OpenCV, MATLAB or AForge ??

Image processing is the process of manipulating image data in order to make it suitable for computer vision applications or to make it suitable to present it to humans. For example, changing brightness or contrast  is a image processing task which make the image visually pleasing for humans or suitable for further processing for a certain computer vision application.

Computer vision which go beyond image processing, helps to obtain relevant information from images and make decisions based on that information. In other words, computer vision is making the computer see as humans do. Basic steps for a typical computer vision application as follows.1. Image acquisition2. Image manipulation3. Obtaining relevant information4. Decision makingIf you are new to computer vision, you may be wondering where to start. First you have to understand the basic principles of image processing and computer vision. Then you have to choose a suitable language to develop your computer vision application. Some of the most popular methods are using OpenCV with C/C++, MATLAB and AForge. If you don't really know why you would choose one over the other, here is my explanation.

MATLAB is the most easiest and the inefficient way to process images and OpenCV is the most efficient and hardest way to process images. AForge has qualities in between OpenCV and MATLAB.

OpenCV has become hardest only because there is no proper documentation and error handling codes.  But OpenCV has lots of basic inbuilt image processing functions so that those who want to learn computer vision can develop their applications through proper understanding about what they do. 

So, I think that it is worthy to learn computer vision with OpenCV. Therefore in this blog, it is presented basic image processing functions and computer vision applications with line by line explanations.

Read & Display ImageRead Image from File and DisplayHere I am going to explain how to read an image from a file and display the content using OpenCV library functions. First of all, open your C++ IDE and create a new project. You have to configure your new project  in order to use OpenCV library functions. If you have not configured the project for OpenCV yet, please refer to Installing & Configuring with Visual Studio.

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////#include "opencv2/highgui/highgui.hpp"#include <iostream>

using namespace cv;using namespace std;

int main( int argc, const char** argv ){  Mat img = imread("MyPic.JPG", CV_LOAD_IMAGE_UNCHANGED); //read the image data in the file "MyPic.JPG" and store it in 'img'

if (img.empty()) //check whether the image is loaded or not { cout << "Error : Image cannot be loaded..!!" << endl;          //system("pause"); //wait for a key press return -1; }

namedWindow("MyWindow", CV_WINDOW_AUTOSIZE); //create a window with the name "MyWindow" imshow("MyWindow", img); //display the image which is stored in the 'img' in the "MyWindow" window

waitKey(0); //wait infinite time for a keypress

     destroyWindow("MyWindow"); //destroy the window with the name, "MyWindow"

return 0;}//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

Before you run this program, put any image file (MyPic.JPG) into the folder where your c++ file is.Otherwise you have to change the first argument of imread() function and give the absolute path to your image file.

You can download this OpenCV visual c++ project from here. (The downloaded file is a compressed .rar folder. So, you have to extract it using Winrar or other suitable software)

Load & Diplay Image from File

Explanation

Let's review the above OpenCV code line by line. #include "stdafx.h"If you are developing your application in Visual Studio, don't forget to put this line above the code.

#include "opencv2/highgui/highgui.hpp"imread(), namedWindow(), imshow() and waitKey() functions are declared in the above header file. So you must include it.

We are using Mat data structure in the above program. It is declared in "opencv2/core/core.hpp" header file. Then why don't we include this? It's

because "opencv2/highgui/highgui.hpp" header file include that header file inside it. So, we don't need to include it again in our program.

using namespace cv;All the data structures and functions in "opencv2/core/core.hpp" and "opencv2/highgui/highgui.hpp" are declared inside cv namespace. So, we have to add the above line in the top of our program. Otherwise we have to append 'cv::' specifier before each OpenCV functions and data structures. (e.g - cv::Mat, cv::imread() , etc). If you are not familiar with namespaces, please refer this article. 

'#include <iostream>' and 'using namespace std' are added because we are using 'cout' to display some strings in the console. This is basic C++ and you should be familiar with this.

Mat img = imread(const string& filename, int flags=CV_LOAD_IMAGE_COLOR)Mat is a data structure to store images in a matrix. It is declared in "opencv2/core/core.hpp" header file.

imread() is a function declared in "opencv2/highgui/highgui.hpp" header file. It loads an image from a file and stores it in Mat data structure.

