Extract frame from video when user presses a key in Open CV

Capturing the frames from video using Open CV is very simple. Just we need to use cvSaveImage method from Open CV library. But some times we need to capture the images interactively like when the user presses a key. For doing this we can use the function kbhit(). this function returns non zero values when there is something in the keyboard buffer. So by using this we can interactively extract the frames.

By using the fallowing code we can accomplish this.

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

void main()
{
IplImage * img;
CvCapture * v;
v = cvCreateFileCapture("E:/v.mp4");
int k = 0;
char text[10];
while(true)
{
if(_kbhit())
{
sprintf(text,"%s%d%s","E:/Image",k,".jpeg");
cvSaveImage(text,img);
getch();
}
else
{
img= cvQueryFrame(v);
cvWaitKey(0);
k++;
}
}
_getch();
}

we have one while loop which continuously reads the frames from the video. The if block will be executed when the user presses a key from keyboard else else block gets executed. In else block we are extracting the frames from the video we have one delay loop as well else the the computer reads all the frames one by one with in fraction of seconds.

To change the saved image file name every time we are using char buffor. By using sprintf we are changing the file name. cvSaveImage saves the image on the specifed path supplied as first argument and the image to be saved as second argument. getch() is used to clear the buffor else the if block executed continuously.

Splitting the Image in to three channels with Open CV and C Language

In image processing some tines we need to separate the channels of image. This is needful to achieve the required contrast of the image. Because some times if we remove the one or more channels from the image we can get better understanding of the image under consideration. In some cases we have to process only few channels of the image rather than the entire image. Open cv provides the functions to achieve the same. We can do the same thing with out using the functions of the open cv.

Fallowing code snippet is useful for splitting the image channels:

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

void main()
{
    IplImage *Simg;
    IplImage *Rimg;
    IplImage *Gimg;
    IplImage *Bimg;

    int nrows,ncols;

    Simg=cvLoadImage("E:/test.jpg");
    Rimg=cvCreateImage(cvSize(Simg->width,Simg->height),Simg->depth,3);
    Gimg=cvCreateImage(cvSize(Simg->width,Simg->height),Simg->depth,3);
    Bimg=cvCreateImage(cvSize(Simg->width,Simg->height),Simg->depth,3);

    cvNamedWindow("Red",0);
    cvNamedWindow("Green",0);
    cvNamedWindow("Blue",0);
    cvResizeWindow("Red",320,320);
    cvResizeWindow("Green",320,320);
    cvResizeWindow("Blue",320,320);

    nrows=Simg->height;
    ncols=Simg->width;
    
     for(int i=0;i<nrows*ncols*3;i++)
    {
        Rimg->imageData[i]=0;
        Gimg->imageData[i]=0;
        Bimg->imageData[i]=0;
    }
    for(int i=0;i<nrows*ncols*3;i+=3)
    {   
            Rimg->imageData[i+2]=Simg->imageData[i+2];
            Gimg->imageData[i+1]=Simg->imageData[i+1];
            Bimg->imageData[i]=Simg->imageData[i];
     }
   
    cvShowImage("Red",Rimg);
    cvShowImage("Green",Gimg);
    cvShowImage("Blue",Bimg);
    cvWaitKey(0);
    _getch();
}

for example consider the image with three channels. Load that image into the memory with the help of Open CV. Next create the three images with the same size,depth and channels. Fill the entire image with the zeros means with black pixels. Now extract the individual channels from the original image and store them in the newly created images. Open cv changes the order of the pixels to BGR instead of RGB.
If we apply the ->height and ->width on the image in Open CV we will get the number of rows and columns. But actually if the image is color we have columns more than what we get. that is if we have width as 20 the actual columns on the disk is 60 because each channel will store in separate byte. In memory they will be stored in interleaved manner. So to get the red pixels we need to get the alternate pixels with the gap of two.
By running the above code on the below image:

  

we will get the fallowing images:

 

Image thresholding using Open CV and C language with track bar

In previous post we have seen the image thresholding opeeeration. In that the user provides the thresholding level before start of the operation after if he wants to change he has to restart the app. In this post I will explain how interactively user can provide the threshold level. Open CV has lot of GUI tools like in built support for windows etc. We can add one track bar for the window to change the threshold value.

