Using ACES to grade for P3 on Rec709 display

[Tom Dowler]

Trying to wrap my head around everything ACES, and how I can use it (or not) and one thought occurred to me, that I wanted to verify.

I understand that ACES takes in all kinds of footage, transforms it into ACES-language, and then, after the grade, spits it out into whatever display format you want.

So is it reasonable to say that if I grade something in ACES, using a rec709 output transform, on a rec709 monitor, and everything looks good, I could then simply switch the ODT to P3, send it DCP, and in the theater it would look exactly as it had on my rec709 during the grade?

So essentially, does ACES take away the need to grade in P3?

Or will there still be parts of the gamut I can't see in rec709 that will then reappear in P3?

 

[Sabovic Adis]

(...)
So is it reasonable to say that if I grade something in ACES, using a rec709 output transform, on a rec709 monitor, and everything looks good, I could then simply switch the ODT to P3, send it DCP, and in the theater it would look exactly as it had on my rec709 during the grade?

Yes.

So essentially, does ACES take away the need to grade in P3?

No. The thing is, You grade for 709 on 709 screen, and after 709=>P3 transformation You'll see the same 709 image, but on P3 screen.

Or will there still be parts of the gamut I can't see in rec709 that will then reappear in P3?

No. You'll have to grade for P3, on P3 screen, in order to make them gamut parts "reappear".

 

[M Most]

The environment used by a monitor, and that of a dark theater are vastly different. As has been pointed out here numerous times, there is no way any color management system, regardless of its sophisitcation and accuracy, can make them look "identical." On studio level projects, there is always a trim pass done after the DI is completed (usually in a theatrical projection environment) to yield other deliverables, such as video, HDR, and the like. Using a color management system like ACES can minimize the trims needed, but one should never expect everything to be "identical" right out of the box, particularly when you're talking about significantly different viewing environments. As far as colors that are out of gamut for one color space but visible in another, once you've graded in a less wide color space (like Rec709), you've essentially "locked in" the look you want. The color management system will simply yield a "P3" version of your Rec709 grade via gamut mapping. There won't be any "surprise" colors because you didn't use any.

 

[Tom Dowler]

Yes.



No. The thing is, You grade for 709 on 709 screen, and after 709=>P3 transformation You'll see the same 709 image, but on P3 screen.



No. You'll have to grade for P3, on P3 screen, in order to make them gamut parts "reappear".

Gooootcha... that all makes sense.

So on smaller projects, it might make sense to grade in the widest gamut available, and expect that the outputs to smaller gamuts like rec709 would be pretty faithful.

 

[M Most]

Gooootcha... that all makes sense.

So on smaller projects, it might make sense to grade in the widest gamut available, and expect that the outputs to smaller gamuts like rec709 would be pretty faithful.

Only if you do it in an environment that permits accuracy in the wider space. So if you're talking about P3, what you say is valid if you have access to a theater environment. It is not necessarily advisable if you're using a monitor, assuming that your ultimate intent is theatrical projection (which it seems to be).

Your statement is actually the opposite of what I would consider accurate. If you grade in a wider color space, those colors will not be accurate in a narrower space if they're extreme and out of gamut for the less wide space. In other words, you'll be able to see things in the wider space that you cannot see in the smaller space. The opposite is not true. Anything you see in Rec 709 also exists in P3.

 

[Tom Dowler]

If you grade in a wider color space, those colors will not be accurate in a narrower space if they're extreme and out of gamut for the less wide space.

The ACES output transform for rec709 doesn't gracefully deal with anything that's out of gamut?

 

[M Most]

The ACES output transform for rec709 doesn't gracefully deal with anything that's out of gamut?

I didn't say that. I said the out of gamut colors would not be accurate, and they won't because they will need to be mapped into "perceptually similar" colors that are visible in the narrower space. Similar but not the same. i.e., not accurate. Not that an average viewer would notice, though.

 

[Sabovic Adis]

Gooootcha... that all makes sense.

So on smaller projects, it might make sense to grade in the widest gamut available, and expect that the outputs to smaller gamuts like rec709 would be pretty faithful.

It's best to always ask for "target" and grade for it. :smile5:

 

[Steve Shaw]

This may help with background info: https://www.lightillusion.com/aces_overview.html

Steve

 

[Christoffer Glans]

Yes.



No. The thing is, You grade for 709 on 709 screen, and after 709=>P3 transformation You'll see the same 709 image, but on P3 screen.



No. You'll have to grade for P3, on P3 screen, in order to make them gamut parts "reappear".

So, you are implying to work with a P3 or HDR REC2020 screen and then go backwards for REC709 output? I.e you grade in greater color spaces and conversions and then limit for limited outputs like REC709, instead of working in limited color spaces and then expanding.

Might this be the reason why Annihilation looks like shit on Netflix? They graded in HDR REC2020 and screwed up the limit conversion down to REC709?

 

[Sabovic Adis]

So, you are implying to work with a P3 or HDR REC2020 screen and then go backwards for REC709 output? I.e you grade in greater color spaces and conversions and then limit for limited outputs like REC709, instead of working in limited color spaces and then expanding.

I would suggest that, but only in case You don't have a 709 monitor, and Your delivery has to be 709. And I'd hope for the best...

Might this be the reason why Annihilation looks like shit on Netflix? They graded in HDR REC2020 and screwed up the limit conversion down to REC709?

Could be anything, Chris!
The thing is, I don't believe in conversions. Not the correct ones, at least. That's why I always ask for target before I pull the trigger - I have to aim first. If the target is 709, aim for it. If 2020, aim for it...

See this:

Video
ID : 1
Format : HEVC
Format/Info : High Efficiency Video Coding
Format profile : Main 10@L5.1@High
Codec ID : V_MPEGH/ISO/HEVC
Duration : 2 h 0 min
Bit rate : 44.1 Mb/s
Width : 3 840 pixels
Height : 2 160 pixels
Display aspect ratio : 16:9
Frame rate mode : Constant
Frame rate : 23.976 (24000/1001) FPS
Color space : YUV
Chroma subsampling : 4:2:0 (Type 2)
Bit depth : 10 bits
Bits/(Pixel*Frame) : 0.222
Stream size : 37.1 GiB (90%)
Title : MM
Default : Yes
Forced : No
Color range : Limited
Color primaries : BT.2020
Transfer characteristics : PQ
Matrix coefficients : BT.2020 non-constant
Mastering display color pri : Display P3
Mastering display luminance : min: 0.0050 cd/m2, max: 4000 cd/m2
Maximum Content Light Level : 9918 cd/m2
Maximum Frame-Average Light : 3241 cd/m2

How exactly do You "convert" 9918 nits to fit the 100 nits lightbudget if not by clipping (or regrading)? It's like cropping: You want same size but on smaller display - You'll have to crop the shit out of it. :smile5: