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

Michael Niedermayer michaelni
Fri Aug 15 21:37:07 CEST 2008


On Fri, Aug 15, 2008 at 08:43:15PM +0300, Kostya wrote:
> On Thu, Aug 14, 2008 at 04:40:27PM +0200, Michael Niedermayer wrote:
> [...]
> > 
> > > /**
> > >  * Initialize filter coefficients.
> > >  *
> > >  * @param coeffs filter coefficients
> > >  * @param freq   input frequency (sample rate/2)
> > >  * @param cutoff cutoff frequency
> > >  *
> > >  * @return pointer to filter coefficients structure or NULL if filter cannot be created
> > >  */
> > > void* ff_lowpass_filter_init_coeffs(int order, int freq, int cutoff);
> > 
> > it should have some type not void* (and yes it can be done without putting the
> > whole struct in the header see libavutil/*.h)
> > you also forget to docuent order not that this is imporatnt i just noticed
> 
> Used the scheme after libavutil/tree.h
> 
> [...]
>  
> > > 
> > > void ff_lowpass_filter(void *coeffs, void *state, int size, int16_t *src, int sstep, int16_t *dst, int dstep)
> > > {
> > >     int i, j;
> > >     float in, res;
> > >     LPFilterCoeffs *c = (LPFilterCoeffs*)coeffs;
> > >     LPFilterState  *s = (LPFilterState*) state;
> > > 
> > >     for(i = 0; i < size; i++, src += sstep, dst += dstep){ 
> > >         for(j = 0; j < LOWPASS_FILTER_ORDER - 1; j++){
> > >             s->x[j] = s->x[j+1];
> > >             s->y[j] = s->y[j+1];
> > >         }
> > 
> > luckily iam in good mood
> > please unroll the loop and remove this
> > 
> > its supposed to look something like
> > 
> > #define FILTER(a,b,c,d, src)
> >     s->x[a]=  *src++ * c->gain;
> >     s->y[a]=  s->x[a] + 123*s->y[a] + 321*s->y[b] + ...
> >     *dst++= av_clip_int16(s->y[a]);
> > 
> > FILTER(0,1,2,3)
> > FILTER(1,2,3,0)
> > FILTER(2,3,0,1)
> > FILTER(3,0,1,2)
> 
> Here it is.
>  
> > [...]
> > -- 
> > Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
> > 
> > The misfortune of the wise is better than the prosperity of the fool.
> > -- Epicurus

> /*
>  * 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

ok

>  * @param coeffs filter coefficients

no such parameter


>  * @param freq   input frequency (sample rate/2)
>  * @param cutoff cutoff frequency
>  *
>  * @return pointer to filter coefficients structure or NULL if filter cannot be created
>  */
> struct FFLPFilterCoeffs* ff_lowpass_filter_init_coeffs(int order, int freq, int cutoff);

freq and cutoff are redundant relative to each other, only the ratio should
be needed


> 
> /**
>  * 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);

ok


> 
> /**
>  * 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);

we do not need wrapers around av_free()
this is not public API where it would be hard to replace later


> 
> /**
>  * 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(struct FFLPFilterCoeffs *coeffs, struct FFLPFilterState *state, int size, int16_t *src, int sstep, int16_t *dst, int dstep);
> 
> #endif /* FFMPEG_LOWPASS_H */
> 

ok


> /*
>  * Lowpass IIR filter

actually, this could do more generic IIR filtering one day ...


