[FFmpeg-user] convert from bt601 to bt709

[Rio Kierkels]

On 11 June 2014 19:34, Rio Kierkels <riokierkels at gmail.com> wrote:

> > Except that I was already working in 709 space.  Yes the EXR is linear,
> > and was interpreted as such and converted into 709 and looks correct.
> >
> > The DPX on the other hand was tagged as already being 709, yet I had to
> > manually "undo" the gamma to get it right in 709, which is why it seems
> > to me like it was applied twice.  It should have just come in correct to
> > begin with and look like the EXR snapshot did, if it really was 709.
> >
> > Bob
>
> This is interesting. It looks to me that your DPX doesn't have a gamma
> applied at all. But I've only been able to check using ffmpeg's histogram
> filter that just shows the pixel values without any transforms (right,
> people with better knowledge about it than I have?). And the lightworks
> scopes. I'll compare yours and mine on the nuke, flame, smoke and lustre
> machines tomorrow if I can get some time with them.
>
> Rio
>
I've confirmed that your DPX doesn't contain a gamma curve by running it
through both Flame and Nuke. But here is where the fun begins.
Nuke on output does apply a gamma curve and a BT.709 transformation on the
primaries however Flame doesn't do anything it seems.
It just sets some metadata in the DPX headers but doesn't do any transforms
at all. It is again a difference in implementations.
So both the DPX sequence I made available and the DPX file rendered by Bob
are fine in the primaries but you'll have to remember that only
the sequence contains the gamma curve as per spec. It would be nice if we
all used the same sequence for testing.


On 5 June 2014, Gregory Ducatel <gducatel at gmail.com> wrote:
> Could it be that the color atom are not valid from ffmpeg? Can we set them
> manually?
>
> The same quick time file outputed with Nuke, provide a prefered transfer
> set at 1.8
>
> Ffmpeg on the other hand provide a prefered transfer always set at 2.2?
>
> Is there a way to change that?

Actually the Gamma 1.8 setting in nuke is misleading when exporting
QuickTime files from Nuke.
If left on that default the gamma correction/handling is done by QuickTime
itself.
So It might not be 1.8 at all depending on QuickTime's handling of things.
Actually got this from the guy that programmed it when I was on the
fxphd.com forums ;)

I didn't have the time to test a working ffmpeg commandline with the DPX
test sequence today.
Will do tomorrow. We actually stopped using DNxHD through ffmpeg internally
in the company because of these inconsistencies...


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