Dialing Out The Arri Alexa Film Matrix

How To Use the RGB Mixer To Remove The Arri Alexa Film Matrix

December 15, 2013

Are you fighting a green color cast with Arri Alexa footage? This may be due to the Arri Alexa Film Matrix. Learn how to use Resolve's RGB mixer to fix it.


Series

Day 15: 20 Insights in 20 Days Holiday Marathon

Using The RGB Mixer To Remove The Arri Alexa Film Matrix

These days, at least 60% of the broadcast, political and corporate stuff that I work on is shot on Arri Alexa Cameras. You can imagine this make me a very happy colorist as the image quality (especially in RAW, or Log C) from the Alexa is just astounding.

In other Insights we’ve explored working with LOG C footage from an Alexa in Premiere, Resolve and SpeedGrade. But there is something that many colorists aren’t aware of with the Alexa when it comes to Log C recording.

Film Matrix In, Film Matrix Out

When shooting Log C on the Alexa there is an additional option that DPs and camera operators have – that’s to have the Arri Alexa Film Matrix enabled or disabled. What does that mean? Well, who better than Arri to describe it.

[blockquote]Log C (film matrix off) is a Log C signal with a wide gamut color space. This option provides great flexibility in color grading, as it preserves the most color information in the recorded image. However, you should be aware that Log C is an intermediate color format and not designed as a display standard. Viewed on a regular video monitor, Log C images look flat and desaturated.[/blockquote]

[blockquote]Log C (film matrix on) applies a color matrix that makes the resulting image resemble a film negative scanned on an ARRISCAN. While this option somewhat reduces the color gamut in contrast to Log C (film matrix off), it provides an easy and fast way for colorists who are used to scanned negative, thus speeding up color grading. It is also a great option when combining ALEXA images with film originated images.[/blockquote]

In this Insight, it’s the second method that I want to explore. Notice in the above description the line “a fast way for colorists who are used to scanned negative”. While I’m sure there are a few of you out there who this sentence applies to, for the rest of you what do you do if you’re not familiar with a scanned negative?

Removing The Green

In practical terms, when a shot as been recorded on the Alexa with the film matrix on in LOG C, after you dial in contrast and apply saturation, the footage will have a green look to it – that’s normal! This is a result of the Film Matrix being applied and is something you’ll see all the time with Alexa shots.

While you can dial this out with lots of different methods in Resolve, one of my favorites – mainly for its precision, is to use the RGB mixer.  Check out the video to see how this works.

Thanks to Nick Shaw from Antler Post-Production Services for providing these numbers. You should also follow Nick On Twitter. Also thanks to Getzels Gordon for the shot!

Questions? Comments? Use the comments below to start a discussion on this Insight. Or ask a question if something I did confuses you.

– Robbie

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Comments

Homepage Forums How To Use the RGB Mixer To Remove The Arri Alexa Film Matrix

Viewing 18 reply threads

    • Lee N
      Member

      Thanks Robbie, great tip! Would you mind uploading your 2 presets so we don’t have to dial it in ourselves. I tried now for 10 minutes, but couldn’t do the exact numbers… 🙂

      Thanks,

      lee


    • RobbieCarman
      Guest

      Thanks Lee! Yes, its a bit tricky. The full resolve panels make this really easy to dial in but in testing this out on my home machine it took a while to get it dialed in.

      Your suggestion of course is great! I think Patrick actually mentioned to me to upload these but I kind of just spaced – sorry! Anyway, up above in the post I’ve put in a link to a zip that contains the two RGB mixer corrections. They are DRX files so you should be able to bring them right in. Hope this helps!


    • Lee N
      Member

      Thank you Robbie. I thought that it’d be probably easier with the big panels.
      If just my Element panel would get a proper remapping 😉


    • Guest
      Guest

      Any particular reason to use this method over applying the Arri supplied LUT for to converting from film matrix to standard log-c?


    • Patrick Inhofer
      Guest

      Damon – Looking over the LUT Generator I’m not seeing an option that generates a different LUT if the Film Matrix is enabled. Not sure if I’m just not seeing it (or it’s called something different).


