How to design a 5by5 ,5cross and X-1 median filter

2013 3 23
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I need to filter speckle noise from a noisy image by the application of 5*5 median filter,cross shaped median filter and a X-1 shaped median filter. I just cannot understand and do the code for it. Request for code.Thank you.

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Anand
Anand 2013-3-23
编辑：Anand 2013-3-25

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You can use the following two functions:
medfilt2 and ordfilt2
Here's an example:
out = medfilt2(im,[5 5]); %5x5 neighborhood
For a neighborhood that is not all 1's, use
nhood = [1 0 0 0 1;...
0 1 0 1 0;...
0 0 1 0 0;...
0 1 0 1 0;...
1 0 0 0 1;];
out = ordfilt2(im,ceil(nnz(nhood)/2),nhood); %cross-shaped neighborhood

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chitra
chitra 2013-3-23
Thank you Anand.Going by your idea,I implemented a 5x5 Plus shaped and your Cross shaped mask.Both of them give a black image as output after filtering!Is there something wrong?
Image Analyst
Image Analyst 2013-3-24
How do you know? Did you examine the variable, or did you just use imshow()? If you used imshow, then it may be because it's a floating point array and floating point arrays need to use [], like imshow(medianFilteredArray, []); though you would probably get all white rather than all black.
chitra
chitra 2013-3-24
编辑：chitra 2013-3-24
filtered_img =ordfilt2(gaus_noiseimg,ceil(numel(nhood_cross)/2),nhood_cross); imshow(filtered_img) is what I did and it outputs a black image.The 5x5 median filtered works fine by using the medfilt2 command.
Anand
Anand 2013-3-24
As ImageAnalyst suggested, try
imshow(filtered_img,[])
Image Analyst
Image Analyst 2013-3-24
chitra, you did not answer my question about whether you actually examined the variable, and you did not take my suggestion about how to use imshow(). Why did you ignore my attempt to help you? Try this:
maxValue = max(filtered_img(:)) % Don't use a semicolon
minValue = min(filtered_img(:)) % Don't use a semicolon
What do you see in the command window?
chitra
chitra 2013-3-24
I did imshow(filtered_img) the output of which is a black image. The values of maxValue = 0 and minValue = 0. The original iamge is subjected to speckle noise and I intend to filter is with a cross shaped/plus filter.
Anand
Anand 2013-3-25
I just realized the mistake in my call to ordfilt2. Here's how you use it:
out = ordfilt2(im,ceil(nnz(nhood)/2),nhood);
The domain has nnz(nhood) non-zero elements, and so the median is the (nnz(nhood)/2)th element.
chitra
chitra 2013-3-25
Yes now it works.So,could you please explain why you are dividing the mask by 2 and using nnz?

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