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

    int rows,cols,level,i,j,threshold_value = 0;

    IplImage * image;
    IplImage * img;

    char path[20];
    const int max_value=255;
    char* trackbar_value = "Value";
    char* imagedata;
    char * name="Threshold";
   

int main()
{
   
    puts("Enter the path of the image");
    gets(path);

    image=cvLoadImage(path,CV_LOAD_IMAGE_GRAYSCALE);
    rows=image->height;
    cols=image->width;
   
    img=cvLoadImage(path,CV_LOAD_IMAGE_GRAYSCALE);
       
    cvNamedWindow(name,0);
    cvResizeWindow(name,352,288);
    cvCreateTrackbar( trackbar_value,"Threshold", &threshold_value,max_value);
   
    while(true)
    {
    imagedata=(char*)image->imageData;
   
    for(i=0;i<rows*cols;i++)
    {
        img->imageData[i]=imagedata[i]>threshold_value?imagedata[i]:0;
    }
    cvShowImage(name,img);
    int c;
    c = cvWaitKey( 20 );
    if( (char)c == 27 )
    {
        cvReleaseImage(&image);
        cvReleaseImage(&img);
        cvDestroyWindow("Threshold");
          break;
    }
   }

    getch();
    return 0;
}

To add the track bar for the window we can use the cvCreateTrackbar function from the Open CV library. The first argument is the name to be displayed for the bar. Second one is the name of the window on which track must be showed. Third one is thee value we want to change with bar. fourth one is the maximum allowed value on track bar.

ImageThresholding uisng Open CV and C

Thresholding is the very basic image processing technique. This splits the pixels of image into two subsets. One set has the pixel values less than the thresholding level and other has the pixel values greater than or equal to the thresholding value. Means thresholding produces binary images having only two values of pixels.

If we draw the histogram of the image after thresholding we will get only two impulse functions in the histogram because we have only two sets of pixels.

By using Open CV and any other programming language like C or C++ we can threshold the images. This is very simple operation. In the fallowing code snippet I explained how we can achieve the image thresholding on images.

The thresholded image looks like:

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

int main()
{
    IplImage * image;
    char path[20];
    int rows,cols,level,i;
    char* imagedata;

    puts("Enter the path of the image");
    gets(path);

    image=cvLoadImage(path,CV_LOAD_IMAGE_GRAYSCALE);
    rows=image->height;
    cols=image->width;

    puts("Enter the threshold level");
    scanf("%d",&level);
    imagedata=(char*)image->imageData;

    for(i=0;i<rows*cols;i++)
    {
        imagedata[i]=imagedata[i]>level?level:0;
    }

    cvShowImage("Threshold",image);
    if(cvWaitKey(1)==27)
    {
        exit(0);
    }

    getch();
    return 0;
}

I think by now you have gone through the code. As usual we started the coding with including the most important header files like cv.h and highgui.h. In the starting of the code we have one char array it stores the path of the image. After we have five integer variables. These are used for storing the height, width of the images and two more variables to iterate the image data and other one is for holding the threshold value given by the user.

The code work like this user enters the path of the image next he/she enters the threshold value. Based on the threshold the image is segmented into two separate pixel values. Here we used inplace modification. We directly changed the pixel values of image with out using other image to store the result.

We compared the image pixel value with the user entered threshold value if the pixel value is greater than the threshold we replaced that pixel with the threshold value other wise we replaced with zero value i.e black pixel.

In thresolding we have variations. The variations include threshold binary, threshold binary inverted,truncate, threshold to zero, threshold to zero inverted*.
We can implement all these types of thresholding methods just by changing the code line

imagedata[i]=imagedata[i]>level?level:0;

for threshold binary use   imagedata[i]=imagedata[i]>level?255:0; 

for threshold binary inverted use   imagedata[i]=imagedata[i]>level?0:255; 

for truncate use   imagedata[i]=imagedata[i]>level?level:imagedata[i]; 

for threshold to zero use   imagedata[i]=imagedata[i]>level?imagedata[i]:0; 

for threshold to zero inverted use   imagedata[i]=imagedata[i]>level?0:imagedata[i];

* according to Open CV documentation.

