Notes on Color #7: Revisiting James Gurney's Gamut Mask

Gamut masks, or gamut mapping, is a color managing tool made popular by James Gurney. It is a set of practical instructions, which allows us to easily create a palette of harmonic colors.

James has explained the method in various formats:

• 2008 blog post
• 2011 series 1, 2, 3
• Video on Youtube.
• His book Color and Light: A Guide for the Realist Painter

I found this method so inspiring when I first learned about it around 2014. Recently it came back to my mind when I started developing the digital Zorn palette, which turned out to work quite well. I decided to revisit the cool method, in the hope of getting better understanding the method and some color therories.

The goal includes:

• Recognizing the limitation of physical paints.
• Figuring out an idealized model of the method.
• Adapting the method for digtal painting.

The Original Method

I'd summarize the original gamut mapping method as the following 3 steps:

• Start with a color wheel.
• Maskthe color wheel with a simple shape, typically a triangle.
• Use only colors in the mask.

This is it. Believing or not, these super simple steps actual work! 

James once mentioned that the method could go back (at least) to the 1920s. He adapted the method from the book The Enjoyment and Use of Color by Walter Sargent.

On the other hand, there is some hidden, ambiguous information that are often overlooked or misinterpreted. This could be well explained by examining the typical digital implementation.

The Typical Digital Version

The gamut mask is available in Krita, which I will examine in details. There are also a few other versions, online, plugins or standalone binaries, which are basically the same.

Gamut Mask in Krita.

In Krita, we start with a HSV (or HSL, HSY) color wheel. For the mask, the user may choose from a few predefined shape, or draw a custom version. In the UI there is a slider where you can adjust value/lightness/luma. More details can be found here.

Well this digital adaption look so natural and intuitive that I didn't have any doubt, until recently.

What Is Wrong? 

The first issue invovles the choice of the color wheel. In previous posts (1, 2) I discussed issues of value/brightness in HSV/HSL/HSY. However for gamut mask, we need something else, namely uniform distribution of the hues.

In the book Color and Light: A Guide for the Realist Painter, James mentioned that the traditional RYB color wheel suffers from uneven distribution of hues. The red-orange-yellow section is too "loose", while the green-blue secction is too "crowded".

Prior to modern color spaces, the Munsell color system was the best hue-chroma-value system that is perceptually uniform. Even today, the Munsell colors are often used to test modern color spaces. It is easy to observe the difference between HSL and CAM16UCS (a modern uniform color system), if we plot the Munsell colors:

Munsell Colors in HSL

Munsell Colors in CAM16UCS

The second issue is about chroma. Note that there is difference between saturation and chroma. Briefly speaking, chroma is independent and absolute, while saturation is relative and depends on hue and/or value.

In the digital version, when we adjust the V/L/Y channel, the H(ue) and S(aturation) channels remain the same. This means chroma would change along. (Well I didn't even mention the poor performance of uniformity in these models, the weird defintion of "saturation" in HSL and the horrendous stretching of chroma in HSY)

In the original version, however, James explicitly mentioned maintaining chroma when mixing colors. Well sometimes he also mentioned intensity or saturation, but I do believe he meant chroma. A solid evidence is that James obtained lighter/darker versions of the base colors by mixing other high-chroma colors, instead of with pure white/black. 

Next, I would justify my arguments by analyzing the idealized model.