Arri Alexa Log C Film Matrix LUT

[M Joel W]

Hi, I remember reading that Arri distributed a LUT designed to emulate color negative film, but later discontinued it. They advertised this LUT as advantageous for chroma keying.

Does anyone have it? Can you send it to me? Is it something I can apply to log footage to get more of a "color negative" feel. Thanks in advance to anyone who has it.

[David Mullen ASC]

A display gamma LUT isn't really advantageous for chroma keying if the original RGB log file information wasn't.

[M Joel W]

Hi David, can you explain in more detail? Are the log c files themselves either film matrix or standard matrix? They are different variants of log c? 

This is what I read from the white paper, I’m not clear what it means:

"Also a new feature in SUP 3 .0 is a film style matrix that can be applied to the Log C output . The same transform is also available as a 3DLUT for post-processing of Log C footage ."

https://www.samys.com/images/pdf/ALEXA-Color-Processing-White-Paper.pdf

In the white paper example, the colors look very close to the film scan they compare it with. I took this to mean there was an alternate rec709 transform LUT that more closely emulates color negative film scans.

Edited September 26, 2020 by M Joel W

[David Mullen ASC]

Sure it may more closely emulate a film scan but why does that mean it would be superior for a chroma key?

I just remember from the days of HD on the Sony F900 that applying any sort of color matrix to boost saturation actually made worse keys because artificial color edges would be created. So it struck me that the color differences had to be in the original data and boosting color wasn’t going to make keying better.

But I’m not a compositor, you’d want to talk to a VFX person...

[M Joel W]

I was more curious about using it to get a "film look" than for keying, but Arri recommends it for vfx here:

Beginning with ALEXA SUP 3.0 until SUP 7.0 ARRI offered a film style matrix that could be applied to the Log C output. The same transform was also available as a 3D-LUT for post-processing of Log C footage.

The film style matrix makes the color characteristics of the Log C image similar to negative film scanned on an ARRISCAN. The matrix is most usefully applied when the data is previewed or converted with a print film emulation (PFE) . This is the common workflow in Digital Intermediate where the PFE is applied as a 3DLUT in the display path.
The film-matrix can in some situations achieve a bigger color separation and therefore make the keying of some shots easier.

Due to new developments in color grading and digital workflow our film style matrix has been discontinued.

https://www.arri.com/en/learn-help/learn-help-camera-system/frequently-asked-questions/visual-effects-faq#accordion-44000

 

[Mark Kenfield]

All of the old digital grading workflows sucked. There was so much trial and error going on - that's why results were so inconsistent. Better to stick with whatever's current.

[David Mullen ASC]

Sounds like from the release that ARRI doesn’t feel it is necessary anymore.