Image corruption in matlab parfor loop - matlab

Question

I have some code for an adaptive median filter (see below) which works perfectly until I attempt to run it as a parallel loop, in which case there are values missing or corrupted and the image I get at the end is incorrect (see http://i.stack.imgur.com/Rt6dV.jpg).
Matlab doesn't throw up any errors but appears to be either dropping random data, or overwriting them. The only thing I could find which would even vaguely match this problem was information on custom type definitions and set functions, but all of my input images are uint8 so I don't have any custom set functions or type definitions. (http://fluffynukeit.com/problems-with-matlab-parfor-data-disappearing/)
The error occurs specifically between lines 63 and 86, when I attempt to change
for Index = 1:length(ss)
to
parfor Index = 1:length(ss)
and I can't for the life of me figure out why.
I usually call the function with an image, I, in uint8 format (although any format should work), w_size is an odd number relating to the size of the window for filtering, and M can be set from 0 to 2, where lower numbers reduce the amount of pixels considered when median filtering by increasing the weight of the standard deviation of the current block. So a typical function call would be thus:
I = imread('image.tif');
J = Par_AMF(I, 5, 1);
figure;
imshow(J)
I should finish by saying that typically the size of images I'm using make the parallel processing a huge time saver, usually about half the time of the non-parallel execution, so although there isn't much point for smaller images the ones I'm using would really benefit from it.
function [FIm] = Par_AMF(Im, w_size, M)
% PAR_AMF Adaptive median filter
% Performs a local adaptive median filtering operation
% Inputs: Im - Image to be filtered
% w_size - Window size for filter
% M - Sigma weighting, range from 0 to 2
%
% Output: FIm - Filtered image
tic;
sz_oldim = size(Im);
w_option = floor(w_size/2);
% ----------------------------------------------------------------------- %
% Padding edges of image matrix
Pad_im = padarray(Im,[w_option, w_option],0,'both');
% ----------------------------------------------------------------------- %
% Stage 1
% Calculate the local Mean and Standard Deviation for finding Speckle
% Pixels
% ----------------------------------------------------------------------- %
sz = size(Pad_im);
start_index = w_option+1;
end_index1=(sz(1)-w_option);
end_index2=(sz(2)-w_option);
LB = zeros(sz_oldim);
UB = LB;
first_loop1 = start_index:end_index1;
first_loop2 = start_index:end_index2;
ss = cell(numel(LB), 1);
Index = 1;
for c = first_loop2
for r = first_loop1
ss{Index} = Pad_im(r-w_option:r+w_option,c-w_option:c+w_option);
Index = Index+1;
end
end
% This parfor functions as expected:
parfor Index = 1:length(ss)
ss1=ss{Index}(:);
if any(ss1 > 0)
local_mn = mean(ss1);
local_sig = std(double([ss1;local_mn]));
LB(Index) = local_mn - (M*local_sig);
UB(Index) = local_mn + (M*local_sig);
end
end
% ----------------------------------------------------------------------- %
% UB is the Upper Bound Limit; LB is the Lower Bound Limit
% ----------------------------------------------------------------------- %
% ----------------------------------------------------------------------- %
% Using UB and LB mark pixels as speckle or valid pixels and update only if
% central pixel of window is speckle pixels with median value calculated
% again with only speckle pixels with the window
% ----------------------------------------------------------------------- %
TempOut = Im;
% When this for loop is changed to a parfor (which should definitely work)
% the output is corrupted horribly:
% parfor Index = 1:length(ss)
for Index = 1:length(ss)
ss1 = ss{Index}(:);
ss2 = ss1;
if any(ss1>0)
zz = 0;
for z = 1:length(ss1)
if (ss1(z) < LB(Index) || ss1(z) > UB(Index))
ss1(z) = 0;
ss2(z) = 0;
else
ss1(z)=1;
zz= zz+1;
end
end
if zz > 0
new_ss1 = ss2(ss2~=0);
if(ss1(ceil(length(ss1)/2)) == 0)
md = median(new_ss1);
TempOut(Index) = md;
end
end
end
end
looptime = toc;
disp(['Total processing time ', num2str(looptime./60), ' minutes'])
FIm = TempOut;
end
EDIT: In Matlab version R2015a