| Re: I think my circular median filter is very inefficient [message #89205 is a reply to message #89204] |
Sun, 03 August 2014 20:15   |
JRP
Messages: 3
Registered: August 2014
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Junior Member |
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On Monday, 4 August 2014 08:35:12 UTC+10, Mats Löfdahl wrote:
> Den söndagen den 3:e augusti 2014 kl. 05:00:59 UTC+2 skrev JRP:
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>> Hi, I have written a circular median filter for removing noise from a noisy signal, which runs through a loop of radii (r=2 -> r=20) and then calculates a peak signal-noise ratio to determine which radius does well (this does not take long). I am by no means at all experienced in any kind of programming, so if anyone is able to offer me any assistance in reducing the time it would take me to do this it would be greatly appreciated! Here is the part of the median filter code:
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>> for k=0,size3[1]-size2[1]-1 do begin
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>> for l=0,size3[2]-size2[2]-1 do begin
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>> for i=0, size2[1]-1 do begin
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>> for j=0, size2[2]-1 do begin
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>> holder[i,j] = (se[i,j]*padding[i+k,j+l])
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>> med = MEDIAN(holder)
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>> clean[k,l] = med
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>> endfor
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>> endfor
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>> endfor
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>> endfor
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>> Here, size2 is the size of the circular structuring element and size3 is the size of the padded noisy image. So basically I multiply the structuring element by the noisy image (which is padded), store in the values that fill the circle, then find the median and assign it to a "cleaned" image. Then the loop moves the structuring element 1 unit over...
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>> At the moment, the program has done up to r=17, and has been running for 7.2 hours. I've been printing the time it takes for each radii to complete and it will take something like a further 5.5 hours to complete! :(
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>> Cheers!
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> Like Fabien, I think you should lose the inner two loops. They are equivalent to something like:
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> holder = (se*padding[k:k+size2[1]-1,l:l+size2[2]-1])
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> med = MEDIAN(holder)
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> clean[k,l] = med
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> But I wonder if your code really does calculate the medians you want, with or without this change. If the array "se" is your "structuring element", which is a binary mask defining the circular area over which you want to calculate the median, then the number you calculate will be biased toward zero by the elements that are zeroed by multiplication with the mask.
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> So if I understand what you are trying to do, you may actually want to write the whole thing as:
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> indx = where(se)
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> for k=0,size3[1]-size2[1]-1 do begin
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> for l=0,size3[2]-size2[2]-1 do begin
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> clean[k,l] = median( ( padding[k:k+size2[1]-1,l:l+size2[2]-1] )[indx] )
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> endfor
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> endfor
Thank you so so much for this. Amazing how the run time can go from life time scales to coffee scales :D.
Also I realised that the 0's in my mask will create a bias and shift in the median, I was going to change all the zero's in my SE to NaN's, but will it suffice to replace:
indx = where(se)
with
indx = where(se EQ 1)?
Cheers.
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