java - 如何使用 Graphics2D 类绘制字符串？

Question

您好，我正在开发一个简单的应用程序，用于从无人机获取视频反馈。所以这是我的问题。我不知道为什么我无法在面板上显示“无视频连接”字符串。知道这里发生了什么吗？当我运行这个框架时，它只向我显示一个没有任何内容的普通空框架。谢谢。

代码:

package dronetest;

import com.codeminders.ardrone.ARDrone;
import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.image.BufferedImage;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicReference;
import javax.swing.*;

public class testStringVideo extends javax.swing.JFrame {

    private AtomicReference<BufferedImage> image = new AtomicReference<BufferedImage>();
    private AtomicBoolean preserveAspect = new AtomicBoolean(true);
    private BufferedImage noConnection = new BufferedImage(500, 500, BufferedImage.TYPE_INT_RGB);
    /**
     * Creates new form testStringVideo
     */
    public testStringVideo() {
        initComponents();

        Graphics2D g2d = (Graphics2D) noConnection.getGraphics();
        Font f = g2d.getFont().deriveFont(24.0f);
        System.err.println(f);

        g2d.drawString("No video connection", 40, 100); 
        image.set(noConnection);
        System.err.println(image);
    }

    public void setDrone(ARDrone drone) {
        drone.addImageListener(this);
    }

    public void setPreserveAspect(boolean preserve) {
        preserveAspect.set(preserve);
    }

    public void frameReceived(BufferedImage im) {
        image.set(im);
        repaint();
    }

    public void paintComponent(Graphics g) {
        Graphics2D g2d = (Graphics2D) g;
        g2d.setRenderingHint(RenderingHints.KEY_TEXT_ANTIALIASING, RenderingHints.VALUE_TEXT_ANTIALIAS_ON);
        g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        int width = getWidth();
        int height = getHeight();
        drawDroneImage(g2d, width, height);
    }

    private void drawDroneImage(Graphics2D g2d, int width, int height) {
        BufferedImage im = image.get();
        if (im == null) {
            return;
        }
        int xPos = 0;
        int yPos = 0;
        if (preserveAspect.get()) {
            g2d.setColor(Color.BLACK);
            g2d.fill3DRect(0, 0, width, height, false);
            float widthUnit = ((float) width / 4.0f);
            float heightAspect = (float) height / widthUnit;
            float heightUnit = ((float) height / 3.0f);
            float widthAspect = (float) width / heightUnit;

            if (widthAspect > 4) {
                xPos = (int) (width - (heightUnit * 4)) / 2;
                width = (int) (heightUnit * 4);
            } else if (heightAspect > 3) {
                yPos = (int) (height - (widthUnit * 3)) / 2;
                height = (int) (widthUnit * 3);
            }
        }
        if (im != null) {
            g2d.drawImage(im, xPos, yPos, width, height, null);
        }
    }  

    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                        javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(testStringVideo.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(testStringVideo.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(testStringVideo.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(testStringVideo.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new testStringVideo().setVisible(true);
            }
        });
    }
    // Variables declaration - do not modify                     
    private javax.swing.JPanel jPanel1;
    // End of variables declaration                   
}  

Answer 1

一般来说，您不会直接调用getGraphics():

Graphics2D g2d = (Graphics2D) noConnection.getGraphics();

这为您提供了一个指向 Graphics 对象的指针，但不是在绘制窗口时提供的。

相反，您应该重写内部 JLabel 的 paintComponent() 方法，例如 this tutorial演示(注意其他人建议的 super.paintComponent() 调用)。

public void paintComponent(Graphics g) {
  super.paintComponent(g);       

  // Draw Text
  g.drawString("This is my custom Panel!",10,20);
} 

paintComponent() 方法由 Swing 调用 Event Dispatch Thread当 Swing 准备好绘制该组件时，传递一个 Graphics 实例。

Oracle 网站(以及其他地方)上有许多有用的教程，其中包含更多详细信息:

• Performing Custom Painting
• Overview of the Java 2D API Concepts
• Working with Text APIs

请务必阅读这些内容； Swing 在幕后为您做了很多工作。它可以节省你很多时间，但如果你不明白它在做什么，你就会与它作斗争，而不是让它为你工作。