For a number of my custom filters users need to pick a number of cells from an unstructured grid interactively. So far I am using the ParaView selection system for this: the user picks cells and the filter can process it.
One problem with this was initially the fact that selections cannot be saved to a state file, so I managed to save the list of cell IDs as a property, and the fact that always only one selection can exist in a pipeline is overcome by some tricky ways to restore the selection programmatically if the user displays an item, based on that IDs property.
However, it turns out that this “hackish” feature tends to always add some instability to the pipelines: users tend to lose their IDs property and the function of the pipeline “breaks down”. Mostly not, but sometimes, and people always have to be careful.
So what I am dreaming about: Some kind of separate “interaction system” that allows to select grid cells in a way that does not depend on the common ParaView selection system. A bit in a way like the “clip” filter has some interactive ways to specify the parameters of the clip surface etc.
I know that such things can be rather tricky, so I did not “dare” such a project yet. Anyway, if somebody could give me some helpful hints towards examples and/or tutorials, I would consider to give it a try!