void gluPickMatrix( GLdoublex, GLdoubley, GLdoubledelX, GLdoubledelY, GLint*viewport)

,*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).

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