Mat image = imread("D:/ju.jpg");

    imshow("素材图", image);

    int bins = 256;    //直条为256

    int hist_size[] = { bins };

    float range[] = { 0,256 };

    const float* ranges[] = { range };

    MatND redHist, greenHist, blueHist;

    //MAT数据为BGR

    int channels_r[] = { 2 }; //r通道

    calcHist(&image, 1, channels_r, Mat(), redHist, 1, hist_size, ranges, true, false);

    int channels_g[] = { 1 };

    calcHist(&image, 1, channels_g, Mat(), greenHist, 1, hist_size, ranges, true, false);

    int channels_b[] = { 0 };

    calcHist(&image, 1, channels_b, Mat(), blueHist, 1, hist_size, ranges, true, false);

    //准备参数绘制三色直方图

    double maxValue_red, maxValue_green, maxValue_blue;

    minMaxLoc(redHist, 0, &maxValue_red, 0, 0);

    minMaxLoc(greenHist, 0, &maxValue_green, 0, 0);

    minMaxLoc(blueHist, 0, &maxValue_blue, 0, 0);

    int scale = 1;

    int histHeight = 256;

    //bins * 3 是因为要绘制三个通道,每个通道的像素取值在 0-bins

    Mat histImage = Mat::zeros(histHeight, bins * 3, CV_8UC3);

    //开始绘制

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

        float binValue_red = redHist.at<float>(i);

        float binValue_green = greenHist.at<float>(i);

        float binValue_blue = blueHist.at<float>(i);

        //计算高度时的乘除与下面绘图的 histHeight - intensity 是为了便于显示，否则有的色度很低

        //要绘制的高度

        int intensity_red = cvRound(binValue_red * histHeight / maxValue_red);

        int intensity_green = cvRound(binValue_green * histHeight / maxValue_green);

        int intensity_blue = cvRound(binValue_blue * histHeight / maxValue_blue);

        rectangle(histImage, Point(i * scale, histHeight - 1),

            Point((i + 1) * scale - 1, histHeight - intensity_red),

            Scalar(255, 0, 0));

        rectangle(histImage, Point((i + bins) * scale, histHeight - 1),

            Point((i + bins + 1) * scale - 1, histHeight - intensity_green),

            Scalar(0, 255, 0));

        rectangle(histImage, Point((i + bins * 2) * scale, histHeight - 1),

            Point((i + bins * 2 + 1) * scale - 1, histHeight - intensity_blue),

            Scalar(0, 0, 255));

    }

    imshow("图像的 RGB 直方图", histImage);