Working with ARRIRAW
Shooting with Looks
It is often advantageous or desirable to monitor your camera footage with the creative look and feel that the final output might have. On an ALEXA production, you can do this in a few different ways. An ARRI Look file can be loaded directly in camera, transforming the traditional Log C output into something similar to the final grade. You can also apply an ASC CDL color correction to a clean monitoring feed using external hardware.
ARRI Look Files
When an ARRI Look File is enabled for an ALEXA output, the look gets embedded as metadata into the recording output. While it’s possible to apply a look to any of the different output options, looks are typically applied only to the monitoring outputs. The recorded image would then remain unmodified, allowing maximum flexibility downstream in the editing bay and/or during color grading (in non-destructive mode). When recording in ARRIRAW, look files will be embedded in the metadata of each frame, allowing for complete non-destructive image enhancement.
To create an ARRI Look File, you need to store a Log C DPX frame grab (REC OUT set to HD422/444 Log C extended range) or an ARRIRAW grab to an SD. Load the grab or clip into the free ARRI Look Creator (ALC) or ARRIRAW Converter and create your look. Finally, save the color correction on the SD card and load the ARRI Look File into the ALEXA. Optionally, you can export an image with the look for reference.
The ARRI Look File contains parameters for saturation and printer lights controls (RGB offsets) and controls for lift, gamma, and gain. Optionally, a look file may also include a freeform curve (grey scale tone map LUT) that will be applied instead of the standard Log C to Video tonemapping curve to adjust the contrast or e.g. the highlight roll-off. Looks can also be created with third party software, such as Pomfort Live Grade and Silverstack SET or Colorfront On-Set Dailies. Live Grade in addition allows to create a look based on a live signal from the camera.
ASC CDL Looks
On-set color correction systems can apply an ASC CDL (American Society of Cinematographers Color Decision List) color correction to a live signal and output it on connected monitors. When the internal recording is used, a clean Log C feed can be taken from the REC OUT of an ALEXA. When ARRIAW is sent out via T-Link, the clean feed needs to be provided either from the ALEXA MON OUT or from the monitoring output of the ARRIRAW recorder. The ASC CDL standard is supported by a wide range of devices.
The color correction settings that were made on the live image during the shoot can be logged with time code and then output in an Avid Log Exchange (ALE) file. The ALE can then be used to apply the CDL corrections when creating the deliverables.
When a Codex Onboard recorder is connected to a color correction system via Ethernet, it can automatically capture the ASC CDL correction values with the camera footage*. The VFS can then use the color correction settings and automatically apply them when generating dailies clips. The new XR Module retains the same functionality as its legacy system predecessors.
*Note: Codex onboard recorders support different protocols for submission of CDL values. The XR Module only supports the Truelight protocol, which is available in different color correction systems.
An ARRI Look File and an ASC CDL color correction offer different parameter sets for color correction. The only overlap are the controls for lift, gamma, and gain (which can be converted into slope, offset, power). The ARRI Look File provides a rich set of controls, but the combination of all options includes operations that are not supported by most color grading tools. When an ARRI Look File is brought into grading, it therefore needs to be converted into a 3D LUT using the ARRI LUT Generator. A 3D LUT acts like a black box, meaning you cannot modify what it does. The colorist can only apply the 3D LUT to see a reference of the intended look and then needs to start from scratch and recreate the look. To be fair, it should be said, that most colorists would start from scratch anyway.
The advantage of the ASC CDL is, that it is fully supported by most grading tools and therefore can be loaded and tweaked in grading. The downside being e.g. the lack of primary color correction controls.
The Best of Both Worlds
You can decide in favor of one method or the other, but it is also possible to combine the advantages of both methods. You can use primary color correction of the Log C signal from the ARRI Look File to set the basic look and adjust the resulting Rec 709 video image with an on-set color corrector using the ASC CDL controls. A colorist will then be able to recreate the same color correction in his grading tool, using the numerical values from ARRI Look File and ASC CDL. If saturation is also adjusted in the ARRI Look File, the colorist will not be able to use the numerical value, but shouldn't have much of a problem to find a setting with matching results.