ACES otd - REC709 to SRGB (Davinci Resolve)

[Orash Rahnema]

Hi all!

I would say that now half of my jobs are ACES, not that anyone asked me to do it, but I quite like it.

Many times I need to output for the web, so once the job is done (graded on a FSI gamma 2.4 using Resolve) I have tried to change the odt from rec709 to sRgb.

Unfortunately never worked as expected, The gamma change is very strong and the signal gets crashed down really hard.

So my workflow so far has been not to change the odt and simply do a gamma transform from 2.4 to 2.2 on the timeline level.

Has anyone experience this problem as well?

Am I doing something wrong or i didin't understand the principle of the odt?

Other system, like baselight, do behave different?

cheers,

Orash

[Margus Voll]

What version of Resolve and ACES?

[Orash Rahnema]

Resolve 12.5.3

aces cct 1.0.2

I have just done a quick test to show what i mean:

Same scene, rec709, rec709 with a gamma transform from 2.4 to 2.2, and ODT sRGB

I have done it with normal exposure and under exposed.
I noticed that in a normal exposure environment the transform it's acceptable.
the real problem appear within the black zones, so in an underexposed scene I don't think it really work

As you can see the sRGB it's a lot more contrasty and with the blacks completely crashed.

I would not expect that the 2.2 and the sRGB images look the same as I know that ACES takes in account how the colours are perceived and not just the math behind it.
But still I would not expect something like this as I don't think it's right

 

REC709

REC709 GAMMA 2.2

sRGB

REC709

REC709 GAMMA 2.2

sRGB

 

Edited June 25, 2017 by Orash Rahnema

[Cary Knoop]

I would forget about sRGB and use Rec.709 with a 2.2 gamma (or 2.4, as opinions seems to differ) as the ODT. 

For sRGB you need to raise the blacks as they will go below zero.  ACEScc handles sRGB better than ACEScct.

 

 

 

 

[Orash Rahnema]

Thanks Cary!

Would be good if there was a odt option to change between a R709 gamma 2.4 to 2.2.

I do a color transform node on the timeline level to do that, but it becomes a pain in the ass once there are multiple timeline to manage. 

[Cary Knoop]

It would take only one constant (DISPGAMMA) to change to make a Rec.709 2.2. ODT, but as far as I know there is no way to add custom ctl files to Resolve.

How do you do the transform in ACES?  A transform would have to take an ACEScc(t) gamma and make a change that will in effect become equivalent to Rec.709 2.2 once transformed with Rec.709 2.4. 

 

[Orash Rahnema]

I use a color transform ofx node on the timeline level keepin the colorspace as r709 and Just changing the gamma. 

I know its not "right" but i thougt, davinci still transform the Linear aces into a working gamma space and that ofx works with the same math, so It can't Be top much of a problem. 

At the end It might not be a perfect gamma transform but It did the trick so far. 

 

In davinci its possibile to add a dctl which is a custom ctl. 

But that works before the node correction i believe. 

I expect the odt to work After. 

By changing gamma before would not create some problems ti what comes After?

 

 

Edited June 30, 2017 by Orash Rahnema

[Steve Shaw]

Just so you know the ONLY difference between Rec709 and sRGB is Rec709 is (nominally) gamma 2.4, and sRGB Gammas 2.2.

There is no other difference.

Steve

[Orash Rahnema]

Hi @Steve Shaw

Yes, exactely, i know, in fact my original question was why in aces (davinci resolve) seams like the gamma change is a lot stronger and have a stronger effect in the Shadow area. 

I work around It not changing the odt but Just changing the output gamma

[Cary Knoop]

9 hours ago, Steve Shaw said:

Just so you know the ONLY difference between Rec709 and sRGB is Rec709 is (nominally) gamma 2.4, and sRGB Gammas 2.2.

There is no other difference.

Steve

In Resolve the transforms are different:

Edited August 15, 2017 by Cary Knoop
Removing whitespace

[Steve Shaw]

Well, that's wrong...

And why is there an 'S' profile to the curves? sRGB and Rec709 are pure power law. (but as I don't know how the graphs were created I'm probably missing something).

Steve

[Cary Knoop]

24 minutes ago, Steve Shaw said:

Well, that's wrong...

