Use overhang detection threshold for support material

[alranel]

As discussed with @jonaskuehling in #2301, overhang detection for perimeters and support material generation use different logic, so they might not catch the same things. The former requires that the overhang is equal or greater to one nozzle diameter (i.e. it's an overhang if nozzle is pushing plastic entirely on the void), while the latter triggers support whenever the loop centerline lies on void. Support material is thus generated more often. This level of sensitivity would be too much for perimeter overhang detection (it was implemented like this, and it produced lots of changes in flow causing bad quality on vases) so I think it would be correct to use the nozzle diameter logic for support material as well.

[jonaskuehling]

Ah ok, I get it. But...

I totally agree that the current implementation partly generates support that is non-effective since the to-be-supported perimeters won't touch it as they won't sag onto it due to being partly supported by perimeters underneath. Thus, as you suggested, avoiding those ineffective support regions will likely not make anything worse.

But this will not solve the fact, that there is no working support for certain overhang angles, at least for manual choice of the overhang angle threshold. The current implementation requires supported regions to kind of sag onto the support surface, which works great for plane bottom model surfaces and larger regions that are supported. This doesen't help for those perimeters that are not entirely overhanging as you described, but still overlapping by e.g. half with the layer below for example - think of 45° overhang angle.

We may need two different kinds of support at the same time i guess: One for perimeters/surfaces printed completely on the void (like on top of raft..), and one for perimeters overlapping with lower layers. Maybe this could be made kind of similar to the brim feature? So some kind of support anchoring from the side (support printed before supported perimeter, but in same layer), possibly also with some zig-zag to allow easier removal later..
This could dramatically improve overhangs with e.g. ABS which tends to contract quite a lot during cooldown, leading to high upcurling tendencies on overhang angles like 40-60°.

Hope you get my idea..? Let me know, could add some quick drawings otherwise for explanation.

[simonkuehling]

@alexrj: just to ensure i follow your proposal correctly: do you suggest to limit support material generation to perimeters that have no overlap with the previous layer?

I think this would be counter-productive, as our observations clearly indicate that the current detection of support-needing surfaces is quite good. Provided that this material would indeed always correctly attach to the model perimeters (through slow extrusion speeds like we do with bridging at the moment), the current support would work really well.

(Please correct me if i am on the wrong track here..)