ColourSpace & ACES

LUT Generation from ACES data

One of the most common uses for the ColourSpace ACES tools is the generation of Look LUTs for DIT use on-set when shooting with specific cameras, that have vendor supplied IDT ctl data.

The simplest, and actually rather effective, ACES workflows is to simply make a LUT from the concatenation of an IDT, with RRT and ODT. This can be performed within ColourSpace very simply.

To generate LUTs using just ACES CTL files, with no other data or manipulations, is a very straightforward process. It is the simplest and easiest of the ACES based Light Space CMS workflows, and will generate a LUT that faithfully represents the source ACES CTL data.

First, place the CTL files to be used into a single folder within the PC. Here we are using an Arri Alexa IDT, with a Rec709 100 Nits ODT, and the standard ACES RRT.

(Note: The order the ctl files are processed in alphabetical order, so it will help prefixing the ctl files with incremental numbering - 001-IDT..., 002-RRT..., 003-ODT...)

Next, within ColourSpace make a new Unity LUT with LUT Tools, as this will be the blank canvas upon which the ACES based changes will be made.

At this point, if wanted, a Reference image can be added into the Unity LUT image, using the Add Picture function. The added image will immediately show the result of any LUT manipulation, including the applied ACES CTL data. It not a required step, as the generated LUT can be applied to any user image within ColourSpace after generation.

Here, we are using an Alexa image from one of Geoff Boyle's CML Camera Tests.

The next step is to apply the ACES CTL data, using LUT Adjust, ACES, ACES ctl, and Import.

The Import button opens a navigation window to allow external ACES ctl/clf files to be imported, and applied the selected LUT.

Importing and processing ACES ctl files into a LUT is based on the alphabetical order of the ctl files, and needs the IDT first, followed by RRT, and ODT.
(Prefixing the ctl files with incremental numbering - 001-IDTxxx, 002-RRTxxx, 003-ODTxxx - can be used to correctly sort the import ordering and processing.)

The result of the applied ACES CTL data will be seen immediately after Open has been clicked.

And the resulting LUT can be seen via the 3D Cube and 1D LUT Views.

The generated LUT can be Saved into the Library, it exported in any LUT format as required, or directly uploaded to integrated LUT boxes or monitors.
(Note the colour temperature variation shown in the 1D graph - this is due to using the ACES D60 ODT within this example, to show the colour temperature change within the 1D graph.)

Converting LUTs into ACES

Where will the LUT be uses in the workflow pipeline?
Before the RRT?
After the ODT
Is the RRT Look desired, or not, etc?

The simplest process within ColourSpace for LUT Conversion to ACES is to use the LUT Generation menu, and burn the required colour space conversion into the LUT.

For example, if you have a Look LUT that is a Rec709 LUT and you want to use the same Look in an ACES workflow.

• First, load the Rec709 Look LUT into the LUT Tools window
• Set Source as Rec709, and Destination and ACES
• When performing the LUT Generation use the Apply to Existing LUT option

Remember this generated LUT will be mapping the original LUT Colour Space into ACES, and will not be expanding the original colour space.

Another alternative workflow is to concatenate two LUTs together, using the Add function within LUT Adjustment, Maths.

• Make a Colour Space Conversion LUT
  (This would be the same as the above LUT Generation settings, but use Make New to generate a new LUT with just the colour space conversion data)
• Save the LUT into the Library
• Select the first LUT (The Look LUT) and use the Add function within LUT Adjustment, Maths to concatenate the Colour Space Conversion LUT with the Look LUT

The order in which the LUTs are combined will alter the end result. The way to visualise this is to imagine an image being passed through each LUT in turn. If the image is first passed through the Look LUT, and the result is then passed through the Colour Space Conversion LUT, the result will be different if the image is first passed through the Colour Space Conversion LUT, and the result is then passed through the Look LUT.

ACES LMT (Look Modification Transforms) Generation

The ACES tools within ColourSpace can be used for many different CTL workflows, and the use of, and generation of LMT (Look Modification Transform), is another example.

However, generating an LMT from existing LUTs is not as simple as you may think or like...

The issue is making sure the LUT is in a format that works within the ACES workflow, and this is where ColourSpace can help.

