PGraphics pg;

void setup() {
  size(640, 640, P2D);
  noSmooth();
  pg = createGraphics(32, 32);
  pg.noSmooth();
}

void draw() {
  pg.beginDraw();
  pg.background(128);
  pg.rect(4, 4, 16, 16);
  pg.endDraw();
  image(pg, 0, 0, width, height);
}