ACES: Try it, it might make your life simpler!

workshops-275 ACES: Try it, it might make your life simpler!

ACES is a workflow for modern digital cinema cameras. It’s designed to act as a common standard that will work with any camera so that colourist can use the same grades on any camera with the same results.

A by-product of the way ACES works is that it can actually simplify your post production workflow as ACES takes care of an necessary conversions to and from different colour spaces and gammas. Without ACES when working with raw or log footage you will often need to use LUT’s to convert your footage to the right output standard. Where you place these LUT’s in your workflow path can have a big impact on your ability to grade your footage and the quality or your output. ACES takes care of most of this for you, so you don’t need to worry about making sure you are grading “under the LUT” etc.

ACES works on footage in Scene Referred Linear, so on import in to an ACES workflow conventional gamma or log footage is either converted on the fly from Log or Gamma to Linear by the IDT (Input Device Transform) or you use something like Sony’s Raw Viewer to pre convert the footage to ACES EXR. If the camera shoots linear raw, as can the F5/F55 then there is still an IDT to go from Sony’s variation of scene referenced linear to the ACES variation, but this is a far simpler conversion with fewer losses or image degradation as a result.

The IDT is a type of LUT that converts from the camera’s own recording space to ACES Linear space. The camera manufacturer has to provide detailed information about the way it records so that the IDT can be created. Normally it is the camera manufacturer that creates the IDT, but anyone with access to the camera manufacturers colour science or matrix/gamma tables can create an IDT. In theory, after converting to ACES, all cameras should look very similar and the same grades and effects can be applied to any camera or gamma and the same end result achieved. However variations between colour filters, dynamic range etc will mean that there will still be individual characteristics to each camera, but any such variation is minimised by using ACES.

“Scene Referred” means linear light as per the actual light coming from the scene. No gamma, no color shifts, no nice looks or anything else. Think of it as an actual measurment of the true light coming from the scene. By converting any camera/gamma/gamut to this we should be making them as close as possible as now the pictures should be a true to life linear representation of the scene as it really is. The F5/F55/F65 when shooting raw are already scene referred linear, so they are particularly well suited to an ACES workflow.

Most conventional cameras are “Display Referenced” where the recordings or output are tailored through the use of gamma curves and looks etc so that they look nice on a monitor that complies to a particular standard, for example 709. To some degree a display referenced camera cares less about what the light from the scene is like and more about what the picture looks like on output, perhaps adding a pleasing warm feel or boosting contrast. These “enhancements” to the image can sometimes make grading harder as you may need to remove them or bypass them. The ACES IDT takes care of this by normalising the pictures and converting to the ACES linear standard.

After application of an IDT and conversion to ACES, different gamma curves such as Sony’s SLog2 and SLog3 will behave almost exactly the same. But there will still be differences in the data spread due to the different curves used in the camera and due to differences in the recording Gamut etc. Despite this the same grade or corrections would be used on any type of gamma/gamut and very, very similar end results achieved. (According to Sony’s white paper, SGamut3 should work better in ACES than SGamut. In general though the same grades should work more or less the same whether the original is Slog2 or Slog3).

In an ACES workflow the grade is performed in Linear space, so exposure shifts etc are much easier to do. You can still use LUT’s to apply a common “Look” to a project, but you don’t need a LUT within ACES for the grade as ACES takes care of the output transformation from the Linear, scene referenced grading domain to your chosen display referenced output domain. The output process is a two stage conversion. First from ACES linear to the RRT or Reference Rendering Transform. This is a very computationally complex transformation that goes from Linear to a “film like” intermediate stage with very large range in excess of most final output ranges. The idea being that the RRT is a fixed and well defined standard and all the complicated maths is done getting to the RRT. From the RRT you then add a LUT called the ODT or Output Device Transform to convert to your final chosen output type. So Rec709 for TV, DCI-XYZ for cinema DCP etc. This means you just do one grading pass and then just select the type of output look you need for different types of master.

Very often to simplify things the RRT and ODT are rolled into a single process/LUT so you may never see the RRT stage.

This all sounds very complicated and complex and to a degree what’s going on under the hood of your software is quite sophisticated. But for the colourist it’s often just as simple as choosing ACES as your grading mode and then just selecting your desired output standard, 709, DCI-P3 etc. The software then applies all the necessary LUT’s and transforms in all the right places so you don’t need to worry about them. It also means you can use exactly the same workflow for any camera that has an ACES IDT, you don’t need different LUT’s or Looks for different cameras. I recommend that you give ACES a try.

4 thoughts on “ACES: Try it, it might make your life simpler!”

  1. I’m really interested to use ACES with Sony F55 Slog3 Footage in a Davinci Resolve Workflow. But at the moment theres only an IDT for slog2 in Resolve. Is there a way to import IDTs into Resolve for example if Sony releases an IDT for Slog3? And is there a big difference between choosing the Alexa IDT since LogC and Slog3 are very similiar gamma curves?

  2. Alister: thank you so much for taking all the time you do to explain things. I am confused about how to use the ACES setting in Catylyst Browse when I am working with my FS7 footage. How would I use it? What would be the process?

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