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LOWPASSF,

Section: LOWPASSIDEALF, (HIGHPASSF,)
Updated: BANDPASSF,
Index

NAME

lowpassf, highpassf, bandpassf, bandstopf, lowpassIdealf,butterworthf, exponentialf, parseFilterTypeName, fftMakeLp,fftMakeHp, fftMakeBp, fftMakeBs - make a filter in theFourier domain

SYNTAX

#include <xite/ideal.h>

int lowpassf( IBAND band,
double cut_frequency, int filter_size,
window_type win_type );

int highassf( IBAND band,
double cut_frequency, int *filter_size,
window_type win_type );

int bandpassf( IBAND band,
double low_cut_frequency, double high_cut_frequency,
int filter_size, window_type win_type );

int bandstopf( IBAND band,
double low_cut_frequency, double high_cut_frequency,
int *filter_size, window_type win_type );

int lowpassIdealf( IBAND band,
double cut_frequency );

int butterworthf( IBAND band, double cut_frequency,
int filter_order );

int exponentialf( IBAND band, double cut_frequency,
int filter_order );

filter_type parseFilterTypeName( char *name );

#include <xite/fft.h>

int fftMakeLp( IC_BAND band,
double cut_frequency, int filter_size );

int fftMakeHp( IC_BAND band,
double cut_frequency, int filter_size );

int fftMakeBp( IC_BAND band,
double low_cut_frequency,
double high_cut_frequency, int filter_size );

int fftMakeBs( IC_BAND band,
double low_cut_frequency,
double high_cut_frequency, int filter_size );

DESCRIPTION

lowpassfmakes a lowpass-filter in the Fourier domain. Thefilter used is an ideal lowpass-filter truncated with awindow in the spatial domain. lowpassfcalls lowpasstogenerate the filter in the spatial domain, then calls fft2dto get the Fourier transform. The scaling in the fft is 1.0,although the standard/default scaling in fft2dforwardtransform is 1/(xsize*ysize). The choice of scaling will givea DC (frequency (0,0)) value of 1.0.

filter_sizespecifies filter diameter. cut_frequencyisrelative to the Nyquist frequency, i.e. a cut_frequencyof 1.0 means half the sampling frequency. win_typespecifies the type of window function. Legal value arethe constants

RECTANGLE

Circularly symmetric rectangular (box, boxcar) window function

BARTLETT

Circularly symmetric Bartlett window

TRIANGLE

Circularly symmetric triangular window function

HAMMING

Circularly symmetric Hamming window

HANNING

Circularly symmetric Hanning (von Hann) window

The circular symmetry is achieved using Huang's method.

highpassf,bandpassfand bandstopfall call theirspatial-domain counterparts and fft2dwith scaling equal to1.0.

lowpassIdealfmakes an ideal lowpass-filter in the Fourierdomain. Unlike lowpassf,lowpassIdealfis not made froma window-truncated spatial-domain filter. It is the box-shapedideal frequency response which corresponds to a non-realizablespatial-domain filter.

butterworthfmakes a Butterworth filter in the Fourierdomain.

exponentialfmakes an exponential filter in the Fourierdomain.

parseFilterTypeNamereturns a symbolic integer value forvarious forms of filter-type name.Legal forms of nameare

Full name    Short name  Number
lowpass         lp       LOWPASS
highpass        hp       HIGHPASS
bandpass        bp       BANDPASS
bandstop        bs       BANDSTOP
lowpass_ideal   ilp      LOWPASS_IDEAL
butterworth_lp  blp      BUTTER_LP
exponential_lp  elp      EXP_LP

fftMakeLpwas a function similar (but not identical) tolowpassfin previous XITE versions. It is retained forcompatibility, but now as a macro call to lowpassfwithwin_typeequal to HAMMING. The old fftMakeLpgaveresults slightly different from the results of lowpassf.This is due to differences in the implementation of the windowfunction. The main difference is that filter_sizewill bereduced by two in lowpass,compared to fftMakeLp.

RESTRICTIONS

Horizontal and vertical size of bandmust be power oftwo.

filter_sizemay not be smaller than horizontal or verticalsize of band.

For highpassfand bandstopf,filter_sizeshould beodd. If it is even, it will be made odd and a warning isissued. For these two functions, filter_sizeis a reference.

bandmust be of pixeltype complex.

RETURN VALUE

0 - OK
1 - Bad pixel type
2 - Unknown window type
3 - Filter_size larger than bandsize
4 - Error in fft2d

REFERENCE

Jae S. Lim

"Two-dimensional Signal And Image Processing"Prentice-Hall, 1990

AUTHOR

Trond Muri Sřrensen

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