Convert the video into images using Open CV and C

 In previous post we have seen how we can use Open CV to show images of a folder continuously so that the viewer can fell like as a video. We can use the Open CV for extracting the frames of a video. After extracting we can save them with the desired file extension. You can use the fallowing code for that.

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

int main()
{
    char  video_path[100];
    char  destination[100];
    char file_name[20];
    int count=100000;

    CvCapture *capture;
    IplImage * image;
   

    printf("Enter the path to the Video file");
    gets(video_path);

    printf("Enter the destinatio folder to save images");
    gets(destination);

    capture=cvCreateFileCapture(video_path);
    for(;;)
    {
        sprintf(file_name,"%s%d%s",destination,count,".jpg");
        image=cvQueryFrame(capture);
        cvSaveImage(file_name,image);
        printf("%s is saved.. \n",file_name);
        count++;
    }
    cvReleaseCapture(&capture);
    puts("Video is converted to images...");
    getch();
    return 0;
}

In this code we used two character arrays. One is for storing the path for the video. Second one is for storing the path to the folder to store images. You might have observed the integer variable this is for giving the file names sequentially for the images. file_name is another char array for temporally storing the file name.

By using sprintf we are appending the .jpg extension to the integer and we are saving the extracted frame from the video with the name containing in the file_name char array. This data changes for each frame.
We are using the very beautiful function provided by Open CV to save the image on the secondary memory that is out HDD. The function is cvSaveImage. This function takes two arguments first one is the filename and the second one is the image data typically variable of type IplImage*.

We are using the infinity loop to get all the frames from the video. After extracting all the frames the loop automatically breaks. Next we are releasing all the resources used by the Open CV by using  cvReleaseCapture.

Displaying the video with Open CV and C

In previous posts you have seen how we can load and display images with the Open CV and C language. Open CV can be used to process videos as well. Processing the videos is simple as the images with the Open CV. This is because the videos are made with the sequence of images. These images are called frames. Generally the videos has the frame rate of 24-30 called as fps. fps represents frames per seconds.

By using Open CV we will grab the each frame from the video from starting frame. After getting the frame from the video to memory we display it as a normal image. If we want to process the grabbed frame we can pass that frame to the processing function and we get result back.

The fallowing code snippet helps you to under stand this process.


#include "stdafx.h" 
#include "cv.h"
#include "highgui.h"
#include <string>
#include <dirent.h>
using namespace std;
CvCapture * capture;
    char* filename;
    string fullpath,path;
    DIR *dp;
    struct dirent *ep;
    const char* full_path;
class base
{
public:
virtual void input()=0;
virtual void process()=0;
virtual void play()=0;
virtual void destroy()=0;
};

class localfile:public base
{
public:
    localfile(char* file);
void input();
void process();
void play();
void destroy();
};
void localfile::input()
{
capture = cvCreateFileCapture(filename);
}
void localfile::process()
{
for ( ; ; )
    {
        play();
        cvWaitKey(33);
    }
}
void localfile::play()
{
 IplImage * frame = cvQueryFrame(capture);
 cvShowImage("Video",frame);
}
void localfile::destroy()
{
cvDestroyWindow("Video");
}
localfile::localfile(char* file)
{
filename=file;
}

int main(int argc,char** argv)
{
    char* path;
    printf("Enter the path to the Video");
    gets(path);
    localfile obj1(path);
    obj1.input();
    obj1.process();
    obj1.destroy();
    return 0;
}

When we run this code it asks for the path to the video file. Based on the path to the video the CvCapture structure will be initialized with the function cvCreateFileCapture which is available in Open CV . Now this structure helps us to get the frames as well as lot information called meta data of the video.

Open CV provides simple function called cvQueryFrame(); this function takes the CvCapture type variable as the argument. and returns the frame for each call to this function. It automatically returns the next frame from the video. After getting the frame we can display this frame as described in the previous posts.

Display Images as video with Open CV part-2

In previous post we have seen how we can display the sequence of images as video. But in that process we have seen a problem which can break our code. The basic condition we posed on our code is the folder contains only images. It does not have any other files which are not supported by Open CV.

In this post I will try to explain how we can get rid of this problem by using the regular expressions. These regular expressions are available now in c++ as well.To use this feature we need to include the regex header file. and we have to use the name space std::tr1

#include "stdafx.h"
#include "cv.h"
#include <regex>
#include "highgui.h"
#include <string>
#include <dirent.h>

using namespace std;
using namespace std::tr1;

char* filename;
string fullpath,path;
DIR *dp;
struct dirent *ep;
const char* full_path;

class base
{
public:
    virtual void input()=0;
    virtual void process()=0;
    virtual void destroy()=0;
};

class imagetovideo:public base
{
public:
    imagetovideo(char* folderpath);
    void input();
    void process();
    void destroy();
};

void imagetovideo::input()
{
    const char* mpath=path.c_str();
    dp=opendir(mpath);
}

void imagetovideo::process()
{
    std::tr1::regex rgx("(.jpg$)|(.png$)|(.jpeg$)|(.bmp$)|(.gif)");
    cmatch result;
    cvNamedWindow("Video",0);
    cvResizeWindow("Video",640,320);
        while(ep=readdir(dp))
        {
            regex_search(ep->d_name, result, rgx);
            if(!result.empty())   
            {
                puts(ep->d_name);
                fullpath=path+(ep->d_name);
                full_path=fullpath.c_str();
                puts(full_path);
                IplImage* img=cvLoadImage(full_path);
                cvShowImage("Video",img);
                    if(cvWaitKey(1000)==27)
                    {
                        cvDestroyWindow("Video");
                        break;
                    }
                cvReleaseImage(&img);
            }
            }
}

void imagetovideo::destroy()
{
    closedir (dp);
    cvDestroyWindow("Video");
}

imagetovideo::imagetovideo(char* folderpath)
{
    path=folderpath;
}

int main(int argc,char** argv)
{
    char* path;
    printf("Enter the path to the image");
    gets(path);
    imagetovideo obj1(path);
    obj1.input();
    obj1.process();
    obj1.destroy();
    return 0;
}

If you observe the above code nothing changed from the previous post code snippet. Only we have added the few lines of code which can control the flow of file paths to the cvShowImage function. Now the code is robust. This code will process only the image files. The regular expression filters out the other files which are not images.

The input for the path must be like D:/x/ in this D is the local disk drive number and the x is the folder containing the images. If the folder path is not specified in this manner code will not work.

In this code I used regular expressions to identify the file format. If the file is image then only file is passed to the cvShowImage function. I will explain the regular expression code lines used in this post.

std::tr1::regex rgx("(.jpg$)|(.png$)|(.jpeg$)|(.bmp$)|(.gif)");

observe the above line. You can see the extensions for the images as well as they ended with the dollar symbol. and they are separated with the | symbol. Here $ symbol indicates that in the given string search for the .jpg or .png or .jpeg etc at the end of the string.

cmatch result;

here result is the resul coming out from the  regex_search(ep->d_name, result, rgx); expression. This expression processes the string passed in the first argument the third argument is the regular expression and the second one is the cmatch type.

If this method finds the match it will store that in the cmatch type variable. If the match is found the match will be stored here. So if this not empty the file is image file. else the file is not image.

Display Images as Video with Open CV

Till now we have seen how we can use Open CV with the C and C++ to display images. You may wonder is there any way to display the images continuously. Yes definitely we can do. Just we have to place the image display code in a loop. Is this enough... Think for a while...
How to write a loop we can use for loop or while loop or any other loop. But before that we must have knowledge of number of images as well as names of the images moreover we must know the directory where the images are stored. From this we can infer that we must get the names of the images from the desired directory. for this we can use dirent.h header file. Which hides all the implementation details of the process. It returns the file names with the file format as well. You can refer this link for more details.
The fallowing code is useful for this.

