[FFmpeg-devel] [PATCH] Lowpass functionality for lavc

Kostya kostya.shishkov
Sun Aug 17 14:19:19 CEST 2008


On Sat, Aug 16, 2008 at 06:01:45PM +0200, Michael Niedermayer wrote:
> On Sat, Aug 16, 2008 at 06:35:43PM +0300, Kostya wrote:
[...]
> > > 
> > > > #define FILTER(i0, i1, i2, i3)                  \
> > > >     in = *src * c->gain;                        \
> > > 
> > > >     res =  (s->x[i0] + in      )*1              \
> > > >          + (s->x[i1] + s->x[i3])*4              \
> > > >          +  s->x[i2]            *6              \
> > > >          + c->c[0]*s->y[i0] + c->c[1]*s->y[i1]  \
> > > >          + c->c[2]*s->y[i2] + c->c[3]*s->y[i3]; \
> > > >     *dst = av_clip_int16(lrintf(res));          \
> > > >     s->x[i0] = in;                              \
> > > >     s->y[i0] = res;                             \
> > > 
> > > i think the followng is equivalent and needs a few memory accesses less
> > > 
> > > in +=  c->c[0]*s->y[i0] + c->c[1]*s->y[i1]
> > >      + c->c[2]*s->y[i2] + c->c[3]*s->y[i3];
> > > res =   s->y[i0] + in
> > >      + (s->y[i1] + s->y[i3])*4
> > >      +  s->y[i2]            *6
> > > s->y[i0]= in;
> >  
> > err, I am not sure that will work at all
> > how those s->x[] and s->y[] are merged into one array?
> 
> Wikipedia has pretty pictures about the trick:
> http://en.wikipedia.org/wiki/Digital_biquad_filter#Direct_Form_1
> http://en.wikipedia.org/wiki/Digital_biquad_filter#Direct_Form_2

indeed, thanks for a pointer

implemented in this way
 
> [...]
> 
> -- 
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
> 
> I count him braver who overcomes his desires than him who conquers his
> enemies for the hardest victory is over self. -- Aristotle
-------------- next part --------------
/*
 * Lowpass 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 lowpass.h
 * lowpass filter interface
 */

#ifndef FFMPEG_LOWPASS_H
#define FFMPEG_LOWPASS_H

#include "avcodec.h"

struct FFLPFilterCoeffs;
struct FFLPFilterState;

/**
 * Initialize filter coefficients.
 *
 * @param order        filter order
 * @param cutoff_ratio cutoff to input frequency ratio
 *
 * @return pointer to filter coefficients structure or NULL if filter cannot be created
 */
const struct FFLPFilterCoeffs* ff_lowpass_filter_init_coeffs(int order, float cutoff_ratio);

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

#if 0 //enable with arbitrary order filter implementation, use av_free() for filter state only for now
/**
 * Free filter coefficients.
 *
 * @param coeffs pointer allocated with ff_lowpass_filter_init_coeffs()
 */
void ff_lowpass_filter_free_coeffs(struct FFLPFilterCoeffs *coeffs);

/**
 * Free filter state.
 *
 * @param state pointer allocated with ff_lowpass_filter_init_state()
 */
void ff_lowpass_filter_free_state(struct FFLPFilterState *state);
#endif

/**
 * 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_lowpass_filter(const struct FFLPFilterCoeffs *coeffs, struct FFLPFilterState *state, int size, int16_t *src, int sstep, int16_t *dst, int dstep);

#endif /* FFMPEG_LOWPASS_H */

-------------- next part --------------
/*
 * Lowpass 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 lowpass.c
 * lowpass filter implementation
 */

#include "lowpass.h"

/**********************
 *         TODO:
 * support filters with order != 4
 * calculate coefficients for filter instead of taking approximate ones from the table
 *********************/

/** filter order */
#define LOWPASS_FILTER_ORDER 4

/**
 * IIR filter global parameters
 */
typedef struct FFLPFilterCoeffs{
    float gain;
    float c[LOWPASS_FILTER_ORDER];
}FFLPFilterCoeffs;

