ACES FAQ

Yes. It’s a bit more contrasty, which can bring out more detail.

In the ARRI color processing architecture, the basic system tonal response comes from a sigmoidal curve influenced by camera 'black gamma', 'gamma' and 'knee' controls. In the ACES color processing architecture, the basic system tonal response comes from the Reference Rendering Transform [RRT]. 

The tonal response curves are somewhat different, because of a philosophical difference between the systems' authors: ARRI delivers by default a flatter image in midtones and highlights, as shown in the graph (see image 1).

In the image 2 below, a cropped region of an ALEXA-captured frame is shown rendered with the traditional ALEXA rendering on the left and an ACES rendering on the right. The ACES rendering shows increased contrast at the edges of the paving stones, and the separation of the curb from the sidewalk is much more evident. The contrast between lit and shaded blades of grass is also increased in the ACES rendering.

The lower-contrast ARRI rendering is by design, not by accident: most ARRI camera users will be doing a later grade, and the ARRI rendering transform emphasizes showing latitude to a cinematographer over showing a near-final image. In another crop from an ALEXA-captured frame (see image 3), a close-up of a rim-lit sweater sleeve shows that the ACES viewing transform renders the fabric "higher" on the tone reproduction curve. 

This can be seen more clearly in an exposure wedge (see image 4), where it is evident all the image detail shown in the ARRI rendering is still there in the ACES image and can be revealed with a simple color grade that moves that content down the exposure curve. Once again the ARRI rendering prioritizes the cinematographer’s decisions about capture – "Am I done shooting this?" over decisions about finishing – "Is this the look I want to deliver?" 

They are different. The ACES skin tone rendering, when applied to ARRI camera output, tends to be "tawny", that is to say, for ARRI camera output ACES will render skin a bit more yellow. Whether this is a welcome or unwelcome change will depend on the subject and the context. 

In the crop from a winter scene below, the ARRI rendering on the left is both less contrasty and less saturated. In the ACES rendering on the right, contrast and saturation are increased, and the skin tone shifts to be slightly more tawny. The right rendering looks, plausibly, like a colder day than the left, bringing out more color to the cheeks. See image 5.

In some cases the ACES rendering will render a "cold" subject somewhat warmer, as seen below. See image 6.

As one would expect, if someone has a naturally florid complexion, or is sunburned, such that their ruddiness in the ARRI rendering is at the limit of what a cinematographer would desire, then the ACES rendering could require some "dialing back" in the grade. This can be true for both the skin itself, and the highlights on the skin. In the tungsten-lit scene below, the ACES rendering shifts the skin highlights so that they are more yellow. See image 7.

To sum up: the ARRI rendering and the ACES rendering process skin tones slightly differently, and which is preferable will depend on the cinematographer’s taste for contrast and saturation in their subject’s default rendering.

Very similar on the front end; mostly similar on the back end. Both systems are flexible but achieve that flexibility differently.

The diagram below compares the high-level ARRI and ACES processing pipelines. Starting from a common buffer of image data, each pipeline transforms the raw data into its own encoding space – ALEXA Wide-Gamut for the former, ACES for the latter – then applies any creatively-determined color adjustment, and then transforms the possibly-adjusted image for viewing on some specific display device (e.g. a monitor or projector). See image 8.

In the ARRI pipeline, systematic changes to the look of the image, such as a sepia-tone look for older material, are blended in to the 3D LUT that implements the default "look". The ACES pipeline explicitly separates creative color grading from more constant and uniform-across-the-frame color grading, by formalizing the latter as Look Modification Transforms (LMTs). Nevertheless, it is easy to see that the pipelines are very similar. 

The ACES pipeline in the lower part of the diagram is actually more general, in that in theory there is no limit to the number of LMTs that could be inserted into the pipeline. This contrasts with the ARRI pipeline in the upper part of the diagram, which is less flexible by design – since it has to run in real-time inside the camera.

Productions that are willing to take the time to test ARRI’s native workflow with content appropriate for their needs, and who can similarly test ACES-based workflows with that same content. Since ARRI does not record native ACES container files (OpenEXR files with ACES imagery and metadata) in their cameras, an initial test can be done without having to shoot the content twice. 

ACES is the best vendor-agnostic solution to the problem of how to mix the output of sometimes very different cameras. Productions trying to do this without ACES are either in for an enormous amount of trial-and-error color matching, or an ambitious exercise in reverse engineering, some of which may be proscribed by the camera manufacturer.

ACES may also be the best solution if production is dealing with VFX vendors who don’t have staff that understand concepts such as scene-referred compositing and consistent color spaces for camera output. While it is true that all the large VFX vendors have understood these ideas for years if not decades, it is common for ‘garage shop’ small vendors to be at a loss when confronted with challenging composites mixing the output of disparate devices. ACES can help these facilities ‘punch above their weight’.

This question has two answers.

A1: Chromatic abberation in some wide-open lenses can produce very saturated blue or purple fringes around strong light sources adjacent to very dark areas, such as the headlights of the cars in the nighttime scene below. When these colors are mapped into the ACES color space, they can fall outside the gamut of colors that the ACES rendering transform is designed to handle. The resulting color clipping manifests itself as an unpleasant artifact. 

The ARRI rendering (again, the ARRI crop is on the left) was specifically designed to be more forgiving in this circumstance, in that it provides a "soft falloff" to the color boundary; the ACES rendering on the right has a very sharp edge to the purple region surrounding the bright white of the headlight. See image 9.

Going from about a 5X blowup to a 15X blowup, in extreme close-up the difference is clear. See image 10.

It should be noted that, in a subsequent take with the lens closed down one stop, the purple fringing was much reduced, and the problem virtually eliminated. The point here is not to show the better handling of the edge case in the ARRI rendering, but rather that to demonstrate the edge case so that it may easily be avoided. 

A2: ACES isn’t a ‘universal cure’. The difficult problems that come with near-monochromatic stimuli, mixed lighting, etc., are still present in ACES; there is no ‘free lunch’. If you’re aware of this, and avoid the ‘gotcha’ of being buzzword-driven rather than needs-driven, then do your standard pre-production tests, use our documentation, and you will get predictable, solid results.

Yes. ARRI provides a downloadable zip file containing a sample ARRIRAW file and derivative images, including a Log C DPX frame, an ACES OpenEXR frame and display-ready frames. These can be used to verify that your ACES pipeline is producing results that match ARRI’s reference results.

ACES is an alternative workflow to ARRI’s standard workflow. The standard tools almost all support ACES now. We think that if you do choose after testing to go with an ACES workflow, that ARRI provides the best and most comprehensive ACES support of any camera vendor.