gluPickMatrix -- define a picking region.
void gluPickMatrix (GLdouble x,
GLdouble y,
GLdouble delX,
GLdouble delY,
GLint *viewport);
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 identi- fied 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 effec- tively use the generated pick matrix for picking, first call glLoadI- dentity 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.
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, gluPerspective
GLUPICKMATRIX(3G)