gluPickMatrix

The gluPickMatrix function defines a picking region.

void gluPickMatrix(
  GLdouble x,  GLdouble y,  GLdouble width,  GLdouble height,  GLint viewport[4]);

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 glGetIntegerv call).

Remarks

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

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 glRenderMode), 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 glMultMatrix were called with the generated matrix.

To use the generated pick matrix effectively for picking

1. Call glLoadIdentity to load an identity matrix onto the perspective matrix stack.
2. Call gluPickMatrix.
3. Call a function (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, 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 */ 

Requirements

**  Windows NT/2000:** Requires Windows NT 3.5 or later.
**  Windows 95/98:** Requires Windows 95 or later. Available as a redistributable for Windows 95.
**  Header:** Declared in Glu.h.
**  Library:** Use Glu32.lib.

See Also

glGetIntegerv, glLoadIdentity, glMultMatrix, glRenderMode, gluPerspective