Feature request: Intersection Polyline

[yetigit]

Hi, thank you for your efforts on doing a fast and robust csg lib

what would be a killer feature is being able to retrieve the intersection data, precisely as naturally ordered polyline(s).
which boils down to just having the intersection points in the right order ; with a topological predicate since there can be several intersection loops.

here a sphere vs a cube

cube vs cube

[elalish]

Check out my GetMeshRelation() function. It returns this data (in a different format, suitable for multiple uses like texture mapping), even after a whole series of Boolean operations. It should be pretty simple to convert it to the polyline format you desire; feel free to add a PR!

[hrgdavor]

I am still on the sidelines watching manifold progress, and have used it only a bit, but I am wondering about a thing regarding union and filet.

There is a use case when unioning two shapes to filet the intersection edge like this issue: openscad/openscad#884 ...

Is it possible to get something in that direction by finding the edge after union and using smooth in manifold ?
if possible, it would make for a great example script on monifoldcad web.

[elalish]

Yes, part of why I developed our smoothing algorithms is to eventually enable features like fillet. However, it still needs considerable work: beyond just identifying the intersection edges, you'll also need to create new vertices that represent the edges of the fillet and remove the actual intersection verts. Basically we need a MakeChamfer function first, then from that we can use smoothing to turn the chamfer into a fillet.

[hrgdavor]

@elalish that is great to hear. I am glad you are looking to tackling this eventually :)

[elalish]

If you know anything about chamfer algorithms, I'm all ears. I think that will still be pretty hard.

[hrgdavor]

aah, sorry :) my knowledge is lacking in computational geometry. Best I can do for now is to cheer you :) and hope you can find something workable for chamfer.

[ofmetal]

So, what is the best way to get that intersection polyline in late 2024 ?

[elalish]

GetMeshGL(), pair up the halfedges (I wonder if it would be enough added convenience to expose these someday?), find any pairs whose triangles are from different runs - these are your intersection edges. Connect the halfedges from one run head-to-tail to form the polylines.

[ofmetal]

The halfedges - the ones that could be extracted from triVerts array?
Plus triangle's parent mesh id from the runIndex array? Or is it runOriginalID?

[elalish]

Yes and yes. The runOriginalID tells you the parent mesh id for each run. If you happen to intersect two copies of the same mesh, that ID will be the same, so safest to just check which part of the runIndex array you're in - as those will always be distinct even if they correspond to the same ID.