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Manual Reference Pages  - GLUPICKMATRIX (3G)

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gluPickMatrix - define a picking region

CONTENTS

C Specification
Parameters
Description
Example
See Also

void gluPickMatrix( GLdouble x,

                  GLdouble y,
                  GLdouble delX,
                  GLdouble delY,
                  GLint *viewport )

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 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:

.Ex glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluPerspective(...); glMatrixMode(GL_MODELVIEW);

.Ee

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

.Ex glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluPickMatrix(x, y, width, height, viewport); gluPerspective(...); glMatrixMode(GL_MODELVIEW);

.Ee

glGet(3G), glLoadIndentity(3G), glMultMatrix(3G), glRenderMode(3G),  gluPerspective(3G)

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