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LOWPASS,

Section: LOWPASSU (HIGHPASS,)
Updated: BANDPASS,
Index

NAME

lowpass, highpass, bandpass, bandstop, lowpassu - "Ideal"filters truncated with window function in the spatial domain

SYNTAX

#include <xite/ideal.h>

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

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

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

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

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

DESCRIPTION

lowpassgenerates a lowpass filter in the spatial domain.The filter is made from an ideal lowpass-filter which istruncated with a window function in the spatial domain.

filter_sizespecifies filter diameter. cut_frequencyisrelative to the Nyquist frequency, i.e. a cut_frequencyof 1.0 means half the sampling frequency. win_typespecifies window function. Legal values for win_typeare the constants

BARTLETT

Circularly symmetric Bartlett window

HAMMING

Circularly symmetric Hamming window

HANNING

Circularly symmetric Hanning (von Hann) window

RECTANGLE

Circularly symmetric rectangular (box, boxcar) window function

TRIANGLE

Circularly symmetric triangular window function

The circular symmetry is achieved using Huang's method.

lowpassis normalized so that the sum of the filtercoefficients equals 1.0. lowpassuis not normalized. Forlowpassu,sumis returned as the sum of the filtercoefficients.

For odd filter_size,the symmetry-center of the filter islocated in pixel (1,1) in band.For even filter_size,the symmetry-center of the filter is located at the upper-leftcorner of pixel (1,1), i.e. the symmetry-center does notfall into the center of a pixel.

For even filter_sizeand even horizontal band-size, theresulting filter_sizemay become smaller than you'ldexpect, because of the location of the symmetry-center of thefilter, as explained above.

highpass,bandpassand bandstopbehave correspondingly.highpassis implemented as delta(n1, n2) - lowpass,withsum of coefficients equal to 0.0. bandpassis implemented asthe difference between two lowpass-filters. bandstopisimplemented as delta(n1, n2) - bandpass.

For highpassand bandstop,filter_sizeshould be odd.If it is even, a warning is issued and filter_sizewill bemade odd. For these functions, filter_sizeis a reference.

RESTRICTIONS

bandmust be one of the pixeltypes real, double, complex ordouble complex.

filter_sizemust not exceed horizontal or vertical size ofband.

For highpassand bandstop,filter_sizemust be odd.If it is even, it will be increased by one (or decreased byone if this exceeds the size of band).

RETURN VALUE

0 : Ok
1 : Bad pixeltype
2 : Unknown window type
3 : Filtersize larger than bandsize

REFERENCE

Jae S. Lim

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

AUTHOR

Trond Muri Sřrensen. Major revision by Svein Bře.

DOC

Svein Bře

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