Odd Results from Barycentric Coordinates and Mesh Normals [4.2 dev6]

Scylla-Leeezard Harden the side edges in Blender. Normals on the mesh look smoothed even for 90 deg edges, so 3 vertex normals for each triangle will differ greatly and the interpolation from barycentric weights will cause the hit normal to fluctuate a lot as you traverse each triangle.

@Scylla-Leeezard - Were you able to resolve this issue? May I ask that you share the solution with us?

I'm experiencing the same problem, despite hardening the side edges and smoothing the shading in Blender. The moment my character comes in contact with my track and starts using barycentric coordinates to determine gravity, I get extremely erratic results. None of which seem to correlate to the expected normal vector.

chetbeigemeister Geometry3D.get_triangle_barycentric_coords() is a generalized function that doesn't know anything about transformations. It just returns barycentric interpolation coords for a given point in a triangle. It's responsibility of the caller to put all arguments into the same transformations space. Otherwise the results won't be correct if mesh instance have any non-identity transforms.

The original code made a mistake of taking the triangle coordinates in mesh/object space while keeping the interpolated point position (gotten from raycasting) in global space.

If you need the normal in global space you can either:
1) transform the triangle vertices from mesh space to global space prior to passing them to bary function, using mesh instance's global_transform. And then transform vertex normals from mesh space to global space prior to interpolating them to get the final normal.
Or
2) transform the raycast point from global space to mesh space, get barycentric coordinates in the mesh space and interpolate the normal in the mesh space as well. Then transform the final normal from mesh space to global space, again using mesh instance's global_transform.

xyz Hello! I am so sorry to necropost; however, you are the only person I've been able to find giving good advice on this topic.

I'm getting similar results (mainly post 4) as OP using mostly similar code. I've experimented with your answer and I cannot seem to figure out where exactly you would convert between local and global spaces. I've tried to think about it logically, but I can't seem to get it right.

1. Could one just multiply the vertices in the for loop by global_transform and leave it that? For example: var vertices: Array[Vector3] = other.get_vertex_positions_at_face_index(ray_cast.get_collision_face_index()) * global_transform

2. When you say "transform vertex normals from mesh space to global space prior to interpolating them to get the final normal," are you saying that you could do the same thing as the vertices array (multiply the normals during iteration by global_transform)? I think what I'm not understanding is the "prior to interpolation" part - as in, what step in the process that would be. Wouldn't that be when OP calls and passes the normal align_up_direction?

xyz Yes it does!

To give you an idea of what is working, here is the core code. Adapted from OP's code, but I made the global changes.

You can see I multiply by each vertex by GlobalTransform, and I also multiply it to upNormal when I pass it to SetUpDirection().

Vector3[] vertices = new Vector3[3];
Vector3[] normals = new Vector3[3];

for (int i = 0; i < 3; i++)
{
    vertices[i] = meshData.GetVertex(meshData.GetFaceVertex(normRay.GetCollisionFaceIndex(), i)) * GlobalTransform;
    normals[i] = meshData.GetVertexNormal(meshData.GetFaceVertex(normRay.GetCollisionFaceIndex(), i));
}

Vector3 baryCoords = Geometry3D.GetTriangleBarycentricCoords(normRay.GetCollisionPoint(), vertices[0], vertices[1], vertices[2]);

Vector3 upNormal = (normals[0] * baryCoords.X) + (normals[1] * baryCoords.Y) + (normals[2] * baryCoords.Z);
upNormal = upNormal.Normalized();
SetUpDirection(upNormal * GlobalTransform, delta);

In SetUpDirection(), you can see I'm using GlobalTransform as well.

  private void SetUpDirection(Vector3 upNormal, double delta)
  {
      Transform3D normTransform = GlobalTransform;
      normTransform.Basis.Y = upNormal;
      normTransform.Basis.X = -normTransform.Basis.Z.Cross(normTransform.Basis.Y);
      normTransform = normTransform.Orthonormalized();
      GlobalTransform = GlobalTransform.InterpolateWith(normTransform, .5f);
  }

paftdunk Matrix multiplication is not commutative. The order of operands is important. To transform a vertex by a matrix the order should be matrix * vertex. Your code is doing vertex * matrix. This is equivalent to multiplying with a transposed matrix which would result in inverse transformation (if the matrix is orthonormal).

Also to transform a normal properly you need to nullify the translation part of the matrix or use only 3x3 submatrix aka the basis. So you can do matrix.basis * normal_vector. If there is some proportional scaling in the basis you either need to orthonormalize the basis prior to multiplication or normalize the resulting normal vector. If there is non proportional scaling in the matrix, you need to use transposed inverse of the actual matrix. Since Godot's transform class doesn't implement transpose function, you can do normal_vector * matrix.basis.affine_inverse(), which now reverses the operand order to transpose the matrix.

paftdunk Post a better image and complete code. Hard to see what's happening there. Remove the interpolation in SetUpDirection for now. First make it work instantly. Just assign the final transform you calculated. Interpolation may introduce other problems. You interpolate barycentric coords anyway so it should work smooth (when on smooth surfaces) without that last interpolation. Always try to isolate the issue into minimum of code. Oh, and you need a second cross product there when calculating the final basis to ensure the wanted orthogonality.