Finding displacement/distance and velocity afterwards

I am doing a fall detection program for my project. Currently, I have tracked a moving object in my video and along with the tracking rectangle, I am able to display the centroid of that box as the object moves.

How do I go about finding the displacement of the centroid of the current frame and the previous frame? After that, how do I find velocity and display a graph showing the velocity of the object from the moment it appears in the video until the end of video? I understand to find velocity is Displacement/Frame Rate, but I am having trouble how to write the codes.

This are my codes so far:

videoObject = VideoReader(movieFullFileName); numberOfFrames = videoObject.NumberOfFrames; vidHeight = videoObject.Height; vidWidth = videoObject.Width; numberOfFramesWritten = 0; figure; set(gcf, 'units','normalized','outerposition',[0 0 1 1]); centry=[];

for frame = 1 : 100 : numberOfFrames

        intFrame = read(videoObject, 5);
    thisFrame = read(videoObject, frame);
    %Display it
    subplot(221);
    imshow(thisFrame);
        caption = sprintf('Original video\nFrame %4d of %d.', frame, numberOfFrames);
    title(caption, 'FontSize', fontSize);
        %background substraction
        i_sub=intFrame-thisFrame;
        subplot(222);
        imshow(i_sub);
        title('Template Background Subtracted', 'FontSize', fontSize);
        igray=rgb2gray(i_sub); 
        level=graythresh(i_sub); 
        binarylevel=im2bw(i_sub,level); 
        subplot(223);   
        imshow(binarylevel); 
        title('After binary', 'FontSize', fontSize);

        BinaryImage = bwareaopen(BinaryImage,10000);
        subplot(224);
        imshow(BinaryImage)
        title('Centroid position of object', 'FontSize', fontSize);
        hold on
        % Label the image.
        labeledImage = bwconncomp(BinaryImage,8);
        % Call bounding box and centroid function
        measurements = regionprops(labeledImage,'BoundingBox','Centroid');
        totalNumberOfBlobs = length(measurements);
        for blobNumber = 1:totalNumberOfBlobs
        bb = measurements(blobNumber).BoundingBox;
        bco = measurements(blobNumber).Centroid;
        x=bco(1);
        y=bco(2);
        rectangle('Position',bb,'EdgeColor','y','LineWidth',1)
        plot(x,y,'-m+');
        centry=[centry bco(2)];
        myCaption = sprintf('(x=%.1f, y=%.1f)', x, y);
        text(x+30,y, myCaption,'FontName','Arial','FontWeight','normal','FontSize',8,'Color','red');
        set(gca,'xdir','normal')
        set(gca,'ydir','reverse')
        axis on;
        end
        drawnow;
        hold off
    end
        figure;
        set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
        subplot(121);
        plot(centry,'m', 'LineWidth', 2);
        title('Motion Tracking', 'FontSize',fontSize);