Skip to main content

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:
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.


Comments

Popular posts from this blog

Determine Perspective Lines With Off-page Vanishing Point

In perspective drawing, a vanishing point represents a group of parallel lines, in other words, a direction. For any point on the paper, if we want a line towards the same direction (in the 3d space), we simply draw a line through it and the vanishing point. But sometimes the vanishing point is too far away, such that it is outside the paper/canvas. In this example, we have a point P and two perspective lines L1 and L2. The vanishing point VP is naturally the intersection of L1 and L2. The task is to draw a line through P and VP, without having VP on the paper. I am aware of a few traditional solutions: 1. Use extra pieces of paper such that we can extend L1 and L2 until we see VP. 2. Draw everything in a smaller scale, such that we can see both P and VP on the paper. Draw the line and scale everything back. 3. Draw a perspective grid using the Brewer Method. #1 and #2 might be quite practical. #3 may not guarantee a solution, unless we can measure distances/p...

Hardware Password Manager

[Updates 2025-01-20] The original blog post assumes that all passwords are stored in one password manager, and the password manager either unlocks everything or nothing (e.g. Keepass). After discussing with friends, I realized that if I use something like pass , and I use a hardware GPG token, I can actually store and sync all encrypted passwords to all devices, because I will only decrypt the passwords on demand, and the computer will not see the GPG private key. The compromise is that the computer will see the list of all password entries (e.g. accounts), as well as a few other issues . I've been using Keepass for many years. I don't use online password services because I cannot fully trust them. Besides, I may not always have Internet connection, which is why I also don't use a self-hosted service. Everything has been working fine, until I turn my paranoid knob to the max. Here's the thought experiment . The Imaginary Scenario Let's say I have 100 PCs for differe...

ESP32S3: Flash Encryption and Secure Boot

Flash encryption and secure boot are useful security features for ESP32S3 chip. While not perfect, they definitely make it harder to extract the secrets in the chip. However, it is tricky to enable both features at the same time. The topic is actually discussed in the official documentation: ESP32S3 Security Features Security Features Enablement Workflows Especially, the second one mentioned it is recommended to enable flash encryption before secure boot. But I still find the documentation confusing. In the end I was able to successfully enable both, here's my findings. My Understanding After my adventure, here's what I think could have worked. WARNING, this is untested. Follow  Security Features Enablement Workflows : Burn all the keys, as long as their purpose eFuses and read/write protections Burn other security eFuses, but DO NOT burn ENABLE_SECURITY_DOWNLOAD in the middle, which is mentined at the end of the instruction for both flash encryption and secure boot. Burn...