BufferedImage转int[]再转回后部分灰度图变黑灰色的问题排查

我正在基于图像处理核的维基百科内容，进行卷积矩阵图片滤波的基础实验。为计算RGB变换，我通过BufferedImage读取位图，调用getRgb()获取像素数组。测试恒等滤镜时发现，部分图片原本的黑色变成了灰色，另一部分图片则显示正常。进一步测试发现，即使不做任何变换，仅执行BufferedImage -> int[] -> BufferedImage的操作也会出现该灰度异常问题。

使用ImageMagick的identify命令查看图片信息：

betty1.png PNG 339x600 339x600+0+0 8-bit Gray 256c 24526B 0.000u 0:00.000
betty2.jpg JPEG 603x797 603x797+0+0 8-bit Gray 256c 126773B 0.000u 0:00.001

两张图均为无Alpha通道的8位256色灰度图。其中一张处理后结果正常：

另一张处理后黑色变为灰色：

以下是复现该问题的SSCCE测试类：

import java.awt.BorderLayout;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.WindowConstants;

/* simple test class for convolution matrix */
public class CopyPic {

    public static void main(String args[]) throws FileNotFoundException, IOException {
        if (args.length < 1) {
            System.err.println("Usage: CopyPic <picture_file>");
            System.exit(1);
        }
        String        imgPath   = args[0];
        String        inputName = imgPath.substring(0, imgPath.lastIndexOf("."));
        File          ifile     = new File(imgPath);
        InputStream   fis_in    = new FileInputStream(ifile);
        BufferedImage bi_in     = ImageIO.read(fis_in);
        fis_in.close();

        int width  = bi_in.getWidth();
        int height = bi_in.getHeight();
        System.out.println(String.format("%s = %d x %d", imgPath, width, height));
        int[] rgb_in = new int[width * height];
        bi_in.getRGB(0, 0, width, height, rgb_in, 0, width);

        BufferedImage bi_out = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
        // for (int y = 0; y < height; y++) {
            // for (int x = 0; x < width; x++) {
                // bi_out.setRGB(x, y, rgb_out[y * width + x]);
            // }
        // }
        bi_out.setRGB(0, 0, width, height, rgb_in, 0, width);
        
        display(bi_in, bi_out);
        
        String outputName = inputName + "-copy.png";
        File         ofile   = new File(outputName);
        OutputStream fos_out = new FileOutputStream(ofile);
        ImageIO.write(bi_out, "PNG", fos_out);
        fos_out.flush();
        fos_out.close();
        System.out.println("Wrote " + outputName);
    }
    // use that to have internal viewer
    private static JFrame frame;
    private static JLabel label1, label2;
    private static void display(BufferedImage img1, BufferedImage img2) {
        if (frame == null) {
            frame = new JFrame();
            frame.setTitle(String.format("%dx%d Original / Copy", img1.getWidth(), img1.getHeight()));
            frame.setSize(img1.getWidth() + img2.getWidth(), img1.getHeight());
            frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
            label1 = new JLabel();
            label1.setIcon(new ImageIcon(img1));
            frame.getContentPane().add(label1, BorderLayout.WEST);
            label2 = new JLabel();
            label2.setIcon(new ImageIcon(img2));
            frame.getContentPane().add(label2, BorderLayout.EAST);
            frame.setLocationRelativeTo(null);
            frame.pack();
            frame.setVisible(true);
        } else {
            label1.setIcon(new ImageIcon(img1));
            label2.setIcon(new ImageIcon(img2));
        }
    }

}

内容的提问来源于stack exchange，提问作者Seki