void setup(){
  size(300,300,P2D);
  noFill();
  background(255);
  rectMode(CENTER);
  stroke(0,32);
  smooth(8);
  // instantiate and start rendering thread
  new PNGRenderer(this);
}

void draw(){
  // draw moving shapes
  translate(width * 0.5, height * 0.5);
  rotate(sin(frameCount * 0.075));
  scale(sin(frameCount * 0.01));
  translate(cos(frameCount * 0.01) * width * 0.1,sin(frameCount * 0.01) * height * 0.1);
  rect(0,0,width * .9,height * .9);
}

public class PNGRenderer implements Runnable{

  PApplet parent;
  PImage  frame;
  boolean shouldSave = false;

  int savedFrameCount;

  boolean isRunning = true;

  PNGRenderer(PApplet parent){
    this.parent = parent;
    this.parent.registerMethod("draw",this);

    frame = createImage(parent.width,parent.height,ARGB);
    frame.loadPixels();

    Thread renderThread = new Thread(this);
    renderThread.setName("Renderer-Thread");
    renderThread.start();
  }

  public void draw(){
    // all is well if I sample pixels in the same OpenGL thread
    //parent.loadPixels();
    shouldSave = true;
  }

  synchronized void sampleAndSavePixels(){
    if(shouldSave){
      // program crashes if I try to save in a separate thread
      parent.loadPixels();
      arrayCopy(parent.pixels,frame.pixels);
      frame.updatePixels();
      frame.save(dataPath("frames/frame_"+nf(savedFrameCount++,4)+".png"));
      println("saved frame",savedFrameCount);
      shouldSave = false;
    }
  }

  public void run(){
    while(isRunning){
      sampleAndSavePixels();
    }
  }

}