gluPickMatrix — define a picking region
Specify the center of a picking region in window coordinates.
Specify the width and height, respectively, of the picking region in window coordinates.
Specifies 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.
restrict drawing to a small region around the cursor.
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.
and, finally, call a command (such as gluPerspective)
to multiply the perspective matrix by the pick matrix.
gluPickMatrix to pick NURBS, be careful to turn off the NURBS
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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