Help with understanding a low pass filter code

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abich 2017-6-24

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/346139-help-with-understanding-a-low-pass-filter-code

评论： Image Analyst 2017-6-26

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

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Answer 1

Image Analyst 2017-6-24

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https://ww2.mathworks.cn/matlabcentral/answers/346139-help-with-understanding-a-low-pass-filter-code#answer_271851

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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abich 2017-6-26

Thank you for your help. Well this is an algorithm for car plate extraction from an image. Why do you think the author used the low pass filter in the first place? What do we normaly use it for?