correlation matrices after ensemble correlation

[Gladys Diaz]

Hi William,

I've been using PIVlab for a couple years now, trying to get the velocity of RBCs in a microchannel. We are using ensemble correlation with 100 pairs of images. I am doing 3 passes with sizes of 16, 8 and 8. 

We are getting an overestimation of the velocity close to the walls, and we believe it is due to the high shear and a higher variability of velocities inside the interrogation window, so we were interested in seeing the correlation matrices and the shape of the correlation peak. 

I have tried to save the matrices in piv_FFtensemble but they all seem to show a displacement of ~0, so I believe I am saving the wrong matrix. 

I am currently saving result_conv_ensemble after line 850. I tried changing do_correlation_matrices to 1 in PIVlab_GUI but it wasn't saving anything. 

I am using PIVlab 3.10 in matlab 2025a

Any help would be much appreciated 

Thanks! 

Gladys Diaz 

[William]

Hi, I can check this later. But what is the displacement in pixels in your images? 16 px as interrogation area seems very small. And you should not have two passes with the same size, as this will only increase noise.

[Gladys Diaz]

Hi William, 

My displacement depends on the location in the channel. They should all be between 1 and 4 pixels. My windows are small because we are trying to get as many vectors in the radial direction and our channel is 68 pixels wide, so my images are 68x5000 pixels. I can reduce my passes to only two

[William]

Hi, when I uncomment these lines in piv_FFTensemble, then I get all correlation matrices in Matlabs workspace as soon as the analysis finishes:

%% Write correlation matrices to the workspace

%{

try

counter=evalin('base','counter');

counter=counter+1;

assignin('base','counter',counter);

all_matrices=evalin('base','all_matrices');

all_matrices{end+1}=result_conv_ensemble;

assignin('base','all_matrices',all_matrices);

disp('appended matrix')

catch

assignin('base','counter',1);

all_matrices{1}=result_conv_ensemble;

assignin('base','all_matrices',all_matrices);

disp('created new matrix')

end

%}

[Erfan Saeedian Sar]

Hello Dear William, 

I have been using the PIVlab in my research for internal combustion engine (thanks for the fantastic app, it makes life much easier:), and for uncertaitny analysis of PIV based on some scholars (like Vlachos,2013 paper on PIV uncertainty based on correlations peak), I need to have full access to the correlation matcies, from the method Multipass FFT windows deformation. Is there a similar way to have access ot it? I tried to modfiy the codes a bit, but got no resluts so far. 

Thanks for time and help,

Erfan

[thielickeoptolution]

Hi, afaik, you just set do_correlation_matrices to 1 in PIVlab_GUI.m That is only possible when using Matlab. Then they should appear in the matlab workspace.

[Erfan Saeedian Sar]

Hello, Thanks for the response. It works now!