Hey,
So I subclassed the effects to provide an easier way to just add filters on a group of objects. Hopefully bringing this into the realm where anyone who understands the filter dialog could build their own filter effect. It needs to documentation and overall is less than complete, but I thought I'd put it out there for people to play with.
I took all the examples from Mauve's excellent example file[1] and turned them into effects on the effects menu. Now you should be able to just grab and object and apply these to your objects.
To create your own I suggest copying "sepia.h" in /src/extensions/internal/filter and replacing the strings. All you need is a non-translated name (ID), Menu name, description and then the filter that you want to apply. You'll then need to add it to "filter-all.cpp" but that is also pretty straight-forward. If someone isn't faster, I'll make better docs. [2]
It is a little more difficult to use parameters, but not that hard. There is one example of that, drop shadow, which of course works with live-effects also. I would also like to clean this up and make it easier.
The biggest limitation of the implementation right now is that it will ONLY apply if there isn't a filter on the object currently. Long term, I'd like it to merge the filters.
That's all for now!
--Ted
[1] I didn't take Drop Shadow from there because I was looking at the SVG file and the filter for Drop Shadow is above the linear gradient where I started looking, so I missed it. No seriously, stop laughing.
[2] I would like for someone with a light programming background to do this as they're likely not to skip over things that other people would need to get started.