This if for Greg; Happy Birthday.
Tools needed (besides the ones built into GIMP): G'MIC
, Filter Factory Convertor
(believe it's called User Filter in Linux), and Saul's Mean Fill Script-fu
(can just use the G'MIC's Shape Average and create the the appropriate Mask).
I actually thought that I created a tut, but I did not. They are not hard. Will Keep it basic and will add my current workflow and not what I did when I originally did these (since I've gotten a lot of help from G'MIC and Saul's Selection Fill Script-fu (also could use the mask equivilent technique in G'MIC, but I still like Sauls Script-fu better).
First Aquire a target (hope you don't mind me borrowing yours Greg).
Now you run Blur>Pixelize (Here, there's rool for play, but I choose a radius of 20). Follow this with Poster set to 10.
pixel_poster.png [ 9.17 KiB | Viewed 2398 times ]
Now run it through Filter Factory>Vasarely Mosaics set to 12 (play here too; value chosen depends on Pixelize radius; you can also choose 30 to go the other way).
Vasarely.png [ 9.41 KiB | Viewed 2398 times ]
Now run G'MIC>Contours>Isophotes (default settings) and then choose the select regions tool and just click on a transparent part that is a results of Isophotes (may want to Ctrl-T to hide the ants since the ants significanly slows down the next step). Also, reduce selection size (play with this too, I chose 2). Delete the Isophotes Layer (no longer needed).
Now that you have your selection islands, run Saul's cool Script-fu then copy and Paste as new (desregarding your original of course; I know; sloppy editing. lol).
selection_islands.png [ 19.63 KiB | Viewed 2398 times ]
To add dimension, just dup, run motion Blur (5 and angle of 235) on the base layer and dup that layer a few times til solid and then Merge just those layers. Then create a new Mid layer with either texture or just Black set to Overlay and then blend opacity to taste. This step gives you bricks. Fill the base layer with white and you are done (much simplified; texturizing is also the other option of course).
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(had to since we only can upload 4 here; lol).