[FFmpeg-user] Encode mov from exr [linear>logC>rec709]

[Paul B Mahol]

On 2/22/19, Clement Poulain <clpo at dupevfx.com> wrote:
> Hey guys,
>
> I am working with an exr image sequence (colorspace: linear)
> I have a lut file to go from AlexaV3LogC to rec709.

Could use lut1d filter, if you have .cube 1D LUT file.

> Obviously to do the encoding i need to change the colorspace first.
> I can't use apply_trc flag as AlexaV3LogC is different from ffmpeg logC
> colorspace.
>
> To counter that in Nuke i used a Expression node with this value in every
> channels (channel r example):
> *r>0.0106232?(log10((r + 0.00937677) / 0.18)*0.2471896) +
> 0.385537:(((r+0.00937677) / 0.18) + 0.04378604) * 0.9661776*
>
> Which is the same than using a OCIOColorSpace node with in=linear and
> out=AlexaV3LogC
>
> As i want to do that with ffmpeg, i updated those expressions with ffmpeg
> convention:
> *"lutrgb=r='if(gt(val, 0.0106232), log((val + 0.00937677) /
> 0.18)/log(10)*0.2471896 + 0.385537, (((val + 0.00937677) / 0.18) +
> 0.04378604) * 0.9661776)':g='if(gt(val, 0.0106232), log((val + 0.00937677)
> / 0.18)/log(10)*0.2471896 + 0.385537, (((val + 0.00937677) / 0.18) +
> 0.04378604) * 0.9661776)':b='if(gt(val, 0.0106232), log((val + 0.00937677)
> / 0.18)/log(10)*0.2471896 + 0.385537, (((val + 0.00937677) / 0.18) +
> 0.04378604) * 0.9661776)'"*
>
> Theoretically, and double checked in a python shell:
> for a val=0.2 val will be 0.40..
> for a val=0.01 val will be 0.14..
>
> But it seems that I can't use the log(x) function in lutrgb.
> I get black frames with no data: every pixel [0,0,0]
>
> By removing all log(x) functions, obviously the values are wrong, but I can
> see my images..
> And I don't have any error message.
>
> Did someone already use log(x) function in lutrgb?
> Is there a clever way to do it?

Pixel values are in range 0-255 not 0-1.
At least only integer pixels are supported by lutrgb filter.

> Any help is welcome.
>
> Thank you.
> Clement
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