D60 Sim for 709 and sRGB


(Charles Boileau) #1

Hi to all,

I was wondering if anyone could give me a quick explanation about the (D60 sim) options for 709 and sRGB. To my understanding this WP will give me a WB that will be closer to my original DCI-P3 grade (aka greener). But, I’m not sure about this… We always use P3-DCI for our P3 grades as our projector is calibrated to DCI WP.

And why SIM? It’s a simulation? It’s not really applied?

Thanks!


(Scott Dyer) #2

“ODT.Academy.P3DCI_48nits” ODT should actually be labeled as “D60_sim” as well. DCI white is the calibration white point for the projector but a neutral ACES value put through that setup will result in a white with a chromaticity of x=0.32168, y=0.33767 (ACES white point).

The “D60_sim” 709 and RGB_monitor ODTs do a similar thing: the calibration white for those displays is D65 (x=0.3127, y=0.329), but a neutral ACES value sent through will result in a chromaticity of x=0.32168, y=0.33767 (ACES white point).

The non-D60_sim 709 and RGB monitor ODTs chromatically adapt the ACES white point to D65. Therefore, a neutral ACES value sent through will result in a chromaticity of x=0.3127, y=0.329 (D65).


(Charles Boileau) #3

Thanks Scott! So this does mean that 709 D60 sim will look “closer” to my P3-DCI ODT Grade. Correct?

Thanks!


(Jim Houston) #4

yes, correct. ACES white point is D60 for application of the tone curves, so there was not a thought that it
needed to be in the name. It is in the header file. _sim to me was probably not a great naming convention.
I think it would have been better to have adapted whites listed explicitly, _adt_D65. They all start from a D60
somewhere in the chain. Getting the better match to the cinema grade at a D60 adopted white is often a starting point. Now if you are working on a native D65 show, that is a whole different thing. Adapting to D65 can cause slight shifts in the result which is especially obvious when you are in the same viewing environment.


(Charles Boileau) #5

Thanks Jim! Do you mean a P3-D65 show? or a 709 D65?

Thanks!


(Jim Houston) #6

Either. If the whole show is using D65 then everything at D60 in ACES should be adapted to D65.


(Charles Boileau) #7

Thanks again @jim_houston!

I’m sorry… I’m not sure I’m following.

When I grade a 709 TV show with a D65 calibrated monitor and the Rec709 ODT (not D60 sim) am I changing the white point? Or am I still at D65?

In this scenario everything is « the same » there would be no need to convert anything to any other WP.

Or do you mean if you have a show with mostly D65 elements any Previously graded D60 elements should be converted to D65?

Thanks!


(Jim Houston) #8

No problem in seeking clarity.

The ACES white point is D60. ODTs that are going to a D65 device change the appearance of the white point unless you are using D60sim on a D65 device. If you want to grade everything in D65, then all ODTs should use the same white point method for output. So D65 all the way without any D60sim anywhere. Previous ungraded elements in ACES are always in nominal D60 (though they are really at the white point that was captured), if you have graded ACES files in D65, then the creative adjustments for white point is ‘baked’ into ACES as part of a grade. If you ‘go back’ to looking at the ACES file with a D60sim, it may be warmer than desired.

The idea about white point handling was that D65 was an adaptation from dark to dim surround and moved things to handle the brightness change better, but some shows choose D65 as the creative white target for everything everywhere, so only D65 ODTs should be applied.

Hope this helps.

Jim


(Charles Boileau) #9

Thanks Jim! So when using the D60sim ODTs the WP is not baked into the file (Hence the SIM)? I think I’m missing something…

So let’s say I’m starting a new project on a DCI-P3 calibrated projector. I turn on ACES and used the DCI-P3 ODT. Suddenly, the image will change to a “less greenish” WP. Then I grade with this SIMULATED WP (correct?) and change the WP to the desired LOOKS (hence artistic WP). Once I’m done I render but the WP shift (to D60) is not rendered with it? Meaning I default back to DCI-P3 (.314, .351)?

Would this not mean that whatever I see after will look too warm/greenish?

I don’t understand why one would want this…

Thank you!


(Jim Houston) #10

I understand the confusion I think. Separate out the concept of the calibration (device) white from the
adopted (creative) white. So on a DCI projector, ACES adopted white is D60. It will look like D60 even on a
DCI projector. Similarly, on a D65 calibrated monitor, D60sim will give you a D60 adopted white. (it is different than the white point of the device).

So in your example, if you have already graded direct onto a P3 projector with the DCI white, then your grade has a compensation for the greenish white of the projector. In other words, you added some magenta to get a clean neutral. Now if you turn on ACES with these already graded files, the files will show with a magenta correction – the neutral can be graded back to the now ACES D60 adopted white. The equivalent name for a P3DCI Odt is fully a P3D60sim_atDCIcal.

So if you grade using ACES to start, the white point you ‘see’ will look like a D60 white point even if it is on a DCI white projector. You can add any creative white at that point and it will shift away from the adopted white of ACES – the code values will shift to the white point you want. On a DCI projector, the scopes will not show your visual white as being equal. When you hit the edge of the available gamut (i.e. the DCI white P3 gamut), you may see some clipping. This clipping if is it on a neutral will look like the native greenish white of the projector.

