Swing Hacks

Turn the Spotlight on Swing #64

Chapter 8, Rendering

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HACK

The next step is to draw the shape containing the spotlights. Before doing

that, though, you need to compute the opacity of

mask. When you create a

new

Color

instance, you can define the opacity as a number between

0.0

(fully transparent), and

1.0

(fully opaque). To compute the required opac-

ity, the program first divides the area of the spotlights by the total area.

Thereby, if spotlights occupy 20% of the total area, you obtain the value 0.2.

Subtract this from 1.0 to compute the final opacity. In this example, the

opacity would be 0.8, or 80%.

The final step of the rendering depends on whether blurring is activated.

When blurring is off, the

mask is simply filled with shieldColor. Note that

the rendering hints defined in the constructor are used at this moment to

draw anti-aliased shapes. If blurring is activated, an extra step is required.

Instead of drawing the spotlights directly onto the glass pane’s graphics sur-

face, an offscreen buffer is created, with the same size as the panel itself.

After having drawn

mask on this offscreen buffer, the resulting picture is

drawn onto the panel’s graphics surface. All the magic happens when you

call the

drawImage( ) method with a ConvolveOp as the second parameter. The

method applies our blur filter on the picture while drawing it.

—Romain Guy