Compositing Graphics

The


AlphaComposite

class encapsulates different 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 is totally transparent (clear).


AlphaComposite

supports the standard Porter-Duff compositing rules:

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 destination overlap, only the source pixels in the overlapping area are rendered.
Source-out (SRC_OUT)	If pixels in the source and 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 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 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 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 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

The Composite program illustrates the effects of different compositing style and alpha combinations.

A new AlphaComposite object ac is constructed 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 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 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);
You can find the complete code for this program in Composite.java and an HTML file that includes the applet in Composite.html.