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Trail: 2D Graphics
Lesson: Advanced Topics in Java2D
Advanced Topics in Java2D
Transforming Shapes, Text, and Images
Clipping the Drawing Region
Compositing Graphics
Controlling Rendering Quality
Constructing Complex Shapes from Geometry Primitives
Supporting User Interaction
Home Page > 2D Graphics > Advanced Topics in Java2D
Compositing Graphics

> The `AlphaComposite` class encapsulates various compositing styles, which determine how overlapping objects are rendered. An `AlphaComposite` can also have an alpha value that specifies the degree of transparency: alpha = 1.0 is totally opaque, alpha = 0.0 totally transparent (clear). `AlphaComposite` supports most of the standard Porter-Duff compositing rules shown in the following table.

> Source-over (`SRC_OVER`) | If pixels in the object being rendered (the source) have the same location as previously rendered pixels (the destination), the source pixels are rendered over the destination pixels.
> Source-in (`SRC_IN`) | If pixels in the source and the destination overlap, only the source pixels in the overlapping area are rendered.
> Source-out (`SRC_OUT`) | If pixels in the source and the destination overlap, only the source pixels outside of the overlapping area are rendered. The pixels in the overlapping area are cleared.
> Destination-over (`DST_OVER`) | If pixels in the source and the destination overlap, only the source pixels outside of the overlapping area are rendered. The pixels in the overlapping area are not changed.
> Destination-in (`DST_IN`) | If pixels in the source and the destination overlap, the alpha from the source is applied to the destination pixels in the overlapping area. If the alpha = 1.0, the pixels in the overlapping area are unchanged; if the alpha is 0.0, pixels in the overlapping area are cleared.
> Destination-out (`DST_OUT`) | If pixels in the source and the destination overlap, the alpha from the source is applied to the destination pixels in the overlapping area. If the alpha = 1.0, the pixels in the overlapping area are cleared; if the alpha is 0.0, the pixels in the overlapping area are unchanged.
> Clear (`CLEAR`) | If the pixels in the source and the destination overlap, the pixels in the overlapping area are cleared.

> To change the compositing style used by the `Graphics2D` class, create an `AlphaComposite` object and pass it into the `setComposite` method.

### Example: Composite

> This program illustrates the effects of various compositing style and alpha combinations.

> > Note: If you don't see the applet running above, you need to install release 6 of the JDK.

> > `Composite.java`. contains the full code for this applet.

> > A new `AlphaComposite` object ac is constructed by calling `AlphaComposite.getInstance` and specifying the desired compositing rule.

> > >     AlphaComposite ac =
> > >                    AlphaComposite.getInstance(AlphaComposite.SRC);

> > When a different compositing rule or alpha value is selected, `AlphaComposite.getInstance` is called again, and the new `AlphaComposite` is assigned to ac. The selected alpha is applied in addition to the per-pixel alpha value and is passed as a second parameter to `AlphaComposite`.`getInstance`.

> > >     ac = AlphaComposite.getInstance(getRule(rule), alpha);

> > The composite attribute is modified by passing the `AlphaComposite` object to `Graphics 2D` `setComposite`. The objects are rendered into a `BufferedImage` and are later copied to the screen, so the composite attribute is set on the `Graphics2D` context for the `BufferedImage`:

> > >     BufferedImage buffImg = new BufferedImage(w, h,
> > >                                 BufferedImage.TYPE_INT_ARGB);
> > >     Graphics2D gbi = buffImg.createGraphics();
> > >     ...
> > >     gbi.setComposite(ac);

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