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andrew sableton
confused over arri alexa log c behavior
on Sep 12, 2013 at 9:35:59 pm

I generally deal with rec 709 HD video but am currently grading ARRI ALEXA log C (Pro Res 444) 1080p footage.

In my node tree I have the last node at the Arri log C to rec 709 LUT

I am monitoring on a rec 709 monitor and have an external scope

In the video monitoring settings I have it set to video levels

The thing is, with a lot of the footage, whatever I do the blacks will not crush down to zero. It varies on the shot, but a lot of the time the blacks do not seem to want to go below the '128' line on the waveform and so the shot looks milky on the monitor. If I change the video monitoring settings to 'data levels' the blacks go to zero on my monitor and external scope, but do not seem to change on the internal davinci scopes.

I will be outputting a pro-res 422HQ for laying down to HDcam for delivery of this project and am nervous about how to make sure the black levels are correct.

Can anyone simply explain how I should be dealing with this situation?

thx

AS


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Kevin Cannon
Re: confused over arri alexa log c behavior
on Sep 12, 2013 at 9:46:30 pm

Hi Andrew,

It should feel more familiar if you use the Alexa LUT as the first node - all the corrections made downstream from there will not be going through the LUT and you won't have that problem.

Even better might be to use it in the second or third node - then you can make corrections before and after the LUT. Corrections made before the LUT will have a nice soft feel, corrections after the LUT will feel like grading rec.709 material.

Cheers,

KC

Prehistoric Digital


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andrew sableton
Re: confused over arri alexa log c behavior
on Sep 12, 2013 at 10:11:48 pm

Hmmm.....you are right....that works.....strange because I am sure I read somewhere that you should put the LUT in the last node because the LUT adjustments are not 32bit float but 10bit calculations so any adjustments after that point work on inferior math - all this is a little esoteric to me but seems to make sense.

Well...thanks anyway....looks good to me this way....

AS


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Joakim Ziegler
Re: confused over arri alexa log c behavior
on Sep 12, 2013 at 11:03:45 pm

LUTs are not inherently any bit depth, they do not quantize your material. However, things like the Alexa LogC to Rec.709 LUT do clip your material, so you will irrevocably lose range.

In general, it's often not necessary to use the LUT, you will get better results just grading with custom curves directly on the LogC material, and you have much more control.

--
Joakim Ziegler - Postproduction Supervisor


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Dmitry Kitsov
Re: confused over arri alexa log c behavior
on Sep 13, 2013 at 1:55:58 am

Might be usefull, from a couple of weeks ago:
http://forum.blackmagicdesign.com/viewtopic.php?f=3&t=11262


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Chris Kenny
Re: confused over arri alexa log c behavior
on Sep 13, 2013 at 3:52:27 pm

[Joakim Ziegler] "In general, it's often not necessary to use the LUT, you will get better results just grading with custom curves directly on the LogC material, and you have much more control."

You can also have the Alexa LUT generator give you a LUT that applies the proper color matrix but retains the log gamma so nothing clips.

--
Digital Workflow/Colorist, Nice Dissolve.

You should follow me on Twitter here. Or read our blog.


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Nate Weaver
Re: confused over arri alexa log c behavior
on Sep 13, 2013 at 9:50:00 pm

[Chris Kenny] "You can also have the Alexa LUT generator give you a LUT that applies the proper color matrix but retains the log gamma so nothing clips."

As primarily a DP, I view a LUT as the other half of the camera, especially if I was using a specific LUT on set to monitor with.

I can usually tell in skintones when a colorist just put some curves on and pumped the sat, and I politely ask to use a correct and vetted LUT to get the color response and matrix where I want them, and where I expect them.

As I view it, Arri built the Alexa+LogC+provided LUTs as a system and were constructed with care and likely many revisions. When I choose the Alexa, I want that image as a baseline.

This is all doubly true with the new Sony cameras and SLog2, because SLog2 with just curves and sat on it is not nearly as pretty as what the cameras are capable of.

Nate Weaver
Director/D.P., Los Angeles
http://www.nateweaver.net


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Joakim Ziegler
Re: confused over arri alexa log c behavior
on Sep 13, 2013 at 10:20:53 pm

While the Arri LUTs are a good baseline, you're most likely monitoring in rec.709 on set, so if your final grade is to P3, which it often is, it's not going to match anyway (an exact match between P3 and rec.709 is basically impossible, it's a matter of getting something that feels and seems the same).

So, it all depends on your workflow. For most high-end DI I've been involved with, with Alexa as the source, especially in the last year or two, we have not used the Arri LUTs, preferring instead to work by hand, or in some cases with a film emulation LUT, even though we're not going to 35mm. This applies both to things we've done in-house and at other facilities, like EFILM. But it's a matter of preference, of course.

--
Joakim Ziegler - Postproduction Supervisor


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