Arguments of imread() function filename - location of the file. If you just give the filename only, that image should be in the same folder as your C++ file. Otherwise you have to give the full path to your image. flags - There are four possible inputso CV_LOAD_IMAGE_UNCHANGED - image-depth=8 bits per pixel in each channel,  no. of channels=unchanged o CV_LOAD_IMAGE_GRAYSCALE - image depth=8 bits,  no. of channels=1o CV_LOAD_IMAGE_COLOR - image-depth=?,  no. of channels=3o CV_LOAD_IMAGE_ANYDEPTH - image-depth=unchanged ,  no. of channels=?o CV_LOAD_IMAGE_ANYCOLOR - image-depth=?,  no. of channels=unchanged 

You can combine these above parameters to get desired image output.e.g -CV_LOAD_IMAGE_ANYDEPTH | CV_LOAD_IMAGE_ANYCOLOR - image-depth=unchanged,  no. of channels=unchanged CV_LOAD_IMAGE_COLOR | CV_LOAD_IMAGE_ANYDEPTH - image-depth=unchanged,  no. of channels=3

If you are not sure what to do, use CV_LOAD_IMAGE_COLOR as the 2nd parameter of

imread() function.

To understand image-depth and concept of channels, you should be familiar with theory of image processing. So, let's discuss little bit of theory of image processing.

Any digital image consists of pixels. Any pixel should have some value. The minimum value for a pixel is 0 and it represents black.When the value of the pixel is increased, the intensity of that pixel is also increased.  In a computer memory, ). In decimal, it is 255. fixed number of bits are allocated for every pixel. Say the number of allocated bits per pixel is 8. Then the maximum number that a pixel can have is 255 (11111111 in binary)

Now what is image-depth? The image-depth means the number of bits allocated for each pixel. If it is 8, each pixel can have a value between 0 and 255. If it is 4, each pixel can have a value between 0 to 15 (1111 in binary). 

Here is a simple model of a image with image-depth of 8 bits. Each small box represents a pixel. So, each box may contain a value between 0 to 255.

Here is some properties of the following image.o  Image-depth 8 bito 1 channel ( So, this is a grayscale image )o The height is 4 pixel o The width is 5 pixelso The resolution of this image is 4x5.This is a grayscale image (black and white image) because this image has no color content. If the value of this pixel is higher, it will be shown more brighter. If the value is low, it will be shown more darker.

Grayscale Image with image depth of 8

Following image is a simple model of a color image. Color image should consist of at least 3 planes; Red, Green and Blue. Any color can be created using a particular combination of these 3 colors. Any pixel is a combination of three 3 values. (255, 0, 0) represent pure red.

(0, 255, 0) represent pure green. (255, 0, 255) represents pure violate. In the same way, you can create many color. Image-depth is 24 because each pixel is represented with 8 x 3 bits (8 bits from each channel).

Here is some properties of the following image.o  Image-depth 24 bito 3 channels ( So, this is a color image )o The height is 4 pixel o The width is 5 pixelso The resolution of this image is 4x5.

R, G, B planes of a color image

In the above model, top left pixel is (23, 231, 46). It will be shown as a greenish color because the green value(231) of that pixel is larger than the red(23) and blue(46) value.

if (img.empty())If imread() function fails to load the image, 'img' will not be loaded any data. Therefore 'img.empty()' should return true. It's a good practice to check whether the image is loaded successfully and if not exit the program. Otherwise your program will crash when executing imshow()function.

bool Mat::empty()This function returns true, if Mat::data==NULL or Mat::total() == 0

 //system("pause");If you are using Visual Studio, it's better to uncomment this line because it will pause the program until user press any key. If we don't uncomment it, the program will exit immediately so that user will not see the error message. void namedWindow(const string& winname, int flags = WINDOW_AUTOSIZE);This function creates a window.

Parameters -o winname - Title of the window. That name will display in the title bar of the newly created windowo flags - determine the size of the window. There are two options

WINDOW_AUTOSIZE - User cannot resize the image. Image will be displayed in its original size CV_WINDOW_NORMAL - Image will resized if you resize the the window

void imshow(const string& winname, InputArray mat);This function shows the image which is stored in the 'mat' in a window specified by winname. If the window is created withWINDOW_AUTOSIZE flag, image will be displayed in its original size. Otherwise image may be scaled to the size of the window.

