如何在JLabel中使ImageIcon的背景透明

Question

我想使用JLabels和ImageIcon将Image放到另一个Image上。一切正常，但我无法找到如何使图像的背景透明。基本上我想编写一个游戏，但我不希望玩家成为一个完美的矩形。所以这是我的代码到目前为止工作：

import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class Game {
    JFrame frame = new JFrame();
    JLabel label1 = new JLabel();
    JLabel label2 = new JLabel();

    public Game() {
        frame.setVisible(true);
        frame.setDefaultCloseOperation(frame.EXIT_ON_CLOSE);
        frame.setTitle("just a test");
        frame.setResizable(true);
        frame.getContentPane().setLayout(null);
        frame.setBounds(400, 200, 915, 600);

        label1.setBounds(0, -195, 900, 890);
        label2.setBounds(50, 200, 260, 240);

        frame.getContentPane().add(label2);
        frame.getContentPane().add(label1);

        ImageIcon icon1 = new ImageIcon("icons/landscape.png");
        ImageIcon icon2 = new ImageIcon("icons/mario1.png");

        label1.setIcon(icon1);
        label2.setIcon(icon2);
    }
    public static void main(String[] args) {
        Game game = new Game();

    }
}

Answer 1

我推荐和Hovercraft Full Of Eels一样，不过我会解释它背后的原因。根据您的设置，您的JLabel将填充所有透明像素，并使用不透明的颜色。 mario1.png是否具有透明背景是无关紧要的，因为它实现的JLabel用其背景颜色填充透明像素。这有两个可能的解决方案。像Hovercraft建议的那样使用单个JLabel，或者使用JComponent代替。我建议使用后者，就好像你正在编写一个游戏，然后你不希望你的Mario sprite与后台一起移动，并且你在当前情况下对JComponent类有更多的控制。

这是JComponent类的Oracle Doc：https://docs.oracle.com/javase/tutorial/uiswing/components/jcomponent.html

如果你愿意，我愿意给你简化的示例代码。

Answer 2

JLabel本质上是透明的 - 它的opaque属性默认是false（不像JPanel，默认情况下是不透明的），所以如果你将透明图像放入ImageIcon并通过.setIcon(...)将它变成JLabel的Icon并添加JLabel对于诸如JPanel的容器，显示的图标的透明区域将保持透明，显示背景图像。例如，将此图像指定为精灵，会在其周围显示带有透明像素的实心圆圈：

因此，如果将鼠标侦听器添加到JLabel，则可以将其拖动到容器周围。

EG，

import java.awt.*;
import java.awt.event.*;
import java.io.IOException;
import java.net.URL;
import javax.imageio.ImageIO;
import javax.swing.*;

@SuppressWarnings("serial")
public class SpriteOnBackground extends JPanel {
    // Image attribution:
    // By Adam Evans - M31, the Andromeda Galaxy (now with h-alpha)
    // Uploaded by NotFromUtrecht, CC BY 2.0, 
    // https://commons.wikimedia.org/w/index.php?curid=12654493
    public static final String ANDROMEDA_IMAGE = "https://upload.wikimedia.org/wikipedia/commons/"
            + "thumb/9/98/Andromeda_Galaxy_%28with_h-alpha%29.jpg/"
            + "1280px-Andromeda_Galaxy_%28with_h-alpha%29.jpg";
    public static final String SPRITE_IMAGE = "https://upload.wikimedia.org/wikipedia/commons/"
            + "thumb/a/a1/Glossy_3d_blue_blue2.png/"
            + "120px-Glossy_3d_blue_blue2.png";
    private Image background;
    private JLabel spriteLabel = new JLabel();

    public SpriteOnBackground(Image bg, Image spriteImg) {
        background = bg;
        spriteLabel.setIcon(new ImageIcon(spriteImg));
        spriteLabel.setSize(spriteLabel.getPreferredSize());
        setLayout(null);
        add(spriteLabel);

        MyMouse myMouse = new MyMouse();
        spriteLabel.addMouseListener(myMouse);
        spriteLabel.addMouseMotionListener(myMouse);
    }

    @Override
    public Dimension getPreferredSize() {
        if (isPreferredSizeSet() || background == null) {
            return super.getPreferredSize();
        }
        // make JPanel the size of the image
        int w = background.getWidth(this);
        int h = background.getHeight(this);
        return new Dimension(w, h);
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        // draw background image
        g.drawImage(background, 0, 0, this);
    }

    // mouse listener to drag the JLabel around the GUI
    private class MyMouse extends MouseAdapter {
        private Point p1;
        private Point pSprite;

        @Override
        public void mousePressed(MouseEvent e) {
            p1 = e.getLocationOnScreen();
            pSprite = spriteLabel.getLocation();
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            if (p1 != null) {
                moveSprite(e);
            }
        }

        private void moveSprite(MouseEvent e) {
            Point p2 = e.getLocationOnScreen();
            int x = pSprite.x + p2.x - p1.x;
            int y = pSprite.y + p2.y - p1.y;
            Point newP = new Point(x, y);
            spriteLabel.setLocation(newP);
            repaint();
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            if (p1 != null) {
                moveSprite(e);
            }
            p1 = null;
        }
    }

    private static void createAndShowGui() {
        SpriteOnBackground mainPanel = null;
        try {
            URL backgroundUrl = new URL(ANDROMEDA_IMAGE);
            Image backGroundImg = ImageIO.read(backgroundUrl);

            URL spriteUrl = new URL(SPRITE_IMAGE);
            Image spriteImg = ImageIO.read(spriteUrl);
            mainPanel = new SpriteOnBackground(backGroundImg, spriteImg);
        } catch (IOException e) {
            e.printStackTrace();
        }

        JFrame frame = new JFrame("Sprite On Background");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }
}