Fixing over-saturated, out-of-gamut colours in Resolve

ARRI Alexa footage transformed by the standard ARRI Alexa LUT.

An issue you might encounter from time to time is over-saturated, out-of-gamut colours. An issue that usually manifests as solid blocks or solid fringing around bright saturated elements such as LED lights. It can happen with footage from any camera, from an iPhone to an Alexa. It can be caused at the camera capture stage, at the grading stage or at the output transform stage. Luckily it’s an issue that’s easy to fix.

In Resolve there are several way to address it. Today we will look at what are probably the three easiest methods: enabling Gamut Compression in the Camera Raw tab, Gamut Mapping via a Color Space Transform (CST) node, and using a dedicated Gamut Mapping node.

Gamut Compression in the Camera raw tab

If your footage is in the BRAW format you can fix over-saturation in the raw tab by toggling on gamut compression. As the name implies, enabling this compresses the outer edges of the gamut, desaturating over-saturated elements back into range.

Use this method if you need a quick fix. It’s quick and simple to use and can produce decent results. But as it’s only a simple on/off switch, it doesn’t allow you to fine tune the process, so it can’t be adjusted if its not working as desired.

I find gamut compression to be quite aggressive, desaturating elements that are not causing problems and that are not even close to being out-of-gamut. This can cause deep, rich colours to lose depth, making them look thin and washed out.

Another issue is that applying gamut compression at the raw source footage stage removes information that can never be brought back within the node graph. I’ve designed PowerGrades like FilmUnlimited to use this out-of-gamut information. And it can actually restore some of the information that initially appeared clipped (see image below). Enabling gamut compression affects this process and actually prevents any information from being brought back.

Image processed through FilmUnlimited

A better method is to address gamut issues in the node graph, after the key image processing is complete. This can be achieved using the methods below.

Gamut Mapping via a CST Node

If your node graph already contains a CST node, you can enable Gamut Mapping within the CST. Gamut mapping via a CST produces similar results to gamut compression above, except it can be adjusted and it will usually be applied at a better stage from an image processing point of view.

If you’re not using any CSTs, use the dedicated Gamut Mapping node method below.

If your node graph contains two or more CST nodes, I recommend enabling Gamut Mapping in the final CST node. If you’re using my 2Alexa PowerGrades this will be the node that converts Linear back to Log (the example below). If you’re using FilmUnlimited this will be node 04.

There are three main options within the gamut mapping section of a CST, Gamut Mapping Method, Saturation Knee and Saturation Max.

• Gamut Mapping Method should be set to Saturation Compression.

• Saturation Knee controls the saturation level at which the saturation compression begins to take effect. Set this at or just below the saturation level that is causing the issue.

• Saturation Max sets the new maximum saturation level for the values that are above Saturation Knee.

Left is original, right is with gamut mapping enabled.

To set Saturation Knee and Saturation Max correctly you might need to go back and forth between the two, adjusting both until you find the balance that achieves your desired result. I usually aim to compress the saturation enough to make the out-of-gamut elements look less distracting. But not too much that it removes all saturation and makes the elements pure white.

Dedicated Gamut Mapping Node

A dedicated gamut mapping node is identical to the gamut mapping section of a CST. It features the exact same controls and is used in the same exact way.

After adding the Gamut Mapping effect, select the Gamma that matches your camera’s gamma. If you’re not sure, leave it as Use timeline. Since you will be adjusting the result by eye, it likely won’t matter too much. If you are using a colour managed workflow leave it as Use timeline. Also make sure you set the Tone Mapping Method to None, as this section is not required.

Use a dedicated Gamut Mapping node if your node graph doesn’t contain any CSTs. Add the Gamut Mapping node towards the end of the node graph. I recommend adding it just prior to your output transform, whether that is being handled by an output LUT or by a collection of nodes. If you’re using a colour managed project, set it at or around the last node. If your output transform is a CST use the CST method above.

Final Words

Feel free to experiment with the methods above, and remember there is no right or wrong when it comes to how much saturation compression to use. Be guided by what looks good to your eye. If it looks good, then it is good.