Parameters -o winname - Title of the window. This name is used to identify the window created by namedWindow() function.o mat - hold the image data

int waitKey(int delay = 0)waitKey() function wait for keypress for certain time, specified by delay (in milliseconds). If delay is zero or negative, it will wait for infinite time. If any key is pressed, this function returns the ASCII value of the key and your program will continue. If there is no key press for the specified time, it will return -1 and program will continue. void destroyWindow(const string& winname)This function closes the opened window, with the title of winname and deallocate any associated memory usage. This function is not essential for this application because when the program exits, operating system usually close all the opened windows and deallocate any associated memory usage.

Summary

When running this program, the image of 'MyPic.JPG' is loaded into the variable, 'img' of type Mat. Then a window named 'MyWindow' is opened. After that  'img' is loaded to that window. The window with the image will be displayed until any key is pressed.

Create a Blank Image & DisplayThis program is also very much similar to the previous application. The only difference is that this program creates a blank image instead of loading an existing image from a file.

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

#include "opencv2/highgui/highgui.hpp"#include <iostream>

using namespace cv;using namespace std;

int main( int argc, const char** argv ){  Mat img(500, 1000, CV_8UC3, Scalar(0,0, 100)); //create an image ( 3 channels, 8 bit image depth, 500 high, 1000 wide, (0, 0, 100) assigned for Blue, Green and Red plane respectively. )

if (img.empty()) //check whether the image is loaded or not { cout << "Error : Image cannot be loaded..!!" << endl;          //system("pause"); //wait for a key press return -1; }

namedWindow("MyWindow", CV_WINDOW_AUTOSIZE); //create a window with the name "MyWindow" imshow("MyWindow", img); //display the image which is stored in the 'img' in the "MyWindow" window

waitKey(0);  //wait infinite time for a keypress

     destroyWindow("MyWindow"); //destroy the window with the name, "MyWindow"

return 0;}//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

Before you run this program, put any image file (MyPic.JPG) into the folder where your c++ file is.Otherwise you have to change the first argument of imread() function and give the absolute path to your image file.

You can download this OpenCV visual c++ project from here. (The downloaded file is a compressed .rar folder. So, you have to extract it using Winrar or other suitable software)

Create & Display Image

New OpenCV functions  Mat::Mat(int rows, int cols, int type, const Scalar& s);This is the one of the many constructor available in Mat class. It initialize the Mat object with the value given by the Scalar object

Parameters :o rows - Number of rows in the 2D array ( height of the image in pixels)o cols - Number of columns in the 2D array ( width of the image in pixels)o type - specify the bit depth, data type and number of channels of the image. I gave CV_8UC3 and it specify 8 bit unsigned integers with 3 channels. Here are some of possible inputs for this parameter CV_8UC1 - 8 bit unsigned integers with single channel CV_8UC3 - 8 bit unsigned integers with 3 channels CV_64FC1 - 64 bit floating point value with 1 channelsIf you want more details about this, please refer to Data Types for Arrays in the Basics of OpenCV API

o s - Initialize each array element with the value given by s. In the above application, I gave Scalar(0,0,100). So, it initialize my first channel (Blue plane) with 0, 2nd channel (Green plane) with 0 and 3rd channel (Red Plane) with 100. So, my final image is red. You can try different combinations of these three and see the output image.

SummaryIn this program, I created a 3 channel image with 500 height and 1000 width. 8 bit unsigned integer is allocated for each pixel in each channel. (8x3 = 24 bits per each pixel)  And each pixel is assigned with (0,0,100) scalar value. That means 1st channel is all zero, 2nd channel is also all zero and the 3rd channel is all 100. Therefore we can see a red image as the output of the program.

Capture Video from File or CameraCapture Video From FileIn this lesson, I am going to show you how to read a video file. It's fairly simple. You just need to initialize a VideoCapture object which will open a video file and read frame by frame from that opened video. Here is the sample OpenCV code. If you are using Visual Studio, you will need to include "stdafx.h" header file.

