The Academy Color Encoding System (ACES) is an image interchange framework developed by the Science and Technology Council of the Academy of Motion Picture Arts and Sciences.
At the core of the ACES system is the Academy Color Encoding Specification (also known as ACES 2065-1). This is an extremely wide-gamut scene-linear space that can encode any visible color without using negative values.
The ACES system also includes a set of color transforms that can provide an innovative color managed workflow. In particular, the ACES system provides the first standard high-quality tone-mapping algorithm for converting scene-linear images to output-referred color spaces for viewing.
ACES in Autodesk Color Management
Autodesk Color Management presents components of the ACES framework in applications such as Maya, Flame, and Lustre.
| A | Autodesk Color Management applies an input transform to convert image sources to a working space, based on their specified input color spaces. Some applications also allow you to specify a custom chain of color transforms. |
| B | ACES 2065-1 is used as a connection space to convert to a suitable working space such as ACEScg, ACEScc, or ACEScct, or as a working space itself. |
| C | Output transforms can be used to render different output types, such as the digital cinema distribution master (DCDM) or HD video (Rec. 709). Depending on the Autodesk application, the output transform that is used for display might be specified as a single option, or as separate view and display transforms. |
The Details of the ACES Framework
The full details of the ACES framework include additional steps, which are combined in the streamlined version presented in application interfaces. The following diagram shows how these detailed pieces fit together, using historical acronyms from the development of the standard.
| A | Inputs from different sources may use different color spaces and encodings. |
| B | Input Device Transforms (IDTs) convert images to ACES. IDTs may be supplied by device vendors, software vendors such as Autodesk, other third parties, and AMPAS itself. In addition, you can author your own.
The ADX to ACES transform is known as the "universal unbuild". Its inverse is the "universal build". |
| C | The Academy Color Encoding Specification (ACES2065-1) is a scene-linear encoding and can be used as both a connection space and working space. The reference viewing environment is outdoor daylight with a white point of D60. |
| D | ACES images can be saved for exchange or archiving in a constrained, or limited, version of the OpenEXR file format with extra metadata standardized as SMPTE ST-2065-4. The only allowable channel layouts are stereo and non-stereo RGB and RGBA. |
| E | The Look Modification Transform (LMT) is optional. It may be inserted before the RRT to provide an aesthetic effect, such as a shot-specific color grade that needs to be shared throughout a workflow. It is typically established by the cinematographer on set. It can be baked in to the final output, or simply used for dailies and as a reference for the final color grade. |
| F | The reference rendering transform (RRT) applies a transformation as a first step in preparing the images for viewing. Among other things, the RRT:
There is only one RRT allowed in the ACES 1.0 standard. However, the Autodesk Color Management transform collection also includes beta versions of the RRT to support older projects that may be using them. |
| G | The Output Color Encoding Specification (OCES) represents the desired output colors if there were no dynamic range limitations. These values are the colors you would project in a cinema viewing environment if you had an idealized wide-gamut, high-dynamic range display device. |
| H | The reference device transform (RDT) converts images from OCES so that they are viewable on a SMPTE reference projector. The RDT is used as a reference for developing ODTs for other devices. |
| I | The output of the RDT forms the digital cinema distribution master. |
| J | Different output device transforms (ODTs) are applied to prepare the images for display on other devices. As with IDTs, the ODTs may be supplied by device vendors, software vendors such as Autodesk, other third parties, and AMPAS itself. |
The ACES2065-1 and ADX color encodings, and the constrained OpenEXR container specification are published in the 2065 family of SMPTE standards. Additional documents available from the Academy website describe the ACEScg, ACEScc, and ACEScct working spaces as well as other topics such as the Common LUT Format (CLF). The native color transform format of Autodesk Color Management (the CTF format) is a superset of the Common LUT Format.
ACES in On-set "Look" Workflows
When working on set, it is generally not possible to use floating-point values for monitoring. To solve this issue, the ACES system includes an integer encoding that can be transmitted over an HD-SDI link for on-set monitoring, called ACESproxy. This is a lower-quality ACES encoding that should never be stored, but only used for on-the-fly previews.
Also on set, it is becoming common for the cinematographer to establish a "look", or basic color grade, so that the production team can see an approximation of the intended final grade early in the process. It is typically baked into dailies and footage for editorial. This look also serves as an initial reference for the final grade during the digital intermediate (DI) process. Because the effect of the look transform depends on what color space it is applied in, it is important to specify the intended space as part of the workflow.
The ACES system includes a standard for applying the look on-set in ACESproxy space, and then applying it again in DI such that the original look is preserved. This is the intended workflow for using ASC CDL looks.
In Autodesk Color Management, most of the output transforms include a reference to the look transform (LMT). The transform currently set as the defaultLook alias in the Autodesk Color Management preferences is applied before the RRT. The look gets applied in ACEScc space as recommended by the ACES specifications.
The output transforms reference the look transform in such a way that the look is not applied for the final output, but can be activated for viewing. Some Autodesk applications, such as Flame and Smoke, allow you to toggle the look transform on and off interactively for display. This provides a way to quickly preview the on-set look in VFX, in editorial, and as a starting point for final color grading.
All internal processing is performed at 32-bit floating-point precision, so the conversion to ACEScc for application of a look transform (and then back to ACES) avoids the quantization and clipping imposed by the integer-based implementation of ACESproxy used on set.
If you need to bake in the look for output, you can use a custom color transform to apply the default_look-ACESproxy.ctf transform from the misc/ directory before the RRT+ODT transform. See Custom Color Transforms.