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once again - RGB XYZ rendering for DCI output

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Andy Winter
once again - RGB XYZ rendering for DCI output
on Oct 5, 2012 at 11:34:44 am

dear fellow colorists, i know this topic was raised before, and i did read the posts, but nevertheless
i would love if someone could help me out with a couple of things i do not really understand...

what i know:
it is a common worklflow to grade in rec709 (2.2 or 2.35, there is no real given standard) and then
deliver the graded master to a posthouse for DCP mastering. (eg prores 444)

but for smaller projects i would love to make my own dcps, and i already did using tools like opendcp.

so, instead of letting the posthouse do the magic i want to understand the magic itself. so, as far as i know
there are a couple of things to do (picturewise, assuming a rec709 workflow, 25fps):
1. bring your rec709 master with the help of a lut to rgb XYZ and doing some kind of gamma conversion (2.2 -> 2.6, 2.35 -> 2.6)
2. resize your image so it fits the DCI specs (1.85/Flat: 1998x1080, 2.38/Scope: 2048x858)
3. convert them to 24 fps if you would like to be 100% sure every cinema can play it (mxf interop norm)
3a. convert to 24fps or leave it at 25fps and make SMPTE cinema package, newer standard, 4 % of the cinemas worldwide can't ingest and play it
4. render jpeg2000s
5. wrap it in an mxf container
6. generate xmls etc. according to the DCI norm

converting to 24fps is obviously optional, most films are shot as 24fps for cinema. point 4 to 6 i can easily manage in eg opendcp.
but point 1 and 2 i would love to do in davinci (4 as well, but davinci can't render j2ks by now). and point 1 is the crux where i am
not quite sure how to achieve it accurately.

there is a 3d-lut-called rec709 to DCI XYZ but i don't know which gamma is assumed as basis for the rec709.
and if i use that lut and render out as TIFF XYZ 16bit, am i doing some double conversion? what does the TIFF XYZ
render setting do exactly? does it only change the gamut or the gamma as well? if it does change the gamma, again,
on which basis (2.2 or 2.35). and what i nearly forgot: rec709 is D65, DCI is D5798. do you have a lut for that operation
as well?

of course i can just output it as dpx rgb 10bit and let opendcp do the conversion, but it is accurate enough?

what are your workflows regarding DCI?

would love if people would share their knowledge and in case i did write something wrong, correct me!


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Pepijn Klijs
Re: once again - RGB XYZ rendering for DCI output
on Oct 5, 2012 at 7:18:41 pm

I just happen to have finished my first DCP on my workstation at home. I think the most problematic part of the process is the colorspace conversion. Like you already mentioned it is difficult to just apply a lut because doesn't tell you what it is supposed to be fed. What I did was testing. I used a free copy of easydcp player to check my results/dcp against the renders from Resolve.
Too often the dcp was lighter (gamma issue) and warmer (colortempurature/kelvin issue). The only time I had a perfect match was when I took the rgb 16 bit tiffs from davinci, convert them to xyz tiff in scratch and then use opendcp without color conversion to transcode them to jpeg2000. Probably there are other options to get a good result, but that was what I came up with. I think the reason why scratch does such a good job is because it lets you define the gamma of your source, which I think is crucial.

But I hope other people will respond to this thread and come with easier workflows...

Editor/Colorist, Amsterdam, The Netherlands

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Pepijn Klijs
Re: once again - RGB XYZ rendering for DCI output
on Oct 5, 2012 at 7:22:50 pm

And... The render to xyz tiff function does do a color conversion so doing that with a lut applied would be double trouble;).

Editor/Colorist, Amsterdam, The Netherlands

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