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ACES EDUCATIONAL PIECE
What is ACES?
ACES stands for Academy Color Encoding Science. ACES is a free, open, device independent colour management and image interchange system developed by scientists, engineers and end users under the Academy of Motion Picture Arts and Sciences. To simplify, ACES is a combination of functions that work together to provide an effective colour management solution.
How does ACES work?
ACES is based on the use of IDTs, RRTs and ODTs.
IDTs (Input Device Transforms) are camera specific transforms created by the camera manufacturer that translate the image from its original colour space to the ACES colour space.
RRTs (Reference Rendering Transform) is a transform controlled by the ACES committee and is intended to be the universal standard for transforming images from their scene referred values to high dynamic range output referred values. The RRT is where the images are rendered but not optimised for any one output format.
ODTs (Output Device Transforms) are transforms that maps the high dynamic range output of the RRT to display referred values for specific colour spaces and display devices. Like IDTs each ODT is specifically designed for each target i.e. Rec 709, Rec 2020 or P3. The ACES RRT and ODT work together like a display LUT or output LUT.
What are the benefits of ACES?
ACES simplifies the interchange of imagery by using a standardised colour space. By having every application use the same regulated specification, it eliminates uncertainty between on set looks and downstream colour correction. As an example, if you were handing off a project or working on a round trip where you were sending files from the colourist to your VFX team and back, they would traditionally need to use a variety of LUTs to correctly be able to visualise how the final image would look. If the colourist and VFX team are all working in ACES, they can simply apply the same IDT.
ACES also preserves the full range of highlights, shadows and colours captured in the image throughout post-production so there’s no danger of clipping any detail! It can also simplify matching images from different cameras by converting them all into ACES and then out to your desired deliverable space.
Another advantage of ACES is that it is a fantastic way to archive files and projects. As ACES is standardised, you can reopen your project and not have to scramble to find the LUT or transform that was used for that specific project.
ACEScc and ACEScct
ACEScc applies a standard Cineon-style log encoding to the ACES data before it is processed. This common encoding makes it possible for ASC CDL values to be used across systems using the same ACEScc encoding. After processing a reverse encoding is applied in order to output ACES linear data.
ACEScct is a variation of ACEScc that adds a roll off to the toe of the image that differs from the encoding of ACEScc. This makes lift operations “feel” more like they do with film scans and LogC encoded images. This makes it easier to raise the darkest values and get milky shadows. After processing a reverse encoding is applied in order to output ACES linear data.