Re: [Gimp-developer] Adding better LCH support to GIMP 2.10

[Elle Stone <ellestone ninedegreesbelow com>]

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 
around 252.

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: 
http://ninedegreesbelow.com/photography/lch-rgb-hsv/lch-hue-252.jpg

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): 
http://ninedegreesbelow.com/photography/lch-rgb-hsv/hsv-hue-252.jpg

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 
see http://harvardforest.fas.harvard.edu/leaves/pigment).

* 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: 
http://ninedegreesbelow.com/photography/lch-rgb-hsv/lch-plant-green-hues.jpg

Using HSV and dialing in these same Hue values produces decidedly more 
blue shades of green (yellow-greens are completely missing): 
http://ninedegreesbelow.com/photography/lch-rgb-hsv/hsv-green-hues.jpg

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 
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1404500/).

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: 
http://ninedegreesbelow.com/photography/lch-rgb-hsv/unique-blue-and-srgb-blue-on-lch-color-wheel.jpg 
- 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 
Hue-Chroma tool.

Best,
Elle