Image Processing, Draw Line, Find Angle

Hi all, I currently doing a project to find the angle for the leg of a cyclist in video.

Below is the code and the output I able to get

video = VideoReader('BikeFit1.mp4');
nFrame = video.NumFrames;
for img = 1:nFrame;
    frames = read(video,img);
    imshow (frames); %Able to detect the red area but not able to run all the frames after detected
 
    myVideo = VideoWriter ('AfterDetected.mp4'); %To create video
    myVideo.FrameRate = 10; %To create video
    open(myVideo) %To create video
    
    %Filter the frames to detect the red marker
    diff_im = imsubtract(frames(:,:,1), rgb2gray(frames));
    diff_im = medfilt2(diff_im, [3 3]);
    diff_im = imbinarize(diff_im,0.18);
    diff_im = bwareafilt(diff_im,[1000 2200]);
    bw = bwlabel(diff_im, 8);
    stats = regionprops(bw, 'BoundingBox', 'Centroid');
    
    %Mark the red marker
    hold on
    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', 'white');
    end
    hold off
   
    pause(0.01) %To create video
    frames2 = getframe(gcf); %To create video
    writeVideo (myVideo,frames2); %To create video
    close(myVideo) %To create video
end

** Now I need

1. filter the unwanted red areas, only the three markers on leg is needed
2. draw line between 2 markers (will have 2 lines)
3. get the angle between 2 lines
4. Save it in video, the code above with comment %to create video is not function.

But until now, I not able to find the suitable code to do these, may anyone help?

Thank you very much

[Teo Chou Wen]

Really needs help, thanks