How do you track a tip of a pendulium and plot its displacement against time?

I'm trying to track the horizontal motion of a tip of an oscillating pendulum and plot it's displacement versus time graph. This y-axis is fixed through the frames.

I know i have to : 1) upload the video 2) read each frame 3) track a point on each frame 4) plot the displacement of that tip vs time

video: https://www.dropbox.com/scl/fi/9kauaf0lxm8i69ru04xsl/FR.mp4?rlkey=mzrt6ffxjli7c30e90ir0bcv6&st=sxodfpxh&dl=0

And so far, i've used this:

close all;clear all;

% read video file; xyobj is an matlab object
%there are files:
filename='FR1-10-2.mp4';


xyobj = VideoReader(filename);

Error using VideoReader/initReader (line 734)
The filename specified was not found in the MATLAB path.

Error in audiovideo.internal.IVideoReader (line 136)
initReader(obj, fileName, currentTime);

Error in VideoReader (line 104)
[e-mail](varargin{:});

% number available frames
nframes = xyobj.NumberOfFrames-1

% frame rate
framerate=xyobj.FrameRate

vidHeight = xyobj.Height
vidWidth = xyobj.Width


frame(1) = struct('cdata', zeros(vidHeight, vidWidth, 3, 'uint8'),'colormap',[]);

figure(1)
% Specify the boundary of the region to cut
b3=220;b4=350;
b1=375;b2=640;
xmax=b4-b3;
ymax=b2-b1;

hold on
grid on
xlabel('x')
ylabel('y')
axis([ 1 xmax 1 ymax]);
kk=0;
for k1 = 1:nframes-1,
k=nframes-k1;
k
frame(1).cdata=read(xyobj,k);
oneframe=frame(1).cdata;

 oneframecut=oneframe(b1:b2,b3:b4,:); % cut an area with ball only to help for recognition

for jp=1:206,
    jp1=jp+50-1;
greenChannel = oneframecut(jp1,:, 2);
[xm,im]=min(greenChannel);
xedge(k,jp)=im;
pedge(k,jp)=xm;
end
%pause(0.1)


end


save FR.pos xedge -ascii
pedge1=cast(pedge,"double");
save FR.val pedge1 -ascii

Your help is really appreciated!

Answer 1

[Image Analyst]

It all comes down to extracting one frame at a time and then segmenting that frame and using regionprops to find the centroid.

See my attached demos on video tracking.

Also see my Image Segmentation Tutorial in my File Exchange:

Image Analyst's File Exchange

It's a generic, general purpose demo of how to threshold an image to find blobs, and then measure things about the blobs, and extract certain blobs based on their areas or diameters.