1、calcHist三种函数原型：opencv中自带了求算图像直方图的函数calhist()，函数原型如下：①void calcHist( const Mat* images, int nimages, const int* channels, InputArray mask, OutputArray hist, int dims, const int* histSize, const float** ranges, bool uniform=true, bool accumulate=false );②void calcHist( const Mat* images, int nimages, const int* channels, InputArray mask, SparseMat& hist, int dims, const int* histSize, const float** ranges, bool uniform=true, bool accumulate=false );③void calcHist( InputArrayOfArrays images, const vector<int>& channels, InputArray mask, OutputArray hist, const vector<int>& histSize, const vector<float>& ranges, bool accumulate=false );
2、calcHist参数详解：以第一个函数原型为例：①const Mat* images：为输入图像的指针；②int nimages：要计算直方图的图像的个数。此函数可以为多图像求直方图，我们通常情况下都只作用于单一图像，所以通常nimages=1。③const int* channels：图像的通道是一个数组，如果是灰度图像则channels[1]={0};如果是彩色图像则channels[3]={0,1,2}；如果只是求彩色图像第2个通道的直方图则channels[1]={1};④IuputArray mask：掩膜，仅对指定元素操作，默认情况设置为一个空图像，即：Mat()。⑤OutArray hist：计算得到的直方图⑥int dims：得到的直方图的维数，灰度图像为1维，彩色图像为3维⑦const int* histSize：直方图横坐标的区间数。如果是10，则它会横坐标分为10份，然后统计每个区间的像素点总和⑧const float** ranges：这是一个二维数组，用来指出每个区间的范围⑨uniform参数表明直方图是否等距，⑩accumulate与多图像下直方图的显示与存储有关；
3、注意的地方：关于calHist计算直方图的结果hist，hist的结果为统计的灰度级0,1，…，255的数量；表示一维或三维的矩阵；对于直方图结果矩阵，访问形式为：cout<<hist.at<float>(i)<<endl;
4、灰度图的直方图绘制：#include <opencv2\opencv.hpp>#include <opencv2\highgui\highgui.hpp>#include <opencv2\features2d\features2d.hpp>#include <opencv2\core\core.hpp>using namespace std;using namespace cv;int main(){ Mat srcImage = imread("mask.jpg"); imshow("【原图】", srcImage); int channels = 0; MatND dstHist; //接下来是直方图的每一个维度的柱条的数目 int histSize[] = { 256 }; //如果这里写成int histSize = 256; //那么下面调用计算直方图的函数的时候，该变量需要写 &histSize //定义变量用来存储单个维度的数值的取值范围 float midRanges[] = { 0, 256 }; const float *ranges[] = { midRanges }; calcHist(&srcImage, 1, &channels, Mat(), dstHist, 1, histSize, ranges, true, false); //calcHist 函数调用结束后， //dstHist变量中将储存了直方图的信息; //用dstHist的模版函数 at<Type>(i)得到第i个柱条的值 //at<Type>(i, j)得到第i个并且第j个柱条的值 Mat drawImage = Mat::zeros(Size(256, 256), CV_8UC3); double g_dHistMaxValue; minMaxLoc(dstHist, 0, &g_dHistMaxValue, 0, 0); for (int i = 0; i < 256; i++) { int value = cvRound(dstHist.at<float>(i) * 256 * 0.9 / g_dHistMaxValue); line(drawImage, Point(i, drawImage.rows - 1), Point(i, drawImage.rows - 1 - value), Scalar(255, 0, 0)); } imshow("【直方图】", drawImage); waitKey(0); return 0; }
5、彩色直方图的绘制：int main(){ Mat srcImage = imread("mask.jpg"); imshow("【原图】", srcImage); int channels = 0; MatND dstHist; int histSize = 256 ; float midRanges[] = { 0, 256 }; const float *ranges[] = { midRanges }; calcHist(&srcImage, 1, &channels, Mat(), dstHist, 1, &histSize, ranges, true, false); //绘制蓝色通道 Mat b_drawImage = Mat::zeros(Size(256, 256), CV_8UC3); double g_dHistMaxValue = 0; minMaxLoc(dstHist, 0, &g_dHistMaxValue, 0, 0); for (int i = 0; i < histSize; i++) { int value = cvRound(dstHist.at<float>(i) * 256 * 0.9 / g_dHistMaxValue); rectangle(b_drawImage, Point(i * 8, b_drawImage.rows - 1), Point(i * 8 + 256 / histSize, b_drawImage.rows - 1 - value) ,Scalar(255, 0, 0), -1); } imshow("【蓝色通道的直方图】", b_drawImage); //绘制绿色通道 channels = 1; calcHist(&srcImage, 1, &channels, Mat(), dstHist, 1, &histSize, ranges, true, false); Mat g_drawImage = Mat::zeros(Size(256, 256), CV_8UC3); minMaxLoc(dstHist, 0, &g_dHistMaxValue, 0, 0); for (int i = 0; i < histSize; i++) { int value = cvRound(dstHist.at<float>(i) * 256 * 0.9 / g_dHistMaxValue); rectangle(g_drawImage, Point(i * 8, g_drawImage.rows - 1), Point(i * 8 + 256 / histSize, g_drawImage.rows - 1 - value), Scalar(0, 255, 0), -1); } imshow("【绿色通道的直方图】", g_drawImage); //绘制红色通道 channels = 2; calcHist(&srcImage, 1, &channels, Mat(), dstHist, 1, &histSize, ranges, true, false); Mat r_drawImage = Mat::zeros(Size(256, 256), CV_8UC3); minMaxLoc(dstHist, 0, &g_dHistMaxValue, 0, 0); for (int i = 0; i < histSize; i++) { int value = cvRound(dstHist.at<float>(i) * 256 * 0.9 / g_dHistMaxValue); rectangle(r_drawImage, Point(i * 8, r_drawImage.rows - 1), Point(i * 8 + 256 / histSize, r_drawImage.rows - 1 - value), Scalar(0, 0, 255), -1); } imshow("【红色通道的直方图】", r_drawImage); waitKey(0); return 0;}
6、【注】：calhist()函数求得的直方图返回值是一个一维或三维等的矩阵；不是二维；通过hist.at<float>(i)格式访问；
