Border around holes / Smoothing objects

[TemperMichael]

Is there a way to efficiently create a border around holes? My current approach seems quite performance heavy as it lags for a more complex model. I tried all day to find a solution to get the path of the hole and use Mesh.stroke, but I couldn't figure it out myself.

My current approach:

        let border = hull                        // hull = OriginalMesh.scaled(by: 1.05).subtracting(OriginalMesh.scaled(by: 0.95)
            .intersection(holeSphere)            // holeSphere = sphere.subtracting(sphere.scaled(by: 0.8))
            .subtracting(secondBorderMesh)       // Subtract overlapping border
            .withMaterial(Euclid.MaterialWrapper(material))

And since the model is quite low-poly to prevent lags, the resulting cut out border also has quite some edges in it as this comes from the "hull" of the model. Or is there any way to make a Mesh smoother afterwards? Thanks in advance!

[nicklockwood]

Hi, sorry for the late response. I don't think there's currently an easier way to do this, but I think I could probably expose an API to return the intersection between shapes as a path.

Could you possibly share what you have so far (or a similar sample) so I can tinker with it? It'll save me some time trying to create a test project.

[TemperMichael]

No problem, I am just happy to get help!

Here is the example code I use in my project - I tried to highlight my questions with a "TODO" so you can simply search for them.

BorderExample.zip

[nicklockwood]

Awesome! Thanks so much ☺️

[TemperMichael]

@nicklockwood I was wondering if you already found some solutions?

There is no rush on this - I will just start with beta testing this week and just wanted to see if I eventually can already improve some things before giving them the current build.

[nicklockwood]

@TemperMichael I'm still working on it, but sorry I don't have any updates right now.

I'm ~100% confident that I can provide an API that returns a path representing the intersection boundary between two meshes, but I'm a little less certain about when I'll find the time to ship it 😅

[TemperMichael]

Sure, no worries, take your time! I am happy to get any help at all 😄

[nicklockwood]

@TemperMichael it turned out to be simpler than I expected! I've not stress tested it yet, but I shipped a preliminary API for this in the Euclid release today.

You use it like this (I'm writing this on mobile so apologies for any errors but hopefully you should get the gist):

let a = // some mesh
let b = // some other mesh

// edges is a set of undirected line segments
// representing all the points at which a polygon
// from mesh a intersects a polygon from mesh b
let edges = a.edges(intersecting: b)

// you can do various things with line segments
// directly, but probably for your use case it would
// be more useful to convert them to a path
let boundary = Path(edges)

// the Path constructor will do its best to stitch the
// assorted edges into one or more continuous contours
// you can then apply a stroke to this path, or use it as
// input into other geometry generation functions
let borderMesh = Mesh.stroke(boundary, color: .red)

The performance of the edges(intersecting:) function should be comparable to a regular CSG clip operation - maybe a bit faster since it can discard most polygons early, and doesn't need to generate a BSP from both meshes.

You will probably still need to do a regular CSG difference operation as well to actually make a hole in the mesh. You should probably generate the border first before applying the difference to avoid weird edge cases (literally! 😅)

If you are making several holes and trying to create a unified border mesh then you may have to play with the correct order of operations - it'll be something like:

first hole

• Generate edges 1 for hole mesh 1
• Subtract hole mesh 1 from blob mesh
• create border path 1 from edges 1

second hole

• Generate edges 2 for hole mesh 2
• Subtract hole mesh 2 from blob mesh
• Also subtract hole mesh 2 from edges 1
• discard border path 1
• merge edges 1 and edges 2 sets
• create border path 2 from merged edges

And so on. You should be able to clip edges directly to a mesh but I don't recall. If not let me know and I'll add that. It's more efficient to do that and regenerate the path than to try to clip the already - generated path, and it will probably look better too.

There may also be some performance benefit to reusing the same mesh instance for successive operations, but I'm not 💯 sure about that.

[TemperMichael]

Awesome, thank you very much for the quick solution, really appreciate that! 🙏 I will immediately give it a try! 😄

Oh and I will start with the beta tests of my game this week probably, so if you have a Vision Pro & would like to give it a try I am happy to share a TestFlight link 🙂

Update: I tried it now and it's AMAZING! It's super fast, I can use my high poly mesh and it makes the border immediately 🙌 (Actually, with my low poly mesh, creating the path didn't really work - it made some crazy object. Creating a stroke with only the edges had at least the correct shape)

The only last step I am missing is what you described last, I think it's the clipping functionality you mentioned? So basically, I don't want to merge the edges, I want two separate subtracted borders like in the original picture. I just couldn't find a way to subtract a mesh from edges. I was able to kinda get the result by creating the stroke first and then subtracting the hole mesh 2 from that stroke mesh - but then again I had small lags and also sometimes the app crashed.

So here is how I would like to do it if I got your suggestion right:

let edges1 = blobMesh.edges(intersecting: holeMesh1)
        
// This method does not exist, how can I do that correctly?
let remainingEdges = edges1.subtracting(holeMesh2)
        
let path = Path(remainingEdges)
                
let border = Mesh.stroke(path, width: 0.03, detail: 6)

[TemperMichael]

Sorry for bumping this again, but since I want to release the game next week and the high poly mesh would be a great final touch, I just wanted to ask again about the clipping issue.

I totally get it if you just have no time currently, it's also fine if I add this polish at some later time. After all your efforts there will be definitely some coffees on me, thanks again for your help! 🙏

[nicklockwood]

@TemperMichael hi, sorry I somehow completely missed your previous reply - I'll take a look now

[nicklockwood]

@TemperMichael I've added a method to subtract a mesh from a set of line segments. I've not had much chance to test it yet, but I've pushed it to the main branch so you can try it.

[TemperMichael]

Awesome thank you! I will test it then directly later.

[nicklockwood]

FYI, I've now released this in version 0.7.13

[TemperMichael]

Thanks for letting me know! I played around with it all day and I am getting closer and closer to what I want. The performance is great now with your latest changes!

The only issue is, that I have sometimes some kind of fragments as shown in this picture:

It especially appears on (rounded) edges and I think I was able to pinpoint it down to the .edges(intersecting: subMesh) method.

This is how I do it now:

        var final = box
            .edges(intersecting: cutoutSphere)

        if !previousBorder.polygons.isEmpty, previousBorder.isWatertight {
            final = final.subtracting(previousBorder)
        }
        
        let path = Path(final)

        let border = Mesh.stroke(path, width: 0.03, detail: 16)
            .makeWatertight()

If I do the same with Mesh.stroke(final, width: 0.03, detail: 16) instead of using the path, the dotted line segments sometimes do not show up at all (which I think is the equivalent of the fragments with using the path)

I also tried rounding up all kind of different transforms I use on the box/sphere but that also didn't fix the issue.

Lastly, I also think it is also somehow related to how fast it can be computed since everything works correctly if I reduce the slices from 64 to 34:

    let cutoutSphere: Mesh = Mesh.sphere(
        radius: 0.1,
        slices: 34,
        faces: .front,
        wrapMode: .none,
        material: MaterialWrapper(UnlitMaterial.clear)
    ).withoutTexcoords()

I just thought I mention all that in case you know a quick fix for that, otherwise I am also already happy with the 34 slices solution - so thank you very much for taking the time to help me! 🙏

[nicklockwood]

I think it's less likely to be related to performance and more that with smaller polygons there's probably some edge case where a very small value yields a large error (like how divide by zero -> infinity)

It would be super helpful if you were able to isolate some particular example geometry that causes the problem - e.g. the exact relative mesh positions or the resultant LineSegment Set