The “I render but the WP shift” is where you go astray. ACES is rendered with a D60 system white assumption always. This is always present but the neutral axis is changed for the output ODT in some D65 ODTs (i.e. the non-sim ones) The question of what white is the device calibrated to determines what code values have to be sent to the projector and the calibration white point is where the R G and B will be equal. But the ODT handles that math.

So back to the grade, if you grade to a ‘shifted’ white, like the D65 adopted whites, when you go over to a DCI projector, the grade may turn out to look a little bit warmer and the amount of that is the distance between D60 and D65. (DCI white doesn’t enter into it, because it is only a calibration white point - what code values produce what color).

The adopted system white of D60 is a center point with respect to the grade, but you can take any creative choice and grade to there and that change will be preserved. If you turn off ACES and look at it on a DCI white projector, then it will look greenish white because the equal code values of ACES are now at the DCI white and not D60 where they belong and were designed for.

Hope this helps…

Jim


(Nick Shaw) #11

If you render a P3 master with the same ODT you were viewing through when grading, the same values are baked into the render as were being sent to the projector when you graded. It’s only if you change ODT when you render that there is the possibility for confusion. But if you change to a DCDM ODT, the X’Y’Z’ equivalents of the values being sent to the P3 projector when grading are rendered to the DCDM.

White point adaptation is potentially confusing. When you use e.g. the Rec.709 ODT which “adapts the white to D65” it is actually the viewer who is “adapted”, that is their eyes have been looking at D65 for long enough that D65 is what they consider to be white. For them to perceive a white in the image as being white, in the same way as somebody whose eyes had adjusted to consider D60 as white perceived the D60 image, the values in the image are processed through a 3x3 chromatic adaptation matrix (normally using Bradford CAT for ACES).

BUT, the potential confusion comes because the result of the chromatic adaptation matrix is that an ACES neutral (R=G=B) becomes a Rec.709 neutral (R=G=B). Without the chromatic adaptation (D60 sim) ACES neutrals map to unequal R’G’B’ values in Rec.709. So although intuitively you might think that “adaptation” would be the one which “skews” the R’G’B’ values of white, in fact it is the one which keeps them equal, because the viewer is “adapted”. It does modify other colours to make them appear perceptually the same to the differently adapted viewers.

Has this made things clearer or more confusing? I don’t know!


(Charles Boileau) #12

Haha… Yes super confusing… But I like confusing. It makes me work…

So please just answer this:

REC709 ODT: is the WP D60 or D65? (I’m calibrated to D65)
Could I start a grade on 709 D60 sim if I wanted?

Thanks!


(Scott Dyer) #13

Equal ACES triplets (always D60) will come out of the Rec.709 ODT as equal Rec.709 code value triplets with the chromaticity of D65.


(Charles Boileau) #14

Some lingo in there I don’t understand… Triplets? As equal RGB code values? It’s hard to get a yes and no answer with ACES technicalities. :wink:


(Scott Dyer) #15

Three channels. [R,G,B]. R=G=B

When R=G=B in ACES, the chromaticity is D60.
When R=G=B out of the Rec.709 ODT, the chromaticity is D65.


(Charles Boileau) #16

Finally! Now I get the difference! Thanks @sdyer!

And grading from the D60sim in 709? Personal preference?

And, why not change the DCI-P3 ODT to D60sim? It’s effectively a little bit confusing.

Thanks!


(Jim Houston) #17

grading from the D60sim in Rec709 will give EXACTLY the same white point response
as the digital cinema output. [the surround compensation will mean codes and contrast
are not exactly the same]

The 709 version is a little bluer than the cinema version.

In theory, this should compensate for a dim viewing surround, but in practice, I personally
prefer to have the colorist time just the amount of white for 709. There are certain borderline
colors that go from warm to cool artistically (in a small way) and I think that is better to be
decided by a human in the context of the scene. Much conversion in Post is done in the manner
of D60sim.

Yes, DCIP3 name should be expanded.

Another way to look at the difference is there might be a set of ODTs that are “absolute colorimetry” in a dark surround (all at say D60 or all at D65) and there is a set that are “relative colorimetry” where there are slight shifts (such as D60 in cinema but D65 for dim).


(Charles Boileau) #18

Thanks @jim_houston!

Now of the other P3 ODTs

P3-D60 is assuming that you projector is already at D60?
And does P3-D65 Shift to D60?

And my initial question was: Should we use sRGB for the web? It seems like certain computers react well to the curve but others seem to be way off. I know computer screens are very far apart. But I’m wondering what others are doing.

Thanks!

Thanks!


(Jim Houston) #19

Since I assume you are discussing the Resolve ODTs, I’ll have to check later.
Don’t want to say what I think should happen versus what is in there.

As far as sRGB goes, I have had problems with it too. I have had images that look
really crushed because that system is not really implementing the sRGB curve but is
just using an approximate g2.2. This unfortunately shows up more than I like. While
generally either work for web graphics when you start making subtle color changes and
the monitor is not tracking the sRGB curve, you can have problems. sRGB is the standard
for Web pages though. I still use Rec709 g2.4 as much as possible on my own displays.

If using sRGB, I would just hope you can confirm that your system and your display are both
actually tracking correctly especially in the shadow region. Gamma 2.2 works for the top end
as an approximation but…

Jim


(Charles Boileau) #20

That’s exactly what I was getting… Even with macs it’s never the same. I guess the displays will dictate the end results! Thanks again!