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ACES vs YRGB exposure differences in Davinci Resolve?

Resolve ACES YRGB SLOG3

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#1 Mark Kenfield

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Posted 19 May 2015 - 12:12 PM

Hi Guys,

 

I’ve been running into some very strange discrepancies between grading in the ACES and YRGB colour spaces in Resolve, and was wondering if anyone might be able to shed some light on them?

 

In a nutshell, what I’m seeing is the following:

- in YRGB, images respond predictably to LUTs, but yield less subtle variation in skintones
- in ACES, images become violently overexposed when applying LUTs, and contain boosted red tones
- After correction, images in ACES appear to yield more accurate and nuanced colour reproduction

 

Here’s the original log image (slog3 from a Sony F5) and it’s waveform. The exposure values for her skin and the wall in the background are sitting where I’d expect them to be:

 

WcYprNg.jpg

 

Now in YRGB mode, when I apply the LC709A LUT to the log image, exposure values are lifted to approximately where I’d expect them to go:

 

vxTCGvt.jpg

 

LC709A is designed to mimic Alexa’s Rec709 output, and does a nice job of normalising slog3 images, it doesn’t crush the blacks though, so a slight contrast curve is needed to put the darker tones where they belong:

 

xjWuf77.jpg

 

There’s a slight overall green tint to the colour here, but it’s easily corrected out, and we have a normalised image. When apply a straight Filmconvert grade on top of the image, everything ends up right where we’d expect it to go without any further corrections or fine-tuning:

 

YTQYk0N.jpg

 

When we flip Resolve over into ACES v1.0 mode though, and apply the same LUTs/Looks, we get very different results. (note I’m not using any IDT or ODT on the footage, merely grading it in the ACES colour space). Applying the LC709A LUT in ACES results in this:

 

nubvv3U.jpg

 

Exposure values blown sky high. Applying the straight Filmconvert look on top does the same thing, only worse:

 

T6HMYfz.jpg

 

Now with some contrast curves, we can correct the ACES image back to normalised levels:

 

gZVA1KQ.jpg

 

However, there is an excessive amount of red in this normalised ACES image. Now for this scene, I had uncorrected 650w tungsten lights coming through daytime windows to simulate late afternoon light, and that would have increased the red in the image, but given the light wasn’t hitting the actress directly, it seems odd that the red levels are so boosted. With the excess red dialled out however, the ACES image yield more nuanced and appealing skintones. Here’s the corrected Filmconvert look, with the excess red removed and the exposure levels brought down to normalised levels:

 

aEMcDGW.jpg

 

Compare that to the YRGB Filmconvert image, and you can see how much more nuanced and realistic the skintones look:

 

YTQYk0N.jpg

 

And again, ACES with a straight Filmconvert look applied, brightness corrected and excess red removed:

 

WMjhKBG.jpg

 

And the YRGB version:

 

http://i.imgur.com/adukanA.jpg

 

Again the skin looks much less wooden, and more realistic in the ACES version. The richer reds of her lips are more clearly defined from the skin on her face.

 

Is anyone else experiencing similar strange responses to LUTs in ACES in Resolve? And does anyone know what might be causing the discrepancy?

 

Any insights would be greatly appreciated.

 

Cheers,

 

Mark


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#2 Dennis Hingsberg

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Posted 19 May 2015 - 04:53 PM

Once you work in ACES color space, the Sony 3D LUTs, and even other 3rd party LUTs (impulz, etc..) are expecting a different color space (usually for Sony s-gamut or s-gamut3) so of course the results end up being wonky. 

 

I do find working under ACES space in Resolve gives you a great point for starting a grade manually, but still occasionally feel torn between YRGB (non-ACES) with 3D LUT support vs straight up ACES without 3D LUT support.

 

 

I don't think anything you've noticed is abnormal.

 

 

 

Hope this helps Mark, Cheers,


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#3 Mark Kenfield

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Posted 20 May 2015 - 09:57 AM

Thanks Dennis. My reading on the subject is turning my mathematically uninclined brain to mush, but from what I can make out, colour gamut and colour space are separate things. 

 

I came across a fantastic thread on the liftgammagain forums, which put some elements into clearer layman's terms: http://www.liftgamma...d-resolve.1220/

 

To date I'm finding ACES with the Sony IDTs quite comparable to YRGB with the LC-709A LUT as a nice neutral starting point for a grade. But what really interests me is the findings I posted about above, where ACES (without an IDT) + LUTs is yielding nicer, more nuanced (and I think more accurate) colours once the red spike and luminance spikes are compensated for.

 

Granted it's not using ACES at all in the way it was intended, but I think the results speak for themselves - which makes me want to use that particular workflow for the sake of nicer final grades. The problem remains the added complexity of having to correct the red/luminance spikes (though to be fair, they're not difficult corrections to make).

 

I wonder if anyone is working on adjusting some existing LUTs to interact with ACES IDT'd footage?


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