OpenGL man pages
gluPickMatrix - define a picking region
void gluPickMatrix( GLdouble x,
GLint viewport )
x, y Specify the center of a picking region in window coordinates.
Specify the width and height, respectively, of the picking region in
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. 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
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 */
glGet, glLoadIndentity, glMultMatrix, glRenderMode, gluPerspective
Fri Dec 6 11:18:03 EST 1996
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