Archive notice: This article was originally published on January 10, 2019. Links and embedded videos are preserved as part of the historical record.
One of the most advanced and latest developments in this area is the introduction of ACES. After more than ten years of industry-driven development, version 1.0 of the Academy Colour Encoding System (ACES) was released in December 2014. As a free and open standard, ACES offers a comprehensive colour management infrastructure. A key application of ACES is the production of visual effects. The addition of ACEScg encoding for compositing, lighting, rendering and the rest of the CG workflow simplifies the exchange of elements and enables a true colour preview process across all preview media. Recent major projects that relied on ACES colour management include “Black Panther”, “Avengers: Infinity War” and “Ant Man and The Wasp” – to name just a few examples. ACES promises a lot, namely photographic work processes for CG artists, an easier exchange of work files between departments and studios, e.g. for split scenes, future-proof archiving and more independent, flexible asset production as well as generally fewer problems in the colour workflow. In total, the ACES standard consists of six colour spaces, which in turn use two different sets of RGB primaries (maximum colour values that define the scope of the colour spectrum / gamut). The two colour spaces relevant to us are
ACES 2065-1
This is the ACES standard colour space. With its generously set AP0 primaries, it not only covers the entire visible colour range, but also areas that are not visible to the human eye, with the aim of being equipped for all future developments.
ACEScg
The ACEScg colour space was created when it became apparent that the enormous scope of the AP0 gamut was causing problems in the use of most render engines. The AP1 gamut is similar in scope to the usual high-end digital cameras, but is independent of these devices and appears to be more suitable for Physically Plausible Rendering.
The ACES flowchart provides the following sequence:
Input Transform
This process converts the image information from the existing format into the ACES colour space. The appropriate IT should be selected according to the source. The current OCIO Nuke Config contains default settings for the following camera manufacturers: ARRI, Canon, GoPro, Panasonic, RED and Sony.
Reference Rendering Transform
The RRT contains a look defined by ACES, which is intended to mimic the traditional appearance of film. ACES version 1 has reduced the excessive film look of ACES V0.1, but a look is still included in the RRT.
Output Transform
The ODTs map the image data in the expected colour space of the display (e.g. Rec. 709 or sRGB) in order to display the image correctly.
How does a current compositing pipeline work without ACES?
To be able to judge to what extent these developments are helpful, you have to take a closer look at the usual work processes. Three colour spaces play an important role in the VFX processing of feature films. These are projection or presentation colour spaces. This means that the entire colour pipeline is based on which output medium is used.
DCI-P3 is a common target colour space for film. It uses the same blue primary colour as Rec. 709, sRGB and the Adobe RGB colour space. However, the red and green primaries are significantly further out compared to sRGB and Rec. 709 and therefore offer a wider gamut. According to the manufacturer, high-quality work monitors can usually reproduce a maximum of 99% of the DCI-P3 standard.
The other colour space is Rec. 709 and sRGB. These two colour spaces share the same primaries and therefore have the same colour gamut. However, the gamma curve is different. While sRGB was developed for computer monitors, Rec. 709 originates from the HDTV standard.
Even as a newcomer, you learn that linear work processes have creative advantages. Optical effects such as light and shadow are calculated more realistically and colour adjustments in compositing, for example, do not have to be made against the implemented gamma curve.
In Nuke, this linear process can be mapped in three steps ..
The colour workflow in Nuke begins with the read node. An inverse 1D look-up table (LUT for short) can be used in this node, which converts the material from its original gamma into a linear representation. As an example: image material from the ARRI Alexa is usually encoded with an AlexaV3LogC colour space, which should be offset when importing into Nuke in order to obtain a linear image. 1D LUTs are not able to influence the gamut, so the colour gamut remains untouched. The image material is linearised from this step onwards and can be processed as desired. During the writing process, the original gamma value is usually reapplied in order to bring the supplied plate and the finished compositing into the same format.
Utility – sRGB – Texture korrekt interpretieren.” width=”1772″ height=”2682″ /> An sRGB image can be interpreted correctly using the Utility > Utility – sRGB – Texture setting, for example.The linear images are displayed with a Viewing LUT. There are presets for sRGB and Rec. 709 screens in Nuke. These presets are able to display values from 0 to 1. Higher values are available but are not displayed (clipped)
(clipped).
