Home

Tutorial: C++ basics for OpenCV

Tutorial: C++ basics for OpenCV

Tutorial: C++ basics

Deep Learning Image Processing. Updated. 2024.2

I. Introduction

The OpenCV Library has >2500 algorithms, extensive documentation, and sample code for real-time computer vision. You can see basic information about OpenCV at the following sites,

In this tutorial, you will learn fundamental concepts of the C++ language to use the OpenCV API. You will learn namespace, class, C++ syntax to use image reading, writing and displaying.

OpenCV Example Code Image File Read / Write / Display

#include <iostream>
#include <opencv.hpp>

using namespace std;
using namespace cv;

int main()
{
  /*  read image  */
  String filename1 = "image.jpg";  // class
  Mat img = imread(filename1);  //Mat class
  Mat img_gray = imread("image.jpg", 0);  // read in grayscale
  
  /*  write image  */
  String filename2 = "writeTest.jpg";  // C++ class/syntax (String, cout, cin)
  imwrite(filename2, img);
 
  /*  display image  */
  namedWindow("image", WINDOW_AUTOSIZE);
  imshow("image", img);
  
  namedWindow("image_gray", WINDOW_AUTOSIZE);
  imshow("image_gray", img_gray);
  
  waitKey(0);
}

C++ for OpenCV

OpenCV is provided in C++, Python, Java. We will learn how to use OpenCV in

  1. C++ (general image processing)

  2. Python (for Deep learning processing)

For C++, we need to learn

  • Basic C++ syntax

  • Class

  • Overloading, namespace, template

  • Reference

C++

C++ is a general-purpose programming language created by Bjarne Stroustrup as an extension of the C programming language. C++ is portable and can be used to develop applications that can be adapted to multiple platforms. You can see basic C++ tutorials in following site,

Project Workspace Setting

Create the lecture workspace as C:\Users\yourID\source\repos

  • e.g. C:\Users\ykkim\source\repos

Then, create sub-directories such as :

  • C:\Users\yourID\source\repos\DLIP

  • C:\Users\yourID\source\repos\DLIP\Tutorial

  • C:\Users\yourID\source\repos\DLIP\Include

  • C:\Users\yourID\source\repos\DLIP\Assignment

  • C:\Users\yourID\source\repos\DLIP\LAB

  • C:\Users\yourID\source\repos\DLIP\Image

Define and Declare Functions in Header Files

We will learn how to declare and define functions in the header file

Exercise 1

  1. Create a new C++ project in Visual Studio Community

    • Project Name: DLIP_Tutorial_C++_Ex1

    • Project Folder: C:\Users\yourID\source\repos\DLIP\Tutorial\

  2. Create a new C+ source file

    • File Name: DLIP_Tutorial_C++_Ex1.cpp

  3. Create new header files

    • File Names: TU_DLIP.h, TU_DLIP.cpp

    • Lib Folder: C:\Users\yourID\source\repos\DLIP\Include\

  4. Declare the sum function in the header file(TU_DLIP.h) as

int sum(int val1, int val2);

  1. Define the sum function in the header source file(TU_DLIP.cpp) as

int sum(int val1, int val2){...}

  1. Include the header file in the main source code of DLIP_Tutorial_C++_Ex1.cpp.

  2. Modify the source main code to print the sum value of any two numbers

  3. Compile and run.

//#include "TU_DLIP.h"
#include "../../../Include/TU_DLIP.h"

#include <iostream>

int main()
{
	// =============================
	// Exercise 1 :: Define Function
	// =============================
	int val1 = 11;
	int val2 = 22;

	// Add code here


	std::cout << out << std::endl;
}

C++ Class

Class is similar to C structure:

  • Structure: Cannot inclue functions. Only variables

  • Class: Can include variables, functions definition/declaration, other classes

#include <stdio.h>
#include <stdlib.h>

typedef struct {
  char number[20];
  char password[20];
  char name[20];
  int balance;
}Account;

Class Constructor

Constructor is special method automatically called when an object of a class is created.

  • Use the same name as the class, followed by parentheses ()

  • It is always public.

