java - 在没有清晰背景的情况下旋转图像

Question

我已经阅读了很多有关绘制图像的文章，但是当我需要保留背景时，我无法使它们起作用。单击 JButton 后，我试图旋转位于另一个图像上方的图像。背景图像是通过以下方式在 JPanel 上生成的:

public void paintComponent(Graphics g){

    int index = 0;

    Graphics2D g2 = (Graphics2D) g;

    super.paintComponent(g);    

    try {
        image = ImageIO.read(url1);
        image2 = ImageIO.read(url2);
        image3 = ImageIO.read(url3);
    } catch (IOException e) {
    }


    g.drawImage(image, 0, 0, null);
    g.drawImage(image3, 0, 0, null);

    if(scaleDrawnFlag == 0){

        for(index = 0; index < 60; index ++){
            tx = AffineTransform.getRotateInstance(Math.toRadians(6*index), this.getHeight()/2, this.getWidth()/2);
            op = new AffineTransformOp(tx, AffineTransformOp.TYPE_BILINEAR);
            g.drawImage(op.filter(image3, null), 0, 0, null);
        }       

        scaleDrawnFlag = 1;
    }

    g.drawImage(image2, 0, 0, null);

}

其中进入一个名为JPanel的面板，只绘制一次图像以保持刷新性能，仅针对动画图像。这为转速表绘制了一个刻度，总共 60 行，其中每行都是 image3 的副本

动画图像是通过按下 JButton 生成的，由以下人员制作:

public void paintComponent(Graphics g){

    super.paintComponent(g);        
    BufferedImage img = new BufferedImage(370, 370, BufferedImage.TRANSLUCENT);
    Graphics2D g2d = img.createGraphics();
    g2d.setComposite(AlphaComposite.getInstance(AlphaComposite.CLEAR, 0.0f));
    g2d.fillRect(0, 0, img.getWidth(), img.getHeight());
    g2d.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 0.0f));
    Graphics2D temp = (Graphics2D) g;
    tx = AffineTransform.getRotateInstance(Math.toRadians(degrees), this.getHeight()/2, this.getWidth()/2);
    op = new AffineTransformOp(tx, AffineTransformOp.TYPE_BILINEAR);
    temp.drawImage(op.filter(image2, null), 0, 0, null);
    temp.dispose();
}

这是另一个名为 overPanel 的 JPanel，它位于初始 JPanel 之上。

但是，当我调用这些方法时:

public void up(){
    degrees ++;
    if(degrees == 360) degrees = 0;
    repaint();
}

public void down(){
    degrees --;
    if(degrees == -360) degrees = 0;
    repaint();
}

在 overPanel 类上，JPanel 被完全清除。动画运行良好，但背景消失了。

我必须怎么做才能保留背景？

我还尝试了另一种解决方案，即在每次 up() 和 down() 调用时再次绘制 60 条线。背景已重新绘制，但需要花费太多时间才能完成，因此旋转转速表指示器的动画会滞后。

Answer 1

切勿处置 JVM 提供给您的 Graphics 对象。你这样做:

// temp **is** the same object as g and is the Graphics object given by the JVM
Graphics2D temp = (Graphics2D) g; 
//....
temp.dispose();

并且不应该这样做，因为它会完全破坏绘画链。相反，您应该处理您创建的 g2d 对象。

另外，这样也行

Graphics2D temp = (Graphics2D) g.create(); // temp **is** a new object
//....
temp.dispose();  // this is OK

其他问题:

• 我也不会在 paintComponent 中创建我的 BufferedImage，而是将其作为类的一个字段，并在 paintComponent 中显示它。
• 您的顶级代码显示忽略的关键异常——您不想这样做。
• 它还展示了在绘画方法中读取图像文件，这会不必要地减慢图形速度。同样，不要这样做，在任何绘画方法之外读入一次图像，存储结果，并在绘画方法中使用它们。
• paintComponent 方法是protected，而不是public。避免不必要地提高其知名度。

---

在您的最小示例程序中，您的 scaleDrawnFlag 变量及其关联的 if block 似乎让您感到困惑。该变量和 if block 的目的是什么？如果你摆脱变量和 if block ，你的背景将持续存在。我自己会通过创建稳定的背景图像并每次在 paintComponent(...) 方法中绘制它来做不同的事情。我不会重写 update(...) ，因为那是 AWT 的拼凑而不是用于 Swing 图形。我还尽量避免 null 布局和 setBounds(...) 就像瘟疫一样，因为这会导致非常难以调试、维护和增强的不灵活、僵化的 GUI。例如:

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GridLayout;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;
import java.awt.geom.AffineTransform;
import java.awt.image.AffineTransformOp;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.URL;

import javax.imageio.ImageIO;
import javax.swing.*;

@SuppressWarnings("serial")
public class MyMainPanel extends JPanel {
   private MyDrawingPanel myDrawingPanel;

   public MyMainPanel() {
      try {
         myDrawingPanel = new MyDrawingPanel();
      } catch (IOException e) {
         e.printStackTrace();
         System.exit(-1);
      }

