My current workflow:
1) choose photos in Digikam, copy them to another folder
2) open photo in RawTherapee
3) try to get good colour and contrast in RT, fix highlights, etc,
then export to Gimp (RT has no layers)
4) use RGB curves in Gimp, sometimes decompose to RGB and combine
layers to create BW version
5) use color mixer in GMIC, LAB mixer is good to create "toned down"
version of colors, CMYK is mixer is good to fix colors different way
(or use GMIC for quick BW version)
6) healing tool (only Gimp can do this)
7) save xcf, export jpgs
The thing I miss are LAB curves and CMYK curves. I asked GMIC
developer for LAB/CMYK curves support and it is there, but UI is hard
to use. RawTherapee has support for LAB curves and they work very good
(and in 16bit mode), but there are no layers in RT.
So I am fan of RT, Gimp and GMIC, this is very good, quite mature free
software. I am also aware of Photivo / LAB Curves. Never tried
Darktable and Krita for photos.
After looking at all available solutions the only way I see for me is
to develop simple application (of course GPL) which will open
jpg/tiff, process image in RGB/LAB/CMYK with curves/mixers/blend modes
then save tiff/jpg again. But I will announce it when ready (or won't
if I fail).
PS. CMYK is also best colorspace for skin color retouch by numbers,
that's why I wanted to fix CMYK values in Gimp colorpicker, but there
was big discussion on this mailing on this subject
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