alpha channel : Java Glossary

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alpha channel
The Color class stores colours internally as a 32-bit integer, 8 bits each of red, green, blue and alpha, each value 0..255. Alpha measures opacity. 0=transparent 255=opaque. The default for the alpha value is 255, completely opaque.

The combined ARGB value consists of the alpha component in bits 24-31, the red component in bits 16-23, the green component in bits 8-15, and the blue component in bits 0-7 where bit 0 is the least significant bit.

// Creating a colour with an alpha component.
// Note order of parameters is RGBA, not ARGB as they are internally.
// Dark orange is #ff8c00 in HTML.
Color almostTransparentDarkOrange
= new Color( 0xff /* red */,
             0x8c /* green */,
             0x00 /* blue */,
             0x1f /* alpha */ );

In the Masker Applet, I create an image, an email address painted on a transparent background, defined with alpha=0, then write the whole thing to a *.png image file on disk.

AlphaComponsite

The AlphaComposite class lets define how to blend images together, taking into account transparency. It lets you create an AlphaComposite rule for combining source and destination. You then use Graphics2D. setComposite( anAlphaComposite) to apply that rule to your Graphics2D object to use in all subsquent calls to drawing methods such as drawImage, drawString, draw, and fill.

Learning More

Oracle’s Javadoc on Color class : available:
Oracle’s Javadoc on AlphaComposite class : available:
opaque
transparent
transparent backgrounds
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