如何将图像转换为灰度而不丢失透明度？

我在将带有一些透明像素的彩色图像转换为灰度时遇到问题。我已经在该网站上搜索并找到了相关问题，但我无法用来解决我的问题。

我定义了一个方法“convertType”，如下所示：

/*------------------------------
attempts to convert the type of the image argument to the chosen type
for example: BufferedImage newImage= ImageUtilities.convertType(oldImage, BufferedImage.TYPE_3BYTE_BGR);
11/28/2013 WARNING-it remains to be seen how well this method will work for various type conversions
------------------------------*/
public static BufferedImage convertType (BufferedImage image,int newType){
 if (image.getType() == newType){
   return (image);
 }else {
    int w= image.getWidth();
    int h= image.getHeight ();
    BufferedImage modifiedImage = new BufferedImage( w,h,newType);
    Graphics2D g = ( Graphics2D)modifiedImage.getGraphics();
    g.drawImage( image, null, 0,0);
    g.dispose();
    return (modifiedImage);
}
}

我从 BufferedImage 类型开始TYPE_4BYTE_ABGR命名为“结果”，然后：

 result= convertType (result, BufferedImage.TYPE_BYTE_GRAY);//transparency is lost
 result=convertType (result, BufferedImage.TYPE_INT_ARGB);//pixels will now support transparency

对于原始图像中的彩色不透明像素，上述序列工作正常： 例如，(32, 51, 81, 255)-> (49, 49, 49, 255)

然而，原始图像中的透明像素变成不透明： 例如，(0, 0, 0, 0)-> (0, 0, 0, 255)

我明白发生了什么，如果我可以利用用于转换为灰度的 Java 算法，我就可以解决这个问题。我已经下载了源代码并四处查看，但无法找到该算法。如果有人可以：我将不胜感激：

1. 告诉我哪个类包含灰度转换算法或
2. 建议一种替代方法来完成我正在尝试做的事情。

您是否考虑过使用ColorConvertOp filter?

例如...

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.EventQueue;
import java.awt.GridLayout;
import java.awt.color.ColorSpace;
import java.awt.image.BufferedImage;
import java.awt.image.ColorConvertOp;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class TestAlphaGrayScale {

    public static void main(String[] args) {
        new TestAlphaGrayScale();
    }

    public TestAlphaGrayScale() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new BorderLayout());
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private BufferedImage master;
        private BufferedImage gray;

        public TestPane() {
            setBackground(Color.RED);
            try {
                master = ImageIO.read(new File("C:\\hold\\thumbnails\\Miho_Alpha.png"));
                gray = new BufferedImage(master.getWidth(), master.getHeight(), BufferedImage.TYPE_INT_ARGB);

                // Automatic converstion....
                ColorConvertOp op = new ColorConvertOp(ColorSpace.getInstance(ColorSpace.CS_GRAY), null);
                op.filter(master, gray);

                setLayout(new GridLayout(1, 2));

                add(new JLabel(new ImageIcon(master)));
                add(new JLabel(new ImageIcon(gray)));
            } catch (IOException exp) {
                exp.printStackTrace();
            }
        }
    }

}