Views using the same offscreen buffer, interfering with each other

Question

Hi all,
I am creating an application where I display simulations. In order to keep the view of the simulation ticking along nicely, I paint the view to an offscreen buffer, and then paint the offscreen buffer to the screen.
This has worked fine for me, so long as I was only viewing one single simulation on the screen at a time. I have now started displaying multiple simulations, and the views have started to interact with eachother -- one will paint over the other, particularly when I click or drag the simulation.
I'm not using any static variables, so my guess is that the fault is the offscreen buffer that I'm using. I think my various views are using the same buffer. Is it possible that this is the problem?
Here's how I paint my view:
private BufferedImage offImg;
private Graphics2D offG;
public Graphics2D createGraphics2D(){
          if (offImg != null) {
               offImg.flush();
               offImg = null;
          RepaintManager repaintManager = RepaintManager.currentManager(this);
          try {
          offImg = (BufferedImage) repaintManager.getOffscreenBuffer(
                    this, this.getWidth(), this.getHeight());
          } catch (java.lang.NullPointerException e){
               return null;
          offG = offImg.createGraphics();
          // .. clear canvas ..
          offG.clearRect(0, 0, getSize().width, getSize().height);
          return offG;
private void draw() {
      if (offG == null)
               createGraphics2D();
     offG.drawImage(...) //etc
public void paintComponent(Graphics g){
           g.drawImage(offImg, 0, 0, this);
      }My first thought was to create a list of all the offImg's created and compare them. The different views are indeed getting the same image from repaintManager.getOffscreenBuffer, even though the component passed in as 'this' is different. The offG's created by each view are different, but since they belong to the same image, I assume it's the image that's important.
Is this likely to be the source of my woes, and, assuming that I want to keep using the offscreen buffer (I do) is there a way around this problem?
Thanks so much for any advice.

Answer 1

Hello,
I am creating an application where I display simulations. In order to keep the view of the simulation ticking along nicely, I paint the view to an offscreen buffer, and then paint the offscreen buffer to the screen.You may know that Swing uses double-buffering by default. So you usually don't really need to tweak Swing's own offscreen buffers.
Painting conservatively on an applicative offscreen buffer is sounds (if painting is slow), but then your application has to create a dedicated offscreen buffer, not use Swing's ones.
This has worked fine for me, so long as I was only viewing one single simulation on the screen at a time. I have now started displaying multiple simulations, and the views have started to interact with eachother -- one will paint over the other, particularly when I click or drag the simulation.
I'm not using any static variables, so my guess is that the fault is the offscreen buffer that I'm using. I think my various views are using the same buffer. Is it possible that this is the problem?I can't tell for sure, but I'm not surprised if that's the case: all Swing ("lightweight") widgets within a same JFrame (or any other "heavyweight" top-level container) share the same native graphical resources, so presumably share the same offscreen buffer. It is Swing's machinery that arranges so that each widget paints onto a part of this buffer that corresponds to the widget's bounds.
          offImg = (BufferedImage) repaintManager.getOffscreenBuffer(
                    this, this.getWidth(), this.getHeight());You don't have to, and probably should not, ask Swing's RepaintManager for a buffer. You vould create a BufferedImage yourself, then draw onto it.
offImg = new BufferedImage(...);Then your paintComponent method itself is fine:
public void paintComponent(Graphics g){
           g.drawImage(offImg, 0, 0, this);
Is this likely to be the source of my woes, and, assuming that I want to keep using the offscreen buffer (I do) is there a way around this problem?See above. And look for "offscreen graphics" on this forums, you'll probably find a lot of examples that do this way (and give example code, my sample above is sketchy and untested).
Regards,
J.
Edited by: jduprez on Jun 2, 2011 12:10 AM
Read this article if you need to kick the best out of your fast painting code (in particular, it mentions more informed ways than mine to create Image instances suitable for offscreen drawing):
http://java.sun.com/products/java-media/2D/perf_graphics.html