Rawtherapee: Add color wheels

[elecprog]

Color wheels are a feature commonly found in color grading and correction software for film, however can be of great use for photographers too and is often more intuitive as it uses hue instead of RGB values.

Color wheels (see Speedgrade, Davinci Resolve, CameraBag etc.) are probably the easiest way to correct and grade color in an image. they allow you to push a certain color into the image using a different function (lift, gamma, gain and sometimes offset) per wheel. Most programs also have a luminance slider for every wheel which gives you the ability to quickly correct exposure.

They are a cruder tool than curves, but are a lot faster and usually get the job done, very quickly.

One of the best implementations of this is found in Davinci Resolve --- of which there is a (only slightly limited) free version (also runs on Linux).

The color wheels look something like this:

The slider under the wheels is for luminance.

(Note: originally part of issue #4763)

[Thanatomanic]

I assigned myself because I am very fond to include this in RT. It's going to be a longer term project... Are you okay with that @ElecProg ?

[elecprog]

@Thanatomanic Please take as long as you need, the thing I'm enjoying the most about RT is the near perfection of all the features it has, and getting software (especially a side project) to such a point takes time.

It would speed my edits up by quite a bit when it's finished of course, but it's not stopping me from using RT :smile:

[elecprog]

Potentially useful: https://docs.blender.org/manual/en/dev/compositing/types/color/color_balance.html

[elecprog]

About #4893:

@Thanatomanic As far as I can tell it's actually fairly difficult to achieve the same effect as the color wheels using this method (and as far as I can tell even impossible to achieve the exact effect of gamma).

Does this lower the necessity of color wheels? Yes it does, however I think that color wheels have a use as a primary color correction, the fine-grained control that this option provides is a bit overwhelming and in general I want to get the image roughly where I want before I get to the details :slightly_smiling_face:

[agriggio]

I didn't follow the conversation fully, but did you have a look at the new colour correction module of darktable? maybe that's something we can borrow...