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Home Page > 2D Graphics > 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

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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.

Compositing Rule Description

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, you need to install at least the Java SE Development Kit (JDK) 7 release.

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

 A newAlphaComposite 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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