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

[Michael Niedermayer]

On Tue, Aug 12, 2008 at 07:58:20PM +0300, Kostya wrote:
> On Tue, Aug 12, 2008 at 02:31:35PM +0200, Michael Niedermayer wrote:
> > On Tue, Aug 12, 2008 at 02:23:55PM +0300, Kostya wrote:
> > > $subj, made as a separate module.
> > 
> > [...]
> > 
> > > /**
> > >  * Initialize filter coefficients.
> > >  *
> > >  * @param coeffs filter coefficients
> > >  * @param freq   input frequency (sample rate/2)
> > >  * @param cutoff cutoff frequency
> > >  *
> > >  * @return zero if filter creation succeeded, a negative value if filter could not be created
> > >  */
> > > int ff_lowpass_filter_init_coeffs(LPFilterCoeffs *coeffs, int freq, int cutoff);
> > 
> > the filter order should also be a parameter
> 
> added 

not in the attached patch


>  
> > > 
> > > /**
> > >  * Filter input value.
> > >  *
> > >  * @param coeffs filter coefficients
> > >  * @param s      filter state
> > >  * @param in     input value
> > >  *
> > >  * @return filtered value
> > >  */
> > > static av_always_inline float ff_lowpass_filter(LPFilterCoeffs *coeffs, LPFilterState *s, float in)
> > > {
> > >     int i;
> > >     for(i = 0; i < LOWPASS_FILTER_ORDER; i++){
> > >         s->x[i] = s->x[i+1];
> > >         s->y[i] = s->y[i+1];
> > >     }
> > >     s->x[LOWPASS_FILTER_ORDER] = in * coeffs->gain;
> > >     //FIXME: made only for 4th order filter
> > >     s->y[LOWPASS_FILTER_ORDER] = (s->x[0] + s->x[4])*1
> > >                                + (s->x[1] + s->x[3])*4
> > >                                +  s->x[2]           *6
> > >                                + coeffs->c[0]*s->y[0] + coeffs->c[1]*s->y[1]
> > >                                + coeffs->c[2]*s->y[2] + coeffs->c[3]*s->y[3];
> > >     return s->y[LOWPASS_FILTER_ORDER];
> > > }
> > 
> > as already said in my other reply this should be unrolled so it needs no
> > moving of samples.
> 
> unrolled

neither in the attached patch


>  
> > > int ff_lowpass_filter_init_coeffs(LPFilterCoeffs *coeffs, int freq, int cutoff)
> > > {
> > >     int i, j, size;
> > >     float cutoff_ratio;
> > > 
> > 
> > >     //since I'm too lazy to calculate coefficients, I take more or less matching ones from the table
> > >     //TODO: generic version
> > 
> > hmm
>  
> Calculating actual filter parameters requires complex math
> (I've looked into code of mkfilt by T.Fisher which I used for obtaining
> filter coefficients and I should remember more of complex numbers than
> Euler's formula to understand it).
> I will do generic version when I get free time and refresh my algebra.

fine, its not important ATM

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

Awnsering whenever a program halts or runs forever is
On a turing machine, in general impossible (turings halting problem).
On any real computer, always possible as a real computer has a finite number
of states N, and will either halt in less than N cycles or never halt.
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