Caught my pet turtle, KV, doing his yoga under the basking light. Shell was so square-on I decided to play with it.

Crop, cut out background and body bits, rotate shell, then trace plates on a new layer.
Apply to shell:
[G'MIC] Recursive median : -iain_recursive_median_p 9,1,0,0,1
(All G'MIC results as new layers, of course)
Copy traced layer, select lines and grow a couple pixels wider, gaussian blur, merge with smoothed shell layer, and apply:
[G'MIC] Edges offsets : -gimp_edge_offsets 0.117647,20.2941,4,2,0,0
Then use selection to trim edge layer, add coloring and shading layers, fiddle a bit, add a black background, and here it is:

Nice effect!

Nice Matera

That's lovely Matera. It's interesting finding patterns in nature and yours could be the start of a whole set of Gimp experiments.