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    How does Resolve Color Management work when it comes to Slog2 files?
    #1
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    Hi all:

    I loaded some Slog2/Sgamut footage shot with an Sony a7s into Resolve 16, and tried testing the differences between working with a LUT (which is how I've been doing it until now) and with Resolve Color Management, which is the way recommended in the manual. The LUTs I use are the ones developed originally by Alister Chapman when the a7s first came out (http://www.xdcam-user.com/2014/08/se...-the-sony-a7s/).

    So, I first load a clip, add a node with the LUT applied to it, and then I add a node before it and use the log controls. I lower the offset and I see the entire signal getting darker in the waveform monitor, including the highlights. So far so good.

    Then I reset everything, enable Resolve Color Management and use Slog2/Sgamut as input colorspace and 709 gamma 2.4 as output colorspace. The first thing I notice is that the clip is quite brighter than with the LUT. I then take the log controls and start lowering the offset... and lowering... and lowering... and the highlights in my image *still* have detail left. Tne entire image is already dark, and there are still highlights at 100% and beyond in the waveform monitor.

    Looks like RCM handles highlights very differently than the LUT I've been using, but how exactly? I'd appreciate a technical explanation. What does it do exactly?


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    #2
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    Basically, a LUT is someone else's Preset. Depending on how the levels were originally set when the LUT was created will determine how much grading room you have to play with regards to reclaiming highlights etc. Compared to Color Space Transforms LUTs are destructive. That is why you have to make any lift, gamma, gain, and black level adjustments prior to the LUT. None of those "baked in" predetermined LUT limitations are there when you are working directly in Color Transforms. I look at LUTs much like a preset in a camera. It's a predetermined 'thing'. It's either useful to you to not. There are so many variables when you use a predetermined LUT. If you have LOG footage that was overexposed by 1-2 stops a LUT that was created for Log footage that was exposed for the stock 2000 ISO of let's say an FS7 is not going to be the right LUT to use on Log footage that was rated at 1000 or 500 ISO. Also with the A7 cameras. You would have to check this next statement as I can't recall but weren't some models rating S-LOG2 at 800 ISO and some at 1600 IOS?. Whatever the ISO numbers are a LUT has to be designed to work with the Log footage you want to work with. For that reason, for over-exposure footage, Alister Chaman developed S-LOG2 and S-LOG3 "sets" of LUTs that run from 3 stops over to 3 stops under in 1/2 stop increments. At least by using them you had a chance of choosing a LUT that was pretty close to the exposure value of the footage you were working with. He also developed the same range of LUTs with minus green corrections. Sets -G1 and -G2. You can chase them down on his http://www.xdcam-user.com/ site. Buy him a coffee. A lot of time spent making those. Seriously if you want to pull the best from almost any LOG footage Color Transforms are the only way to go IMHO.

    Chris Young

    Alister SLOG2 set.jpg


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    #3
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    Thanks for the response. Truth is, I know what a LUT does , and as mentioned, I've used extensively Alistair Chapman's LUT's for the last years. My question was mostly about what Resolve does instead. For example: where does Resolve Color Management kick in in the processing order? Does it do its thing before the user's adjustments or later? As we all know (and as you mentioned), when working with LUTs you have to make your adjustments before the LUT comes in. So, in the case of Resolve Color Management, does it also do its thing internally *after* the user's adjustments have been processed?

    And also, what are exactly those transformations that Resolve does? If a LUT is just a table that converts from one set of numbers to another, what is Resolve doing instead? Is it converting from one color space to another, doing the math in real time?


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    #4
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    It all happens mathematically. The Input Transform takes the capture-referred data of a camera and transforms it into scene linear, ACES color space. Camera manufacturers are responsible for developing IDTs for their cameras. Basically, the color management path is as follows:

    Camera Data > Input Transform > Color Grading > Look Transform (optional) > Reference Rendering Transform > Output Transform.

    Probably read Charles Haine's writeup at frame.io, LUTs vs Transforms for a better understanding of how color transforms work as opposed to LUTs

    https://blog.frame.io/2020/04/27/luts-vs-transforms/

    and

    Ben Bailey's frame.io article that covers ACES which is a unifying standard, which allows you to transform footage captured from many different cameras, into a scene-referred color science with a common linear starting point.

    https://blog.frame.io/2019/09/09/guide-to-aces/

    How to use DCTls in Resolve? Short overview video here:

    https://salamifish.com/how-to-set-up...esolve-studio/

    For a much more in-depth understanding go to:

    https://community.acescentral.com/t/...lugins/1385/16
    and
    https://community.acescentral.com/t/...lugins/1385/54

    A lot of the resources listed on the two pages above are now dead links. You can download those DCTL resources as ZIP files from:

    https://github.com/baldavenger/Balda...ns/find/master.

    If you get on top of the workflow it is really "the way" to do it. Spent hours learning the 'this art' but I often ask myself to what end as most of my type of clients wouldn't know a DCTL “DaVinci Colorspace Transform Language” if it hit them in the head!

    Happy reading, experimenting, and learning.

    Chris Young


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    #5
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    A bit delayed, but just wanted to thank you for the resources you posted, specially the first one (LUTs vs. transforms), which is what I was looking for.


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    #6
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    No probs! Glad some of it was useful.

    Chris Young


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