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

[Michael Niedermayer]

On Sun, Aug 17, 2008 at 03:19:19PM +0300, Kostya wrote:
> 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

[...]

> 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);
>     }
> }

please remove all unused variables (like 2 j)
move variables into the loops where they are used when not used outside

except these patch ok

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

If a bugfix only changes things apparently unrelated to the bug with no
further explanation, that is a good sign that the bugfix is wrong.
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