ACES 1.3—Fixing Those Color Management Issues

Since I launched my ACES Explained video series, ACES version 1.3 has come out, and it’s actually a much bigger deal than a simple point release. On top of this, ACES 2.0 is right around the corner. So whether you’re starting from zero or just brushing up, now is a good time to start paying more attention to ACES.

In this article, we’ll focus on two of the biggest changes in ACES 1.3 and will be building upon concepts from my ACES Explained series. So, if you want to go even deeper on ACES, be sure to check out that full three-part series.

But now, let’s dive into ACES 1.3.

Setting up color management via project settings

As always, let’s start with color management. In my original ACES series, we covered how to use the ACES Transform FX Node to provide input and output transforms on a clip-by-clip basis. Today, we’ll handle things a little bit differently by setting up global color management at the project settings level.

To start, navigate to File->Project Settings and select the Color Management tab. Here, we’ll select ACEScct for our color science and confirm we’re using ACES version 1.3. 

For the moment, turn off the checkbox for “Apply ACES Reference Gamut Compress.” We’ll talk about what this means later, but for now, we’ll leave it off.

Next, we’ll configure our input and output transform options. Anytime we’re using ACES, or indeed any color management framework, we’ll need to do some form of this. We need to tell the framework where we’re coming from and where we’re headed. 

For today, I’ll specify ARRI Log C3 as my input because that’s the color space of my sample footage, and I’ll use Rec 709 as my output because that is the monitor that I’m using for mastering. You’ll need to configure your own settings based on the footage and system you are working with, so choose what’s right for you.

Limitations of the ACES color space

With our ACES 1.3 color management framework now set up in our project settings, we see the magic of color management at work in the below sample image.

Resolve is displaying a normalized image in the viewer, an image that looks correct for our display. It may not represent any creative intent or feel particularly artful, but it is normalized for our display in a way that it was not a moment ago.

However, the first thing that pops up when reviewing this transform is some artifacting across this light beam element in the middle of the image. This is a problem. 

Where did this come from? Was it there a moment ago? If we used an FX node to do our color transform, we could simply disable that node and review that change. Thankfully, Resolve offers a similar workflow when our input transform is handled in our project settings. 

Right-click the clip thumbnail and select Bypass Color Management. This returns the clip to the original camera negative, where we can observe that this artifacting is actually not present in the source image. 

That’s not an artifact from the sensor or the capture; that’s an artifact from what we’re doing with our ACES color management.

Let’s look at another example below, where in the upper left corner, we see a bright cyan area that is getting this artifacting as well. 

Once again, this a great example of a color that’s perfectly within the camera gamut, but with color management enabled, that color is now displayed with an undesired result.

This is something that really plagued ACES all the way up until ACES 1.3, as a result of the following: some professional cinema cameras, such as Arri or RED, actually have a larger capture color space than the working color space that is specified out by the ACES standard. So there’s a mismatch where a color is perfectly valid inside of the camera color space but also falls outside of the color gamut for ACES. When that happens, the result is the clipping or artifacting we saw in our sample images.

In ACES 1.2 or earlier, you’d have to come up with some sort of hand-spun solution to deal with this issue on a clip-by-clip basis. There’s a better solution in ACES 1.3 that we’re going to look at in just a moment.

Working with ACES linear footage 

Let’s review our last sample image, shot number three. Shot number three has got big problems. Shot number three is all kinds of wrong, right? 

What’s happening here is a mismatch between the color space of this clip and the input transform of our color management settings. To correct this, we need to specify a different input color space than we used with our previous clips.

Before we do that, note that shot number one is actually from a RED camera, which is also a different color space from our input transform. However, that clip displayed correctly because it’s a .R3D file, RED’s raw file format. When Resolve encounters a raw file, it thinks, “Oh, you’re a raw file, I can unpack you directly into my working color space for whatever framework has been specified in the project settings.”

We do not need to change our input color space, and indeed we cannot change our input color space.

So in the case of raw files, such as .R3D, we do not need to change our input color space, and indeed we cannot change our input color space. This is one of the benefits of configuring color management at the project settings level.

It’s a different story here in shot number three which is not a .R3D file or some other raw format. This is an .EXR file, and so we need to tell Resolve that this is an ACES linear input, not CLog3.

This should be an easy fix, right? We just need to right-click on the clip thumbnail, go to the ACES Input Transform, and select the setting for ACES in the dropdown. But that’s when you discover that you can’t find an input space for ACES anymore. 

So, what are you supposed to do when you have an ACES linear source? This is a very common format to exchange and receive files in, especially with visual effects vendors.

When you want to specify a clip as ACES linear, you now need to right-click the clip and select ACES Input Transform->No Input Transform. This is one of the biggest changes in ACES 1.3, even though it’s just an interface change, and the net result is the same input transform that you had before when selecting ACES as your input.

Utilizing the ACES Reference Gamut Compress option

So we’ve properly mapped our third clip, but we’re once again getting some tearing in our image. Generally, you’ll notice these chroma clipping issues popping up in the right-hand quadrant of the vectorscope, primarily in the magentas and blues, but you’ll often see it in cyan colors as well.

Regardless of what color is clipping, it can be a real issue when you see it. However, what’s even worse is when you don’t spot such an issue, and then you might be sitting in the room with clients and notice it for the first time. That’s not ideal, right?

So let’s talk about what was introduced in ACES 1.3 to solve this on an automatic global basis as opposed to a shot-by-shot incidental basis. You may have guessed the answer, which is, of course, that checkbox we turned off initially. 

Return to File->Project Settings, select the Color Management tab, and check the box for Apply ACES reference gamut compress. With this new option enabled, notice that all the detail and color that was present in the camera negative has now been restored into our mapped image. We’re no longer getting that breaking or tearing.

Looking back at shot number two, the nasty artifacting has now been tamed, and we’ve got a reasonable reproduction of that high saturation, high luminance area of the image. 

We’ll see the same thing here on shot number one. The light tube that was clipping and artifacting before is now looking nice and smooth. Every clip in the project will now correctly display any areas with a mismatched color space issue.

In conclusion

I started by saying that ACES 1.3, to me, is really bigger than a simple point release. In fact, I moved my professional practice almost entirely off of ACES because of the uncertainty around gamut clipping issues like these. Either I would notice them and not have time to deal with an individual workaround, or worse yet, I wouldn’t notice it right away and end up embarrassing myself in front of clients or delivering something that had a technical flaw to it. That risk became untenable for me, so I really put ACES aside.

This new gamut compression option allows me to use ACES with confidence again.

Even though ACES 1.3 is mostly cosmetic changes, this new gamut compression option allows me to use ACES with confidence again. So, if you’ve been sidelining ACES for the same reasons that I have, I encourage you to start reconsidering that decision. With ACES 2.0 right around the corner, it’s a good time to get up to speed with ACES, as there are a bunch of exciting changes coming that are really going to have a positive impact on our workflow.

Cullen Kelly

Cullen Kelly is a Los Angeles-based senior colorist with credits spanning film, television, and commercials, for clients and outlets including Netflix, HBO, Hulu, Microsoft, McDonald’s, and Sephora. With a background in image science as well as the arts, he’s passionate about the intersection of the creative and technical, and its role in great visual storytelling. In addition to his grading work, Cullen is an educator and proven thought leader, with platforms including his podcast The Color Code as well as his YouTube channel.