No way to opt-out of SimplifyTopology when creating Manifold from MeshGL

[wrongbad]

So I'm stuck again getting my verts and triangles auto-deleted before I can use them.

This time I prototyped a refine_to_length() function in python using MeshGL interface - if the longest edge is >threshold, cut it in half.

from badcad import *
t = text('yo', size=10, font='monospace')
yo = t.offset(1,'round').extrude(2) - t.extrude(2)
yom = yo.to_mesh()
verts = yom.vert_properties.astype(np.float32)
tris = yom.tri_verts.astype(np.uint32)

def refine_to_length(verts, tris, target=1):
    verts = verts.tolist()
    tris = tris.tolist()
    print(len(tris))
    mids = {}
    i = 0
    while i < len(tris):
        tri = tris[i]
        v = [verts[i] for i in tri]
        dv = v - np.roll(v, 1, 0)
        l = np.linalg.norm(dv, axis=-1)
        mi = np.argmax(l)
        ml = l[mi]
        if ml > target:
            key = (min(tri[mi],tri[mi-1]), max(tri[mi],tri[mi-1]))
            if key not in mids:
                mididx = len(verts)
                midv = [(v[mi][j] + v[mi-1][j])/2 for j in [0,1,2]]
                verts += [midv]
                mids[key] = mididx
            else:
                mididx = mids[key]
            tri2 = [*tri]
            tri2[mi-1] = mididx
            tris += [tri2]
            tri[mi] = mididx
        else:
            i += 1
    print(len(tris))
    return np.array(verts, np.float32), np.array(tris, np.int32)

verts, tris = refine_to_length(verts, tris, 1)

display((verts, tris), wireframe=True)

But when I import it into a Manifold in order to use for CSG and Warp, it forcibly simplifies back

display(Manifold(manifold3d.Mesh(verts, tris)), wireframe=True)

[wrongbad]

I'm thinking this might be another case where we should disable simplify by default, since importing Meshes doesn't inherently create redundant faces, and it's more likely the existing mesh structure is intentional by the user.

[wrongbad]

On a semi related note, is it possible to tell which triangles are touching the last boolean op surface? I wonder if it makes sense to apply simplification only to affected triangles and leave the rest untouched.

[wrongbad]

I found a work-around - assigning a unique face_id for every triangle stops simplification here.

m = manifold3d.Mesh(verts, tris, face_id=np.arange(len(tris)))

[pca006132]

I do think that this is desirable, and in general we should probably not collapse triangles that are not created due to boolean operations. @elalish what do you think?

[elalish]

As a performance thing I do think it may be useful to only look for edge collapses among the newly-created verts in a Boolean. I think that shouldn't even be too hard, since we start by putting all the new verts at the end of the list.

I think assigning unique face IDs is the right approach to the OP issue though. Otherwise the behavior would change when exporting and importing a mesh in the middle of several Boolean ops.

[pca006132]

Not sure what you mean by the behavior would change when exporting and importing a mesh in the middle of several boolean ops. I think the mesh is still simplified at the end of each boolean operations (before export), and ideally importing the mesh should not require any further simplification? Currently we do have different behavior if we export and import the mesh again, because our simplification is only best-effort and applying it multiple times can have different results.