Canvas stops displaying after a few seconds.

[ZHANG Erkang]

Hello everyone,


I'm trying to display a million ovals in a canvas dynamicly every second. I use 
a thread as the rendering thread, but my problem is, after a few secondes, the 
canvas freezes and stops displaying. I guess that the buffer is full so it 
can't display anymore, but how can I clear up the buffer? The test source code 
is as follow:


public class Main extends Application {

    public void start(Stage primaryStage) {
        primaryStage.setTitle("Drawing Operations Test");
        Group root = new Group();
        Canvas canvas = new Canvas(800, 800);
        GraphicsContext gc = canvas.getGraphicsContext2D();
        drawShapes(gc);
        root.getChildren().add(canvas);
        primaryStage.setScene(new Scene(root));
        primaryStage.show();
        Task task2 = new Task<Void>()
        {
            public synchronized Void call() throws Exception {
                while (true) {
                    Thread.sleep(1000);
                    Canvas canvas = gc.getCanvas();
                    canvas.getGraphicsContext2D().clearRect(0, 0, 
canvas.getHeight(), canvas.getWidth());
                    drawShapes(canvas.getGraphicsContext2D());

                }
            }
        };
        Thread t = new Thread(task2);
        t.start();
    }

    private void drawShapes(GraphicsContext gc) {
        gc.setFill(Color.GREEN);
        gc.setStroke(Color.BLUE);
        gc.setLineWidth(1);
        double widthOval=1;
        double heightOval = 1;
        for (int i = 0; i < 500; ++i) {
            for (int j = 0; j < 500; ++j) {

                if (Math.random() < 0.5) {
                    gc.fillOval(i * widthOval, j * heightOval, widthOval, 
heightOval);
                }
                else {
                    gc.strokeOval(i * widthOval, j * heightOval, widthOval, 
heightOval);
                }
            }
        }
    }

    public static void main(String[] args) {
        launch(args);
    }
}



Can anyone help me?

Thanks in advance,

Pierre