PGraphics pg;

void setup() {
  size(400, 400, P3D);

  pg = createGraphics(400, 400, P3D);
  pg.smooth(4);
}

void draw() {
  background(0);

  pg.beginDraw();
  pg.background(255, 0, 0);
  pg.ellipse(mouseX, mouseY, 100, 100);
  pg.endDraw();

  image(pg, 0, 0, 400, 400);
}