I'm confused by this bit: "The primary reason for a different extension is to not store the rendered images in the wiki database."
Is there a reason GraphViz or PlantUML couldn't just store the images they produce in the same way that the Math extension does?
I wasn't aware of this being an issue until just now. Since I see you've been working on this since July, but I, personally, haven't looked at it closely otherwise, I hope you can provide some insight.
AFAIK, Math just creates a separate, unique directory in the images directory and doesn't try to use the entire WikiImage infrastructure.
This seems like a simpler solution than creating an additional service that people need to install
or the image handling in plantuml and graphviz we need access via the API in read mode
see samwilson/diagrams-extension#9
So if a local image is mentioned we need to get access to it which in the past could be done like in https://www.mediawiki.org/wiki/Topic:Pzvu1613ewnfahcd
For storing images it would be good to allow different methods of reference:
by name
by name / version
by crc32 code
and then it should not make much of a difference whether a local or remote service delivers the result. This would give us quite some flexibility. I find it interesting that there are so many extensions with similar issues:
the extensions have in common that the input is some markup and the output a graphical representation - in case of the 3D extensions even in an interactive manner - and that some external software is needed for rendering.