如何使 java JPanel 和 graphics2d 透明？
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how to make java JPanel and graphics2d transparent?
提问by vondip
Well the title is quite self explanatory. I want to build two panels one on-top of each other in layers using java. I want the top layer to contain a JPanel which will contain a graphics2d object. I'd like both the JPanel and graphics2d to have transparent background (I still want the content drawn by the graphics2d visible). Does anyone have an idea how it can be done?
那么标题是不言自明的。我想使用 java 在层中构建两个面板，一个在彼此的顶部。我希望顶层包含一个 JPanel，它将包含一个 graphics2d 对象。我希望 JPanel 和 graphics2d 都具有透明背景（我仍然希望 graphics2d 绘制的内容可见）。有谁知道如何做到这一点？
回答by Nate
Call setOpaque(false) on the JPanel - that will not paint the JPanel's background.
在 JPanel 上调用 setOpaque(false) - 这不会绘制 JPanel 的背景。
Depending on what method you're overriding to get at the Graphics2D (JPanel's don't containa Graphics2D object like a component - a Graphics2D object is used to paint the JPanel) - if it's paintComponent() you should read the JavaDocs for JComponent- and call super.paintComponent(g) first so that opacity is honored - and then do the rest of your painting.
取决于您要覆盖的获取 Graphics2D 的方法（JPanel 不包含像组件一样的 Graphics2D 对象 - Graphics2D 对象用于绘制 JPanel） - 如果它是paintComponent()，您应该阅读 JComponent 的 JavaDocs-并首先调用 super.paintComponent(g) 以保证不透明度 - 然后进行其余的绘画。
Working example:
工作示例：
package com.stackoverflow.opaque;
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
public class OpaqueExample extends JFrame {
private JLayeredPane layers;
private JPanel up, down;
private JButton toggleOpaque;
public OpaqueExample() {
layers = new JLayeredPane();
down = new JPanel();
down.setBackground(Color.GREEN);
down.setBounds(100, 100, 200, 200);
layers.add(down, new Integer(1));
up = new JPanel() {
public void paintComponent(Graphics og) {
super.paintComponent(og);
Graphics2D g = (Graphics2D)og;
GradientPaint gradient = new GradientPaint(0, 0, Color.BLUE, 10, 0,
Color.WHITE, true );
Polygon poly = new Polygon();
poly.addPoint(10, 10);
poly.addPoint(100, 50);
poly.addPoint(190, 10);
poly.addPoint(150, 100);
poly.addPoint(190, 190);
poly.addPoint(100, 150);
poly.addPoint(10, 190);
poly.addPoint(50, 100);
poly.addPoint(10, 10);
g.setPaint(gradient);
g.fill(poly);
g.setPaint(Color.BLACK);
g.draw(poly);
}
};
up.setBackground(Color.RED);
up.setBounds(150, 150, 200, 200);
layers.add(up, new Integer(2));
getContentPane().add(layers, BorderLayout.CENTER);
JPanel buttonPanel = new JPanel(new FlowLayout(FlowLayout.CENTER));
toggleOpaque = new JButton("Toggle Opaque");
toggleOpaque.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
up.setOpaque(!up.isOpaque());
layers.repaint();
}
});
buttonPanel.add(toggleOpaque);
getContentPane().add(buttonPanel, BorderLayout.EAST);
}
public static void main(String[] args) {
JFrame f = new OpaqueExample();
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.setSize(500, 500);
f.setLocationRelativeTo(null);
f.setVisible(true);
}
}
