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ACES or the Academy Color Encoding System is an open color management and interchange system developed by the Academy Of Motion Picture Arts & Sciences (AMPAS) and industry partners.
ACES standardizes the color science used in projects of all types and is designed to give the industry a standardized color management system that encompasses production, post-production, presentation, and archiving.
In 2004 when the ACES project started, the Academy Of Motion Picture Arts & Sciences (AMPAS) and many industry experts realized a need for an open color management system that was suitable to handle the complexity of modern digital production.
In an industry with dozens of different camera systems, multiple encoding options, different display devices, etc, the Academy saw a need for a system like ACES to manage color in a precise, yet straightforward way, no matter the camera or display being used.
Many software developers have tackled color management in their own applications, but none of those solutions are industry-wide and open. ACES is an industry-standard color management solution that anyone can use to manage the color pipeline of any project.
Since ACES 1.0 was released in 2014, ACES pipelines have been used on dozens of Hollywood blockbusters, TV series and independent films. The ACES pipeline is available today in many applications including DaVinci Resolve, Filmlight’s Baselight, Nucoda, and many others.
One of the most important things to understand about ACES is that its processing pipeline utilizes capture-referred data – i.e. the color science (and sometimes secret sauce) that each camera system uses and bakes into the signal. ACES reverse-engineers that data (through what’s known as an ACES Input Transform or IDT) back into the pure linear light information that was in the actual scene in front of the camera. Theoretically, without any camera bias.
This is why ACES is often discussed as being scene-referred or, with the more technical phrase, scene-linear. Abstracting away the camera’s bias allows us to get closer to the actual scene that the camera was pointing at.
Additionally, ACES color uses a color space so large it actually encompasses the entire visual locus (everything humans can see) and even light we can’t even see! And even the smaller working color spaces are also very large (the particulars of ACES color space(s) will be discussed in a later section).
ACES is a unifying standard – transforming captured referred data from many cameras into scene linear with a common starting point and shared color science.
ACES is also broad – in an ACES workflow you never run into the limitations of working in smaller color spaces. For client handoffs and archival purposes, the ACES color space is so large it is essentially ‘future proof’.
The scene linear approach of ACES is only part of the overall pipeline.
The human eye doesn’t work in a linear fashion and ultimately projects are viewed on TVs, projectors, etc., that assign gamma or EOTF (Electro Optical Transfer Function) curves that aren’t linear. These devices also have much more limited color spaces compared to ACES.
Because of this, ACES also incorporates display-referred color management as part of the overall pipeline. ACES data is parsed through different transforms for different color spaces and display devices.
When it comes to precision, ACES uses Open EXR 16-bit half-float processing which results in 30+ stops of scene referred exposure.
Keep in mind that even though EXR is used, often this processing is just internal to the app you’re using and no EXR files are created for you to manage – except in renders.
In sum, these are the benefits of ACES pipelines:
Even though ACES and its various transforms are mathematically complex, you can understand ACES better by understanding what each part or transform in the pipeline does.
Here’s the terminology for each of these transforms (after we define the transforms, in the next section, you’ll see how they work together in an ACES pipeline):
There are also three main subsets of ACES used for finishing workflows called ACEScc, ACEScct and ACEScg:
While ACEScc, ACEScct and ACEScg and the transforms they use are what you’ll most often see when it comes to ACES, there are some additional terms you may encounter:
Now that we’ve defined the transforms used for ACES, understanding how the various transforms combine to form an ACES processing pipeline is pretty straightforward:
Camera Data -> Input Transform -> Color Grading -> Look Transform (optional) -> Reference Rendering Transform -> Output Transform
As mentioned, ACES is hybrid color management system of scene referred/scene linear and display referred data.
In the graphic below, the various ACES transforms fit into the scene referred (top section) and display referred (bottom section) part of the pipeline:
When ACES is discussed, you’ll often hear some highly technical phrases to describe ACES: 2065-1, AP0, AP1, Rec.2020+.
What do these terms mean?
SMPTE ST 2065-1 is the SMPTE standardization of ACES.
While this standard has many parts, in daily usage ACES 2065 has come to mean the full linear version of ACES that has a larger gamut than the visual locus set of primaries (red, green & blue). ACES 2065 has a set of primaries known as AP0 (ACES Primaries 0).
A standard CIE1931 plot is a good way to compare ACES to other gamuts. The horseshoe of colors represents the human visual locus – all the colors we can see. The plot on the left shows ACES AP0 primaries compared to other gamuts, you can also see it’s larger than the entire visual locus. The plot on the right shows ACES AP1 primaries compared to other color gamuts. AP1 primaries are used in ACEScc, ACEScct & ACEScg
2065-1 AP0 is mainly meant for archival and file exchange. In real-world usage – grading, vfx etc, 2065-1 AP0 is not used. Instead, a smaller set of primaries called AP1 (ACES Primaries 1) are used. Also known as Rec2020+, AP1 primaries are slightly larger than Rec. 2020 which is championed as an ultra wide gamut for the future, and it’s the AP1 Primaries that are used in ACEScc, ACEScct, and ACEScg.
Currently, the official version of ACES is 1.1 but many applications are not supporting that version yet, although version 1.03 is widely supported. Unless there are major future changes and/or needs for end users as determined by AMPAS, you will continue to see the major release versions in software and probably not the minor .0x releases even though software manufacturers behind the scenes may implement the .0X releases for performance and bug fixes.
Projects created using older versions of ACES can be opened using a newer version of ACES that an application uses, but this may cause some differences in image representation compared to the original project. Some applications offer the ability to switch the version of ACES being used to an older one to match the ACES version that was used in the original project.
It’s also worth noting that ACES development is ‘open source’. AMPAS manages and curates the development of the system but if you’re mathematically/color science inclined, you can contribute to the development of the system or observe the development process by visiting the GitHUB repository the Academy maintains for ACES:
Also, as mentioned, ACES transforms are written in CTL or Color Transform Language. The Academy also maintains a GitHub repository for CTL:
If you’re less technically-inclined, you can also visit ACES Central. ACES central is a site that AMPAS maintains geared towards discussion of ACES and is frequented by end users and ACES certified partners.
Video collaboration solved.