mkfftfilter - Builds 2D frequency-domain filters

Calling Sequence

transfer_function = mkfftfilter(image,filtername,frequency1[,frequency2])

Parameters

• image: A gray-level image.
• filtername: a string, the name can be 'binary', 'butterworth1', 'butterworth2', 'exp', 'gauss', or 'trapeze'
• frequency1 and frequency2: 1st and 2nd cut-off frequencies which set the filter characteristics.
• transfer_function: is a matrix with values between 0 and 1. These values can then be applied on the fft spectrum of an image.

Description

This function gives some popular filters to be applied on the spectrum (fft) of an image.

The Fourier Transform gives informations about which frequencies are present in a signal (=spectrum).

A great property of the spectrum is that the original image can be reconstructed from it. Of course, modifications in the spectrum will result in a modified image, but spectrum modifications can be easier and more intuitive.

A combination of several filters is possible.

All these filters are cylindrical and act only on amplitude (not on phase). The following filters are available (h is the trasfer function):

h=1/(1+(f/frequency1)^(2*n)) n=1,2 or 3 for 'butterworth1', 'butterworth2' or 'butterworth3'.

The exponential filter: h=exp(-(f/frequency1)^1);

The gaussian filter which is a particular case of the exponential: h=exp(-(f/frequency1)^2);

h=1 if f<=frequency1 h=-(f-frequency2)/(frequency2-frequency1) h=0 if f>=frequency2

Examples

   stacksize(4e7); // increase the stack size because 
                   // images are very much memory consumming

   image=gray_imread(SIPDIR+'images/ararauna.png');
   xset("window",0);xbasc();imshow(image);
   xtitle("Original Image");

   IM=fft(image,-1);

   //calculate the power spectrum
   spectrum=real((IM).*conj(IM));
   //for visualisation: the low frequencies are moved to the center of the image
   //with sip_fftshift,
   //the use of log(spectrum+1) allows to observe great amplitude variations.
   xset("window",1);xbasc();imshow(sip_fftshift(log(spectrum+1)),[]);
   xtitle("Power Spectrum");

   //transfer function
   h=mkfftfilter(image,'binary',20);
   xset("window",2);xbasc();imshow(h);
   xtitle("Transfer Function");

   IM2=IM.*sip_fftshift(h);//spectrum modification

   //reverse transform
   im2=real(fft(IM2,1));
   xset("window",3);xbasc();xselect();imshow(im2,[]);
   xtitle("Low-pass binary filtering");

   //High-pass filter
   IM3=IM.*sip_fftshift(1-h);//spectrum modification with (1-h)
   im3=real(fft(IM3,1));
   xset("window",4);xbasc();xselect();imshow(im3,[]);
   xtitle("High-pass binary filtering");

   //Combination of 2 filters
   h1=mkfftfilter(image,'binary',30);
   h2=mkfftfilter(image,'binary',5);
   h=h1-h2;
   IM4=IM.*sip_fftshift(h);//spectrum modification
   im4=real(fft(IM4,1));
   xset("window",5);xbasc();xselect();imshow(im4,[]);
   xtitle("Band-pass binary filtering");
   

Authors

Jocelyn Druel <jocelyn.druel1@libertysurf.fr>

Availability

http://siptoolbox.sourceforge.net

See Also

sip_fftshift,  gsm2d,  imconv,