Attention: Here be dragons
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Helper class for creating and clearing navigation meshes.
This class is responsible for creating and clearing 3D navigation meshes used as NavigationMesh resources inside NavigationRegion3D. The NavigationMeshGenerator has very limited to no use for 2D as the navigation mesh baking process expects 3D node types and 3D source geometry to parse.
The entire navigation mesh baking is best done in a separate thread as the voxelization, collision tests and mesh optimization steps involved are very performance and time hungry operations.
Navigation mesh baking happens in multiple steps and the result depends on 3D source geometry and properties of the NavigationMesh resource. In the first step, starting from a root node and depending on NavigationMesh properties all valid 3D source geometry nodes are collected from the SceneTree. Second, all collected nodes are parsed for their relevant 3D geometry data and a combined 3D mesh is build. Due to the many different types of parsable objects, from normal MeshInstance3Ds to CSGShape3Ds or various CollisionObject3Ds, some operations to collect geometry data can trigger RenderingServer and PhysicsServer3D synchronizations. Server synchronization can have a negative effect on baking time or framerate as it often involves Mutex locking for thread security. Many parsable objects and the continuous synchronization with other threaded Servers can increase the baking time significantly. On the other hand only a few but very large and complex objects will take some time to prepare for the Servers which can noticeably stall the next frame render. As a general rule the total number of parsable objects and their individual size and complexity should be balanced to avoid framerate issues or very long baking times. The combined mesh is then passed to the Recast Navigation Object to test the source geometry for walkable terrain suitable to NavigationMesh agent properties by creating a voxel world around the meshes bounding area.
Note: Using meshes to not only define walkable surfaces but also obstruct navigation baking does not always work. The navigation baking has no concept of what is a geometry "inside" when dealing with mesh source geometry and this is intentional. Depending on current baking parameters, as soon as the obstructing mesh is large enough to fit a navigation mesh area inside, the baking will generate navigation mesh areas that are inside the obstructing source geometry mesh.