Render offscreen high resolution canvas and preview on screen

offscreen_pgraphics.pde

PGraphics canvas;

void setup() {
  size(255,330);
  
  canvas = createGraphics(2550,3300);
  
  canvas.beginDraw();
  canvas.background(0);
  for (int i = 0; i < 1000; i++) {
    canvas.fill(255,100);
    canvas.rect(random(canvas.width),random(canvas.height),200,200);
  }
  canvas.endDraw();
  canvas.save("test.png");
  image(canvas,0,0,width,height);
}