How can I finding the x and y coordinate of a curvature frame-by-frame in a clip?

I am trying to digitalize a recording of an experiment in which a line of particle tracers demonstrating the flow of a fluid in a cup.

At the moment, I load the movie frame by frame and use the imageprocessing tools " drawassisted" to manually draw an "roi" over the curvature. Then I extract the coordinate from the "roi". But this method is very slow and time consuming and it is involved with a lot of error due to hand vibratation.

Any suggestion to make this process better and more accurate. The tracers' shape is critical and must be preserved for my analysis.

        p  = []; XY = {}; t = [];
        for i = 1 : ((vidObj.NumFrames))
            v = read(vidObj,i);
            [BW,maskedImage] = segmentImage(v);
            imshow(maskedImage,'XData',xrangeNew,'YData',yrangeNew);
        %         roi = drawpolyline;
            roi = drawassisted;
            XY{end+1} = roi.Position;
            t(end+1) = vidObj.current; 
        end