1991-2006 Silicon Graphics, Inc.

gluPickMatrix

3G

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.

`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).

## 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 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.

## Example

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 */

## See Also

gluPerspective

,

glGet

,

glLoadIdentity

,

glMultMatrix

,

glRenderMode

## Copyright

Copyright 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.