[FFmpeg-devel] [PATCH v3] lavu: add an AV_FRAME_DATA_GAMMA side data type

[Michael Niedermayer]

On Tue, Nov 07, 2017 at 08:13:57PM +0000, Rostislav Pehlivanov wrote:
> On 6 November 2017 at 18:03, Nicolas George <george at nsup.org> wrote:
> 
> > Le sextidi 16 brumaire, an CCXXVI, Rostislav Pehlivanov a écrit :
> > > Signed-off-by: Rostislav Pehlivanov <atomnuker at gmail.com>
> > > ---
> > >  doc/APIchanges      | 3 +++
> > >  libavutil/frame.h   | 6 ++++++
> > >  libavutil/version.h | 2 +-
> > >  3 files changed, 10 insertions(+), 1 deletion(-)
> >
> > Sorry if it has come up before, but why not just add
> >
> >         AVRational gamma;
> >
> > with 0/0 meaning unspecified? It seems like a relevant information, at
> > least as much as AVColor*. And overall much simpler.
> >
> > Regards,
> >
> > --
> >   Nicolas George
> >
> 
> Gamma info is related to mastering info so API users expect to get both
> using the same API rather than look for new fields in avframe and evaluate
> whether they're different on a per-frame basis.
> Also, it prevents from adding more fields to avframe and making a bigger
> mess.

If a filter changes gamma, it will then have to update the side data
if its in side data

I think i understand that you may see gamma more as a input device
characteristic. But really its part of the mapping of the physical
worlds light spectrum shape for each pixel to the 3 element vector
used to represent pixels.

As such if the representation of pixels change, gamma may change

Take a very simple example, simple rescaling or similar filtering,
all these filters should be performed with a flat 1.0 gamma to be
physically correct. So often a whole filter chain would be more
correct by converting the gamma != 1.0 8bit per sample to a gamma = 1.0
bps > 8bit first and at the end of the chain convert back to a more
commonly used gamma != 1.0.
We dont do this but we should support it at least optionally and
having gamma only in side data would make it a bit unwieldy


[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Why not whip the teacher when the pupil misbehaves? -- Diogenes of Sinope
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