How to use estimateFlow between two images that are not video frames

I am interested in computing optical flow between images that are not part of a video sequence using the built-in MATLAB function estimateFlow. That is, I would like to compute optical flow between two images in my workspace, Img1 and Img2. How can I do this?

Currently, I am combining the images into an array and running a for loop to obtain the optical flow:

ImgStack(:,:,1) = Img1; % combine the images to a sequence
ImgStack(:,:,2) = Img2;
reset(opticFlow); % clear previous optical flow
opticFlow = opticalFlowLKDoG; % At this point, I don't realy care what method is used
for t = 1:2
    flow = estimateFlow(opticFlow,ImgStack(:,:,t));
end

This procedure is inelegant and quite cumbersome. I am doing this on a large number of images and I would like a more efficient way to do this. It would be nice if I could just do something like flow = estimateFlow(opticFlow,[Img1,Img2]).

Dose anyone know how to do this?

[Image Analyst]

What do you have to start with? A bunch of arrays called Img1, Img2, Img3, etc.? Or one array ImgStack?

[chris crowley]

This operation is nested inside of another operation within the code that only generates two images at a time. Img1 and Img2 are grayscale images of the same size. I could rewrite the code to produce an array that is all the Img1s and Img2s appended in an interlaced way into one very long array. This will limit how many images I can analyze because of memory constraints.

Answer 1

[Divya Gaddipati]

You can use opticalFlow function for this purpose.

img1 = im2double(Img1); % the images should be in double
img2 = im2double(Img2);
opflow = opticalFlow(img1, img2);

h = figure;
movegui(h);
hViewPanel = uipanel(h,'Position',[0 0 1 1],'Title','Plot of Optical Flow Vectors');
hPlot = axes(hViewPanel);

imshow(img1)
hold on
plot(opflow,'DecimationFactor',[5 5],'ScaleFactor',60,'Parent',hPlot);
hold off

For more information, refer to the documentation on opticalFlow

https://www.mathworks.com/help/vision/ref/opticalflowobject.html

[Julius Å]

Hello!

I'm trying to understand this solution.

According to the documentation, the input to the opticalFlow()-function is supposed to be the x- and y-components of velocity. In the above example, you insert the actual image frames instead. Does this really give a correct plot of the optical flow?

[Qingyuan Li]

I would also like to know the answer of Julius's questoin.

[Walter Roberson]

@Julius Å

It depends exactly what is in the images. The images just might happen to be sensor readings from a flow meter that is directly estimating velocity at each point, perhaps using sonar or polarization techniques. The image files would have to be TIFF in order to store negative values properly...

So the call here is not definitely wrong... just likely wrong.