/////////////////////////////////////////////////////////////////////////////////////////////////////////////#include "opencv2/highgui/highgui.hpp"#include <iostream>

using namespace cv;using namespace std;

int main(int argc, char* argv[]){    VideoCapture cap("C:/Users/SHERMAL/Desktop/SampleVideo.avi"); // open the video file for reading

    if ( !cap.isOpened() )  // if not success, exit program    {         cout << "Cannot open the video file" << endl;         return -1;    }

    //cap.set(CV_CAP_PROP_POS_MSEC, 300); //start the video at 300ms

    double fps = cap.get(CV_CAP_PROP_FPS); //get the frames per seconds of the video

     cout << "Frame per seconds : " << fps << endl;

    namedWindow("MyVideo",CV_WINDOW_AUTOSIZE); //create a window called "MyVideo"

    while(1)    {        Mat frame;

        bool bSuccess = cap.read(frame); // read a new frame from video

if (!bSuccess) //if not success, break loop {                        cout << "Cannot read the frame from video file" << endl;

break; }

        imshow("MyVideo", frame); //show the frame in "MyVideo" window

        if(waitKey(30) == 27) //wait for 'esc' key press for 30 ms. If 'esc' key is pressed, break loop {                cout << "esc key is pressed by user" << endl; break;  }    }

    return 0;

}////////////////////////////////////////////////////////////////////////////////////////////////////////////You can download this OpenCV visual c++ project from here. (The downloaded file is a compressed .rar folder. So, you have to extract it using Winrar or other suitable software) 

This is how  I play a video using my OpenCV program

New OpenCV functions which are not found earlier are explained here VideoCapture::VideoCapture(const string& filename)This is one of few constructors available in VideoCapture class. This constructor open the video file and initializes the VideoCapture object for reading the video stream from the specified file. 

The destructor of this class will deallocated any associated memory with this object. Therefore you don't need to deallocate memory explicitly in your program.

bool VideoCapture::IsOpened()If the previous call to VideoCapture constructor is successful, this method will return true. Otherwise it will return false. It is essential to check that whether the VideoCapture initialize successfully. If it is unsuccessful, program should be exited. Otherwise when you try to read a frame from the VideoCapture object, your program will crash.

bool VideoCapture::set(int propId, double value)You can change some properties of VideoCapture object. If it is successful, this method will return true. Otherwise it will return false. You should try to change some properties of the video stream in your code. In my code, I have shown you how to change the position of the video, by changing the CV_CAP_PROP_POS_MSEC property.

Parameters :o int propID - This argument specify the property you are going to change. There are many options for this argument. Some of them are listed here.   CV_CAP_PROP_POS_MSEC - current position of the video in milliseconds CV_CAP_PROP_POS_FRAMES - current position of the video in frames CV_CAP_PROP_FRAME_WIDTH - width of the frame of the video stream CV_CAP_PROP_FRAME_HEIGHT - height of the frame of the video stream CV_CAP_PROP_FPS - frame rate (frames per second) CV_CAP_PROP_FOURCC - four character code  of codeco double value - This is the new value you are going to assign to the property, specified by the propID

doubleVideoCapture::get(int propId)This function returns the value of the property which is specified by propId. You should try to obtain some properties of the video stream in your code. In my code, I have shown you how to obtain the frame rate (frames per second) of the video, by using the CV_CAP_PROP_FPS argument.

Parameters - o int propID - This argument specify the property you are going to obtain. There are many options for this argument. Some of them are listed here.   CV_CAP_PROP_POS_MSEC - current position of the video in milliseconds CV_CAP_PROP_POS_FRAMES - current position of the video in frames CV_CAP_PROP_FRAME_WIDTH - width of the frame of the video stream CV_CAP_PROP_FRAME_HEIGHT - height of the frame of the video stream CV_CAP_PROP_FPS - frame rate (frames per second) CV_CAP_PROP_FOURCC - four character code  of codec

bool VideoCapture::read(Mat& image);The function grabs the next frame from the video, decodes it and stores it in the 'image' variable. Inside this function, VideoCapture::grap() and VideoCapture::retrieve() will be called. If you want, you can use these two functions instead of VideoCapture::read() function. If the operation successful, it will return true. Otherwise it will return false.

waitKey(30)The function waits for 30 milliseconds. If a key was pressed before the specified time, it returns the ASCII value of the pressed key. If that value is 27 (ASCII value of 'esc' key is 27), the program will execute inside the if block. If no key is pressed during that 30ms, the function returns -1 program will continue the while loop.

