/**
 * Create Graphics. 
 * 
 * The createGraphics() function creates an object from 
 * the PGraphics class. PGraphics is the main graphics and 
 * rendering context for Processing. The beginDraw() method 
 * is necessary to prepare for drawing and endDraw() is
 * necessary to finish. Use this class if you need to draw 
 * into an off-screen graphics buffer or to maintain two 
 * drawing surfaces with different properties.
 */

PGraphics pg;

void setup() {
  size(640, 360);
  pg = createGraphics(400, 200);
}

void draw() {
  fill(0, 12);
  rect(0, 0, width, height);
  fill(255);
  noStroke();
  ellipse(mouseX, mouseY, 60, 60);
  
  pg.beginDraw();
  pg.background(51);
  pg.noFill();
  pg.stroke(255);
  pg.ellipse(mouseX-120, mouseY-60, 60, 60);
  pg.endDraw();
  
  // Draw the offscreen buffer to the screen with image() 
  image(pg, 120, 60); 
}