Previous image does not clear on repainting Canvas in a loop with different positions

[Ahbab Alam]

Hi All,
I am new to android and trying to animate small image.
I have written below piece of code but during execution i see the the traces of previous drawn images

        protected void onDraw(Canvas canvas){
		if (x < getWidth() - bmp.getWidth()){
			x+=2;
		}
		canvas.drawBitmap(bmp, x, 10, null);
		invalidate();
	}

this method is invoked from another thread:

public void run(){
		while(flag){
		Canvas canvas = null;
		try{
		canvas = view.getHolder().lockCanvas();
		synchronized (view.getHolder()) {
			view.onDraw(canvas);
		}
		}finally{
			if (canvas != null)
				view.getHolder().unlockCanvasAndPost(canvas);
		}
	  }
	}

Do you guys see anything wrong in this ? Any comment will be appreciated.

[Peter Johnson]

Unless you are dealing with sprites , you have to erase the first image before painting the second image. If your background is a solid color, to erase an image, repaint the exact same image using the background color. Then paint the new image.

[Przemek Boryka]

Hi!

I think you can do it in one thread, call him animatThread, for example : animateUpdate(), animateRender(), paintScreen() methods invoked from thread. All those metods should be in while loop for example:

... animateThread ...

public void run() {
   animateStart = true;

   while(animateStart) {
        animateUpdate(); // here you perform calculation
        animateRender(); // put to buffer images, objects etc
      
        paintScreen();  // paint the buffer 
        try {
        Thread.sleep(20); // sleep a bit for finish unfinished operations
        } catch (InterruptedException e){}
   }

}

this piece of code is from Java, but I think that, the mechanism will be the same both in Java and Android. Maybe this will help you