Possibly an unanswerable meta-question:
I was playing with stitch sub-paths LPE for creating hatching in line art. I accidentally stumbled into some effects I don't see documented anywhere but seem potentially useful. My question is how worried should I be that they are effects that were never intended and therefore may disappear in some future release.

In the image below, I drew a curve, duplicated, spread them apart a bit, hit Ctrl-K, then applied the stitch sub-paths LPE.
The left-most image only uses documented features.
In the central image, I simply set the fill (previously unset) to black and slightly moved the edit-on-canvas stitch path. I would never have guessed that setting the fill would create this (potentially useful for shading effects) behavior. I'm worried this is a bug that might get fixed.
In the right-most image, I also added nodes to the edit-on-canvas stitch path and tweaked them to create two shadows with a highlight between. I'm guessing adding nodes might be an intended feature, but can't be highly confident as it's not documented.

It's neither a bug, nor an undocumented feature. It's just an arrangement of filled, multiplicated, interpolated paths.

Documentation is here:
http://tavmjong.free.fr/INKSCAPE/MANUAL ... Paths.html

It's a nice effect / trick, though. I haven't seen it used in this way before. Cool discovery!

Ah, I see. I was thinking "line" and couldn't see how turning on the fill should matter. My mental model for "path" needs to move closer to reality.

Thanks, I'll use it without fear then!

It is more complicated than it seems, but you can be fairly sure it's not going away. It's working this way because each segment is a sub path, not a path. When you bend them on screen they overlap each other creating a large filled path that self intersects. in the fill dialog you can choose one of two options for how inkscape handles self intersecting filled paths. Either option should give you the result you are seeing but if you bend them more you are likely to see triple overlaps which will require you to select the correct option in the fill dialog to get the result you want. Here are two versions of the same path filled using the two different rules



One laast edit: in the interest of thoroughness, when I said this capability is unlikely to go away, there is the remote possibility that at some time there may be an addition to SVG that specifies 'Only fill closed paths' which would end what you are seeing here.

@druban: I cannot see any overlaps, it works just the same when you do object to path and Path > Break apart, to have separate paths (in the depicted case). But somehow it looks like you knew that - and still gave it as a reason for it looking how it does. You're contradicting yourself a bit here.

I have attached an image above. I'm sorry if I have not made myself clear but since you know a lot more than I do I will just accept your statement instead of beginning some sort of time wasting argument.

For that newly attached image, it's true. Only not for the one from the OP - and your image wasn't there last time I looked.