Using a BufferedImage for Double Buffering

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Trail: 2D Graphics
Lesson: Manipulating and Displaying Images

## Using a BufferedImage for Double Buffering

> When a graphic is complex or is used repeatedly, you can reduce the time it takes to display it by first rendering it to an off-screen buffer and then copying the buffer to the screen. This technique, called double buffering, is often used for animations.

> > Note: When you are rendering into a Swing component, Swing automatically double-buffers the display.

> A `BufferedImage` can easily be used as an off-screen buffer. To create a `BufferedImage` whose color space, depth, and pixel layout exactly match the window into which you're drawing, call the `Component` `createImage` method. If you need control over the off-screen image's type or transparency, you can construct a `BufferedImage` object directly and use it as an off-screen buffer.

> To draw into the buffered image, you call the `BufferedImage createGraphics` method to get a `Graphics2D` object; then you call the appropriate rendering methods on the `Graphics2D`. All of the Java 2D API rendering features can be used when you're rendering to a `BufferedImage` that's being used as an off-screen buffer.

> When you're ready to copy the `BufferedImage` to the screen, you simply call `drawImage` on your component's `Graphics2D` and pass in the `BufferedImage`.

### Example: BufferedShapeMover

> The following applet allows the user to drag a rectangle around within the applet window. Instead of rendering the rectangle at every mouse location to provide feedback as the user drags it, a `BufferedImage` is used as an off-screen buffer. As the rectangle is dragged, it is re-rendered into the `BufferedImage` at each new location and the `BufferedImage` is blitted to the screen.

> This is a picture of the applet's GUI. To run the applet, click the picture. The applet will appear in a new browser window.

> The source code for the applet is in `BufferedShapeMover.java`.

> Here is the code used to render into the `BufferedImage` and display the image on the screen:

> >     public void updateLocation(MouseEvent e){
> >         rect.setLocation(last_x + e.getX(),
> >                          last_y + e.getY());
> >         ...
> >         repaint();
> >         ...
> >         // In the update method...
> >         if(firstTime) {
> >             Dimension dim = getSize();
> >             int w = dim.width;
> >             int h = dim.height;
> >             area = new Rectangle(dim);
> >             bi = (BufferedImage)createImage(w, h);
> >             big = bi.createGraphics();
> >             rect.setLocation(w/2-50, h/2-25);
> >             big.setStroke(new BasicStroke(8.0f));
> >             firstTime = false;
> >         }

> >         // Clears the rectangle that was previously drawn.
> >         big.setColor(Color.white);
> >         big.clearRect(0, 0, area.width, area.height);

> >         // Draws and fills the newly positioned rectangle
> >         // to the buffer.
> >         big.setPaint(strokePolka);
> >         big.draw(rect);
> >         big.setPaint(fillPolka);
> >         big.fill(rect);

> >         // Draws the buffered image to the screen.
> >         g2.drawImage(bi, 0, 0, this);
> >     }

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