NAME

gluPickMatrix - define a picking region

C SPECIFICATION

void gglluuPPiicckkMMaattrriixx( GLdouble _x,
                    GLdouble _y,
                    GLdouble _d_e_l_X,
                    GLdouble _d_e_l_Y,
                    GLint _*_v_i_e_w_p_o_r_t )

delim $$

PARAMETERS

_x, _y
Specify the center of a picking region in window coordinates.
_d_e_l_X, _d_e_l_Y
Specify the width and height, respectively, of the picking region in window coordinates.
_v_i_e_w_p_o_r_t
Specifies the current viewport (as from a ggllGGeettIInntteeggeerrvv call).

DESCRIPTION

gglluuPPiicckkMMaattrriixx 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 gglluuPPiicckkMMaattrriixx to restrict drawing to a small region around the cursor. Then, enter selection mode (with ggllRReennddeerrMMooddee) 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 gglluuPPiicckkMMaattrriixx is multiplied by the current matrix just as if ggllMMuullttMMaattrriixx is called with the generated matrix. To effectively use the generated pick matrix for picking, first call ggllLLooaaddIIddeennttiittyy to load an identity matrix onto the perspective matrix stack. Then call gglluuPPiicckkMMaattrriixx, and finally, call a command (such as gglluuPPeerrssppeeccttiivvee) to multiply the perspective matrix by the pick matrix.

When using gglluuPPiicckkMMaattrriixx to pick NURBS, be careful to turn off the NURBS property GGLLUU__AAUUTTOO__LLOOAADD__MMAATTRRIIXX. If GGLLUU__AAUUTTOO__LLOOAADD__MMAATTRRIIXX 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_MODELVIEW);

SEE ALSO

ggllGGeett((33GG)), ggllLLooaaddIInnddeennttiittyy((33GG)), ggllMMuullttMMaattrriixx((33GG)), ggllRReennddeerrMMooddee((33GG)), gglluuPPeerrssppeeccttiivvee((33GG))