Question about "binormal" calculation.

I use the following code to draw tangent, normal,binormal.but the w of tangent always be -1.
Vector3[] vertices = mesh.vertices;
Vector3[] normals = mesh.normals;
Vector4[] tangents = mesh.tangents;
for (int i = 0; i < vertices.Length; i++) {
ShowTangentSpace(
transform.TransformPoint(vertices),
transform.TransformDirection(normals),
transform.TransformDirection(tangents),
tangents.w
);
}
void ShowTangentSpace (
Vector3 vertex, Vector3 normal, Vector3 tangent, float binormalSign
) {
Gizmos.color = Color.green;
Gizmos.DrawLine(vertex, vertex + normal * scale);
Gizmos.color = Color.red;
Gizmos.DrawLine(vertex, vertex + tangent * scale);
Vector3 binormal = Vector3.Cross(normal, tangent) * binormalSign;
Gizmos.color = Color.blue;
Gizmos.DrawLine(vertex, vertex + binormal * scale);
}
the result is right ,However the tangents.w is -1,which seems like make the local space form left-handed to right-handed.Then why not just tangents.w=1 and modify the binormal calculation?using Vector3.Cross(tangent,normal).I think it is more simple.

 

The w component is used to handle mirrored UVs, so it might be -1 or 1 depending on the UVs. Performance wise, since the value needs to exist, it doesn't matter if it defaults to -1 or 1. Correctness wise, the bitangent / binormal is defined as the cross product of the normal and tangent, so it makes sense to keep the calculations correctly in the space & handedness it’s actually in, and use -1 to be explicit about the orientation flip of the spaces.

Ultimately all that matters is that there are an even number of sign errors.