I've also added an option in the G'MIC
Patterns/Op art filter to allow an user to add its own custom shapes defined in top layers over the image. The entry 'Custom shapes' must be selected for the parameter 'Shape', like I did in the screenshot below :
Attachment:
gmic_opart.png
It's a bit similar to the way the
Pack sprites works.
You can still have a control on the minimal and maximal size of your shapes with the sliders.
Finally, you get something like this (added a black background to get rid of the transparency) :
Attachment:
gmic_custom_opart.jpg
Awesome David; was waiting for the G'MIC equivalent. Would be close to packed sprites except that you don't pack them; just align them. lol

There are still some minor problems with the preview, but I hope I can fix them soon.
Hope you will enjoy it.