skip to main content

Notes on using LCH to pick colors (tutorial in progress)

I'm trying to write one or two tutorials on using GIMP's LCH color sliders to pick colors, that hopefully can be added to the tutorials on gimp.org and/or pixls.us. Help in making an actually readable tutorial for GIMP users is greatly, greatly appreciated.

If you'd like to help with writing this tutorial (comments, questions to be answered, what to include, what to leave out, suggested text, additional images or links, etc), probably the easiest thing to do is participate in this pixls.us forum discussion: GIMP 2.9 now has an LCH Hue-Chroma tool plus LCH Color Sliders.

Images and text in this "tutorial in progress" are released under the Attribution-ShareAlike 4.0 International (CC BY-SA 4.0) license (the same license used on the pixls.us website).

Written June 2017.

Picking colors (why we like HSV more than RGB)

For picking colors, most people find thinking in terms of hue, saturation (chroma/colorfulness/etc), and value (lightness/brightness/etc) more convenient than thinking in terms of a color's RGB channel values. For example, an artist might want to:

Like most image editors, GIMP allows to select colors using the HSV (Hue/Saturation/Value) color space, and GIMP's color picking tools provide HSV color sliders and also an HSV color wheel. GIMP 2.9 now also allows to select colors using the LCH color space ("Lightness, Chroma, Hue"), using LCH color sliders, but we don't yet have an LCH color wheel panel.

GIMP's new LCH color sliders

GIMP's HSV, RGB, and LCH color sliders.

On the one hand, GIMP's new LCH color sliders can be used for all the same color picking tasks for which artists have been using the HSV color wheel and color sliders. On the other hand, using GIMP's LCH color sliders isn't nearly as easy as using the HSV color sliders, and "how" to use the LCH color sliders isn't exactly obvious:

If LCH is more difficult to use than HSV, why would anyone bother with LCH? HSV is a computationally inexpensive transform of RGB (designed to run on computers made in the 1990s), and so inherits all the problems of RGB working spaces:

In stark contrast to HSV, LCH is a perceptually uniform reference color space based on actual studies of how people perceive colors:

For a very short and readable introduction to the idea of a four-color opponent-based color wheel, see Color Wheels are wrong? How color vision actually works. And then follow the author's advice and spend your "color study" time perusing the handprint.com website's treasure trove of color information, starting with Vision and color are at the heart of painting. For excellent, practical information on color wheels and choosing colors, see the handprint.com pages on color wheels, color harmony & color design, and color temperature. Many thanks to Alexandre Prokoudine for referencing Color Wheels are wrong? in one of his very fine Libre Graphics World articles.

A tour of GIMP's sRGB-based HSV color wheel

GIMP's sRGB-based HSV color wheel is shown below on the left. I want to focus on just the "Hue ring" part of the sRGB HSV color wheel, which is shown below on the right:

Left: GIMP's sRGB HSV color wheel; Right: HSV Hue ring with color dots smarking every 30 "HSV Hue degrees".

The sRGB HSV color wheel is familiar and comfortable. It's also misleading:

The sRGB HSV Hue ring shows the brightest, most saturated possible colors in the sRGB color space:

  1. The three sRGB Red, Green, and Blue primaries are located around the HSV Hue ring at 120-degree intervals:
    • The sRGB Red primary (1,0,0) is located at HSV Hue 0/360.
    • The sRGB Green primary (0,1,0) is located at HSV Hue 120.
    • The sRGB Blue primary (0,0,1) is at HSV Hue 240.
  2. The three sRGB secondary colors Yellow, Cyan, and Magenta also are located around the HSV Hue ring at 120-degree intervals, equidistant from the primaries:
    • sRGB Yellow (1,1,0) is located at HSV Hue 60.
    • sRGB Cyan (0,1,1) is located at HSV Hue 180.
    • sRGB Magenta (1,0,1) is at HSV Hue 300.
  3. The six sRGB tertiary colors (HSV's orange-red, cyan-green, violet-blue, etc) are located around the HSV Hue ring at 60-degree intervals midway between the color dots for the primaries and secondaries, at HSV Hues 30, 90, 150, 210, 270, and 330.

The primary, secondary, and tertiary sRGB colors on the LCH color wheel

The sRGB HSV Hue ring wrapped around the LCH Color circle, showing the sRGB HSV primary, secondary, and tertiary colors on the HSV Hue ring and the LCH color wheel.

