Order of operations when grading in ACES

(Fabián Matas González) #1

Hi, im fairly new to ACES grading. When I grade in other color spaces(mainly in LOG). I tend to work like this:

LOG controls corrections->Normalization(mainly contrast and saturation)->Look work.

I like share the Normalization since it helps me to keep consistence in my grading. In ACES I’m working all time before a normalized image since there is a ODT, but I understand I’m still working in Log space. Should I consider my order of operations like I did on standard Log space? I tried to find answer to this question and I didn’t find any solid answer, sorry if it’s to basic.

Thanks in advance,

(dado) #2

You should work in ACEScct for the closest experience to grading in Log. Contrast and Saturation will behave in the same way, you just won’t need to dial them as much as when grading in LOG ( I assume that you are currently grading LOG without a print LUT or Monitor LUT on the output )

(Nick Shaw) #3

When you say “normalisation” here, are you referring to “making the image look normal”, i.e. “not flat and log”? Your normalisation process here is doing the job that a print emulation LUT or camera LUT would do if you used one, correct? I ask because the term “normalisation” is more commonly used for the “correction” step you refer to before your normalisation, to even out differences of exposure or white balance, to get every shot to the same “normal” start point.

In ACES, what you call “normalisation” is done for you by the combination of the RRT and ODT.

With your previous approach you then apply your look after this, working on a “display referred” image, i.e. one which is already a “normal video image”. You cannot do this with ACES, as the RRT + ODT is the last step, applied by the grading system, and you cannot add anything after it.

In ACES all your grading happens in what would be your first set of nodes in your previous approach. So you are not working directly on the pixel values, but rather seeing the effect of your changes through the RRT and ODT. This can be slightly counterintuitive at first for somebody who is used to a more “video” approach. It is more akin to a traditional DI grade, where a LUT emulated the look of a film-out, and you had to grade before this, because clearly you couldn’t add changes after printing to film.

It takes a bit of getting used to, but once you become familiar with how the various grading controls respond, it will become just as intuitive to you as your previous approach.

(Walter Arrighetti) #4

Hi Fabián.
If you use ACES-compliant products for grading ‒and not building a color-pipeline on your own inside an application not natively supporting ACES (there are fewer and fewer, but they are there)‒ you shouldn’t mind of anything in the “normalization” process as you call it, as the application does that automatically by means of the Output Transform for the viewing/display device of your choice.
As Nick says, the Output Transform is a combination of two things, a fixed standard “RRT” + an “ODT” for your viewing device, but this is a technical detail that you might not be concerned about.

The grading application should also have already set the grading color-space for you to be a “log” one ‒either ACEScc or ACEScct, you might have to choose among the two‒ so effectively you will be grading in a logarithmic space. The response of grading operations will be a bit different from grading in other “log” spaces you may be used to (LogC, S-Log3, REDlogFilm, etc.), but this is okay.

Please also bear in mind that pictures saved in an ACES workflow (OpenEXRs encoded in ACES2065-1 color-space) are not “log”: they are just temporaneally brought to “log” (ACEScc or ACEScct) in the background by the color-grading application, and just for the purpose of grading/finishing.

(Fabián Matas González) #5

Thanks for both your answers, I’m refering to normalisation as the process to go from Log to REC709(or others) as far as I read about it, using a LUT or a Curve+Saturation. Before that process we work with Log controls, after it I work in LGG( usually for doing the look). As Nick pointed RRT´+ ODT is doing that process so I’m working all time before that.

So as I understand I have to do everything in with Log controls. For my process is really important to do a contrast curve by myself and do the color correction before that since it helps me to maintain the consistency of my grade, after that curve I do the look with LOG controls.

Do you think is still viable using that curve or is “useless” since I already have a curve applied on the RRT+ODT?

Walter you just answered me when I already wrote everything above, thanks for the answer. I’m actually using Davinci Resolve and I use ACEScc and the REC709. I didn’t explain myself properly in the first post, I already worked in ACES(and I love it) but I wanted to make sure my current grading operation order is still viable since I’m used to it ( I’m aware of ACEScc and ACEScct behaviour).

Thanks again for all the answer.

(Nick Shaw) #6

Nothing wrong with using a curve as part of your grade, but it won’t be the same kind of s-curve you used previously, as the tone mapping is already being done by the RRT and ODT. Doubling up on that would make the image too contrasty.