#include "stdafx.h"
#include "cv.h"
#include "highgui.h"
#include <string>
#include <dirent.h>
using namespace std;

char* filename;
    string fullpath,path;
    DIR *dp;
    struct dirent *ep;
const char* full_path;
class base
{
public:
virtual void input()=0;
virtual void process()=0;
virtual void destroy()=0;
};

class imagetovideo:public base
{
public:
imagetovideo(char* folderpath);
void input();
void process();
void destroy();
};

void imagetovideo::input()
{
const char* mpath=path.c_str();
dp=opendir(mpath);
}
void imagetovideo::process()
{
cvNamedWindow("Video",0);
cvResizeWindow("Video",640,320);
int c=0;
while(ep=readdir(dp))
{

c++;
if(c>2)
{
puts(ep->d_name);
fullpath=path+(ep->d_name);
full_path=fullpath.c_str();
puts(full_path);
IplImage* img=cvLoadImage(full_path);
cvShowImage("Video",img);
if(cvWaitKey(1000)==27)
{
cvDestroyWindow("Video");
break;
}
cvReleaseImage(&img);
}
}
}

void imagetovideo::destroy()
{
closedir (dp);
cvDestroyWindow("Video");
}
imagetovideo::imagetovideo(char* folderpath)
{
path=folderpath;
}
int main(int argc,char** argv)
{
char* path;
printf("Enter the path to the image");
gets(path);
imagetovideo obj1(path);
obj1.input();
obj1.process();
obj1.destroy();
return 0;
}

Here we have a base class. Which is implemented as abstract class. Our main class which has code is inherited from the base class. This derived class provides the functionality for our program. It contains three methods and one constructor. The constructor initializes the  path to the folder path. init method initializes the variables available in the dirent header file. The process method gets the file names one by one from the folder and displays the corresponding image in the window. The destroy method releases all the resources used by the Open CV as well as the dirent header file.

Caution: If the folder contains files other than the images the code breaks. Because the dirent header file gets all the files irrespective of the file format even hidden files. We are directly displaying the images with out any filtering. we assumed that the folder contains only images. The reason behind code breaking is the cvload image display only images. If we passed files other than the images this function having no knowledge of the processing. So the code breaks.

Displaying Image based on user input

In previous post I explained how to display the image with the help of Open CV and C. In this post I will explain how we can give the user control so that he can control the output based on his input. To do this we need to provide the user a choice to select the window size as well as the path to the image to be displayed.
Fallowing is the code snippet:

void main()
{
char* path;
int choice;
printf("Enter the path to the image");
gets(path);
IplImage* image=cvLoadImage(path);
printf("Select the behaviour of window\n");
printf("1. For WINDOW_NORMAL\n");
printf("2. For WINDOW_AUTOSIZE\n");
printf("3. For WINDOW_OPENGL\n");
scanf("%d",&choice);
switch(choice)
{
case 1:
cvNamedWindow("Image",WINDOW_NORMAL);
break;
case 2:
cvNamedWindow("Image",WINDOW_AUTOSIZE);
break;
case 3:
cvNamedWindow("Image",WINDOW_OPENGL);
break;
default :
cvNamedWindow("Image",WINDOW_NORMAL);
}

cvShowImage("Image",image);
printf("Press ESC key to close the window");
if(cvWaitKey(1000)==27)
{
cvDestroyWindow("Image");
}
printf("Press any key to exit");
getch();
}

I will explain the code line by line. We need two variables to hold the path to the image to be displayed and second one is for to hold the user choice for window behavior. char * path is for holding the path for the image int choice for holding the user choice for holding the window behavior. The rest of the code explanation is same as for the previous post code snippet.
We can add one more functionality to the previous code snippet. that is we can add one more condition that user can use to load the image as color or gray scale image. In first post I explained how to specify the second argument for the cvLoadImage function. which felicitates us to control the behavior of function.

CV_LOAD_IMAGE_COLOR the loaded image is forced to be a 3-channel color image
CV_LOAD_IMAGE_GRAYSCALE the loaded image is forced to be grayscale
CV_LOAD_IMAGE_UNCHANGED the loaded image will be loaded as is

