Constructing Complex Shapes from Geometry Primitives

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Trail: 2D Graphics
Lesson: Displaying Graphics with Graphics2D

## Constructing Complex Shapes from Geometry Primitives

> Constructive area geometry (CAG) is the process of creating new geometric shapes by performing boolean operations on existing ones. In the Java 2D API a special type of `Shape` called an `Area` supports boolean operations. You can construct an `Area` from any `Shape` .

> `Areas` support the following boolean operations:

> | Union |  | Subtraction
> | Intersection |  | Exclusive-or (`XOR`)

### Example: Areas

> In this example `Area` objects construct a pear shape from several ellipses.

> 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.

> `Pear.java` contains the complete code for this applet.

> The leaves are each created by performing an intersection on two overlapping circles.

> >     leaf = new Ellipse2D.Double();
> >     ...
> >     leaf1 = new Area(leaf);
> >     leaf2 = new Area(leaf);
> >     ...
> >     leaf.setFrame(ew-16, eh-29, 15.0, 15.0);
> >     leaf1 = new Area(leaf);
> >     leaf.setFrame(ew-14, eh-47, 30.0, 30.0);
> >     leaf2 = new Area(leaf);
> >     leaf1.intersect(leaf2);
> >     g2.fill(leaf1);
> >     ...
> >     leaf.setFrame(ew+1, eh-29, 15.0, 15.0);
> >     leaf1 = new Area(leaf);
> >     leaf2.intersect(leaf1);
> >     g2.fill(leaf2);

> Overlapping circles are also used to construct the stem through a subtraction operation.

> >     stem = new Ellipse2D.Double();
> >     ...
> >     stem.setFrame(ew, eh-42, 40.0, 40.0);
> >     st1 = new Area(stem);
> >     stem.setFrame(ew+3, eh-47, 50.0, 50.0);
> >     st2 = new Area(stem);
> >     st1.subtract(st2);
> >     g2.fill(st1);

> The body of the pear is constructed by performing a union operation on a circle and an oval.

> >     circle = new Ellipse2D.Double();
> >     oval = new Ellipse2D.Double();
> >     circ = new Area(circle);
> >     ov = new Area(oval);
> >     ...
> >     circle.setFrame(ew-25, eh, 50.0, 50.0);
> >     oval.setFrame(ew-19, eh-20, 40.0, 70.0);
> >     circ = new Area(circle);
> >     ov = new Area(oval);
> >     circ.add(ov);
> >     g2.fill(circ);

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