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Beginning Java 2- JDK 1.3 Version : Images and Animation

Buy this book Title: Beginning Java 2- JDK 1.3 Version ISBN: 1861003668 US Price: $ 49.99 Canadian Price: C$ 74.95 UK Price: £ 35.99 Publication Date: March 2000 Pages: 1230 © Wrox Press Limited, US and UK.

Beginning Java 2- JDK 1.3 Version
Images and Animation

DST_OVER With this rule, the fraction of each source pixel in the result is 1 - AlphaD, and the fraction of the corresponding destination pixel is 1. Where the source overlaps the destination, the destination is effectively rendered over the source. The operation of this rule is defined by:

ColorR = ColorS*(1 - AlphaD) + ColorD
AlphaR = AlphaS*(1 - AlphaD) + AlphaD

Since in our illustration the destination has an alpha of 1.0f, the fraction of the source pixel is 0.0 so the destination completely hides the part of the source that is underneath.

DST_IN With this rule, the fraction of the source in the result is 0 and the fraction of the destination is Alphas. The operations are:

ColorR = ColorD*AlphaS
AlphaR = AlphaD*AlphaS

Thus the destination is rendered inside the boundary of the source, but with the alpha from the source, so it looks lighter than the rest of the destination.

DST_OUT The source fraction in the result is zero, and the destination fraction is 1 – AlphaS. Thus the rule operation is:

ColorR = ColorD*(1 – AlphaS)
AlphaR = AlphaD*(1 – AlphaS)

CLEAR The fractions of the source and destination pixels involved in the operation of this rule are both zero. The effect is that pixels corresponding to the source pixels are cleared, so they will have all color components at zero.

The AlphaComposite class also defines static members that are AlphaComposite objects. Each object corresponds to one of the rules we have just discussed and they all have an alpha of 1.0f. These members are:

As the Clear object has an alpha of 1.0f, using this for alpha compositing will result in an opaque black result.

Since the alpha for an image determines the transparency, we can fade an image into the background by repainting the image with a shrinking value for the alpha. Let's see how that works with an example.

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