<JAVA图像学习笔记>关于Graphics/Graphics2D以及简单的几何图像制作（一个简单钟表的实现）

题外话：正好赶上OperatingSystem的作业要做一个模拟线程/进程调度的问题，决定用JAVA实现才发现这些内容和之前学过的GUI制作是两码事儿- -b

　　通过学习java.swing库的ActionListener接口我们初步了解了一些关于java框体程序的各个部件JFrame,JPanel,JComponent和控件之间是如何联系通讯的，然而这次我们要从另一个视角来看java程序框体。

　　从一个框体实现“表”的代码我们来初步看一下java是如何在JFrame平台上制作出自己想要的图案的吧！

　　（实现效果图）

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有几个可能会比较陌生的类先筛选出来：

（1）GeneralPath：是java.awt.geom.Path2D的一个子类，path的设定过程主要用两个函数moveTo（double x，double y）和linkTo（double x，double y），当然如果需要实现path的动态绘制的话需要的方法远不止这两个，可以自行去javadoc查阅。

（2）Ellipse2D:是java.awt.geom下的一个类，可以用来绘制理论上的椭圆构造参数为（double x，double y，double width，double height）意义不多赘述。

（3）AffineTransform：这个是整个代码中最有分量的一个类，这个类的引用的获取方法是用静态方法AffineTransform.getInstance（double theta，double x，double y）这里的x和y指的是锚点也就是整个旋转过程参照的旋转中心，theta是要用弧度制表示的旋转角度。具体使用方法下文中会详细总结。

（4）Timer类是在我接触到的新的控件，和JButton或者JLabel等等不同的是这个空间不要手动加上ActionListener的接口来进行事件驱动。使用方法在代码中有详述。

 import java.awt.*;
 import java.awt.event.*;
 import java.awt.geom.*;
 import java.util.Calendar;
 import javax.swing.*;

 public class Main extends JPanel implements ActionListener
 {
     // Create a shape for the face of the clock
     protected static Ellipse2D face = new Ellipse2D.Float(3, 3, 94, 94);

     // Create a path that represents a tick mark
     protected static GeneralPath tick = new GeneralPath();
     static
     {
         tick.moveTo(100, 100);
         tick.moveTo(49, 0);
         tick.lineTo(51, 0);
         tick.lineTo(51, 6);
         tick.lineTo(49, 6);
         tick.lineTo(49, 0);

     }

     // Create a cool hour hand
     protected static GeneralPath hourHand = new GeneralPath();
     static
     {
         hourHand.moveTo(50, 15);
         hourHand.lineTo(53, 50);
         hourHand.lineTo(50, 53);
         hourHand.lineTo(47, 50);
         hourHand.lineTo(50, 15);
     }

     // Create a cool minute hand
     protected static GeneralPath minuteHand = new GeneralPath();
     static
     {
         minuteHand.moveTo(50, 2);
         minuteHand.lineTo(53, 50);
         minuteHand.lineTo(50, 58);
         minuteHand.lineTo(47, 50);
         minuteHand.lineTo(50, 2);
     }

     // And a cool second hand
     protected static GeneralPath secondHand = new GeneralPath();
     static
     {
         secondHand.moveTo(49, 5);
         secondHand.lineTo(51, 5);
         secondHand.lineTo(51, 62);
         secondHand.lineTo(49, 62);
         secondHand.lineTo(49, 5);
     }

     // Create some colors for the pieces of the clock
     protected static Color faceColor = new Color(220, 220, 220);
     protected static Color hourColor = Color.red.darker();
     protected static Color minuteColor = Color.blue.darker();
     protected static Color secondColor = new Color(180, 180, 0);
     protected static Color pinColor = Color.gray.brighter();

     // Create circles for the pivot and center pin
     protected Ellipse2D pivot = new Ellipse2D.Float(47, 47, 6, 6);
     protected Ellipse2D centerPin = new Ellipse2D.Float(49, 49, 2, 2);

     // Create three transforms that center around the pivot point
     protected AffineTransform hourTransform =
             AffineTransform.getRotateInstance(0, 50, 50);
     protected AffineTransform minuteTransform =
             AffineTransform.getRotateInstance(0, 50, 50);
     protected AffineTransform secondTransform =
             AffineTransform.getRotateInstance(0,50,50);

     // Create a timer that fires once a second and a Calendar
     // instance for getting the time values
     protected Timer timer = new Timer(1000, this);
     protected Calendar calendar = Calendar.getInstance();

     // Constructor - hardcode a preferred size of 100x100
     public Main()
     {
         setPreferredSize(new Dimension(100, 100));
     }

     // Invoked when panel is added to a container
     public void addNotify()
     {
 // Call the superclass and start the timer
         super.addNotify();
         timer.start();
     }

     // Invoked when panel is removed from a container
     public void removeNotify()
     {
 // Call the superclass and stop the timer
         timer.stop();
         super.removeNotify();
     }

