This java tip illustrates how to improve efficiency of mobile application while drawing complex image. If you create a mutable Image, you can use Graphics methods to draw onto it and then copy the result to the screen. It is called double-buffering. This technique can be used to improve performance by drawing complex shapes that do not change or change rarely offline so that they can be quickly copied to the screen when required in the paint( ) method of the Canvas class.An example that uses the technique of drawing onto a mutable image.

 public void paint(Graphics g) {

  int width = getWidth( );
  int height = getHeight( );

  // Create an Image the same size as the Canvas.
  Image image = Image.createImage(width, height);
  Graphics imageGraphics = image.getGraphics( );

  // Fill the background of the image black
  imageGraphics.fillRect(0, 0, width, height);

  // Draw a pattern of lines
  int count = 10;
  int yIncrement = height/count;
  int xIncrement = width/count;

  for (int i = 0, x = xIncrement, y = 0; i < count; i++) {
    imageGraphics.setColor(0xC0 + ((128 + 10 * i) << 8) +
      ((128 + 10 * i) << 16));
    imageGraphics.drawLine(0, y, x, height);
    y += yIncrement;
    x += xIncrement;
  }

  // Add some text
  imageGraphics.setFont(Font.getFont(Font.FACE_PROPORTIONAL,
  Font.STYLE_UNDERLINED, Font.SIZE_SMALL));
  imageGraphics.setColor(0xffff00);
  imageGraphics.drawString("Image Graphics", width/2, 0, Graphics.TOP |
  Graphics.HCENTER);
  
  // Copy the Image to the screen
  g.drawImage(image, 0, 0, Graphics.TOP | Graphics.LEFT);
}