Optical flow between two frames

I have used the following code to estimate a velocity of a moving box, u and v values of different points at the same block are not close. How can I estimate the representive u & v values?

obj = VideoReader('bigbox.mp4');
for k = 1 : 2  
    this_frame = readFrame(obj);
    thisfig = figure();
    thisax = axes('Parent', thisfig);
    image(this_frame, 'Parent', thisax);
    title(thisax, sprintf('Frame #%d', k));
end
%% optical flow
I1=imread('Framep#1.png');
I2=imread('Framep#2.png');
modelname = 'ex_blkopticalflow.slx';
open_system(modelname)
out = sim(modelname);
Vx = real(out.simout);
Vy = imag(out.simout);
img = out.simout1;
flow = opticalFlow(Vx,Vy);
figure
imshow(img)
hold on
plot(flow,'DecimationFactor',[5 5],'ScaleFactor',40);

[Image Analyst]

If you have any more questions, then attach your data ('bigbox.mp4') with the paperclip icon after you read this:

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