  • It does not have any return values.

class MyNum{
  public:
    MyNum();  // Constructor 1
    MyNum(int x);  // Constructor 2

    int num;
};

// Class Constructor 1
MyNum::MyNum(){}

// Class Constructor 2
MyNum::MyNum(int x)
{
  num = x;
}

int main(){
  // Creating object by constructor 1
  MyNum mynum;
  mynum.num = 10;

  // Creating object by constructor 2
  MyNum mynum2(10);
}

Mat Class in OpenCV

The image data are in forms of 1D, 2D, 3D arrays with values 0~255 or 0~1

OpenCV provides the Mat class for operating multi dimensional images

Example Printing out informations about the source image using OpenCV

#include "opencv.hpp"
#include <iostream>

using namespace cv;
using namespace std;

int main(int argc, char* argv[])
{
  cv::Mat src, gray, dst;
  src = cv::imread("testImage.jpg");

  if (src.empty())
    std::cout << "src is empty!!" << std::endl;

  // Print result
  std::cout << "is empty? \t: " << src.empty() << std::endl;
  std::cout << "channels \t: " << src.channels() << std::endl;
  std::cout << "row of src \t: " << src.rows << std::endl;
  std::cout << "col of src \t: " << src.cols << std::endl;
  std::cout << "type of src \t: " << src.type() << std::endl;

  cv::namedWindow("src");
  cv::imshow("src", src);

  cv::waitKey(0);
}

Results

Exercise 2

  1. Create a new C++ project in Visual Studio Community

  • Project Name: DLIP_Tutorial_C++_Ex2

  • Project Folder: C:\Users\yourID\source\repos\DLIP\Tutorial\

  1. Create a new C+ source file

  • File Name: DLIP_Tutorial_C++_Ex2.cpp

Create a Class 'MyNum'

  1. Modify the header file TU_DLIP.h and TU_DLIP.cpp to declare a class member named as MyNum.

  • Constructor : MyNum()

  • Member variables: val1, val2 // integer type

  • Member functions: int sum() // returns the sum of val1 and val2

  • Member functions: void print() // prints values of val1, val2, and sum

  1. Then, compile and run the program.

//#include "TU_DLIP.h"
#include "../../../Include/TU_DLIP.h"

int main()
{
	// =============================
	// Exercise 1: Define Function
	// =============================

	int val1 = 11;
	int val2 = 22;

	int out = sum(val1, val2);

	std::cout << out << std::endl;

	// ====================================
	// Exercise 2: Create a Class 'MyNum'
	// ====================================

	MyNum mynum(10, 20);
	mynum.print();

}

Solution

#ifndef _TU_DLIP_H		// same as "#if !define _TU_DLIP_H" (or #pragma once) 
#define _TU_DLIP_H

#include <iostream>

// =============================
// Exercise 1: Define Function
// =============================

int sum(int val1, int val2);


// ====================================
// Exercise 2: Create a Class "MyNum"
// ====================================

class MyNum 
{
	public:
		MyNum(int x1, int x2);
		int val1;
		int val2;
		int sum(void);
		void print(void);
};

#endif // !_TU_DLIP_H

Namespace

A namespace provides a scope to the identifiers (the names of types, functions, variables, etc) inside it.

  • Uses :: as scope resolution operator

  • Use namespace in order to avoid collision using functions with the same name e.g. KimHandong --> Student::KimHandong, TA::KimHandong

// Method 1. Calling specific function(recommended)
int main(void){
  project_A::add_value(3, 7);
  project_A::subtract_value(10, 2);
  return 0;
}

// Method 2. Calling all functions in the namespace
using namespace project_A;

int main(void){
  add_value(3, 7);
  subtract_value(10, 2);
  return 0;
}

  • std::cout, std::cin, std::endl are also defined in iostream

// Method 1
std::cout<<"print this"<<std::endl;

// Method 2
using namespace std
cout<<"print this"<<endl;

Exercise 3

Create another Class 'MyNum'

In this exercise, you create the MyNum class, previously implemented in Exercise 2, with the same class name in different namespaces, proj\_A, and proj\_B.

  1. Create a new C++ project in Visual Studio Community

  • Project Name: DLIP_Tutorial_C++_Ex3

  • Project Folder: C:\Users\yourID\source\repos\DLIP\Tutorial\

  1. Create a new C+ source file

  • File Name: DLIP_Tutorial_C++_Ex3.cpp

  1. Modify the header file TU_DLIP.h and TU_DLIP.cpp to declare two class members named as MyNum in proj\_A and proj\_B.

  2. Use namespace to identify two classes clearly

    • First MyNum class: namespace name proj_A

    • Second MyNum class: namespace name proj_B

  3. Also, declare class member variables for each MyNum class: Constructor / val1 / val2 / val3 / sum / print

    • Constructor MyNum(int in1, int in2, int in3): A constructor for specifying values val1, val2, val3

    • val1, val2, val3: member variable of integer type

    • sum(void): member function that returns the sum of val1, val2, and val3

    • `print(void): member function that prints val1, val2, val3, and sum

#include "../../../Include/TU_DLIP.h"
#include "TU_DLIP.h"

void main()
{
	proj_A::MyNum mynum1(1, 2, 3);
	proj_B::MyNum mynum2(4, 5, 6);

	mynum1.print();
	mynum2.print();

	system("pause");
}
#ifndef _TU_DLIP_H		// same as "#if !define _TU_DLIP_H" (or #pragma once) 
#define _TU_DLIP_H