The first stage is understanding the process required to generate a LMT, which basically means converting the original LUT into ACES space. This needs to be done in stages.

The first step is to combine the RRT and ODT into a single concatenated LUT/CTL. As before, all the different CTL file that are going to be used need to be held within the same PC folder.

• Generate a Unity LUT with LUT Tools, as this will be the blank canvas upon which the ACES based changes will be made
  (Switching to the 1D LUT Viewer can help in visually understanding the processes being applied)
• Using LUT Adjust, ACES, ACES ctl, and Import, select the CTL files to be concatenated using Ctrl, Left Click
  (Prefixing the ctl files with incremental numbering - 002-RRTxxx, 003-ODTxxx - can be used to correctly sort the import ordering and processing)
• With both files selected, click Open to start the file concatenation process.
• The concatenation result can be saved as a LUT into the Library, and directly exported as an ACES CTL via the Export function

The second step is to combine the InvODT and InvRRT (in that order) into a single concatenated LUT/CTL.

• Generate a Unity LUT with LUT Tools, as this will be the blank canvas upon which the ACES based changes will be made
  (Switching to the 1D LUT Viewer can help in visually understanding the processes being applied)
• Using LUT Adjust, ACES, ACES ctl, and Import, select the CTL files to be concatenated using Ctrl, Left Click
  (Prefixing the ctl files with incremental numbering - 001-InvODTxxx, 002-IncRRTxxx - can be used to correctly sort the import ordering and processing)
• With both files selected, click Open to start the file concatenation process.
• The concatenation result can be saved as a LUT into the Library, and directly exported as an ACES CTL via the Export function

Getting the Look LUT into the correct format for use with the ACES workflow can be a bit more tricky, and requires some thought.

In this example we will use a D65 Film Print emulation Look LUT, as can be downloaded via the Customer Downloads, or General Downloads, page of the website. The standard LUT is a Log LUT, which expect the source images to be Log Rec709 colour space, while the ODT we are using is standard Rec709, and is expecting to work with standard gamma based images. As a result, the LUT needs to be modified to work with standard gamma Rec709 images.

There are many ways to perform such a LUT modification, and here we will use the in-built LUT Adjustment tools.

• Load the Film Log Look LUT into ColourSpace, in this example via LUT Management, LUT Image
• Save the LUT into the Library, and directly export as an ACES CTL via the LUT Adjustment, ACES, Export function

We now have the three separate components that are required to generate the final LMT CTL, which can be performed using the saved CTL files, or via the saved LUTs.

• To use the CTL files, generate a Unity LUT with LUT Tools, as this will be the blank canvas upon which the CTL files will be processed
• From the LUT Adjustment, ACES, Import function, navigate to the folder holding the three CTL files, hold down the keyboard Ctrl button, and select each CTL file in turn with a mouse Left Click
  (Prefixing the ctl files with incremental numbering can be used to correctly sort the import ordering and processing)
• With the files selected, click Open to start the file concatenation process.
• The concatenation result can be saved as a LUT into the Library, and directly exported as an ACES CTL via the Export function

The order the CTL files are processed is based on their alphabetical ordering, and incorrect ordering will generate incorrect end results.

The LUTs saved into the Library can also be used to generate the LMT, using LUT Tools, LUT Adjustment, Maths, and the Add function.

If wanted, a LUT can be made using the full ACES CTL concatenation workflow to see the final end result.

• Generate a Unity LUT with LUT Tools, as this will be the blank canvas upon which the CTL files will be processed
• From the LUT Adjustment, ACES, Import function, navigate to the folder holding the CTL files, hold down the keyboard Ctrl button, and select each CTL file in turn with a mouse Left Click
  (Prefixing the ctl files with incremental numbering can be used to correctly sort the import ordering and processing)
• The files need to be processed in order of IDT, LMT, RRT, and ODT

ACES CLF/CTF

ACES CLF/CTF capability within the Edit menu of ColourSpace enables the import and export of any .clf/.ctf (ACES based or otherwise) files, as well as the conversion of LUTs held within ColourSpace into .clf/.ctf files.

Note: the .ctf format is an Autodesk variation on the ACES .clf format.