java把彩色图片变成黑白图片

package cn.moqi.test;

import java.awt.image.BufferedImage;
import java.io.File;

import javax.imageio.ImageIO;

public class Test {

    public static void main(String[] args) throws Exception {

        BufferedImage img = ImageIO.read(new File("d:/id_card.jpg"));

        final int width = img.getWidth();
        final int height = img.getHeight();

        for (int i = 0; i < width; i++) {

            for (int j = 0; j < height; j++) {

                int rgb = img.getRGB(i, j);
                img.setRGB(i, j, grayRGB(Integer.toHexString(rgb)));
            }

        }

        ImageIO.write(img, "jpg", new File("d:/id.jpg"));
    }

    private static int grayRGB(String argb) {
        //ARGB前两位是透明度,后面开始是RGB
        int r = Integer.parseInt(argb.substring(2, 4), 16);
        int g = Integer.parseInt(argb.substring(4, 6), 16);
        int b = Integer.parseInt(argb.substring(6, 8), 16);
        //平均值
        String average = Integer.toHexString((r + g + b) / 3);

        if (average.length() == 1) {
            average = "0" + average;
        }
        //RGB都变成平均值
        return Integer.parseInt(average + average + average, 16);
    }

}