Re: [FFmpeg-devel] [PATCH] avcodec/exr: tag gamma=1.0 output as linear light

["Tomas Härdin" <git@haerdin.se>]

tor 2023-08-17 klockan 14:04 -0400 skrev Leo Izen:
> On 8/17/23 08:59, Tomas Härdin wrote:
> > ons 2023-08-16 klockan 01:20 -0400 skrev Leo Izen:
> > > By default the OpenEXR decoder outputs linear light pixel data by
> > > applying a gamma=1.0 transfer (i.e. a no-op). When it does so, it
> > > should tag the data as linear so color-managed filters or other
> > > tools
> > > can work with it correctly.
> > > 
> > > Signed-off-by: Leo Izen <leo.izen@gmail.com>
> > > ---
> > >   libavcodec/exr.c | 2 ++
> > >   1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/libavcodec/exr.c b/libavcodec/exr.c
> > > index fae1d08ab0..518066facf 100644
> > > --- a/libavcodec/exr.c
> > > +++ b/libavcodec/exr.c
> > > @@ -2088,6 +2088,8 @@ static int decode_frame(AVCodecContext
> > > *avctx,
> > > AVFrame *picture,
> > >   
> > >       if (s->apply_trc_type != AVCOL_TRC_UNSPECIFIED)
> > >           avctx->color_trc = s->apply_trc_type;
> > > +    else if (s->gamma > 0.9999f && s->gamma < 1.0001f)
> > > +        avctx->color_trc = AVCOL_TRC_LINEAR;
> > 
> > I'm going to be difficult here and point out that gamma=0.99991 is
> > not
> > linear. It's probably linear *enough* most of the time, but also
> > 1.0
> > can be exactly represented by float so an equality check seems
> > appropriate.
> 
> This exact check exists elsewhere in the source file. There's a
> branch 
> where it sets up a LUT if gamma is not between those values, and has
> a 
> no-op track otherwise - so in the event you request 0.99995 or
> something 
> of that form, the code that does the color conversion treats it as
> 1.0f.

Alright. I just find myself wondering how gamma could ever become some
value that is close to 1.0 but not quite equal.

/Tomas
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