Workflow walkthrough: In-camera grading with ARRI Alexa 35 and Livegrade

Welcome to the second article in our three-part blog series on in-camera grading! If you haven’t already, we recommend checking out part 1 for a comprehensive introduction to the topic. In this article, we’re moving from theory to practice by walking you through a hands-on example of an in-camera grading workflow. 

As we mentioned in part 1, most professional camera manufacturers support in-camera grading, but workflows vary depending on the underlying look files and technical approaches unique to each system. It would be impossible to cover all the different approaches within the scope of this article, so we focused on one specific example instead: In the following, we’ll discuss how to grade an Alexa 35 directly in-camera to manage dual-monitoring setups for SDR and HDR. In doing so, we will stick to ARRI’s built-in color management for now. However, come back for the third and last part of this series, in which we will explore the custom color management capabilities of an Alexa 35. 

One last thing before we start: Please keep in mind that the scenario presented here is intentionally simplified to illustrate the general process of in-camera grading. In reality, you might need to adjust the mentioned steps to suit your specific requirements. So, without further ado, let’s dive in! 

Understanding ARRI Look File 4 (ALF4)

As previously mentioned, a camera’s underlying look is always the basis for grading that camera remotely. Depending on the type of camera you’re working with, you will come across different types of look files. Since we will explain the process using the ARRI Alexa 35 as an example, we must first understand its underlying look file: ARRI Look File 4, or ALF4 for short. 

Before introducing the Alexa 35 with ALF4 look files, ARRI’s look files always consisted of a CDL and a 3D LUT part. The colorspace conversion to a certain output colorspace (e.g., Rec.709) was part of the 3D LUT, which was also used to create a desired look. A separation between the creative look and the colorspace conversion was not possible, so looks for different colorspaces had to be designed for each colorspace separately (e.g., one for SDR and another one for HDR). That changed with the introduction of ALF4 files, which key concepts are:

One look for multiple output colorspaces: ALF4 looks can be used for multiple colorspaces, as the creative look is separated from the colorspace conversion. A certain look can, therefore, easily be used for different output colorspaces.

One look for dual monitoring: ALF4 looks always include two display rendering transforms, which can be used for dual monitoring on set. Dual monitoring with one single look file is now possible, as the camera’s individual video outputs can be configured separately for HDR or SDR colorspaces. 

The graphic below visualizes the different components of an ALF4. In the following, we will take a closer look at each individual part. 

CDL & Saturation:

• This part consists of ASC CDL values, which are usually used to adjust color values on a shot basis.
• Example: Warming up a shot a little bit.

CMT:

• The Color Modification Transform (CMT) is located in the middle section of the ALF4. It’s a 3D LUT intended to creatively adjust the look as a Log-to-Log adjustment.
• Example: Emulation of a film stock in terms of contrast and color.

DRT SDR & DRT HDR: 

• At the end of the look, we find the Display Render Transforms (DRTs) for SDR and HDR. DRT SDR and HDR are both 3D LUTs that convert the log colorspace into a display referred colorspace. To enable dual monitoring for SDR and HDR pipelines, there is one DRT for SDR and another one for the HDR transform.
• Both of these DRTs must always be set. This applies to looks that use ARRI’s color management as well as looks that use custom color management. 
• When working with ALF4 looks that use ARRI’s color management, those DRTs will always be set automatically by the camera, depending on the colorspace you have set for the SDI output. This ensures that the transform from LogC4 to the respective display colorspace is technically accurate. As already mentioned, it is also possible to use custom color management, which would then be responsible for the display rendering. However, there are a few things to bear in mind when taking this route, which we will explore in detail during the third part of this blog series.
• Examples:
  • DRT SDR: Colorspace conversion LUT from LogC4 to Rec.709
  • DRT HDR: Colorspace conversion LUT from LogC4 to Rec.2100 PQ

Step by step: Controlling ALF4 with Livegrade on a dual-monitoring setup

Now that we’ve got the basics straight, let’s apply these learnings in the hypothetical scenario depicted below. We will walk you through each step of adjusting an ALF4 look interactively with Livegrade and in a dual monitoring setup without any additional LUT box. We’ll also explain how to extract the look from the recorded clips using Silverstack. 

Step 1: Design your setup for dual-monitoring with Livegrade and Alexa 35

The scenario we are discussing is based on the setup pictured above and consists of the following components: 

• The camera’s SDI 1 (configured to SDR Rec.709) and SDI 2 (configured to HDR Rec.2100 PQ) feed an SDI router with one SDR- and one HDR-graded signal. 
• The SDI router feeds SDR and HDR monitors on the DIT’s cart and in the video village.
• In order to work with a log encoded image source in Livegrade, Livegrade receives a log encoded frame grab via ethernet from the camera.
• In this setup, the viewfinder of the Alexa 35 can show either the HDR- or the SDR-graded image at any time.

Step 2: Prepare the camera

With the hardware all set up, let’s take care of the camera next. To enable the communication with Livegrade, please check the following:

Network connection: To exchange look information with Livegrade, the camera needs to have a working network connection with the computer that’s running Livegrade.

Enable CAP: To connect Livegrade to the camera, you have to enable the Camera Access Protocol (CAP) in the ARRI camera’s menu at „System > Camera Access Protocol“. You can also set a CAP server password there to protect access to the camera.

Editable look selected in the camera: Before grading an ARRI camera, make sure that you have selected an editable look (e.g., by duplicating the “Default” look) in your camera.

Example: Duplicate the Default look and name it “Example_ALF4”.

