Not much to say, Thank you Issabella for sharing the tutorial. I didn't like the Octopus, so I created some "Cat's eyes" pattern.
Instructions
***Important***
There is a known problem with GIMP trying to choose anther brush on the pop up Brushes dialog, making the plugin impossible to run twice in a row.(we hope it gets fixed). In the mean time, to run the plugin again go to "Filter> Reset All Filters" if want to run the plugin and you're set to choose a different brush.




Version 2
Changes:
1. You can choose the brush of your choice before running the plugin.
2. The plugin will run every time you call it.




Version 3
1. Added an option to merge all layer's effects into one layer
2. Fixed a typo "Text size" to "Brush size"(sorry guys) Reduce brush size and now you can be able to change the size up to 550 inside the Marble.

So, you did it!
Issabella will be very happy.
Why not to create a new "joint venture" between you two for the creation of interesting objects?
(no conflict with our JVID, I'm only interested in photo manipulations)

Thank you Pocholo, good job...how can I possibly use a different brush in the glass marble? I've already tried a few things but it always uses the same image.

Pocholo! Wow!!!!!!!
You did a wonderful difficult job with superb success!
I love the result. Your object inside is so attractive, too.
I'm going to do my first result with your filter.

Here my first results.
Thank you so much

Pocholo, when I want to get another design from your three options, the filter will always choose the first option.
Thanks Pocholo.

Wow!!!
Thank you so much Pocholo
What I like a lot is that we got all layers, thus allowing us to modify them as we wish

If you wish you can add to your "Read-Me.txt"
Linux paths for all users
Python script > /usr/lib/gimp/2.0/plug-ins
Brushes > /usr/share/gimp/2.0/brushes

Linux path for a single user
Python script > /home/username/.config/GIMP/2.10/plug-ins
Brushes > /home/username/.config/GIMP/2.10/brushes

My first try

It's "hard coded" @ lines
208 pdb.gimp_brushes_set_brush("Marble ball design #1")
261 pdb.gimp_brushes_set_brush("Marble ball design #1")

While waiting for a fix, we can play by creating a brush and call it Marble ball design #1 (as you did it with your beautiful butterfly )

Sorry I can't wait... so I fixed the brushes' problem (just the choice problem), sorry Pocholo

As usual, any script passing by me can burn your computer, your house and may even nuke your town, even if I changed just 2 tuples


It works with any brush you got (I have some lightning brushes )

You only need to add some information.
Try redoing the plugin without restarting GIMP.
This causes a small "known" problem Issues #6317

Thank you for the head up in any mistake I made or something I missed.
@Jappaloe63 Thank you. The plugin was fixed by PixLab and you can either redownload it from the initial post or from PixLab post #9

@Issabella; same as Jappaloe63. Download the fixed files again from the initial post or from PixLab post #9

@PixLab, Thank you for the catch and fixed. Also, thank you for pointing out the directory for the Linux users, I will write it on the "Read me please file"

@MareroQ, I totally forgot about that PF_BRUSH problem. By the way, you don't have to re-start GIMP if you want to run the plugin again. I found out that going to: Filters> Reset All Filters, will do the trick. I will included in the "Read me please" file. Thank you for reminding me!

Pocholo, you did an excellent job. Thanks a lot.
PixLab, thanks for fixing the problem.
MareroQ, Thanks, too

I did a few more glass balls. a bit gripping, I think.
In colour, too.

Very nicely done Issa. I like how you used the colors on the glass bubble

Thank you for the script Pocholo



Added a loop on top and turned off the bottom shadow

Very nice Teapot, you have that early Christmas spirit!

Really nice, teapot.

Pocholo, Using "Reset All Filters" every time I want to use the plugin would get a bit tiresome. Is there another solution for this problem? Also, how can brushes that users already have, be resized to fit inside the marble? Also, I think there should be a flatten image option. That way users can continue using the plugin on the same size canvas if they wanted to.

I used the fixed version of the plugin and the plugin still only runs once. Also, not all brushes that users have will fit into the dimensions of the glass ball.

An example would be this:



As you can see, the dropbox logo brush doesn't fit into the glass marble.

Of course (at least two):
1. Remind the GIMP authors to fix the error (especially as there is a suggestion to fix the problem) - Recommended
2. Change the code so that the plugin uses the default active brush (so You can also customize the size...).
It will be a minor matter for the plugin author - if he wants to change it?.