/**
 * filter data for 4th order IIR lowpass Butterworth filter
 */
static const FFLPFilterCoeffs lp_filter_coeffs[] = {
    { 9.398085e-01, { -0.0176648009,  0.0000000000, -0.4860288221,  0.0000000000 } },
    { 6.816645e-01, { -0.4646665999, -2.2127207402, -3.9912017501, -3.2380429984 } },
    { 4.998150e-01, { -0.2498216698, -1.3392807613, -2.7693097862, -2.6386277439 } },
    { 3.103469e-01, { -0.0965076902, -0.5977763360, -1.4972580903, -1.7740085241 } },
    { 2.346995e-01, { -0.0557639007, -0.3623690447, -1.0304538354, -1.3066051440 } },
    { 1.528432e-01, { -0.0261686639, -0.1473794606, -0.6204721225, -0.6514716536 } },
    { 6.917529e-02, { -0.0202414073,  0.0780167640, -0.5277442247,  0.3631641670 } },
    { 6.178391e-02, { -0.0223681543,  0.1069446609, -0.5615167033,  0.4883976841 } },
    { 5.298685e-02, { -0.0261686639,  0.1473794606, -0.6204721225,  0.6514716536 } },
    { 2.229030e-02, { -0.0647354087,  0.4172275190, -1.1412129810,  1.4320761385 } },
    { 1.693903e-02, { -0.0823177861,  0.5192354923, -1.3444768251,  1.6365345642 } },
    { 7.374053e-03, { -0.1481421788,  0.8650973862, -1.9894244796,  2.1544844308 } },
    { 5.541768e-03, { -0.1742301048,  0.9921936565, -2.2090801108,  2.3024482658 } },
};

/** cutoff ratios for lp_filter_data[] */
static const float lp_cutoff_ratios[] = {
    0.5000000000, 0.4535147392, 0.4166666667, 0.3628117914,
    0.3333333333, 0.2916666667, 0.2267573696, 0.2187500000,
    0.2083333333, 0.1587301587, 0.1458333333, 0.1133786848,
    0.1041666667,
};

/**
 * IIR filter state
 */
typedef struct FFLPFilterState{
    float x[LOWPASS_FILTER_ORDER];
}FFLPFilterState;

const struct FFLPFilterCoeffs* ff_lowpass_filter_init_coeffs(int order, float cutoff_ratio)
{
    int i, j, size;

    //we can create only order-4 filters with cutoff ratio <= 0.5 for now
    if(order != LOWPASS_FILTER_ORDER) return NULL;

    size = sizeof(lp_cutoff_ratios) / sizeof(lp_cutoff_ratios[0]);
    if(cutoff_ratio > lp_cutoff_ratios[0])
        return NULL;
    for(i = 0; i < size; i++){
        if(cutoff_ratio >= lp_cutoff_ratios[i])
            break;
    }
    if(i == size)
        i = size - 1;
    return &lp_filter_coeffs[i];
}

struct FFLPFilterState* ff_lowpass_filter_init_state(int order)
{
    if(order != LOWPASS_FILTER_ORDER) return NULL;
    return av_mallocz(sizeof(FFLPFilterState));
}

#define FILTER(i0, i1, i2, i3)                  \
    in =   *src * c->gain                       \
         + c->c[0]*s->x[i0] + c->c[1]*s->x[i1]  \
         + c->c[2]*s->x[i2] + c->c[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_lowpass_filter(const struct FFLPFilterCoeffs *c, struct FFLPFilterState *s, int size, int16_t *src, int sstep, int16_t *dst, int dstep)
{
    int i, j;
    float in, res;

    for(i = 0; i < size; i += 4){
        FILTER(0, 1, 2, 3);
        FILTER(1, 2, 3, 0);
        FILTER(2, 3, 0, 1);
        FILTER(3, 0, 1, 2);
    }
}



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