I dont understand.
you get the trails because you dont draw a background.
If you draw one you get what you want.
thats the normal behavior. i wouldnt call it a hack. thats the way it is.
whats the problem with calling background? what do you want to achieve?

Thanks for the responses.

What I am trying to do is overlay a smaller PGraphics layer within the base sketch layer. I want the background of this PGraphics layer to keep its alpha so that the the base layer is visible behind it, without having the trails left behind by the PGraphic layer. I guess what I am seeking is a way to redraw and update the overlayed PGraphics layer before it is drawn onto the screen.

My plan is to have several off-screen PGraphics buffers that will be composited over each other. I'm starting to think that this approach is not the way to do this.

I think I found a solution. I noticed that adding an alpha value to  pg.background allows contol over the PGraphics opacity. I wasn't aware that RGBA values could be applied to the background.

Thanks again for helping me out.

Code:

PGraphics pg;
float x;

void setup() {
  size(200, 200,P3D);
  pg = createGraphics(160, 180, P3D);

}

void draw() {
  background(255, 0, 255);
  rect( x, 100, 20, 20);
  x +=1;
  if( x == width) {
    x = 0;
  }

  pg.beginDraw();

  pg.background(255, 255, 0, 128);
  pg.fill(0,0, 255, 127);
  pg.noStroke();
  pg.ellipse(mouseX, mouseY,30,30);

  pg.stroke(255,255,0,127);
  pg.strokeWeight(4);
  pg.line(100, 100, mouseX, mouseY);

  pg.fill(0,255, 0, 127);
  pg.noStroke();
  pg.rect(50,50,200,200);

  pg.endDraw();

  image(pg, 20, 10);


}