finding a velocity of moving object using optical flow

I have a matlab based application in which I compute the optical flow by the following code. How can I find the resultant vector of velocity for the moving object and what are the units of u and v?

%%Extraction of frames
>> obj = VideoReader(filename);
for k = 1 : 2  %fill in the appropriate number
    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)

[Cris LaPierre]

You might be interested in our Computer Vision for Engineering and Science specialization. It's free to enroll.

Here's a relevant video from Course 3: Applying Optical Flow