gluPickMatrix  WA9PWM

[New - Windows 95, OEM Service Release 2]

The gluPickMatrix function defines a picking region.

void gluPickMatrix(

Parameters

x, y

The center of a picking region in window coordinates.

width, height

The width and height, respectively, of the picking region in window coordinates.

viewport

The current viewport (as from a glGetIntegerv8KNKTW call).

Remarks

The gluPickMatrix function creates a projection matrix you can use to restrict drawing to a small region of the viewport.

{bmc bm4.MRB}        To determine which objects are being drawn near the cursor

  1.  Use gluPickMatrix to restrict drawing to a small region around the cursor.

  2.  Enter selection mode (with glRenderMode4MC301H), and then 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 glMultMatrix1G2XCGH were called with the generated matrix.

{bmc bm4.MRB}        To effectively use the generated pick matrix for picking

  1.  Call glLoadIdentity22_90WL to load an identity matrix onto the perspective matrix stack.

  2.  Call gluPickMatrix.

  3.  Call a function (such as gluPerspective_SGTR1) 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, any NURBS surface rendered is subdivided differently with the pick matrix from how 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 */

the following code selects a portion of the viewport:

glMatrixMode(GL_PROJECTION); 

glLoadIdentity( ); 

gluPickMatrix(x, y, width, height, viewport); 

gluPerspective(. . .); 

glMatrixMode(GL_MODELVIEW); 

/* Draw the scene */

See Also