Tracking cars in a video using Optical flow. Vectors not displaying properly on the image

Hi I am new to computer vision and optical flow in general. So I am basically trying to plot motion vectors on moving vehicles in a video. It is based on one of the MATLAB tutorial for computer vision. So the problem I am having is that the vectors are not plotting properly. Can anyone help me?

if true
  % code
end
Here is the code:
%%initialization
close all
clear all

% Create reader

reader = vision.VideoFileReader; reader.Filename = 'viptraffic.avi';

% Create viewer
viewer = vision.DeployableVideoPlayer;
%%viewer.FrameRate = 10;

 %Create Optical Flow
optical = vision.OpticalFlow; %how pixels are moving from one frame to the next
optical.OutputValue = 'Horizontal and vertical components in complex form'; %will allow us to draw a vector
 %%%on the vision so that we see how the pixels are moving from one frame to the next

 %%We pass the horizontal and vertical components to the shape inserter
 %%below

 % Display vector fields
 shapes = vision.ShapeInserter;
 shapes.Shape = 'Lines';
 shapes.BorderColor = 'white';

 R = 1:4:120;%%downsample the optical flow field
 C = 1:4:160;%%downsample the optical flow field

 [Cv, Rv] = meshgrid (C, R); %%%display a grid on the image and take every fourth value

 Rv = Rv(:)';
 Cv = Cv(:)';

 %%Execution
 reset(reader)

 %Set up for stream
 while ~isDone(reader)

    I = step(reader);
    of = step(optical,rgb2gray(I));
    size(of)

    ofd = of(R,C);
    size(ofd)

    H = imag(ofd)*20;
    V = real(ofd)*20;

    %Draw lines on top of image
    lines = [Rv;Cv; Rv+H(:)'; Cv+V(:)']; %%start and a finish , start+movement, end+movement
   % lines = [Cv;Rv;Cv;Rv];
    Ishp = step(shapes,I,lines);

    step(viewer,Ishp);

end release(viewer);