[FFmpeg-devel] [PATCH] Fast half-float to float conversion

[Reimar Döffinger]

On Wed, Jul 01, 2009 at 01:21:57PM +0200, Jimmy Christensen wrote:
> On 2009-07-01 13:15, Reimar D?ffinger wrote:
> > On Wed, Jul 01, 2009 at 12:52:08PM +0200, Jimmy Christensen wrote:
> >> I modified for the purpose I need it for. Converting from half-float to
> >> unsigned int.
> >>
> >> uint16_t av_halflt2uint(uint16_t v){
> >>        uint16_t nosign = v+v;
> >>        if (v>>15)
> >>            return 0; // negatives are not interesting so clamp it to 0
> >>        if (nosign>= 0xfc00)
> >>            return 65535; // Anything above 1 should be clamped to 65535
> >>        if (nosign<  0x0200)
> >>            return ldexp((v&0x3ff), 1-25)*65535; // denormal or 0
> >>        return ldexp((v&0x3ff) + (1<<10), (v>>10&0x1f)-25)*65535;
> >> }
> >
> > Btw. a probably much faster but +-1 inaccurate version is:
> > int exp = v>>  10;
> > if ((v&  0x8000) || !exp)
> >      return 0;
> > if (exp>= 15)
> >      return 0xffff;
> > v<<= 6;
> > return (v+(1<<16))>>  (15-exp);
> 
> Thanks. Alot faster than the table based. Gives 66fps.

That's why I said that you have to optimize when you know the code and
what you need, i.e. after everything else is done.
Obviously if you go for speed the function absolutely should be either
in the same file or in a header as "static inline" function.

> I really appreciate this. I can live with the inaccuracy since it's 
> rounded off to integer anyway.

Um, the +-1 error is in the generated value. E.g. the 16-bit value "1"
can never appear.
I also just think that the error is +-1 at most, I haven't tested it.

> Will submit the decoder soon.

It would be possible to use the "bitexact" flag to switch to a more
accurate version, though this may be a bit of a misuse of "bitexact".