Re: [Gimp-developer] Adding LAB support to GIMP

On 11/30/2017 07:11 AM, Alexandre Prokoudine wrote:
On Thu, Nov 30, 2017 at 2:44 PM Elle Stone wrote:

It seems to me that full LAB support would require:

* The option to open images that are in the LAB color space, keeping in
mind that there are various LAB color space ICC profiles floating
around, and some LAB images don't have any embedded ICC profile, but
instead just have metadata that indicates the color space.
      Upon opening a LAB image, it could be immediately converted to RGB
for processing, as the conversions from RGB to LAB or LCH for further
processing in these color spaces are already handled by babl.

Hi Alex, and thanks much! for responding.

So you are not in the scene referred / display referred camp now? :)

After some reflection, I realized that I don't understand the question :)

* Scene-referred editing needs to be done using linear RGB.

* To avoid gamma artifacts, many color mixing/modifying operations need to be done using linear RGB.

* Color mixing/modifying covers a lot more territory in painting and photography than the specific operations that preserve the scene-referred nature of scene-referred output from the camera ("as scene-referred as possible" given the limitations of the camera, the lens, the raw file, and the interpolation algorithms).

* For my own photography workflow I start with an image that's "as scene-referred as possible". But usually that's not the final image but rather the starting point for further editing. I use GIMP's LCH blend modes and tools a lot, for selecting colors and for modifying colors, in paintings and in photographs.

As far as adjusting colors in the LAB color space, and specifically using the A and B channels, the people who've asked me about adding better LAB support to GIMP are people who make their living editing images. It would be odd to tell someone who finds this type of operation useful "Well, that's not a scene-referred operation and it doesn't work on linear RGB, so GIMP shouldn't include it".

* The ability to do Curves and Levels operations on the LAB channels and
perhaps on the CH channels.

We briefly discussed this on #gimp a while ago, it's seems to be doable
presently already (but is likely to be somewhat slow).

Given the intensity with which people request LAB curves, they might be very happy with "slow" if the other alternative is "not even available".

* Adding L/A/B/C/H histograms to the Histograms dialog: We already have
Luminance in the histograms dialog. Lightness is just the perceptually
uniform transform of Luminance.

That's a nice write-up!

Regarding "does an a* and b* histogram actually make sense?", this article showing a sample use case for the LAB A and B histograms:

The actual modification in the article was done using Levels, but seeing the histogram is required to make the adjustments, and does convey information all by itself regarding color distribution.

I have some old PSD files for which digiKam can't display a thumbnail, so I used GIMP to create sidecar "png" files for each of these PSD files - I was surprised at the fairly large percentage of these old PSD files that were in the LAB color space, which GIMP couldn't open, so I don't have any idea what kind of LAB adjustments I might have been making.

Admittedly, I got sidetracked from this.

What else might be required to add full support for the LAB space to GIMP?

There's probably a place for LAB/LCH-specific tools, like some iops in
darktable (e.g. color contrast, CLUT, and especially color zones).

Quite awhile back you suggested adding a certain darktable module to GIMP, and I said something like "personally I'd never use that". Several months later I was processing an image to make a black and white rendition and wishing very much that GIMP had that darktable module :)

Except now I can't seem to find the module even in darktable, so maybe I'm not remembering correctly what it was. I wanted the ability to selectively depress the Lightness of all colors within a given range of LCH Hues, trying to somewhat emulate what an actual red or yellow or etc filter would do when shooting black and white film.

Personally, I'm also a big fan of conditional blending (as available in
both darktable and e.g. Photoshop). In darktable, it operates on LAB

Conditional blending would be wonderful. I used to use it in Corel's PhotoPaint, and then in PhotoShop - wonderfully easy way to fade out an adjustment towards the shadows and highlights for example, and to limit an adjustment to a given portion of a given channel's histogram.

In Photoshop, it appears to work on whatever channels as per current color



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