Monday, May 29, 2023

ACES workflow in DaVinci Resolve

This is the second of four tutorials on optimising DaVinci Resolve for Panasonic Lumix footage. In the first article I conducted a practical test of Lumix S5 footage and stabilisation. This article will explain how to set up an industry-standard ACES workflow in Resolve. The third post will explain the film look and provide specific settings for Lumix cameras. The final article develops a Film Look PowerGrade that you can use in your Resolve projects.

The ACES model

How do we ensure consistent colour from ingesting clips to delivering finished output? It's certainly not easy, considering the huge variety of cameras, displays, and other equipment you may need to work with. Thankfully the folks who give out the Oscars, the Academy of Motion Picture Arts and Sciences, started on a project to solve this way back in 2004. They established the Academy Color Encoding System (ACES) to handle the proliferation of digital standards.

ACES uses an RGB color model with a huge colour space and over 30 stops of exposure. This model can represent visual perception in its entirety. This image data is "scene-referred", which means that the data refers to the lighting and colours as they actually existed in front of the camera. To do this, an idealised "standard camera" is referenced. When you bring footage into an ACES workflow, the profile of the camera you used is mapped onto this reference. This mapping can happen automatically if the camera footage is tagged appropriately. 

In fact ACES offers several different colour spaces. ACEScc applies a Cineon-style log encoding, which apparently helps how colour wheels and other controls respond to the footage. Apparently it's a very "film-like" approach, that makes experienced operators comfortable. I wouldn't know, having never edited film.

ACEScct goes further by adding a roll-off at the toe of the image to improve shadows. This colour space is nonetheless larger than the ITU Rec.2020 colour space, so we lose nothing to get nicer behaviour from our footage.

Setting up ACES in Resolve

The setup is easy once you know how, though I had to read several chapters in the enormous Resolve manual to figure this out. But that's why you are here reading this article!

When starting a new project, go to the "Project Settings" and click the "Color Management" tab. Set up your "Color Space & Transforms" as follows.

We use the ACEScct colour science and the latest version 1.3. The Input Transform should be chosen to match the footage you most often expect. I am using a Lumix S5, so I set this to "Panasonic Varicam VLog VGamut". In fact, these gamma and gamut mappings are the same for all Panasonic cameras.

Since I am producing output for screen, I choose "sRGB" for the output transform. If instead you are producing output for standard broadcast, you will likely choose "Rec.709". But do whatever you are told by your distributor. 

Now we can specify the space we'll be using as an intermediate between input and output, which is to say the space in which our processing takes place. Confusingly, there is no "ACEScct" so instead choose "ACEScc". (The terminology is not always consistent in different places in Resolve. This should be fixed.)

That's all you need to change in the settings. But there is one check you should make when ingesting footage. In the Media workspace right-click on a clip and confirm the "ACES Input Transform". This should be automatically identified as Panasonic Varicam. But if not, you can change the transform manually. 

And that is all.

Now, when you work on colour grading there is no need for Colour Space Transforms or LUTs. The ACES workflow automatically transforms your clip from the input space to our intermediary work space, and then to the output space for delivery. Simple!

Of course if you want to use a LUT for aesthetic reasons, go ahead. It's best to use a LUT designed for the ACES space, but this is not strictly necessary.

About gamut and gamma

It's important to realise that for every transform, there are actually two characteristics that are being mapped. The first is gamut, the range of colour available in the colour space. The second is the gamma, which specifies a luminance (brightness) curve.

The broadcast standard Rec. 709 has a gamma of 2.4, which specifies a logarithmic curve that allows for greater resolution in the most important range. To accommodate this, the midpoint of the luminance curve is shifted to 18% gray. But computers with sRGB colour space have a gamma of 2.2, which was design for the brighter viewing conditions of an office space.

Output designed for one gamma will not look the same on a device set to a different gamma.

Alternative workflows

ACES is not the only possible workflow system. Blackmagic offer their own methods.

The first of these was "DaVinci YRGB" which is "Display Referred". This means that accuracy can only be judged using a calibrated display; Resolve has no absolute knowledge of the colour. This mode requires hat you manage all transforms from one colour space to another manually.

The second system is "DaVinci YRGB Color Managed" which, like ACES, is "Scene Referred" and requires matching ingested media to a known colour profile, after which Resolve manages the workflow. Unfortunately I had some difficulties ensuring consistency in this mode. While this is likely due to my lack of understanding, the ACES method presented no such impediments.

Thanks to CharlesH on the L-mount Forum for encouraging me in this direction.


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