Hands-on notes - the basic usage of OpenCV

illustrate

The following operations are demonstrated: image reading, display, pixel traversal, copy, assignment, etc. Most of the comments are already written in the code. When compiling the program, you need to add the OpenCV header file in CMakeLists.txt, and then link the program to the library file. At the same time, due to the use of C++11 standards (such as nullptr and chrono), some compiler settings are also required.

CMakeLists.txt key content

cmake_minimum_required(VERSION 2.8)

set(CMAKE_BUILD_TYPE "Release")
# 添加c++ 11标准支持
set(CMAKE_CXX_FLAGS "-std=c++11 -O2")

# Eigen
include_directories("/usr/include/eigen3")

# 寻找OpenCV库
find_package(OpenCV REQUIRED)
# 添加头文件
include_directories(${OpenCV_INCLUDE_DIRS})

add_subdirectory(imageBasics)

source code

#include <iostream>
#include <chrono>

using namespace std;

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>

int main(int argc, char **argv) {
  // 读取argv[1]指定的图像
  cv::Mat image;
  image = cv::imread(argv[1]); //cv::imread函数读取指定路径下的图像

  // 判断图像文件是否正确读取
  if (image.data == nullptr) { //数据不存在,可能是文件不存在
    cerr << "文件" << argv[1] << "不存在." << endl;
    return 0;
  }

  // 文件顺利读取, 首先输出一些基本信息
  cout << "图像宽为" << image.cols << ",高为" << image.rows << ",通道数为" << image.channels() << endl;
  cv::imshow("image", image);      // 用cv::imshow显示图像
  cv::waitKey(0);                  // 暂停程序,等待一个按键输入

  // 判断image的类型
  if (image.type() != CV_8UC1 && image.type() != CV_8UC3) {
    // 图像类型不符合要求
    cout << "请输入一张彩色图或灰度图." << endl;
    return 0;
  }

  // 遍历图像, 请注意以下遍历方式亦可使用于随机像素访问
  // 使用 std::chrono 来给算法计时
  chrono::steady_clock::time_point t1 = chrono::steady_clock::now();
  for (size_t y = 0; y < image.rows; y++) {
    // 用cv::Mat::ptr获得图像的行指针
    unsigned char *row_ptr = image.ptr<unsigned char>(y);  // row_ptr是第y行的头指针
    for (size_t x = 0; x < image.cols; x++) {
      // 访问位于 x,y 处的像素
      unsigned char *data_ptr = &row_ptr[x * image.channels()]; // data_ptr 指向待访问的像素数据
      // 输出该像素的每个通道,如果是灰度图就只有一个通道
      for (int c = 0; c != image.channels(); c++) {
        unsigned char data = data_ptr[c]; // data为I(x,y)第c个通道的值
      }
    }
  }
  chrono::steady_clock::time_point t2 = chrono::steady_clock::now();
  chrono::duration<double> time_used = chrono::duration_cast < chrono::duration < double >> (t2 - t1);
  cout << "遍历图像用时：" << time_used.count() << " 秒。" << endl;

  // 关于 cv::Mat 的拷贝
  // 直接赋值并不会拷贝数据
  cv::Mat image_another = image;
  // 修改 image_another 会导致 image 发生变化
  image_another(cv::Rect(0, 0, 100, 100)).setTo(0); // 将左上角100*100的块置零
  cv::imshow("image", image);
  cv::waitKey(0);

  // 使用clone函数来拷贝数据
  cv::Mat image_clone = image.clone();
  image_clone(cv::Rect(0, 0, 100, 100)).setTo(255);
  cv::imshow("image", image);
  cv::imshow("image_clone", image_clone);
  cv::waitKey(0);

  // 对于图像还有很多基本的操作,如剪切,旋转,缩放等,限于篇幅就不一一介绍了,请参看OpenCV官方文档查询每个函数的调用方法.
  cv::destroyAllWindows();
  return 0;
}