Pre-allocate not helping

I am running this piece of code with and without pre-allocating the matrices(res_stroke and res_erase) but after testing :

- Time taken without pre-allocation : 23.97s

- Time taken with pre-allocation : 21.92s

If someone could point to some possible error then it would be really helpful.

The code is for going through each frame of a video and traversing each pixel to store latest positive pixel in res_stroke and the latest negative pixel in res_erase.

mov = VideoReader('C:\Users\Harshil\Desktop\SURA\Resources\gesture.mp4');
numberOfFrames = mov.NumberOfFrames;
%num = numberOfFrames;
num = 24;
res_stroke = ones(num-1, 3);  
% res_stroke = [];
res_erase = ones(num-1, 3);   
% res_erase = [];
tic;
for i = 1:(num-1)
  fr_in = imcrop(bwmorph(edge(rgb2gray(read(mov, i))), 'close'), [130 5 595 470]);
  fr_fin = imcrop(bwmorph(edge(rgb2gray(read(mov, i+1))), 'close'), [130 5 595 470]);
  %figure, imshow( imabsdiff(fr_in, fr_fin), [] );
  diff = imabsdiff(fr_in, fr_fin);
  [x, y] = size(diff);
  progressIndication = sprintf('Processed diff[%d]',i);
  disp(progressIndication);
  if max(max(diff))==0
     continue;
  end
  for l = 1:x
     for m = 1:y
         if diff(l, m) > 0
             res_stroke(i,:) = [l, m, double(i)/double(30)];  
             %res_stroke = [res_stroke; l, m, double(i)/double(30)];
         elseif  diff(l, m) < 0
             res_erase(i,:) = [l, m, double(i)/double(30)];   
             %res_erase = [res_erase; l, m, double(i)/double(30)];
         end
     end           
  end
end
toc

Answer 1

[per isakson]

These two line takes most of the time

&nbsp

and this one very little ( res_stroke is not large)

&nbsp

And after the first turn of the outer loop the array is "preallocated" anyhow.

Use the profiler. See profile, Profile execution time for functions

[harshil meena]

Thank you for your help 'per isakson'