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gluPickMatrix

Chapter 14

323

gluPickMatrix

gluPickMatrix: deﬁne a picking region.

C Speciﬁcation

void gluPickMatrix(

GLdouble x,

GLdouble y,

GLdouble delX,

GLdouble delY,

GLint *viewport)

Parameters

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

xdelX, delY Specify the width and height, respectively, of the picking region in

window coordinates.

viewport Speciﬁes the current viewport (as from a glGetIntegerv 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 re-render 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.

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