Notes on using LCh to pick colors

Written June 2017. Updated May 2018.

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:

• Pick colors that are the same as a given color but brighter or darker.
• Pick colors that are the same as a given color, but more or less saturated.
• Pick colors that have a warmer or cooler hue than a given color, perhaps for painting highlights or shadows.
• Pick harmonious colors based on their relative hue positions on a color wheel.

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 2.10 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

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:

• Just as RGB working spaces are not perceptually uniform, so HSV is not perceptually uniform. In case you are wondering, having a more or less perceptually uniform TRC (such as the sRGB TRC) doesn't magically make the spacing of colors in RGB or HSV perceptually uniform. It just makes the rate at which the individual RGB channel values change perceptually uniform.
• The meaning (the actual color) referred to by any given set of HSV values depends on what RGB working space is used to calculate the HSV values. This means that color palettes composed of sRGB-based HSV values won't work for any other RGB working space. Why should you care? sRGB is based on the display characteristics of consumer-grade CRT monitors made in the 1990s, sRGB never was a very good "fit" for LCD monitors, and Rec.2020-capable monitors eventually will make sRGB obsolete.

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

• LCh is just a polar transform of LAB, and an LCh color wheel is also a LAB color wheel. LAB/LCh is perceptually uniform, or at least very close to perceptually uniform (JAB/JCH is even closer).
• LCh is "RGB color space independent", which means you can use an LCh-based color palette in any RGB working space, including the Rec.2020 color space, which will be very useful when Rec.2020 monitors finally reach the consumer desktop.
• Unlike the RGB-based HSV, the LCh color wheel has four "primary" colors, yellow, cyan, violet-blue, and magenta-red. These four colors reflect the "opponent-color" aspect of how we perceive colors. Of interest to the many artists who want to promote yellow on the HSV color wheel to the status of a primary color, yellow already is one of the four opponent colors that define the LCh color wheel.

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:

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

• The even spacing of the HSV hues around the outer edge of the Hue ring somewhat implies that the twelve "color dots" shown above on the right are perceptually evenly spaced. But does it seem to you that the hue change between the 30- and 90-degree color dots is somehow the same as the hue change between the 90- and 150-degree color dots?
• For all colors on the Hue ring, the HSV Value is 100, which implies that these colors are all equally bright. But looking at the color dots, clearly the blue dot at HSV Hue 240 isn't nearly as bright as the green dot at HSV Hue 120 or the yellow dot at HSV Hue 60.
• For all colors on the Hue ring, the HSV Saturation is 100, which implies that these colors are all equally saturated. For colors as saturated as most of the colors on the HSV Hue ring, judging "more or less saturated" isn't easy to do, at least not on my "not wide gamut" monitor. But the colors between HSV Hues 30 and 60 and near Hue 180 look decidedly less colorful than the Hues near 120 (sRGB greenest green), 240 (sRGB bluest blue) and between 300 (sRGB magenta) and 360/0 (sRGB reddest red).

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

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:

• Monochromatic: pick one hue.
• Analogous: pick a hue and then pick hues that are 30 degrees to either side of the picked Hue.
• Complementary: pick a hue and also the hue that's on the opposite side of the wheel.
• Split-Complementary: pick two hues that are roughly 60 degrees apart, and a third hue that's directly opposite the hue that's half-way between the first two hues.
• Triadic: pick three hues that are evenly spaced (120 degrees apart) on the HSV color wheel.
• Square: pick four hues that are evenly spaced (90 degrees apart) on the HSV color wheel.
• Tetradic: pick two hues that are 60 degrees apart, and then pick each hue's complementary hue, for a total of four hues.

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?