Fixes to CSG robustness

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…iangles

Collaboration with OldBelge

Previously, the code assumed that if the intersection line hit the common edge of 2 triangles, it meant that the intersection entered or exited a shape. However, there is also a case where the intersection just "skirts" the other shape on the outside. To handle this case, we added a check for the direction of the normals of the faces. If the inner product of the normals and the intersection is greater than 0, then the intersection is inside the shape.

Fixes: #58637

Still happens: #41140

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Latest test cases CSGBug_2023-03-11.zip

[akien-mga]

I started testing this PR, as I have a project using CSG which suffers from glitches. This PR seems to fix a lot of the problems!

It's a fairly pathological case, we stitched 40 Path3Ds together, generating CSGPolygon3D from each Path3D and using them to subtract from a big CSGBox3D.

Here's how it looks like in 4.0.2-stable:

(same in 4.1 master as of 6bf94cf)
The glitches can move around, forcing a regeneration of the CSGCombiner can typically make things better or worse, so it's clearly a problem of precision issues.

And here's how it looks like with this PR (rebased on latest master):

So... it's definitely better :D

I'll soon make that project open source so it can be used by others for testing too. It's this game: https://akien.itch.io/godog

The above screenshots is from a level we didn't use in the end due to those glitches. But even the playable build on itch has CSG glitches (weird hard lines going through the level where it's supposed to be subtracting geometry), and I confirmed that this PR fixes them.

[YuriSizov]

Does this also fix #73927? (Which may be a duplicate of the linked issue.)

[akien-mga]

I pushed some changes to avoid breaking compatibility. It's still possible to reduce the snap value down to 1e-6 m, i.e. 1 µm, which should help solve situations with missing triangles on small CSG meshes as seen by @goatchurchprime in #41140.

It's not perfect, but hopefully that makes things more usable.

[akien-mga]

Tested some more projects from previously reported issues / duplicates.

This PR does fix the MRPs from those issues:

• Holes in CSG meshes with coplanar faces and engine freeze when moving them #58637
• CSG box and sphere subtract displays cracking from missing triangle  #41140, when changing the snap value to 0.0001 to match the size of the geometry
• https://github.com/team-godog/NGJ2023_Godog
  • main branch, which had a few holes and most importantly missing triangles in the subtraction which would end up as hard lines on top of the track.
  • pathageddon branch, which a much more complex network of Path3Ds which used to be full of holes in the lower part, as shown in Fixes to CSG robustness #74771 (comment) (this branch is iffy when loading in Godot 4.0 or 4.1.dev, be sure to delete .godot to force a reimport, possibly re-edit the project twice, and you might still need to manually trigger a reimport of the shiba.gltf mesh to be able to open the Game scene).

But doesn't fix the MRPs from those issues:

• CSG flicker / missing triangles / holes in the geometry at certain translations #43755
• CSG Hole #75862
• Odd triangle holes will appear in CSG subtraction #22988 (comment) - this one was fixed in the past, and this PR doesn't introduce regressions, but there's still one visible issue of a missing triangle on the pink surface:
  

So it doesn't solve everything, we should probably reopen one of the issues closed as duplicate of #58637, if we're going to close that one. Or open a new one after this is merged, linking the old and new MRPs which still have issues.

[akien-mga]

Thanks!

[justinwash]

Does this also fix #73927? (Which may be a duplicate of the linked issue.)

Testing with the repro project here from #74343 it does not appear so. I have a branch on my fork that fixes this issue by keeping track of which edges have already been processed by the loop. Will post over on issue #73927 to continue the conversation there.

[akien-mga]

Cherry-picked for 4.0.3.