The Arri Alexa remains one of the most common cameras used in production these days. Its proprietary LogC format captures fantastic highlight detail and exceptionally clean imagery.

But with each new proprietary camera format comes a new process for decoding, viewing, and interacting with the camera's footage. Generally speaking, this involves applying a specific LUT to our footage.

Most applications have these LUTs built in to their media management tools. All it takes to correctly view your footage is to select which LUT to use on your clip.

This is, unfortunately, not the full story when it comes to Alexa footage.

If you've ever imported an Alexa colorspace clip into Nuke, set your Read node to "AlexaV3LogC", and viewed it with the default Viewer settings, you may notice that the highlights look blown out. If you use a color corrector or the Exposure slider on your Viewer, you'll see that the image detail in the highlights is still there, it's just not being displayed correctly.

An Alexa LogC clip being viewed in NukeX with the sRGB Viewer Input Process.

If you import that same clip into DaVinci Resolve, again, set it to Alexa colorspace and view it, you'll notice that it doesn't match the Nuke viewer. In Resolve, the footage looks "correct".

An Alexa LogC clip being viewed in Resolve with the Arri Alexa LogC to Rec709 3D LUT applied.

So, what's going on here?

The Alexa's LogC footage needs to be gamma corrected and tone-mapped to a Rec709 colorspace. In Nuke, this is a 2-step process. The footage gets its gamma linearized in the Read node before work is done, then, after our work has been added, the footage needs to be converted to Rec709 colorspace. In DaVinci Resolve, these 2 steps are performed at the same time.

The problem is that second step in Nuke. There is no built-in Viewer Input Process to properly view Alexa footage. We could toss a OCIOColorSpace node at the end of our script and work in between it and our Read. But we don't want to bake that Rec709 conversion into our render, we just want to view it in the corrected colorspace.

Adding a Custom Input Process

The first thing we're going to need is the Alexa Viewer LUT. No, this is not the same LUT that comes with the application. You can download it here, or build your own with Arri's online LUT generator.

If you only use Nuke/NukeX, adding the Input Process is relatively simple, and bares a striking resemblance to a lot of the Defaults customization we've done in the past. If, however, you also use Nuke Studio or Hiero, you'll want to ignore this section and skip ahead to the OCIOConfig version.

Nuke / NukeX

To get started, create a new Nuke project. Then:

Create a OCIOFileTransform node and add the downloaded LUT file.
Set your "working space" to "AlexaV3LogC". Leave the "direction" on "forward" and "interpolation" on "linear".
After the OCIOFileTransform node, add an OCIOColorSpace node.
Set your "in" to "linear" and your "out" to "AlexaV3LogC"

The nodes for the AlexaLUT Gizmo in Nuke.

Now we need to turn these 2 nodes into a Gizmo. To do that, select them both, hit CMD+G on the keyboard to Group them, then click the "Export Gizmo" button. Save the Gizmo in your .nuke directory. Mine is called Alexa_LUT.gizmo.

Once we've saved our Gizmo, we just need to add the following line to our Init.py file:

nuke.ViewerProcess.register("Alexa", nuke.Node, ("Alexa_LUT", ""))

Now, when you start up Nuke, you'll have your Alexa LUT in the Input Process menu in your Viewer.

The Alexa Input Process in the Nuke Viewer.

And, just so we're clear, if we're working on an Alexa colorspace clip, as a Good VFX Artist, we're going to send back a render that is also in Alexa colorspace. That means setting the "colorspace" on our Write node to "AlexaV3LogC", regardless of the file format.

NukeStudio (and Also Nuke / NukeX)

Welcome, Nuke Studio users. For you, this process is going to be a little more work.

Just like everything in Nuke Studio, am I right?

Sorry. Let's get started.

To add our Alexa LUT to Nuke Studio, we need to create our own custom OCIOConfig. Since we're lazy (read: smart), we'll duplicate and modify the Nuke Default OCIOConfig to save us a lot of time and effort.

The OCIOConfigs that come with Nuke can be found in the app's installation directory under /plugins/OCIOConfigs/configs/. We're going to copy the folder called "nuke-default" and paste it into .nuke/OCIOConfigs/configs/ and let's rename it to something like "default-alexa".

Before we do anything else, we need to put our Alexa Viewer LUT inside the "luts" folder inside our "default-alexa" folder.

Is it there? Good.

Inside our "default-alexa" folder, is a file called "config.ocio". Open that in a text editor of your choice.

Near the top of the file, you'll see a section that looks like this:

displays:
  default:
    - !<View> {name: None, colorspace: raw}
    - !<View> {name: sRGB, colorspace: sRGB}
    - !<View> {name: rec709, colorspace: rec709}
    - !<View> {name: rec1886, colorspace: Gamma2.4}

We need to add this line:

- !<View> {name: Alexa, colorspace: AlexaViewer}

I put mine at the top, first in the list, because I want the Alexa viewer to be my primary Input Process LUT. A good 80% of the footage I work with is Alexa footage. Your use case may vary. Rearranging these lines will have not break anything as long as you keep the indentation the same.

Now, scroll all the way down to the bottom of the file, past all the built-in colorspace configs. Add the following:

- !<ColorSpace>
  name: AlexaViewer
  description: |
    Alexa Log C
  from_reference: !<GroupTransform>
    children:
      - !<ColorSpaceTransform> {src: linear, dst: AlexaV3LogC}
      - !<FileTransform> {src: ARRI_LogC2Video_709_davinci3d.cube, interpolation: linear}

That wasn't so bad, was it. Was it?

Now, all that's left to do is open Nuke and/or Nuke Studio, go to your application preferences, and under the "Color Management" section, select our new OCIOConfig file.

Choosing our custom OCIOConfig in the Nuke application preferences.

Now, you'll have your Alexa LUT in your Input Process dropdown in both Nuke and Nuke Studio and you can finally get to work.

Thanks Are in Order

I've been putting off this blog post for a very long time. Very nearly 2 years, to be specific.

I was deep into a project in Nuke Studio and was losing my mind over not being able to properly view my Alexa raw footage or Alexa-encoded renders. This project also included a large number of motion graphics, so making sure colors and white levels matched was doubly important.

So, I sent an email to Foundry support.

After about a week and a half of unsuccessful back-and-forth with my initial contact, my issue was escalated and I was contacted by Senior Customer Support Engineer Elisabeth Wetchy.

Elisabeth deserves all of the credit for solving this issue. She was possibly the most helpful customer support representative I've ever worked with.

Also, in the process of doing some research for this blog post (yeah, I do that sometimes shut up), I came across an article she wrote the day we figured this stuff out. So I guess I shouldn't feel too bad for making you guys wait 2 years for my post.

Note: Test footage from of Arri can be found here.

Update for Nuke 12 and Up

In case you didn't click through to that support article above, there's a very important update for users of Nuke 12 and up:

As of Nuke 12, the active_views list will now be respected, and this controls which views are visible and the order in which they appear.

So for the custom LUT to appear in the Viewer, you will need to append the LUT to the active_views list in the OCIO config:

active_views: [sRGB, rec709, rec1886, None]

For example:

active_views: [sRGB, rec709, rec1886, AlexaToRec709, None]

This line is also optional and, by default, will set all views to be visible and will respect the order of the views under the display. So if you wish for all LUTs to be visible, you could simply delete this line.

In my case, my active_views line says active_views: [Alexa, sRGB, sRGBf, rec709, rec1886, None] because my Alexa profile is named "Alexa", not "AlexaToRec709". I also put it at the front of the active_views list because I want it to be the default since I primarily work with Alexa LogC footage.