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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 aBufferedImage
whose color space, depth, and pixel layout exactly match the window into which you're drawing, call theComponent
createImage
method. If you need control over the off-screen image's type or transparency, you can construct aBufferedImage
object directly and use it as an off-screen buffer.To draw into the buffered image, you call the
BufferedImage createGraphics
method to get aGraphics2D
object; then you call the appropriate rendering methods on theGraphics2D
. All of the Java 2D API rendering features can be used when you're rendering to aBufferedImage
that's being used as an off-screen buffer.When you're ready to copy the
BufferedImage
to the screen, you simply calldrawImage
on your component'sGraphics2D
and pass in theBufferedImage
.
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, aBufferedImage
is used as an off-screen buffer. As the rectangle is dragged, it is re-rendered into theBufferedImage
at each new location and theBufferedImage
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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