Optical Flow in Matlab

Question

How do you compute and display the optical flow of two images in Matlab? Is their a built in function for this and if so, how do you implement this?

Answer 1

If you are merely interested in how to visualize optical flow as color and/or arrows, here is a simple script to show you how it can be done in Matlab:

% Generate an example flow field
[u,v] = meshgrid(linspace(-1,1)/sqrt(2));
mag = sqrt(u.^2 + v.^2);
an = (atan2(v,u)+pi)/(2*pi);

%display as arrows
subplot(1,2,2);
quiver(u(1:5:end,1:5:end),v(1:5:end,1:5:end));
axis image

%display as color
subplot(1,2,1);
imagesc(hsv2rgb(an,mag,mag));
axis image

My toolbox on optical flow in Matlab (which I am now advertising) has at least 3 different ways of visualizing optical flow with a combination of arrows and color coding. To use it with an avi movie file, do as follows:

in.movieType = 'example.avi'; % assumes a file 'example.avi' in current folder. 
% in.movieType = 'synthetic';  % generate synthetic video. 
% in.movieType = 'camera';     % assumes a camera available in the system. 

in.method =  @Flow1;       %Locally regularized and vectorized method
% in.method =  @FlowLK;      %Lucas and Kanade 
% in.method =  @FlowHS;      %Horn and Schunk, variational (global regularization)

in.bRecordFlow = 1; %record the optical flow, not only display it

% resolution of flow field [Height Width]:
in.flowRes = [24 24];     %flow resolution <= video resolution

% DO THE CALL TO THE TOOLBOX(initiate the session):
pathToSave = vidProcessing(in);

% view interactively the saved session:
FancyFlowPlayer(pathToSave);

Answer 2

If you have access to the Matlab Computer Vision Toolbox, you can use the built in functions of Lucas-Kanade Optical FLow, Horn-Schunck and Farnebäck. They both need a grayscale image sequence. They return a Flow object containing the flow in x,y direction aswell as magnitude and angle. For displaying you can use plot or quiver.