GIMP Version: 2.10.38
Operating System: Windows
GIMP Experience: Intermediate Level

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I would like to process an existing image by adjusting whatever parameters are controlled by the 'Hue' slider in Colors/Hue-Chroma, so that the far left of the layer (x=0) is processed with Hue -180, the center is unprocessed (Hue 0), and the far right (x=x_size) is processed with Hue 180, which means the two edges match.

I've done it manually, dividing the image up into ten steps. Make selection, adjust Colors/Hue-Chroma/Hue. They're far too coarse, but it's the general effect I want. As the layer is 3000 pixels wide, I'd like each band to be one step of hue adjustment wide, so 256 bands of 12 pixels or so. Interpolation or anti-aliasing would be even better, but isn't really necessary. And I suspect higher bit depths would give smoother results, but memory limits currently prevent that.

I can't get the gradient tool to do anything but bitmaps and masks, not hue adjustment to existing data. If there's other tools, or a plugin in in G'Mic, I haven't found them. I have pseudocode, more or less, but that's only about 5%-10% of a functional Gimp plugin or script, unless I'm missing a short version.

Is there a way to use existing Gimp tools or plugins to do this? I also have a black-white gradient on a separate layer, but I can't find a way to get that into a hue control.

(edit) I believe I've found a solution - a one-line script-fu thing that simply brute-forces selection-adjust hue in a loop across the image. It only works for this image, but that's sufficient for now.

I don't know how good you are at writing plugins.

As a possible solution, I propose combining two plugins after a minor modification

Rotate Hue Script
viewtopic.php?f=9&t=20420

Plug-in Striped Selection
viewtopic.php?f=9&t=21469

Not good at writing plugins. I get the algorithms, but I'm totally unfamiliar with Gimp's innards and reference material. The method I came up with uses two methods I know from the menus, and a simple counting loop, in a one-line script. That I can handle, even in Scheme. (Did I mention I hate parentheses ever since Cakewalk CAL?) I'll eventually master registration, error handling, image type inspection, and all the other cruft necessary to make a general-purpose plugin, but not right now.

Two general ways:

* Colors > Components > Decompose/recompose will create an image where three components are layers, and there are a couple of color models where one of the component is the Hue. You then replace the Hue layer, and recompose.
* In the layer blend modes, there are "LCH Hue" and "HSV hue" modes that both result in applying the Hue of the layer to the composite image below it. The `HSV Color` and `LCh color` apply both Hue and Saturation, if necessary. So you only need to add a new layer and fill it with whatever.

A possible, might work for you.

Duplicated layer. A layer mask filled with bilinear gradient for the symmetry.

I got it, eventually, in a 'one-line' script-fu script. It turns out that carriage returns are just whitespace to the script-fu interpreter, so this just pasted in and ran. You have to set img to the image number from the title bar - I never did find a way to get that number from the command line. It's currently 2.

Delta 1 or 2 gets more artifacts than colors. It's not too impressive on large expanses of one color, like the grass at the bottom of the photo - that could as well be a semi-transparent color mask. It's best on multicolored things that obscure the rainbow effect. Or you can set it to a narrower range of hue.

The 180 constant is the hue slider range, ±180 degrees.

A lot of trouble for very little result.


hard

I definitely prefer to use the method presented by Rich2005. You get a live preview of the hue adjustment and the appearance of the mask.



Slicing into strips has another limitation - if the hue step is greater than 10, you will see a dividing line:



If Your goal is just to get a rainbow effect, the Ofnust method is better.

You can also combine both methods (mask to a new layer with a gradient).