[gl4java-usergroup] Problem using picking

[Andy <and...@bt...>]

I'm trying to use picking. I draw three rectangles using drawRects(),
and then use selection mode to determine whether the user has clicked
on a specific rectangle (this is just a very simple example). Under NT
the code works fine, when you click on a square you get a HIT. Under
Win98, it registers a hit, but the next glj.gljCheckGL() throws an
exception (stack underflow or stack overflow).

I've narrowed it down to the glPushMatrix and PopMatrix calls in
drawRects when using GL_SELECT mode, but don't have a clue as to why.
Its got to be something silly I've overlooked (maybe you can't have
two glPushMatrix calls when using perspective mode, or can push a
matrix when in one mode and the another glPushMatrix after switching
modes), but I don't know.

Can anyone tell me what I'm doing wrong. I've include a small bit of
the code (showing the relevant sections) below.

Many thanks
Andy Qua

-----------------

   void drawRects(int mode)
   {
      gl.glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

      gl.glPushMatrix();
      gl.glTranslated( -2.0, 1.0, -5.0 );
       if (mode == GL_SELECT)
           gl.glLoadName (1);
      gl.glColor3f (1.0f, 1.0f, 0.0f);
       gl.glBegin (GL_QUADS);
           gl.glVertex3i (0, 0, 0);
           gl.glVertex3i (0, 0, 1);
           gl.glVertex3i (1, 0, 1);
           gl.glVertex3i (1, 0, 0);
       gl.glEnd();
       if (mode == GL_SELECT)
           gl.glLoadName (2);
           gl.glColor3f (0.0f, 1.0f, 1.0f);
       gl.glBegin (GL_QUADS);
           gl.glVertex3i (2, 0,  0);
           gl.glVertex3i (2, 0,  1);
           gl.glVertex3i (3, 0,  1);
           gl.glVertex3i (3, 0,  0);
       gl.glEnd();
       if (mode == GL_SELECT)
           gl.glLoadName (3);
           gl.glColor3f (1.0f, 0.0f, 1.0f);
       gl.glBegin (GL_QUADS);
           gl.glVertex3i (4, 0,  0);
           gl.glVertex3i (4, 0,  1);
           gl.glVertex3i (5, 0,  1);
           gl.glVertex3i (5, 0,  0);
       gl.glEnd ();
       gl.glPopMatrix();
   }

   void processSelection( int x, int y )
   {
       int BUFSIZE = 512;
       int[] selectBuf = new int[BUFSIZE];
       int hits;
       int[] viewport = new int[4];

       gl.glGetIntegerv (GL_VIEWPORT, viewport);
       gl.glSelectBuffer (BUFSIZE, selectBuf);
       gl.glRenderMode (GL_SELECT);
       gl.glInitNames();
       gl.glPushName(-1);
       gl.glMatrixMode (GL_PROJECTION);
       gl.glPushMatrix ();
         gl.glLoadIdentity ();
           /* create 5x5 pixel picking region near cursor location */
           glu.gluPickMatrix ((double) x, (double) (viewport[3] - y),
5.0, 5.0, viewport);
           glu.gluPerspective(45.0, (float)640/480, 1.0, 100.0);
           drawRects (GL_SELECT);
       gl.glPopMatrix ();
       gl.glFlush ();
       hits = gl.glRenderMode (GL_RENDER);
       gl.glMatrixMode (GL_MODELVIEW);

       if (hits > 0 )
          processHits (hits, selectBuf);
   }


   void processHits (int hits, int buffer[])
   {
      System.out.println( "HIT" );
   }


   public void mousePressed(MouseEvent e)
   {
      System.out.println("Mouse Pressed");
      int xPos = e.getX();
      int yPos = e.getY();

      if(!glj.gljMakeCurrent(true))
         return;

      processSelection(xPos, yPos);

      glj.gljSwap();
      glj.gljCheckGL();
      glj.gljFree();
   }