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]

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

Answer 1

[Ishaan]

Hey @Dalia,

I noticed that the code you are using after frame extraction is same as the one in Compute Optical Flow Velocities Example provided by MathWorks. And I understand that you intend to get the resultant velocity in a vector format and know the units of its components.

I think that you are referring to the horizontal (“Vx”) and vertical component (“Vy”) of the velocity with “u” and “v” respectively, same as the nomenclature used in the video linked by @Cris LaPierre.

The units of “Vx” and “Vy” is “pixels per frame” as no other information is available to the optical flow function.

The resultant velocity vector is available as the following read-only properties of the “opticalFlow” object:

• “flow.Orientation” - Phase angles of optical flow in radians
• “flow.Magnitude” - Magnitude of optical flow in pixels per frame

For more details on “opticalFlow”, please refer to this documentation: https://www.mathworks.com/help/vision/ref/opticalflowobject.html