MATLAB: Help with understanding a low pass filter code

filterhistogramimage processingImage Processing Toolbox

The aim of this piece of code is to smooth a horizontal histogram by applying a low pass filter.
First horizontal and vertical histograms representing the sum of differences of gray values between neighboring pixels of an image, column-wise and row-wise were used. The horizontal histogram is named horz1 so horz1(i)=sum where 'i' is the column number and 'sum' is sum of differences. Then, a low pass filter was applied. I don't understand the 'applying low pass filter' part. Where do the values 20, 21 and 40 come from? if anyone could help me understand I would really appreciate it.
%%horizontal histogram
disp('Processing Edges Horizontally...');
max_horz = 0;
maximum = 0;
for i = 2:cols
  sum = 0;
  for j = 2:rows
    if(I(j, i) > I(j-1, i))
      difference = uint32(I(j, i) - I(j-1, i));
    else
      difference = uint32(I(j-1, i) - I(j, i));
    end
    if(difference > 20)
      sum = sum + difference;
    end
  end
  horz1(i) = sum;
  %%applying low pass filter
  sum = 0;
  horz = horz1;
  for i = 21:(cols-21)
    sum = 0;
    for j = (i-20):(i+20)
      sum = sum + horz1(j);
    end
    horz(i) = sum / 41;
  end
end

Best Answer

That's the window size they chose. A larger window size will give more smoothing. A smaller window size will give less smoothing. We don't know why they chose a window size of 41, all we can guess is that that seemed to be best for the "look" they were after.
Also, don't use "sum" as the name of a variable since it's a built-in function.
Other than that, all I can say is that the code is a poorly thought out mess. I would not recommend using code from this author.
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