Average Optical Flow vectors and plot over multiple frames

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Alex M 2021-4-19

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/806572-average-optical-flow-vectors-and-plot-over-multiple-frames

编辑： Adam Danz 2021-4-28

Hello,

I am trying to get the average optical flow vectors for a whole video (e.g. for a 10 second video). Currently I am using the opticalFlowFarneback method from Estimate optical flow example with the Computer Vision Toolbox (https://www.mathworks.com/help/vision/ref/opticalflowhs.estimateflow.html):

%% Estimating Optical Flow
% This example uses the Farneback Method to to estimate the direction and speed of moving
% cars in the video
% Copyright 2018 The MathWorks, Inc.
%% Read the video into MATLAB
vidReader = VideoReader('visiontraffic.avi');
opticFlow = opticalFlowFarneback;
%% Estimate Optical Flow of each frame
while hasFrame(vidReader)
frameRGB = readFrame(vidReader);
frameGray = rgb2gray(frameRGB);
flow = estimateFlow(opticFlow,frameGray);
imshow(frameRGB)
hold on
% Plot the flow vectors
plot(flow,'DecimationFactor',[25 25],'ScaleFactor', 2)
% Find the handle to the quiver object
q = findobj(gca,'type','Quiver');
% Change the color of the arrows to red
q.Color = 'r';
drawnow
hold off
end

Although this produces a nice visualization of flow vectors between two frames (see attached image), I need more like an average of flow vectors across all frames of the video. I am quite new to MatLab, I would be super thankful for any feedback how to change abovementioned code.

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Adam Danz 2021-4-19

Your question is framed in such a way that only people familiar with the tutorial you mentioned can help.

If you can redefine the question more generally or provide a minimal working example, you might have a better chance of a bite.

Alex M 2021-4-21

thank you for your comment! You are right, I edited my initial post so I hope my question might be easier to understand now.

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Answer 1

Adam Danz 2021-4-21

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/806572-average-optical-flow-vectors-and-plot-over-multiple-frames#answer_681015

编辑：Adam Danz 2021-4-22

在 MATLAB Online 中打开

Here's a demo that shows how to

Store the flow structure within a loop (assumes all flow arrays are the same size)
Average a variable in the flow structure across loop iterations.

% preallocate structure array
flow = opticalFlow;
% Create and store optic flow object within loop
for i = 1:10
    Vx = randn(100,100);
    Vy = randn(100,100);
    flow(i) = opticalFlow(Vx,Vy);
end
% flow is a 1x10 structure array
% Concatenate Orientation along 3rd dimension
% flowOrientation is a 100x100x10 array for 
% 100x100 flow vectors across 10 iterations.
flowOrientation = cat(3,flow.Orientation);
% Average Orientation values across loop iterations
% meanOrientation is 100x100 mean vectors
meanOrientation = mean(flowOrientation,3)

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Alex M 2021-4-23

在 MATLAB Online 中打开

Thanks again for the reply, this was very helpful! This is hopefully my last question. As you recommended, I stored the flow in a structure within the while loop:

while hasFrame(vidReader)
frameRGB = readFrame(vidReader);
frameGray = rgb2gray(frameRGB);
flow(j) = estimateFlow(opticFlow,frameGray);
imshow(frameRGB);
hold on
% Plot the flow vectors
plot(flow(j),'DecimationFactor',[25 25],'ScaleFactor', 5)
drawnow
hold off
j = j + 1;
end

I used very simple and short hamburg taxi sequence as quick benchmark. flow is in my case a 1x41 optical flow array. After the loop, I then concatenate Vx and Vy from flow along the 3rd dimension. Afterwards I create a new optical flow object called flower and hand it over to the plot function.

flowVx = cat(3,flow.Vx);
flowVy = cat(3,flow.Vy);
meanVx = mean(flowVx,3);
meanVy = mean(flowVy,3);
flower = opticalFlow(meanVx,meanVy);
plot(flower,'DecimationFactor',[25 25],'ScaleFactor', 10)
q = findobj(gca,'type','Quiver');
q.Color = 'r';

However the result seems a bit weird. Although the resulting vectors seem reasonable in terms of direction, I would expect them on the lower part on the plot instead of being in the upper part. What I am still doing wrong? Help!

Alex M 2021-4-28

Thanks! I think that did the trick! The result after flipping the y-axis looks very reasonable to me. Although I was wondering why this was not issue with the example Optical Flow code in the original Post. Thank you very much for your support, it helped me a lot!

Adam Danz 2021-4-28

编辑：Adam Danz 2021-4-28

I see that you're using 2020b and that example in 2021a looks like it changed. I'd have to look at the documentation for 2020b which I don't have time to do right now but maybe I'll remember to look into it later.

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