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Thread: Simple animation. Strange behavior

  1. #1
    Junior Member
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    May 2012
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    Default Simple animation. Strange behavior

    When I run the code below with the "thread.sleep", the ball runs from left corner to right corner, leaving a trail behind it as expected.

    but when I remove the thread.sleep part, the balls appears directly at the right side corner, without a trail (similar to the behavior after adding a "fill.rec", but there is no such thing is this code".)

    So I ran the code without "thread.sleep" in DEBUG mode. Then it left a trail as expected. The same code, but in debug mode. What is going on here?

    package mypong;
    import javax.swing.*;
    import java.awt.*;
    import java.awt.event.*;
     * @author Administrator
    public class MyPong extends JFrame
        int x=0; //ball X coordinate at the beginning
        int y=0; //ball's Y coordinate at the beginning
        int xPos=100;
        int yPos=100;
        int circ=20; //width height of the ball
         //initialize the frame
         public void go() 
            JFrame frame = new JFrame();
            MyDrawPanel drawPanel = new MyDrawPanel();     
            while (true)
               if (x<440) 
    //        try
    //        {
    //            Thread.sleep(10);
    //        }
    //        catch(Exception x)
    //        {
    //           x.printStackTrace();
    //        }
            }//end for
         }//end go
         public static void main(String[] args)
             MyPong mypong=new MyPong();
        //inner class drawpanel
        class MyDrawPanel extends JPanel
            public void  paintComponent(Graphics g)
                g.fillOval(x, y, circ, circ);

  2. #2
    Crazy Cat Lady KevinWorkman's Avatar
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    Default Re: Simple animation. Strange behavior

    The call to repaint() is only a *request* for the JPanel to repaint itself. Multiple calls to repaint() will be collapsed into a single paint event. Removing the call to Thread.sleep() causes you to call repaint() a ton of times in a short amount of time, which causes them to be collapsed into a single paint event.

    You should also use a new Thread directly, as right now you're tying up the main thread instead of creating your own. That might work for your current setup, but it won't work when you eventually want to start an animation based on user action.

    You might also consider using a data structure such as a List of positions to draw, or a buffer image that you draw to directly, instead of relying on repaint() to keep track of old frames. Those old frames can be cleared out any time (such as when your program is minimized or behind another component), so relying on them is not a good idea.
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  3. #3
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    Default Re: Simple animation. Strange behavior

    The first call in a paintComponent() method should be:

    super.paintComponent( g );

    That will correctly remove the trail. If you want to include a trail effect, then you should do as KW suggested and add the last (some number) of balls to a data structure, perhaps with decreasing opacity, perhaps only saving every Xth ball depending on the ball's velocity, and print the trail or history in the paintComponent() method.

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