VideoCapture::~VideoCapture()Destructor of VideoCapture object will destroy any associated memory of that particular object. This destructor will be called implicitly on exit of the main method of the above program.

Summary

At first, this program captures a video from a file. Then the program enters into a infinite loop. In that loop, it grabs frames from the captured video sequentially, decodes it, shows it in a window and waits for 30 milliseconds. If the video file has no more frames or if the user presses the 'esc' key, the program will break the infinite loop.

Note:Using waitKey(int) function is very important because imshow(string&, MAT) function need time to paint the image in the window and waitKey(int) will give that necessary time.

Capture Video From CameraThe main difference of the following program from the above program is the argument to the VideoCapture constructor. Here you just need to give the index of your camera to the constructor of the VideoCapture object instead of the filename in the above program. Here is the sample OpenCV code.

///////////////////////////////////////////////////////////////////////////////////////////////////////////////#include "opencv2/highgui/highgui.hpp"#include <iostream>

using namespace cv;using namespace std;

int main(int argc, char* argv[]){    VideoCapture cap(0); // open the video camera no. 0

    if (!cap.isOpened())  // if not success, exit program    { cout << "Cannot open the video cam" << endl;        return -1;    }

   double dWidth = cap.get(CV_CAP_PROP_FRAME_WIDTH); //get the width of frames of the video   double dHeight = cap.get(CV_CAP_PROP_FRAME_HEIGHT); //get the height of frames of the video

    cout << "Frame size : " << dWidth << " x " << dHeight << endl;

    namedWindow("MyVideo",CV_WINDOW_AUTOSIZE); //create a window called "MyVideo"

    while (1)    {        Mat frame;

        bool bSuccess = cap.read(frame); // read a new frame from video

if (!bSuccess) //if not success, break loop { cout << "Cannot read a frame from video stream" << endl; break; }

        imshow("MyVideo", frame); //show the frame in "MyVideo" window

        if (waitKey(30) == 27) //wait for 'esc' key press for 30ms. If 'esc' key is pressed, break loop { cout << "esc key is pressed by user" << endl; break;  }    }    return 0;

}

///////////////////////////////////////////////////////////////////////////////////////////////////////////////

You can download this OpenCV visual c++ project from here. (The downloaded file is a compressed .rar folder. So, you have to extract it using Winrar or other suitable software)

This is how  I capture myself with my webcam using the OpenCV program

New OpenCV functions which are not found earlier are explained here VideoCapture::VideoCapture(int device)This is one of constructors available in VideoCapture class. This constructor open the camera indexed by  the argument of this constructor and initializes the VideoCapture object for reading the video stream from the specified camera.Here the '0' means the index of the camera to be used. You can use 1,2,3.. instead of 0, if your computer is attached to more than 1 camera. The destructor of this class will deallocated any associated memory with this object. Therefore you don't need to deallocate memory explicitly in your program.

if (!cap.isOpened())If the VideoCapture object initialization unsuccessful, the expression inside the 'if 'parentheses will evaluate to true and the statements inside the 'if' block will be executed. It is a good practice to check this scenario and handle it accordingly because otherwise it may cause the program to crash.

double dWidth = cap.get(CV_CAP_PROP_FRAME_WIDTH)This function obtain the width (in pixels) of frames of the camera output.

 double dHeight = cap.get(CV_CAP_PROP_FRAME_HEIGHT)This function obtain the height (in pixels) of frames of the camera output.

 All other OpenCV functions and their arguments are exactly same as the 1st program.

Write Image & Video to FileIn this lesson, I am going to show you how to write images and videos to a file using OpenCV.If you have not install and configure OpenCV yet, please refer to Installing & Configuring with Visual Studio.

Write Image to FileIn the following example, a yellow image is created and written into a file. Let's see how to do it with OpenCV.

