I need to save centroid coordinates in a video

How do I save the coordinates of my centroid as I need to get many coordinates?

a = imaqhwinfo;
%[camera_name, camera_id, format] = getCameraInfo(a);

% Capture the video frames using the videoinput function
% You have to replace the resolution & your installed adaptor name.
vid = videoinput(winvideo, camera_id, format);

% Set the properties of the video object
set(vid, 'FramesPerTrigger', Inf);
set(vid, 'ReturnedColorspace', 'rgb')
vid.FrameGrabInterval = 5;

%start the video aquisition here
start(vid)

% Set a loop that stop after 100 frames of aquisition
while(vid.FramesAcquired<=200)
    
    % Get the snapshot of the current frame
    data = getsnapshot(vid);
    
    % Now to track red objects in real time
    % we have to subtract the red component 
    % from the grayscale image to extract the red components in the image.
    diff_im = imsubtract(data(:,:,1), rgb2gray(data));
    %Use a median filter to filter out noise
    diff_im = medfilt2(diff_im, [3 3]);
    % Convert the resulting grayscale image into a binary image.
    diff_im = im2bw(diff_im,0.18);
    
    % Remove all those pixels less than 300px
    diff_im = bwareaopen(diff_im,300);
    
    % Label all the connected components in the image.
    bw = bwlabel(diff_im, 8);
    
    % Here we do the image blob analysis.
    % We get a set of properties for each labeled region.
    stats = regionprops(bw, 'BoundingBox', 'Centroid');
    
    % Display the image
    imshow(data)
    
    hold on
    
    %This is a loop to bound the red objects in a rectangular box.
    for object = 1:length(stats)
        bb = stats(object).BoundingBox;
        bc = stats(object).Centroid;
        rectangle('Position',bb,'EdgeColor','r','LineWidth',2)
        plot(bc(1),bc(2), '-m+')
        a=text(bc(1)+15,bc(2), strcat('X: ', num2str(round(bc(1))), '    Y: ', num2str(round(bc(2)))));
        set(a, 'FontName', 'Arial', 'FontWeight', 'bold', 'FontSize', 12, 'Color', 'yellow');
        allCentroids = [measurements.Centroid];
         centroidX = allCentroids(1:2:end); % Extract x centroids.
          centroidY = allCentroids(2:2:end); % Extract y centroids.
    end
    
    hold off
end
% Both the loops end here.

% Stop the video aquisition.
stop(vid);

% Flush all the image data stored in the memory buffer.
flushdata(vid);

% Clear all variables
clear all
sprintf('%s','That was all about Image tracking, Guess that was pretty easy :) ')

I want to do so by creating an array. So far I have built on the red object detection code on matlab.

Answer 1

[Image Analyst]

You don't need to do this:

sprintf('%s','That was all about Image tracking, Guess that was pretty easy :) ')

instead just do this:

sprintf('That was all about Image tracking, Guess that was pretty easy :) \n');

To save the coordinates, you need to index it.

Get rid of this from the loop:

        allCentroids = [measurements.Centroid];
         centroidX = allCentroids(1:2:end); % Extract x centroids.
          centroidY = allCentroids(2:2:end); % Extract y centroids.

Then save allCentroids in a cell array (since it may have a variable number of centroids per frame) right after you call regionprops() on that frame:

stats = regionprops(bw, 'BoundingBox', 'Centroid');
allCentroids{frameNumber} = vertcat(stats.Centroid);
frameNumber = frameNumber + 1;

Make sure you initialize frameNumber before the while loop

          frameNumber = 1;          

After that, you can do whatever you want, like call save() to store it in a .mat file on disk, or whatever you want.

[Havva Jabbar]

Thank you so much image analyst. Now, if I want to use the values inside the cell array to do some computation, how can I? What I want to do is detect red points at knee,ankle and hip and calculate angles from it.

[Image Analyst]

You can get the results back out of the cell array like this:

theseCentroids = allCentroids{frameNumber}

Some people in my company are using Cornell University's Open Pose to get angle, location, and distance information from such key locations (jointed locations) on human bodies:

Open Pose Website

It works remarkably well. I don't know all the implementation details since I've only seen the results, and I haven't done the programming myself.