    • RobbieCarman
      Guest

      None other than manual control. I’m not a big LUT guy myself. And I really prefer then manual control that I get with this approach. Lots of times I also just dial things in sans RGB mixer and use other primary controls. Lots of ways to handle the situation.


    • Damon Meledones
      Guest

      I just did the same thing. It appears they have removed the option to create a Film Matrix removal LUT, although you can still create a LUT which adds the Film Matrix. The formula for removing and adding the Film Matrix is still in Log-C white paper, which I suspect is where Nick Shaw derived the RGB settings from.


    • Patrick Inhofer
      Guest

      Damon – You are precisely correct. The White Paper lays out the numbers and correspond to the what Nick Shaw shared. If anyone wants to read the White Paper for themselves go here: http://www.arri.com/camera/digital_cameras/downloads/

      On that web page it’s listed under: Technical Information > ALEXA Log C Curve Usage in VFX

      It’s an Insight for me to see that those numbers in the RGB Mixer correspond so precisely with the White Paper. Interesting.


    • RobbieCarman
      Guest

      thanks Pat! Yes I know that the information was available from Arri – but hopefully it’ll help people who are not so whitepaper inclined. Your right Damon thats were nick picked it out from.


    • Patrick Inhofer
      Guest

      And to be completely fair – I was surprised comparing the values in the White Paper to the values that Nick shared for the RGB Mixer and that they essentially correlated 1:1 (with rounding). Even if I read the White Paper it wouldn’t have occurred to me to enter those values in that tool. Now that I know… interesting possibilities arise whenever we come across values like that elsewhere.

      Thanks for pointing it out Damon!

    • I have been using this tip on few jobs now and god does it make my whites look clean and nice. It is like super precise WB on alexa material that i would love to have to other cams as well. Specially when you go from log to rec you do not get colour smear over image. This is something i’m missing with my BMC. The way to get the WB starting point super precise.


    • Simon Rabeder
      Participant

      Is there a way to use the matrix to do the reverse? I’d like to use this as a starting point but then go back somewhat using key. Some clients ask for the ARRI color and loved it in dailies so I wanna be able to go there.


    • Robbie Carman
      Guest

      I’ll have a think on that….one way would to simply use the key gain control (Key Pallet) which essentially acts as a blend/opacity control. So if you did the RGB mixer correction by itself on a node you could blend the correction in/out based on your preference


    • Simon Rabeder
      Participant

      Sure, I could do it that way, but I would wanna use the correction first, do a pass of everything knowing the image is neutral now, then apply the arri matrix again as part of the look in the end. For sake of my point lets say I’d use a node on the whole timeline.


    • Simon Rabeder
      Participant

      Robbie, did you figure something out? I would love to see an in depth tutorial on the Color Matrix. I think it might be a powerful and underused tool to create looks. One might even go as far as applying the film matrix to other cameras.


    • Christian Cashmir
      Guest

      What are the settings that people have found to be best to remove the green cast in the Alexa Mini footage? I typically dial my green to -4 in-camera. Are people finding it’s better to do it in post? I’m using Lumetri in Premiere for mosts of my projects and sometimes we’ll do a grade in Davinci for particular projects or films. If anyone has specific settings they like for using Lumetry, please share! Thanks!


    • Pat Inhofer
      Guest

      To read the rest of this article and watch the video where Robbie shares his workflow, please log in to the website with an active membership. This Insight focuses on the post-production solution and assumes default camera settings. In-camera is usually better for most post-production problems but the in-camera color adjustment is not addressed here.


    • Christian Cashmir
      Guest

      Thanks Pat, I saw the article. Just wondering if anyone had settings based on their personal experience USING LUMETRI IN PREMIERE PRO and also maybe a comparison of in-camera green adjustment versus adjusting in post?


    • Simon Rabeder
      Guest

      Not working on a lot of Alexa Classic footage these days, but Walter Volpatto sent me a DCTL doing it both ways.

Viewing 18 reply threads
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