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Asked by lama riad on 9 Feb 2012

hi,

i want to apply a high pass filter to image in frequency domain. Here is the code:-

I= imread('cameraman.jpg');

FA = fftshift(fft2(I));

w=0.5; % cutoff frequency

N=2; %filter order

highPassFilter=butter(N,w,'high');

OM=highPassFilter(512,512).*abs(FA); % applying high-pass filter

-------------------------------------------------------------

I'm getting this error:

??? Attempted to access b(512,512); index out of bounds because size(b)=[1,3].

Error in ==> all at 32

OM=b(512,512).*abs(FA);

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Answer by Walter Roberson on 9 Feb 2012

Accepted answer

Your error message does not appear to match your code.

Did you name your file "all.m" ? If so then that would interfere with the internal use of the MATLAB function named "all"

The documentation for butter indicates that the first output argument (your highPassFilter) will be a column vector of the length indicated by your first input argument (here N, value 2). It is not the filter itself, and it is not a function: it is part of the coefficients needed to construct a filter. Accessing it at (512,512) is never going to work because it will be a column vector.

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Walter Roberson on 9 Feb 2012

Good point, Anton.

I usually hide in the back when signal questions are asked, as I am a few decades late in handing in my homework. Gosh, I don't remember _anything_ about that course. It's like I never took it at all...

Walter Roberson on 11 Feb 2012

Ah -- ftrans2() is only for FIR filters, but butter() is an IIR filter.

http://www.mathworks.com/help/toolbox/images/ref/ftrans2.html

Answer by lama riad on 10 Feb 2012

yes the m file name is (all), I changed it to (all_registration). now i made an adjustment on the code:

I= imread('cameraman.jpg');

FA = fftshift(fft2(I));

w=0.5; % frequency normalized

N=2; %filter order

b=butter(N,w,'high');

IA=ftrans2(b);

resize_filter=imresize(IA,[512 512]);

OM=resize_filter.*abs(FA);

--------------------------------------------------------

is this correct????? I'm not that well in matlab,so i need ur help plz

Walter Roberson on 11 Feb 2012

I am certain that this is wrong. butter() is pretty much useless unless you have multiple outputs. The number of outputs to butter() determines the filter representation that will be output. None of the representations can be expressed through a single vector.

ftrans2() is for FIR filters, but butter() is an IIR filter.

You are assuming that ftrans2() is returning something that can be simply resized to provide a 2D filter. That isn't how filters work. _If_ you were working with an FIR filter then you would take the output of ftrans2() and use it with filter2() against the image. However, you are not working with an FIR filter, and in the time I spent looking I have not found the equivalent of a 2D filter for IIR filters.

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