#include <iostream>

// =============================
// Exercise 1: Define Function
// =============================

int sum(int val1, int val2);

// ====================================
// Exercise 2: Create a Class "MyNum"
// ====================================

class MyNum
{
public:
	MyNum(int x1, int x2);
	int val1;
	int val2;
	int sum(void);
	void print(void);
};

// ======================================================
// Exercise 3: Create two Class "MyNum" in proj_A, proj_B
// ======================================================
namespace proj_A
{
	class MyNum
	{
	public:
		MyNum(int x1, int x2, int x3);
		int val1;
		int val2;
		int val3;
		int sum(void);
		void print(void);
	};
}

namespace proj_B
{
	class MyNum
	{
	public:
		MyNum(int x1, int x2, int x3);
		int val1;
		int val2;
		int val3;
		int sum(void);
		void print(void);
	};
}
#endif // !_TU_DLIP_H

Template

A template can make a variable type(int, float, char..) as a variable. How can you use the same function but with a different number type as the input argument? : add(float A, float B), add(int A, int B) &rarr add(T A, T B) where T=int or T=float

Function Overloading

Functions with the same name (but with different types or number of parameters) can be defined.

  • Different return type (with everything else the same) is not a function overloading.

Example

cv::Mat can be created in many different ways. Use the up or down the keyboard to see what the options are.

Function Overloading Reference

#include <opencv2/opencv.hpp>
#include <iostream>

cv::Mat cvtGray(const cv::Mat color);
void cvtGray(cv::Mat color, cv::Mat & gray);

void main() {
	cv::Mat src, gray, dst;
	src = cv::imread("image.jpg");

	if (src.empty())
		std::cout << "src is empty!!" << std::endl;

	cvtGray(src, gray);

	cv::namedWindow("src");
	cv::imshow("src", src);

	cv::namedWindow("gray");
	cv::imshow("gray", gray);

	cv::waitKey(0);
}

cv::Mat cvtGray(cv::Mat color)
{
	cv::Mat gray;
	color = cv::Mat::zeros(color.size(), CV_8UC3);
	cv::cvtColor(color, gray, CV_RGB2GRAY);
	return gray;
}

void cvtGray(cv::Mat color, cv::Mat gray)
{
	cv::cvtColor(color, gray, CV_RGB2GRAY);
	cv::namedWindow("inside_function");
	cv::imshow("inside_function", gray);
}

Default Parameter

Default parameter in OpenCV

Pointer

A pointer is a variable whose value is the address of another variable.

What are Pointers?

A pointer is a variable whose value is the address of another variable. i.e. direct address of the memory locations Pointers are the basis for data structures.

  • Define a pointer variable int *ptr;

  • Assign the address of a variable to a pointer ptr = &var

  • Access the value at the address available in the pointer variable int value = *ptr

Example

#include <iostream>

void swap(int *a, int *b);

int main() {
	int val1 = 10;
	int val2 = 20;

	printf("Before SWAP operation \n");
	printf("val1: %d \n", val1);
	printf("val2: %d \n", val2);

	swap(&val1, &val2);

	printf("After SWAP operation \n");
	printf("val1: %d \n", val1);
	printf("val2: %d \n", val2);

	system("pause");
	return 0;
}

void swap(int *a, int *b) {
	int temp = *a;
	*a = *b;
	*b = temp;
}

Reference

Reference can replace the use of pointer *

  • The disadvantage of reference is "We do not know if a function is call-by-value or call-by-reference"

// C/C++ syntax (Pointer)
#include <iostream>

void swap(int *a, int *b){
	int temp = *a;
	*a = *b;
	*b = temp;
}

int main(void){
	int value1 = 10;
	int value2 = 20;

	std::cout << "value1 :" << value1 << "   ";
	std::cout << "value2 :" << value2 << std::endl;

	swap(&value1, &value2);

	std::cout << "value1 :" << value1 << "   ";
	std::cout << "value2 :" << value2 << std::endl;

	return 0;
}

// C++ only (Reference)
#include <iostream>

void swap(int &a, int &b){
	int temp = a;
	a = b;
	b = temp;
}

int main(void){
	int value1 = 10;
	int value2 = 20;

	std::cout << "value1 :" << value1 << "   ";
	std::cout << "value2 :" << value2 << std::endl;

	swap(value1, value2);

	std::cout << "value1 :" << value1 << "   ";
	std::cout << "value2 :" << value2 << std::endl;

	return 0;
}
PreviousTutorial: Installation for OpenCV C++NextTutorial: OpenCV (C++) Basics

Last updated