图像直方图

直方图用来查看图像整体的变化形式，其具有平移不变性，放缩不变性的特点。

#include <opencv2\opencv.hpp>
#include <iostream>
using namespace cv;
using namespace std;
int main()
{
    Mat img = imread("apple.jpg");
    if (img.empty())
    {
        cout << "请确认图像文件名称是否正确" << endl;
        return -1;
    }
    Mat gray;
    cvtColor(img, gray, COLOR_BGR2GRAY);
    //设置提取直方图的相关变量
    Mat hist;  //用于存放直方图计算结果
    const int channels[1] = { 0 };  //通道索引
    float inRanges[2] = { 0,255 };
    const float* ranges[1] = { inRanges };  //像素灰度值范围
    const int bins[1] = { 256 };  //直方图的维度，其实就是像素灰度值的最大值
    calcHist(&gray, 1, channels, Mat(), hist, 1, bins, ranges);  //计算图像直方图
                                                                //准备绘制直方图
    int hist_w = 512;
    int hist_h = 400;
    int width = 2;
    Mat histImage = Mat::zeros(hist_h, hist_w, CV_8UC3);
    for (int i = 1; i <= hist.rows; i++)
    {
        rectangle(histImage, Point(width*(i - 1), hist_h - 1),
            Point(width*i - 1, hist_h - cvRound(hist.at<float>(i - 1) / 15)),
            Scalar(255, 255, 255), -1);
    }
    namedWindow("histImage", WINDOW_AUTOSIZE);
    imshow("histImage", histImage);
    imshow("gray", gray);
    waitKey(0);
    return 0;
}