[FFmpeg-devel] [PATCH] Implement in lavc a flag which makes avcodec_open() to choose the best framerate
Stefano Sabatini
stefano.sabatini-lala
Sun Aug 31 20:58:53 CEST 2008
On date Sunday 2008-08-31 17:09:12 +0200, Michael Niedermayer encoded:
> On Sun, Aug 31, 2008 at 01:19:31PM +0200, Stefano Sabatini wrote:
> > On date Sunday 2008-08-31 00:11:03 +0200, Michael Niedermayer encoded:
> > [...]
> > > I think we should add a function that takes a list of AVRational and a AVRational
> > > and finds the closest match for that. Its return value should indicate if the
> > > match is exact or approximate
> > > Such function might be usefull for other things as well ...
> >
> > Yes it's the best solution I can see, it's even simpler
> > application-level-wise, I would like to have thought that!
> >
> > I also thought about to return the comparation code rather than an
> > "exact" flag like in av_reduce(), I slightly prefer the attached
> > solution but let me know.
>
> what about returning the actually found value, the user could then run
> av_cmp_q() over it himself when he needs.
Mmh, OK it also simplify the interface.
> > As for the name of the flag to implement in ffmpeg, I'm aware that
> > -choose_best_framerate may sound too long, suggestions are welcome.
>
> Id say always pick the best framerate that the codec supports, and allow
> the user with -force-fps to override it.
OK for it, this has also the advantage that doesn't break backward compatibility.
> [...]
> > Index: libavutil/rational.h
> > ===================================================================
> > --- libavutil/rational.h (revision 15120)
> > +++ libavutil/rational.h (working copy)
> > @@ -113,4 +113,13 @@
> > */
> > AVRational av_d2q(double d, int max) av_const;
> >
> > +/**
> > + * Finds in \p q_list the rational nearest to \p q.
>
> Finds the nearest value in q_list to q.
>
>
> > + * @param nearest_q the pointer to the rational where to copy the
> > + * nearest rational to \p q found
> > + * @param q_list NULL-terminated list of rationals
I realized that it's better to have a {0, 0} terminated list, like
those the AVCodec supported_ramerates ones. From the practical point
of view I think they should be exactly the same, that is I think that:
{ {1, 1}, {2, 2}, NULL }
should be equivalent to:
{ {1, 1}, {2, 2}, { 0, 0} }
for most compilers, anyway in the doubt I preferred the {0, 0}
termination in the documentation.
> > + * @return 1 if the found rational is equivalent to \p q, 0 otherwise
> > + */
> > +int av_find_nearest_q(AVRational *nearest_q, AVRational q, AVRational* q_list);
> > +
> > #endif /* AVUTIL_RATIONAL_H */
> > Index: libavutil/rational.c
> > ===================================================================
> > --- libavutil/rational.c (revision 15119)
> > +++ libavutil/rational.c (working copy)
> > @@ -101,3 +101,60 @@
> >
> > return a;
> > }
> > +
> > +int av_find_nearest_q(AVRational *nearest_q, AVRational q, AVRational* q_list)
> > +{
> > + AVRational *p= q_list;
>
> useless temporary variable
>
>
> > + AVRational best_error= (AVRational){INT_MAX, 1};
>
> > + nearest_q->num = nearest_q->den = 0;
>
> unneeded
>
>
> > + for(; p->den!=0; p++){
>
> the != 0 is superflous
>
>
> > + AVRational error= av_sub_q(q, *p);
> > + if(error.num <0) error.num *= -1;
> > + if(av_cmp_q(error, best_error) < 0){
> > + best_error= error;
>
> > + nearest_q->num = p->num;
> > + nearest_q->den = p->den;
>
> *nearest_q= *p
Fixed all.
> > + }
> > + }
> > + return !av_cmp_q(*nearest_q, q);
> > +}
> > +
>
> > +#if TEST
> > +int main (void) {
> > +
> > + AVRational list_q[] = {
> > + {24000, 1001},
> > + { 24, 1},
> > + { 25, 1},
> > + {30000, 1001},
> > + { 30, 1},
> > + { 50, 1},
> > + {60000, 1001},
> > + { 60, 1},
> > + // Xing's 15fps: (9)
> > + { 15, 1},
> > + // libmpeg3's "Unofficial economy rates": (10-13)
> > + { 5, 1},
> > + { 10, 1},
> > + { 12, 1},
> > + { 15, 1},
> > + { 0, 0},
> > + };
> > +
> > + AVRational q = { 5, 1};
> > + AVRational nearest_q;
> > + int exact= av_find_nearest_q(&nearest_q, q, list_q);
> > +#undef printf
> > + printf ("q=%d/%d(%f), nearest_q=%d/%d(%f), exact=%d\n",
> > + q.num, q.den, (double)q.num/q.den,
> > + nearest_q.num, nearest_q.den, (double)nearest_q.num/nearest_q.den,
> > + exact);
> > +
> > + q.num= 42, q.den = 1;
> > + exact= av_find_nearest_q(&q, q, list_q);
> > + printf ("q=%d/%d(%f), nearest_q=%d/%d(%f), exact=%d\n",
> > + q.num, q.den, (double)q.num/q.den,
> > + q.num, q.den, (double)q.num/q.den,
> > + exact);
> > +}
> > +#endif /* TEST */
>
> hmm, i think if you cant come up with a shorter self test then drop it.
OK, I'm keeping it for practical and pedagogical reasons, I'm going to
remove it before the commit.
Regards.
--
FFmpeg = Furious & Fostering Multimedia Portable Exuberant Game
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