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What should a properly exposed 18% gray card read under the RGB color picker?

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Tim Wilde
What should a properly exposed 18% gray card read under the RGB color picker?
on Nov 22, 2016 at 4:08:49 am

In Resolve 12.5, after enabling "Show picker RGB value", what are the RGB values of a properly exposed (or corrected) 18% gray card?

(Rec709 assumed)


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Joseph Owens
Re: What should a properly exposed 18% gray card read under the RGB color picker?
on Nov 22, 2016 at 9:45:46 pm

[Tim Wilde] "(Rec709 assumed)
"


Seems like a Photoshop kind of question.

https://en.wikipedia.org/wiki/Middle_gray

Its not discussed much in cinematography. Its a legacy thing and with display-referenced grading, the emphasis has not been on hitting absolute RGB values.
In short, the 8-bit RGB values are roughly 128*3 in Gamma 2.2 and about 119*3 for gamma 2.4 and if you want that in 10-bit, the math is fairly straightforward.

X-Rite also publishes the target values represented on their reference charts.

http://xritephoto.com/ph_product_overview.aspx?ID=824&Action=Support&Suppor...

jPo, CSI

"I always pass on free advice -- its never of any use to me" Oscar Wilde.


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Tim Wilde
Re: What should a properly exposed 18% gray card read under the RGB color picker?
on Nov 22, 2016 at 11:26:46 pm

Thank you for the reply, Joseph. I had read the Wikipedia piece prior to posting my question but it didn't refer to any standards I recognize as applicable to video.

While 18% gray cards are a legacy item from still photography using film, there seems to be no shortage of references to 18% gray in the world of video camera color charts by companies like DSC Labs and Gamma & Density Company.


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Mark Sanford
Re: What should a properly exposed 18% gray card read under the RGB color picker?
on Nov 23, 2016 at 12:47:02 am

[Tim Wilde] "there seems to be no shortage of references to 18% gray in the world of video camera color charts by companies like DSC Labs and Gamma & Density Company.
"


Tim -

That's because there's no shortage of opinions regarding where to expose that 18% gray card when using various LOG encoding schemes (S-Log 2, S-Log 3, C-Log, Log C, v-Log and so on). And then throw ISO into the mix and you've got an argument that knows no end.

That said, I would have to question why you'd want to use the RGB picker to determine gray values as opposed to a waveform monitor.



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Bill Ravens
Re: What should a properly exposed 18% gray card read under the RGB color picker?
on Nov 23, 2016 at 1:32:14 am

I might suggest a couple of things, here:
1-Resolve has a pretty good calibration procedure using any one of a number of different color charts. That's the firsts step before trying to read color values with the eyedropper.
2-when exposing video, I have found it more appropriate to expose to 90% white. Every gamma has a different IRE value for 90% white, or for 18% grey, for that matter. So, it's pretty important to know the IRE values for the color gamut/gamma you're shooting in.



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Tim Wilde
Re: What should a properly exposed 18% gray card read under the RGB color picker?
on Nov 23, 2016 at 1:50:40 am

Pure curiosity, Mark.


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Mark Sanford
Re: What should a properly exposed 18% gray card read under the RGB color picker?
on Nov 23, 2016 at 2:31:36 am

Curiosity is a good thing.☺



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Chris Wright
Re: What should a properly exposed 18% gray card read under the RGB color picker?
on Nov 23, 2016 at 3:18:14 pm

this should clear it up
http://www.xdcam-user.com/2014/12/ultimate-guide-for-cine-ei-on-the-sony-px...


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Joseph Owens
Re: What should a properly exposed 18% gray card read under the RGB color picker?
on Nov 23, 2016 at 5:19:24 pm

Well there you go and for the price of a cup of coffee. World Vision must be feeling the heat of competition.

What I did come away with is maybe a better appreciation of what a loaded term "properly exposed" is these days.

It does seem to illustrate the dangers of depending on a single reverse-transform LUT to restore the viewfinder values over a range of exposures and lighting conditions. Which is something we colorists seem to be battling every time we roll into a new scene. Without the EI index as metadata, the LUT gets applied to the wrong exposure range -- and I think anyone who has had to deal with a baked-in ProRes or DnX S-Log source clip has experienced this and realized that the offset has to be applied before linearizing or expanding the compressed dynamic with a reverse LUT. My first guess would be that if you have ever had a scene blow out the top of the scope, that's why.

Two things... the article does not go into how this is affected in ACES.

And can't resist the notion that whoever that guy is that wrote his connoisseurship article on "Why Marvel movies look like mud (or something)" could absorb a few lessons. The main benefit, of course, in writing an article of his type is that it definitively illustrates the perils of a little bit of knowledge. And BTW, did it break LIftGammaGain...? Its been down for days.

jPo, CSI

"I always pass on free advice -- its never of any use to me" Oscar Wilde.


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Marc Wielage
Re: What should a properly exposed 18% gray card read under the RGB color picker?
on Nov 24, 2016 at 7:31:13 am

Note that Mark Sanford above is a 7-times Emmy winner, an expert camera engineer going back more than 40 years. If anybody knows what a gray card is for, he would. Mark does very good work.

To answer the question: it kind of doesn't matter what the eyedropper tool looks like, because it has no meaning in worlds with different color standards. A scope is a better tool, and even then, a lot will depend on how the chart is exposed and the specific color temperature of the lights on the set. If the set is lit with colored gels, and you make the gray chart precisely gray, everything else will be wrong.

I'm a big fan of the DSC charts, but I'm also fine with just a big 10-step grayscale chart for camera matching. If I can get that, I'm 2/3 of the way there in terms of matching the cameras and making everything consistent. In truth, I've been on many shows (including film shows) that had no charts at all, so we just had to do it by guts, eyes, and scopes.


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