      JPanel rightPanel = new JPanel();
      rightPanel.setLayout(new GridLayout(0, 1, 5, 5));
      rightPanel.add(new JButton(new MyUpAction("Up", KeyEvent.VK_U)));
      rightPanel.add(new JButton(new MyDownAction("Down", KeyEvent.VK_D)));

      JPanel rightWrapPanel = new JPanel(new BorderLayout());
      rightWrapPanel.add(rightPanel, BorderLayout.PAGE_START);

      setBorder(BorderFactory.createEmptyBorder(5, 5, 5, 5));
      setLayout(new BorderLayout());
      add(myDrawingPanel, BorderLayout.CENTER);
      add(rightWrapPanel, BorderLayout.LINE_END);
   }

   private class MyUpAction extends AbstractAction {
      public MyUpAction(String name, int mnemonic) {
         super(name);
         putValue(MNEMONIC_KEY, mnemonic);
      }

      @Override
      public void actionPerformed(ActionEvent e) {
         myDrawingPanel.up();
      }
   }

   private class MyDownAction extends AbstractAction {
      public MyDownAction(String name, int mnemonic) {
         super(name);
         putValue(MNEMONIC_KEY, mnemonic);
      }

      @Override
      public void actionPerformed(ActionEvent e) {
         myDrawingPanel.down();
      }
   }

   private static void createAndShowGui() {
      JFrame frame = new JFrame("MyMainPanel");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.getContentPane().add(new MyMainPanel());
      frame.pack();
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

@SuppressWarnings("serial")
class MyDrawingPanel extends JPanel {
   private static final String NEEDLE_IMG_PATH = "http://1.bp.blogspot.com/"
         + "-fq-oPGBSLp4/Ttoj7DoAMWI/AAAAAAAABtc/t7gKJlfRQuo/s400/secondHand.png";
   private static final String ORANGE_DISK_IMG_PATH = "http://upload.wikimedia.org/wikipedia/commons/thumb/b/b6/Circle_Fulvous_Solid.svg/200px-Circle_Fulvous_Solid.svg.png";
   private static final String GREEN_LINE_IMG_PATH = "http://www.xtremeskater.com/math/images/circle_radius.png";
   private static final int MAX_DEGREES = 360;

   private int imgWidth = 0;
   private int imgHeight = 0;
   private BufferedImage needleImg = null;
   private BufferedImage orangeDiskImg = null;
   private BufferedImage greenLineImg = null;
   private BufferedImage backgroundImg = null;
   private int degrees; 

   public MyDrawingPanel() throws IOException {
      URL needleUrl = new URL(NEEDLE_IMG_PATH);
      URL orangeDiskUrl = new URL(ORANGE_DISK_IMG_PATH);
      URL greenLineUrl = new URL(GREEN_LINE_IMG_PATH);
      needleImg = ImageIO.read(needleUrl);
      orangeDiskImg = ImageIO.read(orangeDiskUrl);
      greenLineImg = ImageIO.read(greenLineUrl);

      imgWidth = Math.max(orangeDiskImg.getWidth(), 
            greenLineImg.getWidth());
      imgHeight = Math.max(orangeDiskImg.getHeight(),
            greenLineImg.getHeight());
      backgroundImg = new BufferedImage(imgWidth, imgHeight,
            BufferedImage.TYPE_INT_ARGB);
      Graphics2D g2 = backgroundImg.createGraphics();
      drawBackground(g2, imgWidth, imgHeight);
      g2.dispose();
   }

   @Override
   public Dimension getPreferredSize() {
      if (isPreferredSizeSet()) {
         return super.getPreferredSize();
      }
      return new Dimension(imgWidth, imgHeight);
   }

   @Override
   protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      if (backgroundImg != null) {
         g.drawImage(backgroundImg, 0, 0, null);
      }
      AffineTransform tx = AffineTransform.getRotateInstance(Math.toRadians(degrees),
            this.getHeight() / 2, this.getWidth() / 2);
      AffineTransformOp op = new AffineTransformOp(tx, AffineTransformOp.TYPE_BILINEAR);
      g.drawImage(op.filter(needleImg, null), 0, 0, null);
   }

   public void up() {
      degrees++;
      degrees %= MAX_DEGREES;
      repaint();
   }

   public void down() {
      degrees--;
      degrees += MAX_DEGREES;
      degrees %= MAX_DEGREES;
      repaint();
   }

   public int getDregrees() {
      return degrees;
   }

   private void drawBackground(Graphics2D g2, int biWidth, int biHeight) {
      int index;
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
            RenderingHints.VALUE_ANTIALIAS_ON);
      g2.drawImage(orangeDiskImg, 0, 0, null);
      g2.drawImage(greenLineImg, 0, 0, null);
      AffineTransform tx = AffineTransform.getRotateInstance(Math.toRadians(6),
            biWidth / 2, biHeight / 2);
      for (index = 0; index < 60; index++) {
         g2.transform(tx);
         g2.drawImage(greenLineImg, 0, 0, null);
      }
   }
}