In the fallowing snippet I added the previous condition.
void main()
{
char* path;
int choice,color;
IplImage* image;
printf("Enter the path to the image");
gets(path);
printf("Choose from the fallowing\n");
printf("1. To load as color image\n");
printf("2.To load as gray scale image\n");
printf("3. To load the image as unchanged\n");
scanf("%d",&color);
switch(color)
{
case 1:
image=cvLoadImage(path,CV_LOAD_IMAGE_COLOR);
break;
case 2:
image=cvLoadImage(path,CV_LOAD_IMAGE_GRAYSCALE);
break;
case 3:
image=cvLoadImage(path,CV_LOAD_IMAGE_UNCHANGED);
break;
default:
image=cvLoadImage(path,CV_LOAD_IMAGE_UNCHANGED);
}
printf("Select the behaviour of window\n");
printf("1. For WINDOW_NORMAL\n");
printf("2. For WINDOW_AUTOSIZE\n");
printf("3. For WINDOW_OPENGL\n");
scanf("%d",&choice);
switch(choice)
{
case 1:
cvNamedWindow("Image",WINDOW_NORMAL);
break;
case 2:
cvNamedWindow("Image",WINDOW_AUTOSIZE);
break;
case 3:
cvNamedWindow("Image",WINDOW_OPENGL);
break;
default :
cvNamedWindow("Image",WINDOW_NORMAL);
}

cvShowImage("Image",image);
printf("Press ESC key to close the window");
if(cvWaitKey(1000)==27)
{
cvDestroyWindow("Image");
}
printf("Press any key to exit");
getch();
}

Displaying the Image with Open CV and C

This is the second post in Image processing. In previous post I explained the very basic code written with Open CV in C language which is useful for loading the image into the program and displaying it.

In this post I will elaborate the steps involved in displaying the image with Open CV and C.

void main()

{

IplImage* image=cvLoadImage("E:\\Capture.jpg");

cvNamedWindow("Image",WINDOW_AUTOSIZE);

cvShowImage("Image",image);

if(cvWaitKey(1000)==27)

{

cvDestroyWindow("Image");

}

}

This is same as the program in previous post. only change is you explicitly created the window to show the image. and you changed the integer passed to the cvWaitKey function. I guess you know that the ASCII value of the escape is 27. To close the window we compared the ASCII value of the escape key with the ASCII value returned by the waitkey function. If those are equal you are destroying the window so image no longer visible to you.

The cvNamedWindow function takes the two arguments first one is the name of the window and second is the parameter which controls the behavior of the window.

WINDOW_NORMAL If this is set, the user can resize the window (no constraint).

WINDOW_AUTOSIZE If this is set, the window size is automatically adjusted to fit the displayed image (see imshow() ), and you cannot change the window size manually.

WINDOW_OPENGL If this is set, the window will be created with OpenGL support.

In next post I will explain the enhanced version of the code.

                                                                                                                                         

Loading Images into the Program

   In real life we are dealing with lot of images. Especially after advancement of computers and processing capabilities of computers image processing become more easy. For  easy processing of images we can use lot of image processing libraries like Open CV, Emgu CV or well known MATLAB. In this tutorial I will demonstrate  how to use Open CV for image processing.

Software requirements:

1. Open CV
2. Visual studio

To process any image first we need to load it into the computer memory. This is very easy. Open CV provides lot of functions among those we have one function which helps us to load the image into the program. Basically Open CV supports C,C++,Java as well as Python. we can use any programming language along with Open CV based on your comfort.

I will use C and C++ to demonstrate the loading of image into the program.

The basic program to display the image using Open CV is

void main()
{
IplImage* image=cvLoadImage("E:\\Capture.jpg");
cvShowImage("Image",image);
cvWaitKey(1);
getch();
}

IplImage* is a structure holding the image data. It points to the memory location of the image. cvLoadImage function is useful for loading the image. Generally it takes to arguments first one is the path to the image to be loaded second one specifies weather the image to be loaded as color image or gray scale image or unchanged. 

CV_LOAD_IMAGE_COLOR the loaded image is forced to be a 3-channel color image
CV_LOAD_IMAGE_GRAYSCALE the loaded image is forced to be grayscale
CV_LOAD_IMAGE_UNCHANGED the loaded image will be loaded as is.

cvShowImage is useful for displaying the image on a window. the first argument takes the name of the window. and second argument takes the variable of type iplImage*. We can generate the image explicitly and we can pass the name of that window to the function other wise the function generates it for us implicitly.

cvWaitKey is the delay function which gives the time for the processor to load the image on to the window.
if forget to specify this function image can not be displayed. it takes the integer as the argument and returns the ASCII key value of the pressed key.

In next post i will try to explain the process in detail with more detailed code and its explanation and examples.