Is JAVA2D the only rendering mode to display transparent PGraphics objects in Processing?

Is JAVA2D the only rendering mode in Processing in which PGraphics objects can be displayed with a transparent background (as opposed to P2D and P3D)? I have tried rewriting my code in the other two modes (P3D, P2D) but the backgrounds of the PGraphics turn out opaque (black by default)---perhaps I'm missing a method that would turn them transparent? The pngs I'm displaying in the PGraphics objects are turning out transparent as expected; it's the backgrounds of the PGraphics that are opaque. I am working on a project and have currently selected JAVA2D in order to keep my PGraphics transparent, but wonder if I can alternately use P3D and its features instead.

From looking around online believe JAVA2D was the only mode that could render PGraphics transparent in earlier versions of Processing (1.*), and I'm wondering if this has changed with version 2?

Thanks in advance for your clarification and help!

UPDATE:

Below is a sample of my code (written in Eclipse in Java with Processing as a component). v.k.'s reply works great (with my pngs) in the Processing pde, but I've been unsuccessful getting it to work in my project in Eclipse. Thoughts? :

Main class:

package tester;

import java.util.ArrayList;
import processing.core.*;

public class GraphicsMain extends PApplet{
int screenWidth = 800;
int screenHeight = 500;
PGraphicsMaker pgm = new PGraphicsMaker(this);
ArrayList<PGraphics> motifArray = new ArrayList<PGraphics>();


public void setup() {
    size(screenWidth,screenHeight,P3D);
    background(0);

    motifArray = pgm.makeMotifArray();

    if (motifArray.get(0) == null) {
        System.out.println("The motif array is unfilled!");
    }
}

public void draw() {
    for (int i = 0; i < motifArray.size(); i ++) {
        image(motifArray.get(i),200+(10*i),100);

    }


}

static public void main(String[] passedArgs) {
    String[] appletArgs = new String[] { "GraphicsMain" };
    if (passedArgs != null) {
        PApplet.main(concat(appletArgs, passedArgs));
    } else {
        PApplet.main(appletArgs);
    }
}

Second class:

package tester;

import java.util.ArrayList;
import processing.core.*;

public class PGraphicsMaker {
String filepath = "/a/filepath/here/";
PApplet parent;
int populationSampleSize = 16;
int stageWidth = 100;
int stageHeight = 400;
ArrayList<PGraphics> motifArray;
PImage pic;
PImage pic2;

public PGraphicsMaker(PApplet pa) {
    parent = pa;
    pic = parent.loadImage(filepath + "small_jones6.png");
    pic2 = parent.loadImage(filepath + "small_dresser10.png");
}


public ArrayList makeMotifArray() {
    String filepathTempPngs = "/a/filepath/here/tempPngs/";
    ArrayList<PGraphics> motifArray = new ArrayList<PGraphics>();


    for (int i = 0; i < populationSampleSize; i++) {
               motifArray.add(parent.createGraphics(stageWidth,stageHeight,parent.P3D));
        motifArray.get(i).beginDraw();
        motifArray.get(i).background(255, 255, 255, 0);
        motifArray.get(i).image(pic,10,10,100,90);
        motifArray.get(i).image(pic2,10,50,50,50);
        motifArray.get(i).endDraw();
        motifArray.get(i).save(filepathTempPngs + "motif" + i + ".png");
    }
    return motifArray;
}
}

NOTE: I'm not certain that last save() function is necessary in makeMotifArray() in the second class (just another attempt to preserve transparency).

The code below worked for me in 1.5.1 and 2.0b8... If I got you right it is doing what you need... Check it out:

PGraphics p;

void setup(){
  size(400,400,P3D);
  p = createGraphics(400,400,P3D);
  smooth();
  stroke(180,90,210);

}

void draw(){
  background(180,90,210);
  p.beginDraw();
  p.background(220, 180, 40,0);
  p.noFill();
  p.stroke(220, 180, 40);
  p.translate(width/2 - (width/2 - mouseX), height/2, 400-mouseY*5);
  p.sphere(100);
  p.endDraw();
  pushMatrix();
  translate(width/2, height/2, -200);
  rotateY(radians(frameCount));
  rotateX(radians(frameCount/2));
  box(100);
  popMatrix();
  image(p,0,0);
}