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

[Elle (Elle Stone)]

@afre

Correct me if I am wrong: many tools in the default GIMP are hard-coded to sRGB, so while it would be ideal to be in 32f and a generous color space it is impossible at this time.

This is true. Many operations in default GIMP are hard-coded to sRGB. In fact essentially all default GIMP operations are hard-coded to sRGB because much background processing assumes the image has the sRGB TRC.

What I meant to say, tried to say, is that upon opening an image in GIMP that’s in a larger gamut color space, first promote the precision to 32f and then convert to GIMP’s built-in sRGB color space. And then you can edit in the sRGB color space without having clipped any values. This is not an ideal situation because of two reasons:

1. It’s not easy to learn the nuances of dealing with any resulting out of gamut channel values. But it’s worth doing so because of the things it allows you to do, that you can’t do otherwise. See this article for some examples: Edit tonality and color separately using high bit depth GIMP - the article is somewhat out of date because default GIMP now does have full LCH support.

2. Many - but not all - editing operations produce different results in different RGB working spaces:

• White balancing camera-saved sRGB jpegs
• Limitations of unbounded sRGB as a universal editing space
• Color correction fails in unbounded sRGB.

The good news is that GIMP’s LAB/LCH operations are color space independent - once you’ve done an unbounded conversion from sRGB to LAB/LCH, the resulting LAB/LCH channel values are exactly the same as what you’d have gotten if you had converted to LAB/LCH from some other, larger color space.

Many other editing operations also are color space independent, for example scaling an image larger or smaller, gaussian blur, etc. But taking advantage of editing operations that are color space independent requires knowing which operations fall into which category.

@afre

The gift of 32f is that even though there is the initial clipping we are able to maintain a least some accuracy within the limitations we are given. For this reason, I tend to use pure G’MIC because sorting through this stuff makes my head explode.

Well, 32f also allows to do true HDR processing, which is a field that a lot of people are interested in moving into and many already have moved into.

G’MIC also is hard-coded to sRGB. Take a look at the code, sRGB parameters are all over the place. There’s no provision for extracting the image’s actual ICC profile and using the parameters of the image profile for conversions to XYZ/LAB/LCH. Not to mention the various operations that use sRGB Y values for Luma/Luminance.