Trail: 2D Graphics
Lesson: Advanced Topics in Java2D

从几何图元构造复杂形状

Constructive area geometry (CAG) is the process of creating new geometric shapes by performing boolean operations on existing ones. In the Java 2D API the Area class implements the Shape interface and supports the following boolean operations.

	Union		减法
	Intersection		Exclusive-or (`XOR`)

Example: Areas

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

注意:如果你没有看到 applet 正在运行，则需要至少安装 Java SE Development Kit (JDK) 7 版本。

Pear.javacontains 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);