1. 直方图概念
在24课有详细介绍,此就不做赘述。
1.1 直方图常见的几个属性:
- dims 表示维度,对灰度图像来说只有一个通道值dims=1
- bins 表示在维度中子区域大小划分,bins=256,划分为256个级别
- range 表示值得范围,灰度值范围为[0~255]之间
2. 相关API
2.1 split()
- 把多通道图像分为多个单通道图像
split(
const Mat &src, //输入图像
Mat* mvbegin)// 输出的通道图像数组
2.2 直方图计算calcHist()
calcHist(
const Mat* images,//输入单通道图像指针,
int images,// 图像数目,设为1即可
const int* channels,// 通道数,设为0即可
InputArray mask,// 输入mask,计算掩模内的直方图,设为Mat()即可
OutputArray hist,//输出的直方图数据,为一个Mat型变量
int dims,// 维数,需要统计直方图通道的个数
const int* histsize,// 直方图级数,指的是直方图分成多少个区间,就是 bin的个数
const float* ranges,// 值域范围
bool uniform = true,// 是否对得到的直方图数组进行归一化处理
bool accumulate = false// 在多个图像时,是否累计计算像素值得个数
)
3.例程
#include <opencv2/opencv.hpp>
#include <iostream>
#include <math.h>
using namespace std;
using namespace cv;
int main(int argc, char** argv) {
Mat src = imread("D:/resource/images/face.jpg");
if (!src.data) {
printf("could not load image...\n");
return -1;
}
char INPUT_T[] = "input image";
char OUTPUT_T[] = "histogram demo";
namedWindow(INPUT_T, WINDOW_AUTOSIZE);
namedWindow(OUTPUT_T, WINDOW_AUTOSIZE);
imshow(INPUT_T, src);
// 分通道
vector<Mat> bgr_planes;
split(src, bgr_planes);
// 计算直方图
int histSize = 256;
float range[] = { 0, 256 };
const float *histRanges = { range };
Mat b_hist, g_hist, r_hist;
calcHist(&bgr_planes[0], 1, 0, Mat(), b_hist, 1, &histSize, &histRanges, true, false);
calcHist(&bgr_planes[1], 1, 0, Mat(), g_hist, 1, &histSize, &histRanges, true, false);
calcHist(&bgr_planes[2], 1, 0, Mat(), r_hist, 1, &histSize, &histRanges, true, false);
// 建立直方图画布并归一化
int hist_h = 400;
int hist_w = 512;
int bin_w = hist_w / histSize;
Mat histImage(hist_w, hist_h, CV_8UC3, Scalar(0, 0, 0));
normalize(b_hist, b_hist, 0, hist_h, NORM_MINMAX, -1, Mat());
normalize(g_hist, g_hist, 0, hist_h, NORM_MINMAX, -1, Mat());
normalize(r_hist, r_hist, 0, hist_h, NORM_MINMAX, -1, Mat());
// render histogram chart,画直方图
for (int i = 1; i < histSize; i++) {
line(histImage, Point((i - 1)*bin_w, hist_h - cvRound(b_hist.at<float>(i - 1))),
Point((i)*bin_w, hist_h - cvRound(b_hist.at<float>(i))), Scalar(255, 0, 0), 2, LINE_AA);
line(histImage, Point((i - 1)*bin_w, hist_h - cvRound(g_hist.at<float>(i - 1))),
Point((i)*bin_w, hist_h - cvRound(g_hist.at<float>(i))), Scalar(0, 255, 0), 2, LINE_AA);
line(histImage, Point((i - 1)*bin_w, hist_h - cvRound(r_hist.at<float>(i - 1))),
Point((i)*bin_w, hist_h - cvRound(r_hist.at<float>(i))), Scalar(0, 0, 255), 2, LINE_AA);
}
imshow(OUTPUT_T, histImage);
waitKey(0);
return 0;
}