     //
     public void actionPerformed(ActionEvent event)
     {
         // Update the calendar's time
         this.calendar.setTime(new java.util.Date());

         // Extract the hours minutes and seconds
         int hours = this.calendar.get(Calendar.HOUR);
         int minutes = this.calendar.get(Calendar.MINUTE);
         int seconds = this.calendar.get(Calendar.SECOND);

         // Using a little trigonometry, set the transforms to rotate
         // each hand into the proper position.  Center the rotation
         // around the pivot point (50, 50) instead of the origin
         hourTransform.setToRotation(((double) hours) *
                 (Math.PI / 6.0), 50, 50);
         minuteTransform.setToRotation(((double) minutes) *
                 (Math.PI / 30.0), 50, 50);
         secondTransform.setToRotation(((double) seconds) *
                 (Math.PI / 30.0), 50, 50);

         // Force the component to repaint ASAP
         repaint();
     }

     // This is an alternative to creating a UI delegate.  Since JPanel's
     // paint() method only paints the border and backgound, we can just
     // override the paint method of the component to do the graphics.
     public void paint(Graphics g)
     {
         // Call the superclass first to paint the border (if one is assigned)
         super.paint(g);

         // Get the graphics context and turn on anti-aliasing
         Graphics2D g2 = (Graphics2D) g;
         g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                 RenderingHints.VALUE_ANTIALIAS_ON);

         // Set the paint for the clock face and fill it in
         g2.setPaint(faceColor);
         g2.fill(face);

         // Set the paint to black and draw the clock's outline
         g2.setPaint(Color.black);
         g2.draw(face);

         // Fill in the 12 ticks around the face of the clock
         for (double p = 0.0; p < 12.0; p += 1.0)
         {
             // This is probably terribly inefficient and should be
             // done statically or in the constructor - draw the
             // tick as a transformed shape that is rotated.
             g2.fill(tick.createTransformedShape(
                     AffineTransform.getRotateInstance((Math.PI / 6.0)  * p,
                             50, 50)));
         }

         // Set the paint and draw the hour hand.  It is lowest in the
         // 'z-order' so will appear underneath the other hands.  Notice
         // how each hand is transformed by a different <AffineTransform>.
         g2.setPaint(hourColor);
         g2.fill(hourHand.createTransformedShape(hourTransform));

         // Set the paint and draw the minute hand, the second hand,
         // the pivot and the center pin
         g2.setPaint(minuteColor);
         g2.fill(minuteHand.createTransformedShape(minuteTransform));
         g2.setPaint(secondColor);
         g2.fill(secondHand.createTransformedShape(secondTransform));
         g2.fill(pivot);
         g2.setPaint(pinColor);
         g2.fill(centerPin);
     }

     // A little test frame to show off our fancy clock
     public static void main(String[] args)
     {
         JFrame frame = new JFrame();
         frame.setLocation(700, 400);
         frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
         frame.getContentPane().add(new Main());
         frame.pack();
         frame.show();
     }
 }

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Okay，Here are the points :-P
下面进阶深入探究一下这段代码：

#1#关于每次图形变换之后，变化前图形的擦除？

Ans：是这样的，一般来讲有两种办法实现所谓的擦除效果以完成动画：

1.手动擦除，直接对这个paint出来的geom下的类调用clear**方法，或者直接使用新的颜色/画布将需要擦除的形状删除/覆盖。

2.super.paint（g）这个方法有点神奇，以后需要进一步探讨一下，直接在子类的paint方法之中调用父类的paint的方法来实现“擦除”。

代码中方法其实是第一种，在每次repaint过程中，都会有一次将整个表盘“覆盖”重画的过程，但是不得不说，这么实现的话每秒钟都需要对整个表盘全部的空间进行重画，是很浪费时间/空间的。

#2#怎么递推实现的表盘圆周上的12个标记的绘制？

Ans：这里用了一个新的类叫做AffineTransform。使用这个类的方法是在geom下的几何类中直接调用createTransformedShape（AffineTransform at）返回一个Shape类型，这时匿名调用这个类进行draw（勾勒轮廓）/fill（填充颜色）方法可以完成对转换完成之后的几何体的绘制。关于AffineTransform类的更多用法和数学解释：

http://www.apihome.cn/api/java/AffineTransform.html

#3#怎么实现消除斜线的锯齿状？

Ans：这个我真的不懂，但是我知道是下面这段神奇的代码起到的作用,至于这个“渲染提示”具体的用法还得慢慢学..

g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,RenderingHints.VALUE_ANTIALIAS_ON);

#4#阿西巴，addNotify方法到底有没有用？
Ans：addNotify方法是在JPanel添加到JComponent上的时候用的，本例中是在添加pane的直接启动计时器（Timer），关于代码最后那个若有若无的getContentPane（）为什么会出现，其实还要从JFrame的四层结构开始说起（传送门见下）：

http://tieba.baidu.com/p/2004216123