>  * 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
> 

ok

> /**
>  * filter data for 4th order IIR lowpass Butterworth filter
>  *
>  * data format:
>  * normalized cutoff frequency | inverse filter gain | coefficients
>  */
> static const float lp_filter_data[][LOWPASS_FILTER_ORDER+2] = {
>     { 0.5000000000, 9.398085e-01, -0.0176648009,  0.0000000000, -0.4860288221,  0.0000000000 },
>     { 0.4535147392, 6.816645e-01, -0.4646665999, -2.2127207402, -3.9912017501, -3.2380429984 },
>     { 0.4166666667, 4.998150e-01, -0.2498216698, -1.3392807613, -2.7693097862, -2.6386277439 },
>     { 0.3628117914, 3.103469e-01, -0.0965076902, -0.5977763360, -1.4972580903, -1.7740085241 },
>     { 0.3333333333, 2.346995e-01, -0.0557639007, -0.3623690447, -1.0304538354, -1.3066051440 },
>     { 0.2916666667, 1.528432e-01, -0.0261686639, -0.1473794606, -0.6204721225, -0.6514716536 },
>     { 0.2267573696, 6.917529e-02, -0.0202414073,  0.0780167640, -0.5277442247,  0.3631641670 },
>     { 0.2187500000, 6.178391e-02, -0.0223681543,  0.1069446609, -0.5615167033,  0.4883976841 },
>     { 0.2083333333, 5.298685e-02, -0.0261686639,  0.1473794606, -0.6204721225,  0.6514716536 },
>     { 0.1587301587, 2.229030e-02, -0.0647354087,  0.4172275190, -1.1412129810,  1.4320761385 },
>     { 0.1458333333, 1.693903e-02, -0.0823177861,  0.5192354923, -1.3444768251,  1.6365345642 },
>     { 0.1133786848, 7.374053e-03, -0.1481421788,  0.8650973862, -1.9894244796,  2.1544844308 },
>     { 0.1041666667, 5.541768e-03, -0.1742301048,  0.9921936565, -2.2090801108,  2.3024482658 },
> };

an array of FFLPFilterCoeffs makes more sense i think


> 
> /**
>  * IIR filter global parameters
>  */
> typedef struct FFLPFilterCoeffs{
>     float gain;
>     float c[LOWPASS_FILTER_ORDER];
> }FFLPFilterCoeffs;
> 
> /**
>  * IIR filter state
>  */
> typedef struct FFLPFilterState{
>     float x[LOWPASS_FILTER_ORDER];
>     float y[LOWPASS_FILTER_ORDER];
> }FFLPFilterState;

ok


> 
> struct FFLPFilterCoeffs* ff_lowpass_filter_init_coeffs(int order, int freq, int cutoff)
> {
>     int i, j, size;
>     float cutoff_ratio;
>     FFLPFilterCoeffs *coeffs;
> 
>     //we can create only order-4 filters with cutoff ratio <= 0.5 for now
>     if(order != LOWPASS_FILTER_ORDER) return NULL;
> 
>     size = sizeof(lp_filter_data) / sizeof(lp_filter_data[0]);
>     cutoff_ratio = (float)cutoff / freq;
>     if(cutoff_ratio > lp_filter_data[0][0])
>         return NULL;
>     for(i = 0; i < size; i++){
>         if(cutoff_ratio >= lp_filter_data[i][0])
>             break;
>     }
>     coeffs = (FFLPFilterCoeffs*)av_malloc(sizeof(FFLPFilterCoeffs));
>     if(i == size)
>         i = size - 1;
>     coeffs->gain     = lp_filter_data[i][1];
>     memcpy(coeffs->c, lp_filter_data[i]+2, sizeof(coeffs->c[0])*4);
>     return coeffs;
> }
> 

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

ok


> 
> void ff_lowpass_filter_free_coeffs(struct FFLPFilterCoeffs *coeffs)
> {
>     av_free(coeffs);
> }
> 
> void ff_lowpass_filter_free_state(struct FFLPFilterState *state)
> {
>     av_free(state);
> }
> 

> #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(res);                  \
>     s->x[i0] = in;                              \
>     s->y[i0] = res;                             \
>     src += sstep;                               \
>     dst += dstep;                               \
> 
> void ff_lowpass_filter(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);
>     }
> }

you are mixing float and int, iam not sure if this is ideal, at least
the float->int convertion should use lrintf()

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I have never wished to cater to the crowd; for what I know they do not
approve, and what they approve I do not know. -- Epicurus
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