Background

ACES is a framework for motion picture color management, and is the result of a 12-year development effort led by the Academy of Motion Picture Arts and Sciences’ Science and Technology Council. The Academy recruited volunteers from academia, from film and digital camera vendors, from film manufacturers, and from post-production facilities. Digital camera companies vigorously competing with each other in the marketplace (ARRI, SONY and Canon) cooperated wholeheartedly in ACES projects; similarly, Kodak and FUJIFILM worked together to ensure ACES would support photochemical workflows as well as it would purely digital ones.


Key ideas

There are several key concepts in ACES (and modern color management in general) that were novel to cinema professionals when they were first introduced in the late 2000s. They remain key in understanding ACES, so we review them here, and then use them frequently in the other tabs of these ACES pages.
Image state is a property of an image that describes whether its primary purpose is the representation of the scene being captured, or the depiction of the scene to some viewer. In a way, it describes the purpose of the image.

Scene colorimetry describes the luminances and hues of the real-world or synthetic objects that were photographed with a real or virtual camera. Display colorimetry describes the luminances and hues produced by the device that presents the reproduction of that scene to the viewer.

It may help to think of traditional photochemical filmmaking for a moment. The scene might be the set as lit by a cinematographer. The colors present on that set to a human observer are the scene colorimetry. The image state of the developed negative is (loosely speaking) said to be scene-referred. The image state of the film print is said to be display-referred or output-referred, and the image as it appears on the theater screen is presenting display colorimetry to the moviegoer.

In ACES there is considerable emphasis on assuring that the scene-referred image is as colorimetrically accurate as possible. If this were always done perfectly, then the ACES scene-referred images of a scene captured simultaneously by ARRI, SONY and Canon cameras would be identical. In practice, the images are not identical due to differences in dynamic range, in noise, and in the fine points of sensor design. That said, for that very large number of users who have tried to match the output of multiple types of cameras without having tools or theory to help them, ACES is something of a revelation — ACES brings such disparately-sourced images “within range” of each other, quickly, easily and consistently.

When the brightness of a display-referred image is much less than the brightness of the scene-referred image (think of the bright light on-set for beach scenes in a surfing movie, and then the relatively dim light of that movie when projected), just scaling the brightness of the colors doesn’t produce a very appealing image. To make the displayed image compelling, a rendering transform is used to convert scene colorimetry to display colorimetry.

This idea of rendering probably predates ACES by about five centuries; you can see a non-realistic but visually pleasing darkening of shadows and increase of highlight in Rembrandt’s The Night Watch (to take just one example).

But for cinematographers whose background was in classic, pre-digital electronic acquisition, where display-referred images were recorded, ACES’s providing for an explicit rendering step was something new. Such cinematographers were accustomed to obtaining artistic effects not in a separate step, but by ‘painting the camera’ as a form of rendering.

ACES provides for a two-step explicit rendering process. The overall rendering of scene-referred to display-referred image is done with the viewing transform. The viewing transform is itself composed of two concatenated transforms: the Reference Rendering Transform, often abbreviated RRT, and an Output Display Transform (ODT) that is specific to the particular imaging device. The RRT is responsible for imparting those aesthetically pleasing attributes such as a film-like “toe” and “shoulder” for an imaginary, idealized output device without gamut or luminance limitation; the ODT adapts such idealized images for a particular type of display, such as the SMPTE Standard Projector, or a professional reference monitor such as those used on-set.

ACES was originally known as the Image Interchange Framework, and to that end, a special form of OpenEXR file was designed to hold ACES files. This constrained form of OpenEXR is known as the ACES Container File and is specified in SMPTE ST 2065-4:2013. ACES container files can be produced very early, e.g. at the time a Codex Capture Drive is downloaded from an ALEXA XT; they can also be produced considerably later by transcoding the image data and metadata from ARRIRAW files, Log C ProRes-containing QuickTime or Log C DNxHD MXF clips and ARRIRAW-containing MXF clips. Note that all the traditional metadata available to ARRI camera users, including dynamic lens metadata, is likewise available in ARRI-generated OpenEXR files. 

A second container is defined by ACES for holding densitometric data from film scans. Just as ACES defines a constrained form of OpenEXR for holding colorimetric data, ACES also defines a constrained form of DPX for holding densitometric data, known as ADX. 

ARRI’s ARRISCAN film scanners can be configured to produce these specially-marked ACES-compatible ADX-containing DPX files via an augmented scanner calibration process. Digital Intermediate facilities or their clients who wish such a calibration should contact ARRI's service group. 

No special provision for ACES need be taken during film exposure, and no special provision be taken at the lab. Note that special film lab processing (such as negative bleach bypass) should be avoided, as it can make it nearly impossible for the ACES ADX-to-ACES transform to convert densitometry to scene colorimetry.


Strengths of ACES

ACES has a strong theoretical foundation, with years of camera, film and display manufacturer expertise contributed to the project, as well as the considered judgements of many of the “golden eyes” of Hollywood.

ACES is very well documented, with at least a half-dozen ACES-specific SMPTE  standards, and perhaps twice that many white papers on ACES with the Academy imprint.

ACES is vendor-agnostic, and does not preferentially render for any particular vendor’s camera. (It should be noted that though not all significant camera vendors participated in the ACES design efforts, almost every one contributed test images for “golden eyes” evaluation.)