ACES & Associated Workflows

What is ACES, what workflow benefits can it provide, and what alternatives are there?

Before beginning any ACES based workflows it is worth reviewing what ACES actually is, and what it is/is not suitable for when performing colour orientated workflows.


ACES Workflows

ACES

The first point to be made is that compared to other workflows ACES will not improve the final image quality, or enable improved/better colours, or provided any other image related benefit. It is not a 'magic bullet' that somehow guarantees better end results.

ACES was initially designed and intended to be an 'exchange' format to enable simpler workflow interaction between different VFX houses, originally called IIF (Image Interchange Framework) with ACES just defining the encoding specification. It was not, and never has been, something that can improve the quality of an image from capture through to delivery, especially if the project is being performed within a single post facility, as is often the case.

The best final image quality is always generated from the workflow that involves the least number of image transforms, and performs the creative work at the best possible effective bit-depth.

Scene Referred - Display Referred

One of the fundamental approaches that needs to be understood with respect to ACES, and other Open Colour Space workflows, is the difference between Scene Referred & Display Referred image workflows.

In simple terms, Display Referred means the images being manipulated are immediately transformed into the colour space of the displays being used to perform the image manipulations - Rec709, for example - which means restricting the image colour and dynamic range available during the creative manipulation process. This is how colour grading workflows have been performed for years, with the colourist grading the images to look correct on their calibrated display, so 'forcing' the images into the display's colour space, regardless of the colour space of the capturing camera's image output.

Traditionally, most 'video' cameras output an image that was immediately Display Referred, as the camera output was mapped to look 'correct' on a TV, without any creative involvement.

Digital Cinematography cameras took this to the next stage, and output an image that was not 'processed' into a given colour space, but output in a format designed to deliver the maximum capture range of the camera - colour and dynamic range. This 'Capture Referred', or 'Camera Referred' image could be processed into Display Referred by the application of a simple 3D LUT, often provided by the camera manufacturer, or simply 'graded' by a colourist while reviewing the image on a calibrated display, so again forcing the image into a 'Display Referred' space.

Scene Referred simply means the image data is maintained in a format that as closely as possible represents the original scene, without effective restriction on colour or dynamic range. This is not necessarily the same as the 'raw' image data as exported from the camera (after any necessary debayering, etc), but attempts to 'correct' the image to better match the scene the camera was originally pointing at, which may include white point correction, gamut correction, etc. Theses processes are often referred to as 'Scene Reconstruction' processes.

Scene Referred is also often in Linear Light, which while suitable for computer graphic rendering, is not suitable for grading workflows.

The process used to get images into Scene Referred space is to effectively 'undo' the 'Capture Referred', or 'Camera Referred' image, and reverse engineer it back into Scene Referred space. The theory being any camera pointed at the same scene would generate the 'same' image in 'Scene Referred' space, within the limitations of the capturing camera's imaging capabilities.

The problem is neither Display Referred, or Scene Referred, workflows really work well in the real world, and so inevitably compromises have to be made.

In reality, using a Display Referred workflow, with a suitable viewing LUT to maintain the timeline images in a colour space that is greater than the display colour space, can be a far better workflow, with far less complexity and issues to overcome, with less image manipulations being performed, and so potentially the best final image quality.

For example, a P3 HDR to Rec709 Viewing LUT would effectively push the images being worked on into a P3 HDR colour space, while image manipulations are performed viewing a standard Rec709 display.

It is important to realise that we have been working with High Dynamic Range (HDR) and Wide Gamut (or even Ultra Wide Gamut - UWG) images for a long time, as most Digital Cinematography cameras capture this way, as did Film Negative before it.

The desire to use ACES for any project needs to be fully reviewed and assessed before its adoption, as its use can often prove to be more complex and costly, with no direct discernable benefit, when compared to alternative workflows. ACES is best suited to large productions, that will involve a large number of facilities, with a large quantity of VFX shots.

ACES Workflow

The ACES workflow concept is based on the following sequence steps.

Source IDT Creative
Manipulation
RRT ODT Display
ARRI
Sony
Canon
Etc.
The Input Device Transform is Camera manufacturer defined, and maps the captured image into ACES The creative processes, such as grading The Reference Rendering Transform applies an ACES defined 'Look' to the image The Output Device Transform maps the image the display colour space, such as Rec709, etc The display being used for the creative work

Referring to the above workflow overview, the concept of ACES should be relatively simple to understand, as well as the possible limitations and issues.

