redhat man page for glupickmatrix

GLUPICKMATRIX(3G)														 GLUPICKMATRIX(3G)

NAME
       gluPickMatrix - define a picking region

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

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

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

       viewport
	    Specifies the current viewport (as from a glGetIntegerv call).

DESCRIPTION
       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 iden-
       tified 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
       matrix.

EXAMPLE
       When rendering a scene as follows:

       glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluPerspective(...); glMatrixMode(GL_MODELVIEW);

       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_MOD-
       ELVIEW);

SEE ALSO
       glGet(3G), glLoadIndentity(3G), glMultMatrix(3G), glRenderMode(3G), gluPerspective(3G)

																 GLUPICKMATRIX(3G)