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

#include "opencv2/highgui/highgui.hpp"#include <iostream>

using namespace cv;using namespace std;

int main( int argc, const char** argv ){  Mat img(650, 600, CV_16UC3, Scalar(0,50000, 50000)); //create an image ( 3 channels, 16 bit image depth, 650 high, 600 wide, (0, 50000, 50000) assigned for Blue, Green and Red plane respectively. )

if (img.empty()) //check whether the image is loaded or not { cout << "ERROR : Image cannot be loaded..!!" << endl;          //system("pause"); //wait for a key press return -1; }

     vector<int> compression_params; //vector that stores the compression parameters of the image

     compression_params.push_back(CV_IMWRITE_JPEG_QUALITY); //specify the compression technique

     compression_params.push_back(98); //specify the compression quality

     bool bSuccess = imwrite("D:/TestImage.jpg", img, compression_params); //write the image to file

     if ( !bSuccess )

    {

cout << "ERROR : Failed to save the image" << endl;

         //system("pause"); //wait for a key press

    }

namedWindow("MyWindow", CV_WINDOW_AUTOSIZE); //create a window with the name "MyWindow" imshow("MyWindow", img); //display the image which is stored in the 'img' in the "MyWindow" window

waitKey(0);  //wait for a keypress

     destroyWindow("MyWindow"); //destroy the window with the name, "MyWindow"

return 0;}///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////You can download this OpenCV visual c++ project from here. (The downloaded file is a compressed .rar folder. So, you have to extract it using Winrar or other suitable software)

The above program is very much similar to the program under 'Create a Blank Image & Display' section in the lesson of Read & Display Image. If you need further clarifications of any OpenCV functions which are not explained here, please refer to Read & Display Image lesson.

New OpenCV functions which are not found earlier bool imwrite( const string& filename, InputArray img, const vector<int>& params=vector<int>())The function saves the image in the variable 'img' to a file, specified by 'filename' . If this function fails to save the image, it will return false. On success of writing the file to the harddisk, it will return true.

Parameters -

o filename - specify the location and name of the file to be savedo img - hold the image which is going to save

o params - This is a int vector to which you have to insert some int parameters specifying the format of the image JPEG format - You have to puch_back CV_IMWRITE_JPEG_QUALITY first and then a number between 0 and 100 (higher is the better). If you want the best quality output, use 100. I have used 98 in the above sample program. But higher the value, it will take longer time to write the image PNG format - You have to puch_back CV_IMWRITE_PNG_COMPRESSION first and then a number between 0 and 9 (higher is the better compression, but slower).

The image format is chosen depending on the file name extension. Only images with 8 bit or 16 bit unsigned  single channel or 3 channel ( CV_8UC1, CV_8UC3, CV_8SC1, CV_8SC3, CV_16UC1, CV_16UC3) with 'BGR' channel order, can be saved. If the depth or channel order of the image is different, use 'Mat::convertTo()' or 'cvtColor' functions to convert the image to supporting format before using imwritefunction.

The above program is very much similar to the program under 'Create a Blank Image & Display' section in the lesson of Read & Display Image. If you need further clarifications of any OpenCV functions which are not explained here, please refer to Read & Display Image lesson.

SummaryThis program creates an yellowish image ( 3 channels, 16 bit image depth, 650 high, 600 wide, (0, 50,000, 50,000) assigned for BGR channels). Because the image has 16 bit depth, you can use values between 0 and 65535 for each element in each channel. I have used 50,000 for each element in the green and red planes which gives the yellowish color. You can try different values. Then it specifies the compressing technique. I have used JPEG as the compression technique in the above example. Then it saves the image in the "D:/TestImage.jpg" location. Then it displays the newly created image in a window and wait indefinitely until an user presses a key.

Write Video to FileIn the following example, a video is captured from the webcam and written into a file. Let's see how to do it with OpenCV.