Step 3: Add camera as a device to Livegrade

To access the camera remotely from Livegrade, add the ARRI camera to one of your slots using the „Add ARRI Camera“ dialog in the device manager. When doing so, pay attention to the following settings: 

Enable framegrab: In this example, you want to use the frame grab functionality of the camera, so make sure you activate „Enable framegrab“ in the „Add ARRI Camera” dialog.

Look control: As you want to grade an ALF4 look, where the color management is done entirely by the camera, select „CDL+CMT look control“ within the advanced color pipeline options.

Once you have successfully added the Alexa 35 camera to one of your slots, you can use „Slots > Refresh Framegrab of Current Device“ to get an image source to your slot and start live grading.

Step 4: Use CDL + CMT look control of an ALF4 file

Using CDL+CMT look control, Livegrade will access the CDL and CMT part of the look, whereas the display rendering is handled by the camera using the standard ARRI LUTs.

Let’s take a look at what actually happens to the underlying look in the camera when grading with CDL+CMT look control:

CDL & Saturation:

• All of your CDL and saturation adjustments that you make within the ARRI CAP compatible mode will be mathematically merged and then sent as one CDL value to the camera’s look.
• Example: Use the first CDL node to warm up the scene.

CMT:

• All Log-to-Log LUTs, curve adjustments, and other LUT-based nodes within your grade will be merged into one single 3D LUT, which will be sent as CMT to the camera’s look.
• Example: Load 3110 Film A.cube from ARRI’s LogC4 look library into your 3D LUT node to add a filmic look to your image.

DRT:

• The last LUT node of your look won’t be sent to the camera, as the camera will apply the display render transforms (DRT) accordingly to the camera’s SDI out settings.
• However, if you are working with a LogC4-image source like a framegrab in Livegrade and want to use your looks to be applied to original camera footage at a later point, you should set the last LUT node according to your needs.
• Example 1: If your primary target colorspace is Rec.2100 PQ and you want to hand over looks and stills in this colorspace, set the ARRI LogC4 > Rec.2100 PQ LUT in your last LUT node “DRT”.
• Example 2: To be able to export stills of your looks in Rec.709 via Livegrade, set the ARRI LogC4 > Rec.709 LUT in your last LUT node “DRT”.
• After setting the LUT as DRT, try disabling and enabling this node. 
• You should not see any difference between an enabled or disabled DRT node in the camera’s graded outputs.
• However, you should see a difference between an enabled or disabled DRT node within Livegrade’s viewer, for example when receiving a frame grab.

Finally, check if everything is setup correctly: 

• Request a framegrab via „Slots > Refresh Framegrab of Current Device“ 
• Compare the graded framegrab in Livegrade with the graded output of the camera
• Do they match? Then you are good to go!
• If they look different, you should ensure that your ethernet connection to the camera is stable and that the last LUT node “DRT” in Livegrade was set to convert to the same colorspace as the selected SDI colorspace in the camera.

Step 5: Update looks in camera

After setting up your look and making your first adjustments with Livegrade, let’s explore the two main aspects of interacting with the look loaded in the camera:

Immediate look transmission to clip’s metadata: All changes in Livegrade are immediately transferred and saved within the clip’s file header.

Storing the in-camera look permanently when creating shots: When creating a new shot in Livegrade, you can choose if the looks should be stored permanently on the camera as well. There are two main purposes for storing looks permanently in the camera:

• to update the look’s name in the camera menu and the metadata.
• to be able to retrieve certain looks when Livegrade is currently not connected to the camera.

Please note: If you want to change the behavior when creating shots, this setting can be changed in the devices tab of Livegrade’s settings. Simply set the “Store Look in ARRI Camera” option to your personal needs.

Step 6: Extract the look from recorded clips in Silverstack

As already mentioned, the current look that the camera outputs will always be saved in the clip’s file header. This makes it possible to extract the look from every copy of the original camera footage. In Silverstack, this extraction can be done with the selection “From File” for the look source of a clip. After selecting this option, the look from the recorded clip will unfold in its single parts represented by the individual nodes:

• CDL: will contain all CDL adjustments. When multiple CDL nodes have been used when grading in-camera, those would be mathematically merged together into one single CDL node.
• Saturation: will contain all saturation adjustments. When multiple saturation nodes have been used when grading in-camera, those would be mathematically merged together into one single saturation node.
• CMT: will contain all LUTs and curve adjustments (except the last LUT node) of your in-camera grade, merged into one single 3D LUT file.
• DRT: will contain the ARRI default LUT to convert the image from LogC4 to Rec.709.
• Example: After you have adjusted your look in Livegrade and checked that the camera outputs show the graded image as intended, record a clip and offload it using Silverstack. 

In Silverstack, select the clip, switch to „Color”, select „Look: From File” and click on “Edit Look“. Assuming you graded your image as suggested above, your look should unfold as follows:

• CDL: should contain an adjustment to warm up the image.
• Saturation: should be set to neutral.
• CMT: should contain 3110 Film A.cube, which adds a filmic look to the image.
• DRT: should contain the ARRI default LUTLUT to convert the image from LogC4 to Rec.709.

Conclusion and outlook

As we’ve outlined in this article, in-camera grading offers additional possibilities for live grading on set. With the Alexa 35, this includes managing dual-monitoring setups for SDR and HDR without involving additional LUT boxes. Furthermore, you can easily extract colorspace-independent looks from the original material. We hope this step-by-step walkthrough provided valuable insights into the process and its potential. That said, the extent to which in-camera grading can enhance your workflow will always depend on the specific needs of your production.

In the third and final part of this series, we’ll take a deeper dive into the technical details. We’ll explore the custom color management capabilities of ALF4c looks and how to work with them in Livegrade. Stay tuned!