"parfor" skips or fail with filtfilt function

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Hi, I am trying to use 'parfor' to speedy a loop that performs 3 steps to a simple time series (SignCorr): 1 - Detrend 2 - Linear regression 3 - Low Pass Filtering Apparently, 'parfor' skip the step 3. I can run the same code just changing 'parfor' to 'for' and the results are different. Then I noticed that the difference is the LowPass Filtering Step. I am attaching the code only with the LPF and the 'parfor' results in a generic error: "Struct contents reference from a non-struct array object.".
Any idea? Thanks!
Fs = 1/2; % sampling frequency (Hz)
tp = 180; % Time Points
% LowPass Filter design
FpassLOW = 0.1; % passband frequency (Hz)
FstopLOW = 0.1 + (Fs/tp); % stopband frequency (Hz)
ApassLOW = 1;% passband ripple (dB)
AstopLOW = 50; % stopband attenuation (dB)
hLOW = fdesign.lowpass('Fp,Fst,Ap,Ast',FpassLOW,FstopLOW,ApassLOW,AstopLOW,Fs);
HdLOW = design(hLOW, 'equiripple');
EPIreshTMP = zeros(size(finalEPIresh));
tic
parfor tt3 = 1:size(EPIresh,2) % EPIresh = 180x176004 Matrix (Time x Spatial Dimensions)
SignCorr = EPIresh(:,tt3);
MMM = mean(SignCorr);
% Low Pass Filtering
s_filt2 = repmat(s_corr,6,1); % we use the center copy, avoinding phase distortion
s_filtAB1LOW = filtfilt(HdLOW.Numerator,1,s_filt2);
s_filtAB1LOW = s_filtAB1LOW((1+2*180):(3*180));
EPIreshTMP(:,tt3) = s_filtAB1LOW(:) + MMM; %giving the mean back
end
toc
newEPIresh = EPIreshTMP;
  6 Comments
Brunno Machado de Campos
Brunno Machado de Campos on 13 Sep 2018

@Jan: As I mentioned: attached a print of the error.

And to be crystal clear, the Line 17 is: "parfor tt3 = 1:size(EPIresh,2)"

Not sure of how this is more informative than what I posted before but anyway. Thank you.

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Accepted Answer

OCDER
OCDER on 12 Sep 2018
Looks like your object HdLOW is not being passed to the parfor correctly.
The output of design seems to be a System Object. https://www.mathworks.com/help/dsp/ref/design.html
parfor processes Objects differently than a pure structure. https://www.mathworks.com/help/distcomp/objects-and-handles-in-parfor-loops.html
Try the following:
EPIresh = rand(180,100000); % simulating my image data
Frgval = [rand(180,6),ones(180,1)]; % simulating my
% regression matrix (confounding series + column of ones
Fs = 0.5; % sampling frequency (Hz)
tp = 180; % Time Points
% LowPass Filter design
FpassLOW = 0.1; % passband frequency (Hz)
FstopLOW = 0.1 + (Fs/tp); % stopband frequency (Hz)
ApassLOW = 1; % passband ripple (dB)
AstopLOW = 50; % stopband attenuation (dB)
hLOW = fdesign.lowpass('Fp,Fst,Ap,Ast',FpassLOW,FstopLOW,ApassLOW,AstopLOW,Fs);
HdLOW = design(hLOW, 'equiripple');
EPIreshTMP = zeros(size(EPIresh));
tic
HdLOW_Numerator = HdLOW.Numerator; %<===Pull out what you need from the object
parfor tt3 = 1:size(EPIresh,2)
SignCorr = EPIresh(:,tt3);
MMM = mean(SignCorr);
% Detrend
SignCorr = detrend(SignCorr,'linear');% detrend
SignCorr = SignCorr + MMM; %giving the mean back
% Regression
b = regress(SignCorr,Frgval);
WReg = Frgval*b;
s_corr = SignCorr - WReg;
s_corr = s_corr + b(end); % giving the mean back
% Low Pass Filtering
s_filt2 = repmat(s_corr,6,1); % I put the real data in a
% sanduiche of itself, avoinding phase distortion
s_filtAB1LOW = filtfilt(HdLOW_Numerator,1,s_filt2); %<===Change this to not call the object
s_filtAB1LOW = s_filtAB1LOW((1+2*180):(3*180)); %¨taking the
% center copy of the series
EPIreshTMP(:,tt3) = s_filtAB1LOW(:)+ b(end);%giving the mean back
end
toc
newEPIresh = EPIreshTMP;
  2 Comments
Brunno Machado de Campos
Brunno Machado de Campos on 12 Sep 2018
Just to add another observation: In a 'parfor' loop, we can't even check if a structure variable is empty or not (isempty(XXX)).
Thank you all!

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