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////#include "opencv2/highgui/highgui.hpp"#include <iostream>

using namespace cv;

using namespace std;

int main(int argc, char* argv[]){    VideoCapture cap(0); // open the video camera no. 0

    if (!cap.isOpened())  // if not success, exit program    {        cout << "ERROR: Cannot open the video file" << endl;        return -1;    }

 namedWindow("MyVideo",CV_WINDOW_AUTOSIZE); //create a window called "MyVideo"    double dWidth = cap.get(CV_CAP_PROP_FRAME_WIDTH); //get the width of frames of the video   double dHeight = cap.get(CV_CAP_PROP_FRAME_HEIGHT); //get the height of frames of the video

   cout << "Frame Size = " << dWidth << "x" << dHeight << endl;

   Size frameSize(static_cast<int>(dWidth), static_cast<int>(dHeight));

 VideoWriter oVideoWriter ("D:/MyVideo.avi", CV_FOURCC('P','I','M','1'), 20, frameSize, true); //initialize the VideoWriter object 

   if ( !oVideoWriter.isOpened() ) //if not initialize the VideoWriter successfully, exit the program   { cout << "ERROR: Failed to write the video" << endl; return -1;   }

    while (1)    {         Mat frame;

        bool bSuccess = cap.read(frame); // read a new frame from video

        if (!bSuccess) //if not success, break loop {             cout << "ERROR: Cannot read a frame from video file" << endl;             break;        }

oVideoWriter.write(frame); //writer the frame into the file

        imshow("MyVideo", frame); //show the frame in "MyVideo" window

        if (waitKey(10) == 27) //wait for 'esc' key press for 30ms. If 'esc' key is pressed, break loop       {            cout << "esc key is pressed by user" << endl;            break;        }    }

    return 0;

}///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////You can download this OpenCV visual c++ project from here. (The downloaded file is a compressed .rar folder. So, you have to extract it using Winrar or other suitable software)

Here is how I record my video from my webcam and save to a file in D drive

New OpenCV functions which are not found earlier are explained here Size frameSize(static_cast<int>(dWidth), static_cast<int>(dHeight))

Create a Size object with a given width and height. Here I have cast the width and height to integers because they are originally double values and the Size constructor does not accept double values as its parameters.

VideoWriter::VideoWriter(const string& filename, int fourcc, double fps, Size frameSize, bool isColor=true)This is the constructor of the VideoWriter class. It initializes the object with following parameterso const string& filename - Specify the name and the location of the output file. The video stream is written into this fileo int fourcc - specify the 4 character code for the codec which is used to compress the video. Your computer may not be supported some codecs. So, if you fail to save the video, please try other codecs. Here are some popular codecs. CV_FOURCC('D', 'I', 'V', '3') for DivX MPEG-4 codec CV_FOURCC('M', 'P', '4', '2') for MPEG-4 codec CV_FOURCC('D', 'I', 'V', 'X') for DivX codec  CV_FOURCC('P','I','M','1') for MPEG-1 codec CV_FOURCC('I', '2', '6', '3') for ITU H.263 codec CV_FOURCC('M', 'P', 'E', 'G') for MPEG-1 codecComplete list of codec can be found hereHere I have used CV_FOURCC('P', 'I', 'M', '1') as the four character code of codec to compress the video. If the output file cannot be opened, you can try different four character code of codecs.For Windows users, it is possible to use -1 instead of the above codecs in order to choose compression method and additional compression parameters from a dialog box. It is a best method for Microsoft Windows users.

o double fps - frames per seconds of the video stream. I have used 20. You can try different values. But the codec should support the fps value. So, use an appropriate value.o Size frameSize - Size object which specify the width and the height of each frame of the video stream.o bool isColor - If you want to save a color video, pass the value as true. Otherwise false. Remember codec should support whatever value, you pass. In the above example, you have to give true as the 5th argument. Otherwise it will not work.

 if ( !oVideoWriter.isOpened() ) Check whether the VideoWriter object initialize successfully. If not exit the program immediately.

void write(const Mat& image)Write a frame to the video stream. The size of the frame should be same as the size you specified when initializing the VideoWriter object.

All other OpenCV functions have been discussed in earlier lessons. So, if you are not familiar with those OpenCV functions yet, please go through Capture Video from File or Camera

Filtering ImagesImage filtering is an important part of computer vision. For most of computer vision applications, filtering should be done before anything else. OpenCV supports lots of in-build filtering methods for images. Here is the list of filtering methods that I am going to discuss with you in the following posts (with OpenCV 2.4.5 and C++ )

Change Brightness of Image or Video Change Contrast of Image or Video Histogram Equalization of Grayscale or Color Image Smooth / Blur Images

Here is the list of image filtering methods which are explained using examples with OpenCV 2.1  in C style (not C++) Eroding Dilating InvertingHere is the original image which I am going to filter using above methods.

