Compositing Graphics

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Trail: 2D Graphics
Lesson: Displaying Graphics with Graphics2D

## 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 `Graphics2D`, you 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.

> This is a picture of the applet's GUI. To run the applet, click the picture. The applet will appear in a new browser window.

> `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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