How to do a median projection of a large image stack in Matlab

Question

I have a large stack of 800 16bit gray scale images with 2048x2048px. They are read from a single BigTIFF file and the whole stack barely fits into my RAM (8GB). Now I need do a median projection. That means I want to compute the median of each pixel across all 800 frames. The Matlab median function fails because there is not enough memory left make a copy of the whole array for the function call. What would be an efficient way to compute the median?

I have tried using a for loop to compute the median one pixel at a time, but this is still terribly slow.

Answer 1

Iterating over blocks, as @Shai suggests, may be the most straightforward solution. If you do have this problem frequently, you may want to consider converting the image to a mat-file, so that you can access the pixels as n-d array directly from disk.

%# convert to mat file
matObj = matfile('dest.mat','w');
matObj.data(2048,2048,numSlices) = 0; 
for t = 1:numSlices
    matObj.data(:,:,t) = imread(tiffFile,'index',t);
end

%# load a block of the matfile to take median (run as part of a loop)
medianOfBlock = median(matObj.data(1:128,1:128,:),3);