define a picking region
void gluPickMatrix( | GLdouble | x, |
| GLdouble | y, | |
| GLdouble | delX, | |
| GLdouble | delY, | |
| GLint * | viewport); |
x, ySpecify the center of a picking region in window coordinates.
delX, delYSpecify the width and height, respectively, of the picking region in window coordinates.
viewportSpecifies the current viewport (as from a glGetIntegerv call).
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 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 */
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