Academy Color Encoding System


The Academy Color Encoding System is a color image encoding system created by hundreds of industry professionals under the auspices of the Academy of Motion Picture Arts and Sciences. ACES allows for a fully encompassing color accurate workflow, with "seamless interchange of high quality motion picture images regardless of source".
The system defines its own color primaries that completely encompass the visible spectral locus as defined by the CIE xyY specification. The white point is approximate to the CIE D60 standard illuminant, and ACES compliant files are encoded in 16-bit half-floats, thus allowing ACES OpenEXR files to encode 30 stops of scene information. ACES supports both high dynamic range and wide color gamut.
The version 1.0 release occurred in December 2014, and has been implemented by multiple vendors, and used on multiple motion pictures and television shows. ACES received a Primetime Engineering Emmy Award in 2012. The system is standardized in part by the Society of Motion Picture and Television Engineers standards body.
Hundreds of productions, from films to television series to commercials, and VR content has been produced using ACES, including The Lego Movie, The Lego Batman Movie, Guardians of the Galaxy Vol. 2, ', The Grand Tour, Café Society, Bad Santa 2, The Legend of Tarzan, Chef's Table, Chappie, The Wedding Ringer, ' and The Wave.

Background

The ACES project began its development in 2004 in collaboration with 50 industry technologists. The project began due to the recent incursion of digital technologies into the motion picture industry. The traditional motion picture workflow had been based on film negatives, and with the digital transition, scanning of negatives and digital camera acquisition. The industry lacked a color management scheme for diverse sources coming from a variety of digital motion picture cameras and film. The ACES system is designed to control the complexity inherent in managing a multitude of file formats, image encoding, metadata transfer, color reproduction, and image interchanges that are present in the current motion picture workflow.

System overview

The system comprises several components which are designed to work together to create a uniform workflow:
ACES 1.0 defines a total of five color spaces covering the whole ACES framework as pertains generation, transport, processing, and storing of still and moving images. These color spaces all have a few common characteristics:
  1. They are based on the RGB color-additive model.
  2. Their codevalues are scene-referred, i.e. the numerical values represent some form of color-neutral numerical encoding of light as it is emitted and reflected by real scene objects. As a consequence of this: there is no theoretical upper bound to the codevalues ; the all-zero codevalue triple corresponds to the optical absence of light, though negative codevalues are possible as they correspond to tristimuli outside of the gamut primaries. Usually, scene-referred codevalues captured by a camera relate are directly related to luminous exposure via the same transfer characteristics.
  3. The reference illuminant is chosen to be close to CIE D60 standard illuminant, with chromaticities.
The five color spaces use RGB color primaries from an alternative of two sets called AP0 and AP1 respectively ; more specifically their chromaticity coordinates follow the table below.
primariesAP0 RedAP0 GreenAP0 BlueAP1 RedAP1 GreenAP1 Blue

AP0 is defined as the smallest set of primaries that encloses the whole CIE 1964 standard-observer spectral locus; thus theoretically including, and exceeding, all the color stimuli that can be seen by the average human eye. The concept of using non-realizable or imaginary primaries is not new, and is often employed with color systems that wish to render a larger portion of the visible spectral locus. The ProPhoto RGB and the ARRI Wide Gamut are two such color spaces. Values outside the spectral locus are maintained with the assumption that they will later be manipulated through color timing or in other cases of image interchange to eventually lie within the locus. This results in color values not being “clipped” or “crushed” as a result of post-production manipulation.
AP1 is instead contained well within the CIE standard observer's chromaticity diagram, yet still being considered “wide gamut”. It is conceived with primaries “bent” to be closer to those of display-referred color spaces for mainly two reasons:
This is the standard ACES color space; the only one based on AP0 RGB primaries and the only one intended, by design, for mid- and long-term storing into image/video files. It uses a photometrically linear transfer characteristics (i.e. more simply and improperly said to have
ACES2065-1 codevalues are linear values scaled in an Input Transform so that:
ACES2065-1 codevalues often exceed for ordinary scenes, and a very high range of speculars and highlights can be maintained in the encoding.
The internal processing and storage of ACES2065-1 codevalues must be in floating-point arithmetics with at least 16 bits per channel.
Pre-release versions of ACES, i.e. those prior to 1.0, defined ACES2065-1 as the only color space. Legacy applications might therefore refer to ACES2065-1 when referring to “the ACES color space”. Furthermore, because of its importance and linear characteristics, and being the one based on AP0 primaries, it is also improperly referred to as either “Linear ACES”, “ACES.lin”, “SMPTE2065-1” or even “the AP0 color space”.
Standards are defined for storing images in the ACES2065-1 color space, particularly on the metadata side of things, so that applications honoring ACES framework can acknowledge the color space encoding from the metadata rather than inferring it from other things. For example:

ACEScc

Converting ACES2065-1 RGB values to CIE ''XYZ'' values

Converting CIE ''XYZ'' values to ACES2065-1 values

Standards

ACES is defined by several Standards by SMPTE and documentations by AMPAS, which include:
A SMPTE standard is also under development to allow ACES codestreams to be mapped to the Material Exchange Format container.