Holes in CSG meshes with coplanar faces and engine freeze when moving them

[codecat]

Godot version

4.0.alpha3

System information

Windows 11

Issue description

I am generating a number of CSGMesh3D objects and putting them in a CSGCombiner3D. All meshes are set to be a union. Sometimes, this generates random holes:

And, potentially related, when moving the mesh with the hole on the Z axis, the engine freezes.

Steps to reproduce

1. Open csg_bug.tscn in the editor
2. Look at the red CSG mesh
3. Optionally, try to move the red CSG mesh on the Z axis

Minimal reproduction project

CSGBug.zip

[Calinou]

Coplanar faces and CSG don't mix well. I suggest moving around your CSG nodes slightly (0.001 units will do) to avoid having coplanar faces. This is something that could be handled automatically internally, but I don't know how much complexity it would add to be done properly.

Accurate CSG algorithms (that handle coplanar faces correctly) are much slower, so I don't know if it's a viable tradeoff given many users are using those nodes at run-time.

[codecat]

Thanks! Moving around the nodes seems like a bit of a hack, and kinda breaks the use-case I need CSG for.

Accurate CSG algorithms (that handle coplanar faces correctly) are much slower

Kinda off-topic, but do you happen to know of any such algorithms that I can drop into C or C++?

[Calinou]

Kinda off-topic, but do you happen to know of any such algorithms that I can drop into C or C++?

I don't have any resources for this (as most CSG libraries or implementations are GPL-licensed, such as Blender's or CGAL). reduz had to write a custom CSG implementation for Godot to get something permissively licensed…

[hoontee]

Related to / duplicate of #41140.

[golddotasksquestions]

@Calinou

Accurate CSG algorithms (that handle coplanar faces correctly) are much slower, so I don't know if it's a viable tradeoff given many users are using those nodes at run-time.

There is no point in using CSG or CSG operations if the operation results in a mesh with holes. Not only does it look broken, it also results in bad prototyping functionality because to resulting collision shape is broken too.

I have had CSG blockout prototype levels were I failed to arrange the shapes in such a way there were no holes in the scene, despite knowing about he workaround fix of avoiding coplanar faces. In certain situations there is just always going to be coplanar faces. Also sometimes it is just not feasible to avoid them because non coplanar faces would create collision issues during gameplay.

I would at the very least offer this slower CSG algorithm as an option in the Project Settings. If is reduces this issue drastically it will be definitely worth the time it takes to compute the new mesh. Recalculating CSG shapes in game during runtime is strongly discouraged anyway already (because of performance) and does not really make much sense in most cases unless the user is building an editor, not a game. And if the user is creating an editor, the slower CSG solver will be no or much less of an issue, just like it is almost no issue in the Godot editor right now.

[fire]

I have tested #74771 fixes this bug.

[capnm]

eaa84bc fixes only the visual appearance.

3. Translating some CSG Mesh on the Z axis still causes a deadlock inside the calculation of the shapes.
   (As already assumed in the PR, 9f12e7b doesn't fixes this case.)

* thread #1, name = 'godot.linuxbsd.', stop reason = step over
  * frame #0: 0x0000555556c1ebc4 godot.linuxbsd.editor.dev.x86_64`CowData<CSGBrushOperation::Build2DFaces::Face2D>::insert(this=0x000055556f642f70, p_pos=45, p_val=0x00007fffffffcd80) at cowdata.h:178:11
    frame #1: 0x0000555556c1acb6 godot.linuxbsd.editor.dev.x86_64`CSGBrushOperation::Build2DFaces::_find_edge_intersections(Vector2 const*, Vector<int>&) [inlined] Vector<CSGBrushOperation::Build2DFaces::Face2D>::insert(this=0x000055556f642f68, p_pos=45, p_val=Face2D @ 0x00007fffffffcd80) at vector.h:94:53
    frame #2: 0x0000555556c1acab godot.linuxbsd.editor.dev.x86_64`CSGBrushOperation::Build2DFaces::_find_edge_intersections(this=0x000055556f642f58, p_segment_points=0x00007fffffffceb0, r_segment_indices=0x00007fffffffcea0) at csg.cpp:1149:11
    frame #3: 0x0000555556c16a28 godot.linuxbsd.editor.dev.x86_64`CSGBrushOperation::Build2DFaces::insert(this=0x000055556f642f58, p_brush=0x0000555576e41750, p_face_idx=<unavailable>) at csg.cpp:1348:4
    frame #4: 0x0000555556c15190 godot.linuxbsd.editor.dev.x86_64`CSGBrushOperation::update_faces(this=<unavailable>, p_brush_a=0x0000555576e41750, p_face_idx_a=<unavailable>, p_brush_b=0x0000555576acf3a0, p_face_idx_b=4, p_collection=0x00007fffffffd1b0, p_vertex_snap=0) at csg.cpp:1529:43
    frame #5: 0x0000555556c1183f godot.linuxbsd.editor.dev.x86_64`CSGBrushOperation::merge_brushes(this=<unavailable>, p_operation=OPERATION_UNION, p_brush_a=0x0000555576e41750, p_brush_b=0x0000555576acf3a0, r_merged_brush=0x00005555704b1cf0, p_vertex_snap=0) at csg.cpp:289:5
    frame #6: 0x0000555556be667d godot.linuxbsd.editor.dev.x86_64`CSGShape3D::_get_brush(this=0x0000555575c88820) at csg_shape.cpp:0:5
    frame #7: 0x0000555556be6ca2 godot.linuxbsd.editor.dev.x86_64`CSGShape3D::_update_shape(this=0x0000555575c88820) at csg_shape.cpp:302:16
...