Counting line in video

So I'm doing a project in automatic people counting using optical flow. I've managed to get the optical flow applied to the video and working quite well but I'm really struggling to implement a line onto the video that will actively tell when it has been crossed. I have attached the code i have for applying the optical flow and simply putting a line in where I'd like it to be but the line is purely for visual basis, it doesn't react to people crossing it. I've seen a few videos that have lines that when they are crossed change colour and allow for the count to go up or down. Any help would be greatly appreciated!

%% Initialisation step to clear up command window close all clear clc %% Set up video reader videoReader = vision.VideoFileReader('vtest.avi','ImageColorSpace','Intensity');

%This is a quicker way of carrying out rgb2gray for video %%Trigger line

%RGB = insertShape('videoReader','line',[135 500 335 200]); p1 = [200 300]; p2 = [275 200]; %plot([p1(2),p2(2)],[p1(1),p2(1)],'color','r','LineWidth',2); %% Set up optical flow of = opticalFlowHS; of.Smoothness = 0.1;

%Smoothness is the assumption that neighbouring pixels will have similar %values, so a if the change between frames is large, so should this be

%% Loop algorithm while(~isDone(videoReader)) %looping through for e\ach frame

    % Acquire frame
    vidFrame = step(videoReader);       %Takes a screenshot

    % Estimate flow
    flowField = estimateFlow(of,vidFrame); %Estimate Flow gets the of between 
                                           %the specified frame and the previous one

    % Visualize flow field
    %subplot(2,2,[1 3]);
    imshow(vidFrame)
    hold on
    plot(flowField,...
        'DecimationFactor',[5 5],...    %Divides the amount of arrows for less cluttered view, uses 2x2 matrix for x and y components
        'ScaleFactor',5);               %Scales the of vectors 
    title('Optical Flow')
    hold on
    plot([p1(2),p2(2)],[p1(1),p2(1)],'color','r','LineWidth',2);
    hold off

    % Update figures
    drawnow;

end

%% Clean up release(videoReader);