GLUPICKMATRIX() MachTen Programmer’s Manual GLUPICKMATRIX()

NAME
gluPickMatrix - define a picking region

C SPECIFICATION
void gluPickMatrix( GLdouble x,
GLdouble y,
GLdouble delX,
GLdouble delY,
GLint *viewport )

delim $$

PARAMETERS
x, y Specify the center of a picking region in window
coordinates.

delX, delY
Specify the width and height, respectively, of the
picking region in window coordinates.

viewport
Specifies the current viewport (as from a glGetInte-
gerv call).

DESCRIPTION
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 reren-
der 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.

EXAMPLE
When rendering a scene as follows:

glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluPerspec-
tive(...); glMatrixMode(GL_MODELVIEW); /* Draw the scene
*/

a portion of the viewport can be selected as a pick region
like this:

glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluPickMa-
trix(x, y, width, height, viewport); gluPerspective(...);
glMatrixMode(GL_MODELVIEW); /* Draw the scene */

SEE ALSO
glGet, glLoadIndentity, glMultMatrix, glRenderMode, glu-
Perspective

MachTen 2