Original Image

If you have not install and configure OpenCV yet, please refer to Installing & Configuring with Visual Studio. 

ErodingEroding is a simple way of filtering images. Here is how it can be done with OpenCV.

///////////////////////////////////////////////////////////////////////////////////////#include "stdafx.h"#include <cv.h>#include <highgui.h>

int main(){        //display the original image        IplImage* img = cvLoadImage("C:/MyPic.jpg");        cvNamedWindow("MyWindow");        cvShowImage("MyWindow", img);

        //erode and display the eroded image        cvErode(img, img, 0, 2);        cvNamedWindow("Eroded");        cvShowImage("Eroded", img);                cvWaitKey(0);                //cleaning up        cvDestroyWindow("MyWindow");        cvDestroyWindow("Eroded");        cvReleaseImage(&img);                return 0;} ///////////////////////////////////////////////////////////////////////////////////////

You can download this OpenCV visual c++ project from here. 

Eroded Image

New OpenCV functions which are not found earlier are explained here  cvErode(img, img, 0, 2)The 1st parameter is the source image.The 2nd parameter is the destination image which is to be the eroded image.Here the 3rd parameter is the structuring element used for erosion. If it is 0, a 3×3 rectangular structuring element is used. The 4th parameter is the number of times, erosion is applied.This function can process images in place. That means same variable can be used for the 1st and 2nd parameters.

If you want more explanation about various methods in the above computer application , please refer to Capturing Images & Videos. 

Dilating Dilating is something like opposite of the eroding an image. Here is the OpenCV code.

///////////////////////////////////////////////////////////////////////////////////////#include "stdafx.h"

#include <cv.h>#include <highgui.h>

int main(){        //display the original image        IplImage* img = cvLoadImage("C:/MyPic.jpg");        cvNamedWindow("MyWindow");        cvShowImage("MyWindow", img);

        //dilate and display the dilated image        cvDilate(img, img, 0, 2);        cvNamedWindow("Dilated");        cvShowImage("Dilated", img);

        cvWaitKey(0);                //cleaning up        cvDestroyWindow("MyWindow");        cvDestroyWindow("Dilated");        cvReleaseImage(&img);                return 0;}/////////////////////////////////////////////////////////////////////////////////////// 

You can download this OpenCV visual c++ project from here. 

Dilated Image

New OpenCV functions which are not found earlier are explained here  cvDilate(img, img, 0, 2)The 1st parameter is the source image.The 2nd parameter is the destination image which is to be the dilated image.Here the 3rd parameter is the structuring element used for dilation. If it is 0, a 3×3 rectangular structuring element is used. The 4th parameter is the number of times, dilation is applied.This function can process images in place. That means same variable can be used for the 1st and 2nd parameters.

Inverting Inverting an image is like taking the negative of an image.

///////////////////////////////////////////////////////////////////////////////////////

#include "stdafx.h"#include <cv.h>#include <highgui.h>

int main()

{        //display the original image        IplImage* img = cvLoadImage("C:/MyPic.jpg");        cvNamedWindow("MyWindow");        cvShowImage("MyWindow", img);

        //invert and display the inverted image        cvNot(img, img);        cvNamedWindow("Inverted");        cvShowImage("Inverted", img);

        cvWaitKey(0);               //cleaning up        cvDestroyWindow("MyWindow");        cvDestroyWindow("Inverted");        cvReleaseImage(&img);               return 0;}

///////////////////////////////////////////////////////////////////////////////////////

You can download this OpenCV visual c++ project from here. 

Inverted Image

 New OpenCV functions which are not found earlier are explained here cvNot(img, img)This function inverts every bit in every element of the image in the 1st parameter and places the result in the image in the 2nd parameter.This function can process images in place. That means same variable can be used for the 1st and 2nd parameters.

e.g - For a 8 bit image, the value 0 will be mapped to (255-0)=255                                        the value 46 will be mapped to (255-46)=209

        For a 16 bit image, the value 0 will be mapped to (65535-0)=65535                                   the value 46 will be mapped to (65535-46)=65489