GIMP 2.9 now has an LCH Hue-Chroma tool plus LCH Color Sliders

[Elle (Elle Stone)]

Hi @RawConvert ,

@RawConvert

if I want to experiment with LCH and have GIMP do most of the adjusting of a photo, what practical approach would you suggest re. raw conversion. I use RT. I’m thinking I shouldn’t do too much adjusting in RT otherwise there is little for GIMP LCH to do. But on the other hand, the most resolution I believe you can get out of RT is a 16bit tiff. So if you then do significant shifts in GIMP, perhaps quality will suffer. If I do the adjustments in RT, I think it’s all done via 32bit floating point up to the point of output.

As far as editing in GIMP starting with a 16-bit file, if you promote the precision to 32f upon import, then quality won’t suffer even with significant shifts. Well, at least this has been my experience, and I sometimes subject image files to a whole lot of editing.

You’ve pinpointed two approaches to editing: do as much as possible in the raw processor, or start with scene-referred output (minimal processing in the raw processor) and do the “make pretty” processing in GIMP. The latter is the approach that I use, but that doesn’t mean it’s the right approach for you. It partly depends on what kind of edits you want to use LCH for (about which I’d be very interested to hear what you are doing and how it works out). Anyway, try both approaches and see which one you like.

An important consideration regarding the adequacy of 16-bit integer output is the question of whether your output profile (in which the interpolated file is saved to disk) has a large enough color gamut to include all the colors that were captured by your camera, as interpreted by your camera input profile. It’s an unpleasant fact that the colors in interpolated camera raw files can easily exceed the sRGB color space, and in fact can exceed the color gamut of very large RGB working spaces such as Rec.2020. This is especially true of images that contain saturated bright yellows and yellow green (eg bright yellow flowers), and dark saturated violet blues (eg neon signs). Saving to disk using 32f precision (as eg darktable allows) means colors are not clipped by the act of saving them to disk and opening them with GIMP. But when exporting to disk at 16-bit integer, any out of gamut colors that are still in the file will be clipped upon export.

So an important question is what output color space do you plan to use with RT? Default GIMP requires images that are in the regular sRGB color space. But if the image contains a lot of saturated colors, you can export from RT in a larger color space that hopefully holds all the image colors (but again, bright yellows and dark violet blues can be problematic even for very large output color spaces). Then open the image with GIMP, change the precision to 32f, and then convert to GIMP’s built-in sRGB color space - at 32f colors won’t be clipped by the conversion. That’s the good news. The bad news is you’ll need to learn how to deal with any resulting out of gamut colors to bring them into gamut. Otherwise when you export the finished image to disk as a jpeg or png or integer-precision tiff, you might be surprised and dismayed by color shifts from clipped colors.

@RawConvert

Perhaps you have a fair idea about the GIMP roadmap? Do you have any idea when re-vamped processing will appear in the Curves and Levels tools please?

(My GIMP is 2.9.5 with commit ending …f766)

I don’t know the GIMP roadmap other than what’s on the GIMP website. What revamped processing are you referring to for the Curves and Levels tools? Do you mean defaulting to linear processing instead of processing RGB data that’s encoded using the sRGB TRC? In any event, I don’t know the timetable. When was your version of GIMP compiled? The LCH Hue-Chroma tool was added only a short time ago, and the LCH color sliders were added just a few days ago. So this is really new and worth updating to get.