You can learn a lot from studying the above image:

  1. If HSV were perceptually uniform, the sRGB HSV primary, secondary, and tertiary colors would be distributed uniformly around the LCH Color Wheel. But only one of the twelve plotted colors is in the lower left quadrant of the LCH Color wheel. The remaining eleven colors are squeezed into other three quadrants of the LCH color wheel. In particular, the three green HSV Hues (90, 120, and 150) are squeezed very close together on the LCH Color wheel, as are the blue and violet-blue HSV Hues (240 and 270).
  2. In the HSV color space, all colors on the outer edge of the HSV color wheel have an HSV Saturation of 100 and an HSV Value of 100. This implies that all these colors appear equally bright and colorful:
    • If you know anything about Luminance (the Y of a color's position in the XYZ reference color space) — or if you just look at the colors — you already know that sRGB blue (H=240, S=V=100) is not nearly as bright as sRGB yellow (H=60, S=V=100). So HSV is not perceptually uniform with respect to Lightness.
    • The LCH Chroma values for the twelve plotted colors ranges from a low of Chroma=53 for sRGB "cyan" at HSV Hue 180/LCH Hue 196) to a high of Chroma=131 for sRGB "blue" at HSV Hue 240/LCH Hue 301. So HSV isn't perceptually uniform with respect to Chroma. And if you divide Chroma by Lightness for the twelve plotted colors to get a measure of Colorfulness, HSV is not perceptually uniform with respect to Colorfulness.
  3. Unlike most image editors, GIMP's HSV Color Wheel arranges the HSV Hues in counterclockwise order starting from HSV Hue 0/360 on the right side of the horizontal axis of the HSV Color wheel. This means the Hue numbers for marking degrees on GIMP's HSV Color Wheel and the Hue numbers for marking degrees on an LCH Color wheel line up very nicely.
  4. Even though the Hue numbers for marking degrees on the LCH color wheel and GIMP's HSV color wheel do line up, for any given color in the sRGB color space, the corresponding HSV and LCH Hue values range from "almost lining up" (look at the HSV Hue ring color dot for HSV Hue=150, and follow the line to the color's location on the LCH Color Wheel), to as much as 60 degrees of difference (look at the HSV color dots with HSV Hues 210 and 240, and follow the lines to each color's location on the LCH Color Wheel).

    Please note that the above observations on "how well do the Hues line up" is limited to the most saturated and bright colors on the HSV Hue ring — we haven't yet checked what happens to the LCH Hues as the Hue ring colors are desaturated and/or made lighter or darker.

OK, HSV Hue is not a good way to pick perceptually equally spaced hues. The same is true for the HSL, HSI, and HSY color wheels, as all of these RGB-derived color spaces use the same Hue ring (the calculations for Saturation and for Value/Lightness/Intensity/Luma do vary between the HS"X" color models). Does any of this matters even one little bit when faced with the practical task of picking colors in the digital darkroom? The only way to answer this question is to try for yourself and see what you think.

Making hue-based color palettes from a color wheel

As already noted, the pristinely pure geometric relationship between hues that is implied by the HS"X" color wheel does fall apart as soon as you start looking at studies of how real people actually perceive real colors and color differences. However, in the digital darkroom picking harmonious hues based on such geometric relationships is a very common practice. Again thanking Alexandre Prokoudine for the link, HS"X" color-wheel-derived hue-based color schemes include:

Typically the above-listed HS"X" hue-based color schemes allow to use all hues within 15 degrees of the selected main hue(s). And all colors with the picked hue(s) can be used, all the way from black to white, and all the way from completely desaturated to maximum saturation, as long as the hues are within 15 degrees of the selected main hue(s).

The sRGB HS"X" color wheels space the twelve sRGB primary, secondary, and tertiary colors evenly every 30 degrees around the HSV color wheel. These same colors on the LCH color wheel are very unevenly distributed. So the color wheel you use to make a hue-based color palette does determine which "real world colors" you actually end up with.

The best way for an artist to decide whether she prefers HSV or LCH for making color palettes is to actually make a few color palettes using both wheels, and then try using the resulting palettes. So next up will be some sample color palettes, starting with monochromatic color palettes.

Comparing LCH and HSV color palettes

Next?