[Gimp-developer] Drawing Rec 2020 gradients in the unbounded mode sRGB color space

[Elle Stone <ellestone ninedegreesbelow com>]

Let's say it's the year 2017. Let's say GIMP 2.10 or maybe even 3.0 has 
been released.

Assume a graphic artist wants to use GIMP to prepare an image for 
presenting the Rec 2020 color space as displayed on a spiffy new Ultra 
High Definition display that's being introduced at a Buyers Convention.

She opens GIMP, makes a new blank 32-bit floating point document, 
assigns the Rec 2020 color space profile, and then the image is 
converted to unbounded mode sRGB.

The artist envisions a triangle of colors with Rec 2020 red, green, and 
blue star-bursting from the corners, transitioning to white in the center.

So she needs to draw three gradients, from Rec 2020 reddest red to 
white, from Rec 2020 greenest green to white, and from Rec 2020 bluest 
blue to white.

How does she know what unbounded mode *s*RGB values to select for the 
Rec 2020 red, green, and blue ends of the gradients?

And how does she actually select these colors?

In other words, are there any thoughts for the UI for selecting colors 
when the artist wants to paint and draw in color spaces other than sRGB?

For reference, here are the required linear gamma and regular sRGB 
values for the desired Rec 2020 colors:

Rec 2020 reddest red (1,0,0):
linear gamma sRGB (1.660581, -0.124595, -0.018144)
regular sRGB (1.248252, -1.609913, -0.234435).

Rec 2020 greenest red (0,1,0):
linear gamma sRGB (-0.587709, 1.132939, -0.100573)
regular sRGB (-7.593863, 1.056319, -1.299524).

Rec 2020 bluest blue (0,0,1):
linear gamma sRGB (-0.072872, -0.008343, 1.118717)
regular sRGB (-0.941591, -0.107805, 1.050485).

It seems to me that the simplest, indeed the only possible approach is 
to keep the original Rec 2020 chromaticities available for use with the 
numbers displayed by the Color Selection Dialog.

It seems to me to be completely unrealistic to expect the user to dial 
in (1.660581, -0.124595, -0.018144) or (1.248252, -1.609913, -0.234435) 
when what she really wants is Rec 2020 red (1,0,0).

The same considerations apply to the Color Picker. If the user wants to 
paint/draw an image in the Rec 2020 color space, when she uses the Color 
Picker she needs to know the Rec 2020 colors, not the unbounded mode 
sRGB colors.

No one will understand what (1.660581, -0.124595, -0.018144) means.
Everyone understands what (1,0,0) means.


Elle, puzzled as to how this kind of situation might otherwise be handled.