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.
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.
ACES Workflows
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.
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 display 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. These 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
Continue ACES
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
In ACES workflow the LMT sits before the RRT, and applies a creative Look to the image path.
-
Source
ARRI, Sony, Canon, etc
-
IDT
The Input Device Transform is Camera manufacturer defined, and maps the captured image into ACES
-
Creative Manipulation
The creative processes, such as grading
-
RRT
The Reference Rendering Transform applies an ACES defined Look to the image
-
ODT
The Output Device Transform maps the image the display colour space, such as Rec709, etc
-
Display
The display being used for the creative work
Continue ACES
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.
It is the Viewing LUT that defines the working colour space, by setting a colour space transform from the LUT input to output.
If working on a single camera project, the input to the LUT can be the camera's native colour space, with the LUT output being the display's colour space - Rec709, or P3, etc.
If using multiple cameras, the Viewing LUT can be a defined working colour space, such as P3 Log, with the same output display colour space. In this way, any grading work will map the input images into the LUT's input colour space.
-
Image Source
ARRI, Sony, Canon, etc
-
Working Colour Space
The creative processes, such as grading, performed within a wide gamut and high(er) dynamic range colour space, such as the native camera space, or a defined space, such as P3 Log
-
Viewing LUT
A Display LUT maps the image from its working colour space and dynamic range to the display colour space, such as Rec709, P3, etc., for viewing purposes
-
Grading Display
The display being used to view the creative work
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.
-
Image Source
ARRI, Sony, Canon, etc
-
Match Sources
Match via IDT, Chip Chart, etc
-
Working Colour Space
The creative processes, such as grading, performed within a wide gamut and high(er) dynamic range colour space, such as the native camera space, or a defined space, such as P3 Log
-
Viewing LUT
A Display LUT maps the image from its working colour space and dynamic range to the display colour space, such as Rec709, P3, etc., for viewing purposes
-
Grading Display
The display being used to view the creative work
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.
-
Image Source
ARRI, Sony, Canon, etc
-
Match Sources
Match via IDT, Chip Chart, etc
-
Colour Space Conversion
Input LUT maps from the input (ACES?) colour space to the working colour space
-
Working Colour Space
The creative processes, such as grading, performed within a wide gamut and high(er) dynamic range colour space, such as the native camera space, or a defined space, such as P3 Log
-
Viewing LUT
A Display LUT maps the image from its working colour space and dynamic range to the display colour space, such as Rec709, P3, etc., for viewing purposes
-
Grading Display
The display being used to view the creative work
And with ColourSpace, the generation of suitable LUTs and/or CTLs is very simple.
Continue ACES
The above alternative workflows maintain the image in either the camera Log space, or a defined wide gamut/high dynamic range space, such as the camera's native colour space, or a defined colour space such as P3 Log, as defined by the input of the Viewing LUT. The output of the Viewing LUT just matches the workspace to the display's colour space, be that Rec709 or P3, etc.
If you do chose to uses an ACES based workflow, ColourSpace 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 ColourSpace ACES Tools website page for information on working with ACES within ColourSpace.