And why is there an 'S' profile to the curves? sRGB and Rec709 are pure power law. (but as I don't know how the graphs were created I'm probably missing something).

Steve

Here is a straight line ramp (Gamma 2.4), the resulting difference is equivalent:

I would avoid sRGB and instead use Rec.709 Gamma 2.2

Edited August 15, 2017 by Cary Knoop

[Thomas Singh]

Interesting.

[Steve Shaw]

Looks like the sRGB has been incorrectly based on the encoding Gamma formula.

It should be a pure 2.2 power law gamma.

The encoding gamma fore Rec709 is similar...

Steve

[neila]

Just happened to be reading through a thread on the acescentral.com forums that might be relevant: http://acescentral.com/t/p3-to-709-workflows/1177/6

Doug Walker:

"Note that sRGB is very different from gamma 2.2 in the shadows and the ACES transforms really use the sRGB curve, not 2.2."

 

[Mahesh ghanti]

Hi

i am from animation production (surfacing)

please can anyone reply ,what if we use srgb in preproduction and post are using there video output in rec709 is there any color change between this

[Anton Meleshkevich]

My favorite 'Steve Shaw VS sRGB' thread here also now!
Final argument for me,  that sRGB displays should have (and as I've noticed they have) just pure gamma, was "sRGB was created to display images on CRT pure gamma ~2.2". Many thanks to Steve Shaw!

 

On 6/30/2017 at 11:26 PM, Orash Rahnema said:

I use a color transform ofx node on the timeline level keepin the colorspace as r709 and Just changing the gamma. 

I know its not "right" but i thougt, davinci still transform the Linear aces into a working gamma space and that ofx works with the same math,

 

On 8/15/2017 at 1:54 PM, Orash Rahnema said:

why in aces (davinci resolve) seams like the gamma change is a lot stronger and have a stronger effect in the Shadow area.

Not sure what your signal path was, but keep in mind that ACEScc(t) is not linear. It has log gamma curve.
If I understand you right, you want to add pure gamma adjustment AFTER ODT, but since the only place where you can do that is before ODT. So you should go from timeline ACEScc(t) gamma to pure gamma, then do you gamma transform, then go back from pure gamma to ACEScc(t).
Also it's better to manually set all the gamma settings instead of just leaving it 'use timeline', because resolve ignores timeline colorspace when color science set to ACES and make all the transformations like if timeline color space is rec709 gamma 2.4. It's a bug. I tired to report it with no luck. Well, actually this bug only appears when you use node colorspace settings. As far as I remember, it works as it should with plugins set to 'use timeline'. But ACES still have a lot of bugs in Resolve. So it's more safe to manually set colorspace settings in plugins when using ACES.

 And I've just noticed how old the thread is 🙂

 

On 3/3/2020 at 9:32 AM, Mahesh ghanti said:

Hi

i am from animation production (surfacing)

please can anyone reply ,what if we use srgb in preproduction and post are using there video output in rec709 is there any color change between this

I've started learning 3d (blender) just a couple of months ago. Just as a hobby. Always wanted to make robots with lots of pistons.
But what I've already done - I edited OCIO config file to make it possible to select rec709 as a preview colorspace instead of sRGB.
Of course now I have to do the same for matte painting textures input colorspace. This may be wrong. But this lets me get more predictable shadows, when I export animation in linear gamma EXR, then transform it to LogC and add Alexa LUT or to Cineon and add 2383 LUT.

As a colorist, when I instruct VFX guys how to use preview LUTs the get from me, I always recommend them to switch preview gamma in nuke from sRGB to Rec709. So, the path (for nuke) is: compositing in linear => from linear to some log, logC for example. This is done in the same node where the lut loaded in => preview lut from me => transform from rec709 to linear. This is also done in the same node where the lut loaded in => transform from linear to rec709 for preview (using rec709 setting above the preview window).

Again, I'm not talking this is a correct way. But this is how I work. If something wrong with this workflow, I hope, more experienced colorists will correct me here.

 

Edited July 21, 2020 by Anton Meleshkevich

[cameronrad]

https://www.colour-science.org/posts/srgb-eotf-pure-gamma-22-or-piece-wise-function/

Edited March 5, 2020 by cameronrad