How to analyse optical flow information (2024)

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Life is Wonderful on 2 Aug 2023

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Commented: Life is Wonderful on 24 Aug 2023

Accepted Answer: Praveen Reddy

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Hi there,

I have acquired the output from the optical fow in a structure and would like to learn how to use the data to say there are defects - using magnitude and Orientation, as well as how to use Vector motion Vx and Vy in my analysis.As an example, consider determining the peak and obtaining the wavelength measurement per unit time.

Is it possible to extract the output vectors? I'd like to use static analysis to determine the quality in terms of flicker, blink, and blur. Any suggestions would be highly appreciated.

The information in my sample data structure is as follows:

Vx: [360×640 single]

Vy: [360×640 single]

Orientation: [360×640 single]

Magnitude: [360×640 single]

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Image Analyst on 3 Aug 2023

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If you have any more questions, then attach your data and code to read it in with the paperclip icon after you read this:

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Edited: Life is Wonderful on 3 Aug 2023

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Sure, please accept my apologies. I am using code that is mostly available in the example paper. It would be great if you could offer your thoughts on how to proceed with the problem stated above.

vidReader = VideoReader('visiontraffic.avi');

opticFlow = opticalFlowHS

h = figure;

movegui(h);

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

hPlot = axes(hViewPanel);

fprintf('%15s| %15s| %15s| %15s|\n---------------+----------------+----------------+-----------------+\n', ...

'VidCurrentTime','Magnitude','Orientation','Algexecutiontime' );

while hasFrame(vidReader)

frameRGB = readFrame(vidReader);

frameGray = im2gray(frameRGB);

t1 =tic;

flow = estimateFlow(opticFlow,frameGray);

t2 = toc(t1);

fprintf('%15.4f|%15.8f | %15.8f| %16.8f|\n',vidReader.CurrentTime,std2(sqrt(flow.Magnitude)),std2(angle(flow.Orientation)),t2 );

imshow(frameRGB)

hold on

plot(flow,'DecimationFactor',[5 5],'ScaleFactor',60,'Parent',hPlot);

q = findobj(gca,'type','Quiver');

% Change the color of the arrows to red

q.Color = 'r';

drawnow

hold off

pause(10^-3)

end

Life is Wonderful on 8 Aug 2023

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Could you please show me how to use the above code to generate spatial image gradients in grey levels per pixel and temporal gradients in grey levels per frame and export the results?

Thank you very much

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Accepted Answer

Praveen Reddy on 23 Aug 2023

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Edited: Praveen Reddy on 23 Aug 2023

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Hi,

I understand that you want to compute spatial image gradients, temporal gradients and export the results. You can compute the spatial gradient using the flow obtained and temporal gradient as the difference between current frame and previous frame. Please find the modified code below where the results are exported as “.mat” files.

vidReader = VideoReader('visiontraffic.avi');

opticFlow = opticalFlowHS;

h = figure;

movegui(h);

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

hPlot = axes(hViewPanel);

% Create arrays to store spatial and temporal gradients

spatial_gradients = [];

temporal_gradients = [];

fprintf('%15s| %15s| %15s| %15s|\n---------------+----------------+----------------+-----------------+\n', ...

'VidCurrentTime','Magnitude','Orientation','Algexecutiontime' );

% Initialize previous frame variables

prevFrameRGB = readFrame(vidReader);

prevFrameGray = im2gray(prevFrameRGB);

while hasFrame(vidReader)

frameRGB = readFrame(vidReader);

frameGray = im2gray(frameRGB);

t1 = tic;

flow = estimateFlow(opticFlow,frameGray);

t2 = toc(t1);

fprintf('%15.4f|%15.8f | %15.8f| %16.8f|\n',vidReader.CurrentTime,std2(sqrt(flow.Magnitude)),std2(angle(flow.Orientation)),t2 );

% Calculate spatial gradient (per pixel) using the magnitude of flow

spatial_gradient = sqrt(flow.Magnitude);

spatial_gradients = [spatial_gradients; spatial_gradient(:)];

% Calculate temporal gradient (per frame) using the difference between current and previous frame

temporal_gradient = abs(frameGray - prevFrameGray);

temporal_gradients = [temporal_gradients; temporal_gradient(:)];

imshow(frameRGB)

hold on

plot(flow,'DecimationFactor',[5 5],'ScaleFactor',60,'Parent',hPlot);

q = findobj(gca,'type','Quiver');

q.Color = 'r';

drawnow

hold off

pause(10^-3)

% Store current frame for the next iteration

prevFrameRGB = frameRGB;

prevFrameGray = frameGray;

end

% Export the spatial and temporal gradients as data files

save('spatial_gradients.mat', 'spatial_gradients');

save('temporal_gradients.mat', 'temporal_gradients');

I hope this helps.

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Life is Wonderful on 24 Aug 2023

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Hi @Praveen Reddy,

could you kindly advise on how to rotate the angular component for both temporal and spatial gradients?

I am really grateful.

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How to analyse optical flow information (2024)

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