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

[Frank Barchard]

On Tue, Jun 30, 2009 at 7:08 AM, Reimar D?ffinger
<Reimar.Doeffinger at gmx.de>wrote:

> On Tue, Jun 30, 2009 at 03:56:32PM +0200, Jimmy Christensen wrote:
> > On 2009-06-30 15:50, Reimar D?ffinger wrote:
> > > On Tue, Jun 30, 2009 at 03:09:33PM +0200, Jimmy Christensen wrote:
> > >>> Lastly it seems to me that this code would belong into
> > >>> libavutil/intfloat_readwrite.*
> > >>
> > >> You are probably right.
> > >
> > > Personally I'd suggest doing a stupid implementation for
> > > libavutil/intfloat_readwrite.*, something like this (certainly
> > > not a beauty, possibly buggy, does not support denormals, though since
> > > they are rare it probably wouldn't be a big issue to add handling for
> > > them in if (nosign<  0x0400)).
> > > float av_int2halflt(int16_t v){
> > >      uint16_t nosign = v+v;
> > >      if (nosign>= 0xfc00) {
> > >          if (nosign == 0xfc00) return v>>15 ? -1.0/0.0 : 1.0/0.0;
> > >          else return 0.0/0.0;
> > >      }
> > >      if (nosign<  0x0400) return 0; // denormal or 0
> > >      return ldexp((v&0x3ff) + (1<<11)) * (v>>15|1), (v>>10&0x1f)-26);
> > > }
> > >
> > >
> > > And consider an optimized version only after it is actually used and
> > > thus it is clear what kind of optimizations are necessary/most useful.
> > > E.g. the table based approach probably is hard to SIMD and thus might
> be
> > > a bad idea if you want to convert a lot of data.
> >
> > So I should just put the table into the OpenEXR decoder (eg. disabled by
> > default)?
>
> No, you shouldn't add it at all unless you really need it.
> Personally I don't think there is any need for such optimization for the
> first implementation of such a rare format as OpenEXR, it can still be
> added after the rest has passed review (though Michael sometimes has a
> different opinion on these kinds of things), and hopefully in a way that
> can
> be properly optimized with assembly (whether ARM NEON or SSE2 or
> whatever).


I agree.    A small/simple version that can be done in hardware would be
good for now.
Graphic chips are starting to support half float.  So at some point we'll
just want to pass the images along to be rendered as is.
Conversion could be done in swscaler.
Also a test that converts from half float to float and back again losslessly
would be good.


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