELVIEW); /* Draw the
	  scene	*/

	  a portion of the viewport can	be selected as a pick region
	  like this:

	  glMatrixMode(GL_PROJECTION); glLoadIdentity();
	  gluPickMatrix(x, y, width, height, viewport);
	  gluPerspective(...); glMatrixMode(GL_MODELVIEW); /* Draw the
	  scene	*/

	  glGet, glLoadIndentity, glMultMatrix,	glRenderMode,