[FFmpeg-devel] [RFC] Lowpass filter

Kostya kostya.shishkov
Wed Aug 27 07:00:14 CEST 2008


On Tue, Aug 26, 2008 at 07:33:28PM +0200, Michael Niedermayer wrote:
> On Tue, Aug 26, 2008 at 07:48:07PM +0300, Kostya wrote:
> > /**
> >  * @file filter.h
> >  * IIR filter interface 
> 
> > (maybe FIR will be here too)
> 
> I dont think so, it doesnt fit that well, it might fit better in the
> resample code ...
 
no problems, it's less work for me then
 
> >  */
> > 
> > #ifndef FFMPEG_FILTER_H
> > #define FFMPEG_FILTER_H
> > 
> > #include "avcodec.h"
> > 
> > struct FFFilterCoeffs;
> > struct FFFilterState;
> > 
> > enum FilterType{
> >     FF_FILTER_TYPE_BESSEL,
> >     FF_FILTER_TYPE_BUTTERWORTH,
> >     FF_FILTER_TYPE_CHEBYSHEV,
> >     FF_FILTER_TYPE_ELLIPTIC,
> > };
> > 
> > enum FilterMode{
> >     FF_FILTER_MODE_LOWPASS,
> >     FF_FILTER_MODE_HIGHPASS,
> >     FF_FILTER_MODE_BANDPASS,
> >     FF_FILTER_MODE_BANDSTOP,
> > };
> 
> I think all the stuff should have IIR in their names

added everywhere except filter types and modes names since
it's redundant for names and modes are generic
 
> [...]
> > /**
> >  * IIR filter state
> >  */
> > typedef struct FFFilterState{
> >     float *x;
> > }FFFilterState;
> 
> no, we do not want to do the extra pointer dereference nor do we want an
> extra pointer to be stored, the correct way is:
> 
> typedef struct FFFilterState{
>     float x[];
> }

only with your own compiler, gcc results in
iirfilter.c:45: error: flexible array member in otherwise empty struct

[...]
> -- 
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
> 
> Many things microsoft did are stupid, but not doing something just because
> microsoft did it is even more stupid. If everything ms did were stupid they
> would be bankrupt already.

The only export of NowWhat is the NowWhattian boghog skin, which no one in
their right minds would want to buy, and the export trade only manages to
survive because of the significant number of people in the Galaxy who aren't.
-------------- next part --------------
/*
 * IIR filter
 * Copyright (c) 2008 Konstantin Shishkov
 *
 * This file is part of FFmpeg.
 *
 * FFmpeg is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * FFmpeg is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with FFmpeg; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

/**
 * @file filter.h
 * IIR filter interface (maybe FIR will be here too)
 */

#ifndef FFMPEG_IIRFILTER_H
#define FFMPEG_IIRFILTER_H

#include "avcodec.h"

struct FFIIRFilterCoeffs;
struct FFIIRFilterState;

enum IIRFilterType{
    FF_FILTER_TYPE_BESSEL,
    FF_FILTER_TYPE_BUTTERWORTH,
    FF_FILTER_TYPE_CHEBYSHEV,
    FF_FILTER_TYPE_ELLIPTIC,
};

enum IIRFilterMode{
    FF_FILTER_MODE_LOWPASS,
    FF_FILTER_MODE_HIGHPASS,
    FF_FILTER_MODE_BANDPASS,
    FF_FILTER_MODE_BANDSTOP,
};

/**
 * Initialize filter coefficients.
 *
 * @param filt_type    filter type (e.g. Butterworth)
 * @param filt_mode    filter mode (e.g. lowpass)
 * @param order        filter order
 * @param cutoff_ratio cutoff to input frequency ratio
 * @param stopband     stopband to input frequency ratio (used by bandpass and bandstop filter modes)
 * @param ripple       ripple factor (used only in Chebyshev filters)
 *
 * @return pointer to filter coefficients structure or NULL if filter cannot be created
 */
struct FFIIRFilterCoeffs* ff_iir_filter_init_coeffs(enum IIRFilterType filt_type,
                                                    enum IIRFilterMode filt_mode,
                                                    int order, float cutoff_ratio,
                                                    float stopband, float ripple);

/**
 * Create new filter state.
 *
 * @param order filter order
 *
 * @return pointer to new filter state or NULL if state creation fails
 */
struct FFIIRFilterState* ff_iir_filter_init_state(int order);

/**
 * Free filter coefficients.
 *
 * @param coeffs pointer allocated with ff_iir_filter_init_coeffs()
 */
void ff_iir_filter_free_coeffs(struct FFIIRFilterCoeffs *coeffs);

/**
 * Free filter state.
 *
 * @param state pointer allocated with ff_iir_filter_init_state()
 */
void ff_iir_filter_free_state(struct FFIIRFilterState *state);

/**
 * Perform lowpass filtering on input samples.
 *
 * @param coeffs pointer to filter coefficients
 * @param state  pointer to filter state
 * @param size   input length
 * @param src    source samples
 * @param sstep  source stride
 * @param dst    filtered samples (destination may be the same as input)
 * @param dstep  destination stride
 */
void ff_iir_filter(const struct FFIIRFilterCoeffs *coeffs, struct FFIIRFilterState *state,
                   int size, const int16_t *src, int sstep, int16_t *dst, int dstep);

