draw cumulative histogram output

edit

where is this line and I cannot find it? is it at my code?

Mat c_hist(hist.size(), hist.type()); In this line you passed hist size and type to MAT object , right ?

and my question is that what the difference will be if we write

    c_hist.at<float>(0) = hist.at<float>(30)     ?

@sarmad yes in the beginning I am creating a matrix of the same size and type as your original histogram image. As for your second question

    c_hist.at<float>(0) = hist.at<float>(0) // line 1
    for(size_t k = 1; k < hist.rows; ++k)    // line 2
        c_hist.at<float>(k) = hist.at<float>(k) + c_hist.at<float>(k-1); // line 3

in line 1 I am initializing the first value of the cumulative histogram image with the first value of the normal histogram image. Since if you notice in the the last variable in line 3 I need to use the previous value of the cumulative image all the time for that reason I am starting my loop for k = 1 and not 0 since if I start for k = 0 in the first run the c_hist.at<float>(k-1) will contain garbage.

now if you use c_hist.at<float>(0) = hist.at<float>(30) instead then you will initialize the first value in the cumulative image with the 30st value of you normal histogram image, which is something that it is wrong. Actually, you can replace the below code:

// calculate cumulative histogram
Mat c_hist(hist.size(), hist.type());

c_hist.at<float>(0) = hist.at<float>(0)
for(size_t k = 1; k < hist.rows; ++k)
    c_hist.at<float>(k) = hist.at<float>(k) + c_hist.at<float>(k-1);

with:

// calculate cumulative histogram
Mat c_hist = hist.clone();

for(int k = 1; k < c_hist.rows; ++k)
    c_hist.at<float>(k) += c_hist.at<float>(k-1);

I hope that it is more understandable to you now. Let me know otherwise ;-).

It is clear thanks , I want to use video file to calculate the cumulative histogram , what changes should be done

Mat frame;

cap >> frame; // get a new frame

imshow( "video", frame );

cvtColor(frame, ycbcr, CV_RGB2GRAY);

I have tried it this error is displayed

1>source.cpp(105): warning C4244: 'initializing' : conversion from 'int' to 'float', possible loss of data 1>source.cpp(113): warning C4267: 'argument' : conversion from 'size_t' to 'int', possible loss of data

these are not errors, but warnings. Your code should work though without problem. However, you should be aware of them since they can lead to some strange behavior of your algorithm. This output points you at lines 105 and 113 of your source code in order to see what you are doing there.

I want to apply cumulative histogram to a video file I have made theses changes

   int main()
  {  
      /// Load image
      VideoCapture cap("eye movements.mp4"); // open video

     if(!cap.isOpened())  // check if we succeeded

      return -1;

   while(1)
  {
   Mat frame;
  cap >> frame; // get a new frame
  imshow( "video", frame );

this is the output , it is not updating the histogram and I think only first frame is displayed

outputoutput

please edit the post and add your full while() loop, because with the code snippet that you have added I cannot see if you are doing something wrong later. This code is only the frame capturing part which seems ok.

I have uploaded it here

code

I tested your code and it works perfectly here.