  • The IDTs are intended to map the captured image into a unified colour space (ACES), such that any camera pointed at the same scene generates what is ostensibly the same result.
  • The first issue with the above concept is that few DoPs want all cameras to produce the same 'look', as the look is a key part of any creative cinematography.
  • Further, all IDTs are camera manufacturer specific, but few camera manufacturers actually provide them, and those that are available are not guaranteed accurate.
    (Totally accurate IDTs would provide full information on the camera's underlying image capture and optics capabilities - which manufacturers are obviously not keen to reveal)
  • The RRT contains an ACES defined 'Look', intended to in some way replicate the traditional look and feel of 'film'.
    (The v1 release of ACES has reduced the excessive 'film' look of ACES v0.1, but none the less, a 'look' is still contained with the RRT)
  • This predetermined look actually goes against the desires for most production workflows, where the 'Look' is a creative decision made without imposed restriction.
  • Ideally, the RRT should not exist, or should at least be neutral, imparting no predetermined 'Look' onto the image.
  • The ODTs map the image data from the output of the RRT into the display's expected colour space, so that the image can be viewed within a known and expected colour space, enabling creative work to be performed as desired.
  • Ideally, the ODT should take images directly from ACES space, with no RRT involvement.

As can be seen, there are a number of obvious potential issues and limitations, including a vast array of formulas and conversions within each ACES CTL, which has the result of isolating the creative process from the image being worked on.

A further confusion with ACES is the actual colour space itself.

  • ACES contains negative values, which makes no realistic sense in any colour based workflow.
  • ACES is a Linear Light colour space, which is not suitable for colour grading work.
  • ACEScc is a different 'ACES' colour space, and is both Log based, as well as having different colour primaries to ACES.
  • ACEScg is a another different 'ACES' colour space, which is Linear Light based, but has different colour primaries to ACES.
  • ACESproxy is another different colour space, which is Log based using integer values, and different colour primaries to ACES.

The result of the above potential issues and restriction with ACES has been to prompt some creative equipment manufactures to enable their own 'Open Colour Space' workflows, which potentially offer cleaner and simpler capabilities.

Alternatives to ACES Workflows

Due in part to the potential complex nature of ACES many creative equipment manufacturers, and others, have developed alternative open colour image workflows which potentially offer cleaner and simpler workflows capabilities for projects where ACES is not really suited or needed.

Manufacturer developed open colour workflows include FilmLight's 'Truelight Colour Spaces', BMD Resolve's 'RCM', and SGO Mistika's 'UniColour', which tend provide a simpler, more predictable, but inherently very powerful image path workflows.

The simplest workflow by far is to use a Display Referred workflow, with a suitable viewing LUT to maintain the timeline images in a colour space that is greater than the display colour space. Such simple workflows can often be far easier, with far less complexity and issues to overcome, with less image manipulations being performed, and so potentially the best possible final image quality.

Image Source Working
Colour Space
Viewing LUT Grading Display
ARRI
Sony
Canon
Etc.
The creative processes, such as grading, performed within a wide gamut and high(er) dynamic range colour space A Display LUT maps the image from its working colour space and dynamic range to the display colour space, such as Rec709, etc The display being used to view the creative work
Camera Capture Working Colour Space Viewing LUT Working Colour Space

If a decent 'colour chart' is used for each shot captured, with the same 'lighting exposure' used every time, performing an input grade to 'normalise' the colour chart will also perform the role of an IDT, pushing each shot into a user defined 'Grading' space.

Capture Match
Sources
Working
Colour Space
Viewing LUT Display
ARRI
Sony
Canon
Etc.
Match via IDT, Chip Chart, etc. Using a Viewing LUT 'pushes' the matched shots into a user defined Colour Space A Viewing LUT maps the image from its working colour space and dynamic range to the display colour space The image displayed correctly in the Display Colour Space via the Viewing LUT
Camera Capture Camera Capture Working Colour Space Viewing LUT Working Colour Space

Obviously, if IDT's exist for the cameras used, they can simply be added to the workflow, but without any additional ACES CTLs, making the 'Grading Space' ACES, but without the use of any other ACES components - such as the RRT/ODT, as the Viewing LUT replaces their use.

A further potential step is to use a User Defined LUT or CTL to move the input ACES colour space, as defined by the IDT, into a more user friendly open colour space, that uses a smaller gamut and dynamic range, so making better use of the available 'bits'.

Capture Match
Sources
Colour Space
Conversion
Working
Colour Space
Viewing LUT Display
ARRI
Sony
Canon
Etc.
Match via IDT, Chip Chart, etc. Input LUT maps from the input (ACES?) colour space to the working colour space User defined Colour Space, better suited to the desired workflow A Viewing LUT maps the image from its working colour space and dynamic range to the display colour space The image displayed correctly in the Display Colour Space via the Viewing LUT
Camera Capture Camera Capture Viewing LUT Working Colour Space Viewing LUT Working Colour Space

And with LightSpace CMS, the generation of suitable LUTs and/or CTLs is very simple.

If you do chose to uses an ACES based workflow, LightSpace CMS provides a selection of tools that enables the conversion of any .ctl (ACES based or otherwise) files (IDT/RRT/ODT, etc.) into LUTs, as well as the conversion of LUTs into .ctl files (IDT/RRT/ODT, etc.), enabling the simple generation of user designed Look workflows.

When combined with MatchLight IMS this even enables the automatic generation of user defined camera IDT files.

See the Working with ACES & LightSpace CMS website page for information of the ACES Tools available within LightSpace CMS.

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