GEGL can drive you crazy. It has been around some years so came as a bit of a surprise that generally does not work with (k)ubuntu 16.04 - GEGL version 0.3.4 Spent some time before I realised it was not going to work, but does work with the later version in (k)ubuntu 18.04 ver. 0.4.12 which comes from the otto-kesselgulasch PPA for Gimp.
A quick example with both. 0.3 gives no result, 0.4 complains but gives a result. https://i.imgur.com/dGc8dB2.jpg
Øyvind Kolås had a good basic introduction somewhere (but I cant find it again
so attached a PDF) However, generally, all there on the GEGL site with some examples http://gegl.org/gegl-chain.html
A quick example with both. 0.3 gives no result, 0.4 complains but gives a result. https://i.imgur.com/dGc8dB2.jpg
Øyvind Kolås had a good basic introduction somewhere (but I cant find it again
so attached a PDF) However, generally, all there on the GEGL site with some examples http://gegl.org/gegl-chain.html