How to obtain velocity and magnitude vectors of the optical flow field? I am using Lucas-Kande method of optical flow.

I am working on the following code.

n=0;

folder = fileparts(which('viptraffic.avi'));

movieFullFileName = fullfile(folder, 'viptraffic.avi');

vidReader = VideoReader(movieFullFileName);

opticFlow = opticalFlowLK('NoiseThreshold',0.0039);

while hasFrame(vidReader)

   frameRGB = readFrame(vidReader);
   frameGray = rgb2gray(frameRGB);
   flow = estimateFlow(opticFlow,frameGray); 
   H=imag(flow)
   V=real(flow)
    frameWithFlow = getframe(gca);
    imshow(frameRGB);
    imshow(frameWithFlow.cdata)
    hold on
    plot(flow,'DecimationFactor',[5 5], 'ScaleFactor',10)
    hold off
    n=n+1;
end

Is there a way to get the optical flow estimates of velocity and magnitude for every optical flow field obtained in every image?

[Nachiket Patki]

hey @ Daksh Agarwal I also tried this code can you please tell me one thing whether your output video runs very slowly? Because I am facing that problem. How can I make it faster?

Answer 1

[Vishnu Sreekumar]

If you're still looking for an answer:

x_vel = flow.Vx;
y_vel = flow.Vy;
magnitudes = flow.Magnitude;
orientations = flow.Orientation;

Note that if the image is n x m (n rows, m columns), X direction is top to bottom and Y direction is left to right when using imshow and

plot(flow,'DecimationFactor',[5 5], 'ScaleFactor',10)

is the same as

quiver(1:n,1:m,x_vel,y_vel);

[Nachiket Patki]

I implemented the same code and what I obtained is mxn sized x_vel and y_vel matrix (also magnitude and orientation get the same size) which is same as the frame size. But how can I get the velocity of only the objects (what I mean is particularly the objects only)that are present in the video frames?

[Cheuk Wing Edmond Lam]

do you happen to know the units of the values reported by flow.Magnitude?

[Vishnu Sreekumar]

@Cheuk: No but I imagine it is pixels traveled per timeframe.

So if your magnitude is 0.4, and the distance between pixels is 1 cm (depending on your specific application), and the timestep between two frames is 0.1 s, then the speed is 0.4 x 1 cm / 0.1 s = 4 cm/s.