I think you should use two buffers. P3D/OPENGL PApplet for the display, and P2D or JAVA2D PGraphics for the GUI. Probably you will have the same problems later, so you´ll have to disable depth test before drawing the GUI (and enable it again when rendering 3d shapes). I have to say, though, enabling+disabling the depth test is quite expensive sometimes.
From hint reference ( http://processing.org/reference/hint_.html):
"hint(DISABLE_DEPTH_TEST) - Disable the zbuffer, allowing you to draw on top of everything at will. When depth testing is disabled, items will be drawn to the screen sequentially, like a painting. This hint is most often used to draw in 3D, then draw in 2D on top of it (for instance, to draw GUI controls in 2D on top of a 3D interface). Starting in release 0149, this will also clear the depth buffer. Restore the default with hint(ENABLE_DEPTH_TEST), but note that with the depth buffer cleared, any 3D drawing that happens later in draw() will ignore existing shapes on the screen."

Ale · 60rpm.tv/i

This does exactly what I want (for now). Thank you very much!

You could try using PGraphics http://processing.org/reference/PGraphics.html

or

Do your GUI in the primary sketch and create a class like this one

class My3dStuff extends PApplet implements PConstants
{
   public void setup()
   {
      // your code
   }
   public void draw()
   {
      // your code
   }
}

and add it to your sketch like this

  // Creating class
  sketch = new My3dStuff(this, mywidth, myheight);                                     
  sketch.init();
  sketch.setBounds(20, 30, sketch.width, sketch.height);
 
  // Adding 3D stuff to the current sketch
  this.setLayout(null);
  this.add(sketch); //this adds the embedded sketch to the main sketch

That might be even better. I will definitely check that out as well, probably later this weekend. Thanks!

Hi Handlewithcare, did you ever get this sorted out, and would you share your code/solution? I have the same problem. I want to draw a 2D CP5 interface to one side of a window displaying my rotatable 3D OpenGL object. My controls get drawn in 3D space and are rotated with the model.