On 02/07/2016 06:28 PM, Alexandre Prokoudine wrote:
I can see how this would
irritate people who want numeric inputs for everything though.
I do like numbers.
LCH Color Picker for picking colors "by the numbers":
Maybe most GIMP users are good at picking colors out of thin air, but
personally I appreciate any help I can get. Fortunately the internet is
a treasure trove of color information, and an LCH color picker makes it
easy for digital artists to use this information.
Consider painting a picture of a blue sky and some vegetation. A nice
first step is to make some appropriate blue and green color swatches:
According to the handprint.com website, "the average sky chromaticity .
. . correspond[s] to a dominant wavelength of about 470 to 475 nm"
(http://www.handprint.com/HP/WCL/color12.html). These wavelengths
correspond to a JCH Hue of around 247
(http://handprint.com/HP/WCL/color16.html#tourwheel) and an LCH Hue of
It would be better if GIMP could have a JCH color picker, but JCH and
LCH colors are not usually too far apart, and using LCH is infinitely
better than pretending that HSV hues actually mean anything.
An LCH color picker makes it simple to use the LCH sky hue of 252 to
create a set of sky blue color swatches:
Here's what results from trying to make "sky blue with LCH hue 252"
using the HSV color picker (sorry, these colors are pathetically wrong):
Color information from the internet also can be used to make color
swatches for typical vegetation colors:
* This website says that green plants reflect light in the 500 to 600
nanometer range: http://www.webexhibits.org/causesofcolor/7A.html (also
* The handprint.com watercolor website's "tour of the color wheel"
(http://handprint.com/HP/WCL/color16.html#tourwheel) provides wavelength
and JCH Hue information for watercolor pigments, and colors from 500 to
600 nm cover the JCH Hue range from around 110 to 170. Fortunately the
LCH Hue range is not too far off from the JCH Hue range, so I'll assume
the LCH Hues are "close enough".
Putting these numbers together with the LCH color picker makes it very
easy to construct a set of color swaths for painting vegetation:
Using HSV and dialing in these same Hue values produces decidedly more
blue shades of green (yellow-greens are completely missing):
You can't use an HSV color picker to pick colors based on LCH/JCH color
information. So the internet treasure trove of color information is
pretty much out of reach for anyone who doesn't have access to an
LCH/JCH color picker.
LCH Hue-Chroma tool for modifying colors:
RGB-based tools for modifying saturation also change hues at the same
time. This is exactly why I coded up GIMP LCH color picker and
Hue-Chroma tools. I had picked a particular shade of blue from a
photograph to use for colorizing the sky portions of a black and white
image. I wanted to modify the lightness and increase the saturation but
keep the hue visually the same. Every attempt to to keep the hue
constant and only change the lightness and saturation ended up shifting
the hue towards violet.
http://www.handprint.com/HP/WCL/CIELAB.pdf shows "unique colors" on the
LCH color wheel. "Unique blue" is that shade of blue that most people
would say is neither slightly on the green side nor slightly on the
violet side of blue, "unique red" is neither slightly orange nor
slightly magenta, and so on. An internet search turns up quite a lot of
research into the topic of unique colors (for example
The following graphic shows why adding saturation using the usual RGB
tools (HSV, Channel Mixer, etc) *always* shifts color hues in various
directions - blues move towards violet, bluer greens get more yellow,
and so forth:
- the small dots are sRGB's Reddest Red, Bluest Blue, etc, and the large
dots are unique red, unique blue, etc.
When located on the LCH color wheel, sRGB "Blue" isn't even blue at all.
It's blatantly violet. sRGB Red is an orange-red. And sRGB Green is
decidedly on the yellow side of green.
One major problem with using sRGB/HSV to pick colors for painting and
graphic design is that the colors we pick tend to get pushed towards the
rather warped representations of blue, green, yellow, and red that are
defined by the sRGB primaries (warped from the point of view of picking
colors for painting - color space primaries from other RGB working
spaces are not any better). An LCH color picker provides a way to choose
colors based on how we actually perceive colors out there in the real world.
For adding saturation to a photographic image, GIMP's new Saturation
tool ("Colors/Saturation") is excellent because it operates on LAB/LCH
instead of RGB. But for modifying the Hue, Chroma ("saturation"), and
Lightness of a solid color (for example a color swatch color for digital
painting or a solid color blending layer for colorizing a black and
white image) or a small range of hues, GIMP needs a dedicated LCH
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