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

Reimar Döffinger Reimar.Doeffinger
Tue Jun 30 15:50:43 CEST 2009

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.

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