MATLAB: Time-optimization? Digital image processing

digital image processingImage Processing Toolboxoptimization

Hello,
For each column of the mask, I calculate the first non-zero pixel position (2000 images). Then, I save them to an array and compute the maximum; between other simple operations.
It is working fine, but it is not at all time-efficient. Would you have any suggestion?
Thank you very much,
Best regards,
Santiago
   Base= Mask; 
      i=1; 
      b= zeros (y,1,num_images,'double'); %preallocating 
      rows= x;
      columns= y;
    for k= 1:num_images;
    for col= 1: columns
    a = find(D(:, col,k), 1, 'first'); %find the first non-zero pixel
     if a > 0 
     b(i,k) = a; 
     c = max (b(:,k)); 
     i= i+1;
    else
      continue
       end
      end
      Base(c:rows,:,k)=1; %keeping the top part
      end

Best Answer

The vectorized method of finding the first non-zero pixel for each column is:
leading_zero_count = sum( cumprod( ~D ) );
This will do the operation for all columns for all images simultaneously. The output will be 1 x number_of_columns x number_of_images
This is the count of the number of leading zeros, which is not exactly the same as the location of the first non-zero pixel. For any column that has a non-zero pixel, the leading zero count will be 1 less than the position of the first non-zero pixel. For any column that does not have any non-zero pixels, the leading zero count will be the same as the number of rows, which is a non-empty value, where-as the result of find() for such a case would be [] (the empty vector). A leading zero count can be easier to work with:
D(leading_zero_count(1,J,K)+1:end, J, K)
is easy to code without having to test for the [] that find() can return.
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