You probably need to reset the depth-buffer before you do the 2D overlay, or it could well be "behind" the 3D elements.

What I've done in the past is:

Code:

void draw()
{
  // do all 3D stuff
  camera(); // back to default camera settings
  for(int i=0;i<((PGraphics3D)g).zbuffer.length;i++)
  {
    ((PGraphics3D)g).zbuffer[i]=Float.MAX_VALUE;
  }
  //draw 2D stuff
} 

I did a HUD overlay a few weeks ago. What I did was to create a separate PGraphics object that I then was able to draw into. One of the benefits is that this PGraphics instance can use the JAVA2D renderer which supports antialiasing, while the sketch iselfs runs some 3D renderer. At least antialiased HUD should be possible, I just used the the color "color(129)" as a transparency mask when a copied the pixels from the HUD layer to the main graphics layer.

PGraphics gui;
PFont font;
//-----------------
void setup(){
 size(400,400,P3D);
 gui = createGraphics(width, height, JAVA2D );
 font = loadFont("silkscreen.vlw");
 gui.beginDraw();
 gui.textFont( font );
 gui.endDraw();
}
//-----------------
void draw(){

 translate( width/2, height/2 );
 noFill();
 sphere(100);
 // put arbitrary 3D-code here

 gui.beginDraw();
 gui.background( 129 );
 gui.text("this text resides in a separate graphics object",10,15);
 gui.text("with independent drawing routines",10,25);
 gui.rect( 30,300,40,40);
 // put arbitrary 2D-code here
 gui.endDraw();

 gui.loadPixels();
 loadPixels();
 for(int i=0;i<pixels.length;i++)
   if( gui.pixels[i]!=color(129) ) pixels[i] = gui.pixels[i];
 updatePixels();
 gui.updatePixels();

}

I should have played with this a bit more before posting, here's a bit more info. I moved the camera outside of the cube just to make things a bit clearer.

Turns out, you don't need to disable depth testing, you just need to reset the camera by calling camera(). However, in the example I posted, I'm also setting a custom perspective. If you don't reset the perspective by calling perspective() then the point (0, 0) doesn't map to the top left corner because it is being transformed by the perspective. However, if you *do* reset the perspective before drawing 2D it removes the perspective transform from your 3D drawing. This was problematic when I had the camera inside the box because it meant that I could no longer even see the box, presumably because it was being clipped by the near clip plane.

To see the inside of the box *and* the rectangle, I had to reset *only* the camera and call hint(DISABLE_DEPTH_TEST). But this means that the rectangle is being transformed by the perspective and is therefore not drawing how I want it. Adding a perspective reset to that list make the rectangle appear in the upper left untransformed by the box is not visible.

All of that was with OPENGL. Switching to P3D and the outside the box behaves the same, inside exhibits the same behaviour for the rectangle but the box doesn't render properly. However, as has been pointed out, a 2D overlay can be easily accomplished in P3D with a separate drawing surface.

I think I'm going to make a webpage with pictures and code snippets to illustrate what I'm talking about because probably this is all very poorly described.

roundup page:

http://code.compartmental.net/overlay_test/

will also post a bug because there seems to be some inconsistencies.