OpenCV introduction

• Reference
  • https://opencv.org/releases/
  • https://docs.opencv.org/4.6.0/
• Environment Setup
  https://hackmd.io/4TXk4tknSMmlW77bZCzZtA#Additional-Environment-Setup-If-Required

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Example 1: Display an Image

Python Code

import cv2 as cv
import sys

img = cv.imread(cv.samples.findFile("starry_night.jpg"))

if img is None:
    sys.exit("Could not read the image.")
    
cv.imshow("Display window", img)

k = cv.waitKey(0)

if k == ord("s"):
    cv.imwrite("starry_night.png", img)

With C++

1. Sample Code

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

using namespace cv;

int main()
{
    std::string image_path = samples::findFile("starry_night.jpg");
    Mat img = imread(image_path, IMREAD_COLOR);
    if(img.empty())
    {
        std::cout << "Could not read the image: " << image_path << std::endl;
        return 1;
    }
    
    imshow("Display window", img);
    
    int k = waitKey(0); // Wait for a keystroke in the window
    if(k == 's')
    {
        imwrite("starry_night.png", img);
    }
    
    return 0;
}

2. Create a CMake file, known as CMakeLists.txt
   https://docs.opencv.org/4.6.0/db/df5/tutorial_linux_gcc_cmake.html

   cmake_minimum_required(VERSION 2.8)
   project(DisplayImage)
   find_package(OpenCV REQUIRED)
   include_directories(${OpenCV_INCLUDE_DIRS})
   add_executable(DisplayImage DisplayImage.cpp)
   target_link_libraries(DisplayImage ${OpenCV_LIBS})

3. Build the executable file
   https://docs.opencv.org/4.6.0/db/df5/tutorial_linux_gcc_cmake.html

   cd <DisplayImage_directory>
   cmake .
   make

Results

• Display the image
  

  

• Key 's' to save it
  

  

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Example 2: Capture a Video

Python Code

import numpy as np
import cv2 as cv

cap = cv.VideoCapture(0)
if not cap.isOpened():
    print("Cannot open camera")
    exit()

while True:
    # Capture frame-by-frame
    ret, frame = cap.read()
    
    # if frame is read correctly ret is True
    if not ret:
        print("Can't receive frame (stream end?). Exiting ...")
        break

    # Our operations on the frame come here
    gray = cv.cvtColor(frame, cv.COLOR_BGR2GRAY)

    # Display the original frame
    cv.imshow('color frame', frame)

    # Display the resulting frame
    cv.imshow('gray frame', gray)
    if cv.waitKey(1) == ord('q'):
        break

# When everything done, release the capture
cap.release()
cv.destroyAllWindows()

With C++

1. Sample Code
   Modified from https://docs.opencv.org/4.x/d8/dfe/classcv_1_1VideoCapture.html

#include <iostream> // for standard I/O
#include <opencv2/core.hpp>     // Basic OpenCV structures (cv::Mat)
#include <opencv2/videoio.hpp>  // Video I/O
#include <opencv2/highgui.hpp>  // for gui applications, like imshow()
#include <opencv2/gapi/core.hpp>  // for image processing

using namespace std;
using namespace cv;


int main()
{
    VideoCapture cap_vid;              
    
    int cam_id = 0;  // Camera ID

    cap_vid.open(cam_id);  // Open video

    if (!cap_vid.isOpened())
    {
        cout  << "Could not open the camera, id: " << cam_id << endl;
        return -1;
    }

    Mat frame;
    Mat gray;

    do
    {
        // Capture frames
        bool ret = cap_vid.read(frame);
        if (!ret) {
            cout << "Can't receive frame (stream end?). Exiting ...\n";
            break;
        }
        
        // Color to gray
        cvtColor(frame, gray, COLOR_BGR2GRAY);

        // Display live frames
        imshow("Live", frame);  // Color
        // imshow("live", gray);  // Gray
    } while (waitKey(30) != 'q');
    
    return 0;
}

2. Create a CMake file, known as CMakeLists.txt

cmake_minimum_required(VERSION 2.8)
project(CaptureVideo)
find_package(OpenCV REQUIRED)
include_directories(${OpenCV_INCLUDE_DIRS})
add_executable(CaptureVideo CaptureVideo.cpp)
target_link_libraries(CaptureVideo ${OpenCV_LIBS})

3. Switch to shell and build the executable file

$ cd <CaptureVideo_directory> && mkdir build
$ cd build  # Collect all of the files generated by cmake.
$ cmake ../
$ make

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