Swing Hacks

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Chapter 6, Transparent and Animated Windows

#42 Make Your Frame Dissolve

HACK

Do the Drawing

Now that the component will be repainted for each frame of the animation,

you can finally do some drawing. Fading to nothing is really easy with

Swing. You just need to draw the background image first, and then draw the

frame on top using a

Composite

. A

Composite

is a class that knows to adjust

the standard mechanism in some way. An

AlphaComposite

will draw par-

tially transparent images depending on the alpha value you pass in. An alpha

value of

1 will draw the image fully opaque, while a value of 0 will be com-

pletely transparent. Values between

1 and 0 will draw the image partially

transparent. Always be sure to save the old composite so you can return the

Graphics

object to its original state when you are done:

public void paint(Graphics g) {

Graphics2D g2 = (Graphics2D)g;

// draw the screen, offset in case the window isn't at 0,0

g.drawImage(screen_buffer,-fullscreen.getX( ),

-fullscreen.getY( ),null);

// draw the frame

Composite old_comp = g2.getComposite( );

Composite fade = AlphaComposite.getInstance(

AlphaComposite.SRC_OVER,1.0f-((float)count)/20f);

g2.setComposite(fade);

g2.drawImage(frame_buffer,frame.getX(),frame.getY( ),null);

g2.setComposite(old_comp);

}

The class in Example 6-2 creates a frame with one button called quit. When

you press the Quit button, the dissolve will be activated, fading the window

into nothing and then calling

System.exit( ). If you compile and run the

code, it will look something like Figure 6-4.

Figure 6-4. A fade dissolve