#endif /* FFMPEG_IIRFILTER_H */
-------------- next part --------------
/*
 * IIR filter
 * Copyright (c) 2008 Konstantin Shishkov
 *
 * This file is part of FFmpeg.
 *
 * FFmpeg is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * FFmpeg is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with FFmpeg; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

/**
 * @file filter.c
 * different IIR filters implementation
 */

#include "iirfilter.h"
#include <complex.h>
#include <math.h>

/**
 * IIR filter global parameters
 */
typedef struct FFIIRFilterCoeffs{
    int   order;
    float gain;
    int   *cx;
    float *cy;
}FFIIRFilterCoeffs;

/**
 * IIR filter state
 */
typedef struct FFIIRFilterState{
    float *x;
}FFIIRFilterState;

/// maximum supported filter order
#define MAXORDER 30

struct FFIIRFilterCoeffs* ff_iir_filter_init_coeffs(enum IIRFilterType filt_type,
                                                    enum IIRFilterMode filt_mode,
                                                    int order, float cutoff_ratio,
                                                    float stopband, float ripple)
{
    int i, j, size;
    FFIIRFilterCoeffs *c;
    double wa;
    complex p[MAXORDER + 1];

    if(filt_type != FF_FILTER_TYPE_BESSEL || filt_mode != FF_FILTER_MODE_LOWPASS)
        return NULL;
    if(order <= 1 || (order & 1) || order > MAXORDER || cutoff_ratio >= 1.0)
        return NULL;

    c = av_malloc(sizeof(FFIIRFilterCoeffs));
    c->cx = av_mallocz(sizeof(c->cx[0]) * ((order >> 1) + 1));
    c->cy = av_malloc (sizeof(c->cy[0]) * order);
    c->order = order;

    wa = 2 * tan(M_PI * 0.5 * cutoff_ratio);

    c->cx[0] = 1;
    for(i = 1; i < (order >> 1) + 1; i++)
        c->cx[i] = c->cx[i - 1] * (order - i + 1LL) / i;

    p[0] = 1.0;
    for(i = 1; i <= order; i++)
        p[i] = 0.0;
    for(i = 0; i < order; i++){
        complex zp;
        double th = (i + (order >> 1) + 0.5) * M_PI / order;
        zp = cexp(I*th) * wa;
        zp = (zp + 2.0) / (zp - 2.0);

        for(j = order; j >= 1; j--)
            p[j] = zp*p[j] + p[j - 1];
        p[0] *= zp;
    }
    c->gain = creal(p[order]);
    for(i = 0; i < order; i++){
        c->gain += creal(p[i]);
        c->cy[i] = creal(-p[i] / p[order]);
    }
    c->gain /= 1 << order;

    return c;
}

struct FFIIRFilterState* ff_iir_filter_init_state(int order)
{
    FFIIRFilterState* s = av_mallocz(sizeof(FFIIRFilterState) + sizeof(s->x[0]) * order);
    s->x = (void*)s + sizeof(FFIIRFilterState);
    return s;
}

#define FILTER(i0, i1, i2, i3)                    \
    in =   *src * c->gain                         \
         + c->cy[0]*s->x[i0] + c->cy[1]*s->x[i1]  \
         + c->cy[2]*s->x[i2] + c->cy[3]*s->x[i3]; \
    res =  (s->x[i0] + in      )*1                \
         + (s->x[i1] + s->x[i3])*4                \
         +  s->x[i2]            *6;               \
    *dst = av_clip_int16(lrintf(res));            \
    s->x[i0] = in;                                \
    src += sstep;                                 \
    dst += dstep;                                 \

void ff_iir_filter(const struct FFIIRFilterCoeffs *c, struct FFIIRFilterState *s, int size, const int16_t *src, int sstep, int16_t *dst, int dstep)
{
    int i;

    if(c->order == 4){
        for(i = 0; i < size; i += 4){
            float in, res;
 
            FILTER(0, 1, 2, 3);
            FILTER(1, 2, 3, 0);
            FILTER(2, 3, 0, 1);
            FILTER(3, 0, 1, 2);
        }
    }else{
        for(i = 0; i < size; i++){
            int j;
            float in, res;
            in = *src * c->gain;
            for(j = 0; j < c->order; j++)
                in += c->cy[j] * s->x[j];
            res = s->x[0] + in + s->x[c->order >> 1] * c->cx[c->order >> 1];
            for(j = 1; j < c->order >> 1; j++)
                res += (s->x[j] + s->x[c->order - j]) * c->cx[j];
            for(j = 0; j < c->order - 1; j++)
                s->x[j] = s->x[j + 1];
            *dst = av_clip_int16(lrintf(res));
            s->x[c->order - 1] = in;
            src += sstep;
            dst += sstep;
        }
    }
}

void ff_iir_filter_free_state(struct FFIIRFilterState *state)
{
    av_free(state);
}

void ff_iir_filter_free_coeffs(struct FFIIRFilterCoeffs *coeffs)
{
    if(coeffs){
        av_free(coeffs->cx);
        av_free(coeffs->cy);
    }
    av_free(coeffs);
}




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