When working in film, however, it is common for highlights and pings, such as those that occur when looking into a direct light source, to be far above this maximum displayable value of 1. A customised viewing LUT usually helps here. The LUT offered by ARRI for Rec. 709 monitors, for example, only intervenes very discreetly in the saturations and colour distribution, but has a pleasant roll-off in the bright areas. Visualised as a point cloud, the finer gradations in the bright areas become visible:
When set up correctly, the optical effects work as expected. A controlled setup is most relevant for lighting effects such as pings and glows. These react better and more naturally in a linear image with accurate brightness information.
Film emulation LUTs allow these bright image areas to be visualised in the available viewer spectrum. This does not change the image information, but only how it is displayed. A 3D viewer LUT including creative colour correction adjustments is often supplied by the DI vendor to the VFX studio in order to reproduce the final grading in the VFX previews. Quite often these look stylisations cause problems and the artists have to work against the colour shifts of the LUT. For example: sometimes it is necessary to make colour corrections in a completely different area of the colour spectrum (magenta) in order to achieve the desired result (blue). In other cases, steppings or artefacts occur that interfere with the display of the images. These problems even affect large VFX studios, where dedicated colour specialists usually take care of the creation of these preview LUTs and colour workflows.
What an example workflow for ACES looks like in practice
With the current implementation of OCIO in Nuke, switching to an ACES workflow can be accomplished in just a few steps. The Nuke Project Settings show us the selection options: After the colour management has been set from Nuke standard to OCIO, the current ACES configuration (here aces_1.0.3) should be selected as OCIO Config. As a working space, it is advisable to remain in ACEScg.
What effects does this change have and what do you have to consider with an ACES colour pipeline?
The advantages in the compositing department are obvious: until now, it was usual to bring the various materials into the target colour space by grading and colour corrections. A manual process which, due to the infinitely different transformation possibilities in the colour spectrum, can only ever lead to an imperceptible approximation, but never to a truly accurate solution. ACES offers us a way of bringing these colour transformations under control.
If the colour management is set to OCIO and the OCIO Config is set to aces_1.O.3, the colour space settings of the read and write nodes are also automatically adjusted
In the 3D department, ACEScg offers a well-functioning render colour space that delivers stability even in extreme situations. Another advantage is the controllable preview options: With a standardised OCIO setup, there should be no visible difference between the 3D preview and the compositing preview. When implemented correctly, ACES also offers the possibility of introducing more photographic work processes. This makes the lighting and look development process more similar to the work of a cameraman.
The problem child of a colour workflow is usually the Photoshop used in the matte painting department. A common workflow is to export the images in a log encoding before processing in Photoshop. Without this processing step, values above 1 are clipped by Photoshop. ICC profiles are then used to flatten the image in order to offer the artist a preview option. Photoshop always seems to work most reliably when working with normalised, display-related data that can be managed by the ICC system.
The selection options are quickly explained. For material already transferred to ACES, the corresponding ACES colour space should be selected. For example, an export of the plates encoded to ACES 2O65-1 (VFX pull rendered by conforming or grading) should be transferred to the working space (ACEScg) with the corresponding colour space setting of the read node. Input transfers are now also available for most photo cameras, and the usual formats can also be imported without any problems using the input transfers provided
This leads to a possible workflow that is currently being tested by some studios: The original images are usually converted to Nuke via an output transform. Animal Logic, for example, has created a colour space based on the AP1 primaries and simultaneously assigns the images a synthetic ICC profile that describes this space. This makes it possible to work in Photoshop as usual, and the dynamic range of the material is now only regulated to a tolerable 16 stops. When exporting, an EXR is calculated, which is provided with an adjustment LUT.
For smaller studios that require a more flexible workflow in matte painting, there is also the option of using OCIO for Photoshop. However, this requires that each artist knows the colour workflow and works independently. Download here.
Creative side effects
The first ACES show will probably be an unfamiliar experience for every artist. The main reasons for this are: The RRT contains an S-curve or film look, which is always applied and deviates from the usual sRGB/Rec.-709/P3 display. In direct comparison to a conventional presentation, the result is darker, has stronger contrasts and thus brings out more detail.
The ACES white point is not the usual D65 (6,500 Kelvin), but D60 (6,000 Kelvin). Monitors and projectors need to be recalibrated accordingly. There was already a collective agreement in 2015 to switch to the D65 standard. However, it has not yet been implemented.
By using the IT and converting to the ACES colour space, the look of the camera is removed. In a direct comparison, differences are still visible – especially in the skin tones. Colourists say that although the similarity of the material makes it easier to match, creative design options are lost. The similarities in image aesthetics will redefine camera selection in the future.
