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Problem With Alexav3logc Macro / White Clip After Returning To Log-c


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#16 morazzy

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Posted 27 February 2014 - 07:05 PM

"You can't rebuild all steps yourself so if a client wants rec709 you need to apply ARRI's 3D LUT. Since it requires a logC input you need to convert your linear EXRs to logC first which can be done using Fusion's CineonLog tool."


Hey, I just wondered if you expand on why you "Can't" rebuild these steps yourself?

The conversion from LogC to Linear scene referred can be done with a simple formula, then use the Gamut tool to convert to the particular Gamut that you are targeting, and also apply a gamma that is appropriate for the viewing system being used. I think I am missing the point in your post, I just wanted to make sure in my mind what you where meaning by it. :-)

However, as you stated you can download a 3D Lut that will do the Tone-Mapping, Colour conversion and Gamma correction all at once. This is easier than doing it yourself, but that is no fun ;-)

Linear conversion with no Tone-Mapping can produce less saturated RGB Values, perhaps this is part of the problem? I am going to look into that, I am now curious.

Cheers

p.s. after some further reading, it was said that the formula was where the change happened. When using the formula, there are some variables that could be different or not taken into account, such as, was the encoding done in SUP 3.x or 2.x, the exposure index since ver 3.x can go from 160 to 3200 and version 2.x can go from 160 to 1600, and whether you are using normalised or relative scene exposure values. These can change the final result.

p.s.2
Wow I am an idiot for not reading this all the way through.... oh well. You can ignore p.s. #1

#17 Tilt

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Posted 28 February 2014 - 01:39 AM

I have tried to do all the steps manually that ARRI says their LUT does and that you have described: linearizing, gamut conversion and gamma correction. But unfortunately that didn't match their LUT output. It's close, but especially the soft shoulder for your highlights is something that I couldn't rebuild. If you have a shot that has mostly overexposed stuff (fire) that matters.

If your BT.709 looks less saturated than what ARRI's LUT does, you need to do the gamut conversion from alexa wide gamut to sRGB (or Arri's BT.709 which Stuart from eyeon once told me is from their whitepaper and differs a bit from standard BT.709)

If you manage to figure out a way to rebuild ARRI's LUT let me know! :) Maybe I've missed the part of their whitepaper where they describe their soft shoulder formula?

#18 Tilt

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Posted 28 February 2014 - 01:46 AM

Ok I think I found something: When I go into the first cineonlog after the loader of the Log-C-mov, and change the bitdepth from keep to 32bit float, i get the highlights back.

shit, that means I have to rerender all the comps.

Then the Loader isn't float but int. I can't think of any other explanation. What does the Loader's tooltip say when you hover over it?

Last question: DPX Output as 10bit / logtype bypass/ data is linear checked.

or

10bit/ arrilogC ?


When it's already logC in the viewers you need to use "bypass" mode. The "data is linear" checkbox just sets a flag in the dpx header and it should be disabled (because your data is log).

#19 holgerneuhaeuser

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Posted 28 February 2014 - 03:04 AM

Yes you´re right the original footage is interpreted by fusions loader as 16 bit int

Thanks Tilt for getting me back on track.

#20 xmare

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Posted 28 February 2014 - 06:22 AM

I think You can set-up preference to "switch" all the loaders to 32float as default - for the future...,
also, just for safety reasons, make sure that flow rendering is 32float (in memory settings, i think)...
that is just general case :)

#21 morazzy

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Posted 10 March 2014 - 07:52 PM

I have tried to do all the steps manually that ARRI says their LUT does and that you have described: linearizing, gamut conversion and gamma correction. But unfortunately that didn't match their LUT output. It's close, but especially the soft shoulder for your highlights is something that I couldn't rebuild. If you have a shot that has mostly overexposed stuff (fire) that matters.

If your BT.709 looks less saturated than what ARRI's LUT does, you need to do the gamut conversion from alexa wide gamut to sRGB (or Arri's BT.709 which Stuart from eyeon once told me is from their whitepaper and differs a bit from standard BT.709)

If you manage to figure out a way to rebuild ARRI's LUT let me know! :)/> Maybe I've missed the part of their whitepaper where they describe their soft shoulder formula?


Hey, sorry for not responding sooner. Well, it would seem you are correct that the LUT will make the shoulder conform to a more standard type of file. ie the above 1 values are slowly confined to be under one. This is not unusual, but I am going to see what exactly is going on.

Yeah, the two different rec 709 settings are relating to one which deals with tone mapping and one that does not. In other words if you convert with tone mapping or without, if without you will fine your image is less saturated. So I will presume that is what Stuart was referring to.

Cheers




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